Internet DRAFT - draft-ietf-tsvwg-limited-xmit
draft-ietf-tsvwg-limited-xmit
Internet Engineering Task Force Mark Allman
INTERNET DRAFT NASA GRC/BBN
File: draft-ietf-tsvwg-limited-xmit-00.txt Hari Balakrishnan
MIT
Sally Floyd
ACIRI
August, 2000
Expires: February, 2001
Enhancing TCP's Loss Recovery Using Limited Transmit
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
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
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The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at
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Abstract
This document proposes a new TCP mechanism that can be used to more
effectively recover lost segments when a connection's congestion
window is small, or when a large number of segments are lost in a
single transmission window. The ``Limited Transmit'' algorithm
calls for sending a new data segment in response to each of the
first two duplicate acknowledgments that arrive at the sender.
Transmitting these segments increases the probability that TCP can
recover from a single lost segment using the fast retransmit
algorithm, rather than using a costly retransmission timeout.
Limited Transmit can be used both in conjunction with, and in the
absence of, the TCP selective acknowledgment (SACK) mechanism
[RFC2018].
1 Introduction
A number of researchers have observed that TCP's loss recovery
strategies do not work well when the congestion window at a TCP
sender is small. This can happen, for instance, because there is
only a limited amount of data to send, or because of the limit
imposed by the receiver-advertised window, or because of the
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constraints imposed by end-to-end congestion control over a
connection with a small bandwidth-delay product
[Riz96,Mor97,BPS+98,Bal98,LK98,DMKM00]. When a TCP detects a
missing segment, it enters a loss recovery phase using one of two
methods. First, if an acknowledgment (ACK) for a given segment is
not received in a certain amount of time a retransmission timeout
occurs and the segment is resent [RFC793,PA00]. Second, the ``Fast
Retransmit'' algorithm resends a segment when three duplicate ACKs
arrive at the sender [Jac88,RFC2581]. However, because duplicate
ACKs from the receiver are also triggered by packet reordering in
the Internet, the TCP sender waits for three duplicate ACKs in an
attempt to disambiguate segment loss from packet reordering. Once
in a loss recovery phase, a number of techniques can be used to
retransmit lost segments, including slow start-based recovery or
Fast Recovery [RFC2581], NewReno [RFC2582], and loss recovery based
on selective acknowledgments (SACKs) [RFC2018,FF96].
TCP's retransmission timeout (RTO) is based on measured round-trip
times (RTT) between the sender and receiver, as specified in [PA00].
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
when 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 or if a large number of segments in a window are lost. For
instance, consider a congestion window (cwnd) of three segments. If
one segment is dropped by the network, then at most two duplicate
ACKs will arrive at the sender, assuming no ACK loss. Since three
duplicate ACKs are required to trigger Fast Retransmit, a timeout
will be required to resend the dropped packet.
[BPS+97] found that roughly 56% of retransmissions sent by a busy
web server were sent after the RTO expires, while only 44% were
handled by Fast Retransmit. In addition, only 4% of the RTO-based
retransmissions could have been avoided with SACK, which of course
has to continue to disambiguate reordering from genuine loss. In
contrast, using the technique outlined in this document and in
[Bal98], 25% of the RTO-based retransmissions in that dataset would
have likely been avoided.
The next section of this document outlines small changes to TCP
senders that will decrease the reliance on the retransmission timer,
and thereby improve TCP performance when Fast Retransmit is not
triggered. These changes do not adversely affect the performance of
TCP nor interact adversely with other connections, in other
circumstances.
2 The Limited Transmit Algorithm
When a TCP sender has previously unsent data queued for transmission
it SHOULD use the Limited Transmit algorithm, which calls for a TCP
sender to transmit new data upon the arrival of a duplicate ACK when
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the following conditions are satisfied:
* The receiver's advertised window allows the transmission of the
segment.
* The amount of outstanding data would remain less than the
congestion window plus 2 segments. In other words, the sender
can only send two segments beyond the congestion window (cwnd).
The congestion window (cwnd) MUST NOT be changed when these new
segments are transmitted. Assuming that these new segments and the
corresponding ACKs are not dropped, this procedure allows the sender
to infer loss using the standard Fast Retransmit threshold of three
duplicate ACKs [RFC2581]. This is more robust to reordered packets
than if an old packet were retransmitted on the first or second
duplicate ACK.
Note: If the connection is using selective acknowledgments
[RFC2018], the data sender MUST NOT send new segments in response to
duplicate ACKs that contain no new SACK information, as a
misbehaving receiver can generate such ACKs to trigger inappropriate
transmission of data segments. See [SCWA99] for a discussion of
attacks by misbehaving receivers.
Limited Transmit follows the ``conservation of packets'' congestion
control principle [Jac88]. Each of the first two duplicate ACKs
indicate that a segment has left the network. Furthermore, the
sender has not yet decided that a segment has been dropped and
therefore has no reason to assume that the current congestion
control state is inaccurate. Therefore, transmitting segments does
not deviate from the spirit of TCP's congestion control principles.
[BPS99] shows that packet reordering is not a rare network event.
[RFC2581] does not provide for sending of data on the first two
duplicate ACKs that arrive at the sender. This causes a burst of
segments to be sent when an ACK for new data does arrive following
packet reordering. Using Limited Transmit, data packets will be
clocked out by incoming ACKs and therefore transmission will not be
as bursty.
Note: Limited Transmit is implemented in the ns simulator [NS].
Researchers wishing to investigate this mechanism further can do so
by enabling ``singledup_'' for the given TCP connection.
3 Related Work
Deployment of Explicit Congestion Notification (ECN) [Flo94,RFC2481]
may benefit connections with small congestion window sizes [SA00].
ECN provides a method for indicating congestion to the end-host
without dropping segments. While some segment drops may still
occur, ECN may allow TCP to perform better with small congestion
window sizes because the sender will be required to detect less
segment loss [SA00].
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When ECN-enabled TCP traffic competes with non-ECN-enabled TCP
traffic, ECN-enabled traffic can receive up to 30% higher goodput.
For bulk transfers, the relative performance benefit of ECN is
greatest when on average each flow has 3-4 outstanding packets
during each roundtrip time [ZQ00]. This should be a good estimate
for the performance impact of a flow using Limited Transmit, since
both ECN and Limited Transmit reduce the reliance on the
retransmission timer for signaling congestion.
The Rate-Halving congestion control algorithm [MSML99] uses a form
of limited transmit, as it calls for transmitting a data segment on
every second duplicate ACK that arrives at the sender. The
algorithm decouples the decision of what to send from the decision
of when to send. However, similar to Limited Transmit the algorithm
will always send a new data segment on the second duplicate ACK that
arrives at the sender.
4 Security Considerations
The additional security implications of the changes proposed in this
document, compared to TCP's current vulnerabilities, are minimal.
The potential security issues come from the subversion of end-to-end
congestion control from "false" duplicate ACKs, where a "false"
duplicate ACK is a duplicate ACK that does not actually acknowledge
new data received at the TCP receiver. False duplicate ACKs could
result from duplicate ACKs that are themselves duplicated in the
network, or from misbehaving TCP receivers that send false duplicate
ACKs to subvert end-to-end congestion control [SCWA99,RFC2581].
When the TCP data receiver has agreed to use the SACK option, the
TCP data sender has fairly strong protection against false duplicate
ACKs. In particular, with SACK, a duplicate ACK that acknowledges
new data arriving at the receiver reports the sequence numbers of
that new data. Thus, with SACK, the TCP sender can verify that an
arriving duplicate ACK acknowledges data that the TCP sender has
actually sent, and for which no previous acknowledgment has been
received, before sending new data as a result of that
acknowledgment. For further protection, the TCP sender could keep a
record of packet boundaries for transmitted data packets, and
recognize at most one valid acknowledgment for each packet (e.g.,
the first acknowledgment acknowledging the receipt of all of the
sequence numbers in that packet).
One could imagine some limited protection against false duplicate
ACKs for a non-SACK TCP connection, where the TCP sender keeps a
record of the number of packets transmitted, and recognizes at most
one acknowledgment per packet to be used for triggering the sending
of new data. However, this accounting of packets transmitted and
acknowledged would require additional state and extra complexity at
the TCP sender, and does not seem necessary.
The most important protection against false duplicate ACKs comes
from the limited potential of duplicate ACKs in subverting
end-to-end congestion control. There are two separate cases to
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consider: when the TCP sender receives less than a threshold number
of duplicate ACKs, and when the TCP sender receives at least a
threshold number of duplicate ACKs. In the latter case a TCP with
Limited Transmit will behave essentially the same as a TCP without
Limited Transmit in that the congestion window will be halved and a
loss recovery period will be initiated.
When a TCP sender receives less than a threshold number of duplicate
ACKs a misbehaving receiver could send two duplicate ACKs after each
regular ACK. One might imagine that the TCP sender would send at
three times its allowed sending rate. However, using Limited
Transmit as outlined in section 2 the sender is only allowed to
exceed the congestion window by less than the duplicate ACK
threshold (of three segments), and thus would not send a new packet
for each duplicate ACK received.
Acknowledgments
Jamshid Mahdavi and the Transport Area Working Group provided
valuable feedback on an early version of this document.
References
[Bal98] Hari Balakrishnan. Challenges to Reliable Data Transport
over Heterogeneous Wireless Networks. Ph.D. Thesis, University
of California at Berkeley, August 1998.
[BPS+97] Hari Balakrishnan, Venkata Padmanabhan, Srinivasan Seshan,
Mark Stemm, and Randy Katz. TCP Behavior of a Busy Web Server:
Analysis and Improvements. Technical Report UCB/CSD-97-966,
August 1997. Available from
http://nms.lcs.mit.edu/~hari/papers/csd-97-966.ps. (Also in
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.
[DMKM00] Spencer Dawkins, Gabriel Montenegro, Markku Kojo, Vincent
Magret. End-to-end Performance Implications of Slow Links,
Internet-Draft draft-ietf-pilc-slow-03.txt, March 2000 (work in
progress).
[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.
[Jac88] Van Jacobson. Congestion Avoidance and Control. ACM
SIGCOMM 1988.
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[LK98] Dong Lin, H.T. Kung. TCP Fast Recovery Strategies: Analysis
and Improvements. Proceedings of InfoCom, March 1998.
[MSML99] Matt Mathis, Jeff Semke, Jamshid Mahdavi, Kevin Lahey. The
Rate Halving Algorithm, 1999. URL:
http://www.psc.edu/networking/rate_halving.html.
[Mor97] Robert Morris. TCP Behavior with Many Flows. Proceedings
of the Fifth IEEE International Conference on Network Protocols.
October 1997.
[NS] Ns network simulator. URL: http://www.isi.edu/nsnam/.
[PA00] Vern Paxson, Mark Allman. Computing TCP's Retransmission
Timer, April 2000. Internet-Draft draft-paxson-tcp-rto-01.txt
(work in progress).
[Riz96] Luigi Rizzo. Issues in the Implementation of Selective
Acknowledgments for TCP. January, 1996. URL:
http://www.iet.unipi.it/~luigi/selack.ps
[SA00] Jamal Hadi Salim and Uvaiz Ahmed, Performance Evaluation of
Explicit Congestion Notification (ECN) in IP Networks,
draft-hadi-jhsua-ecnperf-01.txt, March 2000 (work in progress).
[SCWA99] Stefan Savage, Neal Cardwell, David Wetherall, Tom
Anderson. TCP Congestion Control with a Misbehaving Receiver.
ACM Computer Communications Review, October 1999.
[RFC793] Jon Postel, Transmission Control Protocol, September 1981.
RFC 793.
[RFC2018] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn Romanow.
TCP Selective Acknowledgement Options. RFC 2018, October 1996.
[RFC2481] K. K. Ramakrishnan, Sally Floyd. A Proposal to Add
Explicit Congestion Notification (ECN) to IP. RFC 2481, January
1999.
[RFC2581] Mark Allman, Vern Paxson, W. Richard Stevens. TCP
Congestion Control. RFC 2581, April 1999.
[RFC2582] Sally Floyd, Tom Henderson. The NewReno Modification to
TCP's Fast Recovery Algorithm. RFC 2582, April 1999.
[ZQ00] Yin Zhang and Lili Qiu, Understanding the End-to-End
Performance Impact of RED in a Heterogeneous Environment,
Cornell CS Technical Report 2000-1802, July 2000. URL
http://www.cs.cornell.edu/yzhang/papers.htm.
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Author's Addresses:
Mark Allman
NASA Glenn Research Center/BBN Technologies
Lewis Field
21000 Brookpark Rd. MS 54-2
Cleveland, OH 44135
Phone: 216-433-6586
Fax: 216-433-8705
mallman@grc.nasa.gov
http://roland.grc.nasa.gov/~mallman
Hari Balakrishnan
Laboratory for Computer Science
545 Technology Square
Massachusetts Institute of Technology
Cambridge, MA 02139
hari@lcs.mit.edu
http://nms.lcs.mit.edu/~hari/
Sally Floyd
AT&T Center for Internet Research at ICSI (ACIRI)
1947 Center St, Suite 600
Berkeley, CA 94704
Phone: +1 (510) 666-2989
floyd@aciri.org
http://www.aciri.org/floyd/
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