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RFC 2488 - Enhancing TCP Over Satellite Channels using Standard


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Network Working Group                                    M. Allman
Request for Comments: 2488            NASA Lewis/Sterling Software
BCP: 28                                                  D. Glover
Category: Best Current Practice                         NASA Lewis
                                                        L. Sanchez
                                                               BBN
                                                      January 1999

                 Enhancing TCP Over Satellite Channels
                       using Standard Mechanisms

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   The Transmission Control Protocol (TCP) provides reliable delivery of
   data across any network path, including network paths containing
   satellite channels.  While TCP works over satellite channels there
   are several IETF standardized mechanisms that enable TCP to more
   effectively utilize the available capacity of the network path.  This
   document outlines some of these TCP mitigations.  At this time, all
   mitigations discussed in this document are IETF standards track
   mechanisms (or are compliant with IETF standards).

1.  Introduction

   Satellite channel characteristics may have an effect on the way
   transport protocols, such as the Transmission Control Protocol (TCP)
   [Pos81], behave.  When protocols, such as TCP, perform poorly,
   channel utilization is low.  While the performance of a transport
   protocol is important, it is not the only consideration when
   constructing a network containing satellite links.  For example, data
   link protocol, application protocol, router buffer size, queueing
   discipline and proxy location are some of the considerations that
   must be taken into account.  However, this document focuses on
   improving TCP in the satellite environment and non-TCP considerations
   are left for another document.  Finally, there have been many
   satellite mitigations proposed and studied by the research community.
   While these mitigations may prove useful and safe for shared networks
   in the future, this document only considers TCP mechanisms which are

   currently well understood and on the IETF standards track (or are
   compliant with IETF standards).

   This document is divided up as follows: Section 2 provides a brief
   outline of the characteristics of satellite networks.  Section 3
   outlines two non-TCP mechanisms that enable TCP to more effectively
   utilize the available bandwidth.  Section 4 outlines the TCP
   mechanisms defined by the IETF that may benefit satellite networks.
   Finally, Section 5 provides a summary of what modern TCP
   implementations should include to be considered "satellite friendly".

2.  Satellite Characteristics

   There is an inherent delay in the delivery of a message over a
   satellite link due to the finite speed of light and the altitude of
   communications satellites.

   Many communications satellites are located at Geostationary Orbit
   (GSO) with an altitude of approximately 36,000 km [Sta94].  At this
   altitude the orbit period is the same as the Earth's rotation period.
   Therefore, each ground station is always able to "see" the orbiting
   satellite at the same position in the sky.  The propagation time for
   a radio signal to travel twice that distance (corresponding to a
   ground station directly below the satellite) is 239.6 milliseconds
   (ms) [Mar78].  For ground stations at the edge of the view area of
   the satellite, the distance traveled is 2 x 41,756 km for a total
   propagation delay of 279.0 ms [Mar78].  These delays are for one
   ground station-to-satellite-to-ground station route (or "hop").
   Therefore, the propagation delay for a message and the corresponding
   reply (one round-trip time or RTT) could be at least 558 ms.  The RTT
   is not based solely on satellite propagation time.  The RTT will be
   increased by other factors in the network, such as the transmission
   time and propagation time of other links in the network path and
   queueing delay in gateways.  Furthermore, the satellite propagation
   delay will be longer if the link includes multiple hops or if
   intersatellite links are used.  As satellites become more complex and
   include on-board processing of signals, additional delay may be
   added.

   Other orbits are possible for use by communications satellites
   including Low Earth Orbit (LEO) [Stu95] [Mon98] and Medium Earth
   Orbit (MEO) [Mar78].  The lower orbits require the use of
   constellations of satellites for constant coverage.  In other words,
   as one satellite leaves the ground station's sight, another satellite
   appears on the horizon and the channel is switched to it.  The
   propagation delay to a LEO orbit ranges from several milliseconds
   when communicating with a satellite directly overhead, to as much as
   80 ms when the satellite is on the horizon.  These systems are more

   likely to use intersatellite links and have variable path delay
   depending on routing through the network.

   Satellite channels are dominated by two fundamental characteristics,
   as described below:

      NOISE - The strength of a radio signal falls in proportion to the
      square of the distance traveled.  For a satellite link the
      distance is large and so the signal becomes weak before reaching
      its destination.  This results in a low signal-to-noise ratio.
      Some frequencies are particularly susceptible to atmospheric
      effects such as rain attenuation.  For mobile applications,
      satellite channels are especially susceptible to multi-path
      distortion and shadowing (e.g., blockage by buildings).  Typical
      bit error rates (BER) for a satellite link today are on the order
      of 1 error per 10 million bits (1 x 10^-7) or less frequent.
      Advanced error control coding (e.g., Reed Solomon) can be added to
      existing satellite services and is currently being used by many
      services.  Satellite error performance approaching fiber will
      become more common as advanced error control coding is used in new
      systems.  However, many legacy satellite systems will continue to
      exhibit higher BER than newer satellite systems and terrestrial
      channels.

      BANDWIDTH - The radio spectrum is a limited natural resource,
      hence there is a restricted amount of bandwidth available to
      satellite systems which is typically controlled by licenses.  This
      scarcity makes it difficult to trade bandwidth to solve other
      design problems.  Typical carrier frequencies for current, point-
      to-point, commercial, satellite services are 6 GHz (uplink) and 4
      GHz (downlink), also known as C band, and 14/12 GHz (Ku band).  A
      new service at 30/20 GHz (Ka band) will be emerging over the next
      few years.  Satellite-based radio repeaters are known as
      transponders.  Traditional C band transponder bandwidth is
      typically 36 MHz to accommodate one color television channel (or
      1200 voice channels).  Ku band transponders are typically around
      50 MHz.  Furthermore, one satellite may carry a few dozen
      transponders.

   Not only is bandwidth limited by nature, but the allocations for
   commercial communications are limited by international agreements so
   that this scarce resource can be used fairly by many different
   applications.

   Although satellites have certain disadvantages when compared to fiber
   channels (e.g., cannot be easily repaired, rain fades, etc.), they
   also have certain advantages over terrestrial links.  First,
   satellites have a natural broadcast capability.  This gives
   satellites an advantage for multicast applications.  Next, satellites
   can reach geographically remote areas or countries that have little
   terrestrial infrastructure.  A related advantage is the ability of
   satellite links to reach mobile users.

   Satellite channels have several characteristics that differ from most
   terrestrial channels.  These characteristics may degrade the
   performance of TCP.  These characteristics include:

   Long feedback loop

      Due to the propagation delay of some satellite channels (e.g.,
      approximately 250 ms over a geosynchronous satellite) it may take
      a long time for a TCP sender to determine whether or not a packet
      has been successfully received at the final destination.  This
      delay hurts interactive applications such as telnet, as well as
      some of the TCP congestion control algorithms (see section 4).

   Large delay*bandwidth product

      The delay*bandwidth product (DBP) defines the amount of data a
      protocol should have "in flight" (data that has been transmitted,
      but not yet acknowledged) at any one time to fully utilize the
      available channel capacity.  The delay used in this equation is
      the RTT and the bandwidth is the capacity of the bottleneck link
      in the network path.  Because the delay in some satellite
      environments is large, TCP will need to keep a large number of
      packets "in flight" (that is, sent but not yet acknowledged) .

   Transmission errors

      Satellite channels exhibit a higher bit-error rate (BER) than
      typical terrestrial networks.  TCP uses all packet drops as
      signals of network congestion and reduces its window size in an
      attempt to alleviate the congestion.  In the absence of knowledge
      about why a packet was dropped (congestion or corruption), TCP
      must assume the drop was due to network congestion to avoid
      congestion collapse [Jac88] [FF98].  Therefore, packets dropped
      due to corruption cause TCP to reduce the size of its sliding
      window, even though these packet drops do not signal congestion in
      the network.

   Asymmetric use

      Due to the expense of the equipment used to send data to
      satellites, asymmetric satellite networks are often constructed.
      For example, a host connected to a satellite network will send all
      outgoing traffic over a slow terrestrial link (such as a dialup
      modem channel) and receive incoming traffic via the satellite
      channel.  Another common situation arises when both the incoming
      and outgoing traffic are sent using a satellite link, but the
      uplink has less available capacity than the downlink due to the
      expense of the transmitter required to provide a high bandwidth
      back channel.  This asymmetry may have an impact on TCP
      performance.

   Variable Round Trip Times

      In some satellite environments, such as low-Earth orbit (LEO)
      constellations, the propagation delay to and from the satellite
      varies over time.  Whether or not this will have an impact on TCP
      performance is currently an open question.

   Intermittent connectivity

      In non-GSO satellite orbit configurations, TCP connections must be
      transferred from one satellite to another or from one ground
      station to another from time to time.  This handoff may cause
      packet loss if not properly performed.

   Most satellite channels only exhibit a subset of the above
   characteristics.  Furthermore, satellite networks are not the only
   environments where the above characteristics are found.  However,
   satellite networks do tend to exhibit more of the above problems or
   the above problems are aggravated in the satellite environment.  The
   mechanisms outlined in this document should benefit most networks,
   especially those with one or more of the above characteristics (e.g.,
   gigabit networks have large delay*bandwidth products).

3.  Lower Level Mitigations

   It is recommended that those utilizing satellite channels in their
   networks should use the following two non-TCP mechanisms which can
   increase TCP performance.  These mechanisms are Path MTU Discovery
   and forward error correction (FEC) and are outlined in the following
   two sections.

   The data link layer protocol employed over a satellite channel can
   have a large impact on performance of higher layer protocols.  While
   beyond the scope of this document, those constructing satellite

   networks should tune these protocols in an appropriate manner to
   ensure that the data link protocol does not limit TCP performance.
   In particular, data link layer protocols often implement a flow
   control window and retransmission mechanisms.  When the link level
   window size is too small, performance will suffer just as when the
   TCP window size is too small (see section 4.3 for a discussion of
   appropriate window sizes).  The impact that link level
   retransmissions have on TCP transfers is not currently well
   understood.  The interaction between TCP retransmissions and link
   level retransmissions is a subject for further research.

3.1 Path MTU Discovery

   Path MTU discovery [MD90] is used to determine the maximum packet
   size a connection can use on a given network path without being
   subjected to IP fragmentation.  The sender transmits a packet that is
   the appropriate size for the local network to which it is connected
   (e.g., 1500 bytes on an Ethernet) and sets the IP "don't fragment"
   (DF) bit.  If the packet is too large to be forwarded without being
   fragmented to a given channel along the network path, the gateway
   that would normally fragment the packet and forward the fragments
   will instead return an ICMP message to the originator of the packet.
   The ICMP message will indicate that the original segment could not be
   transmitted without being fragmented and will also contain the size
   of the largest packet that can be forwarded by the gateway.
   Additional information from the IESG regarding Path MTU discovery is
   available in [Kno93].

   Path MTU Discovery allows TCP to use the largest possible packet
   size, without incurring the cost of fragmentation and reassembly.
   Large packets reduce the packet overhead by sending more data bytes
   per overhead byte.  As outlined in section 4, increasing TCP's
   congestion window is segment based, rather than byte based and
   therefore, larger segments enable TCP senders to increase the
   congestion window more rapidly, in terms of bytes, than smaller
   segments.

   The disadvantage of Path MTU Discovery is that it may cause a delay
   before TCP is able to start sending data.  For example, assume a
   packet is sent with the DF bit set and one of the intervening
   gateways (G1) returns an ICMP message indicating that it cannot
   forward the segment.  At this point, the sending host reduces the
   packet size per the ICMP message returned by G1 and sends another
   packet with the DF bit set.  The packet will be forwarded by G1,
   however this does not ensure all subsequent gateways in the network
   path will be able to forward the segment.  If a second gateway (G2)
   cannot forward the segment it will return an ICMP message to the
   transmitting host and the process will be repeated.  Therefore, path

   MTU discovery can spend a large amount of time determining the
   maximum allowable packet size on the network path between the sender
   and receiver.  Satellite delays can aggravate this problem (consider
   the case when the channel between G1 and G2 is a satellite link).
   However, in practice, Path MTU Discovery does not consume a large
   amount of time due to wide support of common MTU values.
   Additionally, caching MTU values may be able to eliminate discovery
   time in many instances, although the exact implementation of this and
   the aging of cached values remains an open problem.

   The relationship between BER and segment size is likely to vary
   depending on the error characteristics of the given channel.  This
   relationship deserves further study, however with the use of good
   forward error correction (see section 3.2) larger segments should
   provide better performance, as with any network [MSMO97].  While the
   exact method for choosing the best MTU for a satellite link is
   outside the scope of this document, the use of Path MTU Discovery is
   recommended to allow TCP to use the largest possible MTU over the
   satellite channel.

3.2 Forward Error Correction

   A loss event in TCP is always interpreted as an indication of
   congestion and always causes TCP to reduce its congestion window
   size.  Since the congestion window grows based on returning
   acknowledgments (see section 4), TCP spends a long time recovering
   from loss when operating in satellite networks.  When packet loss is
   due to corruption, rather than congestion, TCP does not need to
   reduce its congestion window size.  However, at the present time
   detecting corruption loss is a research issue.

   Therefore, for TCP to operate efficiently, the channel
   characteristics should be such that nearly all loss is due to network
   congestion.  The use of forward error correction coding (FEC) on a
   satellite link should be used to improve the bit-error rate (BER) of
   the satellite channel.  Reducing the BER is not always possible in
   satellite environments.  However, since TCP takes a long time to
   recover from lost packets because the long propagation delay imposed
   by a satellite link delays feedback from the receiver [PS97], the
   link should be made as clean as possible to prevent TCP connections
   from receiving false congestion signals.  This document does not make
   a specific BER recommendation for TCP other than it should be as low
   as possible.

   FEC should not be expected to fix all problems associated with noisy
   satellite links.  There are some situations where FEC cannot be
   expected to solve the noise problem (such as military jamming, deep
   space missions, noise caused by rain fade, etc.).  In addition, link

   outages can also cause problems in satellite systems that do not
   occur as frequently in terrestrial networks.  Finally, FEC is not
   without cost.  FEC requires additional hardware and uses some of the
   available bandwidth.  It can add delay and timing jitter due to the
   processing time of the coder/decoder.

   Further research is needed into mechanisms that allow TCP to
   differentiate between congestion induced drops and those caused by
   corruption.  Such a mechanism would allow TCP to respond to
   congestion in an appropriate manner, as well as repairing corruption
   induced loss without reducing the transmission rate.  However, in the
   absence of such a mechanism packet loss must be assumed to indicate
   congestion to preserve network stability.  Incorrectly interpreting
   loss as caused by corruption and not reducing the transmission rate
   accordingly can lead to congestive collapse [Jac88] [FF98].

4.  Standard TCP Mechanisms

   This section outlines TCP mechanisms that may be necessary in
   satellite or hybrid satellite/terrestrial networks to better utilize
   the available capacity of the link.  These mechanisms may also be
   needed to fully utilize fast terrestrial channels.  Furthermore,
   these mechanisms do not fundamentally hurt performance in a shared
   terrestrial network.  Each of the following sections outlines one
   mechanism and why that mechanism may be needed.

4.1 Congestion Control

   To avoid generating an inappropriate amount of network traffic for
   the current network conditions, during a connection TCP employs four
   congestion control mechanisms [Jac88] [Jac90] [Ste97].  These
   algorithms are slow start, congestion avoidance, fast retransmit and
   fast recovery.  These algorithms are used to adjust the amount of
   unacknowledged data that can be injected into the network and to
   retransmit segments dropped by the network.

   TCP senders use two state variables to accomplish congestion control.
   The first variable is the congestion window (cwnd).  This is an upper
   bound on the amount of data the sender can inject into the network
   before receiving an acknowledgment (ACK).  The value of cwnd is
   limited to the receiver's advertised window.  The congestion window
   is increased or decreased during the transfer based on the inferred
   amount of congestion present in the network.  The second variable is
   the slow start threshold (ssthresh).  This variable determines which
   algorithm is used to increase the value of cwnd.  If cwnd is less
   than ssthresh the slow start algorithm is used to increase the value
   of cwnd.  However, if cwnd is greater than or equal to (or just
   greater than in some TCP implementations) ssthresh the congestion

   avoidance algorithm is used.  The initial value of ssthresh is the
   receiver's advertised window size.  Furthermore, the value of
   ssthresh is set when congestion is detected.

   The four congestion control algorithms are outlined below, followed
   by a brief discussion of the impact of satellite environments on
   these algorithms.

4.1.1 Slow Start and Congestion Avoidance

   When a host begins sending data on a TCP connection the host has no
   knowledge of the current state of the network between itself and the
   data receiver.  In order to avoid transmitting an inappropriately
   large burst of traffic, the data sender is required to use the slow
   start algorithm at the beginning of a transfer [Jac88] [Bra89]
   [Ste97].  Slow start begins by initializing cwnd to 1 segment
   (although an IETF experimental mechanism would increase the size of
   the initial window to roughly 4 Kbytes [AFP98]) and ssthresh to the
   receiver's advertised window.  This forces TCP to transmit one
   segment and wait for the corresponding ACK.  For each ACK that is
   received during slow start, the value of cwnd is increased by 1
   segment.  For example, after the first ACK is received cwnd will be 2
   segments and the sender will be allowed to transmit 2 data packets.
   This continues until cwnd meets or exceeds ssthresh (or, in some
   implementations when cwnd equals ssthresh), or loss is detected.

   When the value of cwnd is greater than or equal to (or equal to in
   certain implementations) ssthresh the congestion avoidance algorithm
   is used to increase cwnd [Jac88] [Bra89] [Ste97].  This algorithm
   increases the size of cwnd more slowly than does slow start.
   Congestion avoidance is used to slowly probe the network for
   additional capacity.  During congestion avoidance, cwnd is increased
   by 1/cwnd for each incoming ACK.  Therefore, if one ACK is received
   for every data segment, cwnd will increase by roughly 1 segment per
   round-trip time (RTT).

   The slow start and congestion control algorithms can force poor
   utilization of the available channel bandwidth when using long-delay
   satellite networks [All97].  For example, transmission begins with
   the transmission of one segment.  After the first segment is
   transmitted the data sender is forced to wait for the corresponding
   ACK.  When using a GSO satellite this leads to an idle time of
   roughly 500 ms when no useful work is being accomplished.  Therefore,
   slow start takes more real time over GSO satellites than on typical
   terrestrial channels.  This holds for congestion avoidance, as well
   [All97].  This is precisely why Path MTU Discovery is an important
   algorithm.  While the number of segments we transmit is determined by
   the congestion control algorithms, the size of these segments is not.

   Therefore, using larger packets will enable TCP to send more data per
   segment which yields better channel utilization.

4.1.2 Fast Retransmit and Fast Recovery

   TCP's default mechanism to detect dropped segments is a timeout
   [Pos81].  In other words, if the sender does not receive an ACK for a
   given packet within the expected amount of time the segment will be
   retransmitted.  The retransmission timeout (RTO) is based on
   observations of the RTT.  In addition to retransmitting a segment
   when the RTO expires, TCP also uses the lost segment as an indication
   of congestion in the network.  In response to the congestion, the
   value of ssthresh is set to half of the cwnd and the value of cwnd is
   then reduced to 1 segment.  This triggers the use of the slow start
   algorithm to increase cwnd until the value of cwnd reaches half of
   its value when congestion was detected.  After the slow start phase,
   the congestion avoidance algorithm is used to probe the network for
   additional capacity.

   TCP ACKs always acknowledge the highest in-order segment that has
   arrived.  Therefore an ACK for segment X also effectively ACKs all
   segments < X.  Furthermore, if a segment arrives out-of-order the ACK
   triggered will be for the highest in-order segment, rather than the
   segment that just arrived.  For example, assume segment 11 has been
   dropped somewhere in the network and segment 12 arrives at the
   receiver.  The receiver is going to send a duplicate ACK covering
   segment 10 (and all previous segments).

   The fast retransmit algorithm uses these duplicate ACKs to detect
   lost segments.  If 3 duplicate ACKs arrive at the data originator,
   TCP assumes that a segment has been lost and retransmits the missing
   segment without waiting for the RTO to expire.  After a segment is
   resent using fast retransmit, the fast recovery algorithm is used to
   adjust the congestion window.  First, the value of ssthresh is set to
   half of the value of cwnd.  Next, the value of cwnd is halved.
   Finally, the value of cwnd is artificially increased by 1 segment for
   each duplicate ACK that has arrived.  The artificial inflation can be
   done because each duplicate ACK represents 1 segment that has left
   the network.  When the cwnd permits, TCP is able to transmit new
   data.  This allows TCP to keep data flowing through the network at
   half the rate it was when loss was detected.  When an ACK for the
   retransmitted packet arrives, the value of cwnd is reduced back to
   ssthresh (half the value of cwnd when the congestion was detected).

   Generally, fast retransmit can resend only one segment per window of
   data sent.  When multiple segments are lost in a given window of
   data, one of the segments will be resent using fast retransmit and
   the rest of the dropped segments must usually wait for the RTO to
   expire, which causes TCP to revert to slow start.

   TCP's response to congestion differs based on the way the congestion
   is detected.  If the retransmission timer causes a packet to be
   resent, TCP drops ssthresh to half the current cwnd and reduces the
   value of cwnd to 1 segment (thus triggering slow start).  However, if
   a segment is resent via fast retransmit both ssthresh and cwnd are
   set to half the current value of cwnd and congestion avoidance is
   used to send new data.  The difference is that when retransmitting
   due to duplicate ACKs, TCP knows that packets are still flowing
   through the network and can therefore infer that the congestion is
   not that bad.  However, when resending a packet due to the expiration
   of the retransmission timer, TCP cannot infer anything about the
   state of the network and therefore must proceed conservatively by
   sending new data using the slow start algorithm.

   Note that the fast retransmit/fast recovery algorithms, as discussed
   above can lead to a phenomenon that allows multiple fast retransmits
   per window of data [Flo94].  This can reduce the size of the
   congestion window multiple times in response to a single "loss
   event".  The problem is particularly noticeable in connections that
   utilize large congestion windows, since these connections are able to
   inject enough new segments into the network during recovery to
   trigger the multiple fast retransmits.  Reducing cwnd multiple times
   for a single loss event may hurt performance [GJKFV98].

   The best way to improve the fast retransmit/fast recovery algorithms
   is to use a selective acknowledgment (SACK) based algorithm for loss
   recovery.  As discussed below, these algorithms are generally able to
   quickly recover from multiple lost segments without needlessly
   reducing the value of cwnd.  In the absence of SACKs, the fast
   retransmit and fast recovery algorithms should be used.  Fixing these
   algorithms to achieve better performance in the face of multiple fast
   retransmissions is beyond the scope of this document.  Therefore, TCP
   implementers are advised to implement the current version of fast
   retransmit/fast recovery outlined in RFC 2001 [Ste97] or subsequent
   versions of RFC 2001.

4.1.3 Congestion Control in Satellite Environment

   The above algorithms have a negative impact on the performance of
   individual TCP connection's performance because the algorithms slowly
   probe the network for additional capacity, which in turn wastes
   bandwidth.  This is especially true over long-delay satellite

   channels because of the large amount of time required for the sender
   to obtain feedback from the receiver [All97] [AHKO97].  However, the
   algorithms are necessary to prevent congestive collapse in a shared
   network [Jac88].  Therefore, the negative impact on a given
   connection is more than offset by the benefit to the entire network.

4.2 Large TCP Windows

   The standard maximum TCP window size (65,535 bytes) is not adequate
   to allow a single TCP connection to utilize the entire bandwidth
   available on some satellite channels.  TCP throughput is limited by
   the following formula [Pos81]:

                      throughput = window size / RTT

   Therefore, using the maximum window size of 65,535 bytes and a
   geosynchronous satellite channel RTT of 560 ms [Kru95] the maximum
   throughput is limited to:

         throughput = 65,535 bytes / 560 ms = 117,027 bytes/second

   Therefore, a single standard TCP connection cannot fully utilize, for
   example, T1 rate (approximately 192,000 bytes/second) GSO satellite
   channels.  However, TCP has been extended to support larger windows
   [JBB92].  The window scaling options outlined in [JBB92] should be
   used in satellite environments, as well as the companion algorithms
   PAWS (Protection Against Wrapped Sequence space) and RTTM (Round-Trip
   Time Measurements).

   It should be noted that for a satellite link shared among many flows,
   large windows may not be necessary.  For instance, two long-lived TCP
   connections each using a window of 65,535 bytes, as in the above
   example, can fully utilize a T1 GSO satellite channel.

   Using large windows often requires both client and server
   applications or TCP stacks to be hand tuned (usually by an expert) to
   utilize large windows.  Research into operating system mechanisms
   that are able to adjust the buffer capacity as dictated by the
   current network conditions is currently underway [SMM98].  This will
   allow stock TCP implementations and applications to better utilize
   the capacity provided by the underlying network.

4.3 Acknowledgment Strategies

   There are two standard methods that can be used by TCP receivers to
   generated acknowledgments.  The method outlined in [Pos81] generates
   an ACK for each incoming segment.  [Bra89] states that hosts SHOULD
   use "delayed acknowledgments".  Using this algorithm, an ACK is

   generated for every second full-sized segment, or if a second full-
   size segment does not arrive within a given timeout (which must not
   exceed 500 ms).  The congestion window is increased based on the
   number of incoming ACKs and delayed ACKs reduce the number of ACKs
   being sent by the receiver.  Therefore, cwnd growth occurs much more
   slowly when using delayed ACKs compared to the case when the receiver
   ACKs each incoming segment [All98].

   A tempting "fix" to the problem caused by delayed ACKs is to simply
   turn the mechanism off and let the receiver ACK each incoming
   segment.  However, this is not recommended.  First, [Bra89] says that
   a TCP receiver SHOULD generate delayed ACKs.  And, second, increasing
   the number of ACKs by a factor of two in a shared network may have
   consequences that are not yet understood.  Therefore, disabling
   delayed ACKs is still a research issue and thus, at this time TCP
   receivers should continue to generate delayed ACKs, per [Bra89].

4.4 Selective Acknowledgments

   Selective acknowledgments (SACKs) [MMFR96] allow TCP receivers to
   inform TCP senders exactly which packets have arrived.  SACKs allow
   TCP to recover more quickly from lost segments, as well as avoiding
   needless retransmissions.

   The fast retransmit algorithm can generally only repair one loss per
   window of data.  When multiple losses occur, the sender generally
   must rely on a timeout to determine which segment needs to be
   retransmitted next.  While waiting for a timeout, the data segments
   and their acknowledgments drain from the network.  In the absence of
   incoming ACKs to clock new segments into the network, the sender must
   use the slow start algorithm to restart transmission.  As discussed
   above, the slow start algorithm can be time consuming over satellite
   channels.  When SACKs are employed, the sender is generally able to
   determine which segments need to be retransmitted in the first RTT
   following loss detection.  This allows the sender to continue to
   transmit segments (retransmissions and new segments, if appropriate)
   at an appropriate rate and therefore sustain the ACK clock.  This
   avoids a costly slow start period following multiple lost segments.
   Generally SACK is able to retransmit all dropped segments within the
   first RTT following the loss detection.  [MM96] and [FF96] discuss
   specific congestion control algorithms that rely on SACK information
   to determine which segments need to be retransmitted and when it is
   appropriate to transmit those segments.  Both these algorithms follow
   the basic principles of congestion control outlined in [Jac88] and
   reduce the window by half when congestion is detected.

5.  Mitigation Summary

   Table 1 summarizes the mechanisms that have been discussed in this
   document.  Those mechanisms denoted "Recommended" are IETF standards
   track mechanisms that are recommended by the authors for use in
   networks containing satellite channels.  Those mechanisms marked
   "Required' have been defined by the IETF as required for hosts using
   the shared Internet [Bra89].  Along with the section of this document
   containing the discussion of each mechanism, we note where the
   mechanism needs to be implemented.  The codes listed in the last
   column are defined as follows: "S" for the data sender, "R" for the
   data receiver and "L" for the satellite link.

    Mechanism                 Use          Section      Where
   +------------------------+-------------+------------+--------+
   | Path-MTU Discovery     | Recommended | 3.1        | S      |
   | FEC                    | Recommended | 3.2        | L      |
   | TCP Congestion Control |             |            |        |
   |   Slow Start           | Required    | 4.1.1      | S      |
   |   Congestion Avoidance | Required    | 4.1.1      | S      |
   |   Fast Retransmit      | Recommended | 4.1.2      | S      |
   |   Fast Recovery        | Recommended | 4.1.2      | S      |
   | TCP Large Windows      |             |            |        |
   |   Window Scaling       | Recommended | 4.2        | S,R    |
   |   PAWS                 | Recommended | 4.2        | S,R    |
   |   RTTM                 | Recommended | 4.2        | S,R    |
   | TCP SACKs              | Recommended | 4.4        | S,R    |
   +------------------------+-------------+------------+--------+
                                Table 1

   Satellite users should check with their TCP vendors (implementors) to
   ensure the recommended mechanisms are supported in their stack in
   current and/or future versions.  Alternatively, the Pittsburgh
   Supercomputer Center tracks TCP implementations and which extensions
   they support, as well as providing guidance on tuning various TCP
   implementations [PSC].

   Research into improving the efficiency of TCP over satellite channels
   is ongoing and will be summarized in a planned memo along with other
   considerations, such as satellite network architectures.

6.  Security Considerations

   The authors believe that the recommendations contained in this memo
   do not alter the security implications of TCP.  However, when using a
   broadcast medium such as satellites links to transfer user data
   and/or network control traffic, one should be aware of the intrinsic
   security implications of such technology.

   Eavesdropping on network links is a form of passive attack that, if
   performed successfully, could reveal critical traffic control
   information that would jeopardize the proper functioning of the
   network.  These attacks could reduce the ability of the network to
   provide data transmission services efficiently.  Eavesdroppers could
   also compromise the privacy of user data, especially if end-to-end
   security mechanisms are not in use.  While passive monitoring can
   occur on any network, the wireless broadcast nature of satellite
   links allows reception of signals without physical connection to the
   network which enables monitoring to be conducted without detection.
   However, it should be noted that the resources needed to monitor a
   satellite link are non-trivial.

   Data encryption at the physical and/or link layers can provide secure
   communication over satellite channels.  However, this still leaves
   traffic vulnerable to eavesdropping on networks before and after
   traversing the satellite link.  Therefore, end-to-end security
   mechanisms should be considered.  This document does not make any
   recommendations as to which security mechanisms should be employed.
   However, those operating and using satellite networks should survey
   the currently available network security mechanisms and choose those
   that meet their security requirements.

Acknowledgments

   This document has benefited from comments from the members of the TCP
   Over Satellite Working Group.  In particular, we would like to thank
   Aaron Falk, Matthew Halsey, Hans Kruse, Matt Mathis, Greg Nakanishi,
   Vern Paxson, Jeff Semke, Bill Sepmeier and Eric Travis for their
   useful comments about this document.

References

   [AFP98]   Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
             Initial Window", RFC 2414, September 1998.

   [AHKO97]  Mark Allman, Chris Hayes, Hans Kruse, and Shawn Ostermann.
             TCP Performance Over Satellite Links.  In Proceedings of
             the 5th International Conference on Telecommunication
             Systems, March 1997.

   [All97]   Mark Allman.  Improving TCP Performance Over Satellite
             Channels.  Master's thesis, Ohio University, June 1997.

   [All98]   Mark Allman.  On the Generation and Use of TCP
             Acknowledgments. ACM Computer Communication Review, 28(5),
             October 1998.

   [Bra89]   Braden, R., "Requirements for Internet Hosts --
             Communication Layers", STD 3, RFC 1122, October 1989.

   [FF96]    Kevin Fall and Sally Floyd.  Simulation-based Comparisons
             of Tahoe, Reno and SACK TCP.  Computer Communication
             Review, July 1996.

   [FF98]    Sally Floyd, Kevin Fall.  Promoting the Use of End-to-End
             Congestion Control in the Internet.  Submitted to IEEE
             Transactions on Networking.

   [Flo94]   S. Floyd, TCP and Successive Fast Retransmits. Technical
             report, October 1994.
             ftp://ftp.ee.lbl.gov/papers/fastretrans.ps.

   [GJKFV98] Rohit Goyal, Raj Jain, Shiv Kalyanaraman, Sonia Fahmy,
             Bobby Vandalore, Improving the Performance of TCP over the
             ATM-UBR service, 1998.  Sumbitted to Computer
             Communications.

   [Jac90]   Van Jacobson.  Modified TCP Congestion Avoidance Algorithm.
             Technical Report, LBL, April 1990.

   [JBB92]   Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for
             High Performance", RFC 1323, May 1992.

   [Jac88]   Van Jacobson.  Congestion Avoidance and Control.  In ACM
             SIGCOMM, 1988.

   [Kno93]   Knowles, S., "IESG Advice from Experience with Path MTU
             Discovery", RFC 1435, March 1993.

   [Mar78]   James Martin.  Communications Satellite Systems.  Prentice
             Hall, 1978.

   [MD90]    Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
             November 1990.

   [MM96]    Matt Mathis and Jamshid Mahdavi.  Forward Acknowledgment:
             Refining TCP Congestion Control.  In ACM SIGCOMM, 1996.

   [MMFR96]  Mathis, M., Mahdavi, J., Floyd, S. and A.  Romanow, "TCP
             Selective Acknowledgment Options", RFC 2018, October 1996.

   [Mon98]   M. J. Montpetit. TELEDESIC: Enabling The Global Community
             Interaccess. In Proc. of the International Wireless
             Symposium, May 1998.

   [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.

   [Pos81]   Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, September 1981.

   [PS97]    Craig Partridge and Tim Shepard.  TCP Performance Over
             Satellite Links.  IEEE Network, 11(5), September/October
             1997.

   [PSC]     Jamshid Mahdavi.  Enabling High Performance Data Transfers
             on Hosts.  http://www.psc.edu/networking/perf_tune.html.

   [SMM98]   Jeff Semke, Jamshid Mahdavi and Matt Mathis.  Automatic TCP
             Buffer Tuning.  In ACM SIGCOMM, August 1998.  To appear.

   [Sta94]   William Stallings.  Data and Computer Communications.
             MacMillian, 4th edition, 1994.

   [Ste97]   Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
             Retransmit, and Fast Recovery Algorithms", RFC 2001,January
             1997.

   [Stu95]   M. A. Sturza. Architecture of the TELEDESIC Satellite
             System. In Proceedings of the International Mobile
             Satellite Conference, 1995.

Authors' Addresses

   Mark Allman
   NASA Lewis Research Center/Sterling Software
   21000 Brookpark Rd.  MS 54-2
   Cleveland, OH  44135

   Phone: +1 216 433 6586
   EMail: mallman@lerc.nasa.gov
   http://roland.lerc.nasa.gov/~mallman

   Daniel R. Glover
   NASA Lewis Research Center
   21000 Brookpark Rd.
   Cleveland, OH  44135

   Phone: +1 216 433 2847
   EMail: Daniel.R.Glover@lerc.nasa.gov

   Luis A. Sanchez
   BBN Technologies
   GTE Internetworking
   10 Moulton Street
   Cambridge, MA  02140
   USA

   Phone: +1 617 873 3351
   EMail: lsanchez@ir.bbn.com

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