draft-ietf-tcpm-converters-14.txt		draft-ietf-tcpm-converters-15.txt
	TCPM Working Group O. Bonaventure, Ed.		TCPM Working Group O. Bonaventure, Ed.
	Internet-Draft Tessares		Internet-Draft Tessares
	Intended status: Experimental M. Boucadair, Ed.		Intended status: Experimental M. Boucadair, Ed.
	Expires: May 7, 2020 Orange		Expires: August 14, 2020 Orange
	S. Gundavelli		S. Gundavelli
	Cisco		Cisco
	S. Seo		S. Seo
	Korea Telecom		Korea Telecom
	B. Hesmans		B. Hesmans
	Tessares		Tessares
	November 04, 2019		February 11, 2020
	0-RTT TCP Convert Protocol		0-RTT TCP Convert Protocol
	draft-ietf-tcpm-converters-14		draft-ietf-tcpm-converters-15
	Abstract		Abstract
	This document specifies an application proxy, called Transport		This document specifies an application proxy, called Transport
	Converter, to assist the deployment of TCP extensions such as		Converter, to assist the deployment of TCP extensions such as
	Multipath TCP. This proxy is designed to avoid inducing extra delay		Multipath TCP. A Transport Converter may provide conversion service
	when involved in a network-assisted connection (that is, 0-RTT).		for one or more TCP extensions. The conversion service is provided
			by means of the TCP Convert Protocol (Convert).
	This specification assumes an explicit model, where the proxy is
	explicitly configured on hosts.
	-- Editorial Note (To be removed by RFC Editor)
	Please update these statements with the RFC number to be assigned to		This protocol provides 0-RTT (Zero Round-Trip Time) conversion
	this document: [This-RFC]		service since no extra delay is induced by the protocol compared to
			connections that are not proxied. Also, the Convert Protocol does
			not require any encapsulation (no tunnels, whatsoever).
	Please update TBA statements with the port number to be assigned to		This specification assumes an explicit model, where the Transport
	the 0-RTT TCP Convert Protocol.		Converter is explicitly configured on hosts.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on May 7, 2020.
			This Internet-Draft will expire on August 14, 2020.
	Copyright Notice		Copyright Notice
	Copyright (c) 2019 IETF Trust and the persons identified as the		Copyright (c) 2020 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. The Problem . . . . . . . . . . . . . . . . . . . . . . . 3		1.1. The Problem . . . . . . . . . . . . . . . . . . . . . . . 3
	1.2. Network-Assisted Connections: The Rationale . . . . . . . 4		1.2. Network-Assisted Connections: The Rationale . . . . . . . 4
	2. Conventions and Definitions . . . . . . . . . . . . . . . . . 6		2. Differences with SOCKSv5 . . . . . . . . . . . . . . . . . . 6
	3. Architecture & Behaviors . . . . . . . . . . . . . . . . . . 7		3. Conventions and Definitions . . . . . . . . . . . . . . . . . 8
	3.1. Functional Elements . . . . . . . . . . . . . . . . . . . 7		4. Architecture & Behaviors . . . . . . . . . . . . . . . . . . 9
	3.2. Theory of Operation . . . . . . . . . . . . . . . . . . . 9		4.1. Functional Elements . . . . . . . . . . . . . . . . . . . 9
	3.3. Data Processing at the Transport Converter . . . . . . . 12		4.2. Theory of Operation . . . . . . . . . . . . . . . . . . . 11
	3.3.1. Base Behavior . . . . . . . . . . . . . . . . . . . . 12		4.3. Data Processing at the Transport Converter . . . . . . . 14
	3.3.2. Multipath TCP Specifics . . . . . . . . . . . . . . . 14		4.4. Address Preservation vs. Address Sharing . . . . . . . . 16
	4. Sample Examples . . . . . . . . . . . . . . . . . . . . . . . 15		4.4.1. Address Preservation . . . . . . . . . . . . . . . . 16
	4.1. Outgoing Converter-Assisted Multipath TCP Connections . . 15		4.4.2. Address/Prefix Sharing . . . . . . . . . . . . . . . 17
	4.2. Incoming Converter-Assisted Multipath TCP Connection . . 16		5. Sample Examples . . . . . . . . . . . . . . . . . . . . . . . 18
	5. The Convert Protocol (Convert) . . . . . . . . . . . . . . . 17		5.1. Outgoing Converter-Assisted Multipath TCP Connections . . 18
	5.1. The Convert Fixed Header . . . . . . . . . . . . . . . . 18		5.2. Incoming Converter-Assisted Multipath TCP Connection . . 20
	5.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . . . 18		6. The Convert Protocol (Convert) . . . . . . . . . . . . . . . 21
	5.2.1. Generic Convert TLV Format . . . . . . . . . . . . . 18		6.1. The Convert Fixed Header . . . . . . . . . . . . . . . . 22
	5.2.2. Summary of Supported Convert TLVs . . . . . . . . . . 19		6.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . . . 22
	5.2.3. The Info TLV . . . . . . . . . . . . . . . . . . . . 20		6.2.1. Generic Convert TLV Format . . . . . . . . . . . . . 22
	5.2.4. Supported TCP Extensions TLV . . . . . . . . . . . . 20		6.2.2. Summary of Supported Convert TLVs . . . . . . . . . . 23
	5.2.5. Connect TLV . . . . . . . . . . . . . . . . . . . . . 21		6.2.3. The Info TLV . . . . . . . . . . . . . . . . . . . . 24
	5.2.6. Extended TCP Header TLV . . . . . . . . . . . . . . . 23		6.2.4. Supported TCP Extensions TLV . . . . . . . . . . . . 24
	5.2.7. The Cookie TLV . . . . . . . . . . . . . . . . . . . 24		6.2.5. Connect TLV . . . . . . . . . . . . . . . . . . . . . 25
	5.2.8. Error TLV . . . . . . . . . . . . . . . . . . . . . . 24		6.2.6. Extended TCP Header TLV . . . . . . . . . . . . . . . 28
	6. Compatibility of Specific TCP Options with the Conversion		6.2.7. The Cookie TLV . . . . . . . . . . . . . . . . . . . 28
	Service . . . . . . . . . . . . . . . . . . . . . . . . . . . 28		6.2.8. Error TLV . . . . . . . . . . . . . . . . . . . . . . 29
	6.1. Base TCP Options . . . . . . . . . . . . . . . . . . . . 28		7. Compatibility of Specific TCP Options with the Conversion
	6.2. Window Scale (WS) . . . . . . . . . . . . . . . . . . . . 29		Service . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
	6.3. Selective Acknowledgments . . . . . . . . . . . . . . . . 29		7.1. Base TCP Options . . . . . . . . . . . . . . . . . . . . 32
	6.4. Timestamp . . . . . . . . . . . . . . . . . . . . . . . . 29		7.2. Window Scale (WS) . . . . . . . . . . . . . . . . . . . . 33
	6.5. Multipath TCP . . . . . . . . . . . . . . . . . . . . . . 30		7.3. Selective Acknowledgments . . . . . . . . . . . . . . . . 33
	6.6. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . 30		7.4. Timestamp . . . . . . . . . . . . . . . . . . . . . . . . 34
	6.7. TCP User Timeout . . . . . . . . . . . . . . . . . . . . 31		7.5. Multipath TCP . . . . . . . . . . . . . . . . . . . . . . 34
	6.8. TCP-AO . . . . . . . . . . . . . . . . . . . . . . . . . 31		7.6. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . 34
	6.9. TCP Experimental Options . . . . . . . . . . . . . . . . 31		7.7. TCP-AO . . . . . . . . . . . . . . . . . . . . . . . . . 35
	7. Interactions with Middleboxes . . . . . . . . . . . . . . . . 31		8. Interactions with Middleboxes . . . . . . . . . . . . . . . . 35
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 32		9. Security Considerations . . . . . . . . . . . . . . . . . . . 36
	8.1. Privacy & Ingress Filtering . . . . . . . . . . . . . . . 32		9.1. Privacy & Ingress Filtering . . . . . . . . . . . . . . . 36
	8.2. Authorization . . . . . . . . . . . . . . . . . . . . . . 33		9.2. Authorization . . . . . . . . . . . . . . . . . . . . . . 37
	8.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 34		9.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 38
	8.4. Traffic Theft . . . . . . . . . . . . . . . . . . . . . . 34		9.4. Traffic Theft . . . . . . . . . . . . . . . . . . . . . . 38
	8.5. Multipath TCP-specific Considerations . . . . . . . . . . 34		9.5. Authentication Considerations . . . . . . . . . . . . . . 38
	9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35		10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
	9.1. Convert Service Port Number . . . . . . . . . . . . . . . 35		10.1. Convert Service Name . . . . . . . . . . . . . . . . . . 39
	9.2. The Convert Protocol (Convert) Parameters . . . . . . . . 35		10.2. The Convert Protocol (Convert) Parameters . . . . . . . 39
	9.2.1. Convert Versions . . . . . . . . . . . . . . . . . . 35		10.2.1. Convert Versions . . . . . . . . . . . . . . . . . . 40
	9.2.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . 36		10.2.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . 41
	9.2.3. Convert Error Messages . . . . . . . . . . . . . . . 36		10.2.3. Convert Error Messages . . . . . . . . . . . . . . . 41
	10. References . . . . . . . . . . . . . . . . . . . . . . . . . 37		11. References . . . . . . . . . . . . . . . . . . . . . . . . . 42
	10.1. Normative References . . . . . . . . . . . . . . . . . . 37		11.1. Normative References . . . . . . . . . . . . . . . . . . 42
	10.2. Informative References . . . . . . . . . . . . . . . . . 39		11.2. Informative References . . . . . . . . . . . . . . . . . 44
	Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 42		Appendix A. Example Socket API Changes to Support the 0-RTT
	Appendix B. Example Socket API Changes to Support the 0-RTT		Convert Protocol . . . . . . . . . . . . . . . . . . 47
	Convert Protocol . . . . . . . . . . . . . . . . . . 44		A.1. Active Open (Client Side) . . . . . . . . . . . . . . . . 47
	B.1. Active Open (Client Side) . . . . . . . . . . . . . . . . 44		A.2. Passive Open (Converter Side) . . . . . . . . . . . . . . 48
	B.2. Passive Open (Converter Side) . . . . . . . . . . . . . . 45		Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 49
	Appendix C. Some Design Considerations . . . . . . . . . . . . . 46		Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 49
	Appendix D. Address Preservation vs. Address Sharing . . . . . . 46		Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
	D.1. Address Preservation . . . . . . . . . . . . . . . . . . 46
	D.2. Address/Prefix Sharing . . . . . . . . . . . . . . . . . 47
	Appendix E. Differences with SOCKSv5 . . . . . . . . . . . . . . 48
	Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 50
	Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 51
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 52		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 52
	1. Introduction		1. Introduction
	1.1. The Problem		1.1. The Problem
	Transport protocols like TCP evolve regularly [RFC7414]. TCP has		Transport protocols like TCP evolve regularly [RFC7414]. TCP has
	been improved in different ways. Some improvements such as changing		been improved in different ways. Some improvements such as changing
	the initial window size [RFC6928] or modifying the congestion control		the initial window size [RFC6928] or modifying the congestion control
	scheme can be applied independently on clients and servers. Other		scheme can be applied independently on clients and servers. Other
	skipping to change at page 5, line 5 ¶		skipping to change at page 4, line 47 ¶
	1.2. Network-Assisted Connections: The Rationale		1.2. Network-Assisted Connections: The Rationale
	This document specifies an application proxy, called Transport		This document specifies an application proxy, called Transport
	Converter. A Transport Converter is a function that is installed by		Converter. A Transport Converter is a function that is installed by
	a network operator to aid the deployment of TCP extensions and to		a network operator to aid the deployment of TCP extensions and to
	provide the benefits of such extensions to clients. A Transport		provide the benefits of such extensions to clients. A Transport
	Converter may provide conversion service for one or more TCP		Converter may provide conversion service for one or more TCP
	extensions. Which TCP extensions are eligible to the conversion		extensions. Which TCP extensions are eligible to the conversion
	service is deployment-specific. The conversion service is provided		service is deployment-specific. The conversion service is provided
	by means of the 0-RTT TCP Convert Protocol (Convert), that is an		by means of the 0-RTT TCP Convert Protocol (Convert), that is an
	application-layer protocol which uses TCP port number TBA		application-layer protocol which uses a dedicated TCP port number.
	(Section 9).
	The Convert Protocol provides 0-RTT (Zero Round-Trip Time) conversion		The Convert Protocol provides 0-RTT (Zero Round-Trip Time) conversion
	service since no extra delay is induced by the protocol compared to		service since no extra delay is induced by the protocol compared to
	connections that are not proxied. Particularly, the Convert Protocol		connections that are not proxied. Particularly, the Convert Protocol
	does not require extra signaling setup delays before making use of		does not require extra signaling setup delays before making use of
	the conversion service. The Convert Protocol does not require any		the conversion service. The Convert Protocol does not require any
	encapsulation (no tunnels, whatsoever).		encapsulation (no tunnels, whatsoever).
	The Transport Converter adheres to the main principles drawn in		The Transport Converter adheres to the main principles drawn in
	[RFC1919]. In particular, a Transport Converter achieves the		[RFC1919]. In particular, a Transport Converter achieves the
	skipping to change at page 6, line 16 ¶		skipping to change at page 6, line 6 ¶
	configured with one or a list of Transport Converters (statically or		configured with one or a list of Transport Converters (statically or
	through protocols such as [I-D.boucadair-tcpm-dhc-converter]).		through protocols such as [I-D.boucadair-tcpm-dhc-converter]).
	Configuration means are outside the scope of this document.		Configuration means are outside the scope of this document.
	The use of a Transport Converter means that there is no end-to-end		The use of a Transport Converter means that there is no end-to-end
	transport connection between the client and server. This could		transport connection between the client and server. This could
	potentially create problems in some scenarios such as those discussed		potentially create problems in some scenarios such as those discussed
	in Section 4 of [RFC3135]. Some of these problems may not be		in Section 4 of [RFC3135]. Some of these problems may not be
	applicable, for example, a Transport Converter can inform a client by		applicable, for example, a Transport Converter can inform a client by
	means of Network Failure (65) or Destination Unreachable (97) error		means of Network Failure (65) or Destination Unreachable (97) error
	messages (Section 5.2.8) that it encounters a failure problem; the		messages (Section 6.2.8) that it encounters a failure problem; the
	client can react accordingly. An endpoint, or its network		client can react accordingly. An endpoint, or its network
	administrator, can assess the benefit provided by the Transport		administrator, can assess the benefit provided by the Transport
	Converter service versus the risk. This is one reason why the		Converter service versus the risk. This is one reason why the
	Transport Converter functionality has to be explicitly requested by		Transport Converter functionality has to be explicitly requested by
	an endpoint.		an endpoint.
	This document is organized as follows. First, Section 3 provides a		This document is organized as follows. First, Section 2 provides a
	brief explanation of the operation of Transport Converters. Then,		brief overview of the differences between the well-known SOCKS
	Section 5 describes the Convert Protocol. Section 6 discusses how		protocol and the 0-RTT Convert protocol. Section 4 provides a brief
			explanation of the operation of Transport Converters. Then,
			Section 6 describes the Convert Protocol. Section 7 discusses how
	Transport Converters can be used to support different TCP extensions.		Transport Converters can be used to support different TCP extensions.
	Section 7 then discusses the interactions with middleboxes, while		Section 8 then discusses the interactions with middleboxes, while
	Section 8 focuses on the security considerations.		Section 9 focuses on the security considerations. Appendix A
			describes how a TCP stack would need to support the protocol
			described in this document.
	Appendix B describes how a TCP stack would need to support the		2. Differences with SOCKSv5
	protocol described in this document. Appendix C records some
	considerations that impacted the design of the protocol. Appendix E
	provides a comparison with SOCKS proxies that are already used to
	deploy Multipath TCP in some cellular networks (Section 2.2 of
	[RFC8041]).
	2. Conventions and Definitions		Several IETF protocols provide proxy services; the closest to the
			0-RTT Convert protocol being the SOCKSv5 protocol [RFC1928]. This
			protocol is already used to deploy Multipath TCP in some cellular
			networks (Section 2.2 of [RFC8041]).
			A SOCKS Client creates a connection to a SOCKS Proxy, exchanges
			authentication information, and indicates the IP address and port
			number of the target Server. At this point, the SOCKS Proxy creates
			a connection towards the target Server and relays all data between
			the two proxied connections. The operation of an implementation
			based on SOCKSv5 (without authentication) is illustrated in Figure 1.
			Client SOCKS Proxy Server
			| | |
			| --------------------> | |
			| SYN | |
			| <-------------------- | |
			| SYN+ACK | |
			| --------------------> | |
			| ACK | |
			| | |
			| --------------------> | |
			|Version=5, Auth Methods| |
			| <-------------------- | |
			| Method | |
			| --------------------> | |
			|Auth Request (unless "No auth" method negotiated)
			| <-------------------- | |
			| Auth Response | |
			| --------------------> | |
			| Connect Server:Port | --------------------> |
			| | SYN |
			| | <-------------------- |
			| | SYN+ACK |
			| <-------------------- | |
			| Succeeded | |
			| --------------------> | |
			| Data1 | |
			| | --------------------> |
			| | Data1 |
			| | <-------------------- |
			| | Data2 |
			| <-------------------- | |
			| Data2 | |
			...
			Figure 1: Establishment of a TCP Connection through a SOCKS Proxy
			Without Authentication
			When SOCKS is used, an "end-to-end" connection between a Client and a
			Server becomes a sequence of two TCP connections that are glued
			together on the SOCKS Proxy. The SOCKS Client and Server exchange
			control information at the beginning of the bytestream on the Client-
			Proxy connection. The SOCKS Proxy then creates the connection with
			the target Server and then glues the two connections together so that
			all bytes sent by the application (Client) to the SOCKS Proxy are
			relayed to the Server and vice versa.
			The Convert Protocol is also used on TCP proxies that relay data
			between an upstream and a downstream connection, but there are
			important differences with SOCKSv5. A first difference is that the
			0-RTT Convert protocol exchanges all the control information during
			the initial RTT. This reduces the connection establishment delay
			compared to SOCKS which requires two or more round-trip-times before
			the establishment of the downstream connection towards the final
			destination. In today's Internet, latency is a important metric and
			various protocols have ben tuned to reduce their latency
			[I-D.arkko-arch-low-latency]. A recently proposed extension to SOCKS
			leverages the TFO (TCP Fast Open) option
			[I-D.olteanu-intarea-socks-6] to reduce this delay.
			A second difference is that the Convert Protocol explicitly takes the
			TCP extensions into account. By using the Convert Protocol, the
			Client can learn whether a given TCP extension is supported by the
			destination Server. This enables the Client to bypass the Transport
			Converter when the Server supports the required TCP extension(s).
			Neither SOCKSv5 [RFC1928] nor the proposed SOCKSv6
			[I-D.olteanu-intarea-socks-6] provide such a feature.
			A third difference is that a Transport Converter will only confirm
			the establishment of the connection initiated by the Client provided
			that the downstream connection has already been accepted by the
			Server. If the Server refuses the connection establishment attempt
			from the Transport Converter, then the upstream connection from the
			Client is rejected as well. This feature is important for
			applications that check the availability of a Server or use the time
			to connect as a hint on the selection of a Server [RFC8305].
			A fourth difference is that the 0-RTT Convert protocol only allows
			the Client to specify the IP address/port number of the destination
			server and not a DNS name. We evaluated an alternate design that
			included the DNS name of the remote peer instead of its IP address as
			in SOCKS [RFC1928]. However, that design was not adopted because it
			induces both an extra load and increased delays on the Transport
			Converter to handle and manage DNS resolution requests.
			3. Conventions and Definitions
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in BCP		"OPTIONAL" in this document are to be interpreted as described in BCP
	14 [RFC2119][RFC8174] when, and only when, they appear in all		14 [RFC2119][RFC8174] when, and only when, they appear in all
	capitals, as shown here.		capitals, as shown here.
	The information shown between brackets in the figures refers to		4. Architecture & Behaviors
	Convert Protocol messages described in Section 5.
	Only the exchange of control messages is depicted in the figures.
	3. Architecture & Behaviors
	3.1. Functional Elements		4.1. Functional Elements
	The Convert Protocol considers three functional elements:		The Convert Protocol considers three functional elements:
	o Clients;		o Clients;
	o Transport Converters;		o Transport Converters;
	o Servers.		o Servers.
	A Transport Converter is a network function that proxies all data		A Transport Converter is a network function that proxies all data
	exchanged over one upstream connection to one downstream connection		exchanged over one upstream connection to one downstream connection
	and vice versa (Figure 1). The Transport Converter, thus, maintains		and vice versa (Figure 2). The Transport Converter, thus, maintains
	state that associates one upstream connection to a corresponding		state that associates one upstream connection to a corresponding
	downstream connection.		downstream connection.
	A connection can be initiated from both sides of the Transport		A connection can be initiated from both sides of the Transport
	Converter (Internet-facing interface, customer-facing interface).		Converter (Internet-facing interface, customer-facing interface).
	|		|
	:		:
	|		|
	+------------+		+------------+
	Client <- upstream ->| Transport |<- downstream -> Server		Client <- upstream ->| Transport |<- downstream -> Server
	connection | Converter | connection		connection | Converter | connection
	+------------+		+------------+
	|		|
	customer-facing interface : Internet-facing interface		customer-facing interface : Internet-facing interface
	|		|
	Figure 1: A Transport Converter Proxies Data between Pairs of TCP		Figure 2: A Transport Converter Proxies Data between Pairs of TCP
	Connections		Connections
	"Client" refers to a software instance embedded on a host that can		"Client" refers to a software instance embedded on a host that can
	reach a Transport Converter via its customer-facing interface. The		reach a Transport Converter via its customer-facing interface. The
	"Client" can initiate connections via a Transport Converter (referred		"Client" can initiate connections via a Transport Converter (referred
	to as outgoing connections (Section 4.1)). Also, the "Client" can		to as outgoing connections). Also, the "Client" can accept incoming
	accept incoming connections via a Transport Converter (referred to as		connections via a Transport Converter (referred to as incoming
	incoming connections (Section 4.2)).		connections).
	Transport Converters can be operated by network operators or third		Transport Converters can be operated by network operators or third
	parties. Nevertheless, this document focuses on the single		parties. Nevertheless, this document focuses on the single
	administrative deployment case where the entity offering the		administrative deployment case where the entity offering the
	connectivity service to a client is also the entity which owns and		connectivity service to a client is also the entity which owns and
	operates the Transport Converter.		operates the Transport Converter.
	A Transport Converter can be embedded in a standalone device or be		A Transport Converter can be embedded in a standalone device or be
	activated as a service on a router. How such function is enabled is		activated as a service on a router. How such function is enabled is
	deployment-specific. A sample deployment is depicted in Figure 2.		deployment-specific.
	+-+ +-+ +-+
	Client - |R| -- |R| -- |R| - - - Server
	+-+ +-+ +-+
	|
	+-+
	|R|
	+-+
	|
	+---------+
	|Transport|
	|Converter|
	+---------+
	R: Router
	Figure 2: A Transport Converter Can Be Installed Anywhere in the
	Network
	The architecture assumes that new software will be installed on the		The architecture assumes that new software will be installed on the
	Client hosts to interact with one or more Transport Converters.		Client hosts to interact with one or more Transport Converters.
	Furthermore, the architecture allows for making use of new TCP		Furthermore, the architecture allows for making use of new TCP
	extensions even if those are not supported by a given server.		extensions even if those are not supported by a given server.
	A Client is configured, through means that are outside the scope of		A Client is configured, through means that are outside the scope of
	this document, with the names and/or the addresses of one or more		this document, with the names and/or the addresses of one or more
	Transport Converters and the TCP extensions that they support. The		Transport Converters and the TCP extensions that they support. The
	procedure for selecting a Transport Converter among a list of		procedure for selecting a Transport Converter among a list of
	configured Transport Converters is outside the scope of this		configured Transport Converters is outside the scope of this
	document.		document.
	One of the benefits of this design is that different transport		One of the benefits of this design is that different transport
	protocol extensions can be used on the upstream and the downstream		protocol extensions can be used on the upstream and the downstream
	connections. This encourages the deployment of new TCP extensions		connections. This encourages the deployment of new TCP extensions
	until they are widely supported by servers, in particular.		until they are widely supported by servers, in particular.
	The architecture does not mandate anything on the Server side.		The architecture does not mandate anything on the Server side.
	Similar to address sharing mechanisms, the architecture does not		Similar to SOCKS, the architecture does not interfere with end-to-end
	interfere with end-to-end TLS connections [RFC8446] between the		TLS connections [RFC8446] between the Client and the Server
	Client and the Server (Figure 3). In other words, end-to-end TLS is		(Figure 3). In other words, end-to-end TLS is supported in the
	supported in the presence of a Converter.		presence of a Converter.
	Client Transport Server		Client Transport Server
	| Converter |		| Converter |
	| | |		| | |
	/==========================================\		/==========================================\
	| End-to-end TLS |		| End-to-end TLS |
	\==========================================/		\==========================================/
	* TLS messages exchanged between the Client		* TLS messages exchanged between the Client
	and the Server are not shown.		and the Server are not shown.
	Figure 3: End-to-end TLS via a Transport Converter		Figure 3: End-to-end TLS via a Transport Converter
	It is out of scope of this document to elaborate on specific		It is out of scope of this document to elaborate on specific
	considerations related to the use of TLS in the Client-Converter		considerations related to the use of TLS in the Client-Converter
	connection leg to exchange Convert messages (in addition to the end-		connection leg to exchange Convert messages (in addition to the end-
	to-end TLS connection).		to-end TLS connection).
	3.2. Theory of Operation		4.2. Theory of Operation
	At a high level, the objective of the Transport Converter is to allow		At a high level, the objective of the Transport Converter is to allow
	the use a specific extension, e.g., Multipath TCP, on a subset of the		the use a specific extension, e.g., Multipath TCP, on a subset of the
	path even if the peer does not support this extension. This is		path even if the peer does not support this extension. This is
	illustrated in Figure 4 where the Client initiates a Multipath TCP		illustrated in Figure 4 where the Client initiates a Multipath TCP
	connection with the Transport Converter (packets belonging to the		connection with the Transport Converter (packets belonging to the
	Multipath TCP connection are shown with "===") while the Transport		Multipath TCP connection are shown with "===") while the Transport
	Converter uses a regular TCP connection with the Server.		Converter uses a regular TCP connection with the Server.
	Client Transport Server		Client Transport Server
	skipping to change at page 9, line 44 ¶		skipping to change at page 11, line 27 ¶
	| | |		| | |
	|==================>|--------------------->|		|==================>|--------------------->|
	| | |		| | |
	|<==================|<---------------------|		|<==================|<---------------------|
	| | |		| | |
	Multipath TCP packets Regular TCP packets		Multipath TCP packets Regular TCP packets
	Figure 4: An Example of 0-RTT Network-Assisted Outgoing MPTCP		Figure 4: An Example of 0-RTT Network-Assisted Outgoing MPTCP
	Connection		Connection
	The packets belonging to the pair of connections between the Client		The packets belonging to a connection established through a Transport
	and Server passing through a Transport Converter may follow a		Converter may follow a different path than the packets directly
	different path than the packets directly exchanged between the Client		exchanged between the Client and the Server. Deployments should
	and the Server. Deployments should minimize the possible additional		minimize the possible additional delay by carefully selecting the
	delay by carefully selecting the location of the Transport Converter		location of the Transport Converter used to reach a given
	used to reach a given destination.		destination.
	When establishing a connection, the Client can, depending on local		When establishing a connection, the Client can, depending on local
	policies, either contact the Server directly (e.g., by sending a TCP		policies, either contact the Server directly (e.g., by sending a TCP
	SYN towards the Server) or create the connection via a Transport		SYN towards the Server) or create the connection via a Transport
	Converter. In the latter case (that is, the conversion service is		Converter. In the latter case (that is, the conversion service is
	used), the Client initiates a connection towards the Transport		used), the Client initiates a connection towards the Transport
	Converter and indicates the IP address and port number of the Server		Converter and indicates the IP address and port number of the Server
	within the connection establishment packet. Doing so enables the		within the connection establishment packet. Doing so enables the
	Transport Converter to immediately initiate a connection towards that		Transport Converter to immediately initiate a connection towards that
	Server, without experiencing an extra delay. The Transport Converter		Server, without experiencing an extra delay. The Transport Converter
	skipping to change at page 10, line 22 ¶		skipping to change at page 12, line 5 ¶
	establish the connection before confirming it to the Client.		establish the connection before confirming it to the Client.
	The Client places the destination address and port number of the		The Client places the destination address and port number of the
	Server in the payload of the SYN sent to the Transport Converter to		Server in the payload of the SYN sent to the Transport Converter to
	minimize connection establishment delays. The Transport Converter		minimize connection establishment delays. The Transport Converter
	maintains two connections that are combined together:		maintains two connections that are combined together:
	o the upstream connection is the one between the Client and the		o the upstream connection is the one between the Client and the
	Transport Converter.		Transport Converter.
	o the downstream connection is between the Transport Converter and		o the downstream connection is the one between the Transport
	the Server.		Converter and the Server.
	Any user data received by the Transport Converter over the upstream		Any user data received by the Transport Converter over the upstream
	(or downstream) connection is proxied over the downstream (or		(or downstream) connection is proxied over the downstream (or
	upstream) connection. In particular, if the initial SYN message		upstream) connection. In particular, if the initial SYN message
	contains data in its payload (e.g., [RFC7413]), that data MUST be		contains user data in its payload (e.g., [RFC7413]), that data MUST
	placed right after the Convert TLVs when generating the SYN.		be placed right after the Convert TLVs when generating the SYN.
	The Converter associates a lifetime with state entries used to bind
	an upstream connection with its downstream connection.
	Figure 5 illustrates the establishment of an outgoing TCP connection		Figure 5 illustrates the establishment of an outgoing TCP connection
	by a Client through a Transport Converter.		by a Client through a Transport Converter.
			o Note: The information shown between brackets in Figure 5 (and
			other figures in the document) refers to Convert Protocol messages
			described in Section 6.
	Transport		Transport
	Client Converter Server		Client Converter Server
	| | |		| | |
	|SYN [->Server:port]| SYN |		|SYN [->Server:port]| SYN |
	|------------------>|--------------------->|		|------------------>|--------------------->|
	|<------------------|<---------------------|		|<------------------|<---------------------|
	| SYN+ACK [ ] | SYN+ACK |		| SYN+ACK [ ] | SYN+ACK |
	| ... | ... |		| ... | ... |
	Figure 5: Establishment of an Outgoing TCP Connection Through a		Figure 5: Establishment of an Outgoing TCP Connection Through a
	skipping to change at page 11, line 15 ¶		skipping to change at page 12, line 47 ¶
	Server. If this upstream connection succeeds, the Transport		Server. If this upstream connection succeeds, the Transport
	Converter confirms the establishment of the connection to the Client		Converter confirms the establishment of the connection to the Client
	by returning a SYN+ACK and the first bytes of the bytestream contain		by returning a SYN+ACK and the first bytes of the bytestream contain
	information about the TCP options that were negotiated with the		information about the TCP options that were negotiated with the
	Server. Also, a state entry is instantiated for this connection.		Server. Also, a state entry is instantiated for this connection.
	This state entry is used by the Converter to handle subsequent		This state entry is used by the Converter to handle subsequent
	messages belonging to the connection.		messages belonging to the connection.
	The connection can also be established from the Internet towards a		The connection can also be established from the Internet towards a
	Client via a Transport Converter (Figure 6). This is typically the		Client via a Transport Converter (Figure 6). This is typically the
	case when an application on the Client listens to a specific port		case when the Client hosts an application server that listens to a
	(the Client hosts an application server, typically). When the		specific port number. When the Converter receives an incoming SYN
	Converter receives an incoming SYN from a remote host, it checks if		from a remote host, it checks if it can provide the conversion
	it can provide the conversion service for the destination IP address		service for the destination IP address and destination port number of
	and destination port number of that SYN. If the check fails, the		that SYN. The Transport Converter receives this SYN because it is,
	packet is silently ignored by the Converter. If the check is		for example, on the path between the remote host and the Client or it
	successful, the Converter inserts the source IP address and source		provides address sharing service for the Client. If the check fails,
	port number in the SYN packet, rewrites the source IP address to one		the packet is silently ignored by the Converter. If the check is
	of its IP addresses and, eventually (i.e., only when the Converter is		successful, the Converter tries to initiate a TCP connection towards
	configured in an address sharing mode), the destination IP address		the Client from its own address and using its configured TCP options.
	and port number in accordance with any information stored locally.		In the SYN that corresponds to this connection attempt, the Transport
	That SYN is then forwarded to the next hop. A transport session		Convert inserts a TLV message that indicates the source address and
	entry is created by the Converter for this connection. SYN+ACK and		port number of the remote host. A transport session entry is created
	ACK will be then exchanged between the Client, the Converter, and		by the Converter for this connection. SYN+ACK and ACK will be then
	remote host to confirm the establishment of the connection. The		exchanged between the Client, the Converter, and remote host to
	Converter uses the transport session entry to proxy packets belonging		confirm the establishment of the connection. The Converter uses the
	to the connection.		transport session entry to proxy packets belonging to the connection.
	Transport Remote		Transport Remote
	Client Converter Host (RH)		Client Converter Host (RH)
	| | |		| | |
	|SYN [<-RH IP@:port]| SYN |		|SYN [<-RH IP@:port]| SYN |
	|<------------------|<---------------------|		|<------------------|<---------------------|
	|------------------>|--------------------->|		|------------------>|--------------------->|
	| SYN+ACK [ ] | SYN+ACK |		| SYN+ACK [ ] | SYN+ACK |
	| ... | ... |		| ... | ... |
	Figure 6: Establishment of an Incoming TCP Connection Through a		Figure 6: Establishment of an Incoming TCP Connection Through a
	Transport Converter		Transport Converter
	skipping to change at page 12, line 22 ¶		skipping to change at page 14, line 6 ¶
	as noted in Section 7.3 of [RFC7413] where it is possible to accept		as noted in Section 7.3 of [RFC7413] where it is possible to accept
	the payload of SYN packets without creating additional security risks		the payload of SYN packets without creating additional security risks
	such as a network where addresses cannot be spoofed and the Transport		such as a network where addresses cannot be spoofed and the Transport
	Converter only serves a set of hosts that are identified by these		Converter only serves a set of hosts that are identified by these
	addresses.		addresses.
	For these reasons, this specification does not mandate the use of the		For these reasons, this specification does not mandate the use of the
	TCP Fast Open option when the Client sends a connection establishment		TCP Fast Open option when the Client sends a connection establishment
	packet towards a Transport Converter. The Convert Protocol includes		packet towards a Transport Converter. The Convert Protocol includes
	an optional Cookie TLV that provides similar protection as the TCP		an optional Cookie TLV that provides similar protection as the TCP
	Fast Open option without consuming space in the extended TCP header.		Fast Open option without consuming space in the TCP header.
	In particular, this design allows for the use of longer cookies.		Furthermore, this design allows for the use of longer cookies than
			[RFC7413].
	If the downstream (or upstream) connection fails for some reason		If the downstream (or upstream) connection fails for some reason
	(excessive retransmissions, reception of an RST segment, etc.), then		(excessive retransmissions, reception of an RST segment, etc.), then
	the Converter should force the tear-down of the upstream (or		the Converter reacts by forcing the tear-down of the upstream (or
	downstream) connection.		downstream) connection.
	The same reasoning applies when the upstream connection ends. In		The same reasoning applies when the upstream connection ends with an
	this case, the Converter should also terminate the downstream		exchange of FIN packets. In this case, the Converter should also
	connection by using FIN segments. If the downstream connection		terminate the downstream connection by using FIN packets. If the
	terminates with the exchange of FIN segments, the Converter should		downstream connection terminates with the exchange of FIN packets,
	initiate a graceful termination of the upstream connection.		the Converter should initiate a graceful termination of the upstream
			connection.
	3.3. Data Processing at the Transport Converter
	3.3.1. Base Behavior		4.3. Data Processing at the Transport Converter
	As mentioned in Section 3.2, the Transport Converter acts as a TCP		As mentioned in Section 4.2, the Transport Converter acts as a TCP
	proxy between the upstream connection (i.e., between the Client and		proxy between the upstream connection (i.e., between the Client and
	the Transport Converter) and the downstream connection (i.e., between		the Transport Converter) and the downstream connection (i.e., between
	the Transport Converter and the Server).		the Transport Converter and the Server).
	The control messages, discussed in Section 5, establish state		The control messages, discussed in Section 6, establish state
	(called, transport session entry) in the Transport Converter that		(called, transport session entry) in the Transport Converter that
	will enable it to proxy between the two TCP connections.		will enable it to proxy between the two TCP connections.
	The Transport Converter uses the transport session entry to proxy		The Transport Converter uses the transport session entry to proxy
	packets belonging to the connection. An implementation example of a		packets belonging to the connection. An implementation example of a
	transport session entry for TCP connections is shown in Figure 7.		transport session entry for TCP connections is shown in Figure 7.
	(C,c) <--> (T,t), (S,s), Lifetime		(C,c) <--> (T,t), (S,s), Lifetime
	Where:		Where:
	* C and c are the source IP address and source port number		* C and c are the source IP address and source port number
	used by the Client for the upstream connection.		used by the Client for the upstream connection.
	* S and s are the Server's IP address and port number.		* S and s are the Server's IP address and port number.
	* T and t are the source IP address and source port number		* T and t are the source IP address and source port number
	used by the Transport Converter to proxy the connection.		used by the Transport Converter to proxy the connection.
	* Lifetime is the validity lifetime of the entry as assigned		* Lifetime is the validity lifetime of the entry as assigned
	by the Converter.		by the Converter.
	Figure 7: An Example of Transport Session Entry (TCP)		Figure 7: An Example of Transport Session Entry (TCP)
	Clients send packets bound to connections eligible to the conversion		Clients send packets bound to connections eligible to the conversion
	service to the provisioned Transport Converter using TBA as		service to the provisioned Transport Converter and destination port
	destination port number. This applies for both control messages and		number. This applies for both control messages and data. Additional
	data. Additional information is supplied by Clients to the Transport		information is supplied by Clients to the Transport Converter by
	Converter by means of Convert messages as detailed in Section 5.		means of Convert messages as detailed in Section 6. User data can be
	User data can be included in SYN or non-SYN messages. User data is		included in SYN or non-SYN messages. User data is unambiguously
	unambiguously distinguished from Convert TLVs by a Transport		distinguished from Convert TLVs by a Transport Converter owing to the
	Converter owing to the Convert Fixed Header in the Convert messages		Convert Fixed Header in the Convert messages (Section 6.1). These
	(Section 5.1). These Convert TLVs are destined to the Transport		Convert TLVs are destined to the Transport Convert and are, thus,
	Convert and are, thus, removed by the Transport Converter when		removed by the Transport Converter when proxying between the two
	proxying between the two connections.		connections.
	Upon receipt of a Non-SYN (or a secondary subflow for Multipath TCP)		Upon receipt of a packet that belongs to an existing connection
	on port number TBA by the Transport Converter from a Client, the		between a Client and the Transport Converter the Converter proxies
	Converter checks if the packet matches an active transport session		the user data to the Server using the information stored in the
	entry. If no entry is found, the Transport Converter MUST silently		corresponding transport session entry. For example, in reference to
	ignore the packet. If an entry is found, the user data is proxied to		Figure 7, the Transport Converter proxies the data received from (C,
	the Server using the information stored in the corresponding		c) downstream using (T,t) as source transport address and (S,s) as
	transport session entry. For example, in reference to Figure 7, the		destination transport address.
	Transport Converter proxies the data received from (C, c) downstream
	using (T,t) as source transport address and (S,s) as destination
	transport address.
	A similar process happens for data sent from the Server. The		A similar process happens for data sent from the Server. The
	Converter acts as a TCP proxy and sends the data to the Client		Converter acts as a TCP proxy and sends the data to the Client
	relying upon the information stored in a transport session entry.		relying upon the information stored in a transport session entry.
			The Converter associates a lifetime with state entries used to bind
			an upstream connection with its downstream connection.
	Considerations that are specific to Multipath TCP are described in		When Multipath TCP is used between the Client and the Transport
	Section 3.3.2.		Converter, the Converter maintains more state (e.g. information about
			the subflows) for each Multipath TCP connection. The procedure
			described above continues to apply except that the Converter needs to
			manage the establishment/termination of subflows and schedule packets
			among the established ones. These operations are part of the
			Multipath TCP implementation. They are independent of the Convert
			protocol that only processes the Convert messages in the beginning of
			the bytestream.
	A Transport Converter may operate in address preservation mode (that		A Transport Converter may operate in address preservation mode (that
	is, the Converter does not rewrite the source IP address (i.e.,		is, the Converter does not rewrite the source IP address (i.e.,
	C==T)) or address sharing mode (that is, an address pool is shared		C==T)) or address sharing mode (that is, an address pool is shared
	among all Clients serviced by the Converter (i.e., C!=T)); refer to		among all Clients serviced by the Converter (i.e., C!=T)); refer to
	Appendix D for more details. Which behavior to use by a Transport		Section 4.4 for more details. Which behavior to use by a Transport
	Converter is deployment-specific. If address sharing mode is		Converter is deployment-specific. If address sharing mode is
	enabled, the Transport Converter MUST adhere to REQ-2 of [RFC6888]		enabled, the Transport Converter MUST adhere to REQ-2 of [RFC6888]
	which implies a default "IP address pooling" behavior of "Paired" (as		which implies a default "IP address pooling" behavior of "Paired" (as
	defined in Section 4.1 of [RFC4787]) must be supported. This		defined in Section 4.1 of [RFC4787]) MUST be supported. This
	behavior is meant to avoid breaking applications that depend on the		behavior is meant to avoid breaking applications that depend on the
	source address remaining constant.		source address remaining constant.
	3.3.2. Multipath TCP Specifics		4.4. Address Preservation vs. Address Sharing
	Note that for the Multipath TCP case, the Convert TLVs are only		The Transport Converter is provided with instructions about the
	exchanged during the establishment of the initial subflow.		behavior to adopt with regards to the processing of source addresses
			of outgoing packets. The following sub-sections discusses two
			deployment models for illustration purposes. It is out of the scope
			of this document to make a recommendation.
	The Transport Converter identifies an MPTCP connection by means,		4.4.1. Address Preservation
	e.g., of the token assigned to the MPTCP connection (Section 2.2 of
	[RFC6824]). An implementation example of an MPTCP transport session
	entry maintained by a Transport Converter is shown in Figure 8. The
	entry needs to be updated whenever subflows are added to, or deleted
	from, the MPTCP connection.
	token, {(C1,c1), .., (Cn, cn)} <--> (T,t), (S,s), Lifetime		In this model, the visible source IP address of a packet proxied by a
			Transport Converter to a Server is an IP address of the end host
			(Client). No dedicated IP address pool is provisioned to the
			Transport Converter, but the the Transport Converter is located on
			the path between the Client and the Server.
	Where:		For Multipath TCP, the Transport Converter preserves the source IP
	* Token is a locally unique identifier given to a (upstream)		address used by the Client when establishing the initial subflow.
	multipath connection by the Transport Converter. The token		Data conveyed in secondary subflows will be proxied by the Transport
	is a one-way hash of the MPTCP key.		Converter using the source IP address of the initial subflow. An
	* Ci and ci are the source IP address and source port number		example of a proxied Multipath TCP connection with address
	used by the Client for a subflow of an (upstream) MPTCP		preservation is shown in Figure 8.
	connection.
	* S and s are the Server's IP address and port number.
	* T and t are the source IP address and source port number
	used by the Transport Converter to proxy the connection.
	* Lifetime is the validity lifetime of the entry as assigned
	by the Converter.
	Figure 8: An Example of MPTCP Transport Session Entry		Transport
			Client Converter Server
	Upon receipt of a secondary subflow by the Transport Converter from a		@:C1,C2 @:Tc @:S
	Client, the Converter follows the same behavior specified in		|| | |
	Section 3.3.1 for processing Non-SYNs. For example, in reference to		|src:C1 SYN dst:Tc|src:C1 dst:S|
	Figure 8, the Transport Converter proxies the data received from a		|-------MPC [->S:port]------->|-------SYN------->|
	new subflow of an existing Multipath TCP connection (Cn, cn)		|| | |
	downstream using (T,t) as source transport address and (S,s) as		||dst:C1 src:Tc|dst:C1 src:S|
	destination transport address.		|<---------SYN/ACK------------|<-----SYN/ACK-----|
			|| | |
			|src:C1 dst:Tc|src:C1 dst:S|
			|------------ACK------------->|-------ACK------->|
			| | |
			|src:C2 ... dst:Tc| ... |
			||<-----Secondary Subflow---->|src:C1 dst:S|
			|| |-------data------>|
			| .. | ... |
	4. Sample Examples		Legend:
			Tc: IP address used by the Transport Converter on its customer-facing
			interface.
	4.1. Outgoing Converter-Assisted Multipath TCP Connections		Figure 8: Example of Address Preservation
			The Transport Converter must be on the forwarding path of incoming
			traffic. Because the same (destination) IP address is used for both
			proxied and non-proxied connections, the Transport Converter should
			not drop incoming packets it intercepts if no matching entry is found
			for the packets. Unless explicitly configured otherwise, such
			packets are forwarded according to the instructions of a local
			forwarding table.
			4.4.2. Address/Prefix Sharing
			A pool of global IPv4 addresses is provisioned to the Transport
			Converter along with possible instructions about the address sharing
			ratio to apply (see Appendix B of [RFC6269]). An address is thus
			shared among multiple clients.
			Likewise, rewriting the source IPv6 prefix [RFC6296] may be used to
			ease redirection of incoming IPv6 traffic towards the appropriate
			Transport Converter. A pool of IPv6 prefixes is then provisioned to
			the Transport Converter for this purpose.
			Adequate forwarding policies are enforced so that traffic destined to
			an address of such pool is intercepted by the appropriate Transport
			Converter. Unlike Section 4.4.1, the Transport Converter drops
			incoming packets which do not match an active transport session
			entry.
			An example is shown in Figure 9.
			Transport
			Client Converter Server
			@:C @:Tc|Te @:S
			| | |
			|src:C dst:Tc|src:Te dst:S|
			|-------SYN [->S:port]------->|-------SYN------->|
			| | |
			|dst:C src:Tc|dst:Te src:S|
			|<---------SYN/ACK------------|<-----SYN/ACK-----|
			| | |
			|src:C dst:Tc|src:Te dst:S|
			|------------ACK------------->|-------ACK------->|
			| | |
			| ... | ... |
			Legend:
			Tc: IP address used by the Transport Converter for its customer-facing
			interface.
			Te: IP address used by the Transport Converter for its Internet-facing
			interface.
			Figure 9: Address Sharing
			5. Sample Examples
			5.1. Outgoing Converter-Assisted Multipath TCP Connections
	As an example, let us consider how the Convert Protocol can help the		As an example, let us consider how the Convert Protocol can help the
	deployment of Multipath TCP. We assume that both the Client and the		deployment of Multipath TCP. We assume that both the Client and the
	Transport Converter support Multipath TCP, but consider two different		Transport Converter support Multipath TCP, but consider two different
	cases depending on whether the Server supports Multipath TCP or not.		cases depending on whether the Server supports Multipath TCP or not.
	As a reminder, a Multipath TCP connection is created by placing the		As a reminder, a Multipath TCP connection is created by placing the
	MP_CAPABLE (MPC) option in the SYN sent by the Client.		MP_CAPABLE (MPC) option in the SYN sent by the Client.
	Figure 9 describes the operation of the Transport Converter if the		Figure 10 describes the operation of the Transport Converter if the
	Server does not support Multipath TCP.		Server does not support Multipath TCP.
	Transport		Transport
	Client Converter Server		Client Converter Server
	|SYN, | |		|SYN, MPC | |
	|MPC [->Server:port]| SYN, MPC |		|[->Server:port] | SYN, MPC |
	|------------------>|--------------------->|		|------------------>|--------------------->|
	|<------------------|<---------------------|		|<------------------|<---------------------|
	| SYN+ACK,MPC [.] | SYN+ACK |		| SYN+ACK,MPC [.] | SYN+ACK |
	|------------------>|--------------------->|		|------------------>|--------------------->|
	| ACK, MPC | ACK |		| ACK, MPC | ACK |
	| ... | ... |		| ... | ... |
	Figure 9: Establishment of a Multipath TCP Connection Through a		Figure 10: Establishment of a Multipath TCP Connection through a
	Transport Converter towards a Server that Does Not Support Multipath		Transport Converter towards a Server that does not support Multipath
	TCP		TCP
	The Client tries to initiate a Multipath TCP connection by sending a		The Client tries to initiate a Multipath TCP connection by sending a
	SYN with the MP_CAPABLE option (MPC in Figure 9). The SYN includes		SYN with the MP_CAPABLE option (MPC in Figure 10). The SYN includes
	the address and port number of the target Server, that are extracted		the address and port number of the target Server, that are extracted
	and used by the Transport Converter to initiate a Multipath TCP		and used by the Transport Converter to initiate a Multipath TCP
	connection towards this Server. Since the Server does not support		connection towards this Server. Since the Server does not support
	Multipath TCP, it replies with a SYN+ACK that does not contain the		Multipath TCP, it replies with a SYN+ACK that does not contain the
	MP_CAPABLE option. The Transport Converter notes that the connection		MP_CAPABLE option. The Transport Converter notes that the connection
	with the Server does not support Multipath TCP and returns the		with the Server does not support Multipath TCP and returns the
	extended TCP header received from the Server to the Client.		extended TCP header received from the Server to the Client.
	Note that, if the TCP connection fails for some reason, the Converter		Note that, if the TCP connection is reset for some reason, the
	tears down the Multipath TCP connection by transmitting a		Converter tears down the Multipath TCP connection by transmitting a
	MP_FASTCLOSE. Likewise, if the Multipath TCP connection ends with		MP_FASTCLOSE. Likewise, if the Multipath TCP connection ends with
	the transmission of DATA_FINs, the Converter terminates the TCP		the transmission of DATA_FINs, the Converter terminates the TCP
	connection by using FIN segments. As a side note, given that with		connection by using FIN segments. As a side note, given that with
	Multipath TCP, RST only has the scope of the subflow and will only		Multipath TCP, RST only has the scope of the subflow and will only
	close the concerned subflow but not affect the remaining subflows,		close the concerned subflow but not affect the remaining subflows,
	the Converter does not terminate the TCP connection upon receipt of		the Converter does not terminate the downstream TCP connection upon
	an RST over a Multipath subflow.		receipt of an RST over a Multipath subflow.
	Figure 10 considers a Server that supports Multipath TCP. In this		Figure 11 considers a Server that supports Multipath TCP. In this
	case, it replies to the SYN sent by the Transport Converter with the		case, it replies to the SYN sent by the Transport Converter with the
	MP_CAPABLE option. Upon reception of this SYN+ACK, the Transport		MP_CAPABLE option. Upon reception of this SYN+ACK, the Transport
	Converter confirms the establishment of the connection to the Client		Converter confirms the establishment of the connection to the Client
	and indicates to the Client that the Server supports Multipath TCP.		and indicates to the Client that the Server supports Multipath TCP.
	With this information, the Client has discovered that the Server		With this information, the Client has discovered that the Server
	supports Multipath TCP natively. This will enable the Client to		supports Multipath TCP. This will enable the Client to bypass the
	bypass the Transport Converter for the subsequent Multipath TCP		Transport Converter for the subsequent Multipath TCP connections that
	connections that it will initiate towards this Server.		it will initiate towards this Server.
	Transport		Transport
	Client Converter Server		Client Converter Server
	|SYN, | |		|SYN, MPC | |
	|MPC [->Server:port]| SYN, MPC |		|[->Server:port] | SYN, MPC |
	|------------------>|--------------------->|		|------------------>|--------------------->|
	|<------------------|<---------------------|		|<------------------|<---------------------|
	|SYN+ACK, | SYN+ACK, MPC |		|SYN+ACK, MPC | SYN+ACK, MPC |
	|MPC [MPC supported]| |		|[MPC supported] | |
	|------------------>|--------------------->|		|------------------>|--------------------->|
	| ACK, MPC | ACK, MPC |		| ACK, MPC | ACK, MPC |
	| ... | ... |		| ... | ... |
	Figure 10: Establishment of a Multipath TCP Connection Through a		Figure 11: Establishment of a Multipath TCP Connection through a
	Converter Towards an MPTCP-capable Server		Converter towards an MPTCP-capable Server
	4.2. Incoming Converter-Assisted Multipath TCP Connection		5.2. Incoming Converter-Assisted Multipath TCP Connection
	An example of an incoming Converter-assisted Multipath TCP connection		An example of an incoming Converter-assisted Multipath TCP connection
	is depicted in Figure 11. In order to support incoming connections		is depicted in Figure 12. In order to support incoming connections
	from remote hosts, the Client may use PCP [RFC6887] to instruct the		from remote hosts, the Client may use PCP [RFC6887] to instruct the
	Transport Converter to create dynamic mappings. Those mappings will		Transport Converter to create dynamic mappings. Those mappings will
	be used by the Transport Converter to intercept an incoming TCP		be used by the Transport Converter to intercept an incoming TCP
	connection destined to the Client and convert it into a Multipath TCP		connection destined to the Client and convert it into a Multipath TCP
	connection.		connection.
	Typically, the Client sends a PCP request to the Converter asking to		Typically, the Client sends a PCP request to the Converter asking to
	create an explicit TCP mapping for (internal IP address, internal		create an explicit TCP mapping for (internal IP address, internal
	port number). The Converter accepts the request by creating a TCP		port number). The Converter accepts the request by creating a TCP
	mapping (internal IP address, internal port number, external IP		mapping (internal IP address, internal port number, external IP
	skipping to change at page 17, line 21 ¶		skipping to change at page 21, line 9 ¶
	addresses and, eventually, the destination IP address and port number		addresses and, eventually, the destination IP address and port number
	in accordance with the information stored in the mapping. SYN+ACK		in accordance with the information stored in the mapping. SYN+ACK
	and ACK will be then exchanged between the Client and the Converter		and ACK will be then exchanged between the Client and the Converter
	to confirm the establishment of the initial subflow. The Client can		to confirm the establishment of the initial subflow. The Client can
	add new subflows following normal Multipath TCP procedures.		add new subflows following normal Multipath TCP procedures.
	Transport Remote		Transport Remote
	Client Converter Host		Client Converter Host
	| | |		| | |
	|<--------------------|<-------------------|		|<--------------------|<-------------------|
	|SYN, | SYN |		|SYN, MPC | SYN |
	|MPC[Remote Host:port]| |		|[Remote Host:port] | |
	|-------------------->|------------------->|		|-------------------->|------------------->|
	| SYN+ACK, MPC | SYN+ACK |		| SYN+ACK, MPC | SYN+ACK |
	|<--------------------|<-------------------|		|<--------------------|<-------------------|
	| ACK, MPC | ACK |		| ACK, MPC | ACK |
	| ... | ... |		| ... | ... |
	Figure 11: Establishment of an Incoming Multipath TCP Connection		Figure 12: Establishment of an Incoming Multipath TCP Connection
	through a Transport Converter		through a Transport Converter
	It is out of scope of this document to define specific Convert TLVs		It is out of scope of this document to define specific Convert TLVs
	to manage incoming connections. These TLVs can be defined in a		to manage incoming connections. These TLVs can be defined in a
	separate document.		separate document.
	5. The Convert Protocol (Convert)		6. The Convert Protocol (Convert)
	This section defines the Convert Protocol (Convert, for short)		This section defines the Convert Protocol (Convert, for short)
	messages that are exchanged between a Client and a Transport		messages that are exchanged between a Client and a Transport
	Converter.		Converter.
	By default, the Transport Converter listens on TCP port number TBA		The Transport Converter listens on a dedicated TCP port number for
	for Convert messages from Clients.		Convert messages from Clients. That port number is configured by an
			administrator.
	Convert messages may appear only in a SYN, SYN+ACK, or ACK.
	Convert messages MUST be included as the first bytes of the		Convert messages MUST be included as the first bytes of the
	bytestream. All Convert messages starts with a 32 bits long fixed		bytestream. All Convert messages starts with a 32 bits long fixed
	header (Section 5.1) followed by one or more Convert TLVs (Type,		header (Section 6.1) followed by one or more Convert TLVs (Type,
	Length, Value) (Section 5.2).		Length, Value) (Section 6.2).
	5.1. The Convert Fixed Header		o Implementation note 1: Several implementers expressed concerns
			about the use of TFO. As a reminder, the TFO Cookie protects from
			some attack scenarios that affect open servers like web servers.
			The Convert Protocol is different and, as discussed in RFC7413,
			there are different ways to protect from such attacks. Instead of
			using a TFO cookie inside the TCP options, which consumes precious
			space in the extended TCP header, the Convert Protocol supports
			the utilization of a Cookie that is placed in the SYN payload.
			This provides the same level of protection as a TFO Cookie in
			environments were such protection is required.
			o Implementation note 2: Error messages are not included in RST but
			sent in the bytestream. Implementers have indicated that
			processing RST on clients was difficult on some platforms. This
			design simplifies client implementations.
			6.1. The Convert Fixed Header
	The Convert Protocol uses a 32 bits long fixed header that is sent by		The Convert Protocol uses a 32 bits long fixed header that is sent by
	both the Client and the Transport Converter over each established		both the Client and the Transport Converter over each established
	connection. This header indicates both the version of the protocol		connection. This header indicates both the version of the protocol
	used and the length of the Convert message.		used and the length of the Convert message.
	The Client and the Transport Converter MUST send the fixed-sized		The Client and the Transport Converter MUST send the fixed-sized
	header, shown in Figure 12, as the first four bytes of the		header, shown in Figure 13, as the first four bytes of the
	bytestream.		bytestream.
	1 2 3		1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	| Version | Total Length | Unassigned |		| Version | Total Length | Unassigned |
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	Figure 12: The Convert Fixed Header		Figure 13: The Convert Fixed Header
	The Version is encoded as an 8 bits unsigned integer value. This		The Version is encoded as an 8 bits unsigned integer value. This
	document specifies version 1. Version 0 is reserved by this document		document specifies version 1. Version 0 is reserved by this document
	and MUST NOT be used.		and MUST NOT be used.
	The Total Length is the number of 32 bits word, including the header,		The Total Length is the number of 32 bits word, including the header,
	of the bytestream that are consumed by the Convert messages. Since		of the bytestream that are consumed by the Convert messages. Since
	Total Length is also an 8 bits unsigned integer, those messages		Total Length is also an 8 bits unsigned integer, those messages
	cannot consume more than 1020 bytes of data. This limits the number		cannot consume more than 1020 bytes of data. This limits the number
	of bytes that a Transport Converter needs to process. A Total Length		of bytes that a Transport Converter needs to process. A Total Length
	of zero is invalid and the connection MUST be reset upon reception of		of zero is invalid and the connection MUST be reset upon reception of
	a header with such total length.		a header with such total length.
	The Unassigned field MUST be set to zero in this version of the		The Unassigned field MUST be set to zero in this version of the
	protocol. These bits are available for future use [RFC8126].		protocol. These bits are available for future use.
	Data added by the Convert Protocol to the TCP bytestream is		The Total Length field unambiguously marks the number of 32 bits
	unambiguously distinguished from payload data by the Total Length		words that carry Convert TLVs in the beginning of the bytestream.
	field in the Convert messages.
	5.2. Convert TLVs		6.2. Convert TLVs
	5.2.1. Generic Convert TLV Format		6.2.1. Generic Convert TLV Format
	The Convert Protocol uses variable length messages that are encoded		The Convert Protocol uses variable length messages that are encoded
	using the generic TLV format depicted in Figure 13.		using the generic TLV format depicted in Figure 14.
	The length of all TLVs used by the Convert Protocol is always a		The length of all TLVs used by the Convert Protocol is always a
	multiple of four bytes. All TLVs are aligned on 32 bits boundaries.		multiple of four bytes. All TLVs are aligned on 32 bits boundaries.
	All TLV fields are encoded using the network byte order.		All TLV fields are encoded using the network byte order.
	1 2 3		1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	| Type | Length | Value ... |		| Type | Length | Value ... |
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	// ... (optional) Value //		// ... (optional) Value //
	+---------------------------------------------------------------+		+---------------------------------------------------------------+
	Figure 13: Convert Generic TLV Format		Figure 14: Convert Generic TLV Format
	The Length field covers Type, Length, and Value fields. It is		The Length field covers Type, Length, and Value fields. It is
	expressed in units of 32 bits words. If necessary, Value MUST be		expressed in units of 32 bits words. If necessary, Value MUST be
	padded with zeroes so that the length of the TLV is a multiple of 32		padded with zeroes so that the length of the TLV is a multiple of 32
	bits.		bits.
	A given TLV MUST only appear once on a connection. If two or more		A given TLV MUST only appear once on a connection. If a Client
	instances of the same TLV are exchanged over a Convert connection,		receives two or more instances of the same TLV over a Convert
	the associated TCP connections MUST be closed.		connection, it MUST reset the associated TCP connection. If a
			Converter receives two or more instances of the same TLV over a
			Convert connection, it MUST return a Malformed Message Error TLV and
			close the associated TCP connection.
	5.2.2. Summary of Supported Convert TLVs		6.2.2. Summary of Supported Convert TLVs
	This document specifies the following Convert TLVs:		This document specifies the following Convert TLVs:
	+------+-----+----------+------------------------------------------+		+------+-----+----------+------------------------------------------+
	| Type | Hex | Length | Description |		| Type | Hex | Length | Description |
	+------+-----+----------+------------------------------------------+		+------+-----+----------+------------------------------------------+
	| 1 | 0x1 | 1 | Info TLV |		| 1 | 0x1 | 1 | Info TLV |
	| 10 | 0xA | Variable | Connect TLV |		| 10 | 0xA | Variable | Connect TLV |
	| 20 | 0x14| Variable | Extended TCP Header TLV |		| 20 | 0x14| Variable | Extended TCP Header TLV |
	| 21 | 0x15| Variable | Supported TCP Extensions TLV |		| 21 | 0x15| Variable | Supported TCP Extensions TLV |
	| 22 | 0x16| Variable | Cookie TLV |		| 22 | 0x16| Variable | Cookie TLV |
	| 30 | 0x1E| Variable | Error TLV |		| 30 | 0x1E| Variable | Error TLV |
	+------+-----+----------+------------------------------------------+		+------+-----+----------+------------------------------------------+
	Figure 14: The TLVs used by the Convert Protocol		Figure 15: The TLVs used by the Convert Protocol
	Type 0x0 is a reserved valued. Implementations MUST discard messages		Type 0x0 is a reserved value. If a Client receives a TLV of type
			0x0, it MUST reset the associated TCP connection. If a Converter
			receives a TLV of type 0x0, it MUST return an Unsupported Message
			Error TLV and close the associated TCP connection.
			Implementations MUST reset the connection upon reception of messages
	with such TLV.		with such TLV.
	The Client typically sends in the first connection it established		The Client typically sends in the first connection it established
	with a Transport Converter the Info TLV (Section 5.2.3) to learn its		with a Transport Converter the Info TLV (Section 6.2.3) to learn its
	capabilities. Assuming the Client is authorized to invoke the		capabilities. Assuming the Client is authorized to invoke the
	Transport Converter, the latter replies with the Supported TCP		Transport Converter, the latter replies with the Supported TCP
	Extensions TLV (Section 5.2.4).		Extensions TLV (Section 6.2.4).
	The Client can request the establishment of connections to servers by		The Client can request the establishment of connections to servers by
	using the Connect TLV (Section 5.2.5). If the connection can be		using the Connect TLV (Section 6.2.5). If the connection can be
	established with the final server, the Transport Converter replies		established with the final server, the Transport Converter replies
	with the Extended TCP Header TLV (Section 5.2.6). If not, the		with the Extended TCP Header TLV (Section 6.2.6). If not, the
	Transport Converter returns an Error TLV (Section 5.2.8) and then		Transport Converter returns an Error TLV (Section 6.2.8) and then
	closes the connection.		closes the connection. The Transport Converter MUST NOT send a RST
			immediately after the detection of an error to let the Error TLV
			reach the Client. As explained later, the Client will anyway send a
			RST upon reception of the Error TLV.
	When an error is encountered an Error TLV with the appropriate error		When an error is encountered an Error TLV with the appropriate error
	code MUST be returned by the Transport Converter.		code MUST be returned by the Transport Converter.
	5.2.3. The Info TLV		6.2.3. The Info TLV
	The Info TLV (Figure 15) is an optional TLV which can be sent by a		The Info TLV (Figure 16) is an optional TLV which can be sent by a
	Client to request the TCP extensions that are supported by a		Client to request the TCP extensions that are supported by a
	Transport Converter. It is typically sent on the first connection		Transport Converter. It is typically sent on the first connection
	that a Client establishes with a Transport Converter to learn its		that a Client establishes with a Transport Converter to learn its
	capabilities. Assuming a Client is entitled to invoke the Transport		capabilities. Assuming a Client is entitled to invoke the Transport
	Converter, the latter replies with the Supported TCP Extensions TLV		Converter, the latter replies with the Supported TCP Extensions TLV
	described in Section 5.2.4.		described in Section 6.2.4.
	1 2 3		1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	| Type=0x1 | Length | Zero |		| Type=0x1 | Length | Zero |
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	Figure 15: The Info TLV		Figure 16: The Info TLV
	5.2.4. Supported TCP Extensions TLV		6.2.4. Supported TCP Extensions TLV
	The Supported TCP Extensions TLV (Figure 16) is used by a Transport		The Supported TCP Extensions TLV (Figure 17) is used by a Transport
	Converter to announce the TCP options for which it provides a		Converter to announce the TCP options for which it provides a
	conversion service. A Transport Converter SHOULD include in this		conversion service. A Transport Converter SHOULD include in this
	list the TCP options that it accepts from Clients; these options are		list the TCP options that it supports in outgoing SYNs.
	included by the Transport Converter in the SYN packets that it sends
	to initiate connections.
	Each supported TCP option is encoded with its TCP option Kind listed		Each supported TCP option is encoded with its TCP option Kind listed
	in the "TCP Parameters" registry maintained by IANA.		in the "TCP Parameters" registry maintained by IANA.
	1 2 3		1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	| Type=0x15 | Length | Unassigned |		| Type=0x15 | Length | Unassigned |
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	| Kind #1 | Kind #2 | ... |		| Kind #1 | Kind #2 | ... |
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	/ ... /		/ ... /
	/ /		/ /
	+---------------------------------------------------------------+		+---------------------------------------------------------------+
	Figure 16: The Supported TCP Extensions TLV		Figure 17: The Supported TCP Extensions TLV
	TCP option Kinds 0, 1, and 2 defined in [RFC0793] are supported by		TCP option Kinds 0, 1, and 2 defined in [RFC0793] are supported by
	all TCP implementations and thus MUST NOT appear in this list.		all TCP implementations and thus MUST NOT appear in this list.
	The list of Supported TCP Extensions is padded with 0 to end on a 32		The list of Supported TCP Extensions is padded with 0 to end on a 32
	bits boundary.		bits boundary.
	For example, if the Transport Converter supports Multipath TCP,		For example, if the Transport Converter supports Multipath TCP,
	Kind=30 will be present in the Supported TCP Extensions TLV that it		Kind=30 will be present in the Supported TCP Extensions TLV that it
	returns in response to Info TLV.		returns in response to Info TLV.
	5.2.5. Connect TLV		6.2.5. Connect TLV
	The Connect TLV (Figure 17) is used to request the establishment of a		The Connect TLV (Figure 18) is used to request the establishment of a
	connection via a Transport Converter. This connection can be from or		connection via a Transport Converter. This connection can be from or
	to a Client.		to a Client.
	The 'Remote Peer Port' and 'Remote Peer IP Address' fields contain		The 'Remote Peer Port' and 'Remote Peer IP Address' fields contain
	the destination port number and IP address of the Server, for		the destination port number and IP address of the Server, for
	outgoing connections. For incoming connections destined to a Client		outgoing connections. For incoming connections destined to a Client
	serviced via a Transport Converter, these fields convey the source		serviced via a Transport Converter, these fields convey the source
	port number and IP address.		port number and IP address of the SYN packet received by the
			Transport Converter from the server.
	The Remote Peer IP Address MUST be encoded as an IPv6 address. IPv4		The Remote Peer IP Address MUST be encoded as an IPv6 address. IPv4
	addresses MUST be encoded using the IPv4-Mapped IPv6 Address format		addresses MUST be encoded using the IPv4-Mapped IPv6 Address format
	defined in [RFC4291]. Further, Remote Peer IP address field MUST NOT		defined in [RFC4291]. Further, Remote Peer IP address field MUST NOT
	include multicast, broadcast, and host loopback addresses [RFC6890].		include multicast, broadcast, and host loopback addresses [RFC6890].
	Connect TLVs witch such messages MUST be discarded by the Transport		If a Converter receives a Connect TLVs witch such invalid addresses,
	Converter.		it MUST reply with a Malformed Message Error TLV and close the
			associated TCP connection.
	We distinguish two types of Connect TLV based on their length: (1)		We distinguish two types of Connect TLV based on their length: (1)
	the base Connect TLV has a length of 20 bytes and contains a remote		the Base Connect TLV has a length of 20 bytes and contains a remote
	address and a remote port, (2) the extended Connect TLV spans more		address and a remote port (Figure 18), (2) the Extended Connect TLV
	than 20 bytes and also includes the optional 'TCP Options' field.		spans more than 20 bytes and also includes the optional 'TCP Options'
	This field is used to specify how specific TCP options should be		field (Figure 19). This field is used to request the advertisement
	advertised by the Transport Converter to the server.		of specific TCP options to the server.
	1 2 3		1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	| Type=0xA | Length | Remote Peer Port |		| Type=0xA | Length | Remote Peer Port |
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	| |		| |
	| Remote Peer IP Address (128 bits) |		| Remote Peer IP Address (128 bits) |
	| |		| |
	| |		| |
	+---------------------------------------------------------------+		+---------------------------------------------------------------+
	/ TCP Options (Variable) /
	/ ... /
	+---------------------------------------------------------------+
	Figure 17: The Connect TLV		Figure 18: The Base Connect TLV
			1 2 3
			0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
			+---------------+---------------+-------------------------------+
			| Type=0xA | Length | Remote Peer Port |
			+---------------+---------------+-------------------------------+
			| |
			| Remote Peer IP Address (128 bits) |
			| |
			| |
			+---------------------------------------------------------------+
			/ TCP Options (Variable) /
			/ ... /
			+---------------------------------------------------------------+
			Figure 19: The Extended Connect TLV
	The 'TCP Options' field is a variable length field that carries a		The 'TCP Options' field is a variable length field that carries a
	list of TCP option fields (Figure 18). Each TCP option field is		list of TCP option fields (Figure 20). Each TCP option field is
	encoded as a block of 2+n bytes where the first byte is the TCP		encoded as a block of 2+n bytes where the first byte is the TCP
	option Kind and the second byte is the length of the TCP option as		option Kind and the second byte is the length of the TCP option as
	specified in [RFC0793]. The minimum value for the TCP option Length		specified in [RFC0793]. The minimum value for the TCP option Length
	is 2. The TCP options that do not include a length sub-field, i.e.,		is 2. The TCP options that do not include a length sub-field, i.e.,
	option types 0 (EOL) and 1 (NOP) defined in [RFC0793] MUST NOT be		option types 0 (EOL) and 1 (NOP) defined in [RFC0793] MUST NOT be
	placed inside the TCP options field of the Connect TLV. The optional		placed inside the TCP options field of the Connect TLV. The optional
	Value field contains the variable-length part of the TCP option. A		Value field contains the variable-length part of the TCP option. A
	length of two indicates the absence of the Value field. The TCP		length of two indicates the absence of the Value field. The TCP
	options field always ends on a 32 bits boundary after being padded		options field always ends on a 32 bits boundary after being padded
	with zeros.		with zeros.
	1 2 3		1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+---------------+---------------+---------------+---------------+		+---------------+---------------+---------------+---------------+
	| TCPOpt kind | TCPOpt Length | Value (opt) | .... |		| TCPOpt kind | TCPOpt Length | Value (opt) | .... |
	+---------------+---------------+---------------+---------------+		+---------------+---------------+---------------+---------------+
	| .... |		| .... |
	+---------------------------------------------------------------+		+---------------------------------------------------------------+
	| ... |		| ... |
	+---------------------------------------------------------------+		+---------------------------------------------------------------+
	Figure 18: The TCP Options Field		Figure 20: The TCP Options Field
	Upon reception of a Connect TLV, and absent any policy (e.g., rate-		Upon reception of a Base Connect TLV, and absent any policy (e.g.,
	limit) or resource exhaustion conditions, a Transport Converter		rate-limit) or resource exhaustion conditions, a Transport Converter
	attempts to establish a connection to the address and port that it		attempts to establish a connection to the address and port that it
	contains. The Transport Converter MUST use by default the TCP		contains. The Transport Converter MUST use by default the TCP
	options that correspond to its local policy to establish this		options that correspond to its local policy to establish this
	connection. These are the options that it advertises in the		connection. These are the options that it advertises in the
	Supported TCP Extensions TLV.		Supported TCP Extensions TLV.
	Upon reception of an extended Connect TLV, and absent any rate limit		Upon reception of an Extended Connect TLV, a Transport Converter
	policy or resource exhaustion conditions, a Transport Converter MUST		first checks whether it supports the TCP Options listed in the 'TCP
	attempt to establish a connection to the address and port that it		Options' field. If not, it returns an error message (Section 6.2.8).
	contains. It MUST include the options of the 'TCP Options' sub-field		If the above check succeeded and absent any rate limit policy or
	in the SYN sent to the Server in addition to the TCP options that it		resource exhaustion conditions, a Transport Converter MUST attempt to
			establish a connection to the address and port that it contains. It
			MUST include in the SYN that it sends to the Server the options
			listed in the 'TCP Options' sub-field and the TCP options that it
	would have used according to its local policies. For the TCP options		would have used according to its local policies. For the TCP options
	that are listed without an optional value, the Transport Converter		that are included in the TCP Options field without an optional value,
	MUST generate its own value. For the TCP options that are included		the Transport Converter MUST generate its own value. For the TCP
	in the 'TCP Options' field with an optional value, it MUST copy the		options that are included in the 'TCP Options' field with an optional
	entire option for use in the connection with the destination peer.		value, it MUST copy the entire option in the SYN sent to the remote
	This feature is required to support TCP Fast Open.		server. This feature is required to support TCP Fast Open. See
			Section 7 for a detailed discussion of the different types of TCP
			options.
	The Transport Converter may discard a Connect TLV request for various		The Transport Converter may refuse a Connect TLV request for various
	reasons (e.g., authorization failed, out of resources, invalid		reasons (e.g., authorization failed, out of resources, invalid
	address type). An error message indicating the encountered error is		address type, unsupported TCP option). An error message indicating
	returned to the requesting Client (Section 5.2.8). In order to		the encountered error is returned to the requesting Client
	prevent denial-of-service attacks, error messages sent to a Client		(Section 6.2.8). In order to prevent denial-of-service attacks,
	SHOULD be rate-limited.		error messages sent to a Client SHOULD be rate-limited.
	5.2.6. Extended TCP Header TLV		6.2.6. Extended TCP Header TLV
	The Extended TCP Header TLV (Figure 19) is used by the Transport		The Extended TCP Header TLV (Figure 21) is used by the Transport
	Converter to send to the Client the extended TCP header that was		Converter to return to the Client the TCP options that were returned
	returned by the Server in the SYN+ACK packet. This TLV is only sent		by the Server in the SYN+ACK packet. A Transport Converter MUST
	if the Client sent a Connect TLV to request the establishment of a		return this TLV if the Client sent an Extended Connect TLV and the
	connection.		connection was accepted by the server.
	1 2 3		1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	| Type=0x14 | Length | Unassigned |		| Type=0x14 | Length | Unassigned |
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	/ Returned Extended TCP header /		/ Returned Extended TCP header /
	/ ... /		/ ... /
	+---------------------------------------------------------------+		+---------------------------------------------------------------+
	Figure 19: The Extended TCP Header TLV		Figure 21: The Extended TCP Header TLV
	The Returned Extended TCP header field is a copy of the extended		The Returned Extended TCP header field is a copy of the TCP Options
	header that was received in the SYN+ACK by the Transport Converter.		that were included in the SYN+ACK received by the Transport
			Converter.
	The Unassigned field MUST be set to zero by the sender and ignored by		The Unassigned field MUST be set to zero by the sender and ignored by
	the receiver. These bits are available for future use [RFC8126].		the receiver.
	5.2.7. The Cookie TLV		6.2.7. The Cookie TLV
	The Cookie TLV (Figure 20 is an optional TLV which use is similar to		The Cookie TLV (Figure 22) is an optional TLV which is similar to the
	the TCP Fast Open Cookie [RFC7413]. A Transport Converter may want		TCP Fast Open Cookie [RFC7413]. A Transport Converter may want to
	to verify that a Client can receive the packets that it sends to		verify that a Client can receive the packets that it sends to prevent
	prevent attacks from spoofed addresses. This verification can be		attacks from spoofed addresses. This verification can be done by
	done by using a Cookie that is bound to, for example, the IP		using a Cookie that is bound to, for example, the IP address(es) of
	address(es) of the Client. This Cookie can be configured on the		the Client. This Cookie can be configured on the Client by means
	Client by means that are outside of this document or provided by the		that are outside of this document or provided by the Transport
	Transport Converter as follows.		Converter as follows.
	A Transport Converter that has been configured to use the optional		A Transport Converter that has been configured to use the optional
	Cookie TLV MUST verify the presence of this TLV in the payload of the		Cookie TLV MUST verify the presence of this TLV in the payload of the
	received SYN. If this TLV is present, the Transport Converter MUST		received SYN. If this TLV is present, the Transport Converter MUST
	validate the Cookie by means similar to those in Section 4.1.2 of		validate the Cookie by means similar to those in Section 4.1.2 of
	[RFC7413] (i.e., IsCookieValid). If the Cookie is valid, the		[RFC7413] (i.e., IsCookieValid). If the Cookie is valid, the
	connection establishment procedure can continue. Otherwise, the		connection establishment procedure can continue. Otherwise, the
	Transport Converter MUST return an Error TLV set to "Not Authorized"		Transport Converter MUST return an Error TLV set to "Not Authorized"
	and close the connection.		and close the connection.
	If the received SYN did not contain a Cookie TLV, and cookie		If the received SYN did not contain a Cookie TLV, and cookie
	validation is required, the Transport Converter should compute a		validation is required, the Transport Converter MAY compute a Cookie
	Cookie bound to this Client address and return a Convert message		bound to this Client address and return a Convert message containing
	containing the fixed header, an Error TLV set to "Missing Cookie" and		the fixed header, an Error TLV set to "Missing Cookie" and the
	the computed Cookie and close the connection. The Client will react		computed Cookie and close the connection. The Client will react to
	to this error by storing the received Cookie in its cache and attempt		this error by first issuing a reset to terminate the connection. It
	to reestablish a new connection to the Transport Converter that		also stores the received Cookie in its cache and attempts to
	includes the Cookie TLV.		reestablish a new connection to the Transport Converter that includes
			the Cookie TLV.
	The format of the Cookie TLV is shown in Figure 20.		The format of the Cookie TLV is shown in Figure 22.
	1 2 3		1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	| Type=0x16 | Length | Zero |		| Type=0x16 | Length | Zero |
	+---------------+---------------+-------------------------------+		+---------------+---------------+-------------------------------+
	/ Opaque Cookie /		/ Opaque Cookie /
	/ ... /		/ ... /
	+---------------------------------------------------------------+		+---------------------------------------------------------------+
	Figure 20: The Cookie TLV		Figure 22: The Cookie TLV
	5.2.8. Error TLV		6.2.8. Error TLV
	The Error TLV (Figure 21) is meant to provide information about some		The Error TLV (Figure 23) is meant to provide information about some
	errors that occurred during the processing of a Convert message.		errors that occurred during the processing of a Convert message.
	This TLV has a variable length. Upon reception of an Error TLV, a		This TLV has a variable length. Upon reception of an Error TLV, a
	Client MUST close the associated connection.		Client MUST reset the associated connection.
	1 2 3		1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+---------------+---------------+----------------+--------------+		+---------------+---------------+----------------+--------------+
	| Type=0x1E | Length | Error Code | Value |		| Type=0x1E | Length | Error Code | Value |
	+---------------+---------------+----------------+--------------+		+---------------+---------------+----------------+--------------+
	// ... (optional) Value //		// ... (optional) Value //
	+---------------------------------------------------------------+		+---------------------------------------------------------------+
	Figure 21: The Error TLV		Figure 23: The Error TLV
	Different types of errors can occur while processing Convert		Different types of errors can occur while processing Convert
	messages. Each error is identified by an Error Code represented as		messages. Each error is identified by an Error Code represented as
	an unsigned integer. Four classes of error codes are defined:		an unsigned integer. Four classes of error codes are defined:
	o Message validation and processing errors (0-31 range): returned		o Message validation and processing errors (0-31 range): returned
	upon reception of an invalid message (including valid messages but		upon reception of an invalid message (including valid messages but
	with invalid or unknown TLVs).		with invalid or unknown TLVs).
	o Client-side errors (32-63 range): the Client sent a request that		o Client-side errors (32-63 range): the Client sent a request that
	skipping to change at page 25, line 39 ¶		skipping to change at page 30, line 21 ¶
	from fulfilling the Client's request.		from fulfilling the Client's request.
	o Errors caused by the destination server (96-127 range): the final		o Errors caused by the destination server (96-127 range): the final
	destination could not be reached or it replied with a reset.		destination could not be reached or it replied with a reset.
	The following error codes are defined in this document:		The following error codes are defined in this document:
	o Unsupported Version (0): The version number indicated in the fixed		o Unsupported Version (0): The version number indicated in the fixed
	header of a message received from a peer is not supported.		header of a message received from a peer is not supported.
	This error code MUST be generated by a Transport Converter (or		This error code MUST be generated by a peer (e.g. Transport
	Client) when it receives a request having a version number that it		Converter) when it receives a request having a version number that
	does not support.		it does not support.
	The value field MUST be set to the version supported by the		The value field MUST be set to the version supported by the peer.
	Transport Converter (or Client). When multiple versions are		When multiple versions are supported by the peer, it includes the
	supported by the Transport Converter (or Client), it includes the
	list of supported version in the value field; each version is		list of supported version in the value field; each version is
	encoded in 8 bits. The list of supported versions should be		encoded in 8 bits. The list of supported versions should be
	padded with zeros to end on a 32 bits boundary.		padded with zeros to end on a 32 bits boundary.
	Upon receipt of this error code, the Client (or Transport		Upon receipt of this error code, the remote peer (e.g., Client)
	Converter) checks whether it supports one of the versions returned		checks whether it supports one of the versions returned by the
	by the Transport Converter (or Client). The highest common		peer. The highest common supported version MUST be used by the
	supported version MUST be used by the Client (or Transport		remote peer in subsequent exchanges with the peer.
	Converter) in subsequent exchanges with the Transport Converter
	(or Client).
	o Malformed Message (1): This error code is sent to indicate that a		o Malformed Message (1): This error code is sent to indicate that a
	message received from a peer is can not be successfully parsed and		message received from a peer cannot be successfully parsed and
	validated.		validated.
	Typically, this error code is sent by the Transport Converter if		Typically, this error code is sent by the Transport Converter if
	it receives a Connect TLV enclosing a multicast, broadcast, or		it receives a Connect TLV enclosing a multicast, broadcast, or
	loopback IP address.		loopback IP address.
	To ease troubleshooting, the value field MUST echo the received		To ease troubleshooting, the value field MUST echo the received
	message shifted by one byte to keep to original alignment of the		message shifted by one byte to keep to original alignment of the
	message.		message.
	o Unsupported Message (2): This error code is sent to indicate that		o Unsupported Message (2): This error code is sent to indicate that
	a message type received from a peer is not supported.		a message type received from a Client is not supported.
	To ease troubleshooting, the value field MUST echo the received		To ease troubleshooting, the value field MUST echo the received
	message shifted by one byte to keep to original alignment of the		message shifted by one byte to keep to original alignment of the
	message.		message.
	o Missing Cookie (3): If a Transport Converter requires the		o Missing Cookie (3): If a Transport Converter requires the
	utilization of Cookies to prevent spoofing attacks and a Cookie		utilization of Cookies to prevent spoofing attacks and a Cookie
	TLV was not included in the Convert message, the Transport		TLV was not included in the Convert message, the Transport
	Converter MUST return this error to the requesting client. The		Converter MUST return this error to the requesting client. The
	first byte of the value field MUST be set to zero and the		first byte of the value field MUST be set to zero and the
	remaining bytes of the Error TLV contain the Cookie computed by		remaining bytes of the Error TLV contain the Cookie computed by
	the Transport Converter for this Client.		the Transport Converter for this Client.
	A Client which receives this error code MUST cache the received		A Client which receives this error code SHOULD cache the received
	Cookie and include it in subsequent Convert messages sent to that		Cookie and include it in subsequent Convert messages sent to that
	Transport Converter.		Transport Converter.
	o Not Authorized (32): This error code indicates that the Transport		o Not Authorized (32): This error code indicates that the Transport
	Converter refused to create a connection because of a lack of		Converter refused to create a connection because of a lack of
	authorization (e.g., administratively prohibited, authorization		authorization (e.g., administratively prohibited, authorization
	failure, invalid Cookie TLV, etc.). The Value field MUST be set		failure, invalid Cookie TLV, etc.). The Value field MUST be set
	to zero.		to zero.
	This error code MUST be sent by the Transport Converter when a		This error code MUST be sent by the Transport Converter when a
	skipping to change at page 27, line 40 ¶		skipping to change at page 32, line 21 ¶
	o Connection Reset (96): This error indicates that the final		o Connection Reset (96): This error indicates that the final
	destination responded with an RST packet. The Value field MUST be		destination responded with an RST packet. The Value field MUST be
	set to zero.		set to zero.
	o Destination Unreachable (97): This error indicates that an ICMP		o Destination Unreachable (97): This error indicates that an ICMP
	destination unreachable, port unreachable, or network unreachable		destination unreachable, port unreachable, or network unreachable
	was received by the Transport Converter. The Value field MUST		was received by the Transport Converter. The Value field MUST
	echo the Code field of the received ICMP message.		echo the Code field of the received ICMP message.
	Figure 22 summarizes the different error codes.		Figure 24 summarizes the different error codes.
	+-------+------+-----------------------------------------------+		+-------+------+-----------------------------------------------+
	| Error | Hex | Description |		| Error | Hex | Description |
	+-------+------+-----------------------------------------------+		+-------+------+-----------------------------------------------+
	| 0 | 0x00 | Unsupported Version |		| 0 | 0x00 | Unsupported Version |
	| 1 | 0x01 | Malformed Message |		| 1 | 0x01 | Malformed Message |
	| 2 | 0x02 | Unsupported Message |		| 2 | 0x02 | Unsupported Message |
	| 3 | 0x03 | Missing Cookie |		| 3 | 0x03 | Missing Cookie |
	| 32 | 0x20 | Not Authorized |		| 32 | 0x20 | Not Authorized |
	| 33 | 0x21 | Unsupported TCP Option |		| 33 | 0x21 | Unsupported TCP Option |
	| 64 | 0x40 | Resource Exceeded |		| 64 | 0x40 | Resource Exceeded |
	| 65 | 0x41 | Network Failure |		| 65 | 0x41 | Network Failure |
	| 96 | 0x60 | Connection Reset |		| 96 | 0x60 | Connection Reset |
	| 97 | 0x61 | Destination Unreachable |		| 97 | 0x61 | Destination Unreachable |
	+-------+------+-----------------------------------------------+		+-------+------+-----------------------------------------------+
	Figure 22: Convert Error Values		Figure 24: Convert Error Values
	6. Compatibility of Specific TCP Options with the Conversion Service		7. Compatibility of Specific TCP Options with the Conversion Service
	In this section, we discuss how several standard track TCP options		In this section, we discuss how several deployed standard track TCP
	can be supported through the Convert Protocol. The non-standard		options can be supported through the Convert Protocol. The other TCP
	track options and the experimental options will be discussed in other		options will be discussed in other documents.
	documents.
	6.1. Base TCP Options		7.1. Base TCP Options
	Three TCP options were initially defined in [RFC0793]: End-of-Option		Three TCP options were initially defined in [RFC0793]: End-of-Option
	List (Kind=0), No-Operation (Kind=1) and Maximum Segment Size		List (Kind=0), No-Operation (Kind=1) and Maximum Segment Size
	(Kind=2). The first two options are mainly used to pad the TCP		(Kind=2). The first two options are mainly used to pad the TCP
	header. There is no reason for a client to request a Transport		header. There is no reason for a client to request a Transport
	Converter to specifically send these options towards the final		Converter to specifically send these options towards the final
	destination.		destination.
	The Maximum Segment Size option (Kind=2) is used by a host to		The Maximum Segment Size option (Kind=2) is used by a host to
	indicate the largest segment that it can receive over each		indicate the largest segment that it can receive over each
	connection. This value is function of the stack that terminates the		connection. This value is function of the stack that terminates the
	TCP connection. There is no reason for a Client to request a		TCP connection. There is no reason for a Client to request a
	Transport Converter to advertise a specific MSS value to a remote		Transport Converter to advertise a specific MSS value to a remote
	server.		server.
	A Transport Converter MUST ignore options with Kind=0, 1 or 2 if they		A Transport Converter MUST ignore options with Kind=0, 1 or 2 if they
	appear in a Connect TLV. It MUST NOT announce them in a Supported		appear in a Connect TLV. It MUST NOT announce them in a Supported
	TCP Extensions TLV.		TCP Extensions TLV.
	6.2. Window Scale (WS)		7.2. Window Scale (WS)
	The Window Scale (WS) option (Kind=3) is defined in [RFC7323]. As		The Window Scale (WS) option (Kind=3) is defined in [RFC7323]. As
	for the MSS option, the window scale factor that is used for a		for the MSS option, the window scale factor that is used for a
	connection strongly depends on the TCP stack that handles the		connection strongly depends on the TCP stack that handles the
	connection. When a Transport Converter opens a TCP connection		connection. When a Transport Converter opens a TCP connection
	towards a remote server on behalf of a Client, it SHOULD use a WS		towards a remote server on behalf of a Client, it SHOULD use a WS
	option with a scaling factor that corresponds to the configuration of		option with a scaling factor that corresponds to the configuration of
	its stack. A local configuration MAY allow for WS option in the		its stack. A local configuration MAY allow for WS option in the
	proxied message to be function of the scaling factor of the incoming		proxied message to be function of the scaling factor of the incoming
	connection.		connection.
	There is no benefit from a deployment viewpoint in enabling a Client		There is no benefit from a deployment viewpoint in enabling a Client
	of a Transport Converter to specifically request the utilization of		of a Transport Converter to specifically request the utilization of
	the WS option (Kind=3) with a specific scaling factor towards a		the WS option (Kind=3) with a specific scaling factor towards a
	remote Server. For this reason, a Transport Converter MUST ignore		remote Server. For this reason, a Transport Converter MUST ignore
	option Kind=3 if it appears in a Connect TLV. It MUST NOT announce		option Kind=3 if it appears in a Connect TLV. It MUST NOT announce
	it in a Supported TCP Extensions TLV.		it in a Supported TCP Extensions TLV.
	6.3. Selective Acknowledgments		7.3. Selective Acknowledgments
	Two distinct TCP options were defined to support selective		Two distinct TCP options were defined to support selective
	acknowledgments in [RFC2018]. This first one, SACK Permitted		acknowledgments in [RFC2018]. This first one, SACK Permitted
	(Kind=4), is used to negotiate the utilization of selective		(Kind=4), is used to negotiate the utilization of selective
	acknowledgments during the three-way handshake. The second one, SACK		acknowledgments during the three-way handshake. The second one, SACK
	(Kind=5), carries the selective acknowledgments inside regular		(Kind=5), carries the selective acknowledgments inside regular
	segments.		segments.
	The SACK Permitted option (Kind=4) MAY be advertised by a Transport		The SACK Permitted option (Kind=4) MAY be advertised by a Transport
	Converter in the Supported TCP Extensions TLV. Clients connected to		Converter in the Supported TCP Extensions TLV. Clients connected to
	this Transport Converter MAY include the SACK Permitted option in the		this Transport Converter MAY include the SACK Permitted option in the
	Connect TLV.		Connect TLV.
	The SACK option (Kind=5) cannot be used during the three-way		The SACK option (Kind=5) cannot be used during the three-way
	handshake. For this reason, a Transport Converter MUST ignore option		handshake. For this reason, a Transport Converter MUST ignore option
	Kind=5 if it appears in a Connect TLV. It MUST NOT announce it in a		Kind=5 if it appears in a Connect TLV. It MUST NOT announce it in a
	TCP Supported Extensions TLV.		TCP Supported Extensions TLV.
	6.4. Timestamp		7.4. Timestamp
	The Timestamp option was initially defined in [RFC1323] and later		The Timestamp option [RFC7323] can be used during the three-way
	refined in [RFC7323]. It can be used during the three-way handshake		handshake to negotiate the utilization of timestamps during the TCP
	to negotiate the utilization of timestamps during the TCP connection.		connection. It is notably used to improve round-trip-time
	It is notably used to improve round-trip-time estimations and to		estimations and to provide protection against wrapped sequence
	provide protection against wrapped sequence numbers (PAWS). As for		numbers (PAWS). As for the WS option, the timestamps are a property
	the WS option, the timestamps are a property of a connection and		of a connection and there is limited benefit in enabling a client to
	there is limited benefit in enabling a client to request a Transport		request a Transport Converter to use the timestamp option when
	Converter to use the timestamp option when establishing a connection		establishing a connection to a remote server. Furthermore, the
	to a remote server. Furthermore, the timestamps that are used by TCP		timestamps that are used by TCP stacks are specific to each stack and
	stacks are specific to each stack and there is no benefit in enabling		there is no benefit in enabling a client to specify the timestamp
	a client to specify the timestamp value that a Transport Converter		value that a Transport Converter could use to establish a connection
	could use to establish a connection to a remote server.		to a remote server.
	A Transport Converter MAY advertise the Timestamp option (Kind=8) in		A Transport Converter MAY advertise the Timestamp option (Kind=8) in
	the TCP Supported Extensions TLV. The clients connected to this		the TCP Supported Extensions TLV. The clients connected to this
	Transport Converter MAY include the Timestamp option in the Connect		Transport Converter MAY include the Timestamp option in the Connect
	TLV but without any timestamp.		TLV but without any timestamp.
	6.5. Multipath TCP		7.5. Multipath TCP
	The Multipath TCP options are defined in [RFC6824]. [RFC6824]		The Multipath TCP options are defined in [RFC6824]. [RFC6824]
	defines one variable length TCP option (Kind=30) that includes a sub-		defines one variable length TCP option (Kind=30) that includes a sub-
	type field to support several Multipath TCP options. There are		type field to support several Multipath TCP options. There are
	several operational use cases where clients would like to use		several operational use cases where clients would like to use
	Multipath TCP through a Transport Converter [IETFJ16]. However, none		Multipath TCP through a Transport Converter [IETFJ16]. However, none
	of these use cases require the Client to specify the content of the		of these use cases require the Client to specify the content of the
	Multipath TCP option that the Transport Converter should send to a		Multipath TCP option that the Transport Converter should send to a
	remote server.		remote server.
	A Transport Converter which supports Multipath TCP conversion service		A Transport Converter which supports Multipath TCP conversion service
	MUST advertise the Multipath TCP option (Kind=30) in the Supported		MUST advertise the Multipath TCP option (Kind=30) in the Supported
	TCP Extensions TLV. Clients serviced by this Transport Converter may		TCP Extensions TLV. Clients serviced by this Transport Converter may
	include the Multipath TCP option in the Connect TLV but without any		include the Multipath TCP option in the Connect TLV but without any
	content.		content.
	6.6. TCP Fast Open		7.6. TCP Fast Open
	The TCP Fast Open cookie option (Kind=34) is defined in [RFC7413].		The TCP Fast Open cookie option (Kind=34) is defined in [RFC7413].
	There are two different usages of this option that need to be		There are two different usages of this option that need to be
	supported by Transport Converters. The first utilization of the TCP		supported by Transport Converters. The first utilization of the TCP
	Fast Open cookie option is to request a cookie from the server. In		Fast Open cookie option is to request a cookie from the server. In
	this case, the option is sent with an empty cookie by the client and		this case, the option is sent with an empty cookie by the client and
	the server returns the cookie. The second utilization of the TCP		the server returns the cookie. The second utilization of the TCP
	Fast Open cookie option is to send a cookie to the server. In this		Fast Open cookie option is to send a cookie to the server. In this
	case, the option contains a cookie.		case, the option contains a cookie.
	skipping to change at page 31, line 7 ¶		skipping to change at page 35, line 21 ¶
	Converter has advertised the support for TCP Fast Open in its		Converter has advertised the support for TCP Fast Open in its
	Supported TCP Extensions TLV, it needs to be able to process two		Supported TCP Extensions TLV, it needs to be able to process two
	types of Connect TLV. If such a Transport Converter receives a		types of Connect TLV. If such a Transport Converter receives a
	Connect TLV with the TCP Fast Open cookie option that does not		Connect TLV with the TCP Fast Open cookie option that does not
	contain a cookie, it MUST add an empty TCP Fast Open cookie option in		contain a cookie, it MUST add an empty TCP Fast Open cookie option in
	the SYN sent to the remote server. If such a Transport Converter		the SYN sent to the remote server. If such a Transport Converter
	receives a Connect TLV with the TCP Fast Open cookie option that		receives a Connect TLV with the TCP Fast Open cookie option that
	contains a cookie, it MUST copy the TCP Fast Open cookie option in		contains a cookie, it MUST copy the TCP Fast Open cookie option in
	the SYN sent to the remote server.		the SYN sent to the remote server.
	6.7. TCP User Timeout		7.7. TCP-AO
	The TCP User Timeout option is defined in [RFC5482]. The associated
	TCP option (Kind=28) does not appear to be widely deployed.
	6.8. TCP-AO
	TCP-AO [RFC5925] provides a technique to authenticate all the packets		TCP-AO [RFC5925] provides a technique to authenticate all the packets
	exchanged over a TCP connection. Given the nature of this extension,		exchanged over a TCP connection. Given the nature of this extension,
	it is unlikely that the applications that require their packets to be		it is unlikely that the applications that require their packets to be
	authenticated end-to-end would want their connections to pass through		authenticated end-to-end would want their connections to pass through
	a converter. For this reason, we do not recommend the support of the		a converter. For this reason, we do not recommend the support of the
	TCP-AO option by Transport Converters. The only use cases where it		TCP-AO option by Transport Converters. The only use cases where it
	could make sense to combine TCP-AO and the solution in this document		could make sense to combine TCP-AO and the solution in this document
	are those where the TCP-AO-NAT extension [RFC6978] is in use.		are those where the TCP-AO-NAT extension [RFC6978] is in use.
	A Transport Converter MUST NOT advertise the TCP-AO option (Kind=29)		A Transport Converter MUST NOT advertise the TCP-AO option (Kind=29)
	in the Supported TCP Extensions TLV. If a Transport Converter		in the Supported TCP Extensions TLV. If a Transport Converter
	receives a Connect TLV that contains the TCP-AO option, it MUST		receives a Connect TLV that contains the TCP-AO option, it MUST
	reject the establishment of the connection with error code set to		reject the establishment of the connection with error code set to
	"Unsupported TCP Option", except if the TCP-AO-NAT option is used.		"Unsupported TCP Option", except if the TCP-AO-NAT option is used.
	6.9. TCP Experimental Options		8. Interactions with Middleboxes
	The TCP Experimental options are defined in [RFC4727]. Given the
	variety of semantics for these options and their experimental nature,
	it is impossible to discuss them in details in this document.
	7. Interactions with Middleboxes
	The Convert Protocol is designed to be used in networks that do not		The Convert Protocol is designed to be used in networks that do not
	contain middleboxes that interfere with TCP. Under such conditions,		contain middleboxes that interfere with TCP. Under such conditions,
	it is assumed that the network provider ensures that all involved on-		it is assumed that the network provider ensures that all involved on-
	path nodes are not breaking TCP signals (e.g., strip TCP options,		path nodes are not breaking TCP signals (e.g., strip TCP options,
	discard some SYNs, etc.).		discard some SYNs, etc.).
	Nevertheless, and in order to allow for a robust service, this		Nevertheless, and in order to allow for a robust service, this
	section describes how a Client can detect middlebox interference and		section describes how a Client can detect middlebox interference and
	stop using the Transport Converter affected by this interference.		stop using the Transport Converter affected by this interference.
	Internet measurements [IMC11] have shown that middleboxes can affect		Internet measurements [IMC11] have shown that middleboxes can affect
	the deployment of TCP extensions. In this section, we only discuss		the deployment of TCP extensions. In this section, we focus the
	the middleboxes that modify SYN and SYN+ACK packets since the Convert		middleboxes that modify the payload since the Convert Protocol places
	Protocol places its messages in such packets.		its messages at the beginning of the bytestream.
	Consider a middlebox that removes the SYN payload. The Client can		Consider a middlebox that removes the SYN payload. The Client can
	detect this problem by looking at the acknowledgment number field of		detect this problem by looking at the acknowledgment number field of
	the SYN+ACK returned by the Transport Converter. The Client MUST		the SYN+ACK returned by the Transport Converter. The Client MUST
	stop to use this Transport Converter given the middlebox		stop to use this Transport Converter given the middlebox
	interference.		interference.
	Consider now a middlebox that drops SYN/ACKs with a payload. The		Consider now a middlebox that drops SYN/ACKs with a payload. The
	Client won't be able to establish a connection via the Transport		Client won't be able to establish a connection via the Transport
	Converter.		Converter. The case of a middlebox that removes the payload of
			SYN+ACKs or from the packet that follows the SYN+ACK (but not the
	The case of a middlebox that removes the payload of SYN+ACKs (but not		payload of SYN) can be detected by a Client. This is hinted by the
	the payload of SYN) can be detected by a Client. This is hinted by		absence of a valid Convert message in the response.
	the absence of an Error or Extended TCP Header TLV in a response. If
	an Error was returned by the Transport Converter, a message to close
	the connection would normally follow from the Converter. If no such
	message is received, the Client may continue to use this Converter.
	As explained in [RFC7413], some CGNs (Carrier Grade NATs) can affect		As explained in [RFC7413], some CGNs (Carrier Grade NATs) can affect
	the operation of TFO if they assign different IP addresses to the		the operation of TFO if they assign different IP addresses to the
	same end host. Such CGNs could affect the operation of the cookie		same end host. Such CGNs could affect the operation of the cookie
	validation used by the Convert Protocol. As a reminder CGNs, enabled		validation used by the Convert Protocol. As a reminder CGNs, enabled
	on the path between a Client and a Transport Converter, must adhere		on the path between a Client and a Transport Converter, must adhere
	to the address preservation defined in [RFC6888]. See also the		to the address preservation defined in [RFC6888]. See also the
	discussion in Section 7.1 of [RFC7413].		discussion in Section 7.1 of [RFC7413].
	8. Security Considerations		9. Security Considerations
	8.1. Privacy & Ingress Filtering		9.1. Privacy & Ingress Filtering
	The Transport Converter may have access to privacy-related		The Transport Converter may have access to privacy-related
	information (e.g., subscriber credentials). The Transport Converter		information (e.g., subscriber credentials). The Transport Converter
	is designed to not leak such sensitive information outside a local		is designed to not leak such sensitive information outside a local
	domain.		domain.
	Given its function and its location in the network, a Transport		Given its function and its location in the network, a Transport
	Converter has access to the payload of all the packets that it		Converter has access to the payload of all the packets that it
	processes. As such, it MUST be protected as a core IP router (e.g.,		processes. As such, it MUST be protected as a core IP router (e.g.,
	[RFC1812]).		[RFC1812]).
	Furthermore, ingress filtering policies MUST be enforced at the		Furthermore, ingress filtering policies MUST be enforced at the
	network boundaries [RFC2827].		network boundaries [RFC2827].
	This document assumes that all network attachments are managed by the		This document assumes that all network attachments are managed by the
	same administrative entity. Therefore, enforcing anti-spoofing		same administrative entity. Therefore, enforcing anti-spoofing
	filters at these network ensures that hosts are not sending traffic		filters at these network ensures that hosts are not sending traffic
	with spoofed source IP addresses.		with spoofed source IP addresses.
	8.2. Authorization		9.2. Authorization
	The Convert Protocol is intended to be used in managed networks where		The Convert Protocol is intended to be used in managed networks where
	end hosts can be identified by their IP address.		end hosts can be identified by their IP address.
	Stronger mutual authentication schemes MUST be defined to use the		Stronger mutual authentication schemes MUST be defined to use the
	Convert Protocol in more open network environments. One possibility		Convert Protocol in more open network environments. One possibility
	is to use TLS to perform mutual authentication between the client and		is to use TLS to perform mutual authentication between the client and
	the Converter. That is, use TLS when a Client retrieves a Cookie		the Converter. That is, use TLS when a Client retrieves a Cookie
	from the Converter and rely on certificate-based client		from the Converter and rely on certificate-based client
	authentication, pre-shared key based [RFC4279] or raw public key		authentication, pre-shared key based [RFC4279] or raw public key
	skipping to change at page 33, line 27 ¶		skipping to change at page 37, line 27 ¶
	If the authentication succeeds, the Converter returns a cookie to the		If the authentication succeeds, the Converter returns a cookie to the
	Client. Subsequent Connect messages will be authorized as a function		Client. Subsequent Connect messages will be authorized as a function
	of the content of the Cookie TLV.		of the content of the Cookie TLV.
	In deployments where network-assisted connections are not allowed		In deployments where network-assisted connections are not allowed
	between hosts of a domain (i.e., hairpinning), the Converter may be		between hosts of a domain (i.e., hairpinning), the Converter may be
	instructed to discard such connections. Hairpinned connections are		instructed to discard such connections. Hairpinned connections are
	thus rejected by the Transport Converter by returning an Error TLV		thus rejected by the Transport Converter by returning an Error TLV
	set to "Not Authorized". Absent explicit configuration otherwise,		set to "Not Authorized". Absent explicit configuration otherwise,
	hairpinning is enabled by the Converter (see Figure 23.		hairpinning is enabled by the Converter (see Figure 25.
	<===Network Provider===>		<===Network Provider===>
	+----+ from X1:x1 to X2':x2' +-----+ X1':x1'		+----+ from X1:x1 to X2':x2' +-----+ X1':x1'
	| C1 |>>>>>>>>>>>>>>>>>>>>>>>>>>>>>--+---		| C1 |>>>>>>>>>>>>>>>>>>>>>>>>>>>>>--+---
	+----+ | v |		+----+ | v |
	| v |		| v |
	| v |		| v |
	| v |		| v |
	+----+ from X1':x1' to X2:x2 | v | X2':x2'		+----+ from X1':x1' to X2:x2 | v | X2':x2'
	| C2 |<<<<<<<<<<<<<<<<<<<<<<<<<<<<<--+---		| C2 |<<<<<<<<<<<<<<<<<<<<<<<<<<<<<--+---
	+----+ +-----+		+----+ +-----+
	Converter		Converter
	Note: X2':x2' may be equal to		Note: X2':x2' may be equal to
	X2:x2		X2:x2
	Figure 23: Hairpinning Example		Figure 25: Hairpinning Example
	See below for authorization considerations that are specific for		See below for authorization considerations that are specific for
	Multipath TCP.		Multipath TCP.
	8.3. Denial of Service		9.3. Denial of Service
	Another possible risk is the amplification attacks since a Transport		Another possible risk is the amplification attacks since a Transport
	Converter sends a SYN towards a remote Server upon reception of a SYN		Converter sends a SYN towards a remote Server upon reception of a SYN
	from a Client. This could lead to amplification attacks if the SYN		from a Client. This could lead to amplification attacks if the SYN
	sent by the Transport Converter were larger than the SYN received		sent by the Transport Converter were larger than the SYN received
	from the Client or if the Transport Converter retransmits the SYN.		from the Client or if the Transport Converter retransmits the SYN.
	To mitigate such attacks, the Transport Converter SHOULD rate limit		To mitigate such attacks, the Transport Converter SHOULD rate limit
	the number of pending requests for a given Client. It SHOULD also		the number of pending requests for a given Client. It SHOULD also
	avoid sending to remote Servers SYNs that are significantly longer		avoid sending to remote Servers SYNs that are significantly longer
	than the SYN received from the Client. Finally, the Transport		than the SYN received from the Client. Finally, the Transport
	Converter SHOULD only retransmit a SYN to a Server after having		Converter SHOULD only retransmit a SYN to a Server after having
	received a retransmitted SYN from the corresponding Client. Means to		received a retransmitted SYN from the corresponding Client. Means to
	protect against SYN flooding attacks MUST also be enabled [RFC4987].		protect against SYN flooding attacks should also be enabled (e.g.,
			Section 3 of [RFC4987]).
	8.4. Traffic Theft		9.4. Traffic Theft
	Traffic theft is a risk if an illegitimate Converter is inserted in		Traffic theft is a risk if an illegitimate Converter is inserted in
	the path. Indeed, inserting an illegitimate Converter in the		the path. Indeed, inserting an illegitimate Converter in the
	forwarding path allows traffic interception and can therefore provide		forwarding path allows traffic interception and can therefore provide
	access to sensitive data issued by or destined to a host. Converter		access to sensitive data issued by or destined to a host. Converter
	discovery and configuration are out of scope of this document.		discovery and configuration are out of scope of this document.
	8.5. Multipath TCP-specific Considerations		9.5. Authentication Considerations
	Multipath TCP-related security threats are discussed in [RFC6181] and
	[RFC6824].
	The operator that manages the various network attachments (including		The operator that manages the various network attachments (including
	the Transport Converters) can enforce authentication and		the Transport Converters) can enforce authentication and
	authorization policies using appropriate mechanisms. For example, a		authorization policies using appropriate mechanisms. For example, a
	non-exhaustive list of methods to achieve authorization is provided		non-exhaustive list of methods to achieve authorization is provided
	hereafter:		hereafter:
	o The network provider may enforce a policy based on the		o The network provider may enforce a policy based on the
	International Mobile Subscriber Identity (IMSI) to verify that a		International Mobile Subscriber Identity (IMSI) to verify that a
	user is allowed to benefit from the Multipath TCP converter		user is allowed to benefit from the TCP converter service. If
	service. If that authorization fails, the Packet Data Protocol		that authorization fails, the Packet Data Protocol (PDP) context/
	(PDP) context/bearer will not be mounted. This method does not		bearer will not be mounted. This method does not require any
	require any interaction with the Transport Converter for		interaction with the Transport Converter for authorization
	authorization matters.		matters.
	o The network provider may enforce a policy based upon Access		o The network provider may enforce a policy based upon Access
	Control Lists (ACLs), e.g., at a Broadband Network Gateway (BNG)		Control Lists (ACLs), e.g., at a Broadband Network Gateway (BNG)
	to control the hosts that are authorized to communicate with a		to control the hosts that are authorized to communicate with a
	Transport Converter. These ACLs may be installed as a result of		Transport Converter. These ACLs may be installed as a result of
	RADIUS exchanges, e.g., [I-D.boucadair-radext-tcpm-converter].		RADIUS exchanges, e.g., [I-D.boucadair-radext-tcpm-converter].
	This method does not require any interaction with the Transport		This method does not require any interaction with the Transport
	Converter for authorization matters.		Converter for authorization matters.
	o A device that embeds a Transport Converter may also host a RADIUS		o A device that embeds a Transport Converter may also host a RADIUS
	client that will solicit an AAA server to check whether		client that will solicit an AAA server to check whether
	connections received from a given source IP address are authorized		connections received from a given source IP address are authorized
	or not [I-D.boucadair-radext-tcpm-converter].		or not [I-D.boucadair-radext-tcpm-converter].
	A first safeguard against the misuse of Transport Converter resources		A first safeguard against the misuse of Transport Converter resources
	by illegitimate users (e.g., users with access networks that are not		by illegitimate users (e.g., users with access networks that are not
	skipping to change at page 35, line 16 ¶		skipping to change at page 39, line 13 ¶
	Converter for authorization matters.		Converter for authorization matters.
	o A device that embeds a Transport Converter may also host a RADIUS		o A device that embeds a Transport Converter may also host a RADIUS
	client that will solicit an AAA server to check whether		client that will solicit an AAA server to check whether
	connections received from a given source IP address are authorized		connections received from a given source IP address are authorized
	or not [I-D.boucadair-radext-tcpm-converter].		or not [I-D.boucadair-radext-tcpm-converter].
	A first safeguard against the misuse of Transport Converter resources		A first safeguard against the misuse of Transport Converter resources
	by illegitimate users (e.g., users with access networks that are not		by illegitimate users (e.g., users with access networks that are not
	managed by the same provider that operates the Transport Converter)		managed by the same provider that operates the Transport Converter)
	is the Transport Converter to reject Multipath TCP connections		is the Transport Converter to reject Convert connections received on
	received on its Internet-facing interfaces. Only Multipath TCP		its Internet-facing interfaces. Only Convert connections received on
	connections received on the customer-facing interfaces of a Transport		the customer-facing interfaces of a Transport Converter will be
	Converter will be accepted.		accepted.
	9. IANA Considerations		10. IANA Considerations
	9.1. Convert Service Port Number		Note to the RFC Editor: Please replace "THISRFC" in the following
			sub-sections with the RFC number to be assigned to this document.
	IANA is requested to assign a TCP port number (TBA) for the Convert		10.1. Convert Service Name
	Protocol from the "Service Name and Transport Protocol Port Number
	Registry" available at https://www.iana.org/assignments/service-		IANA is requested to assign a service name for the Convert Protocol
	names-port-numbers/service-names-port-numbers.xhtml.		from the "Service Name and Transport Protocol Port Number Registry"
			available at https://www.iana.org/assignments/service-names-port-
			numbers/service-names-port-numbers.xhtml.
	Service Name: convert		Service Name: convert
	Port Number: TBD		Port Number: N/A
	Transport Protocol(s): TCP		Transport Protocol(s): TCP
	Description: 0-RTT TCP Convert Protocol		Description: 0-RTT TCP Convert Protocol
	Assignee: IESG <iesg@ietf.org>		Assignee: IESG <iesg@ietf.org>
	Contact: IETF Chair <chair@ietf.org>		Contact: IETF Chair <chair@ietf.org>
	Reference: RFC XXXX		Reference: THISRFC
	9.2. The Convert Protocol (Convert) Parameters		Clients may use this service name to fed the procedure defined in
			[RFC2782] to discover the IP address(es) and the port number used by
			the Transport Converters of a domain.
	IANA is requested to create a new "The Convert Protocol (Convert)		10.2. The Convert Protocol (Convert) Parameters
			IANA is requested to create a new "The TCP Convert Protocol (Convert)
	Parameters" registry.		Parameters" registry.
	The following subsections detail new registries within "The Convert		The following subsections detail new registries within "The Convert
	Protocol (Convert) Parameters" registry.		Protocol (Convert) Parameters" registry.
	9.2.1. Convert Versions		Registration requests for the 128-191 range (for both "Convert TLVs"
			and "Convert Error Messages" sub-registries) are evaluated after a
			three-week review period on the tcp-convert-review@ietf.org mailing
			list, on the advice of one or more Designated Experts. However, to
			allow for the allocation of values prior to publication, the
			Designated Experts may approve registration once they are satisfied
			that such a specification will be published. New registration
			requests should be sent in the form of an email to the review mailing
			list; the request should use an appropriate subject (e.g., "Request
			to register 0-RTT Convert TLV: example" or "Request to register 0-RTT
			Convert Error Message: example"). IANA will only accept new
			registrations from the Designated Experts, and will check that review
			was requested on the mailing list in accordance with these
			procedures.
			Within the review period, the Designated Experts will either approve
			or deny the registration request, communicating this decision to the
			review list and IANA. Denials should include an explanation and, if
			applicable, suggestions as to how to make the request successful.
			Registration requests that are undetermined for a period longer than
			21 days can be brought to the IESG's attention (using the
			iesg@ietf.org mailing list) for resolution.
			The Designated Expert is expected to ascertain the existence of
			suitable documentation as described in Section 4.6 of [RFC8126] and
			to verify that the document is permanently and publicly available.
			The Designated Expert is also expected to check the clarity of
			purpose and use of the requested code points.
			Also, criteria that should be applied by the Designated Experts
			includes determining whether the proposed registration duplicates
			existing functionality, whether it is likely to be of general
			applicability or whether it is useful only for a private use, and
			whether the registration description is clear. IANA must only accept
			registry updates to the 128-191 range (for both "Convert TLVs" and
			"Convert Error Messages" sub-registries) from the Designated Experts
			and should direct all requests for registration to the review mailing
			list. It is suggested that multiple Designated Experts be appointed.
			In cases where a registration decision could be perceived as creating
			a conflict of interest for a particular Expert, that Expert should
			defer to the judgment of the other Experts.
			10.2.1. Convert Versions
	IANA is requested to create the "Convert versions" sub-registry. New		IANA is requested to create the "Convert versions" sub-registry. New
	values are assigned via IETF Review (Section 4.8 of [RFC8126]).		values are assigned via IETF Review (Section 4.8 of [RFC8126]).
	The initial values to be assigned at the creation of the registry are		The initial values to be assigned at the creation of the registry are
	as follows:		as follows:
	+---------+--------------------------------------+-------------+		+---------+--------------------------------------+-------------+
	| Version | Description | Reference |		| Version | Description | Reference |
	+---------+--------------------------------------+-------------+		+---------+--------------------------------------+-------------+
	| 0 | Reserved by this document | [This-RFC] |		| 0 | Reserved by this document | THISRFC |
	| 1 | Assigned by this document | [This-RFC] |		| 1 | Assigned by this document | THISRFC |
	+---------+--------------------------------------+-------------+		+---------+--------------------------------------+-------------+
	9.2.2. Convert TLVs		10.2.2. Convert TLVs
	IANA is requested to create the "Convert TLVs" sub-registry. The		IANA is requested to create the "Convert TLVs" sub-registry. The
	procedure for assigning values from this registry is as follows:		procedure for assigning values from this registry is as follows:
	o The values in the range 1-127 can be assigned via IETF Review.		o The values in the range 1-127 can be assigned via IETF Review.
	o The values in the range 128-191 can be assigned via Specification		o The values in the range 128-191 can be assigned via Specification
	Required.		Required.
	o The values in the range 192-255 can be assigned for Private Use.		o The values in the range 192-255 are reserved for Private Use.
	The initial values to be assigned at the creation of the registry are		The initial values to be assigned at the creation of the registry are
	as follows:		as follows:
	+---------+--------------------------------------+-------------+		+---------+--------------------------------------+-------------+
	| Code | Name | Reference |		| Code | Name | Reference |
	+---------+--------------------------------------+-------------+		+---------+--------------------------------------+-------------+
	| 0 | Reserved | [This-RFC] |		| 0 | Reserved | THISRFC |
	| 1 | Info TLV | [This-RFC] |		| 1 | Info TLV | THISRFC |
	| 10 | Connect TLV | [This-RFC] |		| 10 | Connect TLV | THISRFC |
	| 20 | Extended TCP Header TLV | [This-RFC] |		| 20 | Extended TCP Header TLV | THISRFC |
	| 21 | Supported TCP Extension TLV | [This-RFC] |		| 21 | Supported TCP Extension TLV | THISRFC |
	| 22 | Cookie TLV | [This-RFC] |		| 22 | Cookie TLV | THISRFC |
	| 30 | Error TLV | [This-RFC] |		| 30 | Error TLV | THISRFC |
	+---------+--------------------------------------+-------------+		+---------+--------------------------------------+-------------+
	9.2.3. Convert Error Messages		10.2.3. Convert Error Messages
	IANA is requested to create the "Convert Errors" sub-registry. Codes		IANA is requested to create the "Convert Errors" sub-registry. Codes
	in this registry are assigned as a function of the error type. Four		in this registry are assigned as a function of the error type. Four
	types are defined; the following ranges are reserved for each of		types are defined; the following ranges are reserved for each of
	these types:		these types:
	o Message validation and processing errors: 0-31		o Message validation and processing errors: 0-31
	o Client-side errors: 32-63		o Client-side errors: 32-63
	skipping to change at page 37, line 12 ¶		skipping to change at page 42, line 12 ¶
	o Errors caused by destination server: 96-127		o Errors caused by destination server: 96-127
	The procedure for assigning values from this sub-registry is as		The procedure for assigning values from this sub-registry is as
	follows:		follows:
	o 0-127: Values in this range are assigned via IETF Review.		o 0-127: Values in this range are assigned via IETF Review.
	o 128-191: Values in this range are assigned via Specification		o 128-191: Values in this range are assigned via Specification
	Required.		Required.
	o 192-255: Values in this range are assigned for Private Use.		o 192-255: Values in this range are reserved for Private Use.
	The initial values to be assigned at the creation of the registry are		The initial values to be assigned at the creation of the registry are
	as follows:		as follows:
	+-------+------+-----------------------------------+-----------+		+-------+------+-----------------------------------+-----------+
	| Error | Hex | Description | Reference |		| Error | Hex | Description | Reference |
	+-------+------+-----------------------------------+-----------+		+-------+------+-----------------------------------+-----------+
	| 0 | 0x00 | Unsupported Version | [This-RFC]|		| 0 | 0x00 | Unsupported Version | THISRFC |
	| 1 | 0x01 | Malformed Message | [This-RFC]|		| 1 | 0x01 | Malformed Message | THISRFC |
	| 2 | 0x02 | Unsupported Message | [This-RFC]|		| 2 | 0x02 | Unsupported Message | THISRFC |
	| 3 | 0x03 | Missing Cookie | [This-RFC]|		| 3 | 0x03 | Missing Cookie | THISRFC |
	| 32 | 0x20 | Not Authorized | [This-RFC]|		| 32 | 0x20 | Not Authorized | THISRFC |
	| 33 | 0x21 | Unsupported TCP Option | [This-RFC]|		| 33 | 0x21 | Unsupported TCP Option | THISRFC |
	| 64 | 0x40 | Resource Exceeded | [This-RFC]|		| 64 | 0x40 | Resource Exceeded | THISRFC |
	| 65 | 0x41 | Network Failure | [This-RFC]|		| 65 | 0x41 | Network Failure | THISRFC |
	| 96 | 0x60 | Connection Reset | [This-RFC]|		| 96 | 0x60 | Connection Reset | THISRFC |
	| 97 | 0x61 | Destination Unreachable | [This-RFC]|		| 97 | 0x61 | Destination Unreachable | THISRFC |
	+-------+------+-----------------------------------+-----------+		+-------+------+-----------------------------------+-----------+
	Figure 24: The Convert Error Codes		Figure 26: The Convert Error Codes
	10. References		11. References
	10.1. Normative References		11.1. Normative References
	[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,		[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
	RFC 793, DOI 10.17487/RFC0793, September 1981,		RFC 793, DOI 10.17487/RFC0793, September 1981,
	<https://www.rfc-editor.org/info/rfc793>.		<https://www.rfc-editor.org/info/rfc793>.
			[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
			Selective Acknowledgment Options", RFC 2018,
			DOI 10.17487/RFC2018, October 1996,
			<https://www.rfc-editor.org/info/rfc2018>.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key		[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
	Ciphersuites for Transport Layer Security (TLS)",		Defeating Denial of Service Attacks which employ IP Source
	RFC 4279, DOI 10.17487/RFC4279, December 2005,		Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
	<https://www.rfc-editor.org/info/rfc4279>.		May 2000, <https://www.rfc-editor.org/info/rfc2827>.
	[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing		[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
	Architecture", RFC 4291, DOI 10.17487/RFC4291, February		Architecture", RFC 4291, DOI 10.17487/RFC4291, February
	2006, <https://www.rfc-editor.org/info/rfc4291>.		2006, <https://www.rfc-editor.org/info/rfc4291>.
	[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
	ICMPv6, UDP, and TCP Headers", RFC 4727,
	DOI 10.17487/RFC4727, November 2006,
	<https://www.rfc-editor.org/info/rfc4727>.
	[RFC4787] Audet, F., Ed. and C. Jennings, "Network Address		[RFC4787] Audet, F., Ed. and C. Jennings, "Network Address
	Translation (NAT) Behavioral Requirements for Unicast		Translation (NAT) Behavioral Requirements for Unicast
	UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January		UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
	2007, <https://www.rfc-editor.org/info/rfc4787>.		2007, <https://www.rfc-editor.org/info/rfc4787>.
	[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common		[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
	Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,		Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
	<https://www.rfc-editor.org/info/rfc4987>.		<https://www.rfc-editor.org/info/rfc4987>.
	[RFC5482] Eggert, L. and F. Gont, "TCP User Timeout Option",
	RFC 5482, DOI 10.17487/RFC5482, March 2009,
	<https://www.rfc-editor.org/info/rfc5482>.
	[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP		[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
	Authentication Option", RFC 5925, DOI 10.17487/RFC5925,		Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
	June 2010, <https://www.rfc-editor.org/info/rfc5925>.		June 2010, <https://www.rfc-editor.org/info/rfc5925>.
	[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,		[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
	"TCP Extensions for Multipath Operation with Multiple		"TCP Extensions for Multipath Operation with Multiple
	Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,		Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
	<https://www.rfc-editor.org/info/rfc6824>.		<https://www.rfc-editor.org/info/rfc6824>.
	[RFC6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,		[RFC6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
	A., and H. Ashida, "Common Requirements for Carrier-Grade		A., and H. Ashida, "Common Requirements for Carrier-Grade
	NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,		NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,
	April 2013, <https://www.rfc-editor.org/info/rfc6888>.		April 2013, <https://www.rfc-editor.org/info/rfc6888>.
	[RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,		[RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
	"Special-Purpose IP Address Registries", BCP 153,		"Special-Purpose IP Address Registries", BCP 153,
	RFC 6890, DOI 10.17487/RFC6890, April 2013,		RFC 6890, DOI 10.17487/RFC6890, April 2013,
	<https://www.rfc-editor.org/info/rfc6890>.		<https://www.rfc-editor.org/info/rfc6890>.
	[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,		[RFC6978] Touch, J., "A TCP Authentication Option Extension for NAT
	Weiler, S., and T. Kivinen, "Using Raw Public Keys in		Traversal", RFC 6978, DOI 10.17487/RFC6978, July 2013,
	Transport Layer Security (TLS) and Datagram Transport		<https://www.rfc-editor.org/info/rfc6978>.
	Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
	June 2014, <https://www.rfc-editor.org/info/rfc7250>.		[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
			Scheffenegger, Ed., "TCP Extensions for High Performance",
			RFC 7323, DOI 10.17487/RFC7323, September 2014,
			<https://www.rfc-editor.org/info/rfc7323>.
	[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP		[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
	Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,		Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
	<https://www.rfc-editor.org/info/rfc7413>.		<https://www.rfc-editor.org/info/rfc7413>.
	[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for		[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
	Writing an IANA Considerations Section in RFCs", BCP 26,		Writing an IANA Considerations Section in RFCs", BCP 26,
	RFC 8126, DOI 10.17487/RFC8126, June 2017,		RFC 8126, DOI 10.17487/RFC8126, June 2017,
	<https://www.rfc-editor.org/info/rfc8126>.		<https://www.rfc-editor.org/info/rfc8126>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	10.2. Informative References		11.2. Informative References
	[ANRW17] Trammell, B., Kuhlewind, M., De Vaere, P., Learmonth, I.,		[ANRW17] Trammell, B., Kuehlewind, M., De Vaere, P., Learmonth, I.,
	and G. Fairhurst, "Tracking transport-layer evolution with		and G. Fairhurst, "Tracking transport-layer evolution with
	PATHspider", Applied Networking Research Workshop 2017		PATHspider", Applied Networking Research Workshop 2017
	(ANRW17) , July 2017.		(ANRW17) , July 2017.
	[Fukuda2011]		[Fukuda2011]
	Fukuda, K., "An Analysis of Longitudinal TCP Passive		Fukuda, K., "An Analysis of Longitudinal TCP Passive
	Measurements (Short Paper)", Traffic Monitoring and		Measurements (Short Paper)", Traffic Monitoring and
	Analysis. TMA 2011. Lecture Notes in Computer Science, vol		Analysis. TMA 2011. Lecture Notes in Computer Science, vol
	6613. , 2011.		6613. , 2011.
	[HotMiddlebox13b]		[HotMiddlebox13b]
	Detal, G., Paasch, C., and O. Bonaventure, "Multipath in		Detal, G., Paasch, C., and O. Bonaventure, "Multipath in
	the Middle(Box)", HotMiddlebox'13 , December 2013,		the Middle(Box)", HotMiddlebox'13 , December 2013,
	<http://inl.info.ucl.ac.be/publications/multipath-		<http://inl.info.ucl.ac.be/publications/
	middlebox>.		multipath-middlebox>.
	[I-D.arkko-arch-low-latency]		[I-D.arkko-arch-low-latency]
	Arkko, J. and J. Tantsura, "Low Latency Applications and		Arkko, J. and J. Tantsura, "Low Latency Applications and
	the Internet Architecture", draft-arkko-arch-low-		the Internet Architecture", draft-arkko-arch-low-
	latency-02 (work in progress), October 2017.		latency-02 (work in progress), October 2017.
	[I-D.boucadair-mptcp-plain-mode]		[I-D.boucadair-mptcp-plain-mode]
	Boucadair, M., Jacquenet, C., Bonaventure, O., Behaghel,		Boucadair, M., Jacquenet, C., Bonaventure, O., Behaghel,
	D., stefano.secci@lip6.fr, s., Henderickx, W., Skog, R.,		D., stefano.secci@lip6.fr, s., Henderickx, W., Skog, R.,
	Vinapamula, S., Seo, S., Cloetens, W., Meyer, U.,		Vinapamula, S., Seo, S., Cloetens, W., Meyer, U.,
	Contreras, L., and B. Peirens, "Extensions for Network-		Contreras, L., and B. Peirens, "Extensions for Network-
	Assisted MPTCP Deployment Models", draft-boucadair-mptcp-		Assisted MPTCP Deployment Models", draft-boucadair-mptcp-
	plain-mode-10 (work in progress), March 2017.		plain-mode-10 (work in progress), March 2017.
	[I-D.boucadair-radext-tcpm-converter]		[I-D.boucadair-radext-tcpm-converter]
	Boucadair, M. and C. Jacquenet, "RADIUS Extensions for		Boucadair, M. and C. Jacquenet, "RADIUS Extensions for
	0-RTT TCP Converters", draft-boucadair-radext-tcpm-		0-RTT TCP Converters", draft-boucadair-radext-tcpm-
	converter-02 (work in progress), April 2019.		converter-02 (work in progress), April 2019.
	[I-D.boucadair-tcpm-dhc-converter]		[I-D.boucadair-tcpm-dhc-converter]
	Boucadair, M., Jacquenet, C., and R. K, "DHCP Options for		Boucadair, M., Jacquenet, C., and T. Reddy.K, "DHCP
	0-RTT TCP Converters", draft-boucadair-tcpm-dhc-		Options for 0-RTT TCP Converters", draft-boucadair-tcpm-
	converter-03 (work in progress), October 2019.		dhc-converter-03 (work in progress), October 2019.
	[I-D.olteanu-intarea-socks-6]		[I-D.olteanu-intarea-socks-6]
	Olteanu, V. and D. Niculescu, "SOCKS Protocol Version 6",		Olteanu, V. and D. Niculescu, "SOCKS Protocol Version 6",
	draft-olteanu-intarea-socks-6-07 (work in progress), July		draft-olteanu-intarea-socks-6-08 (work in progress),
	2019.		November 2019.
	[I-D.peirens-mptcp-transparent]		[I-D.peirens-mptcp-transparent]
	Peirens, B., Detal, G., Barre, S., and O. Bonaventure,		Peirens, B., Detal, G., Barre, S., and O. Bonaventure,
	"Link bonding with transparent Multipath TCP", draft-		"Link bonding with transparent Multipath TCP", draft-
	peirens-mptcp-transparent-00 (work in progress), July		peirens-mptcp-transparent-00 (work in progress), July
	2016.		2016.
	[IETFJ16] Bonaventure, O. and S. Seo, "Multipath TCP Deployment",		[IETFJ16] Bonaventure, O. and S. Seo, "Multipath TCP Deployment",
	IETF Journal, Fall 2016 , n.d..		IETF Journal, Fall 2016 , n.d..
	[IMC11] Honda, K., Nishida, Y., Raiciu, C., Greenhalgh, A.,		[IMC11] Honda, K., Nishida, Y., Raiciu, C., Greenhalgh, A.,
	Handley, M., and T. Hideyuki, "Is it still possible to		Handley, M., and T. Hideyuki, "Is it still possible to
	extend TCP?", Proceedings of the 2011 ACM SIGCOMM		extend TCP?", Proceedings of the 2011 ACM SIGCOMM
	conference on Internet measurement conference , 2011.		conference on Internet measurement conference , 2011.
	[RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
	for High Performance", RFC 1323, DOI 10.17487/RFC1323, May
	1992, <https://www.rfc-editor.org/info/rfc1323>.
	[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",		[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
	RFC 1812, DOI 10.17487/RFC1812, June 1995,		RFC 1812, DOI 10.17487/RFC1812, June 1995,
	<https://www.rfc-editor.org/info/rfc1812>.		<https://www.rfc-editor.org/info/rfc1812>.
	[RFC1919] Chatel, M., "Classical versus Transparent IP Proxies",		[RFC1919] Chatel, M., "Classical versus Transparent IP Proxies",
	RFC 1919, DOI 10.17487/RFC1919, March 1996,		RFC 1919, DOI 10.17487/RFC1919, March 1996,
	<https://www.rfc-editor.org/info/rfc1919>.		<https://www.rfc-editor.org/info/rfc1919>.
	[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and		[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
	L. Jones, "SOCKS Protocol Version 5", RFC 1928,		L. Jones, "SOCKS Protocol Version 5", RFC 1928,
	DOI 10.17487/RFC1928, March 1996,		DOI 10.17487/RFC1928, March 1996,
	<https://www.rfc-editor.org/info/rfc1928>.		<https://www.rfc-editor.org/info/rfc1928>.
	[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP		[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
	Selective Acknowledgment Options", RFC 2018,		specifying the location of services (DNS SRV)", RFC 2782,
	DOI 10.17487/RFC2018, October 1996,		DOI 10.17487/RFC2782, February 2000,
	<https://www.rfc-editor.org/info/rfc2018>.		<https://www.rfc-editor.org/info/rfc2782>.
	[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
	Defeating Denial of Service Attacks which employ IP Source
	Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
	May 2000, <https://www.rfc-editor.org/info/rfc2827>.
	[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.		[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
	Shelby, "Performance Enhancing Proxies Intended to		Shelby, "Performance Enhancing Proxies Intended to
	Mitigate Link-Related Degradations", RFC 3135,		Mitigate Link-Related Degradations", RFC 3135,
	DOI 10.17487/RFC3135, June 2001,		DOI 10.17487/RFC3135, June 2001,
	<https://www.rfc-editor.org/info/rfc3135>.		<https://www.rfc-editor.org/info/rfc3135>.
	[RFC6181] Bagnulo, M., "Threat Analysis for TCP Extensions for		[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
	Multipath Operation with Multiple Addresses", RFC 6181,		Ciphersuites for Transport Layer Security (TLS)",
	DOI 10.17487/RFC6181, March 2011,		RFC 4279, DOI 10.17487/RFC4279, December 2005,
	<https://www.rfc-editor.org/info/rfc6181>.		<https://www.rfc-editor.org/info/rfc4279>.
	[RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and		[RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
	P. Roberts, "Issues with IP Address Sharing", RFC 6269,		P. Roberts, "Issues with IP Address Sharing", RFC 6269,
	DOI 10.17487/RFC6269, June 2011,		DOI 10.17487/RFC6269, June 2011,
	<https://www.rfc-editor.org/info/rfc6269>.		<https://www.rfc-editor.org/info/rfc6269>.
	[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix		[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
	Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,		Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,
	<https://www.rfc-editor.org/info/rfc6296>.		<https://www.rfc-editor.org/info/rfc6296>.
	[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and		[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
	P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,		P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
	DOI 10.17487/RFC6887, April 2013,		DOI 10.17487/RFC6887, April 2013,
	<https://www.rfc-editor.org/info/rfc6887>.		<https://www.rfc-editor.org/info/rfc6887>.
	[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,		[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
	"Increasing TCP's Initial Window", RFC 6928,		"Increasing TCP's Initial Window", RFC 6928,
	DOI 10.17487/RFC6928, April 2013,		DOI 10.17487/RFC6928, April 2013,
	<https://www.rfc-editor.org/info/rfc6928>.		<https://www.rfc-editor.org/info/rfc6928>.
	[RFC6978] Touch, J., "A TCP Authentication Option Extension for NAT		[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
	Traversal", RFC 6978, DOI 10.17487/RFC6978, July 2013,		Weiler, S., and T. Kivinen, "Using Raw Public Keys in
	<https://www.rfc-editor.org/info/rfc6978>.		Transport Layer Security (TLS) and Datagram Transport
			Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
	[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.		June 2014, <https://www.rfc-editor.org/info/rfc7250>.
	Scheffenegger, Ed., "TCP Extensions for High Performance",
	RFC 7323, DOI 10.17487/RFC7323, September 2014,
	<https://www.rfc-editor.org/info/rfc7323>.
	[RFC7414] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.		[RFC7414] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
	Zimmermann, "A Roadmap for Transmission Control Protocol		Zimmermann, "A Roadmap for Transmission Control Protocol
	(TCP) Specification Documents", RFC 7414,		(TCP) Specification Documents", RFC 7414,
	DOI 10.17487/RFC7414, February 2015,		DOI 10.17487/RFC7414, February 2015,
	<https://www.rfc-editor.org/info/rfc7414>.		<https://www.rfc-editor.org/info/rfc7414>.
	[RFC8041] Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and		[RFC8041] Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and
	Operational Experience with Multipath TCP", RFC 8041,		Operational Experience with Multipath TCP", RFC 8041,
	DOI 10.17487/RFC8041, January 2017,		DOI 10.17487/RFC8041, January 2017,
	skipping to change at page 42, line 36 ¶		skipping to change at page 47, line 25 ¶
	Q., and E. Smith, "Cryptographic Protection of TCP Streams		Q., and E. Smith, "Cryptographic Protection of TCP Streams
	(tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,		(tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,
	<https://www.rfc-editor.org/info/rfc8548>.		<https://www.rfc-editor.org/info/rfc8548>.
	[TS23501] 3GPP (3rd Generation Partnership Project), ., "Technical		[TS23501] 3GPP (3rd Generation Partnership Project), ., "Technical
	Specification Group Services and System Aspects; System		Specification Group Services and System Aspects; System
	Architecture for the 5G System; Stage 2 (Release 16)",		Architecture for the 5G System; Stage 2 (Release 16)",
	2019, <https://www.3gpp.org/ftp/Specs/		2019, <https://www.3gpp.org/ftp/Specs/
	archive/23_series/23.501/>.		archive/23_series/23.501/>.
	Appendix A. Change Log		Appendix A. Example Socket API Changes to Support the 0-RTT Convert
	This section to be removed before publication.
	o 00 : initial version, designed to support Multipath TCP and TFO
	only
	o 00 to -01 : added section Section 6 describing the support of
	different standard tracks TCP options by Transport Converters,
	clarification of the IANA section, moved the SOCKS comparison to
	the appendix and various minor modifications
	o 01 to -02: Minor modifications
	o 02 to -03: Minor modifications
	o 03 to -04: Minor modifications
	o 04 to -05: Integrate a lot of feedback from implementors who have
	worked on client and server side implementations. The main
	modifications are the following :
	* TCP Fast Open is not strictly required anymore. Several
	implementors expressed concerns about this requirement. The
	TFO Cookie protects from some attack scenarios that affect open
	servers like web servers. The Convert Protocol is different
	and as discussed in RFC7413, there are different ways to
	protect from such attacks. Instead of using a TFO cookie
	inside the TCP options, which consumes precious space in the
	extended TCP header, this version supports the utilization of a
	Cookie that is placed in the SYN payload. This provides the
	same level of protection as a TFO Cookie in environments were
	such protection is required.
	* the Bootstrap procedure has been simplified based on feedback
	from implementors
	* Error messages are not included in RST segments anymore but
	sent in the bytestream. Implementors have indicated that
	processing such segments on clients was difficult on some
	platforms. This change simplifies client implementations.
	* Many minor editorial changes to clarify the text based on
	implementors feedback.
	o 05 to -06: Many clarifications to integrate the comments from the
	chairs in preparation to the WGLC:
	* Updated IANA policy to require "IETF Review" instead of
	"Standard Action"
	* Call out explicitly that data in SYNs are relayed by the
	Converter
	* Reiterate the scope
	* Hairpinning behavior can be disabled (policy-based)
	* Fix nits
	o 07:
	* Update the text about supplying data in SYNs to make it clear
	that a constraint defined in RFC793 is relaxed following the
	same rationale as in RFC7413.
	* Nits
	* Added Appendix A on example Socket API changes
	o 08:
	* Added short discussion on the termination of connections
	o 09:
	* Address various comments received during last call
	o 10-13:
	* Changes to address the comments from Phil: Add a new section to
	group data plane considerations in one place + add a new
	appendix with more details on address modes + rearrange the
	MPTCP text.
	o 14: fixed nits (the shepherd write-up)
	Appendix B. Example Socket API Changes to Support the 0-RTT Convert
	Protocol		Protocol
	B.1. Active Open (Client Side)		A.1. Active Open (Client Side)
	On the client side, the support of the 0-RTT Converter protocol does		On the client side, the support of the 0-RTT Converter protocol does
	not require any other changes than those identified in Appendix A of		not require any other changes than those identified in Appendix A of
	[RFC7413]. Those modifications are already supported by multiple TCP		[RFC7413]. Those modifications are already supported by multiple TCP
	stacks.		stacks.
	As an example, on Linux, a client can send the 0-RTT Convert message		As an example, on Linux, a client can send the 0-RTT Convert message
	inside a SYN by using sendto with the MSG_FASTOPEN flag as shown in		inside a SYN by using sendto with the MSG_FASTOPEN flag as shown in
	the example below:		the example below:
	skipping to change at page 45, line 12 ¶		skipping to change at page 48, line 9 ¶
	o 0x1: (client) enables sending data in the opening SYN on the		o 0x1: (client) enables sending data in the opening SYN on the
	client.		client.
	o 0x4: (client) send data in the opening SYN regardless of cookie		o 0x4: (client) send data in the opening SYN regardless of cookie
	availability and without a cookie option.		availability and without a cookie option.
	By setting this configuration variable to 0x5, a Linux client using		By setting this configuration variable to 0x5, a Linux client using
	the above code would send data inside the SYN without using a TFO		the above code would send data inside the SYN without using a TFO
	option.		option.
	B.2. Passive Open (Converter Side)		A.2. Passive Open (Converter Side)
	The Converter needs to enable the reception of data inside the SYN		The Converter needs to enable the reception of data inside the SYN
	independently of the utilization of the TFO option. This implies		independently of the utilization of the TFO option. This implies
	that the Transport Converter application cannot rely on the TFO		that the Transport Converter application cannot rely on the TFO
	cookies to validate the reachability of the IP address that sent the		cookies to validate the reachability of the IP address that sent the
	SYN. It must rely on other techniques, such as the Cookie TLV		SYN. It must rely on other techniques, such as the Cookie TLV
	described in this document, to verify this reachability.		described in this document, to verify this reachability.
	[RFC7413] suggested the utilization of a TCP_FASTOPEN socket option		[RFC7413] suggested the utilization of a TCP_FASTOPEN socket option
	the enable the reception of SYNs containing data. Later, Appendix A		the enable the reception of SYNs containing data. Later, Appendix A
	skipping to change at page 46, line 10 ¶		skipping to change at page 49, line 5 ¶
	However, this configuration is system-wide. This is convenient for		However, this configuration is system-wide. This is convenient for
	typical Transport Converter deployments where no other applications		typical Transport Converter deployments where no other applications
	relying on TFO are collocated on the same device.		relying on TFO are collocated on the same device.
	Recently, the TCP_FASTOPEN_NO_COOKIE socket option has been added to		Recently, the TCP_FASTOPEN_NO_COOKIE socket option has been added to
	provide the same behavior on a per socket basis. This enables a		provide the same behavior on a per socket basis. This enables a
	single host to support both servers that require the TFO cookie and		single host to support both servers that require the TFO cookie and
	servers that do not use it.		servers that do not use it.
	Appendix C. Some Design Considerations
	Several implementors expressed concerns about the use of TFO. As a
	reminder, the TFO Cookie protects from some attack scenarios that
	affect open servers like web servers. The Convert Protocol is
	different and, as discussed in RFC7413, there are different ways to
	protect from such attacks. Instead of using a TFO cookie inside the
	TCP options, which consumes precious space in the extended TCP
	header, the Convert Protocol supports the utilization of a Cookie
	that is placed in the SYN payload. This provides the same level of
	protection as a TFO Cookie in environments were such protection is
	required.
	Error messages are not included in RST segments but sent in the
	bytestream. Implementors have indicated that processing such
	segments on clients was difficult on some platforms. This change
	simplifies client implementations.
	Appendix D. Address Preservation vs. Address Sharing
	The Transport Converter is provided with instructions about the
	behavior to adopt with regards to the processing of source addresses
	of outgoing packets. The following sub-sections discusses two
	deployment models for illustration purposes. It is out of the scope
	of this document to make a recommendation.
	D.1. Address Preservation
	In this model, the visible source IP address of a packet proxied by a
	Transport Converter to a Server is an IP address of the end host
	(Client). No dedicated IP address pool is provisioned to the
	Transport Converter.
	For Multipath TCP, the Transport Converter preserves the source IP
	address used by the Client when establishing the initial subflow.
	Data conveyed in secondary subflows will be proxied by the Transport
	Converter using the source IP address of the initial subflow. An
	example of a proxied Multipath TCP connection with address
	preservation is shown in Figure 25.
	Transport
	Client Converter Server
	@:C1,C2 @:Tc @:S
	|| | |
	|src:C1 SYN dst:Tc|src:C1 dst:S|
	|-------MPC [->S:port]------->|-------SYN------->|
	|| | |
	||dst:C1 src:Tc|dst:C1 src:S|
	|<---------SYN/ACK------------|<-----SYN/ACK-----|
	|| | |
	|src:C1 dst:Tc|src:C1 dst:S|
	|------------ACK------------->|-------ACK------->|
	| | |
	|src:C2 ... dst:Tc| ... |
	||<-----Secondary Subflow---->|src:C1 dst:S|
	|| |-------data------>|
	| .. | ... |
	Legend:
	Tc: IP address used by the Transport Converter on its customer-facing
	interface.
	Figure 25: Example of Address Preservation
	The Transport Converter must be on the forwarding path of incoming
	traffic. Because the same (destination) IP address is used for both
	proxied and non-proxied connections, the Transport Converter should
	not drop incoming packets it intercepts if no matching entry is found
	for the packets. Unless explicitly configured otherwise, such
	packets are forwarded according to the instructions of a local
	forwarding table.
	D.2. Address/Prefix Sharing
	A pool of global IPv4 addresses is provisioned to the Transport
	Converter along with possible instructions about the address sharing
	ratio to apply (see Appendix B of [RFC6269]). An address is thus
	shared among multiple clients.
	Likewise, rewriting the source IPv6 prefix [RFC6296] may be used to
	ease redirection of incoming IPv6 traffic towards the appropriate
	Transport Converter. A pool of IPv6 prefixes is then provisioned to
	the Transport Converter for this purpose.
	Adequate forwarding policies are enforced so that traffic destined to
	an address of such pool is intercepted by the appropriate Transport
	Converter. Unlike Appendix D.1, the Transport Converter drops
	incoming packets which do not match an active transport session
	entry.
	An example is shown in Figure 26.
	Transport
	Client Converter Server
	@:C @:Tc|Te @:S
	| | |
	|src:C dst:Tc|src:Te dst:S|
	|-------SYN [->S:port]------->|-------SYN------->|
	| | |
	|dst:C src:Tc|dst:Te src:S|
	|<---------SYN/ACK------------|<-----SYN/ACK-----|
	| | |
	|src:C dst:Tc|src:Te dst:S|
	|------------ACK------------->|-------ACK------->|
	| | |
	| ... | ... |
	Legend:
	Tc: IP address used by the Transport Converter for its customer-facing
	interface.
	Te: IP address used by the Transport Converter for its Internet-facing
	interface.
	Figure 26: Address Sharing
	Appendix E. Differences with SOCKSv5
	At a first glance, the solution proposed in this document could seem
	similar to the SOCKS v5 protocol [RFC1928] which is used to proxy TCP
	connections. The Client creates a connection to a SOCKS proxy,
	exchanges authentication information and indicates the destination
	address and port of the final server. At this point, the SOCKS proxy
	creates a connection towards the final server and relays all data
	between the two proxied connections. The operation of an
	implementation based on SOCKSv5 is illustrated in Figure 27.
	Client SOCKS Proxy Server
	-------------------->
	SYN
	<--------------------
	SYN+ACK
	-------------------->
	ACK
	-------------------->
	Version=5, Auth Methods
	<--------------------
	Method
	-------------------->
	Auth Request (unless "No auth" method negotiated)
	<--------------------
	Auth Response
	-------------------->
	Connect Server:Port -------------------->
	SYN
	<--------------------
	SYN+ACK
	<--------------------
	Succeeded
	-------------------->
	Data1
	-------------------->
	Data1
	<--------------------
	Data2
	<--------------------
	Data2
	Figure 27: Establishment of a TCP connection through a SOCKS proxy
	without authentication
	The Convert Protocol also relays data between an upstream and a
	downstream connection, but there are important differences with
	SOCKSv5.
	A first difference is that the Convert Protocol exchanges all control
	information during the three-way handshake. This reduces the
	connection establishment delay compared to SOCKS that requires two or
	more round-trip-times before the establishment of the downstream
	connection towards the final destination. In today's Internet,
	latency is a important metric and various protocols have been tuned
	to reduce their latency [I-D.arkko-arch-low-latency]. A recently
	proposed extension to SOCKS leverages the TFO option
	[I-D.olteanu-intarea-socks-6].
	A second difference is that the Convert Protocol explicitly takes the
	TCP extensions into account. By using the Convert Protocol, the
	Client can learn whether a given TCP extension is supported by the
	destination Server. This enables the Client to bypass the Transport
	Converter when the destination supports the required TCP extension.
	Neither SOCKS v5 [RFC1928] nor the proposed SOCKS v6
	[I-D.olteanu-intarea-socks-6] provide such a feature.
	A third difference is that a Transport Converter will only accept the
	connection initiated by the Client provided that the downstream
	connection is accepted by the Server. If the Server refuses the
	connection establishment attempt from the Transport Converter, then
	the upstream connection from the Client is rejected as well. This
	feature is important for applications that check the availability of
	a Server or use the time to connect as a hint on the selection of a
	Server [RFC8305].
	A fourth difference is that the Convert Protocol only allows the
	client to specify the address/port of the destination server and not
	a DNS name. We evaluated an alternate design for the Connect TLV
	that included the DNS name of the remote peer instead of its IP
	address as in SOCKS [RFC1928]. However, that design was not adopted
	because it induces both an extra load and increased delays on the
	Transport Converter to handle and manage DNS resolution requests.
	Acknowledgments		Acknowledgments
	Although they could disagree with the contents of the document, we		Although they could disagree with the contents of the document, we
	would like to thank Joe Touch and Juliusz Chroboczek whose comments		would like to thank Joe Touch and Juliusz Chroboczek whose comments
	on the MPTCP mailing list have forced us to reconsider the design of		on the MPTCP mailing list have forced us to reconsider the design of
	the solution several times.		the solution several times.
	We would like to thank Raphael Bauduin, Stefano Secci, Anandatirtha		We would like to thank Raphael Bauduin, Stefano Secci, Anandatirtha
	Nandugudi and Gregory Vander Schueren for their help in preparing		Nandugudi and Gregory Vander Schueren for their help in preparing
	this document. Nandini Ganesh provided valuable feedback about the		this document. Nandini Ganesh provided valuable feedback about the
	handling of TFO and the error codes. Yuchung Cheng and Praveen		handling of TFO and the error codes. Yuchung Cheng and Praveen
	Balasubramanian helped to clarify the discussion on supplying data in		Balasubramanian helped to clarify the discussion on supplying data in
	SYNs. Phil Eardley and Michael Scharf's helped to clarify different		SYNs. Phil Eardley and Michael Scharf's helped to clarify different
	parts of the text.		parts of the text.
			Many thanks to Mirja Kuehlewind for the detailed AD review.
	This document builds upon earlier documents that proposed various		This document builds upon earlier documents that proposed various
	forms of Multipath TCP proxies [I-D.boucadair-mptcp-plain-mode],		forms of Multipath TCP proxies [I-D.boucadair-mptcp-plain-mode],
	[I-D.peirens-mptcp-transparent] and [HotMiddlebox13b].		[I-D.peirens-mptcp-transparent] and [HotMiddlebox13b].
	From [I-D.boucadair-mptcp-plain-mode]:		From [I-D.boucadair-mptcp-plain-mode]:
	Many thanks to Chi Dung Phung, Mingui Zhang, Rao Shoaib, Yoshifumi		Many thanks to Chi Dung Phung, Mingui Zhang, Rao Shoaib, Yoshifumi
	Nishida, and Christoph Paasch for their valuable comments.		Nishida, and Christoph Paasch for their valuable comments.
	Thanks to Ian Farrer, Mikael Abrahamsson, Alan Ford, Dan Wing, and		Thanks to Ian Farrer, Mikael Abrahamsson, Alan Ford, Dan Wing, and
	skipping to change at page 52, line 16 ¶		skipping to change at page 50, line 38 ¶
	The authors of [I-D.peirens-mptcp-transparent] were:		The authors of [I-D.peirens-mptcp-transparent] were:
	o Bart Peirens		o Bart Peirens
	o Gregory Detal		o Gregory Detal
	o Sebastien Barre		o Sebastien Barre
	o Olivier Bonaventure		o Olivier Bonaventure
			Change Log
			This section to be removed before publication.
			o 00 : initial version, designed to support Multipath TCP and TFO
			only
			o 00 to -01 : added section Section 7 describing the support of
			different standard tracks TCP options by Transport Converters,
			clarification of the IANA section, moved the SOCKS comparison to
			the appendix and various minor modifications
			o 01 to -02: Minor modifications
			o 02 to -03: Minor modifications
			o 03 to -04: Minor modifications
			o 04 to -05: Integrate a lot of feedback from implementers who have
			worked on client and server side implementations. The main
			modifications are the following :
			* TCP Fast Open is not strictly required anymore. Several
			implementers expressed concerns about this requirement. The
			TFO Cookie protects from some attack scenarios that affect open
			servers like web servers. The Convert Protocol is different
			and as discussed in RFC7413, there are different ways to
			protect from such attacks. Instead of using a TFO cookie
			inside the TCP options, which consumes precious space in the
			extended TCP header, this version supports the utilization of a
			Cookie that is placed in the SYN payload. This provides the
			same level of protection as a TFO Cookie in environments were
			such protection is required.
			* the Bootstrap procedure has been simplified based on feedback
			from implementers
			* Error messages are not included in RST segments anymore but
			sent in the bytestream. Implementers have indicated that
			processing such segments on clients was difficult on some
			platforms. This change simplifies client implementations.
			* Many minor editorial changes to clarify the text based on
			implementers feedback.
			o 05 to -06: Many clarifications to integrate the comments from the
			chairs in preparation to the WGLC:
			* Updated IANA policy to require "IETF Review" instead of
			"Standard Action"
			* Call out explicitly that data in SYNs are relayed by the
			Converter
			* Reiterate the scope
			* Hairpinning behavior can be disabled (policy-based)
			* Fix nits
			o 07:
			* Update the text about supplying data in SYNs to make it clear
			that a constraint defined in RFC793 is relaxed following the
			same rationale as in RFC7413.
			* Nits
			* Added Appendix A on example Socket API changes
			o 08:
			* Added short discussion on the termination of connections
			o 09:
			* Address various comments received during last call
			o 10-13:
			* Changes to address the comments from Phil: Add a new section to
			group data plane considerations in one place + add a new
			appendix with more details on address modes + rearrange the
			MPTCP text.
			o 14: fixed nits (the shepherd write-up)
			o 15: Various clarifications in the text to address the detailed
			comments provided by Mirja Kuehlewind
	Authors' Addresses		Authors' Addresses
	Olivier Bonaventure (editor)		Olivier Bonaventure (editor)
	Tessares		Tessares
	Email: Olivier.Bonaventure@tessares.net		Email: Olivier.Bonaventure@tessares.net
	Mohamed Boucadair (editor)		Mohamed Boucadair (editor)
	Orange		Orange
	Clos Courtel		Clos Courtel
End of changes. 201 change blocks.
	896 lines changed or deleted		891 lines changed or added
This html diff was produced by rfcdiff 1.47. The latest version is available from http://tools.ietf.org/tools/rfcdiff/