TCP Maintenance (TCPM)                                         D. Borman
Internet-Draft                                       Quantum Corporation
Intended status: Standards Track                               B. Braden
Expires: October 14, 2013                         University of Southern
                                                             V. Jacobson
                                                           Packet Design
                                                   R. Scheffenegger, Ed.
                                                            NetApp, Inc.
                                                          April 12, 2013

                  TCP Extensions for High Performance


   This document specifies a set of TCP extensions to improve
   performance over paths with a large bandwidth * delay product and to
   provide reliable operation over very high-speed paths.  It defines
   TCP options for scaled windows and timestamps.  The timestamps are
   used for two distinct mechanisms, RTTM (Round Trip Time Measurement)
   and PAWS (Protection Against Wrapped Sequences).

   This document updates and obsoletes RFC 1323.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 14, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  TCP Performance  . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  TCP Reliability  . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Using TCP options  . . . . . . . . . . . . . . . . . . . .  6
     1.4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  7
   2.  TCP Window Scale Option  . . . . . . . . . . . . . . . . . . .  8
     2.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . .  8
     2.2.  Window Scale Option  . . . . . . . . . . . . . . . . . . .  8
     2.3.  Using the Window Scale Option  . . . . . . . . . . . . . .  9
     2.4.  Addressing Window Retraction . . . . . . . . . . . . . . . 10
   3.  RTTM -- Round-Trip Time Measurement  . . . . . . . . . . . . . 12
     3.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 12
     3.2.  TCP Timestamp Option . . . . . . . . . . . . . . . . . . . 13
     3.3.  The RTTM Mechanism . . . . . . . . . . . . . . . . . . . . 14
     3.4.  Which Timestamp to Echo  . . . . . . . . . . . . . . . . . 16
   4.  PAWS -- Protection Against Wrapped Sequence Numbers  . . . . . 18
     4.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 18
     4.2.  The PAWS Mechanism . . . . . . . . . . . . . . . . . . . . 18
     4.3.  Basic PAWS Algorithm . . . . . . . . . . . . . . . . . . . 20
     4.4.  Timestamp Clock  . . . . . . . . . . . . . . . . . . . . . 22
     4.5.  Outdated Timestamps  . . . . . . . . . . . . . . . . . . . 23
     4.6.  Header Prediction  . . . . . . . . . . . . . . . . . . . . 24
     4.7.  IP Fragmentation . . . . . . . . . . . . . . . . . . . . . 25
     4.8.  Duplicates from Earlier Incarnations of Connection . . . . 25
   5.  Conclusions and Acknowledgements . . . . . . . . . . . . . . . 26
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 27
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 28
   Appendix A.  Implementation Suggestions  . . . . . . . . . . . . . 30
   Appendix B.  Duplicates from Earlier Connection Incarnations . . . 31
     B.1.  System Crash with Loss of State  . . . . . . . . . . . . . 31
     B.2.  Closing and Reopening a Connection . . . . . . . . . . . . 32
   Appendix C.  Summary of Notation . . . . . . . . . . . . . . . . . 33
   Appendix D.  Event Processing Summary  . . . . . . . . . . . . . . 34
   Appendix E.  Timestamps Edge Cases . . . . . . . . . . . . . . . . 40
   Appendix F.  Window Retraction Example . . . . . . . . . . . . . . 40
   Appendix G.  Changes from RFC 1323 . . . . . . . . . . . . . . . . 41
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43

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1.  Introduction

   The TCP protocol [RFC0793] was designed to operate reliably over
   almost any transmission medium regardless of transmission rate,
   delay, corruption, duplication, or reordering of segments.  Over the
   years, advances in networking technology has resulted in ever-higher
   transmission speeds, and the fastest paths are well beyond the domain
   for which TCP was originally engineered.

   This document defines a set of modest extensions to TCP to extend the
   domain of its application to match the increasing network capability.
   It is an update to and obsoletes [RFC1323], which in turn is based
   upon and obsoletes [RFC1072] and [RFC1185].

   Changes between [RFC1323] and this document are detailed in
   Appendix G.

   For brevity, the full discussions of the merits and history behind
   the TCP options defined within this document have been omitted.
   [RFC1323] should be consulted for reference.  It is recommended that
   a modern TCP stack implements and make use of the extensions
   described in this document.

1.1.  TCP Performance

   TCP performance problems arise when the bandwidth * delay product is
   large.  A network having such paths is referred to as "long, fat
   network" (LFN).

   There are three fundamental performance problems with basic TCP over
   LFN paths:

   (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 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

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        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.

        Selective acknowledgements are specified in a separate document,
        "A Conservative Selective Acknowledgment (SACK)-based Loss
        Recovery Algorithm for TCP" [RFC6675], and not further discussed
        in this document.

   (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 3.2 defines a TCP option, "Timestamp", and then
        specifies 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 Timestamp Option.

1.2.  TCP Reliability

   An especially serious kind of error may result from an accidental
   reuse of TCP sequence numbers in data segments.  TCP reliability
   depends upon the existence of a bound on the lifetime of a segment:
   the "Maximum Segment Lifetime" or MSL.

   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.,

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        using the same pair of port numbers) is immediately reopened.  A
        delayed segment from the terminated connection could fall within
        the current window for the new incarnation and be accepted as

   Duplicates from earlier incarnations, case (2), are avoided by
   enforcing the current fixed MSL of the TCP specification, as
   explained in Section 4.8 and Appendix B.  However, case (1), avoiding
   the reuse of sequence numbers within the same connection, requires an
   upper bound on MSL that depends upon the transfer rate, and at high
   enough rates, a dedicated mechanism is required.

   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 4 presents a different mechanism, which we call PAWS
   (Protection Against Wrapped Sequence numbers), to extend TCP
   reliability to transfer rates well beyond the foreseeable upper limit
   of network bandwidths.  PAWS uses the TCP timestamp option defined in
   Section 3.2 to protect against old duplicates from the same

1.3.  Using TCP options

   The extensions defined in this document all use TCP options.

   When [RFC1323] was published, there was concern that some buggy TCP
   implementation might be crashed by the first appearance of an option
   on a non-<SYN> segment.  However, bugs like that can lead to DOS
   attacks against a TCP, so it is now expected that most TCP
   implementations will properly handle unknown options on non-<SYN>
   segments.  But it is still prudent to be conservative in what you
   send, and avoiding buggy TCP implementation is not the only reason
   for negotiating TCP options on <SYN> segments.

   The window scale option negotiates fundamental parameters of the TCP
   session.  Therefore, it is only sent during the initial handshake.
   Furthermore, the window scale option will be sent in a <SYN,ACK>
   segment only if the corresponding option was received in the initial
   <SYN> segment.

   The timestamp option may appear in any data or <ACK> segment, adding
   12 bytes to the 20-byte TCP header.  We recognize there is a trade-
   off between the bandwidth saved by reducing unnecessary
   retransmission timeouts, and the extra header bandwidth used by this

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   option.  It is required that this TCP option will be sent on non-
   <SYN> segments only after an exchange of options on the <SYN>
   segments has indicated that both sides understand this extension.

   Appendix A contains a recommended layout of the options in TCP
   headers to achieve reasonable data field alignment.

   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

1.4.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   In this document, these words will appear with that interpretation
   only when in UPPER CASE.  Lower case uses of these words are not to
   be interpreted as carrying [RFC2119] significance.

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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 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.

   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 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

   TCP Window Scale Option (WSopt):

   Kind: 3

   Length: 3 bytes

          | Kind=3  |Length=3 |shift.cnt|
               1         1         1

   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).

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   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 option 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 segment where the SYN bit is set (i.e., a <SYN>
   or <SYN,ACK>) is never scaled.

2.3.  Using the Window Scale Option

   A model implementation of window scaling is as follows, using the
   notation of [RFC0793]:

   o  All windows are treated as 32-bit quantities for storage in 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.

   o  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.

   o  If a TCP receives a <SYN> segment containing a Window Scale
      option, it sends its own Window Scale option in the <SYN,ACK>

   o  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.

   o  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.

   o  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 [RFC0793] are met, and using the
      "C" notation "<<" for left-shift).

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   o  The window field (SEG.WND) of every outgoing segment, with the
      exception of <SYN> segments, is right-shifted by Rcv.Wind.Scale

                    SND.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 two
   times the max window size 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

   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.

2.4.  Addressing Window Retraction

   When a non-zero scale factor is in use, there are instances when a
   retracted window can be offered - see Appendix F for a detailed
   example.  The end of the window will be on a boundary based on the
   granularity of the scale factor being used.  If the sequence number
   is then updated by a number of bytes smaller than that granularity,
   the TCP will have to either advertise a new window that is beyond
   what it previously advertised (and perhaps beyond the buffer), or
   will have to advertise a smaller window, which will cause the TCP
   window to shrink.  Implementations MUST ensure that they handle a
   shrinking window, as specified in section of [RFC1122].

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   For the receiver, this implies that:

   1)  The receiver MUST honor, as in-window, any segment that would
       have been in-window for any <ACK> sent by the receiver.

   2)  When window scaling is in effect, the receiver SHOULD track the
       actual maximum window sequence number (which is likely to be
       greater than the window announced by the most recent <ACK>, if
       more than one segment has arrived since the application consumed
       any data in the receive buffer).

   On the sender side:

   3)  The initial transmission MUST be within the window announced by
       the most recent <ACK>.

   4)  On first retransmission, or if the sequence number is out-of-
       window by less than (2^Rcv.Wind.Scale) then do normal
       retransmission(s) without regard to receiver window as long as
       the original segment was in window when it was sent.

   5)  Subsequent retransmissions MAY only be sent, if they are within
       the window announced by the most recent <ACK>.

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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" [RFC0896] 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
   one segment per window or less.  While this yields an adequate
   approximation to the RTT for small windows, it results in an
   unacceptably poor RTT estimate for a 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
   segments, 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 segments, the RTT estimator may be seriously in error,
   resulting in spurious retransmissions.

   If there are dropped segments, the problem becomes worse.  Zhang
   [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)

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   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 Timestamp 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
   Timestamp Option.

   TCP Timestamp Option (TSopt):

   Kind: 8

   Length: 10 bytes

          |Kind=8 |  10   |   TS Value (TSval)  |TS Echo Reply (TSecr)|
              1       1              4                     4

   The Timestamp 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 valid if the ACK bit is set
   in the TCP header; if it is valid, it echoes a timestamp value that
   was sent by the remote TCP in the TSval field of a Timestamp option.
   When TSecr is not valid, its value MUST be zero.  However, a value of
   zero does not imply TSecr being invalid.  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 Timestamp 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 received a TSopt in the
   initial <SYN> or <SYN,ACK> segment for the connection.

   Once TSopt has been successfully negotiated (sent and received)
   during the <SYN>, <SYN,ACK> exchange, TSopt MUST be sent in every
   non-<RST> segment for the duration of the connection.  If a non-<RST>
   segment is received without a TSopt, a TCP MAY drop the segment and
   send an <ACK> for the last in-sequence segment.  A TCP MUST NOT abort
   a TCP connection if a non-<RST> segment is received without a TSopt.

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   If a TSopt is received on a connection where TSopt was not negotiated
   in the initial three-way handshake, the TSopt MUST be ignored and the
   packet processed normally.

   In the case of crossing <SYN> segments where one <SYN> contains a
   TSopt and the other doesn't, both sides MAY send a TSopt in the
   <SYN,ACK> segment.

   TSopt is required for the two mechanisms described in sections 3.3
   and 4.2.  There are also other mechanisms that rely on the presence
   of the TSopt, e.g.  [RFC3522].  If a TCP stopped sending TSopt at any
   time during an established session, it interferes with these
   mechanisms.  This update to [RFC1323] describes explicitly the
   previous assumption (see Section 4.2), that each TCP segment must
   have TSopt, once negotiated.

3.3.  The RTTM Mechanism

   RTTM places a Timestamp Option in every segment, with a TSval that is
   obtained from a (virtual) "timestamp clock".  Values of this clock
   MUST be at least approximately proportional to real time, in order to
   measure actual RTT.

   These TSval values are echoed in TSecr values in the reverse
   direction.  The difference between a received TSecr value and the
   current timestamp clock value provides a RTT measurement.

   When timestamps are used, every segment that is received will contain
   a TSecr value.  However, these values cannot all be used to update
   the measured RTT.  The following example illustrates why.  It shows 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
   Timestamp Option are shown symbolically as <TSval=x,TSecr=y>.  Each
   TSecr field contains the value most recently received in a TSval

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              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>


   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:

      RTTM Rule: A TSecr value received in a segment MAY be used to
      update the averaged RTT measurement only if the segment advances
      the left edge of the send window (e.g.  SND.UNA is increased).

   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.

   Implementers should note that with timestamps multiple RTTMs can be
   taken per RTT.  Many RTO estimators have a weighting factor based on
   an implicit assumption that at most one RTTM will be sampled per RTT.
   When using multiple RTTMs per RTT to update the RTO estimator, the
   weighting factor needs to be decreased to take into account the more
   frequent RTTMs.  For example, an implementation could choose to just
   use one sample per RTT to update the RTO estimator, or vary the gain
   based on the congestion window, or take an average of all the RTT
   measurements received over one RTT, and then use that value to update
   the RTO estimator.  This document does not prescribe any particular
   method for modifying the RTO estimator.

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3.4.  Which Timestamp to Echo

   If more than one Timestamp 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.

   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 <ACK>s 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

   (C)  A filled hole in the sequence space.

        The segment that fills the hole represents the most recent
        measurement of the network characteristics.  A 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 Timestamp Option processing on a synchronized

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   (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
        <ACK>s have been delayed.

   (2)  If:

            SEG.TSval >= TS.recent and SEG.SEQ <= Last.ACK.sent

        then SEG.TSval is copied to TS.Recent; otherwise, it 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  Segments arrive in sequence, and some of the <ACK>s are delayed.

      By case (A), the timestamp from the oldest unacknowledged segment
      is echoed.

                  <A, TSval=1> ------------------->
                  <B, TSval=2> ------------------->
                  <C, TSval=3> ------------------->
                           <---- <ACK(C), TSecr=1>

   o  Segments arrive out of order, and every segment 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.

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

4.  PAWS -- Protection 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 (Protection Against Wrapped Sequence numbers).  PAWS
   operates within a single TCP connection, using state that is saved in
   the connection control block.  Section 4.8 and Appendix G 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 Timestamp 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 monotonically 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-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

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   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 monotonically 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 <ACK>s,
   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
   non-symmetric algorithm would protect against old duplicate <ACK>s:
   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.

   [RFC1323] recommended that <RST> segments NOT carry timestamps, and
   that they be acceptable regardless of their timestamp.  At that time,
   the thinking was that old duplicate <RST> segments should be
   exceedingly unlikely, and their cleanup function should take
   precedence over timestamps.  More recently, discussions about various
   blind attacks on TCP connections have raised the suggestion that if
   the timestamp option is present, SEG.TSecr could be used to provide
   stricter acceptance tests for <RST> segments.  While still under
   discussion, to enable research into this area it is now RECOMMENDED
   that when generating a <RST>, that if the segment causing the <RST>
   to be generated contained a timestamp option, that the <RST> also
   contain a timestamp option.  In the <RST> segment, SEG.TSecr SHOULD

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   be set to SEG.TSval from the incoming segment and SEG.TSval SHOULD be
   set to zero.  If a <RST> is being generated because of a user abort,
   and Snd.TS.OK is set, then a timestamp option SHOULD be included in
   the <RST>.  When a <RST> segment is received, it MUST NOT be
   subjected to PAWS checks, and information from the timestamp option
   MUST NOT be used to update connection state information.  SEG.TSecr
   MAY be used to provide stricter <RST> acceptance checks.

4.3.  Basic PAWS Algorithm

   The PAWS algorithm REQUIRES the following processing to be performed
   on all incoming segments for a synchronized connection.  Also, PAWS
   processing MUST take precedence over the regular TCP acceptablitiy
   check (Section 3.3 in [RFC0793]), which is performed after
   verification of the received timestamp option:

   R1)  If there is a Timestamp Option in the arriving segment,
        SEG.TSval < TS.Recent, TS.Recent is valid (see later discussion)
        and the RST bit is not set, then treat the arriving segment as
        not acceptable:

           Send an acknowledgement in reply as specified in [RFC0793]
           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

   R2)  If the segment is outside the window, reject it (normal TCP

   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

   Steps R2, R4, and R5 are the normal TCP processing steps specified by

   It is important to note that the timestamp MUST be 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.

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   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.Recent 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.Recent is set to

   This rule allows reasonable performance under loss.  A full window of
   data is in transit at all times, and after a loss a full window less
   one segment 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-R5 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 <ACK>s, then the queued
   segments that caused these <ACK>s must have been received already.

   Even if a segment were delayed past the RTO, the Fast Retransmit
   mechanism [Jacobson90c] will cause the delayed segments 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.

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4.4.  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 RTTs.  However, the
   receiver treats the timestamp as simply a monotonically 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.

   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 window extension defined in Section 2.2, 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 1 ms timestamp clock will work at link speeds up to 8 Tbps
        (8*10^12) bps!

   (b)  The timestamp clock must not be "too fast".

        The recycling time of the timestamp clock 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 4.5), 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 in the internet without requiring precise MSL

   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

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   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.

   o  Some hosts have a hardware clock that is guaranteed to be
      monotonic between hardware resets.

   o  A clock interrupt may be used to simply increment a binary integer
      by 1 periodically.

   o  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).

4.5.  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 keep-alive 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 keep-alives, 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

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   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 update a
   clock or timestamp value associated with the connection whenever
   TS.Recent is updated, for example.  The details will be

4.6.  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

   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 window and
        possibly sending acknowledgments, and queuing 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 unsigned modulo arithmetic.  To perform this
   check on every single segment is contrary to the philosophy of header
   prediction.  We believe that this change might produce a measurable
   reduction in CPU time for TCP protocol processing on high-speed

   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 in [RFC1185] to show that the

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   probability of this failure is negligible.

      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 a 100 Mbit/s 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.

4.7.  IP Fragmentation

   At high data rates, the protection against old segments provided by
   PAWS can be circumvented by errors in IP fragment reassembly (see
   [RFC4963]).  The only way to protect against incorrect IP fragment
   reassembly is to not allow the segments to be fragmented.  This is
   done by setting the Don't Fragment (DF) bit in the IP header.
   Setting the DF bit implies the use of Path MTU Discovery as described
   in [RFC1191], [RFC1981], and [RFC4821], thus any TCP implementation
   that implements PAWS MUST also implement Path MTU Discovery.

4.8.  Duplicates from Earlier Incarnations of Connection

   The PAWS mechanism protects against errors due to sequence number
   wrap-around on high-speed connections.  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

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   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

5.  Conclusions and Acknowledgements

   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 stacks that do not
   implement the extensions.

   These mechanisms are implemented using TCP options for scaled windows
   and timestamps.  The timestamps are used for two distinct mechanisms:
   RTTM (Round Trip Time Measurement) and PAWS (Protection 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 [RFC1185].

   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.  Task force
   members and other on the end2end-interest list have made valuable
   contributions by pointing out flaws in the algorithms and the
   documentation.  Continued discussion and development since the
   publication of [RFC1323] originally occurred in the IETF TCP Large
   Windows Working Group, later on in the End-to-End Task Force, and
   most recently in the IETF TCP Maintenance Working Group.  The authors
   are grateful for all these contributions.

6.  Security Considerations

   The TCP sequence space is a fixed size, and as the window becomes
   larger it becomes easier for an attacker to generate forged packets

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   that can fall within the TCP window, and be accepted as valid
   segments.  While use of timestamps and PAWS can help to mitigate
   this, when using PAWS, if an attacker is able to forge a packet that
   is acceptable to the TCP connection, a timestamp that is in the
   future would cause valid segments to be dropped due to PAWS checks.
   Hence, implementers should take care to not open the TCP window
   drastically beyond the requirements of the connection.

   Middle boxes and options: If a middle box removes TCP options from
   the <SYN> segment, such as TSopt, a high speed connection that needs
   PAWS would not have that protection.  In this situation, an
   implementer could provide a mechanism for the application to
   determine whether or not PAWS is in use on the connection, and chose
   to terminate the connection if that protection doesn't exist.

   Mechanisms to protect the TCP header from modification should also
   protect the TCP options.

   A naive implementation that derives the timestamp clock value
   directly from a system uptime clock may unintentionally leak this
   information to an attacker.  This does not directly compromise any of
   the mechanisms described in this document.  However, this may be
   valuable information to a potential attacker.  An implementer should
   evaluate the potential impact and mitigate this accordingly (i.e. by
   using a random offset for the timestamp clock on each connection, or
   using an external, real-time derived timestamp clock source).

   Expanding the TCP window beyond 64K for IPv6 allows Jumbograms
   [RFC2675] to be used when the local network supports packets larger
   than 64K. When larger TCP segments are used, the TCP checksum becomes

7.  IANA Considerations

   This document has no actions for IANA.

8.  References

8.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2.  Informative References

              Garlick, L., Rom, R., and J. Postel, "Issues in Reliable
              Host-to-Host Protocols", Proc. Second Berkeley Workshop on
              Distributed Data Management and Computer Networks,
              May 1977, <>.

              Hamming, R., "Digital Filters", Prentice Hall, Englewood
              Cliffs, N.J. ISBN 0-13-212571-4, 1977.

              Jacobson, V., "Congestion Avoidance and Control", SIGCOMM
              '88, Stanford,  CA., August 1988,

              Jacobson, V., "4BSD Header Prediction", ACM Computer
              Communication Review, April 1990.

              Jacobson, V., "Modified TCP congestion avoidance
              algorithm", Message to the 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,
              August 1987.

              Martin, D., "[Tsvwg] RFC 1323.bis", Message to the tsvwg
              mailing list, September 2003, <

              Mathis, M., "[tcpm] Example of 1323 window retraction
              problem", Message to the tcpm mailing list, March 2008,

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   [RFC0896]  Nagle, J., "Congestion control in IP/TCP internetworks",
              RFC 896, January 1984.

   [RFC1072]  Jacobson, V. and R. Braden, "TCP extensions for long-delay
              paths", RFC 1072, October 1988.

   [RFC1110]  McKenzie, A., "Problem with the TCP big window option",
              RFC 1110, August 1989.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1185]  Jacobson, V., Braden, B., and L. Zhang, "TCP Extension for
              High-Speed Paths", RFC 1185, October 1990.

   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018, October 1996.

   [RFC2581]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.

   [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, August 1999.

   [RFC2883]  Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
              Extension to the Selective Acknowledgement (SACK) Option
              for TCP", RFC 2883, July 2000.

   [RFC3522]  Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
              for TCP", RFC 3522, April 2003.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963, July 2007.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.

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   [RFC6675]  Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
              and Y. Nishida, "A Conservative Loss Recovery Algorithm
              Based on Selective Acknowledgment (SACK) for TCP",
              RFC 6675, August 2012.

   [RFC6691]  Borman, D., "TCP Options and Maximum Segment Size (MSS)",
              RFC 6691, July 2012.

              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.

Appendix A.  Implementation Suggestions

   TCP Option Layout

      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          |

   Interaction with the TCP Urgent Pointer

      The TCP Urgent pointer, like the TCP window, is a 16 bit value.
      Some of the original discussion for the TCP Window Scale option
      included proposals to increase the Urgent pointer to 32 bits.  As
      it turns out, this is unnecessary.  There are two observations
      that should be made:

      (1)  With IP Version 4, the largest amount of TCP data that can be
           sent in a single packet is 65495 bytes (64K - 1 -- size of
           fixed IP and TCP headers).

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      (2)  Updates to the urgent pointer while the user is in "urgent
           mode" are invisible to the user.

      This means that if the Urgent Pointer points beyond the end of the
      TCP data in the current segment, then the user will remain in
      urgent mode until the next TCP segment arrives.  That segment will
      update the urgent pointer to a new offset, and the user will never
      have left urgent mode.

      Thus, to properly implement the Urgent Pointer, the sending TCP
      only has to check for overflow of the 16 bit Urgent Pointer field
      before filling it in.  If it does overflow, than a value of 65535
      should be inserted into the Urgent Pointer.

      The same technique applies to IP Version 6, except in the case of
      IPv6 Jumbograms.  When IPv6 Jumbograms are supported, [RFC2675]
      requires additional steps for dealing with the Urgent Pointer,
      these are described in section 5.2 of [RFC2675].

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
   [RFC0793].  The MSL that is required here does not depend upon the
   transfer speed.  The current TCP MSL of 2 minutes seemed acceptable
   as an operational compromise, when many host systems used to take
   this long to boot after a crash.  Current host systems can boot
   considerably faster.

   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 is 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/

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   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 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 [RFC0793].
   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.

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   (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 (TS.Recent) is
        kept in the connection control block, and 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.

Appendix C.  Summary of Notation

   The following notation has been used in this document.


      WSopt:            TCP Window Scale Option
      TSopt:            TCP Timestamp 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

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      SEG.TSecr:        TSecr field from TSopt in current segment
      SEG.WSopt:        8-bit value in WSopt

   Clock Values

      my.TSclock:       System wide source of 32-bit timestamp values
      my.TSclock.rate:  Period of my.TSclock (1 ms to 1 sec)
      Snd.TSoffset:     A offset for randomizing Snd.TSclock
      Snd.TSclock:      my.TSclock + Snd.TSoffset

   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
      Start.Time:       Snd.TSclock value when segment being timed was
                        sent (used by pre-1323 code).


      Update_SRTT(m)    Procedure to update the smoothed RTT and RTT
                        variance estimates, using the rules of
                        [Jacobson88a], given m, a new RTT measurement

Appendix D.  Event Processing Summary

   OPEN Call


      An initial send sequence number (ISS) is selected.  Send a <SYN>
      segment of the form:



   SEND Call

      CLOSED STATE (i.e., TCB does not exist)


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         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=Snd.TSclock> and
         <WSopt=Rcv.Wind.Scale>.  Set SND.UNA to ISS, SND.NXT to ISS+1.
         Enter SYN-SENT 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 Timestamp
         Option <TSval=Snd.TSclock,TSecr=TS.Recent> in each data

         Scale the receive window for transmission in the segment

                   SEG.WND = (RCV.WND >> Rcv.Wind.Scale).



      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 ...

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            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:


            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=Snd.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

         fourth other text or control


      If the state is SYN-SENT then

         first check the ACK bit



         fourth check the SYN bit

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            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 it 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
            Snd.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:


            and send it.  If the Snd.Echo.OK bit is on, include a TSopt
            option <TSval=Snd.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:


            and send it.  If the Snd.Echo.OK bit is on, include a TSopt
            option <TSval=Snd.TSclock,TSecr=TS.Recent> in this segment.
            If 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.

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      First, check sequence number

         FIN-WAIT-1 STATE
         FIN-WAIT-2 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

            Check whether the segment contains a Timestamp Option and
            bit Snd.TS.OK is on.  If so:

               If SEG.TSval < TS.Recent and the RST bit is off, then
               test whether connection has been idle less than 24 days;
               if all are true, then the segment is not acceptable;
               follow steps below for an unacceptable segment.

               If SEG.SEQ is less than or equal to Last.ACK.sent, then
               save SEG.TSval in variable TS.Recent.

            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):


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            Last.ACK.sent is set to SEG.ACK of the acknowledgment.  If
            the Snd.Echo.OK bit is on, include the Timestamp Option
            <TSval=Snd.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



               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 Snd.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


      Seventh, process the segment text.

         FIN-WAIT-1 STATE
         FIN-WAIT-2 STATE


            Send an acknowledgment of the form:


            If the Snd.TS.OK bit is on, include Timestamp Option
            <TSval=Snd.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.

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Appendix E.  Timestamps Edge Cases

   While the rules laid out for when to calculate RTTM produce the
   correct results most of the time, there are some edge cases where an
   incorrect RTTM can be calculated.  All of these situations involve
   the loss of segments.  It is felt that these scenarios are rare, and
   that if they should happen, they will cause a single RTTM measurement
   to be inflated, which mitigates its effects on RTO calculations.

   [Martin03] cites two similar cases when the returning <ACK> is lost,
   and before the retransmission timer fires, another returning <ACK>
   segment arrives, which aknowledges the data.  In this case, the RTTM
   calculated will be inflated:

             tc=1   <A, TSval=1> ------------------->

             tc=2   (lost) <---- <ACK(A), TSecr=1, win=n>
                 (RTTM would have been 1)

                    (receive window opens, window update is sent)
             tc=5        <---- <ACK(A), TSecr=1, win=m>
                    (RTTM is calculated at 4)

   One thing to note about this situation is that it is somewhat bounded
   by RTO + RTT, limiting how far off the RTTM calculation will be.
   While more complex scenarios can be constructed that produce larger
   inflations (e.g., retransmissions are lost), those scenarios involve
   multiple segment losses, and the connection will have other more
   serious operational problems than using an inflated RTTM in the RTO

Appendix F.  Window Retraction Example

   Consider a established TCP connection with WSCALE=7 (128 byte
   receiver window quantization), that is running with a very small
   windows because the receiver is bottlenecked and both ends are doing
   small reads and writes.

   Consider the ACKs coming back:

   SEG.ACK  SEG.WIN computed SND.WIN   receiver's actual window
   1000     2       1256               1300

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   The sender writes 40 bytes and receiver ACKs:

   1040     2       1296               1300

   The sender writes 5 additional bytes and the receiver has a problem.
   Two choices:

   1045     2       1301               1300   - BEYOND BUFFER

   1045     1       1173               1300   - RETRACTED WINDOW

   This problems is completely general and can in principle happen any
   time the sender does a write which is smaller than the window scale

   In most stacks it is at least partially obscured when the window size
   is larger than some small number of segments because the stacks
   prefer to announce windows that are integral numbers of segments
   (rounded up to the next window quanta).  This plus silly window
   suppression tends to cause less frequent, larger window updates.  If
   the window was rounded down to a segment size there is more
   opportunity to advance it ("beyond buffer" case above) rather than
   retracting it.

Appendix G.  Changes from RFC 1323

   Several important updates and clarifications to the specification in
   RFC 1323 are made in these document.  The technical changes are
   summarized below:

   (a)  Section 2.4 was added describing the unavoidable window
        retraction issue, and explicitly describing the mitigation steps

   (b)  In Section 3.2 the wording how timestamp option negotiation is
        to be performed was updated with RFC2119 wording.  Further, a
        number of paragraphs were added to clarify the expected behavior
        with a compliant implementation using TSopt, as RFC1323 left
        room for interpretation - e.g. potential late enablement of

   (c)  The description of which TSecr values can be used to update the
        measured RTT has been clarified.  Specifically, with timestamps,
        the Karn algorithm [Karn87] is disabled.  The Karn algorithm
        disables all RTT measurements during retransmission, since it is
        ambiguous whether the <ACK> is for the original segment, or the
        retransmitted segment.  With timestamps, that ambiguity is

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        removed since the TSecr in the <ACK> will contain the TSval from
        whichever data segment made it to the destination.

   (d)  RTTM update processing explicitly excludes segments not updating
        SND.UNA.  The original text could be interpreted to allow taking
        RTT samples when SACK acknowledges some new, non-continuous

   (e)  In RFC1323, section 3.4, step (2) of the algorithm to control
        which timestamp is echoed was incorrect in two regards:

        (1)  It failed to update TS.recent for a retransmitted segment
             that resulted from a lost <ACK>.

        (2)  It failed if SEG.LEN = 0.

        In the new algorithm, the case of SEG.TSval >= TS.recent is
        included for consistency with the PAWS test.

   (f)  It is now recommended that Timestamp Options be included in
        <RST> segments if the incoming segment contained a Timestamp

   (g)  <RST> segments are explicitly excluded from PAWS processing.

   (h)  Added text to clarify the precedence between regular TCP
        [RFC0793] and timestamp/PAWS [RFCxxxx] processing.  Discussion
        about combined acceptability checks are ongoing.

   (i)  Snd.TSoffset and Snd.TSclock variables have been added.
        Snd.TSclock is the sum of my.TSclock and Snd.TSoffset.  This
        allows the starting points for timestamp values to be randomized
        on a per-connection basis.  Setting Snd.TSoffset to zero yields
        the same results as [RFC1323].

   (j)  Appendix A has been expanded with information about the TCP
        Urgent Pointer.  An earlier revision contained text around the
        TCP MSS option, which was split off into [RFC6691].

   (k)  One correction was made to the Event Processing Summary in
        Appendix D.  In SEND CALL/ESTABLISHED STATE, RCV.WND is used to
        fill in the SEG.WND value, not SND.WND.

   Editorial changes of the document, that don't impact the
   implementation or function of the mechanisms described in this
   document include:

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   (a)  Removed much of the discussion in Section 1 to streamline the
        document.  However, detailed examples and discussions in
        Section 2, Section 3 and Section 4 are kept as guideline for

   (b)  Removed references to "new" options, as the options were
        introduced in [RFC1323] already.  Changed the text in
        Section 1.3 to specifically address TS and WS options.

   (c)  Section 1.4 was added for RFC2119 wording.  Normative text was
        updated with the appropriate phrases.

   (d)  Added < > brackets to mark specific types of segments, and
        replaced most occurances of "packet" with "segment", where TCP
        segments are referred.

   (e)  Removed the list of changes between RFC 1323 and prior versions.
        These changes are mentioned in appendix C of RFC 1323.

   (f)  Moved Appendix "Changes" at the end of the appendices for easier
        lookup.  In addition, the entries were split into a technical
        and an editorial part, and sorted to roughly correspond with the
        sections in the text where they apply.

Authors' Addresses

   David Borman
   Quantum Corporation
   Mendota Heights  MN 55120


   Bob Braden
   University of Southern California
   4676 Admiralty Way
   Marina del Rey  CA 90292


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   Van Jacobson
   Packet Design
   2465 Latham Street
   Mountain View  CA 94040


   Richard Scheffenegger (editor)
   NetApp, Inc.
   Am Euro Platz 2
   Vienna,   1120


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