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rfc:rfc2760

Network Working Group M. Allman, Editor Request for Comments: 2760 NASA Glenn Research Center/BBN Technologies Category: Informational S. Dawkins

                                                                Nortel
                                                             D. Glover
                                                             J. Griner
                                                               D. Tran
                                            NASA Glenn Research Center
                                                          T. Henderson
                                  University of California at Berkeley
                                                          J. Heidemann
                                                              J. Touch
                                 University of Southern California/ISI
                                                              H. Kruse
                                                          S. Ostermann
                                                       Ohio University
                                                              K. Scott
                                                 The MITRE Corporation
                                                              J. Semke
                                      Pittsburgh Supercomputing Center
                                                         February 2000
             Ongoing TCP Research Related to Satellites

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

 This document outlines possible TCP enhancements that may allow TCP
 to better utilize the available bandwidth provided by networks
 containing satellite links.  The algorithms and mechanisms outlined
 have not been judged to be mature enough to be recommended by the
 IETF.  The goal of this document is to educate researchers as to the
 current work and progress being done in TCP research related to
 satellite networks.

Allman, et al. Informational [Page 1] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

Table of Contents

 1         Introduction. . . . . . . . . . . . . . . . . . . .  2
 2         Satellite Architectures . . . . . . . . . . . . . .  3
 2.1       Asymmetric Satellite Networks . . . . . . . . . . .  3
 2.2       Satellite Link as Last Hop. . . . . . . . . . . . .  3
 2.3       Hybrid Satellite Networks     . . . . . . . . . . .  4
 2.4       Point-to-Point Satellite Networks . . . . . . . . .  4
 2.5       Multiple Satellite Hops . . . . . . . . . . . . . .  4
 3         Mitigations . . . . . . . . . . . . . . . . . . . .  4
 3.1       TCP For Transactions. . . . . . . . . . . . . . . .  4
 3.2       Slow Start. . . . . . . . . . . . . . . . . . . . .  5
 3.2.1     Larger Initial Window . . . . . . . . . . . . . . .  6
 3.2.2     Byte Counting . . . . . . . . . . . . . . . . . . .  7
 3.2.3     Delayed ACKs After Slow Start . . . . . . . . . . .  9
 3.2.4     Terminating Slow Start. . . . . . . . . . . . . . . 11
 3.3       Loss Recovery . . . . . . . . . . . . . . . . . . . 12
 3.3.1     Non-SACK Based Mechanisms . . . . . . . . . . . . . 12
 3.3.2     SACK Based Mechanisms . . . . . . . . . . . . . . . 13
 3.3.3     Explicit Congestion Notification. . . . . . . . . . 16
 3.3.4     Detecting Corruption Loss . . . . . . . . . . . . . 18
 3.4       Congestion Avoidance. . . . . . . . . . . . . . . . 21
 3.5       Multiple Data Connections . . . . . . . . . . . . . 22
 3.6       Pacing TCP Segments . . . . . . . . . . . . . . . . 24
 3.7       TCP Header Compression. . . . . . . . . . . . . . . 26
 3.8       Sharing TCP State Among Similar Connections . . . . 29
 3.9       ACK Congestion Control. . . . . . . . . . . . . . . 32
 3.10      ACK Filtering . . . . . . . . . . . . . . . . . . . 34
 4         Conclusions . . . . . . . . . . . . . . . . . . . . 36
 5         Security Considerations . . . . . . . . . . . . . . 36
 6         Acknowledgments . . . . . . . . . . . . . . . . . . 37
 7         References. . . . . . . . . . . . . . . . . . . . . 37
 8         Authors' Addresses. . . . . . . . . . . . . . . . . 43
 9         Full Copyright Statement. . . . . . . . . . . . . . 46

1 Introduction

 This document outlines mechanisms that may help the Transmission
 Control Protocol (TCP) [Pos81] better utilize the bandwidth provided
 by long-delay satellite environments.  These mechanisms may also help
 in other environments or for other protocols.  The proposals outlined
 in this document are currently being studied throughout the research
 community.  Therefore, these mechanisms are not mature enough to be
 recommended for wide-spread use by the IETF.  However, some of these
 mechanisms may be safely used today.  It is hoped that this document
 will stimulate further study into the described mechanisms.  If, at

Allman, et al. Informational [Page 2] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 some point, the mechanisms discussed in this memo prove to be safe
 and appropriate to be recommended for general use, the appropriate
 IETF documents will be written.
 It should be noted that non-TCP mechanisms that help performance over
 satellite links do exist (e.g., application-level changes, queueing
 disciplines, etc.).  However, outlining these non-TCP mitigations is
 beyond the scope of this document and therefore is left as future
 work.  Additionally, there are a number of mitigations to TCP's
 performance problems that involve very active intervention by
 gateways along the end-to-end path from the sender to the receiver.
 Documenting the pros and cons of such solutions is also left as
 future work.

2 Satellite Architectures

 Specific characteristics of satellite links and the impact these
 characteristics have on TCP are presented in RFC 2488 [AGS99].  This
 section discusses several possible topologies where satellite links
 may be integrated into the global Internet.  The mitigation outlined
 in section 3 will include a discussion of which environment the
 mechanism is expected to benefit.

2.1 Asymmetric Satellite Networks

 Some satellite networks exhibit a bandwidth asymmetry, a larger data
 rate in one direction than the reverse direction, because of limits
 on the transmission power and the antenna size at one end of the
 link.  Meanwhile, some other satellite systems are unidirectional and
 use a non-satellite return path (such as a dialup modem link).  The
 nature of most TCP traffic is asymmetric with data flowing in one
 direction and acknowledgments in opposite direction.  However, the
 term asymmetric in this document refers to different physical
 capacities in the forward and return links.  Asymmetry has been shown
 to be a problem for TCP [BPK97,BPK98].

2.2 Satellite Link as Last Hop

 Satellite links that provide service directly to end users, as
 opposed to satellite links located in the middle of a network, may
 allow for specialized design of protocols used over the last hop.
 Some satellite providers use the satellite link as a shared high
 speed downlink to users with a lower speed, non-shared terrestrial
 link that is used as a return link for requests and acknowledgments.
 Many times this creates an asymmetric network, as discussed above.

Allman, et al. Informational [Page 3] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

2.3 Hybrid Satellite Networks

 In the more general case, satellite links may be located at any point
 in the network topology.  In this case, the satellite link acts as
 just another link between two gateways.  In this environment, a given
 connection may be sent over terrestrial links (including terrestrial
 wireless), as well as satellite links.  On the other hand, a
 connection could also travel over only the terrestrial network or
 only over the satellite portion of the network.

2.4 Point-to-Point Satellite Networks

 In point-to-point satellite networks, the only hop in the network is
 over the satellite link.  This pure satellite environment exhibits
 only the problems associated with the satellite links, as outlined in
 [AGS99].  Since this is a private network, some mitigations that are
 not appropriate for shared networks can be considered.

2.5 Multiple Satellite Hops

 In some situations, network traffic may traverse multiple satellite
 hops between the source and the destination.  Such an environment
 aggravates the satellite characteristics described in [AGS99].

3 Mitigations

 The following sections will discuss various techniques for mitigating
 the problems TCP faces in the satellite environment.  Each of the
 following sections will be organized as follows: First, each
 mitigation will be briefly outlined.  Next, research work involving
 the mechanism in question will be briefly discussed.  Next the
 implementation issues of the mechanism will be presented (including
 whether or not the particular mechanism presents any dangers to
 shared networks).  Then a discussion of the mechanism's potential
 with regard to the topologies outlined above is given.  Finally, the
 relationships and possible interactions with other TCP mechanisms are
 outlined.  The reader is expected to be familiar with the TCP
 terminology used in [AGS99].

3.1 TCP For Transactions

3.1.1 Mitigation Description

 TCP uses a three-way handshake to setup a connection between two
 hosts [Pos81].  This connection setup requires 1-1.5 round-trip times
 (RTTs), depending upon whether the data sender started the connection
 actively or passively.  This startup time can be eliminated by using
 TCP extensions for transactions (T/TCP) [Bra94].  After the first

Allman, et al. Informational [Page 4] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 connection between a pair of hosts is established, T/TCP is able to
 bypass the three-way handshake, allowing the data sender to begin
 transmitting data in the first segment sent (along with the SYN).
 This is especially helpful for short request/response traffic, as it
 saves a potentially long setup phase when no useful data is being
 transmitted.

3.1.2 Research

 T/TCP is outlined and analyzed in [Bra92,Bra94].

3.1.3 Implementation Issues

 T/TCP requires changes in the TCP stacks of both the data sender and
 the data receiver.  While T/TCP is safe to implement in shared
 networks from a congestion control perspective, several security
 implications of sending data in the first data segment have been
 identified [ddKI99].

3.1.4 Topology Considerations

 It is expected that T/TCP will be equally beneficial in all
 environments outlined in section 2.

3.1.5 Possible Interaction and Relationships with Other Research

 T/TCP allows data transfer to start more rapidly, much like using a
 larger initial congestion window (see section 3.2.1), delayed ACKs
 after slow start (section 3.2.3) or byte counting (section 3.2.2).

3.2 Slow Start

 The slow start algorithm is used to gradually increase the size of
 TCP's congestion window (cwnd) [Jac88,Ste97,APS99].  The algorithm is
 an important safe-guard against transmitting an inappropriate amount
 of data into the network when the connection starts up.  However,
 slow start can also waste available network capacity, especially in
 long-delay networks [All97a,Hay97].  Slow start is particularly
 inefficient for transfers that are short compared to the
 delay*bandwidth product of the network (e.g., WWW transfers).
 Delayed ACKs are another source of wasted capacity during the slow
 start phase.  RFC 1122 [Bra89] suggests data receivers refrain from
 ACKing every incoming data segment.  However, every second full-sized
 segment should be ACKed.  If a second full-sized segment does not
 arrive within a given timeout, an ACK must be generated (this timeout
 cannot exceed 500 ms).  Since the data sender increases the size of
 cwnd based on the number of arriving ACKs, reducing the number of

Allman, et al. Informational [Page 5] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 ACKs slows the cwnd growth rate.  In addition, when TCP starts
 sending, it sends 1 segment.  When using delayed ACKs a second
 segment must arrive before an ACK is sent.  Therefore, the receiver
 is always forced to wait for the delayed ACK timer to expire before
 ACKing the first segment, which also increases the transfer time.
 Several proposals have suggested ways to make slow start less time
 consuming.  These proposals are briefly outlined below and references
 to the research work given.

3.2.1 Larger Initial Window

3.2.1.1 Mitigation Description

 One method that will reduce the amount of time required by slow start
 (and therefore, the amount of wasted capacity) is to increase the
 initial value of cwnd.  An experimental TCP extension outlined in
 [AFP98] allows the initial size of cwnd to be increased from 1
 segment to that given in equation (1).
             min (4*MSS, max (2*MSS, 4380 bytes))               (1)
 By increasing the initial value of cwnd, more packets are sent during
 the first RTT of data transmission, which will trigger more ACKs,
 allowing the congestion window to open more rapidly.  In addition, by
 sending at least 2 segments initially, the first segment does not
 need to wait for the delayed ACK timer to expire as is the case when
 the initial size of cwnd is 1 segment (as discussed above).
 Therefore, the value of cwnd given in equation 1 saves up to 3 RTTs
 and a delayed ACK timeout when compared to an initial cwnd of 1
 segment.
 Also, we note that RFC 2581 [APS99], a standards-track document,
 allows a TCP to use an initial cwnd of up to 2 segments.  This change
 is highly recommended for satellite networks.

3.2.1.2 Research

 Several researchers have studied the use of a larger initial window
 in various environments.  [Nic97] and [KAGT98] show a reduction in
 WWW page transfer time over hybrid fiber coax (HFC) and satellite
 links respectively.  Furthermore, it has been shown that using an
 initial cwnd of 4 segments does not negatively impact overall
 performance over dialup modem links with a small number of buffers
 [SP98].  [AHO98] shows an improvement in transfer time for 16 KB
 files across the Internet and dialup modem links when using a larger
 initial value for cwnd.  However, a slight increase in dropped

Allman, et al. Informational [Page 6] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 segments was also shown.  Finally, [PN98] shows improved transfer
 time for WWW traffic in simulations with competing traffic, in
 addition to a small increase in the drop rate.

3.2.1.3 Implementation Issues

 The use of a larger initial cwnd value requires changes to the
 sender's TCP stack.  Using an initial congestion window of 2 segments
 is allowed by RFC 2581 [APS99].  Using an initial congestion window
 of 3 or 4 segments is not expected to present any danger of
 congestion collapse [AFP98], however may degrade performance in some
 networks.

3.2.1.4 Topology Considerations

 It is expected that the use of a large initial window would be
 equally beneficial to all network architectures outlined in section
 2.

3.2.1.5 Possible Interaction and Relationships with Other Research

 Using a fixed larger initial congestion window decreases the impact
 of a long RTT on transfer time (especially for short transfers) at
 the cost of bursting data into a network with unknown conditions.  A
 mechanism that mitigates bursts may make the use of a larger initial
 congestion window more appropriate (e.g., limiting the size of line-
 rate bursts [FF96] or pacing the segments in a burst [VH97a]).
 Also, using delayed ACKs only after slow start (as outlined in
 section 3.2.3) offers an alternative way to immediately ACK the first
 segment of a transfer and open the congestion window more rapidly.
 Finally, using some form of TCP state sharing among a number of
 connections (as discussed in 3.8) may provide an alternative to using
 a fixed larger initial window.

3.2.2 Byte Counting

3.2.2.1 Mitigation Description

 As discussed above, the wide-spread use of delayed ACKs increases the
 time needed by a TCP sender to increase the size of the congestion
 window during slow start.  This is especially harmful to flows
 traversing long-delay GEO satellite links.  One mechanism that has
 been suggested to mitigate the problems caused by delayed ACKs is the
 use of "byte counting", rather than standard ACK counting
 [All97a,All98].  Using standard ACK counting, the congestion window
 is increased by 1 segment for each ACK received during slow start.
 However, using byte counting the congestion window increase is based

Allman, et al. Informational [Page 7] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 on the number of previously unacknowledged bytes covered by each
 incoming ACK, rather than on the number of ACKs received.  This makes
 the increase relative to the amount of data transmitted, rather than
 being dependent on the ACK interval used by the receiver.
 Two forms of byte counting are studied in [All98].  The first is
 unlimited byte counting (UBC).  This mechanism simply uses the number
 of previously unacknowledged bytes to increase the congestion window
 each time an ACK arrives.  The second form is limited byte counting
 (LBC).  LBC limits the amount of cwnd increase to 2 segments.  This
 limit throttles the size of the burst of data sent in response to a
 "stretch ACK" [Pax97].  Stretch ACKs are acknowledgments that cover
 more than 2 segments of previously unacknowledged data.  Stretch ACKs
 can occur by design [Joh95] (although this is not standard), due to
 implementation bugs [All97b,PADHV99] or due to ACK loss.  [All98]
 shows that LBC prevents large line-rate bursts when compared to UBC,
 and therefore offers fewer dropped segments and better performance.
 In addition, UBC causes large bursts during slow start based loss
 recovery due to the large cumulative ACKs that can arrive during loss
 recovery.  The behavior of UBC during loss recovery can cause large
 decreases in performance and [All98] strongly recommends UBC not be
 deployed without further study into mitigating the large bursts.
 Note: The standards track RFC 2581 [APS99] allows a TCP to use byte
 counting to increase cwnd during congestion avoidance, however not
 during slow start.

3.2.2.2 Research

 Using byte counting, as opposed to standard ACK counting, has been
 shown to reduce the amount of time needed to increase the value of
 cwnd to an appropriate size in satellite networks [All97a].  In
 addition, [All98] presents a simulation comparison of byte counting
 and the standard cwnd increase algorithm in uncongested networks and
 networks with competing traffic.  This study found that the limited
 form of byte counting outlined above can improve performance, while
 also increasing the drop rate slightly.
 [BPK97,BPK98] also investigated unlimited byte counting in
 conjunction with various ACK filtering algorithms (discussed in
 section 3.10) in asymmetric networks.

Allman, et al. Informational [Page 8] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

3.2.2.3 Implementation Issues

 Changing from ACK counting to byte counting requires changes to the
 data sender's TCP stack.  Byte counting violates the algorithm for
 increasing the congestion window outlined in RFC 2581 [APS99] (by
 making congestion window growth more aggressive during slow start)
 and therefore should not be used in shared networks.

3.2.2.4 Topology Considerations

 It has been suggested by some (and roundly criticized by others) that
 byte counting will allow TCP to provide uniform cwnd increase,
 regardless of the ACKing behavior of the receiver.  In addition, byte
 counting also mitigates the retarded window growth provided by
 receivers that generate stretch ACKs because of the capacity of the
 return link, as discussed in [BPK97,BPK98].  Therefore, this change
 is expected to be especially beneficial to asymmetric networks.

3.2.2.5 Possible Interaction and Relationships with Other Research

 Unlimited byte counting should not be used without a method to
 mitigate the potentially large line-rate bursts the algorithm can
 cause.  Also, LBC may send bursts that are too large for the given
 network conditions.  In this case, LBC may also benefit from some
 algorithm that would lessen the impact of line-rate bursts of
 segments.  Also note that using delayed ACKs only after slow start
 (as outlined in section 3.2.3) negates the limited byte counting
 algorithm because each ACK covers only one segment during slow start.
 Therefore, both ACK counting and byte counting yield the same
 increase in the congestion window at this point (in the first RTT).

3.2.3 Delayed ACKs After Slow Start

3.2.3.1 Mitigation Description

 As discussed above, TCP senders use the number of incoming ACKs to
 increase the congestion window during slow start.  And, since delayed
 ACKs reduce the number of ACKs returned by the receiver by roughly
 half, the rate of growth of the congestion window is reduced.  One
 proposed solution to this problem is to use delayed ACKs only after
 the slow start (DAASS) phase.  This provides more ACKs while TCP is
 aggressively increasing the congestion window and less ACKs while TCP
 is in steady state, which conserves network resources.

Allman, et al. Informational [Page 9] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

3.2.3.2 Research

 [All98] shows that in simulation, using delayed ACKs after slow start
 (DAASS) improves transfer time when compared to a receiver that
 always generates delayed ACKs.  However, DAASS also slightly
 increases the loss rate due to the increased rate of cwnd growth.

3.2.3.3 Implementation Issues

 The major problem with DAASS is in the implementation.  The receiver
 has to somehow know when the sender is using the slow start
 algorithm.  The receiver could implement a heuristic that attempts to
 watch the change in the amount of data being received and change the
 ACKing behavior accordingly.  Or, the sender could send a message (a
 flipped bit in the TCP header, perhaps) indicating that it was using
 slow start.  The implementation of DAASS is, therefore, an open
 issue.
 Using DAASS does not violate the TCP congestion control specification
 [APS99].  However, the standards (RFC 2581 [APS99]) currently
 recommend using delayed acknowledgments and DAASS goes (partially)
 against this recommendation.

3.2.3.4 Topology Considerations

 DAASS should work equally well in all scenarios presented in section
 2.  However, in asymmetric networks it may aggravate ACK congestion
 in the return link, due to the increased number of ACKs (see sections
 3.9 and 3.10 for a more detailed discussion of ACK congestion).

3.2.3.5 Possible Interaction and Relationships with Other Research

 DAASS has several possible interactions with other proposals made in
 the research community.  DAASS can aggravate congestion on the path
 between the data receiver and the data sender due to the increased
 number of returning acknowledgments.  This can have an especially
 adverse effect on asymmetric networks that are prone to experiencing
 ACK congestion.  As outlined in sections 3.9 and 3.10, several
 mitigations have been proposed to reduce the number of ACKs that are
 passed over a low-bandwidth return link.  Using DAASS will increase
 the number of ACKs sent by the receiver.  The interaction between
 DAASS and the methods for reducing the number of ACKs is an open
 research question.  Also, as noted in section 3.2.1.5 above, DAASS
 provides some of the same benefits as using a larger initial
 congestion window and therefore it may not be desirable to use both
 mechanisms together.  However, this remains an open question.
 Finally, DAASS and limited byte counting are both used to increase

Allman, et al. Informational [Page 10] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 the rate at which the congestion window is opened.  The DAASS
 algorithm substantially reduces the impact limited byte counting has
 on the rate of congestion window increase.

3.2.4 Terminating Slow Start

3.2.4.1 Mitigation Description

 The initial slow start phase is used by TCP to determine an
 appropriate congestion window size for the given network conditions
 [Jac88].  Slow start is terminated when TCP detects congestion, or
 when the size of cwnd reaches the size of the receiver's advertised
 window.  Slow start is also terminated if cwnd grows beyond a certain
 size.  The threshold at which TCP ends slow start and begins using
 the congestion avoidance algorithm is called "ssthresh" [Jac88].  In
 most implementations, the initial value for ssthresh is the
 receiver's advertised window.  During slow start, TCP roughly doubles
 the size of cwnd every RTT and therefore can overwhelm the network
 with at most twice as many segments as the network can handle.  By
 setting ssthresh to a value less than the receiver's advertised
 window initially, the sender may avoid overwhelming the network with
 twice the appropriate number of segments.  Hoe [Hoe96] proposes using
 the packet-pair algorithm [Kes91] and the measured RTT to determine a
 more appropriate value for ssthresh.  The algorithm observes the
 spacing between the first few returning ACKs to determine the
 bandwidth of the bottleneck link.  Together with the measured RTT,
 the delay*bandwidth product is determined and ssthresh is set to this
 value.  When TCP's cwnd reaches this reduced ssthresh, slow start is
 terminated and transmission continues using congestion avoidance,
 which is a more conservative algorithm for increasing the size of the
 congestion window.

3.2.4.2 Research

 It has been shown that estimating ssthresh can improve performance
 and decrease packet loss in simulations [Hoe96].  However, obtaining
 an accurate estimate of the available bandwidth in a dynamic network
 is very challenging, especially attempting to do so on the sending
 side of the TCP connection [AP99].  Therefore, before this mechanism
 is widely deployed, bandwidth estimation must be studied in a more
 detail.

3.2.4.3 Implementation Issues

 As outlined in [Hoe96], estimating ssthresh requires changes to the
 data sender's TCP stack.  As suggested in [AP99], bandwidth estimates
 may be more accurate when taken by the TCP receiver, and therefore
 both sender and receiver changes would be required.  Estimating

Allman, et al. Informational [Page 11] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 ssthresh is safe to implement in production networks from a
 congestion control perspective, as it can only make TCP more
 conservative than outlined in RFC 2581 [APS99] (assuming the TCP
 implementation is using an initial ssthresh of infinity as allowed by
 [APS99]).

3.2.4.4 Topology Considerations

 It is expected that this mechanism will work equally well in all
 symmetric topologies outlined in section 2.  However, asymmetric
 links pose a special problem, as the rate of the returning ACKs may
 not be the bottleneck bandwidth in the forward direction.  This can
 lead to the sender setting ssthresh too low.  Premature termination
 of slow start can hurt performance, as congestion avoidance opens
 cwnd more conservatively.  Receiver-based bandwidth estimators do not
 suffer from this problem.

3.2.4.5 Possible Interaction and Relationships with Other Research

 Terminating slow start at the right time is useful to avoid multiple
 dropped segments.  However, using a selective acknowledgment-based
 loss recovery scheme (as outlined in section 3.3.2) can drastically
 improve TCP's ability to quickly recover from multiple lost segments
 Therefore, it may not be as important to terminate slow start before
 a large loss event occurs.  [AP99] shows that using delayed
 acknowledgments [Bra89] reduces the effectiveness of sender-side
 bandwidth estimation.  Therefore, using delayed ACKs only during slow
 start (as outlined in section 3.2.3) may make bandwidth estimation
 more feasible.

3.3 Loss Recovery

3.3.1 Non-SACK Based Mechanisms

3.3.1.1 Mitigation Description

 Several similar algorithms have been developed and studied that
 improve TCP's ability to recover from multiple lost segments in a
 window of data without relying on the (often long) retransmission
 timeout.  These sender-side algorithms, known as NewReno TCP, do not
 depend on the availability of selective acknowledgments (SACKs)
 [MMFR96].
 These algorithms generally work by updating the fast recovery
 algorithm to use information provided by "partial ACKs" to trigger
 retransmissions.  A partial ACK covers some new data, but not all
 data outstanding when a particular loss event starts.  For instance,
 consider the case when segment N is retransmitted using the fast

Allman, et al. Informational [Page 12] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 retransmit algorithm and segment M is the last segment sent when
 segment N is resent.  If segment N is the only segment lost, the ACK
 elicited by the retransmission of segment N would be for segment M.
 If, however, segment N+1 was also lost, the ACK elicited by the
 retransmission of segment N will be N+1.  This can be taken as an
 indication that segment N+1 was lost and used to trigger a
 retransmission.

3.3.1.2 Research

 Hoe [Hoe95,Hoe96] introduced the idea of using partial ACKs to
 trigger retransmissions and showed that doing so could improve
 performance.  [FF96] shows that in some cases using partial ACKs to
 trigger retransmissions reduces the time required to recover from
 multiple lost segments.  However, [FF96] also shows that in some
 cases (many lost segments) relying on the RTO timer can improve
 performance over simply using partial ACKs to trigger all
 retransmissions.  [HK99] shows that using partial ACKs to trigger
 retransmissions, in conjunction with SACK, improves performance when
 compared to TCP using fast retransmit/fast recovery in a satellite
 environment.  Finally, [FH99] describes several slightly different
 variants of NewReno.

3.3.1.3 Implementation Issues

 Implementing these fast recovery enhancements requires changes to the
 sender-side TCP stack.  These changes can safely be implemented in
 production networks and are allowed by RFC 2581 [APS99].

3.3.1.4 Topology Considerations

 It is expected that these changes will work well in all environments
 outlined in section 2.

3.3.1.5 Possible Interaction and Relationships with Other Research

 See section 3.3.2.2.5.

3.3.2 SACK Based Mechanisms

3.3.2.1 Fast Recovery with SACK

3.3.2.1.1 Mitigation Description

 Fall and Floyd [FF96] describe a conservative extension to the fast
 recovery algorithm that takes into account information provided by
 selective acknowledgments (SACKs) [MMFR96] sent by the receiver.  The
 algorithm starts after fast retransmit triggers the resending of a

Allman, et al. Informational [Page 13] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 segment.  As with fast retransmit, the algorithm cuts cwnd in half
 when a loss is detected.  The algorithm keeps a variable called
 "pipe", which is an estimate of the number of outstanding segments in
 the network.  The pipe variable is decremented by 1 segment for each
 duplicate ACK that arrives with new SACK information.  The pipe
 variable is incremented by 1 for each new or retransmitted segment
 sent.  A segment may be sent when the value of pipe is less than cwnd
 (this segment is either a retransmission per the SACK information or
 a new segment if the SACK information indicates that no more
 retransmits are needed).
 This algorithm generally allows TCP to recover from multiple segment
 losses in a window of data within one RTT of loss detection.  Like
 the forward acknowledgment (FACK) algorithm described below, the SACK
 information allows the pipe algorithm to decouple the choice of when
 to send a segment from the choice of what segment to send.
 [APS99] allows the use of this algorithm, as it is consistent with
 the spirit of the fast recovery algorithm.

3.3.2.1.2 Research

 [FF96] shows that the above described SACK algorithm performs better
 than several non-SACK based recovery algorithms when 1--4 segments
 are lost from a window of data.  [AHKO97] shows that the algorithm
 improves performance over satellite links.  Hayes [Hay97] shows the
 in certain circumstances, the SACK algorithm can hurt performance by
 generating a large line-rate burst of data at the end of loss
 recovery, which causes further loss.

3.3.2.1.3 Implementation Issues

 This algorithm is implemented in the sender's TCP stack.  However, it
 relies on SACK information generated by the receiver.  This algorithm
 is safe for shared networks and is allowed by RFC 2581 [APS99].

3.3.2.1.4 Topology Considerations

 It is expected that the pipe algorithm will work equally well in all
 scenarios presented in section 2.

3.3.2.1.5 Possible Interaction and Relationships with Other Research

 See section 3.3.2.2.5.

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3.3.2.2 Forward Acknowledgments

3.3.2.2.1 Mitigation Description

 The Forward Acknowledgment (FACK) algorithm [MM96a,MM96b] was
 developed to improve TCP congestion control during loss recovery.
 FACK uses TCP SACK options to glean additional information about the
 congestion state, adding more precise control to the injection of
 data into the network during recovery.  FACK decouples the congestion
 control algorithms from the data recovery algorithms to provide a
 simple and direct way to use SACK to improve congestion control.  Due
 to the separation of these two algorithms, new data may be sent
 during recovery to sustain TCP's self-clock when there is no further
 data to retransmit.
 The most recent version of FACK is Rate-Halving [MM96b], in which one
 packet is sent for every two ACKs received during recovery.
 Transmitting a segment for every-other ACK has the result of reducing
 the congestion window in one round trip to half of the number of
 packets that were successfully handled by the network (so when cwnd
 is too large by more than a factor of two it still gets reduced to
 half of what the network can sustain).  Another important aspect of
 FACK with Rate-Halving is that it sustains the ACK self-clock during
 recovery because transmitting a packet for every-other ACK does not
 require half a cwnd of data to drain from the network before
 transmitting, as required by the fast recovery algorithm
 [Ste97,APS99].
 In addition, the FACK with Rate-Halving implementation provides
 Thresholded Retransmission to each lost segment.  "Tcprexmtthresh" is
 the number of duplicate ACKs required by TCP to trigger a fast
 retransmit and enter recovery.  FACK applies thresholded
 retransmission to all segments by waiting until tcprexmtthresh SACK
 blocks indicate that a given segment is missing before resending the
 segment.  This allows reasonable behavior on links that reorder
 segments.  As described above, FACK sends a segment for every second
 ACK received during recovery.  New segments are transmitted except
 when tcprexmtthresh SACK blocks have been observed for a dropped
 segment, at which point the dropped segment is retransmitted.
 [APS99] allows the use of this algorithm, as it is consistent with
 the spirit of the fast recovery algorithm.

3.3.2.2.2 Research

 The original FACK algorithm is outlined in [MM96a].  The algorithm
 was later enhanced to include Rate-Halving [MM96b].  The real-world
 performance of FACK with Rate-Halving was shown to be much closer to

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 the theoretical maximum for TCP than either TCP Reno or the SACK-
 based extensions to fast recovery outlined in section 3.3.2.1
 [MSMO97].

3.3.2.2.3 Implementation Issues

 In order to use FACK, the sender's TCP stack must be modified.  In
 addition, the receiver must be able to generate SACK options to
 obtain the full benefit of using FACK.  The FACK algorithm is safe
 for shared networks and is allowed by RFC 2581 [APS99].

3.3.2.2.4 Topology Considerations

 FACK is expected to improve performance in all environments outlined
 in section 2.  Since it is better able to sustain its self-clock than
 TCP Reno, it may be considerably more attractive over long delay
 paths.

3.3.2.2.5 Possible Interaction and Relationships with Other Research

 Both SACK based loss recovery algorithms described above (the fast
 recovery enhancement and the FACK algorithm) are similar in that they
 attempt to effectively repair multiple lost segments from a window of
 data.  Which of the SACK-based loss recovery algorithms to use is
 still an open research question.  In addition, these algorithms are
 similar to the non-SACK NewReno algorithm described in section 3.3.1,
 in that they attempt to recover from multiple lost segments without
 reverting to using the retransmission timer.  As has been shown, the
 above SACK based algorithms are more robust than the NewReno
 algorithm.  However, the SACK algorithm requires a cooperating TCP
 receiver, which the NewReno algorithm does not.  A reasonable TCP
 implementation might include both a SACK-based and a NewReno-based
 loss recovery algorithm such that the sender can use the most
 appropriate loss recovery algorithm based on whether or not the
 receiver supports SACKs.  Finally, both SACK-based and non-SACK-based
 versions of fast recovery have been shown to transmit a large burst
 of data upon leaving loss recovery, in some cases [Hay97].
 Therefore, the algorithms may benefit from some burst suppression
 algorithm.

3.3.3 Explicit Congestion Notification

3.3.3.1 Mitigation Description

 Explicit congestion notification (ECN) allows routers to inform TCP
 senders about imminent congestion without dropping segments.  Two
 major forms of ECN have been studied.  A router employing backward
 ECN (BECN), transmits messages directly to the data originator

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 informing it of congestion.  IP routers can accomplish this with an
 ICMP Source Quench message.  The arrival of a BECN signal may or may
 not mean that a TCP data segment has been dropped, but it is a clear
 indication that the TCP sender should reduce its sending rate (i.e.,
 the value of cwnd).  The second major form of congestion notification
 is forward ECN (FECN).  FECN routers mark data segments with a
 special tag when congestion is imminent, but forward the data
 segment.  The data receiver then echos the congestion information
 back to the sender in the ACK packet.  A description of a FECN
 mechanism for TCP/IP is given in [RF99].
 As described in [RF99], senders transmit segments with an "ECN-
 Capable Transport" bit set in the IP header of each packet.  If a
 router employing an active queueing strategy, such as Random Early
 Detection (RED) [FJ93,BCC+98], would otherwise drop this segment, an
 "Congestion Experienced" bit in the IP header is set instead.  Upon
 reception, the information is echoed back to TCP senders using a bit
 in the TCP header.  The TCP sender adjusts the congestion window just
 as it would if a segment was dropped.
 The implementation of ECN as specified in [RF99] requires the
 deployment of active queue management mechanisms in the affected
 routers.  This allows the routers to signal congestion by sending TCP
 a small number of "congestion signals" (segment drops or ECN
 messages), rather than discarding a large number of segments, as can
 happen when TCP overwhelms a drop-tail router queue.
 Since satellite networks generally have higher bit-error rates than
 terrestrial networks, determining whether a segment was lost due to
 congestion or corruption may allow TCP to achieve better performance
 in high BER environments than currently possible (due to TCP's
 assumption that all loss is due to congestion).  While not a solution
 to this problem, adding an ECN mechanism to TCP may be a part of a
 mechanism that will help achieve this goal.  See section 3.3.4 for a
 more detailed discussion of differentiating between corruption and
 congestion based losses.

3.3.3.2 Research

 [Flo94] shows that ECN is effective in reducing the segment loss rate
 which yields better performance especially for short and interactive
 TCP connections.  Furthermore, [Flo94] also shows that ECN avoids
 some unnecessary, and costly TCP retransmission timeouts.  Finally,
 [Flo94] also considers some of the advantages and disadvantages of
 various forms of explicit congestion notification.

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3.3.3.3 Implementation Issues

 Deployment of ECN requires changes to the TCP implementation on both
 sender and receiver.  Additionally, deployment of ECN requires
 deployment of some active queue management infrastructure in routers.
 RED is assumed in most ECN discussions, because RED is already
 identifying segments to drop, even before its buffer space is
 exhausted.  ECN simply allows the delivery of "marked" segments while
 still notifying the end nodes that congestion is occurring along the
 path.  ECN is safe (from a congestion control perspective) for shared
 networks, as it maintains the same TCP congestion control principles
 as are used when congestion is detected via segment drops.

3.3.3.4 Topology Considerations

 It is expected that none of the environments outlined in section 2
 will present a bias towards or against ECN traffic.

3.3.3.5 Possible Interaction and Relationships with Other Research

 Note that some form of active queueing is necessary to use ECN (e.g.,
 RED queueing).

3.3.4 Detecting Corruption Loss

 Differentiating between congestion (loss of segments due to router
 buffer overflow or imminent buffer overflow) and corruption (loss of
 segments due to damaged bits) is a difficult problem for TCP.  This
 differentiation is particularly important because the action that TCP
 should take in the two cases is entirely different.  In the case of
 corruption, TCP should merely retransmit the damaged segment as soon
 as its loss is detected; there is no need for TCP to adjust its
 congestion window.  On the other hand, as has been widely discussed
 above, when the TCP sender detects congestion, it should immediately
 reduce its congestion window to avoid making the congestion worse.
 TCP's defined behavior, as motivated by [Jac88,Jac90] and defined in
 [Bra89,Ste97,APS99], is to assume that all loss is due to congestion
 and to trigger the congestion control algorithms, as defined in
 [Ste97,APS99].  The loss may be detected using the fast retransmit
 algorithm, or in the worst case is detected by the expiration of
 TCP's retransmission timer.
 TCP's assumption that loss is due to congestion rather than
 corruption is a conservative mechanism that prevents congestion
 collapse [Jac88,FF98].  Over satellite networks, however, as in many
 wireless environments, loss due to corruption is more common than on
 terrestrial networks.  One common partial solution to this problem is

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 to add Forward Error Correction (FEC) to the data that's sent over
 the satellite/wireless link.  A more complete discussion of the
 benefits of FEC can be found in [AGS99].  However, given that FEC
 does not always work or cannot be universally applied, other
 mechanisms have been studied to attempt to make TCP able to
 differentiate between congestion-based and corruption-based loss.
 TCP segments that have been corrupted are most often dropped by
 intervening routers when link-level checksum mechanisms detect that
 an incoming frame has errors.  Occasionally, a TCP segment containing
 an error may survive without detection until it arrives at the TCP
 receiving host, at which point it will almost always either fail the
 IP header checksum or the TCP checksum and be discarded as in the
 link-level error case.  Unfortunately, in either of these cases, it's
 not generally safe for the node detecting the corruption to return
 information about the corrupt packet to the TCP sender because the
 sending address itself might have been corrupted.

3.3.4.1 Mitigation Description

 Because the probability of link errors on a satellite link is
 relatively greater than on a hardwired link, it is particularly
 important that the TCP sender retransmit these lost segments without
 reducing its congestion window.  Because corrupt segments do not
 indicate congestion, there is no need for the TCP sender to enter a
 congestion avoidance phase, which may waste available bandwidth.
 Simulations performed in [SF98] show a performance improvement when
 TCP can properly differentiate between between corruption and
 congestion of wireless links.
 Perhaps the greatest research challenge in detecting corruption is
 getting TCP (a transport-layer protocol) to receive appropriate
 information from either the network layer (IP) or the link layer.
 Much of the work done to date has involved link-layer mechanisms that
 retransmit damaged segments.  The challenge seems to be to get these
 mechanisms to make repairs in such a way that TCP understands what
 happened and can respond appropriately.

3.3.4.2 Research

 Research into corruption detection to date has focused primarily on
 making the link level detect errors and then perform link-level
 retransmissions.  This work is summarized in [BKVP97,BPSK96].  One of
 the problems with this promising technique is that it causes an
 effective reordering of the segments from the TCP receiver's point of
 view.  As a simple example, if segments A B C D are sent across a
 noisy link and segment B is corrupted, segments C and D may have
 already crossed the link before B can be retransmitted at the link

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 level, causing them to arrive at the TCP receiver in the order A C D
 B.  This segment reordering would cause the TCP receiver to generate
 duplicate ACKs upon the arrival of segments C and D.  If the
 reordering was bad enough, the sender would trigger the fast
 retransmit algorithm in the TCP sender, in response to the duplicate
 ACKs.  Research presented in [MV98] proposes the idea of suppressing
 or delaying the duplicate ACKs in the reverse direction to counteract
 this behavior.  Alternatively, proposals that make TCP more robust in
 the face of re-ordered segment arrivals [Flo99] may reduce the side
 effects of the re-ordering caused by link-layer retransmissions.
 A more high-level approach, outlined in the [DMT96], uses a new
 "corruption experienced" ICMP error message generated by routers that
 detect corruption.  These messages are sent in the forward direction,
 toward the packet's destination, rather than in the reverse direction
 as is done with ICMP Source Quench messages.  Sending the error
 messages in the forward direction allows this feedback to work over
 asymmetric paths.  As noted above, generating an error message in
 response to a damaged packet is problematic because the source and
 destination addresses may not be valid.  The mechanism outlined in
 [DMT96] gets around this problem by having the routers maintain a
 small cache of recent packet destinations; when the router
 experiences an error rate above some threshold, it sends an ICMP
 corruption-experienced message to all of the destinations in its
 cache.  Each TCP receiver then must return this information to its
 respective TCP sender (through a TCP option).  Upon receiving an ACK
 with this "corruption-experienced" option, the TCP sender assumes
 that packet loss is due to corruption rather than congestion for two
 round trip times (RTT) or until it receives additional link state
 information (such as "link down", source quench, or additional
 "corruption experienced" messages).  Note that in shared networks,
 ignoring segment loss for 2 RTTs may aggravate congestion by making
 TCP unresponsive.

3.3.4.3 Implementation Issues

 All of the techniques discussed above require changes to at least the
 TCP sending and receiving stacks, as well as intermediate routers.
 Due to the concerns over possibly ignoring congestion signals (i.e.,
 segment drops), the above algorithm is not recommended for use in
 shared networks.

3.3.4.4 Topology Considerations

 It is expected that corruption detection, in general would be
 beneficial in all environments outlined in section 2.  It would be
 particularly beneficial in the satellite/wireless environment over
 which these errors may be more prevalent.

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3.3.4.5 Possible Interaction and Relationships with Other Research

 SACK-based loss recovery algorithms (as described in 3.3.2) may
 reduce the impact of corrupted segments on mostly clean links because
 recovery will be able to happen more rapidly (and without relying on
 the retransmission timer).  Note that while SACK-based loss recovery
 helps, throughput will still suffer in the face of non-congestion
 related packet loss.

3.4 Congestion Avoidance

3.4.1 Mitigation Description

 During congestion avoidance, in the absence of loss, the TCP sender
 adds approximately one segment to its congestion window during each
 RTT [Jac88,Ste97,APS99].  Several researchers have observed that this
 policy leads to unfair sharing of bandwidth when multiple connections
 with different RTTs traverse the same bottleneck link, with the long
 RTT connections obtaining only a small fraction of their fair share
 of the bandwidth.
 One effective solution to this problem is to deploy fair queueing and
 TCP-friendly buffer management in network routers [Sut98].  However,
 in the absence of help from the network, other researchers have
 investigated changes to the congestion avoidance policy at the TCP
 sender, as described in [Flo91,HK98].

3.4.2 Research

 The "Constant-Rate" increase policy has been studied in [Flo91,HK98].
 It attempts to equalize the rate at which TCP senders increase their
 sending rate during congestion avoidance.  Both [Flo91] and [HK98]
 illustrate cases in which the "Constant-Rate" policy largely corrects
 the bias against long RTT connections, although [HK98] presents some
 evidence that such a policy may be difficult to incrementally deploy
 in an operational network.  The proper selection of a constant (for
 the constant rate of increase) is an open issue.
 The "Increase-by-K" policy can be selectively used by long RTT
 connections in a heterogeneous environment.  This policy simply
 changes the slope of the linear increase, with connections over a
 given RTT threshold adding "K" segments to the congestion window
 every RTT, instead of one.  [HK98] presents evidence that this
 policy, when used with small values of "K", may be successful in
 reducing the unfairness while keeping the link utilization high, when
 a small number of connections share a bottleneck link.  The selection
 of the constant "K," the RTT threshold to invoke this policy, and
 performance under a large number of flows are all open issues.

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3.4.3 Implementation Issues

 Implementation of either the "Constant-Rate" or "Increase-by-K"
 policies requires a change to the congestion avoidance mechanism at
 the TCP sender.  In the case of "Constant-Rate," such a change must
 be implemented globally.  Additionally, the TCP sender must have a
 reasonably accurate estimate of the RTT of the connection.  The
 algorithms outlined above violate the congestion avoidance algorithm
 as outlined in RFC 2581 [APS99] and therefore should not be
 implemented in shared networks at this time.

3.4.4 Topology Considerations

 These solutions are applicable to all satellite networks that are
 integrated with a terrestrial network, in which satellite connections
 may be competing with terrestrial connections for the same bottleneck
 link.

3.4.5 Possible Interaction and Relationships with Other Research

 As shown in [PADHV99], increasing the congestion window by multiple
 segments per RTT can cause TCP to drop multiple segments and force a
 retransmission timeout in some versions of TCP.  Therefore, the above
 changes to the congestion avoidance algorithm may need to be
 accompanied by a SACK-based loss recovery algorithm that can quickly
 repair multiple dropped segments.

3.5 Multiple Data Connections

3.5.1 Mitigation Description

 One method that has been used to overcome TCP's inefficiencies in the
 satellite environment is to use multiple TCP flows to transfer a
 given file.  The use of N TCP connections makes the sender N times
 more aggressive and therefore can improve throughput in some
 situations.  Using N multiple TCP connections can impact the transfer
 and the network in a number of ways, which are listed below.
 1. The transfer is able to start transmission using an effective
    congestion window of N segments, rather than a single segment as
    one TCP flow uses.  This allows the transfer to more quickly
    increase the effective cwnd size to an appropriate size for the
    given network.  However, in some circumstances an initial window
    of N segments is inappropriate for the network conditions.  In
    this case, a transfer utilizing more than one connection may
    aggravate congestion.

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 2. During the congestion avoidance phase, the transfer increases the
    effective cwnd by N segments per RTT, rather than the one segment
    per RTT increase that a single TCP connection provides.  Again,
    this can aid the transfer by more rapidly increasing the effective
    cwnd to an appropriate point.  However, this rate of increase can
    also be too aggressive for the network conditions.  In this case,
    the use of multiple data connections can aggravate congestion in
    the network.
 3. Using multiple connections can provide a very large overall
    congestion window.  This can be an advantage for TCP
    implementations that do not support the TCP window scaling
    extension [JBB92].  However, the aggregate cwnd size across all N
    connections is equivalent to using a TCP implementation that
    supports large windows.
 4. The overall cwnd decrease in the face of dropped segments is
    reduced when using N parallel connections.  A single TCP
    connection reduces the effective size of cwnd to half when a
    single segment loss is detected.  When utilizing N connections
    each using a window of W bytes, a single drop reduces the window
    to:
      (N * W) - (W / 2)
 Clearly this is a less dramatic reduction in the effective cwnd size
 than when using a single TCP connection.  And, the amount by which
 the cwnd is decreased is further reduced by increasing N.
 The use of multiple data connections can increase the ability of
 non-SACK TCP implementations to quickly recover from multiple dropped
 segments without resorting to a timeout, assuming the dropped
 segments cross connections.
 The use of multiple parallel connections makes TCP overly aggressive
 for many environments and can contribute to congestive collapse in
 shared networks [FF99].  The advantages provided by using multiple
 TCP connections are now largely provided by TCP extensions (larger
 windows, SACKs, etc.).  Therefore, the use of a single TCP connection
 is more "network friendly" than using multiple parallel connections.
 However, using multiple parallel TCP connections may provide
 performance improvement in private networks.

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3.5.2 Research

 Research on the use of multiple parallel TCP connections shows
 improved performance [IL92,Hah94,AOK95,AKO96].  In addition, research
 has shown that multiple TCP connections can outperform a single
 modern TCP connection (with large windows and SACK) [AHKO97].
 However, these studies did not consider the impact of using multiple
 TCP connections on competing traffic.  [FF99] argues that using
 multiple simultaneous connections to transfer a given file may lead
 to congestive collapse in shared networks.

3.5.3 Implementation Issues

 To utilize multiple parallel TCP connections a client application and
 the corresponding server must be customized.  As outlined in [FF99]
 using multiple parallel TCP connections is not safe (from a
 congestion control perspective) in shared networks and should not be
 used.

3.5.4 Topological Considerations

 As stated above, [FF99] outlines that the use of multiple parallel
 connections in a shared network, such as the Internet, may lead to
 congestive collapse.  However, the use of multiple connections may be
 safe and beneficial in private networks.  The specific topology being
 used will dictate the number of parallel connections required.  Some
 work has been done to determine the appropriate number of connections
 on the fly [AKO96], but such a mechanism is far from complete.

3.5.5 Possible Interaction and Relationships with Other Research

 Using multiple concurrent TCP connections enables use of a large
 congestion window, much like the TCP window scaling option [JBB92].
 In addition, a larger initial congestion window is achieved, similar
 to using [AFP98] or TCB sharing (see section 3.8).

3.6 Pacing TCP Segments

3.6.1 Mitigation Description

 Slow-start takes several round trips to fully open the TCP congestion
 window over routes with high bandwidth-delay products.  For short TCP
 connections (such as WWW traffic with HTTP/1.0), the slow-start
 overhead can preclude effective use of the high-bandwidth satellite
 links.  When senders implement slow-start restart after a TCP
 connection goes idle (suggested by Jacobson and Karels [JK92]),

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 performance is reduced in long-lived (but bursty) connections (such
 as HTTP/1.1, which uses persistent TCP connections to transfer
 multiple WWW page elements) [Hei97a].
 Rate-based pacing (RBP) is a technique, used in the absence of
 incoming ACKs, where the data sender temporarily paces TCP segments
 at a given rate to restart the ACK clock.  Upon receipt of the first
 ACK, pacing is discontinued and normal TCP ACK clocking resumes.  The
 pacing rate may either be known from recent traffic estimates (when
 restarting an idle connection or from recent prior connections), or
 may be known through external means (perhaps in a point-to-point or
 point-to-multipoint satellite network where available bandwidth can
 be assumed to be large).
 In addition, pacing data during the first RTT of a transfer may allow
 TCP to make effective use of high bandwidth-delay links even for
 short transfers.  However, in order to pace segments during the first
 RTT a TCP will have to be using a non-standard initial congestion
 window and a new mechanism to pace outgoing segments rather than send
 them back-to-back.  Determining an appropriate size for the initial
 cwnd is an open research question.  Pacing can also be used to reduce
 bursts in general (due to buggy TCPs or byte counting, see section
 3.2.2 for a discussion on byte counting).

3.6.2 Research

 Simulation studies of rate-paced pacing for WWW-like traffic have
 shown reductions in router congestion and drop rates [VH97a].  In
 this environment, RBP substantially improves performance compared to
 slow-start-after-idle for intermittent senders, and it slightly
 improves performance over burst-full-cwnd-after-idle (because of
 drops) [VH98].  More recently, pacing has been suggested to eliminate
 burstiness in networks with ACK filtering [BPK97].

3.6.3 Implementation Issues

 RBP requires only sender-side changes to TCP.  Prototype
 implementations of RBP are available [VH97b].  RBP requires an
 additional sender timer for pacing.  The overhead of timer-driven
 data transfer is often considered too high for practical use.
 Preliminary experiments suggest that in RBP this overhead is minimal
 because RBP only requires this timer for one RTT of transmission
 [VH98].  RBP is expected to make TCP more conservative in sending
 bursts of data after an idle period in hosts that do not revert to
 slow start after an idle period.  On the other hand, RBP makes TCP
 more aggressive if the sender uses the slow start algorithm to start
 the ACK clock after a long idle period.

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3.6.4 Topology Considerations

 RBP could be used to restart idle TCP connections for all topologies
 in Section 2.  Use at the beginning of new connections would be
 restricted to topologies where available bandwidth can be estimated
 out-of-band.

3.6.5 Possible Interaction and Relationships with Other Research

 Pacing segments may benefit from sharing state amongst various flows
 between two hosts, due to the time required to determine the needed
 information.  Additionally, pacing segments, rather than sending
 back-to-back segments, may make estimating the available bandwidth
 (as outlined in section 3.2.4) more difficult.

3.7 TCP Header Compression

 The TCP and IP header information needed to reliably deliver packets
 to a remote site across the Internet can add significant overhead,
 especially for interactive applications.  Telnet packets, for
 example, typically carry only a few bytes of data per packet, and
 standard IPv4/TCP headers add at least 40 bytes to this; IPv6/TCP
 headers add at least 60 bytes.  Much of this information remains
 relatively constant over the course of a session and so can be
 replaced by a short session identifier.

3.7.1 Mitigation Description

 Many fields in the TCP and IP headers either remain constant during
 the course of a session, change very infrequently, or can be inferred
 from other sources.  For example, the source and destination
 addresses, as well as the IP version, protocol, and port fields
 generally do not change during a session.  Packet length can be
 deduced from the length field of the underlying link layer protocol
 provided that the link layer packet is not padded.  Packet sequence
 numbers in a forward data stream generally change with every packet,
 but increase in a predictable manner.
 The TCP/IP header compression methods described in
 [DNP99,DENP97,Jac90] reduce the overhead of TCP sessions by replacing
 the data in the TCP and IP headers that remains constant, changes
 slowly, or changes in a predictable manner with a short "connection
 number".  Using this method, the sender first sends a full TCP/IP
 header, including in it a connection number that the sender will use
 to reference the connection.  The receiver stores the full header and
 uses it as a template, filling in some fields from the limited

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 information contained in later, compressed headers.  This compression
 can reduce the size of an IPv4/TCP headers from 40 to as few as 3 to
 5 bytes (3 bytes for some common cases, 5 bytes in general).
 Compression and decompression generally happen below the IP layer, at
 the end-points of a given physical link (such as at two routers
 connected by a serial line).  The hosts on either side of the
 physical link must maintain some state about the TCP connections that
 are using the link.
 The decompresser must pass complete, uncompressed packets to the IP
 layer.  Thus header compression is transparent to routing, for
 example, since an incoming packet with compressed headers is expanded
 before being passed to the IP layer.
 A variety of methods can be used by the compressor/decompressor to
 negotiate the use of header compression.  For example, the PPP serial
 line protocol allows for an option exchange, during which time the
 compressor/decompressor agree on whether or not to use header
 compression.  For older SLIP implementations, [Jac90] describes a
 mechanism that uses the first bit in the IP packet as a flag.
 The reduction in overhead is especially useful when the link is
 bandwidth-limited such as terrestrial wireless and mobile satellite
 links, where the overhead associated with transmitting the header
 bits is nontrivial.  Header compression has the added advantage that
 for the case of uniformly distributed bit errors, compressing TCP/IP
 headers can provide a better quality of service by decreasing the
 packet error probability.  The shorter, compressed packets are less
 likely to be corrupted, and the reduction in errors increases the
 connection's throughput.
 Extra space is saved by encoding changes in fields that change
 relatively slowly by sending only their difference from their values
 in the previous packet instead of their absolute values.  In order to
 decode headers compressed this way, the receiver keeps a copy of each
 full, reconstructed TCP header after it is decoded, and applies the
 delta values from the next decoded compressed header to the
 reconstructed full header template.
 A disadvantage to using this delta encoding scheme where values are
 encoded as deltas from their values in the previous packet is that if
 a single compressed packet is lost, subsequent packets with
 compressed headers can become garbled if they contain fields which
 depend on the lost packet.  Consider a forward data stream of packets
 with compressed headers and increasing sequence numbers.  If packet N
 is lost, the full header of packet N+1 will be reconstructed at the
 receiver using packet N-1's full header as a template.  Thus the

Allman, et al. Informational [Page 27] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 sequence number, which should have been calculated from packet N's
 header, will be wrong, the checksum will fail, and the packet will be
 discarded.  When the sending TCP times out and retransmits a packet
 with a full header is forwarded to re-synchronize the decompresser.
 It is important to note that the compressor does not maintain any
 timers, nor does the decompresser know when an error occurred (only
 the receiving TCP knows this, when the TCP checksum fails).  A single
 bit error will cause the decompresser to lose sync, and subsequent
 packets with compressed headers will be dropped by the receiving TCP,
 since they will all fail the TCP checksum. When this happens, no
 duplicate acknowledgments will be generated, and the decompresser can
 only re-synchronize when it receives a packet with an uncompressed
 header.  This means that when header compression is being used, both
 fast retransmit and selective acknowledgments will not be able
 correct packets lost on a compressed link.  The "twice" algorithm,
 described below, may be a partial solution to this problem.
 [DNP99] and [DENP97] describe TCP/IPv4 and TCP/IPv6 compression
 algorithms including compressing the various IPv6 extension headers
 as well as methods for compressing non-TCP streams.  [DENP97] also
 augments TCP header compression by introducing the "twice" algorithm.
 If a particular packet fails to decompress properly, the twice
 algorithm modifies its assumptions about the inferred fields in the
 compressed header, assuming that a packet identical to the current
 one was dropped between the last correctly decoded packet and the
 current one.  Twice then tries to decompress the received packet
 under the new assumptions and, if the checksum passes, the packet is
 passed to IP and the decompresser state has been re-synchronized.
 This procedure can be extended to three or more decoding attempts.
 Additional robustness can be achieved by caching full copies of
 packets which don't decompress properly in the hopes that later
 arrivals will fix the problem.  Finally, the performance improvement
 if the decompresser can explicitly request a full header is
 discussed.  Simulation results show that twice, in conjunction with
 the full header request mechanism, can improve throughput over
 uncompressed streams.

3.7.2 Research

 [Jac90] outlines a simple header compression scheme for TCP/IP.
 In [DENP97] the authors present the results of simulations showing
 that header compression is advantageous for both low and medium
 bandwidth links.  Simulations show that the twice algorithm, combined
 with an explicit header request mechanism, improved throughput by
 10-15% over uncompressed sessions across a wide range of bit error
 rates.

Allman, et al. Informational [Page 28] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 Much of this improvement may have been due to the twice algorithm
 quickly re-synchronizing the decompresser when a packet is lost.
 This is because the twice algorithm, applied one or two times when
 the decompresser becomes unsynchronized, will re-sync the
 decompresser in between 83% and 99% of the cases examined.  This
 means that packets received correctly after twice has resynchronized
 the decompresser will cause duplicate acknowledgments.  This re-
 enables the use of both fast retransmit and SACK in conjunction with
 header compression.

3.7.3 Implementation Issues

 Implementing TCP/IP header compression requires changes at both the
 sending (compressor) and receiving (decompresser) ends of each link
 that uses compression.  The twice algorithm requires very little
 extra machinery over and above header compression, while the explicit
 header request mechanism of [DENP97] requires more extensive
 modifications to the sending and receiving ends of each link that
 employs header compression.  Header compression does not violate
 TCP's congestion control mechanisms and therefore can be safely
 implemented in shared networks.

3.7.4 Topology Considerations

 TCP/IP header compression is applicable to all of the environments
 discussed in section 2, but will provide relatively more improvement
 in situations where packet sizes are small (i.e., overhead is large)
 and there is medium to low bandwidth and/or higher BER. When TCP's
 congestion window size is large, implementing the explicit header
 request mechanism, the twice algorithm, and caching packets which
 fail to decompress properly becomes more critical.

3.7.5 Possible Interaction and Relationships with Other Research

 As discussed above, losing synchronization between a sender and
 receiver can cause many packet drops.  The frequency of losing
 synchronization and the effectiveness of the twice algorithm may
 point to using a SACK-based loss recovery algorithm to reduce the
 impact of multiple lost segments.  However, even very robust SACK-
 based algorithms may not work well if too many segments are lost.

3.8 Sharing TCP State Among Similar Connections

Allman, et al. Informational [Page 29] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

3.8.1 Mitigation Description

 Persistent TCP state information can be used to overcome limitations
 in the configuration of the initial state, and to automatically tune
 TCP to environments using satellite links and to coordinate multiple
 TCP connections sharing a satellite link.
 TCP includes a variety of parameters, many of which are set to
 initial values which can severely affect the performance of TCP
 connections traversing satellite links, even though most TCP
 parameters are adjusted later after the connection is established.
 These parameters include initial size of cwnd and initial MSS size.
 Various suggestions have been made to change these initial
 conditions, to more effectively support satellite links.  However, it
 is difficult to select any single set of parameters which is
 effective for all environments.
 An alternative to attempting to select these parameters a-priori is
 sharing state across TCP connections and using this state when
 initializing a new connection.  For example, if all connections to a
 subnet result in extended congestion windows of 1 megabyte, it is
 probably more efficient to start new connections with this value,
 than to rediscover it by requiring the cwnd to increase using slow
 start over a period of dozens of round-trip times.

3.8.2 Research

 Sharing state among connections brings up a number of questions such
 as what information to share, with whom to share, how to share it,
 and how to age shared information.  First, what information is to be
 shared must be determined.  Some information may be appropriate to
 share among TCP connections, while some information sharing may be
 inappropriate or not useful.  Next, we need to determine with whom to
 share information.  Sharing may be appropriate for TCP connections
 sharing a common path to a given host.  Information may be shared
 among connections within a host, or even among connections between
 different hosts, such as hosts on the same LAN.  However, sharing
 information between connections not traversing the same network may
 not be appropriate.  Given the state to share and the parties that
 share it, a mechanism for the sharing is required.  Simple state,
 like MSS and RTT, is easy to share, but congestion window information
 can be shared a variety of ways. The sharing mechanism determines
 priorities among the sharing connections, and a variety of fairness
 criteria need to be considered.  Also, the mechanisms by which
 information is aged require further study.  See RFC 2140 for a
 discussion of the security issues in both sharing state within a
 single host and sharing state among hosts on a subnet.  Finally, the
 security concerns associated with sharing a piece of information need

Allman, et al. Informational [Page 30] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 to be carefully considered before introducing such a mechanism.  Many
 of these open research questions must be answered before state
 sharing can be widely deployed.
 The opportunity for such sharing, both among a sequence of
 connections, as well as among concurrent connections, is described in
 more detail in [Tou97].  The state management itself is largely an
 implementation issue, however what information should be shared and
 the specific ways in which the information should be shared is an
 open question.
 Sharing parts of the TCB state was originally documented in T/TCP
 [Bra92], and is used there to aggregate RTT values across connection
 instances, to provide meaningful average RTTs, even though most
 connections are expected to persist for only one RTT.  T/TCP also
 shares a connection identifier, a sequence number separate from the
 window number and address/port pairs by which TCP connections are
 typically distinguished. As a result of this shared state, T/TCP
 allows a receiver to pass data in the SYN segment to the receiving
 application, prior to the completion of the three-way handshake,
 without compromising the integrity of the connection. In effect, this
 shared state caches a partial handshake from the previous connection,
 which is a variant of the more general issue of TCB sharing.
 Sharing state among connections (including transfers using non-TCP
 protocols) is further investigated in [BRS99].

3.8.3 Implementation Issues

 Sharing TCP state across connections requires changes to the sender's
 TCP stack, and possibly the receiver's TCP stack (as in the case of
 T/TCP, for example).  Sharing TCP state may make a particular TCP
 connection more aggressive.  However, the aggregate traffic should be
 more conservative than a group of independent TCP connections.
 Therefore, sharing TCP state should be safe for use in shared
 networks.  Note that state sharing does not present any new security
 problems within multiuser hosts.  In such a situation, users can
 steal network resources from one another with or without state
 sharing.

3.8.4 Topology Considerations

 It is expected that sharing state across TCP connections may be
 useful in all network environments presented in section 2.

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3.8.5 Possible Interaction and Relationships with Other Research

 The state sharing outlined above is very similar to the Congestion
 Manager proposal [BRS99] that attempts to share congestion control
 information among both TCP and UDP flows between a pair of hosts.

3.9 ACK Congestion Control

 In highly asymmetric networks, a low-speed return link can restrict
 the performance of the data flow on a high-speed forward link by
 limiting the flow of acknowledgments returned to the data sender.
 For example, if the data sender uses 1500 byte segments, and the
 receiver generates 40 byte acknowledgments (IPv4, TCP without
 options), the reverse link will congest with ACKs for asymmetries of
 more than 75:1 if delayed ACKs are used, and 37:1 if every segment is
 acknowledged.  For a 1.5 Mb/second data link, ACK congestion will
 occur for reverse link speeds below 20 kilobits/sec.  These levels of
 asymmetry will readily occur if the reverse link is shared among
 multiple satellite receivers, as is common in many VSAT satellite
 networks.  If a terrestrial modem link is used as a reverse link, ACK
 congestion is also likely, especially as the speed of the forward
 link is increased.  Current congestion control mechanisms are aimed
 at controlling the flow of data segments, but do not affect the flow
 of ACKs.
 In [KVR98] the authors point out that the flow of acknowledgments can
 be restricted on the low-speed link not only by the bandwidth of the
 link, but also by the queue length of the router.  The router may
 limit its queue length by counting packets, not bytes, and therefore
 begin discarding ACKs even if there is enough bandwidth to forward
 them.

3.9.1 Mitigation Description

 ACK Congestion Control extends the concept of flow control for data
 segments to acknowledgment segments.  In the method described in
 [BPK97], any intermediate router can mark an acknowledgment with an
 Explicit Congestion Notification (ECN) bit once the queue occupancy
 in the router exceeds a given threshold.  The data sender (which
 receives the acknowledgment) must "echo" the ECN bit back to the data
 receiver (see section 3.3.3 for a more detailed discussion of ECN).
 The proposed algorithm for marking ACK segments with an ECN bit is
 Random Early Detection (RED) [FJ93].  In response to the receipt of
 ECN marked data segments, the receiver will dynamically reduce the
 rate of acknowledgments using a multiplicative backoff.  Once
 segments without ECN are received, the data receiver speeds up
 acknowledgments using a linear increase, up to a rate of either 1 (no

Allman, et al. Informational [Page 32] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 delayed ACKs) or 2 (normal delayed ACKs) data segments per ACK.  The
 authors suggest that an ACK be generated at least once per window,
 and ideally a few times per window.
 As in the RED congestion control mechanism for data flow, the
 bottleneck gateway can randomly discard acknowledgments, rather than
 marking them with an ECN bit, once the queue fills beyond a given
 threshold.

3.9.2 Research

 [BPK97] analyze the effect of ACK Congestion Control (ACC) on the
 performance of an asymmetric network.  They note that the use of ACC,
 and indeed the use of any scheme which reduces the frequency of
 acknowledgments, has potential unwanted side effects.  Since each ACK
 will acknowledge more than the usual one or two data segments, the
 likelihood of segment bursts from the data sender is increased.  In
 addition, congestion window growth may be impeded if the receiver
 grows the window by counting received ACKs, as mandated by
 [Ste97,APS99].  The authors therefore combine ACC with a series of
 modifications to the data sender, referred to as TCP Sender
 Adaptation (SA).  SA combines a limit on the number of segments sent
 in a burst, regardless of window size.  In addition, byte counting
 (as opposed to ACK counting) is employed for window growth.  Note
 that byte counting has been studied elsewhere and can introduce
 side-effects, as well [All98].
 The results presented in [BPK97] indicate that using ACC and SA will
 reduce the bursts produced by ACK losses in unmodified (Reno) TCP.
 In cases where these bursts would lead to data loss at an
 intermediate router, the ACC and SA modification significantly
 improve the throughput for a single data transfer.  The results
 further suggest that the use of ACC and SA significantly improve
 fairness between two simultaneous transfers.
 ACC is further reported to prevent the increase in round trip time
 (RTT) that occurs when an unmodified TCP fills the reverse router
 queue with acknowledgments.
 In networks where the forward direction is expected to suffer losses
 in one of the gateways, due to queue limitations, the authors report
 at best a very slight improvement in performance for ACC and SA,
 compared to unmodified Reno TCP.

Allman, et al. Informational [Page 33] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

3.9.3 Implementation Issues

 Both ACC and SA require modification of the sending and receiving
 hosts, as well as the bottleneck gateway.  The current research
 suggests that implementing ACC without the SA modifications results
 in a data sender which generates potentially disruptive segment
 bursts.  It should be noted that ACC does require host modifications
 if it is implemented in the way proposed in [BPK97].  The authors
 note that ACC can be implemented by discarding ACKs (which requires
 only a gateway modification, but no changes in the hosts), as opposed
 to marking them with ECN.  Such an implementation may, however,
 produce bursty data senders if it is not combined with a burst
 mitigation technique.  ACC requires changes to the standard ACKing
 behavior of a receiving TCP and therefore is not recommended for use
 in shared networks.

3.9.4 Topology Considerations

 Neither ACC nor SA require the storage of state in the gateway.
 These schemes should therefore be applicable for all topologies,
 provided that the hosts using the satellite or hybrid network can be
 modified.  However, these changes are expected to be especially
 beneficial to networks containing asymmetric satellite links.

3.9.5 Possible Interaction and Relationships with Other Research

 Note that ECN is a pre-condition for using ACK congestion control.
 Additionally, the ACK Filtering algorithm discussed in the next
 section attempts to solve the same problem as ACC.  Choosing between
 the two algorithms (or another mechanism) is currently an open
 research question.

3.10 ACK Filtering

 ACK Filtering (AF) is designed to address the same ACK congestion
 effects described in 3.9.  Contrary to ACC, however, AF is designed
 to operate without host modifications.

3.10.1 Mitigation Description

 AF takes advantage of the cumulative acknowledgment structure of TCP.
 The bottleneck router in the reverse direction (the low speed link)
 must be modified to implement AF.  Upon receipt of a segment which
 represents a TCP acknowledgment, the router scans the queue for
 redundant ACKs for the same connection, i.e. ACKs which acknowledge
 portions of the window which are included in the most recent ACK.
 All of these "earlier" ACKs are removed from the queue and discarded.

Allman, et al. Informational [Page 34] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 The router does not store state information, but does need to
 implement the additional processing required to find and remove
 segments from the queue upon receipt of an ACK.

3.10.2 Research

 [BPK97] analyzes the effects of AF.  As is the case in ACC, the use
 of ACK filtering alone would produce significant sender bursts, since
 the ACKs will be acknowledging more previously-unacknowledged data.
 The SA modifications described in 3.9.2 could be used to prevent
 those bursts, at the cost of requiring host modifications.  To
 prevent the need for modifications in the TCP stack, AF is more
 likely to be paired with the ACK Reconstruction (AR) technique, which
 can be implemented at the router where segments exit the slow reverse
 link.
 AR inspects ACKs exiting the link, and if it detects large "gaps" in
 the ACK sequence, it generates additional ACKs to reconstruct an
 acknowledgment flow which more closely resembles what the data sender
 would have seen had ACK Filtering not been introduced.  AR requires
 two parameters; one parameter is the desired ACK frequency, while the
 second controls the spacing, in time, between the release of
 consecutive reconstructed ACKs.
 In [BPK97], the authors show the combination of AF and AR to increase
 throughput, in the networks studied, over both unmodified TCP and the
 ACC/SA modifications.  Their results also strongly suggest that the
 use of AF alone, in networks where congestion losses are expected,
 decreases performance (even below the level of unmodified TCP Reno)
 due to sender bursting.
 AF delays acknowledgments from arriving at the receiver by dropping
 earlier ACKs in favor of later ACKs.  This process can cause a slight
 hiccup in the transmission of new data by the TCP sender.

3.10.3 Implementation Issues

 Both ACK Filtering and ACK Reconstruction require only router
 modification.  However, the implementation of AR requires some
 storage of state information in the exit router.  While AF does not
 require storage of state information, its use without AR (or SA)
 could produce undesired side effects.  Furthermore, more research is
 required regarding appropriate ranges for the parameters needed in
 AR.

Allman, et al. Informational [Page 35] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

3.10.4 Topology Considerations

 AF and AR appear applicable to all topologies, assuming that the
 storage of state information in AR does not prove to be prohibitive
 for routers which handle large numbers of flows.  The fact that TCP
 stack modifications are not required for AF/AR makes this approach
 attractive for hybrid networks and networks with diverse types of
 hosts.  These modifications, however, are expected to be most
 beneficial in asymmetric network paths.
 On the other hand, the implementation of AF/AR requires the routers
 to examine the TCP header, which prohibits their use in secure
 networks where IPSEC is deployed.  In such networks, AF/AR can be
 effective only inside the security perimeter of a private, or virtual
 private network, or in private networks where the satellite link is
 protected only by link-layer encryption (as opposed to IPSEC).  ACK
 Filtering is safe to use in shared networks (from a congestion
 control point-of-view), as the number of ACKs can only be reduced,
 which makes TCP less aggressive.  However, note that while TCP is
 less aggressive, the delays that AF induces (outlined above) can lead
 to larger bursts than would otherwise occur.

3.10.5 Possible Interaction and Relationships with Other Research

 ACK Filtering attempts to solve the same problem as ACK Congestion
 Control (as outlined in section 3.9).  Which of the two algorithms is
 more appropriate is currently an open research question.

4 Conclusions

 This document outlines TCP items that may be able to mitigate the
 performance problems associated with using TCP in networks containing
 satellite links.  These mitigations are not IETF standards track
 mechanisms and require more study before being recommended by the
 IETF.  The research community is encouraged to examine the above
 mitigations in an effort to determine which are safe for use in
 shared networks such as the Internet.

5 Security Considerations

 Several of the above sections noted specific security concerns which
 a given mitigation aggravates.
 Additionally, any form of wireless communication link is more
 susceptible to eavesdropping security attacks than standard wire-
 based links due to the relative ease with which an attacker can watch
 the network and the difficultly in finding attackers monitoring the
 network.

Allman, et al. Informational [Page 36] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

6 Acknowledgments

 Our thanks to Aaron Falk and Sally Floyd, who provided very helpful
 comments on drafts of this document.

7 References

 [AFP98]   Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
           Initial Window", RFC 2414, September 1998.
 [AGS99]   Allman, M., Glover, D. and L. Sanchez, "Enhancing TCP Over
           Satellite Channels using Standard Mechanisms", BCP 28, RFC
           2488, January 1999.
 [AHKO97]  Mark Allman, Chris Hayes, Hans Kruse, Shawn Ostermann.  TCP
           Performance Over Satellite Links.  In Proceedings of the
           5th International Conference on Telecommunication Systems,
           March 1997.
 [AHO98]   Mark Allman, Chris Hayes, Shawn Ostermann.  An Evaluation
           of TCP with Larger Initial Windows.  Computer Communication
           Review, 28(3), July 1998.
 [AKO96]   Mark Allman, Hans Kruse, Shawn Ostermann.  An Application-
           Level Solution to TCP's Satellite Inefficiencies.  In
           Proceedings of the First International Workshop on
           Satellite-based Information Services (WOSBIS), November
           1996.
 [All97a]  Mark Allman.  Improving TCP Performance Over Satellite
           Channels.  Master's thesis, Ohio University, June 1997.
 [All97b]  Mark Allman.  Fixing Two BSD TCP Bugs.  Technical Report
           CR-204151, NASA Lewis Research Center, October 1997.
 [All98]   Mark Allman. On the Generation and Use of TCP
           Acknowledgments.  ACM Computer Communication Review, 28(5),
           October 1998.
 [AOK95]   Mark Allman, Shawn Ostermann, Hans Kruse.  Data Transfer
           Efficiency Over Satellite Circuits Using a Multi-Socket
           Extension to the File Transfer Protocol (FTP).  In
           Proceedings of the ACTS Results Conference, NASA Lewis
           Research Center, September 1995.
 [AP99]    Mark Allman, Vern Paxson.  On Estimating End-to-End Network
           Path Properties. ACM SIGCOMM, September 1999.

Allman, et al. Informational [Page 37] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 [APS99]   Allman, M., Paxson, V. and W. Richard Stevens, "TCP
           Congestion Control", RFC 2581, April 1999.
 [BCC+98]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
           S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
           Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, S.,
           Wroclawski, J. and L. Zhang, "Recommendations on Queue
           Management and Congestion Avoidance in the Internet", RFC
           2309, April 1998.
 [BKVP97]  B. Bakshi and P. Krishna and N. Vaidya and D. Pradham,
           "Improving Performance of TCP over Wireless Networks", 17th
           International Conference on Distributed Computing Systems
           (ICDCS), May 1997.
 [BPK97]   Hari Balakrishnan, Venkata N. Padmanabhan, and Randy H.
           Katz.  The Effects of Asymmetry on TCP Performance.  In
           Proceedings of the ACM/IEEE Mobicom, Budapest, Hungary,
           ACM.  September, 1997.
 [BPK98]   Hari Balakrishnan, Venkata Padmanabhan, Randy H. Katz.  The
           Effects of Asymmetry on TCP Performance.  ACM Mobile
           Networks and Applications (MONET), 1998 (to appear).
 [BPSK96]  H. Balakrishnan and V. Padmanabhan and S. Sechan and R.
           Katz, "A Comparison of Mechanisms for Improving TCP
           Performance over Wireless Links", ACM SIGCOMM, August 1996.
 [Bra89]   Braden, R., "Requirements for Internet Hosts --
           Communication Layers", STD 3, RFC 1122, October 1989.
 [Bra92]   Braden, R., "Transaction TCP -- Concepts", RFC 1379,
           September 1992.
 [Bra94]   Braden, R., "T/TCP -- TCP Extensions for Transactions:
           Functional Specification", RFC 1644, July 1994.
 [BRS99]   Hari Balakrishnan, Hariharan Rahul, and Srinivasan Seshan.
           An Integrated Congestion Management Architecture for
           Internet Hosts.  ACM SIGCOMM, September 1999.
 [ddKI99]  M. deVivo, G.O. deVivo, R. Koeneke, G. Isern.  Internet
           Vulnerabilities Related to TCP/IP and T/TCP.  Computer
           Communication Review, 29(1), January 1999.

Allman, et al. Informational [Page 38] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 [DENP97]  Mikael Degermark, Mathias Engan, Bjorn Nordgren, Stephen
           Pink.  Low-Loss TCP/IP Header Compression for Wireless
           Networks.  ACM/Baltzer Journal on Wireless Networks, vol.3,
           no.5, p. 375-87.
 [DMT96]   R. C. Durst and G. J. Miller and E. J. Travis, "TCP
           Extensions for Space Communications", Mobicom 96, ACM, USA,
           1996.
 [DNP99]   Degermark, M., Nordgren, B. and S. Pink, "IP Header
           Compression", RFC 2507, February 1999.
 [FF96]    Kevin Fall, Sally Floyd.  Simulation-based Comparisons of
           Tahoe, Reno, and SACK TCP.  Computer Communication Review,
           V. 26 N. 3, July 1996, pp. 5-21.
 [FF99]    Sally Floyd, Kevin Fall.  Promoting the Use of End-to-End
           Congestion Control in the Internet, IEEE/ACM Transactions
           on Networking, August 1999.
 [FH99]    Floyd, S. and T. Henderson, "The NewReno Modification to
           TCP's Fast Recovery Algorithm", RFC 2582, April 1999.
 [FJ93]    Sally Floyd and Van Jacobson.  Random Early Detection
           Gateways for Congestion Avoidance, IEEE/ACM Transactions on
           Networking, V. 1 N. 4, August 1993.
 [Flo91]   Sally Floyd.  Connections with Multiple Congested Gateways
           in Packet-Switched Networks, Part 1: One-way Traffic.  ACM
           Computer Communications Review, V. 21, N. 5, October 1991.
 [Flo94]   Sally Floyd.  TCP and Explicit Congestion Notification, ACM
           Computer Communication Review, V. 24 N. 5, October 1994.
 [Flo99]   Sally Floyd.  "Re: TCP and out-of-order delivery", email to
           end2end-interest mailing list, February, 1999.
 [Hah94]   Jonathan Hahn.  MFTP: Recent Enhancements and Performance
           Measurements.  Technical Report RND-94-006, NASA Ames
           Research Center, June 1994.
 [Hay97]   Chris Hayes.  Analyzing the Performance of New TCP
           Extensions Over Satellite Links.  Master's Thesis, Ohio
           University, August 1997.
 [HK98]    Tom Henderson, Randy Katz.  On Improving the Fairness of
           TCP Congestion Avoidance.  Proceedings of IEEE Globecom `98
           Conference, 1998.

Allman, et al. Informational [Page 39] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 [HK99]    Tim Henderson, Randy Katz.  Transport Protocols for
           Internet-Compatible Satellite Networks, IEEE Journal on
           Selected Areas of Communications, February, 1999.
 [Hoe95]   J. Hoe, Startup Dynamics of TCP's Congestion Control and
           Avoidance Schemes. Master's Thesis, MIT, 1995.
 [Hoe96]   Janey Hoe.  Improving the Startup Behavior of a Congestion
           Control Scheme for TCP.  In ACM SIGCOMM, August 1996.
 [IL92]    David Iannucci and John Lakashman.  MFTP: Virtual TCP
           Window Scaling Using Multiple Connections.  Technical
           Report RND-92-002, NASA Ames Research Center, January 1992.
 [Jac88]   Van Jacobson.  Congestion Avoidance and Control.  In
           Proceedings of the SIGCOMM '88, ACM.  August, 1988.
 [Jac90]   Jacobson, V., "Compressing TCP/IP Headers", RFC 1144,
           February 1990.
 [JBB92]   Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for
           High Performance", RFC 1323, May 1992.
 [JK92]    Van Jacobson and Mike Karels.  Congestion Avoidance and
           Control.  Originally appearing in the proceedings of
           SIGCOMM '88 by Jacobson only, this revised version includes
           an additional appendix.  The revised version is available
           at ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z.  1992.
 [Joh95]   Stacy Johnson.  Increasing TCP Throughput by Using an
           Extended Acknowledgment Interval.  Master's Thesis, Ohio
           University, June 1995.
 [KAGT98]  Hans Kruse, Mark Allman, Jim Griner, Diepchi Tran.  HTTP
           Page Transfer Rates Over Geo-Stationary Satellite Links.
           March 1998. Proceedings of the Sixth International
           Conference on Telecommunication Systems.
 [Kes91]   Srinivasan Keshav.  A Control Theoretic Approach to Flow
           Control.  In ACM SIGCOMM, September 1991.
 [KM97]    S. Keshav, S. Morgan. SMART Retransmission: Performance
           with Overload and Random Losses. Proceeding of Infocom.
           1997.

Allman, et al. Informational [Page 40] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 [KVR98]   Lampros Kalampoukas, Anujan Varma, and K. K.Ramakrishnan.
           Improving TCP Throughput Over Two-Way Asymmetric Links:
           Analysis and Solutions.  Measurement and Modeling of
           Computer Systems, 1998, Pages 78-89.
 [MM96a]   M. Mathis, J. Mahdavi, "Forward Acknowledgment: Refining
           TCP Congestion Control," Proceedings of SIGCOMM'96, August,
           1996, Stanford, CA.  Available from
           http://www.psc.edu/networking/papers/papers.html
 [MM96b]   M. Mathis, J. Mahdavi, "TCP Rate-Halving with Bounding
           Parameters" Available from
           http://www.psc.edu/networking/papers/FACKnotes/current.
 [MMFR96]  Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
           Selective Acknowledgment Options", RFC 2018, October 1996.
 [MSMO97]  M. Mathis, J. Semke, J. Mahdavi, T. Ott, "The Macroscopic
           Behavior of the TCP Congestion Avoidance
           Algorithm",Computer Communication Review, volume 27,
           number3, July 1997.  Available from
           http://www.psc.edu/networking/papers/papers.html
 [MV98]    Miten N. Mehta and Nitin H. Vaidya.  Delayed Duplicate-
           Acknowledgments: A Proposal to Improve Performance of TCP
           on Wireless Links.  Technical Report 98-006, Department of
           Computer Science, Texas A&M University, February 1998.
 [Nic97]   Kathleen Nichols.  Improving Network Simulation with
           Feedback.  Com21, Inc. Technical Report.  Available from
           http://www.com21.com/pages/papers/068.pdf.
 [PADHV99] Paxson, V., Allman, M., Dawson, S., Heavens, I. and B.
           Volz, "Known TCP Implementation Problems", RFC 2525, March
           1999.
 [Pax97]   Vern Paxson.  Automated Packet Trace Analysis of TCP
           Implementations.  In Proceedings of ACM SIGCOMM, September
           1997.
 [PN98]    Poduri, K. and K. Nichols, "Simulation Studies of Increased
           Initial TCP Window Size", RFC 2415, September 1998.
 [Pos81]   Postel, J., "Transmission Control Protocol", STD 7, RFC
           793, September 1981.

Allman, et al. Informational [Page 41] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 [RF99]    Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit
           Congestion Notification (ECN) to IP", RFC 2481, January
           1999.
 [SF98]    Nihal K. G. Samaraweera and Godred Fairhurst,
           "Reinforcement of TCP error Recovery for Wireless
           Communication", Computer Communication Review, volume 28,
           number 2, April 1998.
 [SP98]    Shepard, T. and C. Partridge, "When TCP Starts Up With Four
           Packets Into Only Three Buffers", RFC 2416, September 1998.
 [Ste97]   Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
           Retransmit, and Fast Recovery Algorithms", RFC 2001,
           January 1997.
 [Sut98]   B. Suter, T. Lakshman, D. Stiliadis, and A. Choudhury.
           Design Considerations for Supporting TCP with Per-flow
           Queueing.  Proceedings of IEEE Infocom `98 Conference,
           1998.
 [Tou97]   Touch, J., "TCP Control Block Interdependence", RFC 2140,
           April 1997.
 [VH97a]   Vikram Visweswaraiah and John Heidemann.  Improving Restart
           of Idle TCP Connections.  Technical Report 97-661,
           University of Southern California, 1997.
 [VH97b]   Vikram Visweswaraiah and John Heidemann.  Rate-based pacing
           Source Code Distribution, Web page:
           http://www.isi.edu/lsam/publications/rate_based_pacing/README.html
           November, 1997.
 [VH98]    Vikram Visweswaraiah and John Heidemann.  Improving Restart
           of Idle TCP Connections (revised).  Submitted for
           publication.

Allman, et al. Informational [Page 42] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

8 Authors' Addresses

 Mark Allman
 NASA Glenn Research Center/BBN Technologies
 Lewis Field
 21000 Brookpark Rd.  MS 54-2
 Cleveland, OH  44135
 EMail: mallman@grc.nasa.gov
 http://roland.grc.nasa.gov/~mallman
 Spencer Dawkins
 Nortel
 P.O.Box 833805
 Richardson, TX 75083-3805
 EMail: Spencer.Dawkins.sdawkins@nt.com
 Dan Glover
 NASA Glenn Research Center
 Lewis Field
 21000 Brookpark Rd.  MS 3-6
 Cleveland, OH  44135
 EMail: Daniel.R.Glover@grc.nasa.gov
 http://roland.grc.nasa.gov/~dglover
 Jim Griner
 NASA Glenn Research Center
 Lewis Field
 21000 Brookpark Rd.  MS 54-2
 Cleveland, OH  44135
 EMail: jgriner@grc.nasa.gov
 http://roland.grc.nasa.gov/~jgriner
 Diepchi Tran
 NASA Glenn Research Center
 Lewis Field
 21000 Brookpark Rd.  MS 54-2
 Cleveland, OH  44135
 EMail: dtran@grc.nasa.gov

Allman, et al. Informational [Page 43] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 Tom Henderson
 University of California at Berkeley
 Phone: +1 (510) 642-8919
 EMail: tomh@cs.berkeley.edu
 URL: http://www.cs.berkeley.edu/~tomh/
 John Heidemann
 University of Southern California/Information Sciences Institute
 4676 Admiralty Way
 Marina del Rey, CA 90292-6695
 EMail: johnh@isi.edu
 Joe Touch
 University of Southern California/Information Sciences Institute
 4676 Admiralty Way
 Marina del Rey, CA 90292-6601
 USA
 Phone: +1 310-448-9151
 Fax:   +1 310-823-6714
 URL:   http://www.isi.edu/touch
 EMail: touch@isi.edu
 Hans Kruse
 J. Warren McClure School of Communication Systems Management
 Ohio University
 9 S. College Street
 Athens, OH 45701
 Phone: 740-593-4891
 Fax: 740-593-4889
 EMail: hkruse1@ohiou.edu
 http://www.csm.ohiou.edu/kruse
 Shawn Ostermann
 School of Electrical Engineering and Computer Science
 Ohio University
 416 Morton Hall
 Athens, OH  45701
 Phone: (740) 593-1234
 EMail: ostermann@cs.ohiou.edu

Allman, et al. Informational [Page 44] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

 Keith Scott
 The MITRE Corporation
 M/S W650
 1820 Dolley Madison Blvd.
 McLean VA 22102-3481
 EMail: kscott@mitre.org
 Jeffrey Semke
 Pittsburgh Supercomputing Center
 4400 Fifth Ave.
 Pittsburgh, PA  15213
 EMail: semke@psc.edu
 http://www.psc.edu/~semke

Allman, et al. Informational [Page 45] RFC 2760 Ongoing TCP Research Related to Satellites February 2000

9 Full Copyright Statement

 Copyright (C) The Internet Society (2000).  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.

Allman, et al. Informational [Page 46]

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