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Network Working Group                                    M. AllmanRequest for Comments: 2488            NASA Lewis/Sterling SoftwareBCP: 28                                                  D. GloverCategory: Best Current Practice                         NASA Lewis                                                        L. Sanchez                                                               BBN                                                      January 1999Enhancing TCP Over Satellite Channelsusing Standard MechanismsStatus 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 areAllman, 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 moreAllman, 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 (seesection 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 satelliteAllman, 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 (seesection 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 insection 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, pathAllman, 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 (seesection 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 (seesection 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, linkAllman, 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 congestionAllman, 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 inRFC 2001 [Ste97] or subsequent   versions ofRFC 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 satelliteAllman, 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 isAllman, 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 19995.  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   [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.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             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.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             Behavior of the TCP Congestion Avoidance Algorithm",             Computer Communication Review, volume 27, number3, July             1997.  available fromhttp://www.psc.edu/networking/papers/papers.html.   [Pos81]   Postel, J., "Transmission Control Protocol", STD 7,RFC793, 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.Allman, et. al.          Best Current Practice                 [Page 17]

RFC 2488         Enhancing TCP Over Satellite Channels      January 1999Authors' 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.govhttp://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.comAllman, et. al.          Best Current Practice                 [Page 18]

RFC 2488         Enhancing TCP Over Satellite Channels      January 1999Full Copyright Statement   Copyright (C) The Internet Society (1999).  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.Allman, et. al.          Best Current Practice                 [Page 19]