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rfc:bcp:bcp50

Network Working Group S. Dawkins Request for Comments: 3155 G. Montenegro BCP: 50 M. Kojo Category: Best Current Practice V. Magret

                                                             N. Vaidya
                                                           August 2001
      End-to-end Performance Implications of Links with Errors

Status of this Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

 This document discusses the specific TCP mechanisms that are
 problematic in environments with high uncorrected error rates, and
 discusses what can be done to mitigate the problems without
 introducing intermediate devices into the connection.

Table of Contents

 1.0 Introduction .............................................    2
    1.1 Should you be reading this recommendation?  ...........    3
    1.2 Relationship of this recommendation to PEPs ...........    4
    1.3 Relationship of this recommendation to Link Layer
        Mechanisms.............................................    4
 2.0 Errors and Interactions with TCP Mechanisms ..............    5
    2.1 Slow Start and Congestion Avoidance [RFC2581] .........    5
    2.2 Fast Retransmit and Fast Recovery [RFC2581] ...........    6
    2.3 Selective Acknowledgements [RFC2018, RFC2883] .........    7
 3.0 Summary of Recommendations ...............................    8
 4.0 Topics For Further Work ..................................    9
    4.1 Achieving, and maintaining, large windows .............   10
 5.0 Security Considerations ..................................   11
 6.0 IANA Considerations ......................................   11
 7.0 Acknowledgements .........................................   11
 References ...................................................   11
 Authors' Addresses ...........................................   14
 Full Copyright Statement .....................................   16

Dawkins, et al. Best Current Practice [Page 1] RFC 3155 PILC - Links with Errors August 2001

1.0 Introduction

 The rapidly-growing Internet is being accessed by an increasingly
 wide range of devices over an increasingly wide variety of links.  At
 least some of these links do not provide the degree of reliability
 that hosts expect, and this expansion into unreliable links causes
 some Internet protocols, especially TCP [RFC793], to perform poorly.
 Specifically, TCP congestion control [RFC2581], while appropriate for
 connections that lose traffic primarily because of congestion and
 buffer exhaustion, interacts badly with uncorrected errors when TCP
 connections traverse links with high uncorrected error rates.  The
 result is that sending TCPs may spend an excessive amount of time
 waiting for acknowledgement that do not arrive, and then, although
 these losses are not due to congestion-related buffer exhaustion, the
 sending TCP transmits at substantially reduced traffic levels as it
 probes the network to determine "safe" traffic levels.
 This document does not address issues with other transport protocols,
 for example, UDP.
 Congestion avoidance in the Internet is based on an assumption that
 most packet losses are due to congestion.  TCP's congestion avoidance
 strategy treats the absence of acknowledgement as a congestion
 signal.  This has worked well since it was introduced in 1988 [VJ-
 DCAC], because most links and subnets have relatively low error rates
 in normal operation, and congestion is the primary cause of loss in
 these environments.  However, links and subnets that do not enjoy low
 uncorrected error rates are becoming more prevalent in parts of the
 Internet.  In particular, these include terrestrial and satellite
 wireless links.  Users relying on traffic traversing these links may
 see poor performance because their TCP connections are spending
 excessive time in congestion avoidance and/or slow start procedures
 triggered by packet losses due to transmission errors.
 The recommendations in this document aim at improving utilization of
 available path capacity over such high error-rate links in ways that
 do not threaten the stability of the Internet.
 Applications use TCP in very different ways, and these have
 interactions with TCP's behavior [RFC2861].  Nevertheless, it is
 possible to make some basic assumptions about TCP flows.
 Accordingly, the mechanisms discussed here are applicable to all uses
 of TCP, albeit in varying degrees according to different scenarios
 (as noted where appropriate).

Dawkins, et al. Best Current Practice [Page 2] RFC 3155 PILC - Links with Errors August 2001

 This recommendation is based on the explicit assumption that major
 changes to the entire installed base of routers and hosts are not a
 practical possibility.  This constrains any changes to hosts that are
 directly affected by errored links.

1.1 Should you be reading this recommendation?

 All known subnetwork technologies provide an "imperfect" subnetwork
 service - the bit error rate is non-zero.  But there's no obvious way
 for end stations to tell the difference between packets discarded due
 to congestion and losses due to transmission errors.
 If a directly-attached subnetwork is reporting transmission errors to
 a host, these reports matter, but we can't rely on explicit
 transmission error reports to both hosts.
 Another way of deciding if a subnetwork should be considered to have
 a "high error rate" is by appealing to mathematics.
 An approximate formula for the TCP Reno response function is given in
 [PFTK98]:
                              s
 T = --------------------------------------------------
     RTT*sqrt(2p/3) + tRTO*(3*sqrt(3p/8))*p*(1 + 32p**2)
 where
     T = the sending rate in bytes per second
     s = the packet size in bytes
     RTT = round-trip time in seconds
     tRTO = TCP retransmit timeout value in seconds
     p = steady-state packet loss rate
 If one plugs in an observed packet loss rate, does the math and then
 sees predicted bandwidth utilization that is greater than the link
 speed, the connection will not benefit from recommendations in this
 document, because the level of packet losses being encountered won't
 affect the ability of TCP to utilize the link.  If, however, the
 predicted bandwidth is less than the link speed, packet losses are
 affecting the ability of TCP to utilize the link.
 If further investigation reveals a subnetwork with significant
 transmission error rates, the recommendations in this document will
 improve the ability of TCP to utilize the link.

Dawkins, et al. Best Current Practice [Page 3] RFC 3155 PILC - Links with Errors August 2001

 A few caveats are in order, when doing this calculation:
 (1)   the RTT is the end-to-end RTT, not the link RTT.
 (2)   Max(1.0, 4*RTT) can be substituted as a simplification for
       tRTO.
 (3)   losses may be bursty - a loss rate measured over an interval
       that includes multiple bursty loss events may understate the
       impact of these loss events on the sending rate.

1.2 Relationship of this recommendation to PEPs

 This document discusses end-to-end mechanisms that do not require
 TCP-level awareness by intermediate nodes.  This places severe
 limitations on what the end nodes can know about the nature of losses
 that are occurring between the end nodes.  Attempts to apply
 heuristics to distinguish between congestion and transmission error
 have not been successful [BV97, BV98, BV98a].  This restriction is
 relaxed in an informational document on Performance Enhancing Proxies
 (PEPs) [RFC3135]. Because PEPs can be placed on boundaries where
 network characteristics change dramatically, PEPs have an additional
 opportunity to improve performance over links with uncorrected
 errors.
 However, generalized use of PEPs contravenes the end-to-end principle
 and is highly undesirable given their deleterious implications, which
 include the following: lack of fate sharing (a PEP adds a third point
 of failure besides the endpoints themselves), end-to-end reliability
 and diagnostics, preventing end-to-end security (particularly network
 layer security such as IPsec), mobility (handoffs are much more
 complex because state must be transferred), asymmetric routing (PEPs
 typically require being on both the forward and reverse paths of a
 connection), scalability (PEPs add more state to maintain), QoS
 transparency and guarantees.
 Not every type of PEP has all the drawbacks listed above.
 Nevertheless, the use of PEPs may have very serious consequences
 which must be weighed carefully.

1.3 Relationship of this recommendation to Link Layer Mechanisms

 This recommendation is for use with TCP over subnetwork technologies
 (link layers) that have already been deployed.  Subnetworks that are
 intended to carry Internet protocols, but have not been completely
 specified are the subject of a best common practices (BCP) document
 which has been developed or is under development by the Performance

Dawkins, et al. Best Current Practice [Page 4] RFC 3155 PILC - Links with Errors August 2001

 Implications of Link Characteristics WG (PILC) [PILC-WEB].  This last
 document is aimed at designers who still have the opportunity to
 reduce the number of uncorrected errors TCP will encounter.

2.0 Errors and Interactions with TCP Mechanisms

 A TCP sender adapts its use of network path capacity based on
 feedback from the TCP receiver.  As TCP is not able to distinguish
 between losses due to congestion and losses due to uncorrected
 errors, it is not able to accurately determine available path
 capacity in the presence of significant uncorrected errors.

2.1 Slow Start and Congestion Avoidance [RFC2581]

 Slow Start and Congestion Avoidance [RFC2581] are essential to the
 current stability of the Internet.  These mechanisms were designed to
 accommodate networks that do not provide explicit congestion
 notification.  Although experimental mechanisms such as [RFC2481] are
 moving in the direction of explicit congestion notification, the
 effect of ECN on ECN-aware TCPs is essentially the same as the effect
 of implicit congestion notification through congestion-related loss,
 except that ECN provides this notification before packets are lost,
 and must then be retransmitted.
 TCP connections experiencing high error rates on their paths interact
 badly with Slow Start and with Congestion Avoidance, because high
 error rates make the interpretation of losses ambiguous - the sender
 cannot know whether detected losses are due to congestion or to data
 corruption.  TCP makes the "safe" choice and assumes that the losses
 are due to congestion.
  1. Whenever sending TCPs receive three out-of-order

acknowledgement, they assume the network is mildly congested

       and invoke fast retransmit/fast recovery (described below).
  1. Whenever TCP's retransmission timer expires, the sender assumes

that the network is congested and invokes slow start.

  1. Less-reliable link layers often use small link MTUs. This

slows the rate of increase in the sender's window size during

       slow start, because the sender's window is increased in units
       of segments.  Small link MTUs alone don't improve reliability.
       Path MTU discovery [RFC1191] must also be used to prevent
       fragmentation.  Path MTU discovery allows the most rapid
       opening of the sender's window size during slow start, but a
       number of round trips may still be required to open the window
       completely.

Dawkins, et al. Best Current Practice [Page 5] RFC 3155 PILC - Links with Errors August 2001

 Recommendation: Any standards-conformant TCP will implement Slow
 Start and Congestion Avoidance, which are MUSTs in STD 3 [RFC1122].
 Recommendations in this document will not interfere with these
 mechanisms.

2.2 Fast Retransmit and Fast Recovery [RFC2581]

 TCP provides reliable delivery of data as a byte-stream to an
 application, so that when a segment is lost (whether due to either
 congestion or transmission loss), the receiver TCP implementation
 must wait to deliver data to the receiving application until the
 missing data is received.  The receiver TCP implementation detects
 missing segments by segments arriving with out-of-order sequence
 numbers.
 TCPs should immediately send an acknowledgement when data is received
 out-of-order [RFC2581], providing the next expected sequence number
 with no delay, so that the sender can retransmit the required data as
 quickly as possible and the receiver can resume delivery of data to
 the receiving application.  When an acknowledgement carries the same
 expected sequence number as an acknowledgement that has already been
 sent for the last in-order segment received, these acknowledgement
 are called "duplicate ACKs".
 Because IP networks are allowed to reorder packets, the receiver may
 send duplicate acknowledgments for segments that arrive out of order
 due to routing changes, link-level retransmission, etc.  When a TCP
 sender receives three duplicate ACKs, fast retransmit [RFC2581]
 allows it to infer that a segment was lost.  The sender retransmits
 what it considers to be this lost segment without waiting for the
 full retransmission timeout, thus saving time.
 After a fast retransmit, a sender halves its congestion window and
 invokes the fast recovery [RFC2581] algorithm, whereby it invokes
 congestion avoidance from a halved congestion window, but does not
 invoke slow start from a one-segment congestion window as it would do
 after a retransmission timeout.  As the sender is still receiving
 dupacks, it knows the receiver is receiving packets sent, so the full
 reduction after a timeout when no communication has been received is
 not called for.  This relatively safe optimization also saves time.
 It is important to be realistic about the maximum throughput that TCP
 can have over a connection that traverses a high error-rate link.  In
 general, TCP will increase its congestion window beyond the delay-
 bandwidth product.  TCP's congestion avoidance strategy is additive-
 increase, multiplicative-decrease, which means that if additional
 errors are encountered before the congestion window recovers
 completely from a 50-percent reduction, the effect can be a "downward

Dawkins, et al. Best Current Practice [Page 6] RFC 3155 PILC - Links with Errors August 2001

 spiral" of the congestion window due to additional 50-percent
 reductions.  Even using Fast Retransmit/Fast Recovery, the sender
 will halve the congestion window each time a window contains one or
 more segments that are lost, and will re-open the window by one
 additional segment for each congestion window's worth of
 acknowledgement received.
 If a connection's path traverses a link that loses one or more
 segments during this recovery period, the one-half reduction takes
 place again, this time on a reduced congestion window - and this
 downward spiral will continue to hold the congestion window below
 path capacity until the connection is able to recover completely by
 additive increase without experiencing loss.
 Of course, no downward spiral occurs if the error rate is constantly
 high and the congestion window always remains small; the
 multiplicative-increase "slow start" will be exited early, and the
 congestion window remains low for the duration of the TCP connection.
 In links with high error rates, the TCP window may remain rather
 small for long periods of time.
 Not all causes of small windows are related to errors.  For example,
 HTTP/1.0 commonly closes TCP connections to indicate boundaries
 between requested resources.  This means that these applications are
 constantly closing "trained" TCP connections and opening "untrained"
 TCP connections which will execute slow start, beginning with one or
 two segments.  This can happen even with HTTP/1.1, if webmasters
 configure their HTTP/1.1 servers to close connections instead of
 waiting to see if the connection will be useful again.
 A small window - especially a window of less than four segments -
 effectively prevents the sender from taking advantage of Fast
 Retransmits.  Moreover, efficient recovery from multiple losses
 within a single window requires adoption of new proposals (NewReno
 [RFC2582]).
 Recommendation: Implement Fast Retransmit and Fast Recovery at this
 time.  This is a widely-implemented optimization and is currently at
 Proposed Standard level.  [RFC2488] recommends implementation of Fast
 Retransmit/Fast Recovery in satellite environments.

2.3 Selective Acknowledgements [RFC2018, RFC2883]

 Selective Acknowledgements [RFC2018] allow the repair of multiple
 segment losses per window without requiring one (or more) round-trips
 per loss.

Dawkins, et al. Best Current Practice [Page 7] RFC 3155 PILC - Links with Errors August 2001

 [RFC2883] proposes a minor extension to SACK that allows receiving
 TCPs to provide more information about the order of delivery of
 segments, allowing "more robust operation in an environment of
 reordered packets, ACK loss, packet replication, and/or early
 retransmit timeouts".  Unless explicitly stated otherwise, in this
 document, "Selective Acknowledgements" (or "SACK") refers to the
 combination of [RFC2018] and [RFC2883].
 Selective acknowledgments are most useful in LFNs ("Long Fat
 Networks") because of the long round trip times that may be
 encountered in these environments, according to Section 1.1 of
 [RFC1323], and are especially useful if large windows are required,
 because there is a higher probability of multiple segment losses per
 window.
 On the other hand, if error rates are generally low but occasionally
 higher due to channel conditions, TCP will have the opportunity to
 increase its window to larger values during periods of improved
 channel conditions between bursts of errors.  When bursts of errors
 occur, multiple losses within a window are likely to occur.  In this
 case, SACK would provide benefits in speeding the recovery and
 preventing unnecessary reduction of the window size.
 Recommendation: Implement SACK as specified in [RFC2018] and updated
 by [RFC2883], both Proposed Standards.  In cases where SACK cannot be
 enabled for both sides of a connection, TCP senders may use NewReno
 [RFC2582] to better handle partial ACKs and multiple losses within a
 single window.

3.0 Summary of Recommendations

 The Internet does not provide a widely-available loss feedback
 mechanism that allows TCP to distinguish between congestion loss and
 transmission error.  Because congestion affects all traffic on a path
 while transmission loss affects only the specific traffic
 encountering uncorrected errors, avoiding congestion has to take
 precedence over quickly repairing transmission errors.  This means
 that the best that can be achieved without new feedback mechanisms is
 minimizing the amount of time that is spent unnecessarily in
 congestion avoidance.
 The Fast Retransmit/Fast Recovery mechanism allows quick repair of
 loss without giving up the safety of congestion avoidance.  In order
 for Fast Retransmit/Fast Recovery to work, the window size must be
 large enough to force the receiver to send three duplicate
 acknowledgments before the retransmission timeout interval expires,
 forcing full TCP slow-start.

Dawkins, et al. Best Current Practice [Page 8] RFC 3155 PILC - Links with Errors August 2001

 Selective Acknowledgements (SACK) extend the benefit of Fast
 Retransmit/Fast Recovery to situations where multiple segment losses
 in the window need to be repaired more quickly than can be
 accomplished by executing Fast Retransmit for each segment loss, only
 to discover the next segment loss.
 These mechanisms are not limited to wireless environments.  They are
 usable in all environments.

4.0 Topics For Further Work

 "Limited Transmit" [RFC3042] has been specified as an optimization
 extending Fast Retransmit/Fast Recovery for TCP connections with
 small congestion windows that will not trigger three duplicate
 acknowledgments.  This specification is deemed safe, and it also
 provides benefits for TCP connections that experience a large amount
 of packet (data or ACK) loss.  Implementors should evaluate this
 standards track specification for TCP in loss environments.
 Delayed Duplicate Acknowledgements [MV97, VMPM99] attempts to prevent
 TCP-level retransmission when link-level retransmission is still in
 progress, adding additional traffic to the network.  This proposal is
 worthy of additional study, but is not recommended at this time,
 because we don't know how to calculate appropriate amounts of delay
 for an arbitrary network topology.
 It is not possible to use explicit congestion notification [RFC2481]
 as a surrogate for explicit transmission error notification (no
 matter how much we wish it was!).  Some mechanism to provide explicit
 notification of transmission error would be very helpful.  This might
 be more easily provided in a PEP environment, especially when the PEP
 is the "first hop" in a connection path, because current checksum
 mechanisms do not distinguish between transmission error to a payload
 and transmission error to the header.  Furthermore, if the header is
 damaged, sending explicit transmission error notification to the
 right endpoint is problematic.
 Losses that take place on the ACK stream, especially while a TCP is
 learning network characteristics, can make the data stream quite
 bursty (resulting in losses on the data stream, as well).  Several
 ways of limiting this burstiness have been proposed, including TCP
 transmit pacing at the sender and ACK rate control within the
 network.
 "Appropriate Byte Counting" (ABC) [ALL99], has been proposed as a way
 of opening the congestion window based on the number of bytes that
 have been successfully transfered to the receiver, giving more
 appropriate behavior for application protocols that initiate

Dawkins, et al. Best Current Practice [Page 9] RFC 3155 PILC - Links with Errors August 2001

 connections with relatively short packets.  For SMTP [RFC2821], for
 instance, the client might send a short HELO packet, a short MAIL
 packet, one or more short RCPT packets, and a short DATA packet -
 followed by the entire mail body sent as maximum-length packets.  An
 ABC TCP sender would not use ACKs for each of these short packets to
 increase the congestion window to allow additional full-length
 packets.  ABC is worthy of additional study, but is not recommended
 at this time, because ABC can lead to increased burstiness when
 acknowledgments are lost.

4.1 Achieving, and maintaining, large windows

 The recommendations described in this document will aid TCPs in
 injecting packets into ERRORed connections as fast as possible
 without destabilizing the Internet, and so optimizing the use of
 available bandwidth.
 In addition to these TCP-level recommendations, there is still
 additional work to do at the application level, especially with the
 dominant application protocol on the World Wide Web, HTTP.
 HTTP/1.0 (and earlier versions) closes TCP connections to signal a
 receiver that all of a requested resource had been transmitted.
 Because WWW objects tend to be small in size [MOGUL], TCPs carrying
 HTTP/1.0 traffic experience difficulty in "training" on available
 path capacity (a substantial portion of the transfer has already
 happened by the time TCP exits slow start).
 Several HTTP modifications have been introduced to improve this
 interaction with TCP ("persistent connections" in HTTP/1.0, with
 improvements in HTTP/1.1 [RFC2616]).  For a variety of reasons, many
 HTTP interactions are still HTTP/1.0-style - relatively short-lived.
 Proposals which reuse TCP congestion information across connections,
 like TCP Control Block Interdependence [RFC2140], or the more recent
 Congestion Manager [BS00] proposal, will have the effect of making
 multiple parallel connections impact the network as if they were a
 single connection, "trained" after a single startup transient.  These
 proposals are critical to the long-term stability of the Internet,
 because today's users always have the choice of clicking on the
 "reload" button in their browsers and cutting off TCP's exponential
 backoff - replacing connections which are building knowledge of the
 available bandwidth with connections with no knowledge at all.

Dawkins, et al. Best Current Practice [Page 10] RFC 3155 PILC - Links with Errors August 2001

5.0 Security Considerations

 A potential vulnerability introduced by Fast Retransmit/Fast Recovery
 is (as pointed out in [RFC2581]) that an attacker may force TCP
 connections to grind to a halt, or, more dangerously, behave more
 aggressively.  The latter possibility may lead to congestion
 collapse, at least in some regions of the network.
 Selective acknowledgments is believed to neither strengthen nor
 weaken TCP's current security properties [RFC2018].
 Given that the recommendations in this document are performed on an
 end-to-end basis, they continue working even in the presence of end-
 to-end IPsec.  This is in direct contrast with mechanisms such as
 PEP's which are implemented in intermediate nodes (section 1.2).

6.0 IANA Considerations

 This document is a pointer to other, existing IETF standards.  There
 are no new IANA considerations.

7.0 Acknowledgements

 This recommendation has grown out of RFC 2757, "Long Thin Networks",
 which was in turn based on work done in the IETF TCPSAT working
 group.  The authors are indebted to the active members of the PILC
 working group.  In particular, Mark Allman and Lloyd Wood gave us
 copious and insightful feedback, and Dan Grossman and Jamshid Mahdavi
 provided text replacements.

References

 [ALL99]    M. Allman, "TCP Byte Counting Refinements," ACM Computer
            Communication Review, Volume 29, Number 3, July 1999.
            http://www.acm.org/sigcomm/ccr/archive/ccr-toc/ccr-toc-
            99.html
 [BS00]     Balakrishnan, H. and S. Seshan, "The Congestion Manager",
            RFC 3124, June 2001.
 [BV97]     S. Biaz and N. Vaidya, "Using End-to-end Statistics to
            Distinguish Congestion and Corruption Losses: A Negative
            Result," Texas A&M University, Technical Report 97-009,
            August 18, 1997.

Dawkins, et al. Best Current Practice [Page 11] RFC 3155 PILC - Links with Errors August 2001

 [BV98]     S. Biaz and N. Vaidya, "Sender-Based heuristics for
            Distinguishing Congestion Losses from Wireless
            Transmission Losses," Texas A&M University, Technical
            Report 98-013, June 1998.
 [BV98a]    S. Biaz and N. Vaidya, "Discriminating Congestion Losses
            from Wireless Losses using Inter-Arrival Times at the
            Receiver," Texas A&M University, Technical Report 98-014,
            June 1998.
 [MOGUL]    "The Case for Persistent-Connection HTTP", J. C. Mogul,
            Research Report 95/4, May 1995. Available as
            http://www.research.digital.com/wrl/techreports/abstracts/
            95.4.html
 [MV97]     M. Mehta and N. Vaidya, "Delayed Duplicate-
            Acknowledgements:  A Proposal to Improve Performance of
            TCP on Wireless Links," Texas A&M University, December 24,
            1997.  Available at
            http://www.cs.tamu.edu/faculty/vaidya/mobile.html
 [PILC-WEB] http://pilc.grc.nasa.gov/
 [PFTK98]   Padhye, J., Firoiu, V., Towsley, D. and J.Kurose, "TCP
            Throughput: A simple model and its empirical validation",
            SIGCOMM Symposium on Communications Architectures and
            Protocols, August 1998.
 [RFC793]   Postel, J., "Transmission Control Protocol", STD 7, RFC
            793, September 1981.
 [RFC2821]  Klensin, J., Editor, "Simple Mail Transfer Protocol", RFC
            2821, April 2001.
 [RFC1122]  Braden, R., "Requirements for Internet Hosts --
            Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC1191]  Mogul J., and S. Deering, "Path MTU Discovery", RFC 1191,
            November 1990.
 [RFC1323]  Jacobson, V., Braden, R. and D. Borman. "TCP Extensions
            for High Performance", RFC 1323, May 1992.
 [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow "TCP
            Selective Acknowledgment Options", RFC 2018, October 1996.
 [RFC2140]  Touch, J., "TCP Control Block Interdependence", RFC 2140,
            April 1997.

Dawkins, et al. Best Current Practice [Page 12] RFC 3155 PILC - Links with Errors August 2001

 [RFC2309]  Braden, B., Clark, D., Crowcrfot, J., Davie, B., Deering,
            S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
            Partridge, C., Peterson, L., Ramakrishnan, K., Shecker,
            S., Wroclawski, J. and L, Zhang, "Recommendations on Queue
            Management and Congestion Avoidance in the Internet", RFC
            2309, April 1998.
 [RFC2481]  Ramakrishnan K. and S. Floyd, "A Proposal to add Explicit
            Congestion Notification (ECN) to IP", RFC 2481, January
            1999.
 [RFC2488]  Allman, M., Glover, D. and L. Sanchez. "Enhancing TCP Over
            Satellite Channels using Standard Mechanisms", BCP 28, RFC
            2488, January 1999.
 [RFC2581]  Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
            Control", RFC 2581, April 1999.
 [RFC2582]  Floyd, S. and T. Henderson, "The NewReno Modification to
            TCP's Fast Recovery Algorithm", RFC 2582, April 1999.
 [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
            Masinter, L., Leach P. and T. Berners-Lee, "Hypertext
            Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
 [RFC2861]  Handley, H., Padhye, J. and S., Floyd, "TCP Congestion
            Window Validation", RFC 2861, June 2000.
 [RFC2883]  Floyd, S., Mahdavi, M., Mathis, M. and M. Podlosky, "An
            Extension to the Selective Acknowledgement (SACK) Option
            for TCP", RFC 2883, August 1999.
 [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery", RFC
            2923, September 2000.
 [RFC3042]  Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing
            TCP's Loss Recovery Using Limited Transmit", RFC 3042,
            January, 2001.
 [RFC3135]  Border, J., Kojo, M., Griner, J., Montenegro, G. and Z.
            Shelby, "Performance Enhancing Proxies Intended to
            Mitigate Link-Related Degradations", RFC 3135, June 2001.
 [VJ-DCAC]  Jacobson, V., "Dynamic Congestion Avoidance / Control" e-
            mail dated February 11, 1988, available from
            http://www.kohala.com/~rstevens/vanj.88feb11.txt

Dawkins, et al. Best Current Practice [Page 13] RFC 3155 PILC - Links with Errors August 2001

 [VMPM99]   N. Vaidya, M. Mehta, C. Perkins, and G. Montenegro,
            "Delayed Duplicate Acknowledgements: A TCP-Unaware
            Approach to Improve Performance of TCP over Wireless,"
            Technical Report 99-003, Computer Science Dept., Texas A&M
            University, February 1999. Also, to appear in Journal of
            Wireless Communications and Wireless Computing (Special
            Issue on Reliable Transport Protocols for Mobile
            Computing).

Authors' Addresses

 Questions about this document may be directed to:
 Spencer Dawkins
 Fujitsu Network Communications
 2801 Telecom Parkway
 Richardson, Texas 75082
 Phone: +1-972-479-3782
 EMail: spencer.dawkins@fnc.fujitsu.com
 Gabriel E. Montenegro
 Sun Microsystems
 Laboratories, Europe
 29, chemin du Vieux Chene
 38240 Meylan
 FRANCE
 Phone: +33 476 18 80 45
 EMail: gab@sun.com
 Markku Kojo
 Department of Computer Science
 University of Helsinki
 P.O. Box 26 (Teollisuuskatu 23)
 FIN-00014 HELSINKI
 Finland
 Phone: +358-9-1914-4179
 EMail: kojo@cs.helsinki.fi

Dawkins, et al. Best Current Practice [Page 14] RFC 3155 PILC - Links with Errors August 2001

 Vincent Magret
 Alcatel Internetworking, Inc.
 26801 W. Agoura road
 Calabasas, CA, 91301
 Phone: +1 818 878 4485
 EMail: vincent.magret@alcatel.com
 Nitin H. Vaidya
 458 Coodinated Science Laboratory, MC-228
 1308 West Main Street
 Urbana, IL 61801
 Phone: 217-265-5414
 E-mail: nhv@crhc.uiuc.edu

Dawkins, et al. Best Current Practice [Page 15] RFC 3155 PILC - Links with Errors August 2001

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Dawkins, et al. Best Current Practice [Page 16]

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