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rfc:rfc1323

Network Working Group V. Jacobson Request for Comments: 1323 LBL Obsoletes: RFC 1072, RFC 1185 R. Braden

                                                                   ISI
                                                             D. Borman
                                                         Cray Research
                                                              May 1992
                TCP Extensions for High Performance

Status of This Memo

 This RFC specifies an IAB standards track protocol for the Internet
 community, and requests discussion and suggestions for improvements.
 Please refer to the current edition of the "IAB Official Protocol
 Standards" for the standardization state and status of this protocol.
 Distribution of this memo is unlimited.

Abstract

 This memo presents a set of TCP extensions to improve performance
 over large bandwidth*delay product paths and to provide reliable
 operation over very high-speed paths.  It defines new TCP options for
 scaled windows and timestamps, which are designed to provide
 compatible interworking with TCP's that do not implement the
 extensions.  The timestamps are used for two distinct mechanisms:
 RTTM (Round Trip Time Measurement) and PAWS (Protect Against Wrapped
 Sequences).  Selective acknowledgments are not included in this memo.
 This memo combines and supersedes RFC-1072 and RFC-1185, adding
 additional clarification and more detailed specification.  Appendix C
 summarizes the changes from the earlier RFCs.

TABLE OF CONTENTS

 1.  Introduction .................................................  2
 2.  TCP Window Scale Option ......................................  8
 3.  RTTM -- Round-Trip Time Measurement .......................... 11
 4.  PAWS -- Protect Against Wrapped Sequence Numbers ............. 17
 5.  Conclusions and Acknowledgments .............................. 25
 6.  References ................................................... 25
 APPENDIX A: Implementation Suggestions ........................... 27
 APPENDIX B: Duplicates from Earlier Connection Incarnations ...... 27
 APPENDIX C: Changes from RFC-1072, RFC-1185 ...................... 30
 APPENDIX D: Summary of Notation .................................. 31
 APPENDIX E: Event Processing ..................................... 32
 Security Considerations .......................................... 37

Jacobson, Braden, & Borman [Page 1] RFC 1323 TCP Extensions for High Performance May 1992

 Authors' Addresses ............................................... 37

1. INTRODUCTION

 The TCP protocol [Postel81] was designed to operate reliably over
 almost any transmission medium regardless of transmission rate,
 delay, corruption, duplication, or reordering of segments.
 Production TCP implementations currently adapt to transfer rates in
 the range of 100 bps to 10**7 bps and round-trip delays in the range
 1 ms to 100 seconds.  Recent work on TCP performance has shown that
 TCP can work well over a variety of Internet paths, ranging from 800
 Mbit/sec I/O channels to 300 bit/sec dial-up modems [Jacobson88a].
 The introduction of fiber optics is resulting in ever-higher
 transmission speeds, and the fastest paths are moving out of the
 domain for which TCP was originally engineered.  This memo defines a
 set of modest extensions to TCP to extend the domain of its
 application to match this increasing network capability.  It is based
 upon and obsoletes RFC-1072 [Jacobson88b] and RFC-1185 [Jacobson90b].
 There is no one-line answer to the question: "How fast can TCP go?".
 There are two separate kinds of issues, performance and reliability,
 and each depends upon different parameters.  We discuss each in turn.
 1.1  TCP Performance
    TCP performance depends not upon the transfer rate itself, but
    rather upon the product of the transfer rate and the round-trip
    delay.  This "bandwidth*delay product" measures the amount of data
    that would "fill the pipe"; it is the buffer space required at
    sender and receiver to obtain maximum throughput on the TCP
    connection over the path, i.e., the amount of unacknowledged data
    that TCP must handle in order to keep the pipeline full.  TCP
    performance problems arise when the bandwidth*delay product is
    large.  We refer to an Internet path operating in this region as a
    "long, fat pipe", and a network containing this path as an "LFN"
    (pronounced "elephan(t)").
    High-capacity packet satellite channels (e.g., DARPA's Wideband
    Net) are LFN's.  For example, a DS1-speed satellite channel has a
    bandwidth*delay product of 10**6 bits or more; this corresponds to
    100 outstanding TCP segments of 1200 bytes each.  Terrestrial
    fiber-optical paths will also fall into the LFN class; for
    example, a cross-country delay of 30 ms at a DS3 bandwidth
    (45Mbps) also exceeds 10**6 bits.
    There are three fundamental performance problems with the current
    TCP over LFN paths:

Jacobson, Braden, & Borman [Page 2] RFC 1323 TCP Extensions for High Performance May 1992

    (1)  Window Size Limit
         The TCP header uses a 16 bit field to report the receive
         window size to the sender.  Therefore, the largest window
         that can be used is 2**16 = 65K bytes.
         To circumvent this problem, Section 2 of this memo defines a
         new TCP option, "Window Scale", to allow windows larger than
         2**16.  This option defines an implicit scale factor, which
         is used to multiply the window size value found in a TCP
         header to obtain the true window size.
    (2)  Recovery from Losses
         Packet losses in an LFN can have a catastrophic effect on
         throughput.  Until recently, properly-operating TCP
         implementations would cause the data pipeline to drain with
         every packet loss, and require a slow-start action to
         recover.  Recently, the Fast Retransmit and Fast Recovery
         algorithms [Jacobson90c] have been introduced.  Their
         combined effect is to recover from one packet loss per
         window, without draining the pipeline.  However, more than
         one packet loss per window typically results in a
         retransmission timeout and the resulting pipeline drain and
         slow start.
         Expanding the window size to match the capacity of an LFN
         results in a corresponding increase of the probability of
         more than one packet per window being dropped.  This could
         have a devastating effect upon the throughput of TCP over an
         LFN.  In addition, if a congestion control mechanism based
         upon some form of random dropping were introduced into
         gateways, randomly spaced packet drops would become common,
         possible increasing the probability of dropping more than one
         packet per window.
         To generalize the Fast Retransmit/Fast Recovery mechanism to
         handle multiple packets dropped per window, selective
         acknowledgments are required.  Unlike the normal cumulative
         acknowledgments of TCP, selective acknowledgments give the
         sender a complete picture of which segments are queued at the
         receiver and which have not yet arrived.  Some evidence in
         favor of selective acknowledgments has been published
         [NBS85], and selective acknowledgments have been included in
         a number of experimental Internet protocols -- VMTP
         [Cheriton88], NETBLT [Clark87], and RDP [Velten84], and
         proposed for OSI TP4 [NBS85].  However, in the non-LFN
         regime, selective acknowledgments reduce the number of

Jacobson, Braden, & Borman [Page 3] RFC 1323 TCP Extensions for High Performance May 1992

         packets retransmitted but do not otherwise improve
         performance, making their complexity of questionable value.
         However, selective acknowledgments are expected to become
         much more important in the LFN regime.
         RFC-1072 defined a new TCP "SACK" option to send a selective
         acknowledgment.  However, there are important technical
         issues to be worked out concerning both the format and
         semantics of the SACK option.  Therefore, SACK has been
         omitted from this package of extensions.  It is hoped that
         SACK can "catch up" during the standardization process.
    (3)  Round-Trip Measurement
         TCP implements reliable data delivery by retransmitting
         segments that are not acknowledged within some retransmission
         timeout (RTO) interval.  Accurate dynamic determination of an
         appropriate RTO is essential to TCP performance.  RTO is
         determined by estimating the mean and variance of the
         measured round-trip time (RTT), i.e., the time interval
         between sending a segment and receiving an acknowledgment for
         it [Jacobson88a].
         Section 4 introduces a new TCP option, "Timestamps", and then
         defines a mechanism using this option that allows nearly
         every segment, including retransmissions, to be timed at
         negligible computational cost.  We use the mnemonic RTTM
         (Round Trip Time Measurement) for this mechanism, to
         distinguish it from other uses of the Timestamps option.
 1.2 TCP Reliability
    Now we turn from performance to reliability.  High transfer rate
    enters TCP performance through the bandwidth*delay product.
    However, high transfer rate alone can threaten TCP reliability by
    violating the assumptions behind the TCP mechanism for duplicate
    detection and sequencing.
    An especially serious kind of error may result from an accidental
    reuse of TCP sequence numbers in data segments.  Suppose that an
    "old duplicate segment", e.g., a duplicate data segment that was
    delayed in Internet queues, is delivered to the receiver at the
    wrong moment, so that its sequence numbers falls somewhere within
    the current window.  There would be no checksum failure to warn of
    the error, and the result could be an undetected corruption of the
    data.  Reception of an old duplicate ACK segment at the
    transmitter could be only slightly less serious: it is likely to

Jacobson, Braden, & Borman [Page 4] RFC 1323 TCP Extensions for High Performance May 1992

    lock up the connection so that no further progress can be made,
    forcing an RST on the connection.
    TCP reliability depends upon the existence of a bound on the
    lifetime of a segment: the "Maximum Segment Lifetime" or MSL.  An
    MSL is generally required by any reliable transport protocol,
    since every sequence number field must be finite, and therefore
    any sequence number may eventually be reused.  In the Internet
    protocol suite, the MSL bound is enforced by an IP-layer
    mechanism, the "Time-to-Live" or TTL field.
    Duplication of sequence numbers might happen in either of two
    ways:
    (1)  Sequence number wrap-around on the current connection
         A TCP sequence number contains 32 bits.  At a high enough
         transfer rate, the 32-bit sequence space may be "wrapped"
         (cycled) within the time that a segment is delayed in queues.
    (2)  Earlier incarnation of the connection
         Suppose that a connection terminates, either by a proper
         close sequence or due to a host crash, and the same
         connection (i.e., using the same pair of sockets) is
         immediately reopened.  A delayed segment from the terminated
         connection could fall within the current window for the new
         incarnation and be accepted as valid.
    Duplicates from earlier incarnations, Case (2), are avoided by
    enforcing the current fixed MSL of the TCP spec, as explained in
    Section 5.3 and Appendix B.   However, case (1), avoiding the
    reuse of sequence numbers within the same connection, requires an
    MSL bound that depends upon the transfer rate, and at high enough
    rates, a new mechanism is required.
    More specifically, if the maximum effective bandwidth at which TCP
    is able to transmit over a particular path is B bytes per second,
    then the following constraint must be satisfied for error-free
    operation:
        2**31 / B  > MSL (secs)                     [1]
    The following table shows the value for Twrap = 2**31/B in
    seconds, for some important values of the bandwidth B:

Jacobson, Braden, & Borman [Page 5] RFC 1323 TCP Extensions for High Performance May 1992

         Network       B*8          B         Twrap
                    bits/sec   bytes/sec      secs
         _______    _______      ______       ______
         ARPANET       56kbps       7KBps    3*10**5 (~3.6 days)
         DS1          1.5Mbps     190KBps    10**4 (~3 hours)
         Ethernet      10Mbps    1.25MBps    1700 (~30 mins)
         DS3           45Mbps     5.6MBps    380
         FDDI         100Mbps    12.5MBps    170
         Gigabit        1Gbps     125MBps    17
    It is clear that wrap-around of the sequence space is not a
    problem for 56kbps packet switching or even 10Mbps Ethernets.  On
    the other hand, at DS3 and FDDI speeds, Twrap is comparable to the
    2 minute MSL assumed by the TCP specification [Postel81].  Moving
    towards gigabit speeds, Twrap becomes too small for reliable
    enforcement by the Internet TTL mechanism.
    The 16-bit window field of TCP limits the effective bandwidth B to
    2**16/RTT, where RTT is the round-trip time in seconds
    [McKenzie89].  If the RTT is large enough, this limits B to a
    value that meets the constraint [1] for a large MSL value.  For
    example, consider a transcontinental backbone with an RTT of 60ms
    (set by the laws of physics).  With the bandwidth*delay product
    limited to 64KB by the TCP window size, B is then limited to
    1.1MBps, no matter how high the theoretical transfer rate of the
    path.  This corresponds to cycling the sequence number space in
    Twrap= 2000 secs, which is safe in today's Internet.
    It is important to understand that the culprit is not the larger
    window but rather the high bandwidth.  For example, consider a
    (very large) FDDI LAN with a diameter of 10km.  Using the speed of
    light, we can compute the RTT across the ring as
    (2*10**4)/(3*10**8) = 67 microseconds, and the delay*bandwidth
    product is then 833 bytes.  A TCP connection across this LAN using
    a window of only 833 bytes will run at the full 100mbps and can
    wrap the sequence space in about 3 minutes, very close to the MSL
    of TCP.  Thus, high speed alone can cause a reliability problem
    with sequence number wrap-around, even without extended windows.
    Watson's Delta-T protocol [Watson81] includes network-layer
    mechanisms for precise enforcement of an MSL.  In contrast, the IP

Jacobson, Braden, & Borman [Page 6] RFC 1323 TCP Extensions for High Performance May 1992

    mechanism for MSL enforcement is loosely defined and even more
    loosely implemented in the Internet.  Therefore, it is unwise to
    depend upon active enforcement of MSL for TCP connections, and it
    is unrealistic to imagine setting MSL's smaller than the current
    values (e.g., 120 seconds specified for TCP).
    A possible fix for the problem of cycling the sequence space would
    be to increase the size of the TCP sequence number field.  For
    example, the sequence number field (and also the acknowledgment
    field) could be expanded to 64 bits.  This could be done either by
    changing the TCP header or by means of an additional option.
    Section 5 presents a different mechanism, which we call PAWS
    (Protect Against Wrapped Sequence numbers), to extend TCP
    reliability to transfer rates well beyond the foreseeable upper
    limit of network bandwidths.  PAWS uses the TCP Timestamps option
    defined in Section 4 to protect against old duplicates from the
    same connection.
 1.3 Using TCP options
    The extensions defined in this memo all use new TCP options.  We
    must address two possible issues concerning the use of TCP
    options: (1) compatibility and (2) overhead.
    We must pay careful attention to compatibility, i.e., to
    interoperation with existing implementations.  The only TCP option
    defined previously, MSS, may appear only on a SYN segment.  Every
    implementation should (and we expect that most will) ignore
    unknown options on SYN segments.  However, some buggy TCP
    implementation might be crashed by the first appearance of an
    option on a non-SYN segment.  Therefore, for each of the
    extensions defined below, TCP options will be sent on non-SYN
    segments only when an exchange of options on the SYN segments has
    indicated that both sides understand the extension.  Furthermore,
    an extension option will be sent in a <SYN,ACK> segment only if
    the corresponding option was received in the initial <SYN>
    segment.
    A question may be raised about the bandwidth and processing
    overhead for TCP options.  Those options that occur on SYN
    segments are not likely to cause a performance concern.  Opening a
    TCP connection requires execution of significant special-case
    code, and the processing of options is unlikely to increase that
    cost significantly.
    On the other hand, a Timestamps option may appear in any data or
    ACK segment, adding 12 bytes to the 20-byte TCP header.  We

Jacobson, Braden, & Borman [Page 7] RFC 1323 TCP Extensions for High Performance May 1992

    believe that the bandwidth saved by reducing unnecessary
    retransmissions will more than pay for the extra header bandwidth.
    There is also an issue about the processing overhead for parsing
    the variable byte-aligned format of options, particularly with a
    RISC-architecture CPU.  To meet this concern, Appendix A contains
    a recommended layout of the options in TCP headers to achieve
    reasonable data field alignment.  In the spirit of Header
    Prediction, a TCP can quickly test for this layout and if it is
    verified then use a fast path.  Hosts that use this canonical
    layout will effectively use the options as a set of fixed-format
    fields appended to the TCP header.  However, to retain the
    philosophical and protocol framework of TCP options, a TCP must be
    prepared to parse an arbitrary options field, albeit with less
    efficiency.
    Finally, we observe that most of the mechanisms defined in this
    memo are important for LFN's and/or very high-speed networks.  For
    low-speed networks, it might be a performance optimization to NOT
    use these mechanisms.  A TCP vendor concerned about optimal
    performance over low-speed paths might consider turning these
    extensions off for low-speed paths, or allow a user or
    installation manager to disable them.

2. TCP WINDOW SCALE OPTION

 2.1  Introduction
    The window scale extension expands the definition of the TCP
    window to 32 bits and then uses a scale factor to carry this 32-
    bit value in the 16-bit Window field of the TCP header (SEG.WND in
    RFC-793).  The scale factor is carried in a new TCP option, Window
    Scale.  This option is sent only in a SYN segment (a segment with
    the SYN bit on), hence the window scale is fixed in each direction
    when a connection is opened.  (Another design choice would be to
    specify the window scale in every TCP segment.  It would be
    incorrect to send a window scale option only when the scale factor
    changed, since a TCP option in an acknowledgement segment will not
    be delivered reliably (unless the ACK happens to be piggy-backed
    on data in the other direction).  Fixing the scale when the
    connection is opened has the advantage of lower overhead but the
    disadvantage that the scale factor cannot be changed during the
    connection.)
    The maximum receive window, and therefore the scale factor, is
    determined by the maximum receive buffer space.  In a typical
    modern implementation, this maximum buffer space is set by default

Jacobson, Braden, & Borman [Page 8] RFC 1323 TCP Extensions for High Performance May 1992

    but can be overridden by a user program before a TCP connection is
    opened.  This determines the scale factor, and therefore no new
    user interface is needed for window scaling.
 2.2  Window Scale Option
    The three-byte Window Scale option may be sent in a SYN segment by
    a TCP.  It has two purposes: (1) indicate that the TCP is prepared
    to do both send and receive window scaling, and (2) communicate a
    scale factor to be applied to its receive window.  Thus, a TCP
    that is prepared to scale windows should send the option, even if
    its own scale factor is 1.  The scale factor is limited to a power
    of two and encoded logarithmically, so it may be implemented by
    binary shift operations.
    TCP Window Scale Option (WSopt):
       Kind: 3 Length: 3 bytes
              +---------+---------+---------+
              | Kind=3  |Length=3 |shift.cnt|
              +---------+---------+---------+
       This option is an offer, not a promise; both sides must send
       Window Scale options in their SYN segments to enable window
       scaling in either direction.  If window scaling is enabled,
       then the TCP that sent this option will right-shift its true
       receive-window values by 'shift.cnt' bits for transmission in
       SEG.WND.  The value 'shift.cnt' may be zero (offering to scale,
       while applying a scale factor of 1 to the receive window).
       This option may be sent in an initial <SYN> segment (i.e., a
       segment with the SYN bit on and the ACK bit off).  It may also
       be sent in a <SYN,ACK> segment, but only if a Window Scale op-
       tion was received in the initial <SYN> segment.  A Window Scale
       option in a segment without a SYN bit should be ignored.
       The Window field in a SYN (i.e., a <SYN> or <SYN,ACK>) segment
       itself is never scaled.
 2.3  Using the Window Scale Option
    A model implementation of window scaling is as follows, using the
    notation of RFC-793 [Postel81]:
  • All windows are treated as 32-bit quantities for storage in

Jacobson, Braden, & Borman [Page 9] RFC 1323 TCP Extensions for High Performance May 1992

         the connection control block and for local calculations.
         This includes the send-window (SND.WND) and the receive-
         window (RCV.WND) values, as well as the congestion window.
  • The connection state is augmented by two window shift counts,

Snd.Wind.Scale and Rcv.Wind.Scale, to be applied to the

         incoming and outgoing window fields, respectively.
  • If a TCP receives a <SYN> segment containing a Window Scale

option, it sends its own Window Scale option in the <SYN,ACK>

         segment.
  • The Window Scale option is sent with shift.cnt = R, where R

is the value that the TCP would like to use for its receive

         window.
  • Upon receiving a SYN segment with a Window Scale option

containing shift.cnt = S, a TCP sets Snd.Wind.Scale to S and

         sets Rcv.Wind.Scale to R; otherwise, it sets both
         Snd.Wind.Scale and Rcv.Wind.Scale to zero.
  • The window field (SEG.WND) in the header of every incoming

segment, with the exception of SYN segments, is left-shifted

         by Snd.Wind.Scale bits before updating SND.WND:
            SND.WND = SEG.WND << Snd.Wind.Scale
         (assuming the other conditions of RFC793 are met, and using
         the "C" notation "<<" for left-shift).
  • The window field (SEG.WND) of every outgoing segment, with

the exception of SYN segments, is right-shifted by

         Rcv.Wind.Scale bits:
            SEG.WND = RCV.WND >> Rcv.Wind.Scale.
    TCP determines if a data segment is "old" or "new" by testing
    whether its sequence number is within 2**31 bytes of the left edge
    of the window, and if it is not, discarding the data as "old".  To
    insure that new data is never mistakenly considered old and vice-
    versa, the left edge of the sender's window has to be at most
    2**31 away from the right edge of the receiver's window.
    Similarly with the sender's right edge and receiver's left edge.
    Since the right and left edges of either the sender's or
    receiver's window differ by the window size, and since the sender
    and receiver windows can be out of phase by at most the window
    size, the above constraints imply that 2 * the max window size

Jacobson, Braden, & Borman [Page 10] RFC 1323 TCP Extensions for High Performance May 1992

    must be less than 2**31, or
         max window < 2**30
    Since the max window is 2**S (where S is the scaling shift count)
    times at most 2**16 - 1 (the maximum unscaled window), the maximum
    window is guaranteed to be < 2*30 if S <= 14.  Thus, the shift
    count must be limited to 14 (which allows windows of 2**30 = 1
    Gbyte).  If a Window Scale option is received with a shift.cnt
    value exceeding 14, the TCP should log the error but use 14
    instead of the specified value.
    The scale factor applies only to the Window field as transmitted
    in the TCP header; each TCP using extended windows will maintain
    the window values locally as 32-bit numbers.  For example, the
    "congestion window" computed by Slow Start and Congestion
    Avoidance is not affected by the scale factor, so window scaling
    will not introduce quantization into the congestion window.

3. RTTM: ROUND-TRIP TIME MEASUREMENT

 3.1  Introduction
    Accurate and current RTT estimates are necessary to adapt to
    changing traffic conditions and to avoid an instability known as
    "congestion collapse" [Nagle84] in a busy network.  However,
    accurate measurement of RTT may be difficult both in theory and in
    implementation.
    Many TCP implementations base their RTT measurements upon a sample
    of only one packet per window.  While this yields an adequate
    approximation to the RTT for small windows, it results in an
    unacceptably poor RTT estimate for an LFN.  If we look at RTT
    estimation as a signal processing problem (which it is), a data
    signal at some frequency, the packet rate, is being sampled at a
    lower frequency, the window rate.  This lower sampling frequency
    violates Nyquist's criteria and may therefore introduce "aliasing"
    artifacts into the estimated RTT [Hamming77].
    A good RTT estimator with a conservative retransmission timeout
    calculation can tolerate aliasing when the sampling frequency is
    "close" to the data frequency.   For example, with a window of 8
    packets, the sample rate is 1/8 the data frequency -- less than an
    order of magnitude different.  However, when the window is tens or
    hundreds of packets, the RTT estimator may be seriously in error,
    resulting in spurious retransmissions.
    If there are dropped packets, the problem becomes worse.  Zhang

Jacobson, Braden, & Borman [Page 11] RFC 1323 TCP Extensions for High Performance May 1992

    [Zhang86], Jain [Jain86] and Karn [Karn87] have shown that it is
    not possible to accumulate reliable RTT estimates if retransmitted
    segments are included in the estimate.  Since a full window of
    data will have been transmitted prior to a retransmission, all of
    the segments in that window will have to be ACKed before the next
    RTT sample can be taken.  This means at least an additional
    window's worth of time between RTT measurements and, as the error
    rate approaches one per window of data (e.g., 10**-6 errors per
    bit for the Wideband satellite network), it becomes effectively
    impossible to obtain a valid RTT measurement.
    A solution to these problems, which actually simplifies the sender
    substantially, is as follows: using TCP options, the sender places
    a timestamp in each data segment, and the receiver reflects these
    timestamps back in ACK segments.  Then a single subtract gives the
    sender an accurate RTT measurement for every ACK segment (which
    will correspond to every other data segment, with a sensible
    receiver).  We call this the RTTM (Round-Trip Time Measurement)
    mechanism.
    It is vitally important to use the RTTM mechanism with big
    windows; otherwise, the door is opened to some dangerous
    instabilities due to aliasing.  Furthermore, the option is
    probably useful for all TCP's, since it simplifies the sender.
 3.2  TCP Timestamps Option
    TCP is a symmetric protocol, allowing data to be sent at any time
    in either direction, and therefore timestamp echoing may occur in
    either direction.  For simplicity and symmetry, we specify that
    timestamps always be sent and echoed in both directions.  For
    efficiency, we combine the timestamp and timestamp reply fields
    into a single TCP Timestamps Option.

Jacobson, Braden, & Borman [Page 12] RFC 1323 TCP Extensions for High Performance May 1992

    TCP Timestamps Option (TSopt):
       Kind: 8
       Length: 10 bytes
        +-------+-------+---------------------+---------------------+
        |Kind=8 |  10   |   TS Value (TSval)  |TS Echo Reply (TSecr)|
        +-------+-------+---------------------+---------------------+
            1       1              4                     4
       The Timestamps option carries two four-byte timestamp fields.
       The Timestamp Value field (TSval) contains the current value of
       the timestamp clock of the TCP sending the option.
       The Timestamp Echo Reply field (TSecr) is only valid if the ACK
       bit is set in the TCP header; if it is valid, it echos a times-
       tamp value that was sent by the remote TCP in the TSval field
       of a Timestamps option.  When TSecr is not valid, its value
       must be zero.  The TSecr value will generally be from the most
       recent Timestamp option that was received; however, there are
       exceptions that are explained below.
       A TCP may send the Timestamps option (TSopt) in an initial
       <SYN> segment (i.e., segment containing a SYN bit and no ACK
       bit), and may send a TSopt in other segments only if it re-
       ceived a TSopt in the initial <SYN> segment for the connection.
 3.3 The RTTM Mechanism
    The timestamp value to be sent in TSval is to be obtained from a
    (virtual) clock that we call the "timestamp clock".  Its values
    must be at least approximately proportional to real time, in order
    to measure actual RTT.
    The following example illustrates a one-way data flow with
    segments arriving in sequence without loss.  Here A, B, C...
    represent data blocks occupying successive blocks of sequence
    numbers, and ACK(A),...  represent the corresponding cumulative
    acknowledgments.  The two timestamp fields of the Timestamps
    option are shown symbolically as <TSval= x,TSecr=y>.  Each TSecr
    field contains the value most recently received in a TSval field.

Jacobson, Braden, & Borman [Page 13] RFC 1323 TCP Extensions for High Performance May 1992

       TCP  A                                          TCP B
                      <A,TSval=1,TSecr=120> ------>
           <---- <ACK(A),TSval=127,TSecr=1>
                      <B,TSval=5,TSecr=127> ------>
           <---- <ACK(B),TSval=131,TSecr=5>
           . . . . . . . . . . . . . . . . . . . . . .
                      <C,TSval=65,TSecr=131> ------>
           <---- <ACK(C),TSval=191,TSecr=65>
                      (etc)
    The dotted line marks a pause (60 time units long) in which A had
    nothing to send.  Note that this pause inflates the RTT which B
    could infer from receiving TSecr=131 in data segment C.  Thus, in
    one-way data flows, RTTM in the reverse direction measures a value
    that is inflated by gaps in sending data.  However, the following
    rule prevents a resulting inflation of the measured RTT:
         A TSecr value received in a segment is used to update the
         averaged RTT measurement only if the segment acknowledges
         some new data, i.e., only if it advances the left edge of the
         send window.
    Since TCP B is not sending data, the data segment C does not
    acknowledge any new data when it arrives at B.  Thus, the inflated
    RTTM measurement is not used to update B's RTTM measurement.
 3.4  Which Timestamp to Echo
    If more than one Timestamps option is received before a reply
    segment is sent, the TCP must choose only one of the TSvals to
    echo, ignoring the others.  To minimize the state kept in the
    receiver (i.e., the number of unprocessed TSvals), the receiver
    should be required to retain at most one timestamp in the
    connection control block.

Jacobson, Braden, & Borman [Page 14] RFC 1323 TCP Extensions for High Performance May 1992

    There are three situations to consider:
    (A)  Delayed ACKs.
         Many TCP's acknowledge only every Kth segment out of a group
         of segments arriving within a short time interval; this
         policy is known generally as "delayed ACKs".  The data-sender
         TCP must measure the effective RTT, including the additional
         time due to delayed ACKs, or else it will retransmit
         unnecessarily.  Thus, when delayed ACKs are in use, the
         receiver should reply with the TSval field from the earliest
         unacknowledged segment.
    (B)  A hole in the sequence space (segment(s) have been lost).
         The sender will continue sending until the window is filled,
         and the receiver may be generating ACKs as these out-of-order
         segments arrive (e.g., to aid "fast retransmit").
         The lost segment is probably a sign of congestion, and in
         that situation the sender should be conservative about
         retransmission.  Furthermore, it is better to overestimate
         than underestimate the RTT.  An ACK for an out-of-order
         segment should therefore contain the timestamp from the most
         recent segment that advanced the window.
         The same situation occurs if segments are re-ordered by the
         network.
    (C)  A filled hole in the sequence space.
         The segment that fills the hole represents the most recent
         measurement of the network characteristics.  On the other
         hand, an RTT computed from an earlier segment would probably
         include the sender's retransmit time-out, badly biasing the
         sender's average RTT estimate.  Thus, the timestamp from the
         latest segment (which filled the hole) must be echoed.
    An algorithm that covers all three cases is described in the
    following rules for Timestamps option processing on a synchronized
    connection:
    (1)  The connection state is augmented with two 32-bit slots:
         TS.Recent holds a timestamp to be echoed in TSecr whenever a
         segment is sent, and Last.ACK.sent holds the ACK field from
         the last segment sent.  Last.ACK.sent will equal RCV.NXT
         except when ACKs have been delayed.

Jacobson, Braden, & Borman [Page 15] RFC 1323 TCP Extensions for High Performance May 1992

    (2)  If Last.ACK.sent falls within the range of sequence numbers
         of an incoming segment:
            SEG.SEQ <= Last.ACK.sent < SEG.SEQ + SEG.LEN
         then the TSval from the segment is copied to TS.Recent;
         otherwise, the TSval is ignored.
    (3)  When a TSopt is sent, its TSecr field is set to the current
         TS.Recent value.
    The following examples illustrate these rules.  Here A, B, C...
    represent data segments occupying successive blocks of sequence
    numbers, and ACK(A),...  represent the corresponding
    acknowledgment segments.  Note that ACK(A) has the same sequence
    number as B.  We show only one direction of timestamp echoing, for
    clarity.
    o    Packets arrive in sequence, and some of the ACKs are delayed.
         By Case (A), the timestamp from the oldest unacknowledged
         segment is echoed.
                                                    TS.Recent
                  <A, TSval=1> ------------------->
                                                        1
                  <B, TSval=2> ------------------->
                                                        1
                  <C, TSval=3> ------------------->
                                                        1
                           <---- <ACK(C), TSecr=1>
                  (etc)
    o    Packets arrive out of order, and every packet is
         acknowledged.
         By Case (B), the timestamp from the last segment that
         advanced the left window edge is echoed, until the missing
         segment arrives; it is echoed according to Case (C).  The
         same sequence would occur if segments B and D were lost and
         retransmitted..

Jacobson, Braden, & Borman [Page 16] RFC 1323 TCP Extensions for High Performance May 1992

                                                    TS.Recent
                  <A, TSval=1> ------------------->
                                                        1
                           <---- <ACK(A), TSecr=1>
                                                        1
                  <C, TSval=3> ------------------->
                                                        1
                           <---- <ACK(A), TSecr=1>
                                                        1
                  <B, TSval=2> ------------------->
                                                        2
                           <---- <ACK(C), TSecr=2>
                                                        2
                  <E, TSval=5> ------------------->
                                                        2
                           <---- <ACK(C), TSecr=2>
                                                        2
                  <D, TSval=4> ------------------->
                                                        4
                           <---- <ACK(E), TSecr=4>
                  (etc)

4. PAWS: PROTECT AGAINST WRAPPED SEQUENCE NUMBERS

 4.1  Introduction
    Section 4.2 describes a simple mechanism to reject old duplicate
    segments that might corrupt an open TCP connection; we call this
    mechanism PAWS (Protect Against Wrapped Sequence numbers).  PAWS
    operates within a single TCP connection, using state that is saved
    in the connection control block.  Section 4.3 and Appendix C
    discuss the implications of the PAWS mechanism for avoiding old
    duplicates from previous incarnations of the same connection.
 4.2  The PAWS Mechanism
    PAWS uses the same TCP Timestamps option as the RTTM mechanism
    described earlier, and assumes that every received TCP segment
    (including data and ACK segments) contains a timestamp SEG.TSval
    whose values are monotone non-decreasing in time.  The basic idea
    is that a segment can be discarded as an old duplicate if it is
    received with a timestamp SEG.TSval less than some timestamp
    recently received on this connection.
    In both the PAWS and the RTTM mechanism, the "timestamps" are 32-

Jacobson, Braden, & Borman [Page 17] RFC 1323 TCP Extensions for High Performance May 1992

    bit unsigned integers in a modular 32-bit space.  Thus, "less
    than" is defined the same way it is for TCP sequence numbers, and
    the same implementation techniques apply.  If s and t are
    timestamp values, s < t if 0 < (t - s) < 2**31, computed in
    unsigned 32-bit arithmetic.
    The choice of incoming timestamps to be saved for this comparison
    must guarantee a value that is monotone increasing.  For example,
    we might save the timestamp from the segment that last advanced
    the left edge of the receive window, i.e., the most recent in-
    sequence segment.  Instead, we choose the value TS.Recent
    introduced in Section 3.4 for the RTTM mechanism, since using a
    common value for both PAWS and RTTM simplifies the implementation
    of both.  As Section 3.4 explained, TS.Recent differs from the
    timestamp from the last in-sequence segment only in the case of
    delayed ACKs, and therefore by less than one window.  Either
    choice will therefore protect against sequence number wrap-around.
    RTTM was specified in a symmetrical manner, so that TSval
    timestamps are carried in both data and ACK segments and are
    echoed in TSecr fields carried in returning ACK or data segments.
    PAWS submits all incoming segments to the same test, and therefore
    protects against duplicate ACK segments as well as data segments.
    (An alternative un-symmetric algorithm would protect against old
    duplicate ACKs: the sender of data would reject incoming ACK
    segments whose TSecr values were less than the TSecr saved from
    the last segment whose ACK field advanced the left edge of the
    send window.  This algorithm was deemed to lack economy of
    mechanism and symmetry.)
    TSval timestamps sent on {SYN} and {SYN,ACK} segments are used to
    initialize PAWS.  PAWS protects against old duplicate non-SYN
    segments, and duplicate SYN segments received while there is a
    synchronized connection.  Duplicate {SYN} and {SYN,ACK} segments
    received when there is no connection will be discarded by the
    normal 3-way handshake and sequence number checks of TCP.
    It is recommended that RST segments NOT carry timestamps, and that
    RST segments be acceptable regardless of their timestamp.  Old
    duplicate RST segments should be exceedingly unlikely, and their
    cleanup function should take precedence over timestamps.
    4.2.1  Basic PAWS Algorithm
       The PAWS algorithm requires the following processing to be
       performed on all incoming segments for a synchronized
       connection:

Jacobson, Braden, & Borman [Page 18] RFC 1323 TCP Extensions for High Performance May 1992

       R1)  If there is a Timestamps option in the arriving segment
            and SEG.TSval < TS.Recent and if TS.Recent is valid (see
            later discussion), then treat the arriving segment as not
            acceptable:
                 Send an acknowledgement in reply as specified in
                 RFC-793 page 69 and drop the segment.
                 Note: it is necessary to send an ACK segment in order
                 to retain TCP's mechanisms for detecting and
                 recovering from half-open connections.  For example,
                 see Figure 10 of RFC-793.
       R2)  If the segment is outside the window, reject it (normal
            TCP processing)
       R3)  If an arriving segment satisfies: SEG.SEQ <= Last.ACK.sent
            (see Section 3.4), then record its timestamp in TS.Recent.
       R4)  If an arriving segment is in-sequence (i.e., at the left
            window edge), then accept it normally.
       R5)  Otherwise, treat the segment as a normal in-window, out-
            of-sequence TCP segment (e.g., queue it for later delivery
            to the user).
       Steps R2, R4, and R5 are the normal TCP processing steps
       specified by RFC-793.
       It is important to note that the timestamp is checked only when
       a segment first arrives at the receiver, regardless of whether
       it is in-sequence or it must be queued for later delivery.
       Consider the following example.
            Suppose the segment sequence: A.1, B.1, C.1, ..., Z.1 has
            been sent, where the letter indicates the sequence number
            and the digit represents the timestamp.  Suppose also that
            segment B.1 has been lost.  The timestamp in TS.TStamp is
            1 (from A.1), so C.1, ..., Z.1 are considered acceptable
            and are queued.  When B is retransmitted as segment B.2
            (using the latest timestamp), it fills the hole and causes
            all the segments through Z to be acknowledged and passed
            to the user.  The timestamps of the queued segments are
            *not* inspected again at this time, since they have
            already been accepted.  When B.2 is accepted, TS.Stamp is
            set to 2.
       This rule allows reasonable performance under loss.  A full

Jacobson, Braden, & Borman [Page 19] RFC 1323 TCP Extensions for High Performance May 1992

       window of data is in transit at all times, and after a loss a
       full window less one packet will show up out-of-sequence to be
       queued at the receiver (e.g., up to ~2**30 bytes of data); the
       timestamp option must not result in discarding this data.
       In certain unlikely circumstances, the algorithm of rules R1-R4
       could lead to discarding some segments unnecessarily, as shown
       in the following example:
            Suppose again that segments: A.1, B.1, C.1, ..., Z.1 have
            been sent in sequence and that segment B.1 has been lost.
            Furthermore, suppose delivery of some of C.1, ... Z.1 is
            delayed until AFTER the retransmission B.2 arrives at the
            receiver.  These delayed segments will be discarded
            unnecessarily when they do arrive, since their timestamps
            are now out of date.
       This case is very unlikely to occur.  If the retransmission was
       triggered by a timeout, some of the segments C.1, ... Z.1 must
       have been delayed longer than the RTO time.  This is presumably
       an unlikely event, or there would be many spurious timeouts and
       retransmissions.  If B's retransmission was triggered by the
       "fast retransmit" algorithm, i.e., by duplicate ACKs, then the
       queued segments that caused these ACKs must have been received
       already.
       Even if a segment were delayed past the RTO, the Fast
       Retransmit mechanism [Jacobson90c] will cause the delayed
       packets to be retransmitted at the same time as B.2, avoiding
       an extra RTT and therefore causing a very small performance
       penalty.
       We know of no case with a significant probability of occurrence
       in which timestamps will cause performance degradation by
       unnecessarily discarding segments.
    4.2.2  Timestamp Clock
       It is important to understand that the PAWS algorithm does not
       require clock synchronization between sender and receiver.  The
       sender's timestamp clock is used to stamp the segments, and the
       sender uses the echoed timestamp to measure RTT's.  However,
       the receiver treats the timestamp as simply a monotone-
       increasing serial number, without any necessary connection to
       its clock.  From the receiver's viewpoint, the timestamp is
       acting as a logical extension of the high-order bits of the
       sequence number.

Jacobson, Braden, & Borman [Page 20] RFC 1323 TCP Extensions for High Performance May 1992

       The receiver algorithm does place some requirements on the
       frequency of the timestamp clock.
       (a)  The timestamp clock must not be "too slow".
            It must tick at least once for each 2**31 bytes sent.  In
            fact, in order to be useful to the sender for round trip
            timing, the clock should tick at least once per window's
            worth of data, and even with the RFC-1072 window
            extension, 2**31 bytes must be at least two windows.
            To make this more quantitative, any clock faster than 1
            tick/sec will reject old duplicate segments for link
            speeds of ~8 Gbps.  A 1ms timestamp clock will work at
            link speeds up to 8 Tbps (8*10**12) bps!
       (b)  The timestamp clock must not be "too fast".
            Its recycling time must be greater than MSL seconds.
            Since the clock (timestamp) is 32 bits and the worst-case
            MSL is 255 seconds, the maximum acceptable clock frequency
            is one tick every 59 ns.
            However, it is desirable to establish a much longer
            recycle period, in order to handle outdated timestamps on
            idle connections (see Section 4.2.3), and to relax the MSL
            requirement for preventing sequence number wrap-around.
            With a 1 ms timestamp clock, the 32-bit timestamp will
            wrap its sign bit in 24.8 days.  Thus, it will reject old
            duplicates on the same connection if MSL is 24.8 days or
            less.  This appears to be a very safe figure; an MSL of
            24.8 days or longer can probably be assumed by the gateway
            system without requiring precise MSL enforcement by the
            TTL value in the IP layer.
       Based upon these considerations, we choose a timestamp clock
       frequency in the range 1 ms to 1 sec per tick.  This range also
       matches the requirements of the RTTM mechanism, which does not
       need much more resolution than the granularity of the
       retransmit timer, e.g., tens or hundreds of milliseconds.
       The PAWS mechanism also puts a strong monotonicity requirement
       on the sender's timestamp clock.  The method of implementation
       of the timestamp clock to meet this requirement depends upon
       the system hardware and software.
  • Some hosts have a hardware clock that is guaranteed to be

monotonic between hardware resets.

Jacobson, Braden, & Borman [Page 21] RFC 1323 TCP Extensions for High Performance May 1992

  • A clock interrupt may be used to simply increment a binary

integer by 1 periodically.

  • The timestamp clock may be derived from a system clock

that is subject to being abruptly changed, by adding a

            variable offset value.  This offset is initialized to
            zero.  When a new timestamp clock value is needed, the
            offset can be adjusted as necessary to make the new value
            equal to or larger than the previous value (which was
            saved for this purpose).
    4.2.3  Outdated Timestamps
       If a connection remains idle long enough for the timestamp
       clock of the other TCP to wrap its sign bit, then the value
       saved in TS.Recent will become too old; as a result, the PAWS
       mechanism will cause all subsequent segments to be rejected,
       freezing the connection (until the timestamp clock wraps its
       sign bit again).
       With the chosen range of timestamp clock frequencies (1 sec to
       1 ms), the time to wrap the sign bit will be between 24.8 days
       and 24800 days.  A TCP connection that is idle for more than 24
       days and then comes to life is exceedingly unusual.  However,
       it is undesirable in principle to place any limitation on TCP
       connection lifetimes.
       We therefore require that an implementation of PAWS include a
       mechanism to "invalidate" the TS.Recent value when a connection
       is idle for more than 24 days.  (An alternative solution to the
       problem of outdated timestamps would be to send keepalive
       segments at a very low rate, but still more often than the
       wrap-around time for timestamps, e.g., once a day.  This would
       impose negligible overhead.  However, the TCP specification has
       never included keepalives, so the solution based upon
       invalidation was chosen.)
       Note that a TCP does not know the frequency, and therefore, the
       wraparound time, of the other TCP, so it must assume the worst.
       The validity of TS.Recent needs to be checked only if the basic
       PAWS timestamp check fails, i.e., only if SEG.TSval <
       TS.Recent.  If TS.Recent is found to be invalid, then the
       segment is accepted, regardless of the failure of the timestamp
       check, and rule R3 updates TS.Recent with the TSval from the
       new segment.
       To detect how long the connection has been idle, the TCP may

Jacobson, Braden, & Borman [Page 22] RFC 1323 TCP Extensions for High Performance May 1992

       update a clock or timestamp value associated with the
       connection whenever TS.Recent is updated, for example.  The
       details will be implementation-dependent.
    4.2.4  Header Prediction
       "Header prediction" [Jacobson90a] is a high-performance
       transport protocol implementation technique that is most
       important for high-speed links.  This technique optimizes the
       code for the most common case, receiving a segment correctly
       and in order.  Using header prediction, the receiver asks the
       question, "Is this segment the next in sequence?"  This
       question can be answered in fewer machine instructions than the
       question, "Is this segment within the window?"
       Adding header prediction to our timestamp procedure leads to
       the following recommended sequence for processing an arriving
       TCP segment:
       H1)  Check timestamp (same as step R1 above)
       H2)  Do header prediction: if segment is next in sequence and
            if there are no special conditions requiring additional
            processing, accept the segment, record its timestamp, and
            skip H3.
       H3)  Process the segment normally, as specified in RFC-793.
            This includes dropping segments that are outside the win-
            dow and possibly sending acknowledgments, and queueing
            in-window, out-of-sequence segments.
       Another possibility would be to interchange steps H1 and H2,
       i.e., to perform the header prediction step H2 FIRST, and
       perform H1 and H3 only when header prediction fails.  This
       could be a performance improvement, since the timestamp check
       in step H1 is very unlikely to fail, and it requires interval
       arithmetic on a finite field, a relatively expensive operation.
       To perform this check on every single segment is contrary to
       the philosophy of header prediction.  We believe that this
       change might reduce CPU time for TCP protocol processing by up
       to 5-10% on high-speed networks.
       However, putting H2 first would create a hazard: a segment from
       2**32 bytes in the past might arrive at exactly the wrong time
       and be accepted mistakenly by the header-prediction step.  The
       following reasoning has been introduced [Jacobson90b] to show
       that the probability of this failure is negligible.

Jacobson, Braden, & Borman [Page 23] RFC 1323 TCP Extensions for High Performance May 1992

            If all segments are equally likely to show up as old
            duplicates, then the probability of an old duplicate
            exactly matching the left window edge is the maximum
            segment size (MSS) divided by the size of the sequence
            space.  This ratio must be less than 2**-16, since MSS
            must be < 2**16; for example, it will be (2**12)/(2**32) =
            2**-20 for an FDDI link.  However, the older a segment is,
            the less likely it is to be retained in the Internet, and
            under any reasonable model of segment lifetime the
            probability of an old duplicate exactly at the left window
            edge must be much smaller than 2**-16.
            The 16 bit TCP checksum also allows a basic unreliability
            of one part in 2**16.  A protocol mechanism whose
            reliability exceeds the reliability of the TCP checksum
            should be considered "good enough", i.e., it won't
            contribute significantly to the overall error rate.  We
            therefore believe we can ignore the problem of an old
            duplicate being accepted by doing header prediction before
            checking the timestamp.
       However, this probabilistic argument is not universally
       accepted, and the consensus at present is that the performance
       gain does not justify the hazard in the general case.  It is
       therefore recommended that H2 follow H1.
 4.3.  Duplicates from Earlier Incarnations of Connection
    The PAWS mechanism protects against errors due to sequence number
    wrap-around on high-speed connection.  Segments from an earlier
    incarnation of the same connection are also a potential cause of
    old duplicate errors.  In both cases, the TCP mechanisms to
    prevent such errors depend upon the enforcement of a maximum
    segment lifetime (MSL) by the Internet (IP) layer (see Appendix of
    RFC-1185 for a detailed discussion).  Unlike the case of sequence
    space wrap-around, the MSL required to prevent old duplicate
    errors from earlier incarnations does not depend upon the transfer
    rate.  If the IP layer enforces the recommended 2 minute MSL of
    TCP, and if the TCP rules are followed, TCP connections will be
    safe from earlier incarnations, no matter how high the network
    speed.  Thus, the PAWS mechanism is not required for this case.
    We may still ask whether the PAWS mechanism can provide additional
    security against old duplicates from earlier connections, allowing
    us to relax the enforcement of MSL by the IP layer.  Appendix B
    explores this question, showing that further assumptions and/or
    mechanisms are required, beyond those of PAWS.  This is not part
    of the current extension.

Jacobson, Braden, & Borman [Page 24] RFC 1323 TCP Extensions for High Performance May 1992

5. CONCLUSIONS AND ACKNOWLEDGMENTS

 This memo presented a set of extensions to TCP to provide efficient
 operation over large-bandwidth*delay-product paths and reliable
 operation over very high-speed paths.  These extensions are designed
 to provide compatible interworking with TCP's that do not implement
 the extensions.
 These mechanisms are implemented using new TCP options for scaled
 windows and timestamps.  The timestamps are used for two distinct
 mechanisms: RTTM (Round Trip Time Measurement) and PAWS (Protect
 Against Wrapped Sequences).
 The Window Scale option was originally suggested by Mike St. Johns of
 USAF/DCA.  The present form of the option was suggested by Mike
 Karels of UC Berkeley in response to a more cumbersome scheme defined
 by Van Jacobson.  Lixia Zhang helped formulate the PAWS mechanism
 description in RFC-1185.
 Finally, much of this work originated as the result of discussions
 within the End-to-End Task Force on the theoretical limitations of
 transport protocols in general and TCP in particular.  More recently,
 task force members and other on the end2end-interest list have made
 valuable contributions by pointing out flaws in the algorithms and
 the documentation.  The authors are grateful for all these
 contributions.

6. REFERENCES

    [Clark87]  Clark, D., Lambert, M., and L. Zhang, "NETBLT: A Bulk
    Data Transfer Protocol", RFC 998, MIT, March 1987.
    [Garlick77]  Garlick, L., R. Rom, and J. Postel, "Issues in
    Reliable Host-to-Host Protocols", Proc. Second Berkeley Workshop
    on Distributed Data Management and Computer Networks, May 1977.
    [Hamming77]  Hamming, R., "Digital Filters", ISBN 0-13-212571-4,
    Prentice Hall, Englewood Cliffs, N.J., 1977.
    [Cheriton88]  Cheriton, D., "VMTP: Versatile Message Transaction
    Protocol", RFC 1045, Stanford University, February 1988.
    [Jacobson88a] Jacobson, V., "Congestion Avoidance and Control",
    SIGCOMM '88, Stanford, CA., August 1988.
    [Jacobson88b]  Jacobson, V., and R. Braden, "TCP Extensions for
    Long-Delay Paths", RFC-1072, LBL and USC/Information Sciences
    Institute, October 1988.

Jacobson, Braden, & Borman [Page 25] RFC 1323 TCP Extensions for High Performance May 1992

    [Jacobson90a]  Jacobson, V., "4BSD Header Prediction", ACM
    Computer Communication Review, April 1990.
    [Jacobson90b]  Jacobson, V., Braden, R., and Zhang, L., "TCP
    Extension for High-Speed Paths", RFC-1185, LBL and USC/Information
    Sciences Institute, October 1990.
    [Jacobson90c]  Jacobson, V., "Modified TCP congestion avoidance
    algorithm", Message to end2end-interest mailing list, April 1990.
    [Jain86]  Jain, R., "Divergence of Timeout Algorithms for Packet
    Retransmissions", Proc. Fifth Phoenix Conf. on Comp. and Comm.,
    Scottsdale, Arizona, March 1986.
    [Karn87]  Karn, P. and C. Partridge, "Estimating Round-Trip Times
    in Reliable Transport Protocols", Proc. SIGCOMM '87, Stowe, VT,
    August 1987.
    [McKenzie89]  McKenzie, A., "A Problem with the TCP Big Window
    Option", RFC 1110, BBN STC, August 1989.
    [Nagle84]  Nagle, J., "Congestion Control in IP/TCP
    Internetworks", RFC 896, FACC, January 1984.
    [NBS85]  Colella, R., Aronoff, R., and K. Mills, "Performance
    Improvements for ISO Transport", Ninth Data Comm Symposium,
    published in ACM SIGCOMM Comp Comm Review, vol. 15, no. 5,
    September 1985.
    [Postel81]  Postel, J., "Transmission Control Protocol - DARPA
    Internet Program Protocol Specification", RFC 793, DARPA,
    September 1981.
    [Velten84] Velten, D., Hinden, R., and J. Sax, "Reliable Data
    Protocol", RFC 908, BBN, July 1984.
    [Watson81]  Watson, R., "Timer-based Mechanisms in Reliable
    Transport Protocol Connection Management", Computer Networks, Vol.
    5, 1981.
    [Zhang86]  Zhang, L., "Why TCP Timers Don't Work Well", Proc.
    SIGCOMM '86, Stowe, Vt., August 1986.

Jacobson, Braden, & Borman [Page 26] RFC 1323 TCP Extensions for High Performance May 1992

APPENDIX A: IMPLEMENTATION SUGGESTIONS

 The following layouts are recommended for sending options on non-SYN
 segments, to achieve maximum feasible alignment of 32-bit and 64-bit
 machines.
     +--------+--------+--------+--------+
     |   NOP  |  NOP   |  TSopt |   10   |
     +--------+--------+--------+--------+
     |          TSval   timestamp        |
     +--------+--------+--------+--------+
     |          TSecr   timestamp        |
     +--------+--------+--------+--------+

APPENDIX B: DUPLICATES FROM EARLIER CONNECTION INCARNATIONS

 There are two cases to be considered:  (1) a system crashing (and
 losing connection state) and restarting, and (2) the same connection
 being closed and reopened without a loss of host state.  These will
 be described in the following two sections.
 B.1  System Crash with Loss of State
    TCP's quiet time of one MSL upon system startup handles the loss
    of connection state in a system crash/restart.  For an
    explanation, see for example "When to Keep Quiet" in the TCP
    protocol specification [Postel81].  The MSL that is required here
    does not depend upon the transfer speed.  The current TCP MSL of 2
    minutes seems acceptable as an operational compromise, as many
    host systems take this long to boot after a crash.
    However, the timestamp option may be used to ease the MSL
    requirements (or to provide additional security against data
    corruption).  If timestamps are being used and if the timestamp
    clock can be guaranteed to be monotonic over a system
    crash/restart, i.e., if the first value of the sender's timestamp
    clock after a crash/restart can be guaranteed to be greater than
    the last value before the restart, then a quiet time will be
    unnecessary.
    To dispense totally with the quiet time would require that the
    host clock be synchronized to a time source that is stable over
    the crash/restart period, with an accuracy of one timestamp clock
    tick or better.  We can back off from this strict requirement to
    take advantage of approximate clock synchronization.  Suppose that
    the clock is always re-synchronized to within N timestamp clock

Jacobson, Braden, & Borman [Page 27] RFC 1323 TCP Extensions for High Performance May 1992

    ticks and that booting (extended with a quiet time, if necessary)
    takes more than N ticks.  This will guarantee monotonicity of the
    timestamps, which can then be used to reject old duplicates even
    without an enforced MSL.
 B.2  Closing and Reopening a Connection
    When a TCP connection is closed, a delay of 2*MSL in TIME-WAIT
    state ties up the socket pair for 4 minutes (see Section 3.5 of
    [Postel81].  Applications built upon TCP that close one connection
    and open a new one (e.g., an FTP data transfer connection using
    Stream mode) must choose a new socket pair each time.  The TIME-
    WAIT delay serves two different purposes:
    (a)  Implement the full-duplex reliable close handshake of TCP.
         The proper time to delay the final close step is not really
         related to the MSL; it depends instead upon the RTO for the
         FIN segments and therefore upon the RTT of the path.  (It
         could be argued that the side that is sending a FIN knows
         what degree of reliability it needs, and therefore it should
         be able to determine the length of the TIME-WAIT delay for
         the FIN's recipient.  This could be accomplished with an
         appropriate TCP option in FIN segments.)
         Although there is no formal upper-bound on RTT, common
         network engineering practice makes an RTT greater than 1
         minute very unlikely.  Thus, the 4 minute delay in TIME-WAIT
         state works satisfactorily to provide a reliable full-duplex
         TCP close.  Note again that this is independent of MSL
         enforcement and network speed.
         The TIME-WAIT state could cause an indirect performance
         problem if an application needed to repeatedly close one
         connection and open another at a very high frequency, since
         the number of available TCP ports on a host is less than
         2**16.  However, high network speeds are not the major
         contributor to this problem; the RTT is the limiting factor
         in how quickly connections can be opened and closed.
         Therefore, this problem will be no worse at high transfer
         speeds.
    (b)  Allow old duplicate segments to expire.
         To replace this function of TIME-WAIT state, a mechanism
         would have to operate across connections.  PAWS is defined
         strictly within a single connection; the last timestamp is
         TS.Recent is kept in the connection control block, and

Jacobson, Braden, & Borman [Page 28] RFC 1323 TCP Extensions for High Performance May 1992

         discarded when a connection is closed.
         An additional mechanism could be added to the TCP, a per-host
         cache of the last timestamp received from any connection.
         This value could then be used in the PAWS mechanism to reject
         old duplicate segments from earlier incarnations of the
         connection, if the timestamp clock can be guaranteed to have
         ticked at least once since the old connection was open.  This
         would require that the TIME-WAIT delay plus the RTT together
         must be at least one tick of the sender's timestamp clock.
         Such an extension is not part of the proposal of this RFC.
         Note that this is a variant on the mechanism proposed by
         Garlick, Rom, and Postel [Garlick77], which required each
         host to maintain connection records containing the highest
         sequence numbers on every connection.  Using timestamps
         instead, it is only necessary to keep one quantity per remote
         host, regardless of the number of simultaneous connections to
         that host.

Jacobson, Braden, & Borman [Page 29] RFC 1323 TCP Extensions for High Performance May 1992

APPENDIX C: CHANGES FROM RFC-1072, RFC-1185

 The protocol extensions defined in this document differ in several
 important ways from those defined in RFC-1072 and RFC-1185.
 (a)  SACK has been deferred to a later memo.
 (b)  The detailed rules for sending timestamp replies (see Section
      3.4) differ in important ways.  The earlier rules could result
      in an under-estimate of the RTT in certain cases (packets
      dropped or out of order).
 (c)  The same value TS.Recent is now shared by the two distinct
      mechanisms RTTM and PAWS.  This simplification became possible
      because of change (b).
 (d)  An ambiguity in RFC-1185 was resolved in favor of putting
      timestamps on ACK as well as data segments.  This supports the
      symmetry of the underlying TCP protocol.
 (e)  The echo and echo reply options of RFC-1072 were combined into a
      single Timestamps option, to reflect the symmetry and to
      simplify processing.
 (f)  The problem of outdated timestamps on long-idle connections,
      discussed in Section 4.2.2, was realized and resolved.
 (g)  RFC-1185 recommended that header prediction take precedence over
      the timestamp check.  Based upon some scepticism about the
      probabilistic arguments given in Section 4.2.4, it was decided
      to recommend that the timestamp check be performed first.
 (h)  The spec was modified so that the extended options will be sent
      on <SYN,ACK> segments only when they are received in the
      corresponding <SYN> segments.  This provides the most
      conservative possible conditions for interoperation with
      implementations without the extensions.
 In addition to these substantive changes, the present RFC attempts to
 specify the algorithms unambiguously by presenting modifications to
 the Event Processing rules of RFC-793; see Appendix E.

Jacobson, Braden, & Borman [Page 30] RFC 1323 TCP Extensions for High Performance May 1992

APPENDIX D: SUMMARY OF NOTATION

 The following notation has been used in this document.
 Options
     WSopt:       TCP Window Scale Option
     TSopt:       TCP Timestamps Option
 Option Fields
     shift.cnt:   Window scale byte in WSopt.
     TSval:       32-bit Timestamp Value field in TSopt.
     TSecr:       32-bit Timestamp Reply field in TSopt.
 Option Fields in Current Segment
     SEG.TSval:   TSval field from TSopt in current segment.
     SEG.TSecr:   TSecr field from TSopt in current segment.
     SEG.WSopt:   8-bit value in WSopt
 Clock Values
     my.TSclock:      Local source of 32-bit timestamp values
     my.TSclock.rate: Period of my.TSclock (1 ms to 1 sec).
 Per-Connection State Variables
     TS.Recent:       Latest received Timestamp
     Last.ACK.sent:   Last ACK field sent
     Snd.TS.OK:       1-bit flag
     Snd.WS.OK:       1-bit flag
     Rcv.Wind.Scale:  Receive window scale power
     Snd.Wind.Scale:  Send window scale power

Jacobson, Braden, & Borman [Page 31] RFC 1323 TCP Extensions for High Performance May 1992

APPENDIX E: EVENT PROCESSING

Event Processing

OPEN Call
   ...
  An initial send sequence number (ISS) is selected.  Send a SYN
  segment of the form:
      <SEQ=ISS><CTL=SYN><TSval=my.TSclock><WSopt=Rcv.Wind.Scale>
    ...
SEND Call
  CLOSED STATE (i.e., TCB does not exist)
    ...
  LISTEN STATE
    If the foreign socket is specified, then change the connection
    from passive to active, select an ISS.  Send a SYN segment
    containing the options: <TSval=my.TSclock> and
    <WSopt=Rcv.Wind.Scale>.  Set SND.UNA to ISS, SND.NXT to ISS+1.
    Enter SYN-SENT state. ...
  SYN-SENT STATE
  SYN-RECEIVED STATE
    ...
  ESTABLISHED STATE
  CLOSE-WAIT STATE
    Segmentize the buffer and send it with a piggybacked
    acknowledgment (acknowledgment value = RCV.NXT).  ...
    If the urgent flag is set ...
    If the Snd.TS.OK flag is set, then include the TCP Timestamps
    option <TSval=my.TSclock,TSecr=TS.Recent> in each data segment.
    Scale the receive window for transmission in the segment header:
          SEG.WND = (SND.WND >> Rcv.Wind.Scale).

Jacobson, Braden, & Borman [Page 32] RFC 1323 TCP Extensions for High Performance May 1992

SEGMENT ARRIVES
   ...
  If the state is LISTEN then
    first check for an RST
      ...
    second check for an ACK
      ...
    third check for a SYN
      if the SYN bit is set, check the security.  If the ...
       ...
      If the SEG.PRC is less than the TCB.PRC then continue.
      Check for a Window Scale option (WSopt); if one is found, save
      SEG.WSopt in Snd.Wind.Scale and set Snd.WS.OK flag on.
      Otherwise, set both Snd.Wind.Scale and Rcv.Wind.Scale to zero
      and clear Snd.WS.OK flag.
      Check for a TSopt option; if one is found, save SEG.TSval in the
      variable TS.Recent and turn on the Snd.TS.OK bit.
      Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other
      control or text should be queued for processing later.  ISS
      should be selected and a SYN segment sent of the form:
        <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
      If the Snd.WS.OK bit is on, include a WSopt option
      <WSopt=Rcv.Wind.Scale> in this segment.  If the Snd.TS.OK bit is
      on, include a TSopt <TSval=my.TSclock,TSecr=TS.Recent> in this
      segment.  Last.ACK.sent is set to RCV.NXT.
      SND.NXT is set to ISS+1 and SND.UNA to ISS.  The connection
      state should be changed to SYN-RECEIVED.  Note that any other
      incoming control or data (combined with SYN) will be processed
      in the SYN-RECEIVED state, but processing of SYN and ACK should
      not be repeated.  If the listen was not fully specified (i.e.,
      the foreign socket was not fully specified), then the
      unspecified fields should be filled in now.

Jacobson, Braden, & Borman [Page 33] RFC 1323 TCP Extensions for High Performance May 1992

    fourth other text or control
     ...
  If the state is SYN-SENT then
    first check the ACK bit
      ...
    fourth check the SYN bit
       ...
      If the SYN bit is on and the security/compartment and precedence
      are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to
      SEG.SEQ, and any acknowledgements on the retransmission queue
      which are thereby acknowledged should be removed.
      Check for a Window Scale option (WSopt); if is found, save
      SEG.WSopt in Snd.Wind.Scale; otherwise, set both Snd.Wind.Scale
      and Rcv.Wind.Scale to zero.
      Check for a TSopt option; if one is found, save SEG.TSval in
      variable TS.Recent and turn on the Snd.TS.OK bit in the
      connection control block.  If the ACK bit is set, use my.TSclock
      - SEG.TSecr as the initial RTT estimate.
      If SND.UNA > ISS (our SYN has been ACKed), change the connection
      state to ESTABLISHED, form an ACK segment:
          <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
      and send it.  If the Snd.Echo.OK bit is on, include a TSopt
      option <TSval=my.TSclock,TSecr=TS.Recent> in this ACK segment.
      Last.ACK.sent is set to RCV.NXT.
      Data or controls which were queued for transmission may be
      included.  If there are other controls or text in the segment
      then continue processing at the sixth step below where the URG
      bit is checked, otherwise return.
      Otherwise enter SYN-RECEIVED, form a SYN,ACK segment:
          <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
      and send it.  If the Snd.Echo.OK bit is on, include a TSopt
      option <TSval=my.TSclock,TSecr=TS.Recent> in this segment.  If

Jacobson, Braden, & Borman [Page 34] RFC 1323 TCP Extensions for High Performance May 1992

      the Snd.WS.OK bit is on, include a WSopt option
      <WSopt=Rcv.Wind.Scale> in this segment.  Last.ACK.sent is set to
      RCV.NXT.
      If there are other controls or text in the segment, queue them
      for processing after the ESTABLISHED state has been reached,
      return.
    fifth, if neither of the SYN or RST bits is set then drop the
    segment and return.
  Otherwise,
  First, check sequence number
    SYN-RECEIVED STATE
    ESTABLISHED STATE
    FIN-WAIT-1 STATE
    FIN-WAIT-2 STATE
    CLOSE-WAIT STATE
    CLOSING STATE
    LAST-ACK STATE
    TIME-WAIT STATE
      Segments are processed in sequence.  Initial tests on arrival
      are used to discard old duplicates, but further processing is
      done in SEG.SEQ order.  If a segment's contents straddle the
      boundary between old and new, only the new parts should be
      processed.
      Rescale the received window field:
          TrueWindow = SEG.WND << Snd.Wind.Scale,
      and use "TrueWindow" in place of SEG.WND in the following steps.
      Check whether the segment contains a Timestamps option and bit
      Snd.TS.OK is on.  If so:
        If SEG.TSval < TS.Recent, then test whether connection has
        been idle less than 24 days; if both are true, then the
        segment is not acceptable; follow steps below for an
        unacceptable segment.
        If SEG.SEQ is equal to Last.ACK.sent, then save SEG.ECopt in
        variable TS.Recent.

Jacobson, Braden, & Borman [Page 35] RFC 1323 TCP Extensions for High Performance May 1992

      There are four cases for the acceptability test for an incoming
      segment:
        ...
      If an incoming segment is not acceptable, an acknowledgment
      should be sent in reply (unless the RST bit is set, if so drop
      the segment and return):
        <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
      Last.ACK.sent is set to SEG.ACK of the acknowledgment.  If the
      Snd.Echo.OK bit is on, include the Timestamps option
      <TSval=my.TSclock,TSecr=TS.Recent> in this ACK segment.  Set
      Last.ACK.sent to SEG.ACK and send the ACK segment.  After
      sending the acknowledgment, drop the unacceptable segment and
      return.
        ...
  fifth check the ACK field.
    if the ACK bit is off drop the segment and return.
    if the ACK bit is on
      ...
      ESTABLISHED STATE
        If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK.
        Also compute a new estimate of round-trip time.  If Snd.TS.OK
        bit is on, use my.TSclock - SEG.TSecr; otherwise use the
        elapsed time since the first segment in the retransmission
        queue was sent.  Any segments on the retransmission queue
        which are thereby entirely acknowledged...
          ...
  Seventh, process the segment text.
    ESTABLISHED STATE
    FIN-WAIT-1 STATE
    FIN-WAIT-2 STATE
        ...
      Send an acknowledgment of the form:

Jacobson, Braden, & Borman [Page 36] RFC 1323 TCP Extensions for High Performance May 1992

        <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
      If the Snd.TS.OK bit is on, include Timestamps option
      <TSval=my.TSclock,TSecr=TS.Recent> in this ACK segment.  Set
      Last.ACK.sent to SEG.ACK of the acknowledgment, and send it.
      This acknowledgment should be piggy-backed on a segment being
      transmitted if possible without incurring undue delay.
       ...

Security Considerations

 Security issues are not discussed in this memo.

Authors' Addresses

 Van Jacobson
 University of California
 Lawrence Berkeley Laboratory
 Mail Stop 46A
 Berkeley, CA 94720
 Phone: (415) 486-6411
 EMail: van@CSAM.LBL.GOV
 Bob Braden
 University of Southern California
 Information Sciences Institute
 4676 Admiralty Way
 Marina del Rey, CA 90292
 Phone: (310) 822-1511
 EMail: Braden@ISI.EDU
 Dave Borman
 Cray Research
 655-E Lone Oak Drive
 Eagan, MN 55121
 Phone: (612) 683-5571
 Email: dab@cray.com

Jacobson, Braden, & Borman [Page 37]

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