--- 1/draft-ietf-tcpm-converters-04.txt 2019-02-08 09:13:20.533918904 -0800 +++ 2/draft-ietf-tcpm-converters-05.txt 2019-02-08 09:13:20.617920933 -0800 @@ -1,1280 +1,1374 @@ TCPM Working Group O. Bonaventure, Ed. Internet-Draft Tessares Intended status: Experimental M. Boucadair, Ed. -Expires: April 25, 2019 Orange +Expires: August 11, 2019 Orange S. Gundavelli Cisco S. Seo Korea Telecom - October 22, 2018 + B. Hesmans + Tessares + February 07, 2019 0-RTT TCP Convert Protocol - draft-ietf-tcpm-converters-04 + draft-ietf-tcpm-converters-05 Abstract This document specifies an application proxy, called Transport Converter, to assist the deployment of TCP extensions such as Multipath TCP. This proxy is designed to avoid inducing extra delay when involved in a network-assisted connection (that is, 0-RTT). + 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 document: - [This-RFC] + this document: [This-RFC] Please update TBA statements with the port number to be assigned to - the Converter Protocol. + the 0-RTT TCP Convert Protocol. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - - This Internet-Draft will expire on April 25, 2019. + This Internet-Draft will expire on August 11, 2019. Copyright Notice - Copyright (c) 2018 IETF Trust and the persons identified as the + Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 5 - 3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 5 - 3.1. Functional Elements . . . . . . . . . . . . . . . . . . . 5 - 3.2. Theory of Operation . . . . . . . . . . . . . . . . . . . 7 + 3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 6 + 3.1. Functional Elements . . . . . . . . . . . . . . . . . . . 6 + 3.2. Theory of Operation . . . . . . . . . . . . . . . . . . . 8 3.3. Sample Examples of Outgoing Converter-Assisted Multipath - TCP Connections . . . . . . . . . . . . . . . . . . . . . 10 + TCP Connections . . . . . . . . . . . . . . . . . . . . . 11 3.4. Sample Example of Incoming Converter-Assisted Multipath - TCP Connection . . . . . . . . . . . . . . . . . . . . . 11 - 4. The Converter Protocol (Convert) . . . . . . . . . . . . . . 12 - 4.1. The Convert Fixed Header . . . . . . . . . . . . . . . . 12 - 4.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . . . 13 - 4.2.1. Generic Convert TLV Format . . . . . . . . . . . . . 13 - 4.2.2. Summary of Supported Convert TLVs . . . . . . . . . . 14 - 4.2.3. The Bootstrap TLV . . . . . . . . . . . . . . . . . . 15 - 4.2.4. Supported TCP Extension Services TLV . . . . . . . . 15 - 4.2.5. Connect TLV . . . . . . . . . . . . . . . . . . . . . 16 - 4.2.6. Extended TCP Header TLV . . . . . . . . . . . . . . . 18 - 4.2.7. Error TLV . . . . . . . . . . . . . . . . . . . . . . 18 + TCP Connection . . . . . . . . . . . . . . . . . . . . . 12 + 4. The Convert Protocol (Convert) . . . . . . . . . . . . . . . 13 + 4.1. The Convert Fixed Header . . . . . . . . . . . . . . . . 14 + 4.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . . . 14 + 4.2.1. Generic Convert TLV Format . . . . . . . . . . . . . 14 + 4.2.2. Summary of Supported Convert TLVs . . . . . . . . . . 15 + 4.2.3. The Info TLV . . . . . . . . . . . . . . . . . . . . 16 + 4.2.4. Supported TCP Extensions TLV . . . . . . . . . . . . 16 + 4.2.5. Connect TLV . . . . . . . . . . . . . . . . . . . . . 17 + 4.2.6. Extended TCP Header TLV . . . . . . . . . . . . . . . 19 + 4.2.7. The Cookie TLV . . . . . . . . . . . . . . . . . . . 19 + 4.2.8. Error TLV . . . . . . . . . . . . . . . . . . . . . . 20 5. Compatibility of Specific TCP Options with the Conversion - Service . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 - 5.1. Base TCP Options . . . . . . . . . . . . . . . . . . . . 21 - 5.2. Window Scale (WS) . . . . . . . . . . . . . . . . . . . . 22 - 5.3. Selective Acknowledgements . . . . . . . . . . . . . . . 22 - 5.4. Timestamp . . . . . . . . . . . . . . . . . . . . . . . . 23 - 5.5. Multipath TCP . . . . . . . . . . . . . . . . . . . . . . 23 - 5.6. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . 23 - 5.7. TCP User Timeout . . . . . . . . . . . . . . . . . . . . 24 - 5.8. TCP-AO . . . . . . . . . . . . . . . . . . . . . . . . . 24 - 5.9. TCP Experimental Options . . . . . . . . . . . . . . . . 25 - 6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 25 - 7. Security Considerations . . . . . . . . . . . . . . . . . . . 25 - 7.1. Privacy & Ingress Filtering . . . . . . . . . . . . . . . 25 - 7.2. Authorization . . . . . . . . . . . . . . . . . . . . . . 26 - 7.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 26 - 7.4. Traffic Theft . . . . . . . . . . . . . . . . . . . . . . 27 - 7.5. Multipath TCP-specific Considerations . . . . . . . . . . 27 - 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 - 8.1. Convert Service Port Number . . . . . . . . . . . . . . . 28 - 8.2. The Converter Protocol (Convert) Parameters . . . . . . . 28 - 8.2.1. Convert Versions . . . . . . . . . . . . . . . . . . 28 - 8.2.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . 28 - 8.2.3. Convert Error Messages . . . . . . . . . . . . . . . 29 - 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 - 9.1. Contributors . . . . . . . . . . . . . . . . . . . . . . 31 - 10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 32 - 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 32 - 11.1. Normative References . . . . . . . . . . . . . . . . . . 32 - 11.2. Informative References . . . . . . . . . . . . . . . . . 33 - Appendix A. Differences with SOCKSv5 . . . . . . . . . . . . . . 37 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38 + Service . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 + 5.1. Base TCP Options . . . . . . . . . . . . . . . . . . . . 23 + 5.2. Window Scale (WS) . . . . . . . . . . . . . . . . . . . . 24 + 5.3. Selective Acknowledgements . . . . . . . . . . . . . . . 24 + 5.4. Timestamp . . . . . . . . . . . . . . . . . . . . . . . . 25 + 5.5. Multipath TCP . . . . . . . . . . . . . . . . . . . . . . 25 + 5.6. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . 25 + 5.7. TCP User Timeout . . . . . . . . . . . . . . . . . . . . 26 + 5.8. TCP-AO . . . . . . . . . . . . . . . . . . . . . . . . . 26 + 5.9. TCP Experimental Options . . . . . . . . . . . . . . . . 26 + 6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 26 + 7. Security Considerations . . . . . . . . . . . . . . . . . . . 27 + 7.1. Privacy & Ingress Filtering . . . . . . . . . . . . . . . 27 + 7.2. Authorization . . . . . . . . . . . . . . . . . . . . . . 28 + 7.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 28 + 7.4. Traffic Theft . . . . . . . . . . . . . . . . . . . . . . 28 + 7.5. Multipath TCP-specific Considerations . . . . . . . . . . 29 + 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 + 8.1. Convert Service Port Number . . . . . . . . . . . . . . . 29 + 8.2. The Convert Protocol (Convert) Parameters . . . . . . . . 30 + 8.2.1. Convert Versions . . . . . . . . . . . . . . . . . . 30 + 8.2.2. Convert TLVs . . . . . . . . . . . . . . . . . . . . 30 + 8.2.3. Convert Error Messages . . . . . . . . . . . . . . . 31 + 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32 + 9.1. Contributors . . . . . . . . . . . . . . . . . . . . . . 33 + 10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 34 + 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 + 11.1. Normative References . . . . . . . . . . . . . . . . . . 34 + 11.2. Informative References . . . . . . . . . . . . . . . . . 35 + Appendix A. Differences with SOCKSv5 . . . . . . . . . . . . . . 39 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 1. Introduction Transport protocols like TCP evolve regularly [RFC7414]. TCP has been improved in different ways. Some improvements such as changing the initial window size [RFC6928] or modifying the congestion control scheme can be applied independently on clients and servers. Other improvements such as Selective Acknowledgements [RFC2018] or large windows [RFC7323] require a new TCP option or to change the semantics of some fields in the TCP header. These modifications must be deployed on both clients and servers to be actually used on the Internet. Experience with the latter TCP extensions reveals that - their deployment can require many years. [Fukuda2011] reports - results of a decade of measurements showing the deployment of - Selective Acknowledgements, Window Scale and TCP Timestamps. - [ANRW17] describes measurements showing that TCP Fast Open [RFC7413] - (TFO) is still not widely deployed. + their deployment can require many years. Fukuda reports in + [Fukuda2011] results of a decade of measurements showing the + deployment of Selective Acknowledgements, Window Scale and TCP + Timestamps. [ANRW17] describes measurements showing that TCP Fast + Open (TFO) [RFC7413] is still not widely deployed. There are some situations where the transport stack used on clients (resp. servers) can be upgraded at a faster pace than the transport stack running on servers (resp. clients). In those situations, clients would typically want to benefit from the features of an improved transport protocol even if the servers have not yet been - upgraded and conversely. In the past, Performance Enhancing Proxies - have been proposed and deployed [RFC3135] as solutions to improve TCP - performance over links with specific characteristics. + upgraded and conversely. Performance Enhancing Proxies [RFC3135], + and other service functions have been deployed as solutions to + improve TCP performance over links with specific characteristics. - Recent examples of TCP extensions include Multipath TCP - [RFC6824][I-D.ietf-mptcp-rfc6824bis] or TCPINC - [I-D.ietf-tcpinc-tcpcrypt]. Those extensions provide features that - are interesting for clients such as wireless devices. With Multipath - TCP, those devices could seamlessly use WLAN and cellular networks, - for bonding purposes, faster handovers, or better resiliency. - Unfortunately, deploying those extensions on both a wide range of - clients and servers remains difficult. + Recent examples of TCP extensions include Multipath TCP [RFC6824] or + TCPINC [I-D.ietf-tcpinc-tcpcrypt]. Those extensions provide features + that are interesting for clients such as wireless devices. With + Multipath TCP, those devices could seamlessly use WLAN (Wireless + Local Area Network) and cellular networks, for bonding purposes, + faster handovers, or better resiliency. Unfortunately, deploying + those extensions on both a wide range of clients and servers remains + difficult. More recently, experimentation of 5G bonding, which has very scarce coverage, has been conducted into global range of the incumbent 4G (LTE) connectivity in newly devised clients using Multipath TCP proxy. Even if the 5G and the 4G bonding by using Multipath TCP - increases the bandwidth to data transfer, it is as well crucial to - minimize latency for all the way between endhosts regardless of - whether intermediate nodes are inside or outside of the mobile core. - In order to handle uRLLC (Ultra-Reliable Low-Latency Communication) - for the next generation mobile network, Multipath TCP and its proxy - mechanism must be optimised to reduce latency. + increases the bandwidth, it is as well crucial to minimize latency + for all the way between endhosts regardless of whether intermediate + nodes are inside or outside of the mobile core. In order to handle + uRLLC (Ultra-Reliable Low-Latency Communication) for the next + generation mobile network, Multipath TCP and its proxy mechanism such + as the one used to provide Access tTaffic Steering, Switching, and + Splitting (ATSSS) must be optimised to reduce latency. This document specifies an application proxy, called Transport Converter. A Transport Converter is a function that is installed by a network operator to aid the deployment of TCP extensions and to provide the benefits of such extensions to clients. A Transport - Converter may support conversion service for one or more TCP - extensions. This service is provided by means of the Converter - Protocol (Convert), that is an application layer protocol which uses - TBA TCP port number (Section 8). + Converter may provide conversion service for one or more TCP + extensions. Which TCP extensions are eligible to the conversion + service is deployment-specific. The conversion service is provided + by means of the 0-RTT TCP Convert Protocol (Convert), that is an + application-layer protocol which uses TCP port number TBA + (Section 8). - The Transport Converter adheres to the main principles as drawn in - [RFC1919]. In particular, the Converter achieves the following: + The Transport Converter adheres to the main principles drawn in + [RFC1919]. In particular, a Transport Converter achieves the + following: o Listen for client sessions; o Receive from a client the address of the final target server; o Setup a session to the final server; o Relay control messages and data between the client and the server; o Perform access controls according to local policies. - The main advantage of network-assisted Converters is that they enable - new TCP extensions to be used on a subset of the end-to-end path, - which encourages the deployment of these extensions. The Transport - Converter allows the client and the server to directly negotiate TCP - options. + The main advantage of network-assisted conversion services is that + they enable new TCP extensions to be used on a subset of the path + between endpoints, which encourages the deployment of these + extensions. Furthermore, the Transport Converter allows the client + and the server to directly negotiate TCP options for the sake of + native support along the full path. The Convert Protocol is a generic mechanism to provide 0-RTT conversion service. As a sample applicability use case, this document specifies how the Convert Protocol applies for Multipath TCP. It is out of scope of this document to provide a comprehensive - list of all potential conversion services; separate documents may be - edited in the future for other conversion services upon need. + list of all potential conversion services. Applicability document + may defined in the future. This document does not assume that all the traffic is eligible to the network-assisted conversion service. Only a subset of the traffic - will be forwarded to a Converter according to a set of policies. - Furthermore, it is possible to bypass the Converter to connect to the - servers that already support the required TCP extension. + will be forwarded to a Transport Converter according to a set of + policies. These policies, and how they are communicated to + endpoints, are out of scope. Furthermore, it is possible to bypass + the Transport Converter to connect directly to the servers that + already support the required TCP extension(s). This document assumes that a client is configured with one or a list - of Converters (e.g., [I-D.boucadair-tcpm-dhc-converter]). - Configuration means are outside the scope of this document. + of Transport Converters (statically or through protocols such as + [I-D.boucadair-tcpm-dhc-converter]). Configuration means are outside + the scope of this document. This document is organized as follows. We first provide a brief explanation of the operation of Transport Converters in Section 3. - We describe the Converter Protocol in Section 4. We discuss in + We describe the Convert Protocol in Section 4. We discuss in Section 5 how Transport Converters can be used to support different - TCP options. We then discuss the interactions with middleboxes + TCP extensions. We then discuss the interactions with middleboxes (Section 6) and the security considerations (Section 7). Appendix A provides a comparison with SOCKS proxies that are already - used to deploy Multipath TCP in some cellular networks. + used to deploy Multipath TCP in some cellular networks (Section 2.2 + of [RFC8041]). 2. Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. Architecture 3.1. Functional Elements - The architecture considers three types of endhosts: + The Convert Protocol considers three types of endhosts: - o Client endhosts; + o Clients; o Transport Converters; - o Server endhosts. + o Servers. A Transport Converter is a network function that relays all data exchanged over one upstream connection to one downstream connection - and vice versa (Figure 1). The Converter, thus, maintains state that - associates one upstream connection to a corresponding downstream - connection. + and vice versa (Figure 1). The Transport Converter, thus, maintains + state that associates one upstream connection to a corresponding + downstream connection. A connection can be initiated from both sides of the Transport Converter (Internet-facing interface, client-facing interface). +------------+ - <--- upstream --->| Transport |<--- downstream ---> + client <- upstream ->| Transport |<- downstream ->server | Converter | +------------+ Figure 1: A Transport Converter relays data between pairs of TCP connections Transport Converters can be operated by network operators or third parties. Nevertheless, this document focuses on the single administrative deployment case where the entity offering the connectivity service to a client is also the entity which owns and operates the Transport Converter. 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 - deployment-specific (Figure 2). + deployment-specific. A sample deployment is depicted in Figure 2. +-+ +-+ +-+ Client - |R| -- |R| -- |R| - - - Server +-+ +-+ +-+ | - Transport - Converter + +-+ + |R| + +-+ + | + +---------+ + |Transport| + |Converter| + +---------+ Figure 2: A Transport Converter can be installed anywhere in the network The architecture assumes that new software will be installed on the - Client hosts and on Transport Converters. Further, the architecture - allows for making use of TCP new extensions if those are supported by - a given server. + Client hosts to interact with one or more Transport Converters. + Further, the architecture allows for making use of TCP new extensions + even if those are not supported by a given server. The Client is configured, through means that are outside the scope of this document, with the names and/or the addresses of one or more - Transport Converters. + Transport Converters and the TCP extensions that they support. The + procedure for selecting a Transport Converter among a list of + configured Transport Converters is outside the scope of this + document. + + One of the benefits of this design is that different transport + protocol extensions can be used on the upstream and the downstream + connections. This encourages the deployment of new TCP extensions + until they are widely supported by servers, in particular. The architecture does not mandate anything on the server side. Similar to address sharing mechanisms, the architecture does not - interfere with end-to-end TLS connections between the client and the - server. + interfere with end-to-end TLS connections [RFC8446] between the + Client and the Server (Figure 3). - One of the benefits of this design is that different transport - protocol extensions can be used on the upstream and the downstream - connections. This encourages the deployment of new TCP extensions - until they are widely supported by servers. + Client Transport Server + | Converter | + | | | + /==========================================\ + | End-to-end TLS | + \==========================================/ + + * TLS messages exhanged between the Client + and the Server are not shown. + + Figure 3: End-to-end TLS via a Transport Converter 3.2. Theory of Operation At a high level, the objective of the Transport Converter is to allow - the Client to use a specific extension, e.g. Multipath TCP, on a - subset of the end-to-end path even if the Server does not support - this extension. This is illustrated in Figure 3 where the Client - initiates a Multipath TCP connection with the Converter (Multipath - packets are shown with "===") while the Converter uses a regular TCP - connection with the Server. - - The packets belonging to the pair of connections between the Client - and Server passing through a Transport Converter may follow a - different path than the packets directly exchanged between the Client - and the Server. Deployments should minimize the possible additional - delay by carefully selecting the location of the Transport Converter - used to reach a given destination. + the Client to use a specific extension, e.g., Multipath TCP, on a + subset of the path even if the Server does not support this + extension. This is illustrated in Figure 4 where the Client + initiates a Multipath TCP connection with the Transport Converter + (packets belonging to the Multipath TCP connection are shown with + "===") while the Transport Converter uses a regular TCP connection + with the Server. Transport Client Converter Server ======================> --------------------> <-------------------- <====================== Multipath TCP packets Regular TCP packets - Figure 3: Different TCP variants can be used on the Client-Converter - path and on the Converter-Server path + Figure 4: An example of network-assisted MPTCP Connection + + The packets belonging to the pair of connections between the Client + and Server passing through a Transport Converter may follow a + different path than the packets directly exchanged between the Client + and the Server. Deployments should minimize the possible additional + delay by carefully selecting the location of the Transport Converter + used to reach a given destination. When establishing a connection, the Client can, depending on local policies, either contact the Server directly (e.g., by sending a TCP SYN towards the Server) or create the connection via a Transport - Converter. In the latter case, which is the case we consider in this - document, the Client initiates a connection towards the Transport - Converter and indicates the IP address and port number of the - ultimate Server inside the connection establishment packet. Doing so - enables the Transport Converter to immediately initiate a connection - towards that Server, without experiencing an extra delay. The - Transport Converter waits until the confirmation that the Server - agrees to establish the connection before confirming it to the - Client. + Converter. In the latter case, the Client initiates a connection + towards the Transport Converter and indicates the IP address and port + number of the Server within the connection establishment packet. + Doing so enables the Transport Converter to immediately initiate a + connection towards that Server, without experiencing an extra delay. + The Transport Converter waits until the receipt of the confirmation + that the Server agrees to establish the connection before confirming + it to the Client. The client places the destination address and port number of the - target Server in the payload of the SYN sent to the Converter by - leveraging TCP Fast Open [RFC7413]. In accordance with [RFC1919], - the Transport Converter maintains two connections that are combined - together: + Server in the payload of the SYN sent to the Transport Converter to + minimize connection establishment delays. In accordance with + [RFC1919], the Transport Converter maintains two connections that are + combined together: o the upstream connection is the one between the Client and the Transport Converter. o the downstream connection is between the Transport Converter and - the remote Server. + the Server. Any user data received by the Transport Converter over the upstream (resp., downstream) connection is relayed over the downstream (resp., upstream) connection. - Figure 4 illustrates the establishment of a TCP connection by the - Client through a Transport Converter. The information shown between - brackets is part of the Converter Protocol described later in this - document. - - Figure 4 illustrates the establishment of a TCP connection by the - Client through a Transport Converter. The information shown between - brackets is part of the Converter Protocol described later in this - document. + Figure 5 illustrates the establishment of an outbound TCP connection + by a Client through a Transport Converter. The information shown + between brackets denotes Convert Protocol messages described in + Section 4. Transport Client Converter Server --------------------> - SYN TFO [->Server:port] + SYN [->Server:port] --------------------> SYN <-------------------- SYN+ACK <-------------------- SYN+ACK [ ] - Figure 4: Establishment of a TCP connection through a Converter - - The Client sends a SYN destined to the Transport Converter. This SYN - contains a TFO cookie and inside its payload the addresses and ports - of the destination Server. The Transport Converter does not reply - immediately to this SYN. It first tries to create a TCP connection - towards the destination Server. If this second connection succeeds, - the Transport Converter confirms the establishment of + Figure 5: Establishment of a TCP connection through a Transport + Converter (1) - the connection to the Client by returning a SYN+ACK and the first - bytes of the bytestream contain information about the TCP options - that were negotiated with the final Server. This information is sent - at the beginning of the bytestream, either directly in the SYN+ACK or - in a subsequent packet. For graphical reasons, the figures in this - section show that the Converter returns this information in the - SYN+ACK packet. An implementation could also place this information - in a packet that it sent shortly after the SYN+ACK. + The Client sends a SYN destined to the Transport Converter. The + payload of this SYN contains the address and port number of the + Server. The Transport Converter does not reply immediately to this + SYN. It first tries to create a TCP connection towards the target + Server. If this upstream connection succeeds, the Transport + Converter confirms the establishment of the connection to the Client + by returning a SYN+ACK and the first bytes of the bytestream contain + information about the TCP options that were negotiated with the + Server. This information is sent at the beginning of the bytestream, + either directly in the SYN+ACK or in a subsequent packet. For + graphical reasons, the figures in this section show that the + Transport Converter returns this information in the SYN+ACK packet. + An implementation could also place this information in a packet that + it sent shortly after the SYN+ACK. The connection can also be established from the Internet towards a - Client via a Transport Converter. This is typically the case when - the Client embeds a server (video server, for example). - - The procedure described in Figure 4 assumes that the Client has - obtained a TFO cookie from the Transport Converter. This is part of - the Bootstrap procedure which is illustrated in Figure 5. The Client - sends a SYN with a TFO request option to obtain a valid cookie from - the Converter. The Converter replies with a TFO cookie in the - SYN+ACK. Once this connection has been established, the Client sends - a Bootstrap message to request the list of TCP options for which the - Transport Converter provides a conversion service. - - Transport - Client Converter Server - --------------------> - SYN TFO(empty) - - <-------------------- - SYN+ACK TFO(cookie) - - --------------------> - [Bootstrap] - - <-------------------- - [Supported TCP Extension Services] + Client via a Transport Converter. This is typically the case when an + application on the Client listens to a specific port (the Client + hosts a server, typically). - Figure 5: Bootstrapping a Client connection to a Transport Converter - Note that the Converter may rely on local policies to decide whether - it can service a given requesting Client. That is, the Converter - will not return a cookie for that Client. How such policies are - supplied to the Converter are out of scope. + A Transport Converter MAY operate in address preservation or address + sharing modes as discussed in Section 5.4 of + [I-D.nam-mptcp-deployment-considerations]. Which behavior to use by + a Transport Converter is deployment-specific. If address sharing + mode is enabled, the Transport Converter MUST adhere to REQ-2 of + [RFC6888] which implies a default "IP address pooling" behavior of + "Paired" (as defined in Section 4.1 of [RFC4787]) must be supported. + This behavior is meant to avoid breaking applications that depend on + the external address remaining constant. - Also, the Converter may behave in a cookie-less mode when appropriate - means are enforced at the Converter and the network in-between to - protect against attacks such as spoofing and SYN flood. Under such - deployments, the use of TFO is not required. + Standard TCP ([RFC0793], Section 3.4) allows a SYN packet to carry + data inside its payload but forbids the receiver from delivering it + to the application until completion of the three-way-handshake. This + restriction was motivated by two concerns. First, duplicate SYNs can + cause problems for some applications that rely on TCP [RFC7413]. + Second, TCP suffers from SYN flooding attacks [RFC4987]. TCP Fast + Open [RFC7413] solves these two problems for applications that can + tolerate replays by using the TCP Fast Open option that includes a + cookie. However, the utilization of this option consumes space in + the limited TCP extended header. Furthermore, there are situations, + as noted in Section 7.3 of [RFC7413] where it is possible to accept + the payload of SYN packets without creating additional security risks + such as a network where addresses cannot be spoofed and the Transport + Converter only serves a set of hosts that are identified by these + addresses. For these reasons, this specification does not mandate + the use of the TCP Fast Open option when the Client sends a + connection establishment packet towards a Transport Converter. The + Convert protocol includes an optional Cookie TLV that provides + similar protection as the TCP Fast Open option without consuming + space in the extended TCP header. 3.3. Sample Examples of Outgoing Converter-Assisted Multipath TCP Connections - As an example (Figure 6), let us consider how the Convert protocol - can help the deployment of Multipath TCP [RFC6824]. We assume that - both the Client and the Transport Converter support Multipath TCP, - but consider two different cases depending whether the Server - supports Multipath TCP or not. A Multipath TCP connection is created - by placing the MP_CAPABLE (MPC) option in the SYN sent by the Client. + 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 + Transport Converter support Multipath TCP, but consider two different + cases depending on whether the Server supports Multipath TCP or not. + + As a reminder, a Multipath TCP connection is created by placing the + MP_CAPABLE (MPC) option in the SYN sent by the Client. Figure 6 describes the operation of the Transport Converter if the Server does not support Multipath TCP. Transport Client Converter Server --------------------> SYN, MPC [->Server:port] --------------------> SYN, MPC <-------------------- SYN+ACK <-------------------- - SYN+ACK,MPC [ ] + SYN+ACK,MPC [.] --------------------> ACK,MPC --------------------> ACK Figure 6: Establishment of a Multipath TCP connection through a - Converter + Transport Converter towards a Server that does not support Multipath + TCP The Client tries to initiate a Multipath TCP connection by sending a SYN with the MP_CAPABLE option (MPC in Figure 6). The SYN includes - the address and port number of the final Server and the Transport - Converter attempts to initiate a Multipath TCP connection towards - this Server. Since the Server does not support Multipath TCP, it - replies with a SYN+ACK that does not contain the MP_CAPABLE option. - The Transport Converter notes that the connection with the Server - does not support Multipath TCP and returns the TCP options received - from the Server to the Client. + the address and port number of the target Server, that are extracted + and used by the Transport Converter to initiate a Multipath TCP + connection towards this Server. Since the Server does not support + Multipath TCP, it replies with a SYN+ACK that does not contain the + MP_CAPABLE option. The Transport Converter notes that the connection + with the Server does not support Multipath TCP and returns the + extended TCP header received from the Server to the Client. Figure 7 considers a Server that supports Multipath TCP. In this case, it replies to the SYN sent by the Transport Converter with the MP_CAPABLE option. Upon reception of this SYN+ACK, the Transport Converter confirms the establishment of the connection to the Client and indicates to the Client that the Server supports Multipath TCP. With this information, the Client has discovered that the Server - supports Multipath TCP natively. This will enable it to bypass the - Transport Converter for the next Multipath TCP connection that it - will initiate towards this Server. + supports Multipath TCP natively. This will enable the Client to + bypass the Transport Converter for the subsequent Multipath TCP + connections that it will initiate towards this Server. Transport Client Converter Server --------------------> SYN, MPC [->Server:port] --------------------> SYN, MPC <-------------------- SYN+ACK, MPC <-------------------- SYN+ACK, MPC [ MPC supported ] --------------------> ACK, MPC --------------------> ACK, MPC Figure 7: Establishment of a Multipath TCP connection through a - converter + converter towards a server that supports Multipath TCP 3.4. Sample Example of Incoming Converter-Assisted Multipath TCP Connection An example of an incoming Converter-assisted Multipath TCP connection is depicted in Figure 8. In order to support incoming connections from remote hosts, the Client may use PCP [RFC6887] to instruct the - Converter to create dynamic mappings. Those mappings will be used by - the Converter to intercept an incoming TCP connection destined to the - Client and convert it into a Multipath TCP connection. + Transport Converter to create dynamic mappings. Those mappings will + be used by the Transport Converter to intercept an incoming TCP + connection destined to the Client and convert it into a Multipath TCP + connection. + + Typically, the Client sends a PCP request to the Converter asking to + create an explicit TCP mapping for (internal IP address, internal + port number). The Converter accepts the request by creating a TCP + mapping (internal IP address, internal port number, external IP + address, external port number). The external IP address and external + port number will be then advertised using an out-of-band mechanism so + that remote hosts can initiate TCP connections to the Client via the + Converter. Note that the external and internal information may be + the same. + + Then, when the Converter receives an incoming SYN, it checks its + mapping table to verify if there is an active mapping matching the + destination IP address and destination port of that SYN. If an entry + is found, the Converter inserts an MP_CAPABLE option and Connect TLV + in the SYN packet, rewrites the source IP address to one of its IP + addresses and, eventually, the destination IP address and port number + in accordance with the information stored in the mapping. SYN-ACK + and ACK will be then exchanged between the Client and the Converter + to confirm the establishment of the initial subflow. The Client can + add new subflows following normal Multipath TCP procedures. Transport Client Converter Remote Host <------------------- SYN <------------------- SYN, MPC[Remote Host:port] ---------------------> SYN+ACK, MPC ---------------------> SYN+ACK <--------------------- ACK <------------------- ACK, MPC Figure 8: Establishment of an Incoming TCP Connection through a - Converter - -4. The Converter Protocol (Convert) + Transport Converter - This section describes in details the messages that are exchanged - between a Client and a Transport Converter. The Converter Protocol - (Convert, for short) leverages the TCP Fast Open extension [RFC7413]. +4. The Convert Protocol (Convert) - The Converter Protocol uses a 32 bits long fixed header that is sent - by both the Client and the Transport Converter. This header - indicates both the version of the protocol used and the length of the - Convert message. + This section describes the messages that are exchanged between a + Client and a Transport Converter. The Convert Protocol (Convert, for + short) uses a 32 bits long fixed header that is sent by both the + Client and the Transport Converter over each established connection. + This header indicates both the version of the protocol used and the + length of the Convert message. 4.1. The Convert Fixed Header - The Fixed Header is used to exchange information about the version - and length of the messages between the Client and the Transport + The Fixed Header is used to convey information about the version and + length of the messages exchanged between the Client and the Transport Converter. The Client and the Transport Converter MUST send the fixed-sized - header shown in Figure 9 as the first four bytes of the bytestream. + header, shown in Figure 9, as the first four bytes of the bytestream. 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 +---------------+---------------+-------------------------------+ | Version | Total Length | Unassigned | +---------------+---------------+-------------------------------+ - Figure 9: The fixed-sized header of the Converter protocol + Figure 9: The fixed-sized header of the Convert protocol The Version is encoded as an 8 bits unsigned integer value. This document specifies version 1. Version 0 is reserved by this document and MUST NOT be used. The Total Length is the number of 32 bits word, including the header, - of the bytestream that are consumed by the Converter protocol - messages. Since Total Length is also an 8 bits unsigned integer, - those messages 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 zero is invalid and the connection MUST - be reset upon reception of such a header. + of the bytestream that are consumed by the Convert messages. Since + Total Length is also an 8 bits unsigned integer, those messages + 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 zero is invalid and the connection MUST be reset upon reception of + a header with such total length. The Unassigned field MUST be set to zero in this version of the protocol. These bits are available for future use [RFC8126]. 4.2. Convert TLVs 4.2.1. Generic Convert TLV Format The Convert protocol uses variable length messages that are encoded - using the generic TLV format depicted in Figure 10. All TLV fields - are encoded using the network byte order. + using the generic TLV (Type, Length, Value) format depicted in + Figure 10. + + 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. + All TLV fields are encoded using the network byte order. 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 | Length | (optional) Value ... | +---------------+---------------+-------------------------------+ | ... (optional) Value | +---------------------------------------------------------------+ - Figure 10: Converter Generic TLV Format + Figure 10: Convert Generic TLV Format + + The Length field is expressed in units of 32 bits words. In general + zero padding MUST be added if the value's length in bytes can not be + expressed as 2+(4 * n). A given TLV MUST only appear once on a connection. If two or more - instances of the same TLV are exchanged over a Converter connection, + instances of the same TLV are exchanged over a Convert connection, the associated TCP connections MUST be closed. 4.2.2. Summary of Supported Convert TLVs This document specifies the following Convert TLVs: +------+-----+----------+------------------------------------------+ | Type | Hex | Length | Description | +------+-----+----------+------------------------------------------+ - | 1 | 0x1 | 1 | Bootstrap TLV | + | 1 | 0x1 | 1 | Info TLV | | 10 | 0xA | Variable | Connect TLV | | 20 | 0x14| Variable | Extended TCP Header TLV | - | 21 | 0x15| Variable | Supported TCP Extension Services TLV | + | 21 | 0x15| Variable | Supported TCP Extensions TLV | + | 22 | 0x16| Variable | Cookie TLV | | 30 | 0x1E| Variable | Error TLV | +------+-----+----------+------------------------------------------+ - Figure 11: The TLVs used by the Converter protocol + Figure 11: The TLVs used by the Convert protocol Type 0x0 is a reserved valued. Implementations MUST discard messages with such TLV. - To establish a connection via a Transport Converter, a Client MUST - first obtain a valid TFO cookie from that Converter. This is the - bootstrap procedure during which the Client opens a connection to the - Transport Converter with an empty TFO option. According to - [RFC7413], the Transport Converter returns its cookie in the SYN+ACK. - Then the Client sends a Bootstrap TLV (Section 4.2.3) to which the - Transport Converter replies with the Supported TCP Extension Services - TLV described in Section 4.2.4. - - With the TFO cookie of the Transport Converter, the Client can - request the establishment of connections to remote servers with the - Connect TLV (see Section 4.2.5). If the connection can be + The Client can request the establishment of connections to servers by + using the Connect TLV (Section 4.2.5). If the connection can be established with the final server, the Transport Converter replies - with the Extended TCP Header TLV and returns an Error TLV inside a - RST packet (see Section 4.2.7). - - When the Transport Converter receives an incoming connection - establishment from a Client, it MUST process the TCP options found in - the SYN and the Connect TLV. In general, the Transport Converter - MUST add to the proxied SYN the TCP options that were included in the - Connect TLV. It SHOULD add to the proxied SYN the TCP options that - were included in the incoming SYN provided that it supports the - corresponding TCP extension. - - There are some exceptions to these rules given the semantics of some - TCP options. First, TCP options with Kinds 0 (EOL), 1 (NOP), 2 - (MSS), and 3 (WS) MUST be used according to the configuration of the - TCP stack of the Transport Converter. The Timestamps option - (Kind=10) SHOULD be used in the proxied SYN if it was present in the - incoming SYN, but the contents of the option in the proxied SYN - SHOULD be set by the Converter's stack. The MP_CAPABLE option SHOULD - be added to the proxied SYN if it was present in the incoming SYN, - but the content of the option in the proxied SYN SHOULD be set by the - Converter's stack. The TCP Fast Open cookie option SHOULD be handled - as described in Section 6. + with the Extended TCP Header TLV (Section 4.2.4). If not, the + Transport Converter returns an Error TLV (Section 4.2.8) and then + closes the connection. As a general rule, when an error is encountered an Error TLV with the - appropriate error code MUST be returned. + appropriate error code MUST be returned by the Transport Converter. -4.2.3. The Bootstrap TLV +4.2.3. The Info TLV - The Bootstrap TLV (Figure 12 is sent by a Client to request the TCP - extensions that are supported by a Transport Converter and for which - it provides a conversion service. It is typically sent on the first - connection that a Client establishes with a Transport Converter to - learn its capabilities. Assuming a Client is entitled to invoke the - Converter, this latter replies with the Supported TCP Extensions - Services TLV described in Section 4.2.4. + The Info TLV (Figure 12) is an optional TLV which can be sent by a + Client to request the TCP extensions that are supported by a + Transport Converter. It is typically sent on the first connection + that a Client establishes with a Transport Converter to learn its + capabilities. Assuming a Client is entitled to invoke the Transport + Converter, the latter replies with the Supported TCP Extensions TLV + described in Section 4.2.4. 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 | Length | Zero | + | Type=0x1 | Length | Zero | +---------------+---------------+-------------------------------+ - Figure 12: The Bootstrap TLV + Figure 12: The Info TLV -4.2.4. Supported TCP Extension Services TLV +4.2.4. Supported TCP Extensions TLV - The Supported TCP Extension Services TLV (Figure 13) is used by a + The Supported TCP Extensions TLV (Figure 13) is used by a Transport Converter to announce the TCP options for which it provides a - conversion service. Each supported TCP option is encoded with its - TCP option Kind listed in the "TCP Parameters" registry maintained by - IANA. + conversion service. A Transport Converter SHOULD include in this + list the TCP options that it accepts from Clients and that it + includes the SYN packets that it sends to initiate connections. + + Each supported TCP option is encoded with its TCP option Kind listed + in the "TCP Parameters" registry maintained by IANA. 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 | Length | Unassigned | + | Type=0x15 | Length | Unassigned | +---------------+---------------+-------------------------------+ | Kind #1 | Kind #2 | ... | +---------------+---------------+-------------------------------+ / ... / / / +---------------------------------------------------------------+ - Figure 13: The Supported TCP Extension Services TLV + Figure 13: The Supported TCP Extensions TLV TCP option Kinds 0, 1, and 2 defined in [RFC0793] are supported by all TCP implementations and thus MUST NOT appear in this list. - The list of Supported TCP Extension Services is padded with 0 to end - on a 32 bits boundary. + The list of Supported TCP Extension is padded with 0 to end on a 32 + bits boundary. - Typically, if the Converter only supports Multipath TCP conversion - service, solely Kind=30 will be present in the Supported TCP - Extension Services TLV returned by the Converter to a requesting - Client. + For example, if the Transport Converter supports Multipath TCP, + Kind=30 will be present in the Supported TCP Extensions TLV that it + returns in response to Info TLV. 4.2.5. Connect TLV The Connect TLV (Figure 14) is used to request the establishment of a - connection via a Transport Converter. + connection via a Transport Converter. This connection can be from or + to a client. The 'Remote Peer Port' and 'Remote Peer IP Address' fields contain - the destination port number and IP address of the target server for - an outgoing connection towards a server located on the Internet. For - incoming connections destined to a client serviced via a Converter, - these fields convey the source port and IP address. + the destination port number and IP address of the Server, for + outgoing connections. For incoming connections destined to a Client + serviced via a Transport Converter, these fields convey the source + port number and IP address. The Remote Peer IP Address MUST be encoded as an IPv6 address. IPv4 addresses MUST be encoded using the IPv4-Mapped IPv6 Address format defined in [RFC4291]. Further, Remote Peer IP address field MUST NOT include multicast, broadcast, and host loopback addresses [RFC6890]. + Connect TLVs witch such messages MUST be discarded by the Transport + Converter. - The optional 'TCP Options' field is used to specify how specific TCP - Options should be advertised by the Transport Converter to the final - destination of a connection. If this field is not supplied, the - Transport Converter MUST use the default TCP options that correspond - to its local policy. - - The Connect TLV could be designed to be generic to include 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 Converter to handle - and manage DNS resolution requests. + 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 + address and a remote port, (2) the extended Connect TLV spans more + than 20 bytes and also includes the optional 'TCP Options' field. + This field is used to specify how specific TCP options should be + advertised by the Transport Converter to the server. 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 | Length | Remote Peer Port | + | Type=0xA | Length | Remote Peer Port | +---------------+---------------+-------------------------------+ | | | Remote Peer IP Address (128 bits) | | | | | +---------------------------------------------------------------+ | TCP Options (Variable) | | ... | +---------------------------------------------------------------+ Figure 14: The Connect TLV The 'TCP Options' field is a variable length field that carries a list of TCP option fields (Figure 15). Each TCP option field is encoded as a block of 2+n bytes where the first byte is the TCP - option Type 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 is 2. The TCP options that do not include a length subfield, i.e., - option types 0 (EOL) and 1 (NOP) defined in [RFC0793] cannot 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 Value field contains the variable-length part of the TCP option. A length of two indicates the absence of the Value field. The TCP options field always ends on a 32 bits boundary after being padded with zeros. 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 +---------------+---------------+---------------+---------------+ - | TCPOpt type | TCPOpt Length | Value (opt) | .... | + | TCPOpt kind | TCPOpt Length | Value (opt) | .... | +---------------+---------------+---------------+---------------+ | .... | +---------------------------------------------------------------+ | ... | +---------------------------------------------------------------+ Figure 15: The TCP Options field - If a Transport Converter receives a Connect TLV with a non-empty TCP - options field, and the Converter acceptss to process the request, it - SHALL present those options to the destination peer in addition to - the TCP options that it would have used according to its local - policies. For the TCP options that are listed without an optional - value, the Converter MUST generate its own value. For the TCP - options that are included in the 'TCP Options' field with an optional - value, it SHALL copy the entire option for use in the connection with - the destination peer. This feature is required to support TCP Fast - Open. + Upon reception of a Connect TLV, and absent any rate limit policy or + resource exhaustion conditions, a Transport Converter MUST attempt to + establish a connection to the address and port that it contains. The + Transport Converter MUST use by default the TCP options that + correspond to its local policy to establish this connection. These + are the options that it advertises in the Supported TCP Extensions + TLV. - The Converter may discard a Connect TLV request for many reasons - (e.g., bad TFO cookie, authorization failed, out of resources, - invalid address type). An error message indicating the encountered - error is returned to the requesting Client (Section 4.2.7). In order - to prevent denial-of-service attacks, error messages sent to a Client + Upon reception of an extended Connect TLV, and absent any rate limit + policy or resource exhaustion conditions, a Transport Converter MUST + attempt to establish a connection to the address and port that it + contains. It MUST include the options of the 'TCP Options' subfield + in the SYN sent to the Server in addition to the TCP options that it + would have used according to its local policies. For the TCP options + that are listed without an optional value, the Transport Converter + MUST generate its own value. For the TCP options that are included + in the 'TCP Options' field with an optional value, it MUST copy the + entire option for use in the connection with the destination peer. + This feature is required to support TCP Fast Open. + + The Transport Converter may discard a Connect TLV request for various + reasons (e.g., authorization failed, out of resources, invalid + address type). An error message indicating the encountered error is + returned to the requesting Client (Section 4.2.8). In order to + prevent denial-of-service attacks, error messages sent to a Client SHOULD be rate-limited. 4.2.6. Extended TCP Header TLV The Extended TCP Header TLV (Figure 16) is used by the Transport Converter to send to the Client the extended TCP header that was returned by the Server in the SYN+ACK packet. This TLV is only sent if the Client sent a Connect TLV to request the establishment of a connection. 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 | Length | Unassigned | + | Type=0x14 | Length | Unassigned | +---------------+---------------+-------------------------------+ | Returned Extended TCP header | | ... | +---------------------------------------------------------------+ Figure 16: The Extended TCP Header TLV The Returned Extended TCP header field is a copy of the extended header that was received in the SYN+ACK by the Transport Converter. The Unassigned field MUST be set to zero by the transmitter and ignored by the receiver. These bits are available for future use [RFC8126]. -4.2.7. Error TLV +4.2.7. The Cookie TLV - The optional Error TLV (Figure 17) can be used by the Transport - Converter to provide information about some errors that occurred - during the processing of a request to convert a connection. This TLV - appears after the Convert header in a RST segment returned by the - Transport Converter if the error is fatal and prevented the - establishment of the connection. If the error is not fatal and the - connection could be established with the final destination, then the - error TLV will be carried in the payload. + The Cookie TLV (Figure 17 is an optional TLV which use is similar to + the TCP Fast Open Cookie [RFC7413]. A Transport Converter may want + to verify that its Clients can receive the packets that it sends to + prevent attacks from spoofed addresses. This verification can be + done by using a Cookie that is bound to, for example, the IP + address(es) of the Client. This Cookie can be configured on the + Client by means that are outside of this document or provided by the + Transport Converter as follows. + + 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 + 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 + [RFC7413] (i.e., IsCookieValid). If the Cookie is valid, the + connection establishment procedure can continue. Otherwise, the + Transport Converter MUST return an Error TLV set to "Not Authorized" + and close the connection. + + If the received SYN did not contain a Cookie TLV, and cookie + validation is required, the Transport Converter should compute a + Cookie bound to this Client address and return a Convert message + containing the fixed header, an Error TLV set to "Missing Cookie" and + the computed Cookie and close the connection. The Client will react + to this error by storing the received Cookie in its cache and attempt + to reestablish a new connection to the Transport Converter that + includes the Cookie. + + The format of the Cookie TLV is shown in the below figure. + + 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=0x16 | Length | Zero | + +---------------+---------------+-------------------------------+ + | Opaque Cookie | + | ... | + +---------------------------------------------------------------+ + + Figure 17: The Cookie TLV + +4.2.8. Error TLV + + The Error TLV (Figure 18) is used by the Transport Converter to + provide information about some errors that occurred during the + processing of Convert message. This TLV has a variable length. It + appears after the Convert fixed-header in the bytestream returned by + the Transport Converter. Upon reception of an Error TLV, a Client + MUST close the associated connection. 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 | Length | Error | Value | + | Type=0x1E | Length | Error code | Value | +---------------+---------------+----------------+--------------+ - Figure 17: The Error TLV + Figure 18: The Error TLV Different types of errors can occur while processing Convert - messages. Each error is identified by a code represented as an - unsigned integer. Four classes of errors are defined: + messages. Each error is identified by an Error code represented as + an unsigned integer. Four classes of Error codes are defined: o Message validation and processing errors (0-31 range): returned - upon reception of an an invalid message (including valid messages - but with invalid or unknown TLVs). + upon reception of an invalid message (including valid messages but + with invalid or unknown TLVs). o Client-side errors (32-63 range): the Client sent a request that - could not be accepted by the Converter (e.g., unsupported - operation). + could not be accepted by the Transport Converter (e.g., + unsupported operation). o Converter-side errors (64-95 range): problems encountered on the - Converter (e.g., lack of resources) which prevent it from - fulfilling the Client's request. + Transport Converter (e.g., lack of resources) which prevent it + from fulfilling the Client's request. - o Errors caused by destination server (96-127 range): the final - destination could not be reached or it replied with a reset - message. + o Errors caused by the destination server (96-127 range): the final + destination could not be reached or it replied with a reset. The following error codes are defined in this document: o Unsupported Version (0): The version number indicated in the fixed header of a message received from a peer is not supported. - This error code MUST be generated by a Converter when it receives - a request having a version number that it does not support. + This error code MUST be generated by a Transport Converter when it + receives a request having a version number that it does not + support. The value field MUST be set to the version supported by the - Converter. When multiple versions are supported by the Converter, - it includes the list of supported version in the value field; each - version is encoded in 8 bits. + Transport Converter. When multiple versions are supported by the + Transport Converter, it includes the list of supported version in + the value field; each version is encoded in 8 bits. The list of + supported versions should be padded with zeros to end on a 32 bits + boundary. Upon receipt of this error code, the client checks whether it - supports one of the versions returned by the Converter. The - highest common supported version MUST be used by the client in - subsequent exchanges with the Converter. + supports one of the versions returned by the Transport Converter. + The highest common supported version MUST be used by the client in + subsequent exchanges with the Transport Converter. o Malformed Message (1): This error code is sent to indicate that a message can not be successfully parsed and validated. - Typically, this error message is sent by the Converter if it - receives a Connect TLV enclosing a multicast, broadcast, or + Typically, this error code is sent by the Transport Converter if + it receives a Connect TLV enclosing a multicast, broadcast, or loopback IP address. To ease troubleshooting, the value field MUST echo the received - message. The Converter and the Client MUST send a RST containing - this error upon reception of a malformed message. + message shifted by one byte to keep to original alignment of the + message. o Unsupported Message (2): This error code is sent to indicate that - a message type is not supported by the Converter. + a message type is not supported by the Transport Converter. To ease troubleshooting, the value field MUST echo the received - message. The Converter and the Client MUST send a RST containing - this error upon reception of an unsupported message. + message shifted by one byte to keep to original alignment of the + message. - o Not Authorized (32): This error code indicates that the Converter - refused to create a connection because of a lack of authorization - (e.g., administratively prohibited, authorization failure, etc.). - The Value field MUST be set to zero. + o Missing Cookie (3): If a Transport Converter requires the + utilization of Cookies to prevent spoofing attacks and a Cookie + TLV was not included in the Convert message, the Transport + Converter MUST return this error to the requesting client. 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 + the Transport Converter for this Client. - This error code MUST be sent by the Converter when a request - cannot be successfully processed because the authorization failed. + A Client which receives this error code MUST cache the received + Cookie and include it in subsequent Convert messages sent to that + Transport Converter. + + o Not Authorized (32): This error code indicates that the Transport + Converter refused to create a connection because of a lack of + authorization (e.g., administratively prohibited, authorization + failure, invalid Cookie TLV, etc.). The Value field MUST be set + to zero. + + This error code MUST be sent by the Transport Converter when a + request cannot be successfully processed because the authorization + failed. o Unsupported TCP Option (33): A TCP option that the Client - requested to advertise to the final Server cannot be safely used - jointly with the conversion service. + requested to advertise to the final Server cannot be safely used. The Value field is set to the type of the unsupported TCP option. If several unsupported TCP options were specified in the Connect - TLV, only one of them is returned in the Value. + TLV, then the list of unsupported TCP options is returned. The + list of unsupported TCP options MUST be padded with zeros to end + on a 32 bits boundary. o Resource Exceeded (64): This error indicates that the Transport Converter does not have enough resources to perform the request. - This error MUST be sent by the Converter when it does not have - sufficient resources to handle a new connection. + This error MUST be sent by the Transport Converter when it does + not have sufficient resources to handle a new connection. The + Transport Converter may indicate in the Value field the suggested + delay (in seconds) that the Client SHOULD wait before soliciting + the Transport Converter for a new proxied connection. A Value of + zero corresponds to a default delay of at least 30 seconds. - o Network Failure (65): This error indicates that the Converter is - experiencing a network failure to relay the request. + o Network Failure (65): This error indicates that the Transport + Converter is experiencing a network failure to relay the request. - The Converter MUST send this error code when it experiences - forwarding issues to relay a connection. + The Transport Converter MUST send this error code when it + experiences forwarding issues to relay a connection. The + Transport Converter may indicate in the Value field the suggested + delay (in seconds) that the Client SHOULD wait before soliciting + the Transport Converter for a new proxied connection. A Value of + zero corresponds to a default delay of at least 30 seconds. o Connection Reset (96): This error indicates that the final destination responded with a RST packet. The Value field MUST be set to zero. o Destination Unreachable (97): This error indicates that an ICMP destination unreachable, port unreachable, or network unreachable - was received by the Converter. The Value field MUST echo the Code - field of the received ICMP message. - - This error message MUST be sent by the Converter when it receives - an error message that is bound to a message it relayed previously. + was received by the Transport Converter. The Value field MUST + echo the Code field of the received ICMP message. - Figure 18 summarizes the different error codes. + Figure 19 summarizes the different error codes. +-------+------+-----------------------------------------------+ | Error | Hex | Description | +-------+------+-----------------------------------------------+ | 0 | 0x00 | Unsupported Version | | 1 | 0x01 | Malformed Message | | 2 | 0x02 | Unsupported Message | + | 3 | 0x03 | Missing Cookie | | 32 | 0x20 | Not Authorized | | 33 | 0x21 | Unsupported TCP Option | | 64 | 0x40 | Resource Exceeded | | 65 | 0x41 | Network Failure | | 96 | 0x60 | Connection Reset | | 97 | 0x61 | Destination Unreachable | +-------+------+-----------------------------------------------+ - Figure 18: Convert Error Values + Figure 19: Convert Error Values 5. Compatibility of Specific TCP Options with the Conversion Service In this section, we discuss how several standard track TCP options - can be supported through the Converter. The non-standard track - options and the experimental options will be discussed in other + can be supported through the Convert protocol. The non-standard + track options and the experimental options will be discussed in other documents. 5.1. Base TCP Options Three TCP options were initially defined in [RFC0793]: End-of-Option List (Kind=0), No-Operation (Kind=1) and Maximum Segment Size (Kind=2). The first two options are mainly used to pad the TCP extended header. There is no reason for a client to request a - Converter to specifically send these options towards the final - destination. + Transport Converter to specifically send these options towards the + final destination. The Maximum Segment Size option (Kind=2) is used by a host to indicate the largest segment that it can receive over each connection. This value is function of the stack that terminates the TCP connection. There is no reason for a Client to request a - Converter to advertise a specific MSS value to a remote server. + Transport Converter to advertise a specific MSS value to a remote + server. - A Converter MUST ignore options with Kind=0, 1 or 2 if they appear in - a Connect TLV. It MUST NOT announce them in a Bootstrap TLV. + 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 + TCP Extensions TLV. 5.2. Window Scale (WS) The Window Scale option (Kind=3) is defined in [RFC7323]. As for the MSS option, the window scale factor that is used for a connection strongly depends on the TCP stack that handles the connection. When - a Converter opens a TCP connection 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 its stack. A local configuration - MAY allow for WS option in the proxied message to be function of the - scaling factor of the incoming connection. + a Transport Converter opens a TCP connection 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 its stack. A local + configuration MAY allow for WS option in the proxied message to be + function of the scaling factor of the incoming connection. There is no benefit from a deployment viewpoint in enabling a Client - of a Converter to specifically request the utilisation of the WS - option (Kind=3) with a specific scaling factor towards a remote - Server. For this reason, a Converter MUST ignore option Kind=3 if it - appears in a Connect TLV. It MUST NOT announce it in a Bootstrap - TLV. + of a Transport Converter to specifically request the utilisation of + the WS option (Kind=3) with a specific scaling factor towards a + remote Server. For this reason, a Transport Converter MUST ignore + option Kind=3 if it appears in a Connect TLV. It MUST NOT announce + it in a Supported TCP Extensions TLV. 5.3. Selective Acknowledgements Two distinct TCP options were defined to support selective acknowledgements in [RFC2018]. This first one, SACK Permitted (Kind=4), is used to negotiate the utilisation of selective acknowledgements during the three-way handshake. The second one, SACK (Kind=5), carries the selective acknowledgements inside regular segments. The SACK Permitted option (Kind=4) MAY be advertised by a Transport - Converter in the Bootstrap TLV. In this case, Clients connected to + Converter in the Supported TCP Extensions TLV. Clients connected to this Transport Converter MAY include the SACK Permitted option in the Connect TLV. The SACK option (Kind=5) cannot be used during the three-way handshake. For this reason, a Transport Converter MUST ignore option - Kind=5 with if it appears in a Connect TLV. It MUST NOT announce it - in a Bootstrap TLV. + Kind=5 if it appears in a Connect TLV. It MUST NOT announce it in a + TCP Supported Extensions TLV. 5.4. Timestamp - The Timestamp option was initially defined in [RFC1323] which has - been replaced by [RFC7323]. It can be used during the three-way - handshake to negotiate the utilisation of the timestamps during the - TCP connection. It is notably used to improve round-trip-time - estimations and to provide protection against wrapped sequence - numbers (PAWS). As for the WS option, the timestamps are a property - of a connection and there is limited benefit in enabling a client to - request a Converter to use the timestamp option when establishing a - connection to a remote server. Furthermore, the timestamps that are - used by TCP stacks are specific to each stack and there is no benefit - in enabling a client to specify the timestamp value that a Converter + The Timestamp option was initially defined in [RFC1323] and later + refined in [RFC7323]. It can be used during the three-way handshake + to negotiate the utilization of timestamps during the TCP connection. + It is notably used to improve round-trip-time estimations and to + provide protection against wrapped sequence numbers (PAWS). As for + the WS option, the timestamps are a property of a connection and + there is limited benefit in enabling a client to request a Transport + Converter to use the timestamp option when establishing a connection + to a remote server. Furthermore, the timestamps that are used by TCP + stacks are specific to each stack and there is no benefit in enabling + a client to specify the timestamp value that a Transport Converter could use to establish a connection to a remote server. A Transport Converter MAY advertise the Timestamp option (Kind=8) in - the Bootstrap TLV. The clients connected to this Converter MAY - include the Timestamp option in the Connect TLV but without any - timestamp. + the TCP Supported Extensions TLV. The clients connected to this + Transport Converter MAY include the Timestamp option in the Connect + TLV but without any timestamp. 5.5. Multipath TCP The Multipath TCP options are defined in [RFC6824]. [RFC6824] defines one variable length TCP option (Kind=30) that includes a subtype field to support several Multipath TCP options. There are several operational use cases where clients would like to use - Multipath TCP through a Converter [IETFJ16]. However, none of these - use cases require the Client to specify the content of the Multipath - TCP option that the Converter should send to a remote server. + Multipath TCP through a Transport Converter [IETFJ16]. However, none + of these use cases require the Client to specify the content of the + Multipath TCP option that the Transport Converter should send to a + remote server. A Transport Converter which supports Multipath TCP conversion service - MUST advertise the Multipath TCP option (Kind=30) in the Bootstrap - TLV. Clients serviced by this Converter may include the Multipath - TCP option in the Connect TLV but without any content. + MUST advertise the Multipath TCP option (Kind=30) in the Supported + TCP Extensions TLV. Clients serviced by this Transport Converter may + include the Multipath TCP option in the Connect TLV but without any + content. 5.6. TCP Fast Open The TCP Fast Open cookie option (Kind=34) is defined in [RFC7413]. There are two different usages of this option that need to be - supported by Transport Converters. The first utilisation of the Fast - Open cookie is to request a cookie from the server. In this case, - the option is sent with an empty cookie by the client and the server - returns the cookie. The second utilisation of the Fast Open cookie - is to send a cookie to the server. In this case, the option contains - a cookie. + supported by Transport Converters. The first utilization of the TCP + 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 + 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 + case, the option contains a cookie. A Transport Converter MAY advertise the TCP Fast Open cookie option - (Kind=34) in the Bootstrap TLV. If a Transport Converter has - advertised the support for TCP Fast Open in its Bootstrap TLV, it - needs to be able to process two types of Connect TLV. If such a - Transport Converter receives a 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 the SYN sent to the remote server. If - such a Transport Converter 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 the SYN sent to the remote server. - - The Converter may behave in address preservation or address sharing - modes as discussed in Section 5.4 of - [I-D.nam-mptcp-deployment-considerations]. Which behavior to use by - a Converter is deployment-specific. If address sharing mode is - enabled, the Converter MUST adhere to REQ-2 of [RFC6888] which - implies a default "IP address pooling" behavior of "Paired" (as - defined in Section 4.1 of [RFC4787]) must be supported. This - behavior is meant to avoid breaking applications that depend on the - external address remaining constant. Also, maintaining the same - external IP address for a client is meant to preserve the validity of - the TFO cookie. + (Kind=34) in the Supported TCP Extensions TLV. If a Transport + Converter has advertised the support for TCP Fast Open in its + Supported TCP Extensions TLV, it needs to be able to process two + types of Connect TLV. If such a Transport Converter receives a + 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 + the SYN sent to the remote server. If such a Transport Converter + 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 + the SYN sent to the remote server. 5.7. TCP User Timeout The TCP User Timeout option is defined in [RFC5482]. The associated TCP option (Kind=28) does not appear to be widely deployed. - Editor's Note: Feedback requested for the utilisation of this option - by deployed TCP stacks. - 5.8. TCP-AO TCP-AO [RFC5925] provides a technique to authenticate all the packets exchanged over a TCP connection. Given the nature of this extension, it is unlikely that the applications that require their packets to be authenticated end-to-end would want their connections to pass through a converter. For this reason, we do not recommend the support of the - TCP-AO option by Transport Converters. The only use cases where is - makes sense to combine TCP-AO and the solution in this document are - those where the TCP-AO-NAT extension [RFC6978] is in use. + 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 + are those where the TCP-AO-NAT extension [RFC6978] is in use. - A Converter MUST NOT advertise the TCP-AO option (Kind=29) in the - Bootstrap TLV. If a Converter receives a Connect TLV that contains - the TCP-AO option, it MUST reject the establishment of the connection - with error code set to "Unsupported TCP Option", except if the TCP- - AO-NAT option is used. + A Transport Converter MUST NOT advertise the TCP-AO option (Kind=29) + in the Supported TCP Extensions TLV. If a Transport Converter + receives a Connect TLV that contains the TCP-AO option, it MUST + reject the establishment of the connection with error code set to + "Unsupported TCP Option", except if the TCP-AO-NAT option is used. 5.9. TCP Experimental Options 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. 6. Interactions with Middleboxes - The Converter Protocol was designed to be used in networks that do - not contain middleboxes that interfere with TCP. We describe in this - section how a Client can detect middlebox interference and stop using - the Transport Converter affected by this interference. + The Convert Protocol is designed to be used in networks that do not + contain middleboxes that interfere with TCP. Under such conditions, + it is assumed that the network provider ensures that all involved on- + path nodes are not breaking TCP signals (e.g., strip TCP options, + discard some SYNs, etc.). + + Nevertheless, and in order to allow for a robust service, this + section describes how a Client can detect middlebox interference and + stop using the Transport Converter affected by this interference. Internet measurements [IMC11] have shown that middleboxes can affect the deployment of TCP extensions. In this section, we only discuss - the middleboxes that modify SYN and SYN+ACK packets since the - Converter Protocol places its messages in such packets. - - Let us first consider a middlebox that removes the TFO Option from - the SYN packet. This interference will be detected by the Client - during the bootstrap procedure discussed in Section 4.2.3. A Client - should not use a Transport Converter that does not reply with the TFO - option during the Bootstrap. + the middleboxes that modify SYN and SYN+ACK packets since the Convert + Protocol places its messages in such packets. - Consider a middlebox that removes the SYN payload after the bootstrap - procedure. The Client can detect this problem by looking at the - acknowledgement number field of the SYN+ACK returned by the Transport - Converter. The Client should stop to use this Transport Converter - given the middlebox interference. + Consider a middlebox that removes the SYN payload. The Client can + detect this problem by looking at the acknowledgement number field of + the SYN+ACK returned by the Transport Converter. The Client MUST + stop to use this Transport Converter given the middlebox + interference. - As explained in [RFC7413], some carrier-grade NATs can affect the - operation of TFO if they assign different IP addresses to the same - end host. Such carrier-grade NATs could affect the operation of the - TFO Option used by the Converter Protocol. See also the discussion - in Section 7.1 of [RFC7413]. + As explained in [RFC7413], some CGNs (Carrier Grade NATs) can affect + the operation of TFO if they assign different IP addresses to the + same end host. Such CGNs could affect the operation of the TFO + Option used by the Convert Protocol. As a reminder CGNs, enabled on + the path between a Client and a Transport Converter, must adhere to + the address preservation defined in [RFC6888]. See also the + discussion in Section 7.1 of [RFC7413]. 7. Security Considerations 7.1. Privacy & Ingress Filtering - The Converter may have access to privacy-related information (e.g., - subscriber credentials). The Converter MUST NOT leak such sensitive - information outside a local domain. + The Transport Converter may have access to privacy-related + information (e.g., subscriber credentials). The Transport Converter + is designed to not leak such sensitive information outside a local + domain. Given its function and its location in the network, a Transport 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., [RFC1812]). Furthermore, ingress filtering policies MUST be enforced at the network boundaries [RFC2827]. This document assumes that all network attachments are managed by the same administrative entity. Therefore, enforcing anti-spoofing filters at these network ensures that hosts are not sending traffic with spoofed source IP addresses. 7.2. Authorization - The Converter Protocol is intended to be used in managed networks - where end hosts can be identified by their IP address. Thanks to the - Bootstrap procedure, the Transport Converter can verify that the - Client correctly receives packets sent by the Converter. Stronger - authentication schemes MUST be defined to use the Converter Protocol - in more open network environments; such schemes are out of scope of - this document. + The Convert Protocol is intended to be used in managed networks where + end hosts can be identified by their IP address. + + Stronger mutual authentication schemes MUST be defined to use the + Convert Protocol in more open network environments. One possibility + is to use TLS to perform mutual authentication between the client and + the Converter. That is, use TLS when a Client retrieves a Cookie + from the Converter and rely on certificate-based client + authentication, pre-shared key based [RFC4279] or raw public key + based client authentication [RFC7250] to secure this connection. If + the authentication succeeds, the Converter returns a cookie whose + content may be, for example, set to a hash using as input the + representation of the Subject Public Key Info (SPKI) of the client + X.509 certificate, the Client raw public key, or the "Pre-Shared Key + (PSK) identity" used by the Client in the TLS ClientKeyExchange + message. Subsequent Connect messages will be authorized as a + function of the content of the Cookie TLV. The client MUST also + authenticate. See below for authorization considerations that are specific for Multipath TCP. 7.3. Denial of Service Another possible risk is the amplification attacks since a Transport 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 sent by the Transport Converter were larger than the SYN received from the Client or if the Transport Converter retransmits the SYN. To mitigate such attacks, the Transport Converter SHOULD rate limit the number of pending requests for a given Client. It SHOULD also avoid sending to remote Servers SYNs that are significantly longer - than the SYN received from the Client. In practice, Transport - Converters SHOULD NOT advertise to a Server TCP options that were not - specified by the Client in the received SYN. Finally, the Transport + than the SYN received from the Client. Finally, the Transport Converter SHOULD only retransmit a SYN to a Server after having - received a retransmitted SYN from the corresponding Client. - - Upon reception of a SYN that contains a valid TFO cookie and a - Connect TLV, the Transport Converter attempts to establish a TCP - connection to a remote Server. There is a risk of denial of service - attack if a Client requests too many connections in a short period of - time. Implementations SHOULD limit the number of pending connections - from a given Client. Means to protect against SYN flooding attacks - MUST also be enabled [RFC4987]. + received a retransmitted SYN from the corresponding Client. Means to + protect against SYN flooding attacks MUST also be enabled [RFC4987]. 7.4. Traffic Theft Traffic theft is a risk if an illegitimate Converter is inserted in the path. Indeed, inserting an illegitimate Converter in the forwarding path allows traffic interception and can therefore provide access to sensitive data issued by or destined to a host. Converter discovery and configuration are out of scope of this document. 7.5. Multipath TCP-specific Considerations Multipath TCP-related security threats are discussed in [RFC6181] and [RFC6824]. The operator that manages the various network attachments (including - the Converters) can enforce authentication and authorization policies - using appropriate mechanisms. For example, a non-exhaustive list of - methods to achieve authorization is provided hereafter: + the Transport Converters) can enforce authentication and + authorization policies using appropriate mechanisms. For example, a + non-exhaustive list of methods to achieve authorization is provided + hereafter: o The network provider may enforce a policy based on the International Mobile Subscriber Identity (IMSI) to verify that a user is allowed to benefit from the aggregation service. If that authorization fails, the Packet Data Protocol (PDP) context/bearer will not be mounted. This method does not require any interaction - with the Converter. + with the Transport Converter. o The network provider may enforce a policy based upon Access Control Lists (ACLs), e.g., at a Broadband Network Gateway (BNG) to control the hosts that are authorized to communicate with a - Converter. These ACLs may be installed as a result of RADIUS - exchanges, e.g. [I-D.boucadair-radext-tcpm-converter]. This - method does not require any interaction with the Converter. + Transport Converter. These ACLs may be installed as a result of + RADIUS exchanges, e.g. [I-D.boucadair-radext-tcpm-converter]. + This method does not require any interaction with the Transport + Converter. - o A device that embeds the Converter may also host a RADIUS client - that will solicit an AAA server to check whether connections - received from a given source IP address are authorized or not - [I-D.boucadair-radext-tcpm-converter]. + o A device that embeds a Transport Converter may also host a RADIUS + client that will solicit an AAA server to check whether + connections received from a given source IP address are authorized + or not [I-D.boucadair-radext-tcpm-converter]. - A first safeguard against the misuse of Converter resources by - illegitimate users (e.g., users with access networks that are not - managed by the same provider that operates the Converter) is the - Converter to reject Multipath TCP connections received on its - Internet-facing interfaces. Only Multipath TCP connections received - on the customer-facing interfaces of a Converter will be accepted. + A first safeguard against the misuse of Transport Converter resources + by illegitimate users (e.g., users with access networks that are not + managed by the same provider that operates the Transport Converter) + is the Transport Converter to reject Multipath TCP connections + received on its Internet-facing interfaces. Only Multipath TCP + connections received on the customer-facing interfaces of a Transport + Converter will be accepted. 8. IANA Considerations 8.1. Convert Service Port Number - IANA is requested to assign a TCP port number (TBA) for the Converter + IANA is requested to assign a TCP port number (TBA) for the Convert Protocol 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. -8.2. The Converter Protocol (Convert) Parameters +8.2. The Convert Protocol (Convert) Parameters - IANA is requested to create a new "The Converter Protocol (Convert) + IANA is requested to create a new "The Convert Protocol (Convert) Parameters" registry. - The following subsections detail new registries within "The Converter + The following subsections detail new registries within "The Convert Protocol (Convert) Parameters" registry. 8.2.1. Convert Versions IANA is requested to create the "Convert versions" sub-registry. New values are assigned via Standards Action. The initial values to be assigned at the creation of the registry are as follows: @@ -1298,79 +1392,81 @@ o The values in the range 192-255 can be assigned for Private Use. The initial values to be assigned at the creation of the registry are as follows: +---------+--------------------------------------+-------------+ | Code | Name | Reference | +---------+--------------------------------------+-------------+ | 0 | Reserved | [This-RFC] | - | 1 | Bootstrap TLV | [This-RFC] | + | 1 | Info TLV | [This-RFC] | | 10 | Connect TLV | [This-RFC] | | 20 | Extended TCP Header TLV | [This-RFC] | - | 22 | Supported TCP Extension Services TLV | [This-RFC] | + | 21 | Supported TCP Extension TLV | [This-RFC] | + | 22 | Cookie TLV | [This-RFC] | | 30 | Error TLV | [This-RFC] | +---------+--------------------------------------+-------------+ 8.2.3. Convert Error Messages IANA is requested to create the "Convert Errors" sub-registry. Codes in this registry are assigned as a function of the error type. Four types are defined; the following ranges are reserved for each of these types: o Message validation and processing errors: 0-31 o Client-side errors: 32-63 - o Converter-side errors: 64-95 + o Transport Converter-side errors: 64-95 o Errors caused by destination server: 96-127 The procedure for assigning values from this sub-registry is as follows: o 0-191: Values in this range are assigned via Standards Action. o 192-255: Values in this range are assigned via Specification Required. The initial values to be assigned at the creation of the registry are as follows: +-------+------+-----------------------------------+-----------+ | Error | Hex | Description | Reference | +-------+------+-----------------------------------+-----------+ | 0 | 0x00 | Unsupported Version | [This-RFC]| | 1 | 0x01 | Malformed Message | [This-RFC]| | 2 | 0x02 | Unsupported Message | [This-RFC]| + | 3 | 0x03 | Missing Cookie | [This-RFC]| | 32 | 0x20 | Not Authorized | [This-RFC]| | 33 | 0x21 | Unsupported TCP Option | [This-RFC]| | 64 | 0x40 | Resource Exceeded | [This-RFC]| | 65 | 0x41 | Network Failure | [This-RFC]| | 96 | 0x60 | Connection Reset | [This-RFC]| | 97 | 0x61 | Destination Unreachable | [This-RFC]| +-------+------+-----------------------------------+-----------+ - Figure 19: The Convert Error Codes + Figure 20: The Convert Error Codes 9. Acknowledgements Although they could disagree with the contents of the document, we would like to thank Joe Touch and Juliusz Chroboczek whose comments on the MPTCP mailing list have forced us to reconsider the design of the solution several times. - We would like to thank Raphael Bauduin, Stefano Secci, Benjamin - Hesmans and Anandatirtha Nandugudi for their help in preparing this - document. Nandini Ganesh provided valuable feedback about the + We would like to thank Raphael Bauduin, Stefano Secci, Anandatirtha + Nandugudi and Gregory Vander Schueren for their help in preparing + this document. Nandini Ganesh provided valuable feedback about the handling of TFO and the error codes. Thanks to them. This document builds upon earlier documents that proposed various forms of Multipath TCP proxies [I-D.boucadair-mptcp-plain-mode], [I-D.peirens-mptcp-transparent] and [HotMiddlebox13b]. From [I-D.boucadair-mptcp-plain-mode]: Many thanks to Chi Dung Phung, Mingui Zhang, Rao Shoaib, Yoshifumi Nishida, and Christoph Paasch for their valuable comments. @@ -1455,20 +1551,25 @@ [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . + [RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key + Ciphersuites for Transport Layer Security (TLS)", + RFC 4279, DOI 10.17487/RFC4279, December 2005, + . + [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006, . [RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4, ICMPv6, UDP, and TCP Headers", RFC 4727, DOI 10.17487/RFC4727, November 2006, . [RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common @@ -1486,20 +1587,26 @@ [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013, . [RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman, "Special-Purpose IP Address Registries", BCP 153, RFC 6890, DOI 10.17487/RFC6890, April 2013, . + [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., + Weiler, S., and T. Kivinen, "Using Raw Public Keys in + Transport Layer Security (TLS) and Datagram Transport + Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, + June 2014, . + [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, . [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC @@ -1541,43 +1648,37 @@ [I-D.boucadair-radext-tcpm-converter] Boucadair, M. and C. Jacquenet, "RADIUS Extensions for 0-RTT TCP Converters", draft-boucadair-radext-tcpm- converter-01 (work in progress), October 2018. [I-D.boucadair-tcpm-dhc-converter] Boucadair, M., Jacquenet, C., and R. K, "DHCP Options for 0-RTT TCP Converters", draft-boucadair-tcpm-dhc- converter-01 (work in progress), October 2018. - [I-D.ietf-mptcp-rfc6824bis] - Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C. - Paasch, "TCP Extensions for Multipath Operation with - Multiple Addresses", draft-ietf-mptcp-rfc6824bis-12 (work - in progress), October 2018. - [I-D.ietf-tcpinc-tcpcrypt] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack, Q., and E. Smith, "Cryptographic protection of TCP Streams - (tcpcrypt)", draft-ietf-tcpinc-tcpcrypt-13 (work in - progress), September 2018. + (tcpcrypt)", draft-ietf-tcpinc-tcpcrypt-15 (work in + progress), December 2018. [I-D.nam-mptcp-deployment-considerations] Boucadair, M., Jacquenet, C., Bonaventure, O., Henderickx, W., and R. Skog, "Network-Assisted MPTCP: Use Cases, Deployment Scenarios and Operational Considerations", draft-nam-mptcp-deployment-considerations-01 (work in progress), December 2016. [I-D.olteanu-intarea-socks-6] Olteanu, V. and D. Niculescu, "SOCKS Protocol Version 6", - draft-olteanu-intarea-socks-6-04 (work in progress), - August 2018. + draft-olteanu-intarea-socks-6-05 (work in progress), + October 2018. [I-D.peirens-mptcp-transparent] Peirens, B., Detal, G., Barre, S., and O. Bonaventure, "Link bonding with transparent Multipath TCP", draft- peirens-mptcp-transparent-00 (work in progress), July 2016. [IETFJ16] Bonaventure, O. and S. Seo, "Multipath TCP Deployment", IETF Journal, Fall 2016 , n.d.. @@ -1652,35 +1753,44 @@ Scheffenegger, Ed., "TCP Extensions for High Performance", RFC 7323, DOI 10.17487/RFC7323, September 2014, . [RFC7414] Duke, M., Braden, R., Eddy, W., Blanton, E., and A. Zimmermann, "A Roadmap for Transmission Control Protocol (TCP) Specification Documents", RFC 7414, DOI 10.17487/RFC7414, February 2015, . + [RFC8041] Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and + Operational Experience with Multipath TCP", RFC 8041, + DOI 10.17487/RFC8041, January 2017, + . + [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: Better Connectivity Using Concurrency", RFC 8305, DOI 10.17487/RFC8305, December 2017, . + [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol + Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, + . + Appendix A. Differences with SOCKSv5 - At a first glance, the Convert solution 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 20. + 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 21. Client SOCKS Proxy Server --------------------> SYN <-------------------- SYN+ACK --------------------> ACK --------------------> @@ -1703,54 +1813,62 @@ --------------------> Data1 --------------------> Data1 <-------------------- Data2 <-------------------- Data2 - Figure 20: Establishment of a TCP connection through a SOCKS proxy + Figure 21: Establishment of a TCP connection through a SOCKS proxy without authentication - The Converter protocol also relays data between an upstream and a + 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 Converter protocol leverages the TFO + A first difference is that the Convert protocol leverages the TFO option [RFC7413] to exchange 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 also leverages the TFO option [I-D.olteanu-intarea-socks-6]. - A second difference is that the Converter protocol explicitly takes - the TCP extensions into account. By using the Converter 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 + 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. + Authors' Addresses Olivier Bonaventure (editor) Tessares Email: Olivier.Bonaventure@tessares.net Mohamed Boucadair (editor) Orange @@ -1748,19 +1866,24 @@ Olivier Bonaventure (editor) Tessares Email: Olivier.Bonaventure@tessares.net Mohamed Boucadair (editor) Orange Email: mohamed.boucadair@orange.com + Sri Gundavelli Cisco Email: sgundave@cisco.com - SungHoon Seo Korea Telecom Email: sh.seo@kt.com + + Benjamin Hesmans + Tessares + + Email: Benjamin.Hesmans@tessares.net