rfc6675
Internet Engineering Task Force (IETF) E. Blanton
Request for Comments: 6675 Purdue University
Obsoletes: 3517 M. Allman
Category: Standards Track ICSI
ISSN: 2070-1721 L. Wang
Juniper Networks
I. Jarvinen
M. Kojo
University of Helsinki
Y. Nishida
WIDE Project
August 2012
A Conservative Loss Recovery Algorithm Based on
Selective Acknowledgment (SACK) for TCP
Abstract
This document presents a conservative loss recovery algorithm for TCP
that is based on the use of the selective acknowledgment (SACK) TCP
option. The algorithm presented in this document conforms to the
spirit of the current congestion control specification (RFC 5681),
but allows TCP senders to recover more effectively when multiple
segments are lost from a single flight of data. This document
obsoletes RFC 3517 and describes changes from it.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by
the Internet Engineering Steering Group (IESG). Further
information on Internet Standards is available in Section 2 of
RFC 5741.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6675.
Blanton, et al. Standards Track [Page 1]
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Copyright Notice
Copyright (c) 2012 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
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publication of this document. Please review these documents
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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.
1. Introduction
This document presents a conservative loss recovery algorithm for TCP
that is based on the use of the selective acknowledgment (SACK) TCP
option. While the TCP SACK option [RFC2018] is being steadily
deployed in the Internet [All00], there is evidence that hosts are
not using the SACK information when making retransmission and
congestion control decisions [PF01]. The goal of this document is to
outline one straightforward method for TCP implementations to use
SACK information to increase performance.
[RFC5681] allows advanced loss recovery algorithms to be used by TCP
[RFC793] provided that they follow the spirit of TCP's congestion
control algorithms [RFC5681] [RFC2914]. [RFC6582] outlines one such
advanced recovery algorithm called NewReno. This document outlines a
loss recovery algorithm that uses the SACK TCP option [RFC2018] to
enhance TCP's loss recovery. The algorithm outlined in this
document, heavily based on the algorithm detailed in [FF96], is a
conservative replacement of the fast recovery algorithm [Jac90]
[RFC5681]. The algorithm specified in this document is a
straightforward SACK-based loss recovery strategy that follows the
guidelines set in [RFC5681] and can safely be used in TCP
implementations. Alternate SACK-based loss recovery methods can be
used in TCP as implementers see fit (as long as the alternate
algorithms follow the guidelines provided in [RFC5681]). Please
note, however, that the SACK-based decisions in this document (such
as what segments are to be sent at what time) are largely decoupled
from the congestion control algorithms, and as such can be treated as
separate issues if so desired.
This document represents a revision of [RFC3517] to address several
situations that are not handled explicitly in that document. A
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summary of the changes between this document and [RFC3517] can be
found in Section 9.
2. Definitions
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 BCP 14, RFC 2119
[RFC2119].
The reader is expected to be familiar with the definitions given in
[RFC5681].
The reader is assumed to be familiar with selective acknowledgments
as specified in [RFC2018].
For the purposes of explaining the SACK-based loss recovery
algorithm, we define six variables that a TCP sender stores:
"HighACK" is the sequence number of the highest byte of data that
has been cumulatively ACKed at a given point.
"HighData" is the highest sequence number transmitted at a given
point.
"HighRxt" is the highest sequence number which has been
retransmitted during the current loss recovery phase.
"RescueRxt" is the highest sequence number which has been
optimistically retransmitted to prevent stalling of the ACK clock
when there is loss at the end of the window and no new data is
available for transmission.
"Pipe" is a sender's estimate of the number of bytes outstanding
in the network. This is used during recovery for limiting the
sender's sending rate. The pipe variable allows TCP to use
fundamentally different congestion control than the algorithm
specified in [RFC5681]. The congestion control algorithm using
the pipe estimate is often referred to as the "pipe algorithm".
"DupAcks" is the number of duplicate acknowledgments received
since the last cumulative acknowledgment.
For the purposes of this specification, we define a "duplicate
acknowledgment" as a segment that arrives carrying a SACK block that
identifies previously unacknowledged and un-SACKed octets between
HighACK and HighData. Note that an ACK which carries new SACK data
is counted as a duplicate acknowledgment under this definition even
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if it carries new data, changes the advertised window, or moves the
cumulative acknowledgment point, which is different from the
definition of duplicate acknowledgment in [RFC5681].
We define a variable "DupThresh" that holds the number of duplicate
acknowledgments required to trigger a retransmission. Per [RFC5681],
this threshold is defined to be 3 duplicate acknowledgments.
However, implementers should consult any updates to [RFC5681] to
determine the current value for DupThresh (or method for determining
its value).
Finally, a range of sequence numbers [A,B] is said to "cover"
sequence number S if A <= S <= B.
3. Keeping Track of SACK Information
For a TCP sender to implement the algorithm defined in the next
section, it must keep a data structure to store incoming selective
acknowledgment information on a per connection basis. Such a data
structure is commonly called the "scoreboard". The specifics of the
scoreboard data structure are out of scope for this document (as long
as the implementation can perform all functions required by this
specification).
Note that this document refers to keeping account of (marking)
individual octets of data transferred across a TCP connection. A
real-world implementation of the scoreboard would likely prefer to
manage this data as sequence number ranges. The algorithms presented
here allow this, but require the ability to mark arbitrary sequence
number ranges as having been selectively acknowledged.
Finally, note that the algorithm in this document assumes a sender
that is not keeping track of segment boundaries after transmitting a
segment. It is possible that there is a more refined and precise
algorithm available to a sender that keeps this extra state than the
algorithm presented herein; however, we leave this as future work.
4. Processing and Acting Upon SACK Information
This section describes a specific structure and control flow for
implementing the TCP behavior described by this standard. The
behavior is what is standardized, and this particular collection of
functions is the strongly recommended means of implementing that
behavior, though other approaches to achieving that behavior are
feasible.
The definition of Sender Maximum Segment Size (SMSS) used in this
section is provided in [RFC5681].
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For the purposes of the algorithm defined in this document, the
scoreboard SHOULD implement the following functions:
Update ():
Given the information provided in an ACK, each octet that is
cumulatively ACKed or SACKed should be marked accordingly in the
scoreboard data structure, and the total number of octets SACKed
should be recorded.
Note: SACK information is advisory and therefore SACKed data MUST
NOT be removed from the TCP's retransmission buffer until the data
is cumulatively acknowledged [RFC2018].
IsLost (SeqNum):
This routine returns whether the given sequence number is
considered to be lost. The routine returns true when either
DupThresh discontiguous SACKed sequences have arrived above
'SeqNum' or more than (DupThresh - 1) * SMSS bytes with sequence
numbers greater than 'SeqNum' have been SACKed. Otherwise, the
routine returns false.
SetPipe ():
This routine traverses the sequence space from HighACK to HighData
and MUST set the "pipe" variable to an estimate of the number of
octets that are currently in transit between the TCP sender and
the TCP receiver. After initializing pipe to zero, the following
steps are taken for each octet 'S1' in the sequence space between
HighACK and HighData that has not been SACKed:
(a) If IsLost (S1) returns false:
Pipe is incremented by 1 octet.
The effect of this condition is that pipe is incremented for
packets that have not been SACKed and have not been determined
to have been lost (i.e., those segments that are still assumed
to be in the network).
(b) If S1 <= HighRxt:
Pipe is incremented by 1 octet.
The effect of this condition is that pipe is incremented for
the retransmission of the octet.
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Note that octets retransmitted without being considered lost are
counted twice by the above mechanism.
NextSeg ():
This routine uses the scoreboard data structure maintained by the
Update() function to determine what to transmit based on the SACK
information that has arrived from the data receiver (and hence
been marked in the scoreboard). NextSeg () MUST return the
sequence number range of the next segment that is to be
transmitted, per the following rules:
(1) If there exists a smallest unSACKed sequence number 'S2' that
meets the following three criteria for determining loss, the
sequence range of one segment of up to SMSS octets starting
with S2 MUST be returned.
(1.a) S2 is greater than HighRxt.
(1.b) S2 is less than the highest octet covered by any
received SACK.
(1.c) IsLost (S2) returns true.
(2) If no sequence number 'S2' per rule (1) exists but there
exists available unsent data and the receiver's advertised
window allows, the sequence range of one segment of up to SMSS
octets of previously unsent data starting with sequence number
HighData+1 MUST be returned.
(3) If the conditions for rules (1) and (2) fail, but there exists
an unSACKed sequence number 'S3' that meets the criteria for
detecting loss given in steps (1.a) and (1.b) above
(specifically excluding step (1.c)), then one segment of up to
SMSS octets starting with S3 SHOULD be returned.
(4) If the conditions for (1), (2), and (3) fail, but there exists
outstanding unSACKed data, we provide the opportunity for a
single "rescue" retransmission per entry into loss recovery.
If HighACK is greater than RescueRxt (or RescueRxt is
undefined), then one segment of up to SMSS octets that MUST
include the highest outstanding unSACKed sequence number
SHOULD be returned, and RescueRxt set to RecoveryPoint.
HighRxt MUST NOT be updated.
Note that rules (3) and (4) are a sort of retransmission "last
resort". They allow for retransmission of sequence numbers
even when the sender has less certainty a segment has been
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lost than as with rule (1). Retransmitting segments via rule
(3) and (4) will help sustain the TCP's ACK clock and
therefore can potentially help avoid retransmission timeouts.
However, in sending these segments, the sender has two copies
of the same data considered to be in the network (and also in
the pipe estimate, in the case of (3)). When an ACK or SACK
arrives covering this retransmitted segment, the sender cannot
be sure exactly how much data left the network (one of the two
transmissions of the packet or both transmissions of the
packet). Therefore, the sender may underestimate pipe by
considering both segments to have left the network when it is
possible that only one of the two has.
(5) If the conditions for each of (1), (2), (3), and (4) are not
met, then NextSeg () MUST indicate failure, and no segment is
returned.
Note: The SACK-based loss recovery algorithm outlined in this
document requires more computational resources than previous TCP loss
recovery strategies. However, we believe the scoreboard data
structure can be implemented in a reasonably efficient manner (both
in terms of computation complexity and memory usage) in most TCP
implementations.
5. Algorithm Details
Upon the receipt of any ACK containing SACK information, the
scoreboard MUST be updated via the Update () routine.
If the incoming ACK is a cumulative acknowledgment, the TCP MUST
reset DupAcks to zero.
If the incoming ACK is a duplicate acknowledgment per the definition
in Section 2 (regardless of its status as a cumulative
acknowledgment), and the TCP is not currently in loss recovery, the
TCP MUST increase DupAcks by one and take the following steps:
(1) If DupAcks >= DupThresh, go to step (4).
Note: This check covers the case when a TCP receives SACK
information for multiple segments smaller than SMSS, which can
potentially prevent IsLost() (next step) from declaring a segment
as lost.
(2) If DupAcks < DupThresh but IsLost (HighACK + 1) returns true --
indicating at least three segments have arrived above the current
cumulative acknowledgment point, which is taken to indicate loss
-- go to step (4).
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(3) The TCP MAY transmit previously unsent data segments as per
Limited Transmit [RFC5681], except that the number of octets
which may be sent is governed by pipe and cwnd as follows:
(3.1) Set HighRxt to HighACK.
(3.2) Run SetPipe ().
(3.3) If (cwnd - pipe) >= 1 SMSS, there exists previously unsent
data, and the receiver's advertised window allows, transmit
up to 1 SMSS of data starting with the octet HighData+1 and
update HighData to reflect this transmission, then return
to (3.2).
(3.4) Terminate processing of this ACK.
(4) Invoke fast retransmit and enter loss recovery as follows:
(4.1) RecoveryPoint = HighData
When the TCP sender receives a cumulative ACK for this data
octet, the loss recovery phase is terminated.
(4.2) ssthresh = cwnd = (FlightSize / 2)
The congestion window (cwnd) and slow start threshold
(ssthresh) are reduced to half of FlightSize per [RFC5681].
Additionally, note that [RFC5681] requires that any
segments sent as part of the Limited Transmit mechanism not
be counted in FlightSize for the purpose of the above
equation.
(4.3) Retransmit the first data segment presumed dropped -- the
segment starting with sequence number HighACK + 1. To
prevent repeated retransmission of the same data or a
premature rescue retransmission, set both HighRxt and
RescueRxt to the highest sequence number in the
retransmitted segment.
(4.4) Run SetPipe ()
Set a "pipe" variable to the number of outstanding octets
currently "in the pipe"; this is the data which has been
sent by the TCP sender but for which no cumulative or
selective acknowledgment has been received and the data has
not been determined to have been dropped in the network.
It is assumed that the data is still traversing the network
path.
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(4.5) In order to take advantage of potential additional
available cwnd, proceed to step (C) below.
Once a TCP is in the loss recovery phase, the following procedure
MUST be used for each arriving ACK:
(A) An incoming cumulative ACK for a sequence number greater than
RecoveryPoint signals the end of loss recovery, and the loss
recovery phase MUST be terminated. Any information contained in
the scoreboard for sequence numbers greater than the new value of
HighACK SHOULD NOT be cleared when leaving the loss recovery
phase.
(B) Upon receipt of an ACK that does not cover RecoveryPoint, the
following actions MUST be taken:
(B.1) Use Update () to record the new SACK information conveyed
by the incoming ACK.
(B.2) Use SetPipe () to re-calculate the number of octets still
in the network.
(C) If cwnd - pipe >= 1 SMSS, the sender SHOULD transmit one or more
segments as follows:
(C.1) The scoreboard MUST be queried via NextSeg () for the
sequence number range of the next segment to transmit (if
any), and the given segment sent. If NextSeg () returns
failure (no data to send), return without sending anything
(i.e., terminate steps C.1 -- C.5).
(C.2) If any of the data octets sent in (C.1) are below HighData,
HighRxt MUST be set to the highest sequence number of the
retransmitted segment unless NextSeg () rule (4) was
invoked for this retransmission.
(C.3) If any of the data octets sent in (C.1) are above HighData,
HighData must be updated to reflect the transmission of
previously unsent data.
(C.4) The estimate of the amount of data outstanding in the
network must be updated by incrementing pipe by the number
of octets transmitted in (C.1).
(C.5) If cwnd - pipe >= 1 SMSS, return to (C.1)
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Note that steps (A) and (C) can potentially send a burst of
back-to-back segments into the network if the incoming cumulative
acknowledgment is for more than SMSS octets of data, or if incoming
SACK blocks indicate that more than SMSS octets of data have been
lost in the second half of the window.
5.1. Retransmission Timeouts
In order to avoid memory deadlocks, the TCP receiver is allowed to
discard data that has already been selectively acknowledged. As a
result, [RFC2018] suggests that a TCP sender SHOULD expunge the SACK
information gathered from a receiver upon a retransmission timeout
(RTO) "since the timeout might indicate that the data receiver has
reneged." Additionally, a TCP sender MUST "ignore prior SACK
information in determining which data to retransmit." However, since
the publication of [RFC2018], this has come to be viewed by some as
too strong. It has been suggested that, as long as robust tests for
reneging are present, an implementation can retain and use SACK
information across a timeout event [Errata1610]. While this document
does not change the specification in [RFC2018], we note that
implementers should consult any updates to [RFC2018] on this subject.
Further, a SACK TCP sender SHOULD utilize all SACK information made
available during the loss recovery following an RTO.
If an RTO occurs during loss recovery as specified in this document,
RecoveryPoint MUST be set to HighData. Further, the new value of
RecoveryPoint MUST be preserved and the loss recovery algorithm
outlined in this document MUST be terminated. In addition, a new
recovery phase (as described in Section 5) MUST NOT be initiated
until HighACK is greater than or equal to the new value of
RecoveryPoint.
As described in Sections 4 and 5, Update () SHOULD continue to be
used appropriately upon receipt of ACKs. This will allow the
recovery period after an RTO to benefit from all available
information provided by the receiver, even if SACK information was
expunged due to the RTO.
If there are segments missing from the receiver's buffer following
processing of the retransmitted segment, the corresponding ACK will
contain SACK information. In this case, a TCP sender SHOULD use this
SACK information when determining what data should be sent in each
segment following an RTO. The exact algorithm for this selection is
not specified in this document (specifically NextSeg () is
inappropriate during loss recovery after an RTO). A relatively
straightforward approach to "filling in" the sequence space reported
as missing should be a reasonable approach.
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6. Managing the RTO Timer
The standard TCP RTO estimator is defined in [RFC6298]. Due to the
fact that the SACK algorithm in this document can have an impact on
the behavior of the estimator, implementers may wish to consider how
the timer is managed. [RFC6298] calls for the RTO timer to be
re-armed each time an ACK arrives that advances the cumulative ACK
point. Because the algorithm presented in this document can keep the
ACK clock going through a fairly significant loss event
(comparatively longer than the algorithm described in [RFC5681]), on
some networks the loss event could last longer than the RTO. In this
case the RTO timer would expire prematurely and a segment that need
not be retransmitted would be resent.
Therefore, we give implementers the latitude to use the standard
[RFC6298]-style RTO management or, optionally, a more careful variant
that re-arms the RTO timer on each retransmission that is sent during
recovery MAY be used. This provides a more conservative timer than
specified in [RFC6298], and so may not always be an attractive
alternative. However, in some cases it may prevent needless
retransmissions, go-back-N transmission, and further reduction of the
congestion window.
7. Research
The algorithm specified in this document is analyzed in [FF96], which
shows that the above algorithm is effective in reducing transfer time
over standard TCP Reno [RFC5681] when multiple segments are dropped
from a window of data (especially as the number of drops increases).
[AHKO97] shows that the algorithm defined in this document can
greatly improve throughput in connections traversing satellite
channels.
8. Security Considerations
The algorithm presented in this paper shares security considerations
with [RFC5681]. A key difference is that an algorithm based on SACKs
is more robust against attackers forging duplicate ACKs to force the
TCP sender to reduce cwnd. With SACKs, TCP senders have an
additional check on whether or not a particular ACK is legitimate.
While not fool-proof, SACK does provide some amount of protection in
this area.
Similarly, [CPNI309] sketches a variant of a blind attack [RFC5961]
whereby an attacker can spoof out-of-window data to a TCP endpoint,
causing it to respond to the legitimate peer with a duplicate
cumulative ACK, per [RFC793]. Adding a SACK-based requirement to
trigger loss recovery effectively mitigates this attack, as the
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duplicate ACKs caused by out-of-window segments will not contain SACK
information indicating reception of previously un-SACKED in-window
data.
9. Changes Relative to RFC 3517
The state variable "DupAcks" has been added to the list of variables
maintained by this algorithm, and its usage specified.
The function IsLost () has been modified to require that more than
(DupThresh - 1) * SMSS octets have been SACKed above a given sequence
number as indication that it is lost, which is changed from the
minimum requirement of (DupThresh * SMSS) described in [RFC3517].
This retains the requirement that at least three segments following
the sequence number in question have been SACKed, while improving
detection in the event that the sender has outstanding segments which
are smaller than SMSS.
The definition of a "duplicate acknowledgment" has been modified to
utilize the SACK information in detecting loss. Duplicate cumulative
acknowledgments can be caused by either loss or reordering in the
network. To disambiguate loss and reordering, TCP's fast retransmit
algorithm [RFC5681] waits until three duplicate ACKs arrive to
trigger loss recovery. This notion was then the basis for the
algorithm specified in [RFC3517]. However, with SACK information
there is no need to rely blindly on the cumulative acknowledgment
field. We can leverage the additional information present in the
SACK blocks to understand that three segments lying above a gap in
the sequence space have arrived at the receiver, and can use this
understanding to trigger loss recovery. This notion was used in
[RFC3517] during loss recovery, and the change in this document is
that the notion is also used to enter a loss recovery phase.
The state variable "RescueRxt" has been added to the list of
variables maintained by the algorithm, and its usage specified. This
variable is used to allow for one extra retransmission per entry into
loss recovery, in order to keep the ACK clock going under certain
circumstances involving loss at the end of the window. This
mechanism allows for no more than one segment of no larger than 1
SMSS to be optimistically retransmitted per loss recovery.
Rule (3) of NextSeg() has been changed from MAY to SHOULD, to
appropriately reflect the opinion of the authors and working group
that it should be left in, rather than out, if an implementor does
not have a compelling reason to do otherwise.
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10. Acknowledgments
The authors wish to thank Sally Floyd for encouraging [RFC3517] and
commenting on early drafts. The algorithm described in this document
is loosely based on an algorithm outlined by Kevin Fall and Sally
Floyd in [FF96], although the authors of this document assume
responsibility for any mistakes in the above text.
[RFC3517] was co-authored by Kevin Fall, who provided crucial input
to that document and hence this follow-on work.
Murali Bashyam, Ken Calvert, Tom Henderson, Reiner Ludwig, Jamshid
Mahdavi, Matt Mathis, Shawn Ostermann, Vern Paxson, and Venkat
Venkatsubra provided valuable feedback on earlier versions of this
document.
We thank Matt Mathis and Jamshid Mahdavi for implementing the
scoreboard in ns and hence guiding our thinking in keeping track of
SACK state.
The first author would like to thank Ohio University and the Ohio
University Internetworking Research Group for supporting the bulk of
his work on RFC 3517, from which this document is derived.
11. References
11.1. Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
11.2. Informative References
[AHKO97] Mark Allman, Chris Hayes, Hans Kruse, Shawn Ostermann,
"TCP Performance Over Satellite Links", Proceedings of the
Fifth International Conference on Telecommunications
Systems, Nashville, TN, March, 1997.
Blanton, et al. Standards Track [Page 13]
RFC 6675 SACK Loss Recovery Algorithm for TCP August 2012
[All00] Mark Allman, "A Web Server's View of the Transport Layer",
ACM Computer Communication Review, 30(5), October 2000.
[CPNI309] Fernando Gont, "Security Assessment of the Transmission
Control Protocol (TCP)", CPNI Technical Note 3/2009,
<http://www.gont.com.ar/papers/
tn-03-09-security-assessment-TCP.pdf>, February 2009.
[Errata1610]
RFC Errata, Errata ID 1610, RFC 2018,
<http://www.rfc-editor.org>.
[FF96] Kevin Fall and Sally Floyd, "Simulation-based Comparisons
of Tahoe, Reno and SACK TCP", Computer Communication
Review, July 1996.
[Jac90] Van Jacobson, "Modified TCP Congestion Avoidance
Algorithm", Technical Report, LBL, April 1990.
[PF01] Jitendra Padhye, Sally Floyd "Identifying the TCP Behavior
of Web Servers", ACM SIGCOMM, August 2001.
[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
NewReno Modification to TCP's Fast Recovery Algorithm",
RFC 6582, April 2012.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
2914, September 2000.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298, June
2011.
[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based Loss
Recovery Algorithm for TCP", RFC 3517, April 2003.
[RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961, August
2010.
Blanton, et al. Standards Track [Page 14]
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Authors' Addresses
Ethan Blanton
Purdue University Computer Sciences
305 N. University St.
West Lafayette, IN 47907
United States
EMail: elb@psg.com
Mark Allman
International Computer Science Institute
1947 Center St. Suite 600
Berkeley, CA 94704
United States
EMail: mallman@icir.org
http://www.icir.org/mallman
Lili Wang
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
United States
EMail: liliw@juniper.net
Ilpo Jarvinen
University of Helsinki
P.O. Box 68
FI-00014 UNIVERSITY OF HELSINKI
Finland
EMail: ilpo.jarvinen@helsinki.fi
Markku Kojo
University of Helsinki
P.O. Box 68
FI-00014 UNIVERSITY OF HELSINKI
Finland
EMail: kojo@cs.helsinki.fi
Yoshifumi Nishida
WIDE Project
Endo 5322
Fujisawa, Kanagawa 252-8520
Japan
EMail: nishida@wide.ad.jp
Blanton, et al. Standards Track [Page 15]
ERRATA
