draft-ietf-tcpm-early-rexmt-01.txt		draft-ietf-tcpm-early-rexmt-02.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-01.txt Konstantin Avrachenkov		File: draft-ietf-tcpm-early-rexmt-02.txt Konstantin Avrachenkov
	INRIA		Intended Status: Experimental INRIA
	Urtzi Ayesta		Urtzi Ayesta
	LAAS-CNRS		LAAS-CNRS
	Josh Blanton		Josh Blanton
	Ohio University		Ohio University
	Per Hurtig		Per Hurtig
	Karlstad University		Karlstad University
	January 2009		October 2009
	Expires: July 2009		Expires: April 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.
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	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
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	This Internet-Draft will expire on July 13, 2009.		This Internet-Draft will expire on April 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.
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	Abstract		Abstract
	This document proposes a new mechanism for TCP and SCTP that can be		This document proposes a new mechanism for TCP and SCTP that can be
	used to recover lost segments when a connection's congestion window		used to recover lost segments when a connection's congestion window
	is small. The "Early Retransmit" mechanism allows the transport to		is small. The "Early Retransmit" mechanism allows the transport to
	reduce, in certain special circumstances, the number of duplicate		reduce, in certain special circumstances, the number of duplicate
	acknowledgments required to trigger a fast retransmission. This		acknowledgments required to trigger a fast retransmission. This
	allows the transport to use fast retransmit to recover packet losses		allows the transport to use fast retransmit to recover segment
	that would otherwise require a lengthy retransmission timeout.		losses that would otherwise require a lengthy retransmission
			timeout.
	Terminology		Terminology
	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
			[RFC5681].
	1 Introduction		1 Introduction
	Many researchers have studied problems with TCP [RFC793,RFC2581]		Many researchers have studied problems with TCP [RFC793,RFC5681]
	when the congestion window is small and have outlined possible		when the congestion window is small and have outlined possible
	mechanisms to mitigate these problems		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 implemention. TCP		of the loss recovery phase depending on the TCP implemention. TCP
	can use slow start based recovery or Fast Recovery [RFC2581],		can use slow start based recovery or Fast Recovery [RFC5681],
	NewReno [RFC2582], 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
	acknowledgments.		acknowledgments.
	All the above variants have two methods for invoking loss recovery.		All the above variants have two methods for invoking loss recovery.
	First, if an acknowledgment (ACK) for a given segment is not		First, if an acknowledgment (ACK) for a given segment is not
	received in a certain amount of time a retransmission timer fires		received in a certain amount of time a retransmission timer fires
	and the segment is resent [RFC2988,RFC4960]. Second, the ``Fast		and the segment is resent [RFC2988,RFC4960]. Second, the "Fast
	Retransmit'' algorithm resends a segment when three duplicate ACKs		Retransmit" algorithm resends a segment when three duplicate ACKs
	arrive at the sender [Jac88,RFC2581]. Duplicate ACKs are triggered		arrive at the sender [Jac88,RFC5681]. Duplicate ACKs are triggered by
	by out-of-order arrivals at the receiver. However, because		out-of-order arrivals at the receiver. However, because duplicate
	duplicate ACKs from the receiver are triggered by both packet loss		ACKs from the receiver are triggered by both segment loss and
	and packet reordering in the network path, the sender waits for		segment reordering in the network path, the sender waits for three
	three duplicate ACKs in an attempt to disambiguate packet loss from		duplicate ACKs in an attempt to disambiguate segment loss from
	packet reordering. When using small congestion windows it may not		segment reordering. When the congestion window is small it may not
	be possible to generate the required number of duplicate ACKs to		be possible to generate the required number of duplicate ACKs to
	trigger Fast Retransmit when a loss does happen.		trigger Fast Retransmit when a loss does happen.
	Small windows can occur in a number of situations, such as:		Small congestion windows can occur in a number of situations, such
			as:
	(1) The connection is constrained by end-to-end congestion control		(1) The connection is constrained by end-to-end congestion control
	when the connection's share of the path is small, the path has a		when the connection's share of the path is small, the path has a
	small bandwidth-delay product or the transport is ascertaining		small bandwidth-delay product or the transport is ascertaining
	the available bandwidth in the first few round-trip times of		the available bandwidth in the first few round-trip times of
	slow start.		slow start.
	(2) The connection is "application limited" and has only a limited		(2) The connection is "application limited" and has only a limited
	amount of data to send. This can happen any time the		amount of data to send. This can happen any time the
	application does not produce enough data to fill the congestion		application does not produce enough data to fill the congestion
	skipping to change at page 3, line 21		skipping to change at page 3, line 29
	in [RFC2988] (for TCP) and [RFC4960] (for SCTP). To prevent		in [RFC2988] (for TCP) and [RFC4960] (for SCTP). To prevent
	spurious retransmissions of segments that are only delayed and not		spurious retransmissions of segments that are only delayed and not
	lost, the minimum RTO is conservatively chosen to be 1 second.		lost, the minimum RTO is conservatively chosen to be 1 second.
	Therefore, it behooves TCP senders to detect and recover from as		Therefore, it behooves TCP senders to detect and recover from as
	many losses as possible without incurring a lengthy timeout during		many losses as possible without incurring a lengthy timeout during
	which the connection remains idle. However, if not enough duplicate		which the connection remains idle. However, if not enough duplicate
	ACKs arrive from the receiver, the Fast Retransmit algorithm is		ACKs arrive from the receiver, the Fast Retransmit algorithm is
	never triggered---this situation occurs when the congestion window		never triggered---this situation occurs when the congestion window
	is small, if a large number of segments in a window are lost or at		is small, if a large number of segments in a window are lost or at
	the end of a transfer as data drains from the network. For		the end of a transfer as data drains from the network. For
	instance, consider a congestion window (cwnd) of three segments. If		instance, consider a congestion window of three segments worth of
	one segment is dropped by the network, then at most two duplicate		data. If one segment is dropped by the network, then at most two
	ACKs will arrive at the sender. Since three duplicate ACKs are		duplicate ACKs will arrive at the sender. Since three duplicate
	required to trigger Fast Retransmit, a timeout will be required to		ACKs are required to trigger Fast Retransmit, a timeout will be
	resend the dropped packet.		required to resend the dropped segment. Note, delayed ACKs
			[RFC5681] may further reduce the number of duplicate ACKs a receiver
			sends. However, we assume that receivers send immediate ACKs when
			there is a gap in the received sequence space per [RFC5681].
	[BPS+98] shows that roughly 56% of retransmissions sent by a busy		[BPS+98] shows that roughly 56% of retransmissions sent by a busy
	web server are sent after the RTO timer expires, while only 44% are		web server are sent after the RTO timer expires, while only 44% are
	handled by Fast Retransmit. In addition, only 4% of the RTO		handled by Fast Retransmit. In addition, only 4% of the RTO
	timer-based retransmissions could have been avoided with SACK, which		timer-based retransmissions could have been avoided with SACK, which
	has to continue to disambiguate reordering from genuine loss.		has to continue to disambiguate reordering from genuine loss.
	Furthermore, [All00] shows that for one particular web server the		Furthermore, [All00] shows that for one particular web server the
	median transfer size is less than four segments, indicating that		median number of bytes carried by a connection is less than four
	more than half of the connections will be forced to rely on the RTO		segments, indicating that more than half of the connections will be
	timer to recover from any losses that occur. Thus, loss recovery		forced to rely on the RTO timer to recover from any losses that
	that does not rely on the conservative RTO is likely to be		occur. Thus, loss recovery that does not rely on the conservative
	beneficial for short TCP transfers.		RTO is likely to be beneficial for short TCP transfers.
	The Limited Transmit mechanism introduced in [RFC3042] allows a TCP		The Limited Transmit mechanism introduced in [RFC3042] and currently
	sender to transmit previously unsent data upon the reception of each		codified in [RFC5681] allows a TCP sender to transmit previously
	of the two duplicate ACKs that precede a Fast Retransmit. SCTP		unsent data upon the reception of each of the two duplicate ACKs
	[RFC4960] uses SACK information to calculate the number of		that precede a Fast Retransmit. SCTP [RFC4960] uses SACK
	outstanding segments in the network. Hence, when the first two		information to calculate the number of outstanding segments in the
	duplicate ACKs arrive at the sender they will indicate that data has		network. Hence, when the first two duplicate ACKs arrive at the
	left the network and allow the sender to transmit new data (if		sender they will indicate that data has left the network and allow
	available) similar to TCP's Limited Transmit algorithm. In the		the sender to transmit new data (if available) similar to TCP's
	remainder of this document we use "Limited Transmit" to include both		Limited Transmit algorithm. In the remainder of this document we
	TCP and SCTP mechanisms for sending in response to the first two		use "Limited Transmit" to include both TCP and SCTP mechanisms for
	duplicate ACKs. By sending these two new segments the sender is		sending in response to the first two duplicate ACKs. By sending
	attempting to induce additional duplicate ACKs (if appropriate) so		these two new segments the sender is attempting to induce additional
	that Fast Retransmit will be triggered before the retransmission		duplicate ACKs (if appropriate) so that Fast Retransmit will be
	timeout expires. The "Early Retransmit" mechanism outlined in this		triggered before the retransmission timeout expires. The
	document covers the case when previously unsent data is not		sender-side "Early Retransmit" mechanism outlined in this document
	available for transmission or cannot be transmitted due to an		covers the case when previously unsent data is not available for
	advertised window limitation.		transmission (case (2) above) or cannot be transmitted due to an
			advertised window limitation (case (3) above).
	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
	segments, but rather the number of outstanding bytes or messages.		segments, but rather the number of outstanding bytes or messages.
	Therefore, applying the intuitive notion of a transport with less		(Note, SCTP's message boundaries do not necessarily correspond to
	than four segments outstanding is more complicated than it first		segment boundaries.) Therefore, applying the intuitive notion of a
	appears. In section 2.1 we describe a "byte-based" variant of Early		transport with less than four segments outstanding is more
	Retransmit that attempts to roughly map the number of outstanding		complicated than it first appears. In section 2.1 we describe a
	bytes to a number of outstanding packets that is then used when		"byte-based" variant of Early Retransmit that attempts to roughly
	deciding whether to trigger Early Retransmit. In section 2.2 we		map the number of outstanding bytes to a number of outstanding
	describe a "packet-based" variant that represents a more precise		segments that is then used when deciding whether to trigger Early
	algorithm for triggering Early Retransmit. The precision comes at		Retransmit. In section 2.2 we describe a "segment-based" variant
	the cost of requiring additional state to be kept by the TCP sender.		that represents a more precise algorithm for triggering Early
	In both cases we describe SACK-based and non-SACK-based versions of		Retransmit. The precision comes at the cost of requiring additional
	the scheme (of course, the non-SACK version will not apply to SCTP).		state to be kept by the TCP sender. In both cases we describe
			SACK-based and non-SACK-based versions of the scheme (of course, the
			non-SACK version will not apply to SCTP). This document explicitly
			does not prefer one variant over the other, but leaves the choice to
			the implementer.
	2.1 Byte-based Early Retransmit		2.1 Byte-based Early Retransmit
	A TCP or SCTP sender MAY use byte-based Early Retransmit.		A TCP or SCTP sender MAY use byte-based Early Retransmit.
	A sender employing byte-based Early Retransmit MUST use the		A sender employing byte-based Early Retransmit MUST use the
	following two conditions to determine when an Early Retransmit is		following two conditions to determine when an Early Retransmit is
	sent:		sent:
	(2.a) The amount of outstanding data (ownd)---data sent but not yet		(2.a) The amount of outstanding data (ownd)---data sent but not yet
	acknowledged---is less than 4*SMSS bytes.		acknowledged---is less than 4*SMSS bytes.
	(Note that in the byte-based variant of Early Retransmit		Note that in the byte-based variant of Early Retransmit
	'ownd' is equivalent to 'FlightSize' defined in [RFC2581]. We		'ownd' is equivalent to 'FlightSize' defined in [RFC5681]. We
	use different notation because 'ownd' is not consistent with		use different notation because 'ownd' is not consistent with
	FlightSize through this document.)		FlightSize through this document.
			Also note that in SCTP messages will have to be converted to
			bytes to make this variant of Early Retransmit work.
	(2.b) There is either no unsent data ready for transmission at the		(2.b) There is either no unsent data ready for transmission at the
	sender or the advertised window does not permit new segments		sender or the advertised receive window does not permit new
	to be transmitted.		segments to be transmitted.
	When the above two conditions hold and the connection does not		When the above two conditions hold and a TCP connection does not
	support SACK the duplicate ACK threshold used to trigger a		support SACK the duplicate ACK threshold used to trigger a
	retransmission MUST be reduced to:		retransmission MUST be reduced to:
	ER_thresh = ceiling (ownd/SMSS) - 1 (1)		ER_thresh = ceiling (ownd/SMSS) - 1 (1)
	duplicate ACKs, where ownd is in terms of bytes.		duplicate ACKs, where ownd is in terms of bytes. We call this
			reduced ACK threshold enabling "Early Retransimission".
	When conditions (2.a) and (2.b) hold and the connection does support		When conditions (2.a) and (2.b) hold and a TCP connection does
	SACK, Early Retransmit MUST be used only when "ownd - SMSS" bytes		support SACK or SCTP is in use, Early Retransmit MUST be used only
	have been SACKed.		when "ownd - SMSS" bytes have been SACKed.
	When conditions (2.a) and (2.b) do not hold, the transport MUST NOT		When conditions (2.a) and (2.b) do not hold, the transport MUST NOT
	use Early Retransmit, but rather prefer the standard mechanisms,		use Early Retransmit, but rather prefer the standard mechanisms,
	including Limited Transmit.		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 packets.		bytes is a poor estimate of the number of outstanding segments.
	A similar problem occurs for senders that employ SACK as the		A similar problem occurs for senders that employ SACK as the
	expression "ownd - SMSS" will become negative.		expression "ownd - SMSS" will become negative.
	+ Next, consider a non-SACK TCP sender that uses an SMSS of 1460		+ Next, consider a non-SACK TCP sender that uses an SMSS of 1460
	bytes and transmits 10 segments each with 400 bytes of payload.		bytes and transmits 10 segments each with 400 bytes of payload.
	In this case ER_thresh will be two, per equation (1). Thus,		In this case ER_thresh will be two, per equation (1). Thus,
	even though there are enough segments outstanding to trigger		even though there are enough segments outstanding to trigger
	Fast Retransmit with the standard duplicate ACK threshold Early		Fast Retransmit with the standard duplicate ACK threshold Early
	Retransmit will be triggered. This could cause or exacerbate		Retransmit will be triggered. This could cause or exacerbate
	performance problems caused by packet reordering in the network.		performance problems caused by segment reordering in the network.
	2.2 Packet-based Early Retransmit		2.2 Segment-based Early Retransmit
	A TCP or SCTP sender MAY use packet-based Early Retransmit.		A TCP or SCTP sender MAY use segment-based Early Retransmit.
	A sender employing packet-based Early Retransmit MUST use the		A sender employing segment-based Early Retransmit MUST use the
	following two conditions to determine when an Early Retransmit is		following two conditions to determine when an Early Retransmit is
	sent:		sent:
	(3.a) The number of outstanding segments (oseg)---segments sent but		(3.a) The number of outstanding segments (oseg)---segments sent but
	not yet acknowledged---is less than four.		not yet acknowledged---is less than four.
	(3.b) There is either no unsent data ready for transmission at the		(3.b) There is either no unsent data ready for transmission at the
	sender or the advertised window does not permit new segments		sender or the advertised receive window does not permit new
	to be transmitted.		segments to be transmitted.
	When the above two conditions hold and the connection does not		When the above two conditions hold and a TCP connection does not
	support SACK the duplicate ACK threshold used to trigger a		support SACK the duplicate ACK threshold used to trigger a
	retransmission MUST be reduced to:		retransmission MUST be reduced to:
	ER_thresh = oseg - 1 (2)		ER_thresh = oseg - 1 (2)
	duplicate ACKs, where oseg represents the number of outstanding		duplicate ACKs, where oseg represents the number of outstanding
	segments. (We discuss tracking the number of outstanding segments		segments. (We discuss tracking the number of outstanding segments
	below.)		below.) We call this reduced ACK threshold enabling "Early
			Retransimission".
	When conditions (3.a) and (3.b) hold and the connection does support		When conditions (3.a) and (3.b) hold and a TCP connection does
	SACK, Early Retransmit MUST be used only when "oseg - 1" segments		support SACK or SCTP is in use, Early Retransmit MUST be used only
	have been SACKed.		when "oseg - 1" segments have been SACKed. A segment is considered
			to be SACKed when all its data bytes (TCP) or data chunks (SCTP)
			have been indicated as arrived by the receiver.
	When conditions (3.a) and (3.b) do not hold, the transport MUST NOT		When conditions (3.a) and (3.b) do not hold, the transport MUST NOT
	use Early Retransmit, but rather prefer the standard mechanisms,		use Early Retransmit, but rather prefer the standard mechanisms,
	including Limited Transmit.		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 packet boundaries to form		that the implementation will have to track segment boundaries to
	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 tracking the		transmitted, but not acknowledged. This can be done by the sender
	boundaries of the three segments on the right side of the current		tracking the boundaries of the three segments on the right side of
	window (which involves tracking four sequence numbers in TCP). This		the current window (which involves tracking four sequence numbers in
	could be done by keeping a circular list of the packet boundaries,		TCP). This could be done by keeping a circular list of the segment
	for instance. Cumulative ACKs that do not fall within this region		boundaries, for instance. Cumulative ACKs that do not fall within
	indicate that at least four segments are outstanding and therefore		this region indicate that at least four segments are outstanding and
	Early Retransmit MUST NOT be used. When the outstanding window		therefore Early Retransmit MUST NOT be used. When the outstanding
	becomes small enough that Early Retransmit can be invoked, a full		window becomes small enough that Early Retransmit can be invoked, a
	understanding of the number of outstanding packets will be		full understanding of the number of outstanding segments will be
	available from the four sequence numbers retained.		available from the four sequence numbers retained.
	3 Discussion		3 Discussion
			In this section we discuss a number of issues surrounding the Early
			Retransmit algorithm.
			3.1 SACK vs. non-SACK
	The SACK variant of the Early Retransmit algorithm is preferred to		The SACK variant of the Early Retransmit algorithm is preferred to
	the non-SACK variant in TCP due to its robustness in the face of ACK		the non-SACK variant in TCP due to its robustness in the face of ACK
	loss (since SACKs are sent redundantly) and due to interactions with		loss (since SACKs are sent redundantly) and due to interactions with
	the delayed ACK timer (SCTP does not have a non-SACK mode and		the delayed ACK timer (SCTP does not have a non-SACK mode and
	therefore naturally supports SACK-based Early Retransmit). Consider		therefore naturally supports SACK-based Early Retransmit). Consider
	a flight of three segments, S1...S3, with S2 being dropped by the		a flight of three segments, S1...S3, with S2 being dropped by the
	network. When S1 arrives it is in-order and so the receiver may or		network. When S1 arrives it is in-order and so the receiver may or
	may not delay the ACK, leading to two scenarios:		may not delay the ACK, leading to two scenarios:
	(A) The ACK for S1 is delayed: In this case the arrival of S3 will		(A) The ACK for S1 is delayed: In this case the arrival of S3 will
	trigger an ACK to be transmitted covering segment S1 (which was		trigger an ACK to be transmitted covering segment S1 (which was
	previously unacknowledged). In this case Early Retransmit		previously unacknowledged). In this case Early Retransmit
	without SACK will not prevent an RTO because no duplicate ACKs		without SACK will not prevent an RTO because no duplicate ACKs
	will arrive. However, with SACK the ACK for S1 will also		will arrive. However, with SACK the ACK for S1 will also
	include SACK information indicating that S3 has arrived at the		include SACK information indicating that S3 has arrived at the
	receiver. The sender can then invoke Early Retransmit on this		receiver. The sender can then invoke Early Retransmit on this
	ACK because only one packet remains outstanding.		ACK because only one segment remains outstanding.
	(B) The ACK for S1 is not delayed: In this case the arrival of S1		(B) The ACK for S1 is not delayed: In this case the arrival of S1
	triggers an ACK of previously unacknowledged data. The arrival		triggers an ACK of previously unacknowledged data. The arrival
	of S3 triggers a duplicate ACK (because it is out-of-order).		of S3 triggers a duplicate ACK (because it is out-of-order).
	Both ACKs will cover the same segment (S1). Therefore,		Both ACKs will cover the same segment (S1). Therefore,
	regardless of whether SACK is used Early Retransmit can be		regardless of whether SACK is used Early Retransmit can be
	performed by the sender (assuming no ACK loss).		performed by the sender (assuming no ACK loss).
			3.2 Segment Reordering
	Early Retransmit is less robust in the face of reordered segments		Early Retransmit is less robust in the face of reordered segments
	than when using the standard Fast Retransmit threshold. Research		than when using the standard Fast Retransmit threshold. Research
	shows that a general reduction in the number of duplicate ACKs		shows that a general reduction in the number of duplicate ACKs
	required to trigger Fast Retransmit to two (rather than three) leads		required to trigger Fast Retransmit to two (rather than three) leads
	to a reduction in the ratio of good to bad retransmits by a factor		to a reduction in the ratio of good to bad retransmits by a factor
	of three [Pax97]. However, this analysis did not include the		of three [Pax97]. However, this analysis did not include the
	additional conditioning on the event that the ownd was smaller than		additional conditioning on the event that the ownd was smaller than
	4 segments and that no new data was available for transmission.		4 segments and that no new data was available for transmission.
	A number of studies have shown that network reordering is not a rare		A number of studies have shown that network reordering is not a rare
	event across some network paths. Various measurement studies have		event across some network paths. Various measurement studies have
	shown that reordering along most paths is negligible, but along		shown that reordering along most paths is negligible, but along
	certain paths can be quite prevalent [Pax97,BPS99,BS02,Pir05].		certain paths can be quite prevalent [Pax97,BPS99,BS02,Pir05].
	Evaluating Early Retransmit in the face of real packet reordering is		Evaluating Early Retransmit in the face of real segment reordering is
	part of the experiment we hope to instigate with this document.		part of the experiment we hope to instigate with this document.
			3.3 Worst Case
	Next, we note two "worst case" scenarios for Early Retransmit:		Next, we note two "worst case" scenarios for Early Retransmit:
	(1) Persistent reordering of segments coupled with an application		(1) Persistent reordering of segments coupled with an application
	that does not constantly send data can result in large numbers		that does not constantly send data can result in large numbers
	of needless retransmissions when using Early Retransmit. For		of needless retransmissions when using Early Retransmit. For
	instance, consider an application that sends data two segments		instance, consider an application that sends data two segments
	at a time, followed by an idle period when no data is queued for		at a time, followed by an idle period when no data is queued for
	delivery. If the network consistently reorders the two		delivery. If the network consistently reorders the two
	segments, the sender will needlessly retransmit one out of every		segments, the sender will needlessly retransmit one out of every
	two unique segments transmitted when using the above algorithm		two unique segments transmitted when using the above algorithm
	skipping to change at page 7, line 44		skipping to change at page 8, line 22
	and part of the experiments that this document hopes to trigger		and part of the experiments that this document hopes to trigger
	would involve better understanding of whether such theoretical worst		would involve better understanding of whether such theoretical worst
	case scenarios are prevalent in the network and in general to		case scenarios are prevalent in the network and in general to
	explore the tradeoff between spurious fast retransmits and the delay		explore the tradeoff between spurious fast retransmits and the delay
	imposed by the RTO. Appendix A does offer a survey of possible		imposed by the RTO. Appendix A does offer a survey of possible
	mitigations that call for curtailing the use of Early Retransmit		mitigations that call for curtailing the use of Early Retransmit
	when it is making poor retransmission decisions.		when it is making poor retransmission decisions.
	4 Related Work		4 Related Work
			There are a number of similar proposals in the literature that
			attempt to mitigate the same problem Early Retransmit addresses.
	Deployment of Explicit Congestion Notification (ECN) [Flo94,RFC3168]		Deployment of Explicit Congestion Notification (ECN) [Flo94,RFC3168]
	may benefit connections with small congestion window sizes		may benefit connections with small congestion window sizes
	[RFC2884]. ECN provides a method for indicating congestion to the		[RFC2884]. ECN provides a method for indicating congestion to the
	end-host without dropping segments. While some segment drops may		end-host without dropping segments. While some segment drops may
	still occur, ECN may allow a transport to perform better with small		still occur, ECN may allow a transport to perform better with small
	cwnd sizes because the sender will be required to detect less		congestion window sizes because the sender will be required to
	segment loss [RFC2884].		detect less segment loss [RFC2884].
	[Bal98] outlines another solution to the problem of having no new		[Bal98] outlines another solution to the problem of having no new
	segments to transmit into the network when the first two duplicate		segments to transmit into the network when the first two duplicate
	ACKs arrive. In response to these duplicate ACKs, a TCP sender		ACKs arrive. In response to these duplicate ACKs, a TCP sender
	transmits zero-byte segments to induce additional duplicate ACKs.		transmits zero-byte segments to induce additional duplicate ACKs.
	This method preserves the robustness of the standard Fast Retransmit		This method preserves the robustness of the standard Fast Retransmit
	algorithm at the cost of injecting segments into the network that do		algorithm at the cost of injecting segments into the network that do
	not deliver any data, and therefore are potentially wasting network		not deliver any data, and therefore are potentially wasting network
	resources (at a time when there is a reasonable chance that the		resources (at a time when there is a reasonable chance that the
	resources are scarce).		resources are scarce).
			[RFC4653] also defines an orthogonal method for altering the
			duplicate ACK threshold. The mechanisms proposed in this document
			decrease the duplicate ACK threshold when a small amount of data is
			outstanding. Meanwhile, the mechanisms in [RFC4653] increase the
			duplicate ACK threshold (over the standard of 3) when the congestion
			window is large in an effort to increase robustness to segment
			reordering.
	5 Security Considerations		5 Security Considerations
	The security considerations found in [RFC2581] apply to this		The security considerations found in [RFC5681] apply to this
	document. No additional security problems have been identified with		document. No additional security problems have been identified with
	Early Retransmit at this time.		Early Retransmit at this time.
			6 IANA Considerations
			None
	Acknowledgments		Acknowledgments
	We thank Sally Floyd for her feedback in discussions about Early		We thank Sally Floyd for her feedback in discussions about Early
	Retransmit. The notion of Early Transmit was originally sketched in		Retransmit. The notion of Early Transmit was originally sketched in
	an Internet-Draft co-authored by Sally Floyd and Hari Balakrishnan.		an Internet-Draft co-authored by Sally Floyd and Hari Balakrishnan.
	Armando Caro and many members of the TSVWG and TCPM working groups		Armando Caro, Joe Touch and Alexander Zimmermann and many members of
	provided good discussions that helped shape this document. Our		the TSVWG and TCPM working groups provided good discussions that
	thanks to all!		helped shape this document. Our thanks to all!
	Normative References		Normative References
	[RFC793] Jon Postel. Transmission Control Protocol. Std 7, RFC		[RFC793] Jon Postel. Transmission Control Protocol. Std 7, RFC
	793. September 1981.		793. September 1981.
	[RFC2018] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn Romanow.		[RFC2018] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn Romanow.
	TCP Selective Acknowledgement Options. RFC 2018, October 1996.		TCP Selective Acknowledgement Options. RFC 2018, October 1996.
	[RFC2581] Mark Allman, Vern Paxson, W. Richard Stevens. TCP		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Congestion Control. RFC 2581, April 1999.		Requirement Levels", BCP 14, RFC 2119, March 1997.
	[RFC2883] Sally Floyd, Jamshid Mahdavi, Matt Mathis, Matt Podolsky.		[RFC2883] Sally Floyd, Jamshid Mahdavi, Matt Mathis, Matt Podolsky.
	An Extension to the Selective Acknowledgement (SACK) Option for		An Extension to the Selective Acknowledgement (SACK) Option for
	TCP. RFC 2883, July 2000.		TCP. RFC 2883, July 2000.
	[RFC2988] Vern Paxson, Mark Allman. Computing TCP's Retransmission		[RFC2988] Vern Paxson, Mark Allman. Computing TCP's Retransmission
	Timer. RFC 2988, April 2000.		Timer. RFC 2988, April 2000.
	[RFC3042] Mark Allman, Hari Balakrishnan, Sally Floyd. Enhancing		[RFC3042] Mark Allman, Hari Balakrishnan, Sally Floyd. Enhancing
	TCP's Loss Recovery Using Limited Transmit. RFC 3042, January		TCP's Loss Recovery Using Limited Transmit. RFC 3042, January
	2001.		2001.
	[RFC3522] Reiner Ludwig, Michael Meyer. The Eifel Detection
	Algorithm for TCP. RFC 3522, April 2003.
	[RFC4960] R. Stewart. Stream Control Transmission Protocol. RFC		[RFC4960] R. Stewart. Stream Control Transmission Protocol. RFC
	4960, September 2007.		4960, September 2007.
			[RFC5681] Mark Allman, Vern Paxson, Ethan Blanton. TCP Congestion
			Control. RFC 5681, May 2009.
	Informative References		Informative References
	[AA02] Urtzi Ayesta, Konstantin Avrachenkov, "The Effect of the		[AA02] Urtzi Ayesta, Konstantin Avrachenkov, "The Effect of the
	Initial Window Size and Limited Transmit Algorithm on the		Initial Window Size and Limited Transmit Algorithm on the
	Transient Behavior of TCP Transfers", In Proc. of the 15th ITC		Transient Behavior of TCP Transfers", In Proc. of the 15th ITC
	Internet Specialist Seminar, Wurzburg, July 2002.		Internet Specialist Seminar, Wurzburg, July 2002.
	[All00] Mark Allman. A Web Server's View of the Transport Layer.		[All00] Mark Allman. A Web Server's View of the Transport Layer.
	ACM Computer Communications Review, October 2000.		ACM Computer Communications Review, October 2000.
	[Bal98] Hari Balakrishnan. Challenges to Reliable Data Transport		[Bal98] Hari Balakrishnan. Challenges to Reliable Data Transport
	over Heterogeneous Wireless Networks. Ph.D. Thesis, University		over Heterogeneous Wireless Networks. Ph.D. Thesis, University
	of California at Berkeley, August 1998.		of California at Berkeley, August 1998.
	[BPS+98] Hari Balakrishnan, Venkata Padmanabhan, Srinivasan Seshan,		[BPS+98] Hari Balakrishnan, Venkata Padmanabhan, Srinivasan Seshan,
	Mark Stemm, and Randy Katz. TCP Behavior of a Busy Web Server:		Mark Stemm, and Randy Katz. TCP Behavior of a Busy Web Server:
	Analysis and Improvements. Proc. IEEE INFOCOM Conf., San		Analysis and Improvements. Proc. IEEE INFOCOM Conf., San
	Francisco, CA, March 1998.		Francisco, CA, March 1998.
	skipping to change at page 9, line 16		skipping to change at page 10, line 8
	[Bal98] Hari Balakrishnan. Challenges to Reliable Data Transport		[Bal98] Hari Balakrishnan. Challenges to Reliable Data Transport
	over Heterogeneous Wireless Networks. Ph.D. Thesis, University		over Heterogeneous Wireless Networks. Ph.D. Thesis, University
	of California at Berkeley, August 1998.		of California at Berkeley, August 1998.
	[BPS+98] Hari Balakrishnan, Venkata Padmanabhan, Srinivasan Seshan,		[BPS+98] Hari Balakrishnan, Venkata Padmanabhan, Srinivasan Seshan,
	Mark Stemm, and Randy Katz. TCP Behavior of a Busy Web Server:		Mark Stemm, and Randy Katz. TCP Behavior of a Busy Web Server:
	Analysis and Improvements. Proc. IEEE INFOCOM Conf., San		Analysis and Improvements. Proc. IEEE INFOCOM Conf., San
	Francisco, CA, March 1998.		Francisco, CA, March 1998.
			[BPS99] Jon Bennett, Craig Partridge, Nicholas Shectman. Packet
			Reordering is Not Pathological Network Behavior. IEEE/ACM
			Transactions on Networking, December 1999.
	[BS02] John Bellardo, Stefan Savage. Measuring Packet Reordering,		[BS02] John Bellardo, Stefan Savage. Measuring Packet Reordering,
	ACM/USENIX Internet Measurement Workshop, November 2002.		ACM/USENIX Internet Measurement Workshop, November 2002.
	[FF96] Kevin Fall, Sally Floyd. Simulation-based Comparisons of		[FF96] Kevin Fall, Sally Floyd. Simulation-based Comparisons of
	Tahoe, Reno, and SACK TCP. ACM Computer Communication Review,		Tahoe, Reno, and SACK TCP. ACM Computer Communication Review,
	July 1996.		July 1996.
	[Flo94] Sally Floyd. TCP and Explicit Congestion Notification. ACM		[Flo94] Sally Floyd. TCP and Explicit Congestion Notification. ACM
	Computer Communication Review, October 1994.		Computer Communication Review, October 1994.
	[HB08] Per Hurtig, Anna Brunstrom. Enhancing SCTP Loss Recovery: An		[HB08] Per Hurtig, Anna Brunstrom. Enhancing SCTP Loss Recovery: An
	Experimental Evaluation of Early Retransmit. Elsevier Computer		Experimental Evaluation of Early Retransmit. Elsevier Computer
	Communication, 2008, to appear.		Communications, Vol. 31(16), October 2008, pp. 3778-3788.
	[Jac88] Van Jacobson. Congestion Avoidance and Control. ACM		[Jac88] Van Jacobson. Congestion Avoidance and Control. ACM
	SIGCOMM 1988.		SIGCOMM 1988.
	[LK98] Dong Lin, H.T. Kung. TCP Fast Recovery Strategies: Analysis		[LK98] Dong Lin, H.T. Kung. TCP Fast Recovery Strategies: Analysis
	and Improvements. Proceedings of InfoCom, San Francisco, CA,		and Improvements. Proceedings of InfoCom, San Francisco, CA,
	March 1998.		March 1998.
	[Mor97] Robert Morris. TCP Behavior with Many Flows. Proceedings		[Mor97] Robert Morris. TCP Behavior with Many Flows. Proceedings
	of the Fifth IEEE International Conference on Network Protocols.		of the Fifth IEEE International Conference on Network Protocols.
	skipping to change at page 9, line 50		skipping to change at page 10, line 46
	[Pax97] Vern Paxson. End-to-End Internet Packet Dynamics. ACM		[Pax97] Vern Paxson. End-to-End Internet Packet Dynamics. ACM
	SIGCOMM, September 1997.		SIGCOMM, September 1997.
	[Pir05] N. M. Piratla, "A Theoretical Foundation, Metrics and		[Pir05] N. M. Piratla, "A Theoretical Foundation, Metrics and
	Modeling of Packet Reordering and Methodology of Delay Modeling		Modeling of Packet Reordering and Methodology of Delay Modeling
	using Inter-packet Gaps," Ph.D. Dissertation, Department of		using Inter-packet Gaps," Ph.D. Dissertation, Department of
	Electrical and Computer Engineering, Colorado State University,		Electrical and Computer Engineering, Colorado State University,
	Fort Collins, CO, Fall 2005.		Fort Collins, CO, Fall 2005.
	[RFC2582] Sally Floyd, Tom Henderson. The NewReno Modification to
	TCP's Fast Recovery Algorithm. RFC 2582, April 1999.
	[RFC2884] Jamal Hadi Salim and Uvaiz Ahmed. Performance Evaluation		[RFC2884] Jamal Hadi Salim and Uvaiz Ahmed. Performance Evaluation
	of Explicit Congestion Notification (ECN) in IP Networks. RFC		of Explicit Congestion Notification (ECN) in IP Networks. RFC
	2884, July 2000.		2884, July 2000.
	[RFC3150] Spencer Dawkins, Gabriel Montenegro, Markku Kojo, Vincent		[RFC3150] Spencer Dawkins, Gabriel Montenegro, Markku Kojo, Vincent
	Magret. End-to-end Performance Implications of Slow Links. RFC		Magret. End-to-end Performance Implications of Slow Links. RFC
	3150, July 2001.		3150, July 2001.
	[RFC3168] K. K. Ramakrishnan, Sally Floyd, David Black. The		[RFC3168] K. K. Ramakrishnan, Sally Floyd, David Black. The
	Addition of Explicit Congestion Notification (ECN) to IP. RFC		Addition of Explicit Congestion Notification (ECN) to IP. RFC
	3168, September 2001.		3168, September 2001.
	[RFC3517] Ethan Blanton, Mark Allman, Kevin Fall, Lili Wang. A		[RFC3517] Ethan Blanton, Mark Allman, Kevin Fall, Lili Wang. A
	Conservative Selective Acknowledgment (SACK)-based Loss Recovery		Conservative Selective Acknowledgment (SACK)-based Loss Recovery
	Algorithm for TCP. RFC 3517, April 2003.		Algorithm for TCP. RFC 3517, April 2003.
			[RFC3522] Reiner Ludwig, Michael Meyer. The Eifel Detection
			Algorithm for TCP. RFC 3522, April 2003.
			[RFC3782] Sally Floyd, Tom Henderson, Andrei Gurtov. The NewReno
			Modification to TCP's Fast Recovery Algorithm. RFC 3782, April
			2004.
	Author's Addresses:		Author's Addresses:
	Mark Allman		Mark Allman
	International Computer Science Institute		International Computer Science Institute
	1947 Center Street, Suite 600		1947 Center Street, Suite 600
	Berkeley, CA 94704-1198		Berkeley, CA 94704-1198
	Phone: 440-235-1792		Phone: 440-235-1792
	mallman@icir.org		mallman@icir.org
	http://www.icir.org/mallman/		http://www.icir.org/mallman/
	skipping to change at page 11, line 8		skipping to change at page 12, line 9
	per.hurtig@kau.se		per.hurtig@kau.se
	Appendix A: Research Issues in Adjusting the Duplicate ACK Threshold		Appendix A: Research Issues in Adjusting the Duplicate ACK Threshold
	Decreasing the number of duplicate ACKs required to trigger Fast		Decreasing the number of duplicate ACKs required to trigger Fast
	Retransmit, as suggested in section 2, has the drawback of making		Retransmit, as suggested in section 2, has the drawback of making
	Fast Retransmit less robust in the face of minor network reordering.		Fast Retransmit less robust in the face of minor network reordering.
	Two egregious examples of problems caused by reordering are given in		Two egregious examples of problems caused by reordering are given in
	section 3. This appendix outlines several schemes that have been		section 3. This appendix outlines several schemes that have been
	suggested to mitigate the problems caused by Early Retransmit in the		suggested to mitigate the problems caused by Early Retransmit in the
	face of packet reordering. These methods need further research		face of segment reordering. These methods need further research
	before they are suggested for general use (and, current consensus is		before they are suggested for general use (and, current consensus is
	that the cases that make Early Retransmit unnecessarily retransmit a		that the cases that make Early Retransmit unnecessarily retransmit a
	large amount of data are pathological and therefore these		large amount of data are pathological and therefore these
	mitigations are not generally required).		mitigations are not generally required).
	MITIGATION A.1: Allow a connection to use Early Retransmit as long		MITIGATION A.1: Allow a connection to use Early Retransmit as long
	as the algorithm is not injecting "too much" spurious data into		as the algorithm is not injecting "too much" spurious data into
	the network. For instance, using the information provided by		the network. For instance, using the information provided by
	TCP's DSACK option [RFC2883] or SCTP's Duplicate-TSN		TCP's DSACK option [RFC2883] or SCTP's Duplicate-TSN
	notification, a sender can determine when segments sent via		notification, a sender can determine when segments sent via
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