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Network Working Group                                         S. DawkinsRequest for Comments: 3155                                 G. MontenegroBCP: 50                                                          M. KojoCategory: Best Current Practice                                V. Magret                                                               N. Vaidya                                                             August 2001End-to-end Performance Implications of Links with ErrorsStatus 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 Contents1.0 Introduction .............................................21.1 Should you be reading this recommendation?  ...........31.2 Relationship of this recommendation to PEPs ...........4      1.3 Relationship of this recommendation to Link Layer          Mechanisms.............................................42.0 Errors and Interactions with TCP Mechanisms ..............52.1 Slow Start and Congestion Avoidance [RFC2581] .........52.2 Fast Retransmit and Fast Recovery [RFC2581] ...........62.3 Selective Acknowledgements [RFC2018,RFC2883] .........73.0 Summary of Recommendations ...............................84.0 Topics For Further Work ..................................94.1 Achieving, and maintaining, large windows .............105.0 Security Considerations ..................................116.0 IANA Considerations ......................................117.0 Acknowledgements .........................................11   References ...................................................11   Authors' Addresses ...........................................14   Full Copyright Statement .....................................16Dawkins, et al.          Best Current Practice                  [Page 1]

RFC 3155                PILC - Links with Errors             August 20011.0 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 PerformanceDawkins, 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.      -  Whenever sending TCPs receive three out-of-order         acknowledgement, they assume the network is mildly congested         and invoke fast retransmit/fast recovery (described below).      -  Whenever TCP's retransmission timer expires, the sender assumes         that the network is congested and invokes slow start.      -  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 "downwardDawkins, 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 toSection 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 initiateDawkins, 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 20015.0 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 ofRFC 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 ashttp://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 athttp://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,RFC793, September 1981.   [RFC2821]  Klensin, J., Editor, "Simple Mail Transfer Protocol",RFC2821, 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",RFC2309, 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,RFC2488, 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",RFC2923, 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 fromhttp://www.kohala.com/~rstevens/vanj.88feb11.txtDawkins, 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.fiDawkins, 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.eduDawkins, et al.          Best Current Practice                 [Page 15]

RFC 3155                PILC - Links with Errors             August 2001Full Copyright Statement   Copyright (C) The Internet Society (2001).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Dawkins, et al.          Best Current Practice                 [Page 16]