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rfc:rfc2488

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

Allman, et. al. Best Current Practice [Page 1] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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

Allman, et. al. Best Current Practice [Page 2] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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.

Allman, et. al. Best Current Practice [Page 3] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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.

Allman, et. al. Best Current Practice [Page 4] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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

Allman, et. al. Best Current Practice [Page 5] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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

Allman, et. al. Best Current Practice [Page 6] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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

Allman, et. al. Best Current Practice [Page 7] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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

Allman, et. al. Best Current Practice [Page 8] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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.

Allman, et. al. Best Current Practice [Page 9] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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).

Allman, et. al. Best Current Practice [Page 10] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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

Allman, et. al. Best Current Practice [Page 11] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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

Allman, et. al. Best Current Practice [Page 12] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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.

Allman, et. al. Best Current Practice [Page 13] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

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.

Allman, et. al. Best Current Practice [Page 14] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 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

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           Initial Window", RFC 2414, September 1998.
 [AHKO97]  Mark Allman, Chris Hayes, Hans Kruse, and Shawn Ostermann.
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           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.

Allman, et. al. Best Current Practice [Page 15] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 [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
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 [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:
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 [MMFR96]  Mathis, M., Mahdavi, J., Floyd, S. and A.  Romanow, "TCP
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           Symposium, May 1998.

Allman, et. al. Best Current Practice [Page 16] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

 [MSMO97]  M. Mathis, J. Semke, J. Mahdavi, T. Ott, "The Macroscopic
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Allman, et. al. Best Current Practice [Page 17] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

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

Allman, et. al. Best Current Practice [Page 18] RFC 2488 Enhancing TCP Over Satellite Channels January 1999

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Allman, et. al. Best Current Practice [Page 19]

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