Internet DRAFT - draft-ietf-tcpm-rfc3782-bis
draft-ietf-tcpm-rfc3782-bis
TCP Maintenance and Minor T. Henderson
Extensions Working Group Boeing
Internet-Draft S. Floyd
Obsoletes: 3782 (if approved) ICSI
Intended status: Standards Track A. Gurtov
Expires: July 18, 2012 University of Oulu
Y. Nishida
WIDE Project
January 18, 2012
The NewReno Modification to TCP's Fast Recovery Algorithm
draft-ietf-tcpm-rfc3782-bis-05.txt
Abstract
RFC 5681 documents the following four intertwined TCP
congestion control algorithms: slow start, congestion avoidance, fast
retransmit, and fast recovery. RFC 5681 explicitly allows
certain modifications of these algorithms, including modifications
that use the TCP Selective Acknowledgement (SACK) option (RFC 2883),
and modifications that respond to "partial acknowledgments" (ACKs
which cover new data, but not all the data outstanding when loss was
detected) in the absence of SACK. This document describes a specific
algorithm for responding to partial acknowledgments, referred to as
NewReno. This response to partial acknowledgments was first proposed
by Janey Hoe. This document obsoletes RFC 3782.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 18, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as
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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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This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
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Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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1. Introduction
For the typical implementation of the TCP Fast Recovery algorithm
described in [RFC5681] (first implemented in the 1990 BSD Reno
release, and referred to as the Reno algorithm in [FF96]), the TCP
data sender only retransmits a packet after a retransmit timeout has
occurred, or after three duplicate acknowledgments have arrived
triggering the Fast Retransmit algorithm. A single retransmit
timeout might result in the retransmission of several data packets,
but each invocation of the Fast Retransmit algorithm in RFC 5681
leads to the retransmission of only a single data packet.
Two problems arise with Reno TCP when multiple packet losses occur
in a single window. First, Reno will often take a timeout, as
has been documented in [Hoe95]. Second, even if a retransmission
timeout is avoided, multiple fast retransmits and window reductions
can occur, as documented in [F94]. When multiple packet losses
occur, if the SACK option [RFC2883] is available, the TCP sender
has the information to make intelligent decisions about which packets
to retransmit and which packets not to retransmit during Fast
Recovery. This document applies to TCP connections that are
unable to use the TCP Selective Acknowledgement (SACK) option,
either because the option is not locally supported or
because the TCP peer did not indicate a willingness to use SACK.
In the absence of SACK, there is little information available to the
TCP sender in making retransmission decisions during Fast
Recovery. From the three duplicate acknowledgments, the sender
infers a packet loss, and retransmits the indicated packet. After
this, the data sender could receive additional duplicate
acknowledgments, as the data receiver acknowledges additional data
packets that were already in flight when the sender entered Fast
Retransmit.
In the case of multiple packets dropped from a single window of data,
the first new information available to the sender comes when the
sender receives an acknowledgment for the retransmitted packet (that
is, the packet retransmitted when Fast Retransmit was first
entered). If there is a single packet drop and no reordering, then
the acknowledgment for this packet will acknowledge all of the
packets transmitted before Fast Retransmit was entered. However, if
there are multiple packet drops, then the acknowledgment for the
retransmitted packet will acknowledge some but not all of the packets
transmitted before the Fast Retransmit. We call this acknowledgment
a partial acknowledgment.
Along with several other suggestions, [Hoe95] suggested that during
Fast Recovery the TCP data sender responds to a partial
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acknowledgment by inferring that the next in-sequence packet has been
lost, and retransmitting that packet. This document describes a
modification to the Fast Recovery algorithm in RFC 5681 that
incorporates a response to partial acknowledgments received during
Fast Recovery. We call this modified Fast Recovery algorithm
NewReno, because it is a slight but significant variation of the
basic Reno algorithm in RFC 5681. This document does not discuss the
other suggestions in [Hoe95] and [Hoe96], such as a change to the
ssthresh parameter during Slow-Start, or the proposal to send a new
packet for every two duplicate acknowledgments during Fast
Recovery. The version of NewReno in this document also draws on
other discussions of NewReno in the literature [LM97, Hen98].
We do not claim that the NewReno version of Fast Recovery described
here is an optimal modification of Fast Recovery for responding to
partial acknowledgments, for TCP connections that are unable to use
SACK. Based on our experiences with the NewReno modification in the
NS simulator [NS] and with numerous implementations of NewReno, we
believe that this modification improves the performance of the Fast
Retransmit and Fast Recovery algorithms in a wide variety of
scenarios. Previous versions of this RFC [RFC2582, RFC3782] provide
simulation-based evidence of the possible performance gains.
2. Terminology and Definitions
This document assumes that the reader is familiar with the terms
SENDER MAXIMUM SEGMENT SIZE (SMSS), CONGESTION WINDOW (cwnd), and
FLIGHT SIZE (FlightSize) defined in [RFC5681].
This document defines an additional sender-side state variable
called RECOVER:
RECOVER:
When in Fast Recovery, this variable records the send sequence
number that must be acknowledged before the Fast Recovery
procedure is declared to be over.
3. The Fast Retransmit and Fast Recovery Algorithms in NewReno
3.1. Protocol Overview
The basic idea of these extensions to the Fast Retransmit and
Fast Recovery algorithms described in Section 3.2 of [RFC5681]
is as follows. The TCP sender can infer, from the arrival of
duplicate acknowledgments, whether multiple losses in the same
window of data have most likely occurred, and avoid taking a
retransmit timeout or making multiple congestion window reductions
due to such an event.
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The NewReno modification applies to the Fast Recovery procedure that
begins when three duplicate ACKs are received and ends when either a
retransmission timeout occurs or an ACK arrives that acknowledges all
of the data up to and including the data that was outstanding when
the Fast Recovery procedure began.
3.2. Specification
The procedures specified in Section 3.2 of [RFC5681] are followed
with the following modifications. Note that this specification
avoids the use of the key words defined in RFC 2119 [RFC2119] since
it mainly provides sender-side implementation guidance for
performance improvement, and does not affect interoperability.
1) Initialization of TCP protocol control block:
When the TCP protocol control block is initialized, Recover is
set to the initial send sequence number.
2) Three duplicate ACKs:
When the third duplicate ACK is received, the TCP sender first
checks the value of Recover to see if the Cumulative
Acknowledgment field covers more than Recover. If so, the value
of Recover is incremented to the value of the highest sequence
number transmitted by the TCP so far. The TCP then enters Fast
Retransmit (step 2 of Section 3.2 of [RFC5681]). If not, the TCP
does not enter fast retransmit and does not reset ssthresh.
3) Response to newly acknowledged data:
Step 6 of [RFC5681] specifies the response to the next ACK that
acknowledges previously unacknowledged data. When an ACK
arrives that acknowledges new data, this ACK could be the
acknowledgment elicited by the retransmission from step 2, or
elicited by a later retransmission. There are two cases.
Full acknowledgments:
If this ACK acknowledges all of the data up to and including
Recover, then the ACK acknowledges all the intermediate
segments sent between the original transmission of the lost
segment and the receipt of the third duplicate ACK. Set cwnd to
either (1) min (ssthresh, max(FlightSize, SMSS) + SMSS) or
(2) ssthresh, where ssthresh is the value set when Fast
Retransmit was entered, and where FlightSize in (1) is the amount
of data presently outstanding. This is termed "deflating" the
window. If the second option is selected, the implementation
is encouraged to take measures to avoid a possible burst of
data, in case the amount of data outstanding in the network is
much less than the new congestion window allows. A simple
mechanism is to limit the number of data packets that can be sent
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in response to a single acknowledgment. Exit the Fast Recovery
procedure.
Partial acknowledgments:
If this ACK does *not* acknowledge all of the data up to and
including Recover, then this is a partial ACK. In this case,
retransmit the first unacknowledged segment. Deflate the
congestion window by the amount of new data acknowledged by the
cumulative acknowledgment field. If the partial ACK
acknowledges at least one SMSS of new data, then add back SMSS
bytes to the congestion window. This artificially
inflates the congestion window in order to reflect the additional
segment that has left the network. Send a new segment if
permitted by the new value of cwnd. This "partial window
deflation" attempts to ensure that, when Fast Recovery eventually
ends, approximately ssthresh amount of data will be outstanding
in the network. Do not exit the Fast Recovery procedure (i.e.,
if any duplicate ACKs subsequently arrive, execute Step 4 of
Section 3.2 of [RFC5681].
For the first partial ACK that arrives during Fast Recovery, also
reset the retransmit timer. Timer management is discussed in
more detail in Section 4.
4) Retransmit timeouts:
After a retransmit timeout, record the highest sequence number
transmitted in the variable Recover and exit the Fast
Recovery procedure if applicable.
Step 2 above specifies a check that the Cumulative Acknowledgment
field covers more than Recover. Because the acknowledgment field
contains the sequence number that the sender next expects to receive,
the acknowledgment "ack_number" covers more than Recover when:
ack_number - 1 > Recover;
i.e., at least one byte more of data is acknowledged beyond the
highest byte that was outstanding when Fast Retransmit was last
entered.
Note that in Step 3 above, the congestion window is deflated after
a partial acknowledgment is received. The congestion window was
likely to have been inflated considerably when the partial
acknowledgment was received. In addition, depending on the original
pattern of packet losses, the partial acknowledgment might
acknowledge nearly a window of data. In this case, if the congestion
window was not deflated, the data sender might be able to send nearly
a window of data back-to-back.
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This document does not specify the sender's response to duplicate
ACKs when the Fast Retransmit/Fast Recovery algorithm is not
invoked. This is addressed in other documents, such as those
describing the Limited Transmit procedure [RFC3042]. This document
also does not address issues of adjusting the duplicate
acknowledgment threshold, but assumes the threshold specified in
the IETF standards; the current standard is [RFC5681], which
specifies a threshold of three duplicate acknowledgments.
As a final note, we would observe that in the absence of the SACK
option, the data sender is working from limited information. When
the issue of recovery from multiple dropped packets from a single
window of data is of particular importance, the best alternative
would be to use the SACK option.
4. Handling Duplicate Acknowledgments After A Timeout
After each retransmit timeout, the highest sequence number
transmitted so far is recorded in the variable "recover".
If, after a retransmit timeout, the TCP data sender retransmits three
consecutive packets that have already been received by the data
receiver, then the TCP data sender will receive three duplicate
acknowledgments that do not cover more than "recover". In this
case, the duplicate acknowledgments are not an indication of a new
instance of congestion. They are simply an indication that the
sender has unnecessarily retransmitted at least three packets.
However, when a retransmitted packet is itself dropped, the sender
can also receive three duplicate acknowledgments that do not cover
more than "recover". In this case, the sender would have been
better off if it had initiated Fast Retransmit. For a TCP sender
that implements the algorithm specified in Section 3.2 of this
document, the sender does not infer a packet drop from duplicate
acknowledgments in this scenario. As always, the retransmit timer
is the backup mechanism for inferring packet loss in this case.
There are several heuristics, based on timestamps or on the amount of
advancement of the cumulative acknowledgment field, that allow the
sender to distinguish, in some cases, between three duplicate
acknowledgments following a retransmitted packet that was dropped,
and three duplicate acknowledgments from the unnecessary
retransmission of three packets [Gur03, GF04]. The TCP sender may
use such a heuristic to decide to invoke a Fast Retransmit in some
cases, even when the three duplicate acknowledgments do not cover
more than "recover".
For example, when three duplicate acknowledgments are caused by the
unnecessary retransmission of three packets, this is likely to be
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accompanied by the cumulative acknowledgment field advancing by at
least four segments. Similarly, a heuristic based on timestamps uses
the fact that when there is a hole in the sequence space, the
timestamp echoed in the duplicate acknowledgment is the timestamp of
the most recent data packet that advanced the cumulative
acknowledgment field [RFC1323]. If timestamps are used, and the
sender stores the timestamp of the last acknowledged segment, then
the timestamp echoed by duplicate acknowledgments can be used to
distinguish between a retransmitted packet that was dropped and
three duplicate acknowledgments from the unnecessary
retransmission of three packets.
4.1. ACK Heuristic
If the ACK-based heuristic is used, then following the advancement of
the cumulative acknowledgment field, the sender stores the value of
the previous cumulative acknowledgment as prev_highest_ack, and
stores the latest cumulative ACK as highest_ack. In addition, the
following check is performed if, in Step 2 of Section 3.2, the
Cumulative Acknowledgment field does not cover more than "recover".
1*) If the Cumulative Acknowledgment field didn't cover more than
"recover", check to see if the congestion window is greater
than SMSS bytes and the difference between highest_ack and
prev_highest_ack is at most 4*SMSS bytes. If true, duplicate
ACKs indicate a lost segment (enter Fast Retransmit).
Otherwise, duplicate ACKs likely result from unnecessary
retransmissions (do not enter Fast Retransmit).
The congestion window check serves to protect against fast retransmit
immediately after a retransmit timeout.
If several ACKs are lost, the sender can see a jump in the cumulative
ACK of more than three segments, and the heuristic can fail.
[RFC5681] recommends that a receiver should
send duplicate ACKs for every out-of-order data packet, such as a
data packet received during Fast Recovery. The ACK heuristic is more
likely to fail if the receiver does not follow this advice, because
then a smaller number of ACK losses are needed to produce a
sufficient jump in the cumulative ACK.
4.2. Timestamp Heuristic
If this heuristic is used, the sender stores the timestamp of the
last acknowledged segment. In addition, the last sentence of step
2 in Section 3.2 is replaced as follows:
1**) If the Cumulative Acknowledgment field didn't cover more than
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"recover", check to see if the echoed timestamp in the last
non-duplicate acknowledgment equals the
stored timestamp. If true, duplicate ACKs indicate a lost
segment (enter Fast Retransmit). Otherwise, duplicate
ACKs likely result from unnecessary retransmissions (do not
enter Fast Retransmit).
The timestamp heuristic works correctly, both when the receiver
echoes timestamps as specified by [RFC1323], and by its revision
attempts. However, if the receiver arbitrarily echoes timestamps,
the heuristic can fail. The heuristic can also fail if a timeout was
spurious and returning ACKs are not from retransmitted segments.
This can be prevented by detection algorithms such as [RFC3522].
5. Implementation Issues for the Data Receiver
[RFC5681] specifies that "Out-of-order data segments SHOULD be
acknowledged immediately, in order to accelerate loss recovery."
Neal Cardwell has noted that some data receivers do not send an
immediate acknowledgment when they send a partial acknowledgment,
but instead wait first for their delayed acknowledgment timer to
expire [C98]. As [C98] notes, this severely limits the potential
benefit of NewReno by delaying the receipt of the partial
acknowledgment at the data sender. Echoing [RFC5681], our
recommendation is that the data receiver send an immediate
acknowledgment for an out-of-order segment, even when that
out-of-order segment fills a hole in the buffer.
6. Implementation Issues for the Data Sender
In Section 3, Step 5 above, it is noted that implementations should
take measures to avoid a possible burst of data when leaving Fast
Recovery, in case the amount of new data that the sender is eligible
to send due to the new value of the congestion window is large. This
can arise during NewReno when ACKs are lost or treated as pure window
updates, thereby causing the sender to underestimate the number of
new segments that can be sent during the recovery procedure.
Specifically, bursts can occur when the FlightSize is much less than
the new congestion window when exiting from Fast Recovery. One
simple mechanism to avoid a burst of data when leaving Fast Recovery
is to limit the number of data packets that can be sent in response
to a single acknowledgment. (This is known as "maxburst_" in the ns
simulator.) Other possible mechanisms for avoiding bursts include
rate-based pacing, or setting the slow-start threshold to the
resultant congestion window and then resetting the congestion window
to FlightSize. A recommendation on the general mechanism to avoid
excessively bursty sending patterns is outside the scope of this
document.
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An implementation may want to use a separate flag to record whether
or not it is presently in the Fast Recovery procedure. The use of
the value of the duplicate acknowledgment counter for this purpose is
not reliable because it can be reset upon window updates and
out-of-order acknowledgments.
When updating the Cumulative Acknowledgment field outside of
Fast Recovery, the "recover" state variable may also need to be
updated in order to continue to permit possible entry into Fast
Recovery (Section 3, step 1). This issue arises when an update
of the Cumulative Acknowledgment field results in a sequence
wraparound that affects the ordering between the Cumulative
Acknowledgment field and the "recover" state variable. Entry
into Fast Recovery is only possible when the Cumulative
Acknowledgment field covers more than the "recover" state variable.
It is important for the sender to respond correctly to duplicate ACKs
received when the sender is no longer in Fast Recovery (e.g., because
of a Retransmit Timeout). The Limited Transmit procedure [RFC3042]
describes possible responses to the first and second duplicate
acknowledgments. When three or more duplicate acknowledgments are
received, the Cumulative Acknowledgment field doesn't cover more
than "recover", and a new Fast Recovery is not invoked, it is
important that the sender not execute the Fast Recovery steps (3) and
(4) in Section 3. Otherwise, the sender could end up in a chain of
spurious timeouts. We mention this only because several NewReno
implementations had this bug, including the implementation in the NS
simulator.
It has been observed that some TCP implementations enter a slow start
or congestion avoidance window updating algorithm immediately after
the cwnd is set by the equation found in (Section 3, step 5), even
without a new external event generating the cwnd change. Note that
after cwnd is set based on the procedure for exiting Fast Recovery
(Section 3, step 5), cwnd should not be updated until a further
event occurs (e.g., arrival of an ack, or timeout) after this
adjustment.
7. Security Considerations
[RFC5681] discusses general security considerations concerning TCP
congestion control. This document describes a specific algorithm
that conforms with the congestion control requirements of [RFC5681],
and so those considerations apply to this algorithm, too. There are
no known additional security concerns for this specific algorithm.
8. IANA Considerations
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This document has no actions for IANA.
9. Conclusions
This document specifies the NewReno Fast Retransmit and Fast Recovery
algorithms for TCP. This NewReno modification to TCP can even be
important for TCP implementations that support the SACK option,
because the SACK option can only be used for TCP connections when
both TCP end-nodes support the SACK option. NewReno performs better
than Reno (RFC5681) in a number of scenarios discussed in
previous versions of this RFC ([RFC2582], [RFC3782]).
A number of options to the basic algorithm presented in Section 3 are
also referenced in Appendix A to this document. These include the
handling of the retransmission timer, the response to partial
acknowledgments, and whether or not the sender must maintain a state
variable called Recover. Our belief is that the differences
between these variants of NewReno are small compared to the
differences between Reno and NewReno. That is, the important thing
is to implement NewReno instead of Reno, for a TCP connection
without SACK; it is less important exactly which of the variants of
NewReno is implemented.
10. Acknowledgments
Many thanks to Anil Agarwal, Mark Allman, Armando Caro, Jeffrey Hsu,
Vern Paxson, Kacheong Poon, Keyur Shah, and Bernie Volz for detailed
feedback on this document or on its precursor, RFC 2582. Jeffrey
Hsu provided clarifications on the handling of the recover variable
that were applied to RFC 3782 as errata, and now are in Section 8
of this document. Yoshifumi Nishida contributed a modification
to the fast recovery algorithm to account for the case in which
flightsize is 0 when the TCP sender leaves fast recovery, and the
TCP receiver uses delayed acknowledgments. Alexander Zimmermann
provided several suggestions to improve the clarity of the document.
11. References
11.1. Normative References
[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
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[C98] Cardwell, N., "delayed ACKs for retransmitted packets:
ouch!". November 1998, Email to the tcpimpl mailing list,
Message-ID
"Pine.LNX.4.02A.9811021421340.26785-100000@sake.cs.
washington.edu",
archived at "http://tcp-impl.lerc.nasa.gov/tcp-impl".
[FF96] Fall, K. and S. Floyd, "Simulation-based Comparisons of
Tahoe, Reno and SACK TCP", Computer Communication Review,
July 1996.
URL "ftp://ftp.ee.lbl.gov/papers/sacks.ps.Z".
[F94] Floyd, S., "TCP and Successive Fast Retransmits", Technical
report, October 1994. URL
"ftp://ftp.ee.lbl.gov/papers/fastretrans.ps".
[GF04] Gurtov, A. and S. Floyd, "Resolving Acknowledgment
Ambiguity in non-SACK TCP", Next Generation Teletraffic and
Wired/Wireless Advanced Networking (NEW2AN'04), February
2004. URL "http://www.cs.helsinki.fi/u/gurtov/papers/
heuristics.html".
[Gur03] Gurtov, A., "[Tsvwg] resolving the problem of unnecessary
fast retransmits in go-back-N", email to the tsvwg mailing
list, message ID <3F25B467.9020609@cs.helsinki.fi>,
July 28, 2003. URL "http://www1.ietf.org/mail-archive/
working-groups/tsvwg/current/msg04334.html".
[Hen98] Henderson, T., Re: NewReno and the 2001 Revision. September
1998. Email to the tcpimpl mailing list, Message ID
"Pine.BSI.3.95.980923224136.26134A-100000@raptor.CS.
Berkeley.EDU",
archived at "http://tcp-impl.lerc.nasa.gov/tcp-impl".
[Hoe95] Hoe, J., "Startup Dynamics of TCP's Congestion Control and
Avoidance Schemes", Master's Thesis, MIT, 1995.
[Hoe96] Hoe, J., "Improving the Start-up Behavior of a Congestion
Control Scheme for TCP", ACM SIGCOMM, August 1996. URL
"http://www.acm.org/sigcomm/sigcomm96/program.html".
[LM97] Lin, D. and R. Morris, "Dynamics of Random Early
Detection", SIGCOMM 97, September 1997. URL
"http://www.acm.org/sigcomm/sigcomm97/program.html".
[NS] The Network Simulator (NS).
URL "http://www.isi.edu/nsnam/ns/".
[RFC1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.
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[RFC2582] Floyd, S. and T. Henderson, "The NewReno Modification to
TCP's Fast Recovery Algorithm", RFC 2582, April 1999.
[RFC2883] Floyd, S., J. Mahdavi, M. Mathis, and M. Podolsky, "The
Selective Acknowledgment (SACK) Option for TCP, RFC 2883,
July 2000.
[RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's
Loss Recovery Using Limited Transmit", RFC 3042,
January 2001.
[RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
TCP", RFC 3522, April 2003.
[RFC3782] Floyd, S., T. Henderson, and A. Gurtov, "The NewReno
Modification to TCP's Fast Recovery Algorithm", RFC 3782,
April 2004.
Appendix A. Additional Information
Previous versions of this RFC ([RFC2582], [RFC3782]) contained
additional informative material on the following subjects, and
may be consulted by readers who may want more information about
possible variants to the algorithm and who may want references
to specific [NS] simulations that provide NewReno test cases.
Section 4 of [RFC3782] discusses some alternative behaviors for
resetting the retransmit timer after a partial acknowledgment.
Section 5 of [RFC3782] discusses some alternative behaviors for
performing retransmission after a partial acknowledgment.
Section 6 of [RFC3782] describes more information about the
motivation for the sender's state variable Recover.
Section 9 of [RFC3782] introduces some NS simulation test
suites for NewReno. In addition, references to simulation
results can be found throughout [RFC3782].
Section 10 of [RFC3782] provides a comparison of Reno and
NewReno TCP.
Section 11 of [RFC3782] listed changes relative to [RFC2582].
Appendix B. Changes Relative to RFC 3782
In [RFC3782], the cwnd after Full ACK reception will be set to
(1) min (ssthresh, FlightSize + SMSS) or (2) ssthresh. However,
there is a risk in the first option which results in performance
degradation. With the first option, if FlightSize is zero, the
result will be 1 SMSS. This means TCP can transmit only 1 segment
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at this moment, which can cause delay in ACK transmission at receiver
due to delayed ACK algorithm.
The FlightSize on Full ACK reception can be zero in some situations.
A typical example is where sending window size during fast recovery
is small. In this case, the retransmitted packet and new data packets
can be transmitted within a short interval. If all these packets
successfully arrive, the receiver may generate a Full ACK that
acknowledges all outstanding data. Even if window size is not small,
loss of ACK packets or receive buffer shortage during fast recovery
can also increase the possibility of falling into this situation.
The proposed fix in this document, which sets cwnd to at least 2*SMSS
if the implementation uses option 1 in the Full ACK case (Section 3.2
step 3, option 1), ensures that the sender TCP transmits at least two
segments on Full ACK reception.
In addition, errata for RFC3782 (editorial clarification to Section 8
of RFC2582, which is now Section 6 of this document) has been
applied.
The specification text (Section 3.2 herein) was rewritten to more
closely track Section 3.2 of [RFC5681].
Sections 4, 5, 9-11 of [RFC3782] were removed, and instead Appendix
A of this document was added to back-reference this informative
material. A few references that have no citation in the main body
of the draft have been removed.
Appendix C. Document Revision History
To be removed upon publication
+----------+--------------------------------------------------+
| Revision | Comments |
+----------+--------------------------------------------------+
| draft-00 | RFC3782 errata applied, and changes applied from |
| | draft-nishida-newreno-modification-02 |
+----------+--------------------------------------------------+
| draft-01 | Non-normative sections moved to appendices, |
| | editorial clarifications applied as suggested |
| | by Alexander Zimmermann. |
+----------+--------------------------------------------------+
| draft-02 | Better align specification text with RFC5681. |
| | Replace informative appendices by a new appendix |
| | that just provides back-references to earlier |
| | NewReno RFCs. |
+----------+--------------------------------------------------+
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Internet-Draft TCP NewReno (RFC 3782bis) January 2012
| draft-03 | Document refresh and fix id-nits |
+----------+--------------------------------------------------+
| draft-04 | Address editorial comments received from secdir |
| | review (provided by Tom Yu). |
+----------+--------------------------------------------------+
| draft-05 | Address IESG review comments from David |
| | Harrington, and Gen-ART review comments from |
| | Ben Campbell. |
+----------+--------------------------------------------------+
Authors' Addresses
Tom Henderson
The Boeing Company
EMail: thomas.r.henderson@boeing.com
Sally Floyd
International Computer Science Institute
Phone: +1 (510) 666-2989
EMail: floyd@acm.org
URL: http://www.icir.org/floyd/
Andrei Gurtov
University of Oulu
Centre for Wireless Communications CWC
P.O. Box 4500
FI-90014 University of Oulu
Finland
EMail: gurtov@ee.oulu.fi
Yoshifumi Nishida
WIDE Project
Endo 5322
Fujisawa, Kanagawa 252-8520
Japan
Email: nishida@wide.ad.jp
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