draft-ietf-tcpm-early-rexmt-03.txt		draft-ietf-tcpm-early-rexmt-04.txt
	Internet Engineering Task Force Mark Allman		Internet Engineering Task Force Mark Allman
	INTERNET DRAFT ICSI		INTERNET DRAFT ICSI
	File: draft-ietf-tcpm-early-rexmt-03.txt Konstantin Avrachenkov		File: draft-ietf-tcpm-early-rexmt-04.txt Konstantin Avrachenkov
	Intended Status: Experimental INRIA		Intended Status: Experimental INRIA
	Urtzi Ayesta		Urtzi Ayesta
	LAAS-CNRS		BCAM-IKERBASQUE and LAAS-CNRS
	Josh Blanton		Josh Blanton
	Ohio University		Ohio University
	Per Hurtig		Per Hurtig
	Karlstad University		Karlstad University
	November 2009		January 2010
	Expires: May 2010		Expires: July 2010
	Early Retransmit for TCP and SCTP		Early Retransmit for TCP and SCTP
	Status of this Memo		Status of this Memo
	This Internet-Draft is submitted to IETF in full conformance with		This Internet-Draft is submitted to IETF in full conformance with
	the provisions of BCP 78 and BCP 79.		the provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF), its areas, and its working groups. Note that		Task Force (IETF), its areas, and its working groups. Note that
	skipping to change at page 1, line 39		skipping to change at page 1, line 39
	months and may be updated, replaced, or obsoleted by other documents		months and may be updated, replaced, or obsoleted by other documents
	at any time. It is inappropriate to use Internet-Drafts as		at any time. It is inappropriate to use Internet-Drafts as
	reference material or to cite them other than as "work in progress."		reference material or to cite them other than as "work in progress."
	The list of current Internet-Drafts can be accessed at		The list of current Internet-Drafts can be accessed at
	http://www.ietf.org/ietf/1id-abstracts.txt.		http://www.ietf.org/ietf/1id-abstracts.txt.
	The list of Internet-Draft Shadow Directories can be accessed at		The list of Internet-Draft Shadow Directories can be accessed at
	http://www.ietf.org/shadow.html.		http://www.ietf.org/shadow.html.
	This Internet-Draft will expire on May 18, 2010.		This Internet-Draft will expire on July 27, 2010.
	Copyright Notice		Copyright Notice
	Copyright (c) 2009 IETF Trust and the persons identified as the		Copyright (c) 2009 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 24		skipping to change at page 2, line 24
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in RFC 2119 [RFC2119].		document are to be interpreted as described in RFC 2119 [RFC2119].
	The reader is expected to be familiar with the definitions given in		The reader is expected to be familiar with the definitions given in
	[RFC5681].		[RFC5681].
	1 Introduction		1 Introduction
	Many researchers have studied problems with TCP [RFC793,RFC5681]		Many researchers have studied problems with TCP's loss recovery
	when the congestion window is small and have outlined possible		[RFC793,RFC5681] when the congestion window is small and have
	mechanisms to mitigate these problems		outlined possible mechanisms to mitigate these problems
	[Mor97,BPS+98,Bal98,LK98,RFC3150,AA02]. SCTP's [RFC4960] loss		[Mor97,BPS+98,Bal98,LK98,RFC3150,AA02]. SCTP's [RFC4960] loss
	recovery and congestion control mechanisms are based on TCP and		recovery and congestion control mechanisms are based on TCP and
	therefore the same problems impact the performance of SCTP		therefore the same problems impact the performance of SCTP
	connections. When the transport detects a missing segment, the		connections. When the transport detects a missing segment, the
	connection enters a loss recovery phase. There are several variants		connection enters a loss recovery phase. There are several variants
	of the loss recovery phase depending on the TCP implementation. TCP		of the loss recovery phase depending on the TCP implementation. TCP
	can use slow start based recovery or Fast Recovery [RFC5681],		can use slow start based recovery or Fast Recovery [RFC5681],
	NewReno [RFC3782], and loss recovery based on selective		NewReno [RFC3782], and loss recovery based on selective
	acknowledgments (SACKs) [RFC2018,FF96,RFC3517]. SCTP's loss		acknowledgments (SACKs) [RFC2018,FF96,RFC3517]. SCTP's loss
	recovery is not as varied due to the built-in selective		recovery is not as varied due to the built-in selective
	skipping to change at page 4, line 15		skipping to change at page 4, line 15
	use "Limited Transmit" to include both TCP and SCTP mechanisms for		use "Limited Transmit" to include both TCP and SCTP mechanisms for
	sending in response to the first two duplicate ACKs. By sending		sending in response to the first two duplicate ACKs. By sending
	these two new segments the sender is attempting to induce additional		these two new segments the sender is attempting to induce additional
	duplicate ACKs (if appropriate) so that Fast Retransmit will be		duplicate ACKs (if appropriate) so that Fast Retransmit will be
	triggered before the retransmission timeout expires. The		triggered before the retransmission timeout expires. The
	sender-side "Early Retransmit" mechanism outlined in this document		sender-side "Early Retransmit" mechanism outlined in this document
	covers the case when previously unsent data is not available for		covers the case when previously unsent data is not available for
	transmission (case (2) above) or cannot be transmitted due to an		transmission (case (2) above) or cannot be transmitted due to an
	advertised window limitation (case (3) above).		advertised window limitation (case (3) above).
			Note: This document is being published as an experimental RFC as
			part of the process for the TCPM WG and the IETF to assess whether
			the proposed change is useful and safe in the heterogeneous
			environments, including which variants of the mechanism are the most
			effective. In the future, this specification may be updated and put
			on the standards track if the safeness and efficacy can be
			demonstrated.
	2 Early Retransmit Algorithm		2 Early Retransmit Algorithm
	The Early Retransmit algorithm calls for lowering the threshold for		The Early Retransmit algorithm calls for lowering the threshold for
	triggering Fast Retransmit when the amount of outstanding data is		triggering Fast Retransmit when the amount of outstanding data is
	small and when no previously unsent data can be transmitted (such		small and when no previously unsent data can be transmitted (such
	that Limited Transmit could be used). Duplicate ACKs are triggered		that Limited Transmit could be used). Duplicate ACKs are triggered
	by each arriving out-of-order segment. Therefore, Fast Retransmit		by each arriving out-of-order segment. Therefore, Fast Retransmit
	will not be invoked when there are less than four outstanding		will not be invoked when there are less than four outstanding
	segments (assuming only one segment loss in the window). However,		segments (assuming only one segment loss in the window). However,
	TCP and SCTP are not required to track the number of outstanding		TCP and SCTP are not required to track the number of outstanding
	skipping to change at page 5, line 25		skipping to change at page 5, line 33
	ER_thresh = ceiling (ownd/SMSS) - 1 (1)		ER_thresh = ceiling (ownd/SMSS) - 1 (1)
	duplicate ACKs, where ownd is in terms of bytes. We call this		duplicate ACKs, where ownd is in terms of bytes. We call this
	reduced ACK threshold enabling "Early Retransmission".		reduced ACK threshold enabling "Early Retransmission".
	When conditions (2.a) and (2.b) hold and a TCP connection does		When conditions (2.a) and (2.b) hold and a TCP connection does
	support SACK or SCTP is in use, Early Retransmit MUST be used only		support SACK or SCTP is in use, Early Retransmit MUST be used only
	when "ownd - SMSS" bytes have been SACKed.		when "ownd - SMSS" bytes have been SACKed.
	When conditions (2.a) and (2.b) do not hold, the transport MUST NOT		If either (or both) condition (2.a) or (2.b) does not hold, the
	use Early Retransmit, but rather prefer the standard mechanisms,		transport MUST NOT use Early Retransmit, but rather prefer the
	including Fast Retransmit and Limited Transmit.		standard mechanisms, including Fast Retransmit and Limited Transmit.
	As noted above, the drawback of this byte-based variant is precision		As noted above, the drawback of this byte-based variant is precision
	[HB08]. We illustrate this with two examples:		[HB08]. We illustrate this with two examples:
	+ Consider a non-SACK TCP sender that uses an SMSS of 1460 bytes		+ Consider a non-SACK TCP sender that uses an SMSS of 1460 bytes
	and transmits three segments each with 400 bytes of payload.		and transmits three segments each with 400 bytes of payload.
	This is a case where Early Retransmit could aid loss recovery if		This is a case where Early Retransmit could aid loss recovery if
	one segment is lost. However, in this case ER_thresh will		one segment is lost. However, in this case ER_thresh will
	become zero, per equation (1), because the number of outstanding		become zero, per equation (1), because the number of outstanding
	bytes is a poor estimate of the number of outstanding segments.		bytes is a poor estimate of the number of outstanding segments.
	skipping to change at page 6, line 26		skipping to change at page 6, line 34
	segments. (We discuss tracking the number of outstanding segments		segments. (We discuss tracking the number of outstanding segments
	below.) We call this reduced ACK threshold enabling "Early		below.) We call this reduced ACK threshold enabling "Early
	Retransmission".		Retransmission".
	When conditions (3.a) and (3.b) hold and a TCP connection does		When conditions (3.a) and (3.b) hold and a TCP connection does
	support SACK or SCTP is in use, Early Retransmit MUST be used only		support SACK or SCTP is in use, Early Retransmit MUST be used only
	when "oseg - 1" segments have been SACKed. A segment is considered		when "oseg - 1" segments have been SACKed. A segment is considered
	to be SACKed when all its data bytes (TCP) or data chunks (SCTP)		to be SACKed when all its data bytes (TCP) or data chunks (SCTP)
	have been indicated as arrived by the receiver.		have been indicated as arrived by the receiver.
	When conditions (3.a) and (3.b) do not hold, the transport MUST NOT		If either (or both) conditions (3.a) or (3.b) does not hold, the
	use Early Retransmit, but rather prefer the standard mechanisms,		transport MUST NOT use Early Retransmit, but rather prefer the
	including Fast Retransmit and Limited Transmit.		standard mechanisms, including Fast Retransmit and Limited Transmit.
	This version of Early Retransmit solves the precision issues		This version of Early Retransmit solves the precision issues
	discussed in the previous section. As noted previously, the cost is		discussed in the previous section. As noted previously, the cost is
	that the implementation will have to track segment boundaries to		that the implementation will have to track segment boundaries to
	form an understanding as to how many actual segments have been		form an understanding as to how many actual segments have been
	transmitted, but not acknowledged. This can be done by the sender		transmitted, but not acknowledged. This can be done by the sender
	tracking the boundaries of the three segments on the right side of		tracking the boundaries of the three segments on the right side of
	the current window (which involves tracking four sequence numbers in		the current window (which involves tracking four sequence numbers in
	TCP). This could be done by keeping a circular list of the segment		TCP). This could be done by keeping a circular list of the segment
	boundaries, for instance. Cumulative ACKs that do not fall within		boundaries, for instance. Cumulative ACKs that do not fall within
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