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Versions: 00 01 02 03 04 RFC 5827
Internet Engineering Task Force Mark Allman
INTERNET DRAFT ICSI
File: draft-ietf-tcpm-early-rexmt-04.txt Konstantin Avrachenkov
Intended Status: Experimental INRIA
Urtzi Ayesta
BCAM-IKERBASQUE and LAAS-CNRS
Josh Blanton
Ohio University
Per Hurtig
Karlstad University
January 2010
Expires: July 2010
Early Retransmit for TCP and SCTP
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with
the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on July 27, 2010.
Copyright Notice
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Abstract
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This document proposes a new mechanism for TCP and SCTP that can be
used to recover lost segments when a connection's congestion window
is small. The "Early Retransmit" mechanism allows the transport to
reduce, in certain special circumstances, the number of duplicate
acknowledgments required to trigger a fast retransmission. This
allows the transport to use fast retransmit to recover segment
losses that would otherwise require a lengthy retransmission
timeout.
Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
Many researchers have studied problems with TCP's loss recovery
[RFC793,RFC5681] when the congestion window is small and have
outlined possible mechanisms to mitigate these problems
[Mor97,BPS+98,Bal98,LK98,RFC3150,AA02]. SCTP's [RFC4960] loss
recovery and congestion control mechanisms are based on TCP and
therefore the same problems impact the performance of SCTP
connections. When the transport detects a missing segment, the
connection enters a loss recovery phase. There are several variants
of the loss recovery phase depending on the TCP implementation. TCP
can use slow start based recovery or Fast Recovery [RFC5681],
NewReno [RFC3782], and loss recovery based on selective
acknowledgments (SACKs) [RFC2018,FF96,RFC3517]. SCTP's loss
recovery is not as varied due to the built-in selective
acknowledgments.
All the above variants have two methods for invoking loss recovery.
First, if an acknowledgment (ACK) for a given segment is not
received in a certain amount of time a retransmission timer fires
and the segment is resent [RFC2988,RFC4960]. Second, the "Fast
Retransmit" algorithm resends a segment when three duplicate ACKs
arrive at the sender [Jac88,RFC5681]. Duplicate ACKs are triggered by
out-of-order arrivals at the receiver. However, because duplicate
ACKs from the receiver are triggered by both segment loss and
segment reordering in the network path, the sender waits for three
duplicate ACKs in an attempt to disambiguate segment loss from
segment reordering. When the congestion window is small it may not
be possible to generate the required number of duplicate ACKs to
trigger Fast Retransmit when a loss does happen.
Small congestion windows can occur in a number of situations, such
as:
(1) The connection is constrained by end-to-end congestion control
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when the connection's share of the path is small, the path has a
small bandwidth-delay product or the transport is ascertaining
the available bandwidth in the first few round-trip times of
slow start.
(2) The connection is "application limited" and has only a limited
amount of data to send. This can happen any time the
application does not produce enough data to fill the congestion
window. A particular case when all connections become
application limited is as the connection ends.
(3) The connection is limited by the receiver's advertised window.
The transport's retransmission timeout (RTO) is based on measured
round-trip times (RTT) between the sender and receiver, as specified
in [RFC2988] (for TCP) and [RFC4960] (for SCTP). To prevent
spurious retransmissions of segments that are only delayed and not
lost, the minimum RTO is conservatively chosen to be 1 second.
Therefore, it behooves TCP senders to detect and recover from as
many losses as possible without incurring a lengthy timeout during
which the connection remains idle. However, if not enough duplicate
ACKs arrive from the receiver, the Fast Retransmit algorithm is
never triggered---this situation occurs when the congestion window
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
instance, consider a congestion window of three segments worth of
data. If one segment is dropped by the network, then at most two
duplicate ACKs will arrive at the sender. Since three duplicate
ACKs are required to trigger Fast Retransmit, a timeout will be
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
web server are sent after the RTO timer expires, while only 44% are
handled by Fast Retransmit. In addition, only 4% of the RTO
timer-based retransmissions could have been avoided with SACK, which
has to continue to disambiguate reordering from genuine loss.
Furthermore, [All00] shows that for one particular web server the
median number of bytes carried by a connection is less than four
segments, indicating that more than half of the connections will be
forced to rely on the RTO timer to recover from any losses that
occur. Thus, loss recovery that does not rely on the conservative
RTO is likely to be beneficial for short TCP transfers.
The Limited Transmit mechanism introduced in [RFC3042] and currently
codified in [RFC5681] allows a TCP sender to transmit previously
unsent data upon the reception of each of the two duplicate ACKs
that precede a Fast Retransmit. SCTP [RFC4960] uses SACK
information to calculate the number of outstanding segments in the
network. Hence, when the first two duplicate ACKs arrive at the
sender they will indicate that data has left the network and allow
the sender to transmit new data (if available) similar to TCP's
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Limited Transmit algorithm. In the remainder of this document we
use "Limited Transmit" to include both TCP and SCTP mechanisms for
sending in response to the first two duplicate ACKs. By sending
these two new segments the sender is attempting to induce additional
duplicate ACKs (if appropriate) so that Fast Retransmit will be
triggered before the retransmission timeout expires. The
sender-side "Early Retransmit" mechanism outlined in this document
covers the case when previously unsent data is not available for
transmission (case (2) above) or cannot be transmitted due to an
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
The Early Retransmit algorithm calls for lowering the threshold for
triggering Fast Retransmit when the amount of outstanding data is
small and when no previously unsent data can be transmitted (such
that Limited Transmit could be used). Duplicate ACKs are triggered
by each arriving out-of-order segment. Therefore, Fast Retransmit
will not be invoked when there are less than four outstanding
segments (assuming only one segment loss in the window). However,
TCP and SCTP are not required to track the number of outstanding
segments, but rather the number of outstanding bytes or messages.
(Note, SCTP's message boundaries do not necessarily correspond to
segment boundaries.) Therefore, applying the intuitive notion of a
transport with less than four segments outstanding is more
complicated than it first appears. In section 2.1 we describe a
"byte-based" variant of Early Retransmit that attempts to roughly
map the number of outstanding bytes to a number of outstanding
segments that is then used when deciding whether to trigger Early
Retransmit. In section 2.2 we describe a "segment-based" variant
that represents a more precise algorithm for triggering Early
Retransmit. The precision comes at the cost of requiring additional
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
A TCP or SCTP sender MAY use byte-based Early Retransmit.
Upon the arrival of an ACK, a sender employing byte-based Early
Retransmit MUST use the following two conditions to determine when
an Early Retransmit is sent:
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(2.a) The amount of outstanding data (ownd)---data sent but not yet
acknowledged---is less than 4*SMSS bytes.
Note that in the byte-based variant of Early Retransmit
'ownd' is equivalent to 'FlightSize' defined in [RFC5681]. We
use different notation because 'ownd' is not consistent with
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
sender or the advertised receive window does not permit new
segments to be transmitted.
When the above two conditions hold and a TCP connection does not
support SACK the duplicate ACK threshold used to trigger a
retransmission MUST be reduced to:
ER_thresh = ceiling (ownd/SMSS) - 1 (1)
duplicate ACKs, where ownd is in terms of bytes. We call this
reduced ACK threshold enabling "Early Retransmission".
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
when "ownd - SMSS" bytes have been SACKed.
If either (or both) condition (2.a) or (2.b) does not hold, the
transport MUST NOT use Early Retransmit, but rather prefer the
standard mechanisms, including Fast Retransmit and Limited Transmit.
As noted above, the drawback of this byte-based variant is precision
[HB08]. We illustrate this with two examples:
+ Consider a non-SACK TCP sender that uses an SMSS of 1460 bytes
and transmits three segments each with 400 bytes of payload.
This is a case where Early Retransmit could aid loss recovery if
one segment is lost. However, in this case ER_thresh will
become zero, per equation (1), because the number of outstanding
bytes is a poor estimate of the number of outstanding segments.
A similar problem occurs for senders that employ SACK as the
expression "ownd - SMSS" will become negative.
+ Next, consider a non-SACK TCP sender that uses an SMSS of 1460
bytes and transmits 10 segments each with 400 bytes of payload.
In this case ER_thresh will be two, per equation (1). Thus,
even though there are enough segments outstanding to trigger
Fast Retransmit with the standard duplicate ACK threshold Early
Retransmit will be triggered. This could cause or exacerbate
performance problems caused by segment reordering in the network.
2.2 Segment-based Early Retransmit
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A TCP or SCTP sender MAY use segment-based Early Retransmit.
Upon the arrival of an ACK, a sender employing segment-based Early
Retransmit MUST use the following two conditions to determine when
an Early Retransmit is sent:
(3.a) The number of outstanding segments (oseg)---segments sent but
not yet acknowledged---is less than four.
(3.b) There is either no unsent data ready for transmission at the
sender or the advertised receive window does not permit new
segments to be transmitted.
When the above two conditions hold and a TCP connection does not
support SACK the duplicate ACK threshold used to trigger a
retransmission MUST be reduced to:
ER_thresh = oseg - 1 (2)
duplicate ACKs, where oseg represents the number of outstanding
segments. (We discuss tracking the number of outstanding segments
below.) We call this reduced ACK threshold enabling "Early
Retransmission".
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
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.
If either (or both) conditions (3.a) or (3.b) does not hold, the
transport MUST NOT use Early Retransmit, but rather prefer the
standard mechanisms, including Fast Retransmit and Limited Transmit.
This version of Early Retransmit solves the precision issues
discussed in the previous section. As noted previously, the cost is
that the implementation will have to track segment boundaries to
form an understanding as to how many actual segments have been
transmitted, but not acknowledged. This can be done by the sender
tracking the boundaries of the three segments on the right side of
the current window (which involves tracking four sequence numbers in
TCP). This could be done by keeping a circular list of the segment
boundaries, for instance. Cumulative ACKs that do not fall within
this region indicate that at least four segments are outstanding and
therefore Early Retransmit MUST NOT be used. When the outstanding
window becomes small enough that Early Retransmit can be invoked, a
full understanding of the number of outstanding segments will be
available from the four sequence numbers retained. (Note: the
implicit sequence number consumed by the TCP FIN can also included
in the tracking of segment boundaries.)
3 Discussion
In this section we discuss a number of issues surrounding the Early
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Retransmit algorithm.
3.1 SACK vs. non-SACK
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
loss (since SACKs are sent redundantly) and due to interactions with
the delayed ACK timer (SCTP does not have a non-SACK mode and
therefore naturally supports SACK-based Early Retransmit). Consider
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
may not delay the ACK, leading to two scenarios:
(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
previously unacknowledged). In this case Early Retransmit
without SACK will not prevent an RTO because no duplicate ACKs
will arrive. However, with SACK the ACK for S1 will also
include SACK information indicating that S3 has arrived at the
receiver. The sender can then invoke Early Retransmit on this
ACK because only one segment remains outstanding.
(B) The ACK for S1 is not delayed: In this case the arrival of S1
triggers an ACK of previously unacknowledged data. The arrival
of S3 triggers a duplicate ACK (because it is out-of-order).
Both ACKs will cover the same segment (S1). Therefore,
regardless of whether SACK is used Early Retransmit can be
performed by the sender (assuming no ACK loss).
3.2 Segment Reordering
Early Retransmit is less robust in the face of reordered segments
than when using the standard Fast Retransmit threshold. Research
shows that a general reduction in the number of duplicate ACKs
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
of three [Pax97]. However, this analysis did not include the
additional conditioning on the event that the ownd was smaller than
4 segments and that no new data was available for transmission.
A number of studies have shown that network reordering is not a rare
event across some network paths. Various measurement studies have
shown that reordering along most paths is negligible, but along
certain paths can be quite prevalent [Pax97,BPS99,BS02,Pir05].
Evaluating Early Retransmit in the face of real segment reordering is
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:
(1) Persistent reordering of segments coupled with an application
that does not constantly send data can result in large numbers
of needless retransmissions when using Early Retransmit. For
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instance, consider an application that sends data two segments
at a time, followed by an idle period when no data is queued for
delivery. If the network consistently reorders the two
segments, the sender will needlessly retransmit one out of every
two unique segments transmitted when using the above algorithm
(meaning that one-third of all segments sent are needless
retransmissions). However, this would only be a problem for
long-lived connections from applications that transmit in
spurts.
(2) Similar to the above, consider the case of 2 segment transfers
that always experience reordering. Just as in (1) above, one
out of every two unique data segments will be retransmitted
needlessly, therefore one-third of the traffic will be spurious.
Currently this document offers no suggestion on how to mitigate the
above problems. However, the worst cases are likely pathological
and part of the experiments that this document hopes to trigger
would involve better understanding of whether such theoretical worst
case scenarios are prevalent in the network and in general to
explore the tradeoff between spurious fast retransmits and the delay
imposed by the RTO. Appendix A does offer a survey of possible
mitigations that call for curtailing the use of Early Retransmit
when it is making poor retransmission decisions.
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]
may benefit connections with small congestion window sizes
[RFC2884]. ECN provides a method for indicating congestion to the
end-host without dropping segments. While some segment drops may
still occur, ECN may allow a transport to perform better with small
congestion window sizes because the sender will be required to
detect less segment loss [RFC2884].
[Bal98] outlines another solution to the problem of having no new
segments to transmit into the network when the first two duplicate
ACKs arrive. In response to these duplicate ACKs, a TCP sender
transmits zero-byte segments to induce additional duplicate ACKs.
This method preserves the robustness of the standard Fast Retransmit
algorithm at the cost of injecting segments into the network that do
not deliver any data, and therefore are potentially wasting network
resources (at a time when there is a reasonable chance that the
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
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reordering.
5 Security Considerations
The security considerations found in [RFC5681] apply to this
document. No additional security problems have been identified with
Early Retransmit at this time.
6 IANA Considerations
None
Acknowledgments
We thank Sally Floyd for her feedback in discussions about Early
Retransmit. The notion of Early Transmit was originally sketched in
an Internet-Draft co-authored by Sally Floyd and Hari Balakrishnan.
Armando Caro, Joe Touch and Alexander Zimmermann and many members of
the TSVWG and TCPM working groups provided good discussions that
helped shape this document. Our thanks to all!
Normative References
[RFC793] Jon Postel. Transmission Control Protocol. Std 7, RFC
793. September 1981.
[RFC2018] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn Romanow.
TCP Selective Acknowledgement Options. RFC 2018, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2883] Sally Floyd, Jamshid Mahdavi, Matt Mathis, Matt Podolsky.
An Extension to the Selective Acknowledgement (SACK) Option for
TCP. RFC 2883, July 2000.
[RFC2988] Vern Paxson, Mark Allman. Computing TCP's Retransmission
Timer. RFC 2988, April 2000.
[RFC3042] Mark Allman, Hari Balakrishnan, Sally Floyd. Enhancing
TCP's Loss Recovery Using Limited Transmit. RFC 3042, January
2001.
[RFC4960] R. Stewart. Stream Control Transmission Protocol. RFC
4960, September 2007.
[RFC5681] Mark Allman, Vern Paxson, Ethan Blanton. TCP Congestion
Control. RFC 5681, May 2009.
Informative References
[AA02] Urtzi Ayesta, Konstantin Avrachenkov, "The Effect of the
Initial Window Size and Limited Transmit Algorithm on the
Transient Behavior of TCP Transfers", In Proc. of the 15th ITC
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Internet Specialist Seminar, Wurzburg, July 2002.
[All00] Mark Allman. A Web Server's View of the Transport Layer.
ACM Computer Communications Review, October 2000.
[Bal98] Hari Balakrishnan. Challenges to Reliable Data Transport
over Heterogeneous Wireless Networks. Ph.D. Thesis, University
of California at Berkeley, August 1998.
[BPS+98] Hari Balakrishnan, Venkata Padmanabhan, Srinivasan Seshan,
Mark Stemm, and Randy Katz. TCP Behavior of a Busy Web Server:
Analysis and Improvements. Proc. IEEE INFOCOM Conf., San
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,
ACM/USENIX Internet Measurement Workshop, November 2002.
[FF96] Kevin Fall, Sally Floyd. Simulation-based Comparisons of
Tahoe, Reno, and SACK TCP. ACM Computer Communication Review,
July 1996.
[Flo94] Sally Floyd. TCP and Explicit Congestion Notification. ACM
Computer Communication Review, October 1994.
[HB08] Per Hurtig, Anna Brunstrom. Enhancing SCTP Loss Recovery: An
Experimental Evaluation of Early Retransmit. Elsevier Computer
Communications, Vol. 31(16), October 2008, pp. 3778-3788.
[Jac88] Van Jacobson. Congestion Avoidance and Control. ACM
SIGCOMM 1988.
[LK98] Dong Lin, H.T. Kung. TCP Fast Recovery Strategies: Analysis
and Improvements. Proceedings of InfoCom, San Francisco, CA,
March 1998.
[Mor97] Robert Morris. TCP Behavior with Many Flows. Proceedings
of the Fifth IEEE International Conference on Network Protocols.
October 1997.
[Pax97] Vern Paxson. End-to-End Internet Packet Dynamics. ACM
SIGCOMM, September 1997.
[Pir05] N. M. Piratla, "A Theoretical Foundation, Metrics and
Modeling of Packet Reordering and Methodology of Delay Modeling
using Inter-packet Gaps," Ph.D. Dissertation, Department of
Electrical and Computer Engineering, Colorado State University,
Fort Collins, CO, Fall 2005.
[RFC2884] Jamal Hadi Salim and Uvaiz Ahmed. Performance Evaluation
of Explicit Congestion Notification (ECN) in IP Networks. RFC
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2884, July 2000.
[RFC3150] Spencer Dawkins, Gabriel Montenegro, Markku Kojo, Vincent
Magret. End-to-end Performance Implications of Slow Links. RFC
3150, July 2001.
[RFC3168] K. K. Ramakrishnan, Sally Floyd, David Black. The
Addition of Explicit Congestion Notification (ECN) to IP. RFC
3168, September 2001.
[RFC3517] Ethan Blanton, Mark Allman, Kevin Fall, Lili Wang. A
Conservative Selective Acknowledgment (SACK)-based Loss Recovery
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.
[RFC4653] Sumitha Bhandarkar, A. L. Narasimha Reddy, Mark Allman,
Ethan Blanton. Improving the Robustness of TCP to
Non-Congestion Events, August 2006. RFC 4653.
Author's Addresses:
Mark Allman
International Computer Science Institute
1947 Center Street, Suite 600
Berkeley, CA 94704-1198
Phone: 440-235-1792
mallman@icir.org
http://www.icir.org/mallman/
Konstantin Avrachenkov
INRIA
2004 route des Lucioles, B.P.93
06902, Sophia Antipolis
France
Phone: 00 33 492 38 7751
k.avrachenkov@sophia.inria.fr
http://www.inria.fr/mistral/personnel/K.Avrachenkov/moi.html
Urtzi Ayesta
LAAS-CNRS
7 Avenue Colonel Roche
31077 Toulouse
France
urtzi@laas.fr
http://www.laas.fr/~urtzi
Josh Blanton
Ohio University
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301 Stocker Center
Athens, OH 45701
jblanton@irg.cs.ohiou.edu
Per Hurtig
Karlstad University
Department of Computer Science
Universitetsgatan 2 651 88
Karlstad Sweden
per.hurtig@kau.se
Appendix A: Research Issues in Adjusting the Duplicate ACK Threshold
Decreasing the number of duplicate ACKs required to trigger Fast
Retransmit, as suggested in section 2, has the drawback of making
Fast Retransmit less robust in the face of minor network reordering.
Two egregious examples of problems caused by reordering are given in
section 3. This appendix outlines several schemes that have been
suggested to mitigate the problems caused by Early Retransmit in the
face of segment reordering. These methods need further research
before they are suggested for general use (and, current consensus is
that the cases that make Early Retransmit unnecessarily retransmit a
large amount of data are pathological and therefore these
mitigations are not generally required).
MITIGATION A.1: Allow a connection to use Early Retransmit as long
as the algorithm is not injecting "too much" spurious data into
the network. For instance, using the information provided by
TCP's DSACK option [RFC2883] or SCTP's Duplicate-TSN
notification, a sender can determine when segments sent via
Early Retransmit are needless. Likewise, using Eifel [RFC3522]
the sender can detect spurious Early Retransmits. Once spurious
Early Retransmits are detected the sender can either eliminate
the use of Early Retransmit or limit the use of the algorithm to
ensure that an acceptably small fraction of the connection's
transmissions are not spurious. For example, a connection could
stop using Early Retransmit after the first spurious retransmit
is detected.
MITIGATION A.2: If a sender cannot reliably determine if an Early
Retransmitted segment is spurious or not the sender could simply
limit Early Retransmits either to some fixed number per
connection (e.g., Early Retransmit is allowed only once per
connection) or to some small percentage of the total traffic
being transmitted.
MITIGATION A.3: Allow a connection to trigger Early Retransmit using
the criteria given in section 2, in addition to a "small"
timeout [Pax97]. For instance, a sender may have to wait for 2
duplicate ACKs and then T msec before Early Retransmit is
invoked. The added time gives reordered acknowledgments time to
arrive at the sender and avoid a needless retransmit. Designing
a method for choosing an appropriate timeout is part of the
research that would need to be involved in this scheme.
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