Internet Engineering Task Force (IETF)                         D. Borman
Request for Comments: 7323                           Quantum Corporation
Obsoletes: 1323                                                B. Braden
Category: Standards Track              University of Southern California
ISSN: 2070-1721                                              V. Jacobson
                                                            Google, Inc.
                                                   R. Scheffenegger, Ed.
                                                            NetApp, Inc.
                                                          September 2014

                  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
   the TCP Window Scale (WS) option and the TCP Timestamps (TS) option
   and their semantics.  The Window Scale option is used to support
   larger receive windows, while the Timestamps option can be used for
   at least two distinct mechanisms, Protection Against Wrapped
   Sequences (PAWS) and Round-Trip Time Measurement (RTTM), that are
   also described herein.

   This document obsoletes RFC 1323 and describes changes from it.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) 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.  TCP Timestamps Option . . . . . . . . . . . . . . . . . . . .  11
     3.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Timestamps Option . . . . . . . . . . . . . . . . . . . .  12
   4.  The RTTM Mechanism  . . . . . . . . . . . . . . . . . . . . .  14
     4.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .  14
     4.2.  Updating the RTO Value  . . . . . . . . . . . . . . . . .  15
     4.3.  Which Timestamp to Echo . . . . . . . . . . . . . . . . .  16
   5.  PAWS - Protection Against Wrapped Sequences . . . . . . . . .  19
     5.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .  19
     5.2.  The PAWS Mechanism  . . . . . . . . . . . . . . . . . . .  19
     5.3.  Basic PAWS Algorithm  . . . . . . . . . . . . . . . . . .  20
     5.4.  Timestamp Clock . . . . . . . . . . . . . . . . . . . . .  22
     5.5.  Outdated Timestamps . . . . . . . . . . . . . . . . . . .  24
     5.6.  Header Prediction . . . . . . . . . . . . . . . . . . . .  25
     5.7.  IP Fragmentation  . . . . . . . . . . . . . . . . . . . .  26
     5.8.  Duplicates from Earlier Incarnations of Connection  . . .  26
   6.  Conclusions and Acknowledgments . . . . . . . . . . . . . . .  27
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
     7.1.  Privacy Considerations  . . . . . . . . . . . . . . . . .  29
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  30
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  30
   Appendix A.  Implementation Suggestions . . . . . . . . . . . . .  34
   Appendix B.  Duplicates from Earlier Connection Incarnations  . .  35
     B.1.  System Crash with Loss of State . . . . . . . . . . . . .  35
     B.2.  Closing and Reopening a Connection  . . . . . . . . . . .  35
   Appendix C.  Summary of Notation  . . . . . . . . . . . . . . . .  37
   Appendix D.  Event Processing Summary . . . . . . . . . . . . . .  38
   Appendix E.  Timestamps Edge Cases  . . . . . . . . . . . . . . .  44
   Appendix F.  Window Retraction Example  . . . . . . . . . . . . .  44
   Appendix G.  RTO Calculation Modification . . . . . . . . . . . .  45
   Appendix H.  Changes from RFC 1323  . . . . . . . . . . . . . . .  46

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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 have 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 H.  These changes are partly due to errata in [RFC1323], and
   partly due to the improved understanding of how the involved
   components interact.

   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 a "long, fat
   network" (LFN).

   There are two 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 = 64 KiB.  For LFN paths where the bandwidth *
        delay product exceeds 64 KiB, the receive window limits the
        maximum throughput of the TCP connection over the path, i.e.,
        the amount of unacknowledged data that TCP can send in order to
        keep the pipeline full.

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        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.

        It must be noted that the use of large receive windows increases
        the chance of too quickly wrapping sequence numbers, as
        described below in Section 1.2, (1).

   (2)  Recovery from Losses

        Packet losses in an LFN can have a catastrophic effect on

        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 Acknowledgments and their use are specified in
        separate documents, "TCP Selective Acknowledgment Options"
        [RFC2018], "An Extension to the Selective Acknowledgement (SACK)
        Option for TCP" [RFC2883], and "A Conservative Loss Recovery
        Algorithm Based on Selective Acknowledgment (SACK) for TCP"
        [RFC6675], and are not further discussed in this document.

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 of large volumes of data (at least 4 GiB in the
        same session), the 32-bit sequence space may be "wrapped"
        (cycled) within the time that a segment is delayed in queues.

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   (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 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 5.8 and Appendix B.  In addition, the
   randomizing of ephemeral ports can also help to probabilistically
   reduce the chances of duplicates from earlier connections.  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

   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, 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 3.2 to protect against old duplicates from
   the same connection.

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 crash on the first appearance of an option on a
   non-<SYN> segment.  However, bugs like that can lead to denial-of-
   service (DoS) attacks against a TCP.  Research has shown that most
   TCP implementations will properly handle unknown options on non-<SYN>
   segments ([Medina04], [Medina05]).  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.

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   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 Timestamps option may appear in any data or <ACK> segment, adding
   10 bytes (up to 12 bytes including padding) to the 20-byte TCP
   header.  It is required that this TCP option will be sent on all
   non-<SYN> segments after an exchange of options on the <SYN> segments
   has indicated that both sides understand this extension.

   Research has shown that the use of the Timestamps option to take
   additional RTT samples within each RTT has little effect on the
   ultimate retransmission timeout value [Allman99].  However, there are
   other uses of the Timestamps option, such as the Eifel mechanism
   ([RFC3522], [RFC4015]) and PAWS (see Section 5), which improve
   overall TCP security and performance.  The extra header bandwidth
   used by this option should be evaluated for the gains in performance
   and security in an actual deployment.

   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
   document are important for LFNs 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 30 bits and then uses an implicit scale factor to carry this
   30-bit value in the 16-bit window field of the TCP header (SEG.WND in
   [RFC0793]).  The exponent of 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
   both send and receive window scaling, and (2) communicate the
   exponent of 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 and the exponent 0.  The scale
   factor is limited to a power of two and encoded logarithmically, so
   it may be implemented by binary shift operations.  The maximum scale
   exponent is limited to 14 for a maximum permissible receive window
   size of 1 GiB (2^(14+16)).

   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'

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   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).  If a Window Scale option
   was received in the initial <SYN> segment, then this option MAY be
   sent in the <SYN,ACK> segment.  A Window Scale option in a segment
   without a SYN bit MUST be ignored.

   The window field in a segment where the SYN bit is set (i.e., a <SYN>
   or <SYN,ACK>) MUST NOT be scaled.

2.3.  Using the Window Scale Option

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

   o  The connection state is augmented by two window shift counters,
      Snd.Wind.Shift and Rcv.Wind.Shift, 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 SHOULD send its own Window Scale option in the
      <SYN,ACK> segment.

   o  The Window Scale option MUST be sent with shift.cnt = R, where R
      is the value that the TCP would like to use for its receive

   o  Upon receiving a <SYN> segment with a Window Scale option
      containing shift.cnt = S, a TCP MUST set Snd.Wind.Shift to S and
      MUST set Rcv.Wind.Shift to R; otherwise, it MUST set both
      Snd.Wind.Shift and Rcv.Wind.Shift to zero.

   o  The window field (SEG.WND) in the header of every incoming
      segment, with the exception of <SYN> segments, MUST be left-
      shifted by Snd.Wind.Shift bits before updating SND.WND:

                    SND.WND = SEG.WND << Snd.Wind.Shift

      (assuming the other conditions of [RFC0793] are met, and using the
      "C" notation "<<" for left-shift).

   o  The window field (SEG.WND) of every outgoing segment, with the
      exception of <SYN> segments, MUST be right-shifted by
      Rcv.Wind.Shift bits:

                    SEG.WND = RCV.WND >> Rcv.Wind.Shift

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   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.  The same is true of 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 maximum 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 GiB).  If a
   Window Scale option is received with a shift.cnt value larger than
   14, the TCP SHOULD log the error but MUST use 14 instead of the
   specified value.  This is safe as a sender can always choose to only
   partially use any signaled receive window.  If the receiver is
   scaling by a factor larger than 14 and the sender is only scaling by
   14, then the receive window used by the sender will appear smaller
   than it is in reality.

   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
   (see [RFC5681]) 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.Shift, then do normal
       retransmission(s) without regard to the 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>.

3.  TCP Timestamps Option

3.1.  Introduction

   The Timestamps option is introduced to address some of the issues
   mentioned in Sections 1.1 and 1.2.  The Timestamps option is
   specified in a symmetrical manner, so that Timestamp Value (TSval)
   timestamps are carried in both data and <ACK> segments and are echoed
   in Timestamp Echo Reply (TSecr) fields carried in returning <ACK> or
   data segments.  Originally used primarily for timestamping individual
   segments, the properties of the Timestamps option allow for taking
   time measurements (Section 4) as well as additional uses (Section 5).

   It is necessary to remember that there is a distinction between the
   Timestamps option conveying timestamp information and the use of that
   information.  In particular, the RTTM mechanism must be viewed
   independently from updating the Retransmission Timeout (RTO) (see
   Section 4.2).  In this case, the sample granularity also needs to be
   taken into account.  Other mechanisms, such as PAWS or Eifel, are not
   built upon the timestamp information itself but are based on the
   intrinsic property of monotonically non-decreasing values.

   The Timestamps option is important when large receive windows are
   used to allow the use of the PAWS mechanism (see Section 5).

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   Furthermore, the option may be useful for all TCPs, since it
   simplifies the sender and allows the use of additional optimizations
   such as Eifel ([RFC3522], [RFC4015]) and others ([RFC6817],
   [Kuzmanovic03], [Kuehlewind10]).

3.2.  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.

   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
   TSval field contains the current value of the timestamp clock of the
   TCP sending the option.

   The TSecr field is valid if the ACK bit is set in the TCP header.  If
   the ACK bit is not set in the outgoing TCP header, the sender of that
   segment SHOULD set the TSecr field to zero.  When the ACK bit is set
   in an outgoing segment, the sender MUST echo a recently received
   TSval sent by the remote TCP in the TSval field of a Timestamps
   option.  The exact rules on which TSval MUST be echoed are given in
   Section 4.3.  When the ACK bit is not set, the receiver MUST ignore
   the value of the TSecr field.

   A TCP MAY send the TSopt in an initial <SYN> segment (i.e., segment
   containing a SYN bit and no ACK bit), and MAY send a TSopt in
   <SYN,ACK> only if it received a TSopt in the initial <SYN> segment
   for the connection.

   Once TSopt has been successfully negotiated, that is both <SYN> and
   <SYN,ACK> contain TSopt, the TSopt MUST be sent in every non-<RST>
   segment for the duration of the connection, and SHOULD be sent in an
   <RST> segment (see Section 5.2 for details).  The TCP SHOULD remember
   this state by setting a flag, referred to as Snd.TS.OK, to one.  If a

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   non-<RST> segment is received without a TSopt, a TCP SHOULD silently
   drop the segment.  A TCP MUST NOT abort a TCP connection because any
   segment lacks an expected TSopt.

   Implementations are strongly encouraged to follow the above rules for
   handling a missing Timestamps option and the order of precedence
   mentioned in Section 5.3 when deciding on the acceptance of a

   If a receiver chooses to accept a segment without an expected
   Timestamps option, it must be clear that undetectable data corruption
   may occur.

   Such a TCP receiver may experience undetectable wrapped-sequence
   effects, such as data (payload) corruption or session stalls.  In
   order to maintain the integrity of the payload data, in particular on
   high-speed networks, it is paramount to follow the described
   processing rules.

   However, it has been mentioned that under some circumstances, the
   above guidelines are too strict, and some paths sporadically suppress
   the Timestamps option, while maintaining payload integrity.  A path
   behaving in this manner should be deemed unacceptable, but it has
   been noted that some implementations relax the acceptance rules as a
   workaround and allow TCP to run across such paths [RE-1323BIS].

   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 4 and
   5.  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 5.2) that each TCP segment must have a TSopt, once

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4.  The RTTM Mechanism

4.1.  Introduction

   One use of the Timestamps option is to measure the round-trip time
   (RTT) of virtually every packet acknowledged.  The RTTM mechanism
   requires a Timestamps option in every measured 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.

   TCP measures the RTT, primarily for the purpose of arriving at a
   reasonable value for the RTO timer interval.  Accurate and current
   RTT estimates are necessary to adapt to changing traffic conditions,
   while a conservative estimate of the RTO interval is necessary to
   minimize spurious RTOs.

   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 an 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
   Timestamps 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, i.e., SND.UNA is

   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.

4.2.  Updating the RTO Value

   When [RFC1323] was originally written, it was perceived that taking
   RTT measurements for each segment, and also during retransmissions,
   would contribute to reduce spurious RTOs, while maintaining the
   timeliness of necessary RTOs.  At the time, RTO was also the only
   mechanism to make use of the measured RTT.  It has been shown that
   taking more RTT samples has only a very limited effect to optimize
   RTOs [Allman99].

   Implementers should note that with timestamps, multiple RTTMs can be
   taken per RTT.  The [RFC6298] RTT estimator has weighting factors,
   alpha and beta, based on an implicit assumption that at most one RTTM
   will be sampled per RTT.  When multiple RTTMs per RTT are available

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   to update the RTT estimator, an implementation SHOULD try to adhere
   to the spirit of the history specified in [RFC6298].  An
   implementation suggestion is detailed in Appendix G.

   [Ludwig00] and [Floyd05] have highlighted the problem that an
   unmodified RTO calculation, which is updated with per-packet RTT
   samples, will truncate the path history too soon.  This can lead to
   an increase in spurious retransmissions, when the path properties
   vary in the order of a few RTTs, but a high number of RTT samples are
   taken on a much shorter timescale.

4.3.  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.

   There are three situations to consider:

   (A)  Delayed ACKs.

        Many TCPs acknowledge only every second 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) has 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 RCV.NXT.

        The same situation occurs if segments are reordered by the

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   (C)  A filled hole in the sequence space.

        The segment that fills the hole and advances the window
        represents the most recent measurement of the network
        characteristics.  An RTT computed from an earlier segment would
        probably include the sender's retransmit timeout, 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

   (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.

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   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.

                <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>

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5.  PAWS - Protection Against Wrapped Sequences

5.1.  Introduction

   Another use for the Timestamps option is the PAWS mechanism.
   Section 5.2 describes a simple mechanism to reject old duplicate
   segments that might corrupt an open TCP connection.  PAWS operates
   within a single TCP connection, using state that is saved in the
   connection control block.  Section 5.8 and Appendix H discuss the
   implications of the PAWS mechanism for avoiding old duplicates from
   previous incarnations of the same connection.

5.2.  The PAWS Mechanism

   PAWS uses the TCP Timestamps option 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 timestamps recently received on this

   In the PAWS 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 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 non-decreasing.  For
   example, an implementation 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.  For simplicity, the value TS.Recent
   introduced in Section 4.3 is used instead, as using a common value
   for both PAWS and RTTM simplifies the implementation.  As Section 4.3
   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.

   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

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   last segment whose ACK field advanced the left edge of the send
   window.  This algorithm was deemed to lack economy of mechanism and

   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 Timestamps 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 an <RST>, if the segment causing
   the <RST> to be generated contains a Timestamps option, the <RST>
   should also contain a Timestamps option.  In the <RST> segment,
   SEG.TSecr SHOULD be set to SEG.TSval from the incoming segment and
   SEG.TSval SHOULD be set to zero.  If an <RST> is being generated
   because of a user abort, and Snd.TS.OK is set, then a Timestamps
   option SHOULD be included in the <RST>.  When an <RST> segment is
   received, it MUST NOT be subjected to the PAWS check by verifying an
   acceptable value in SEG.TSval, and information from the Timestamps
   option MUST NOT be used to update connection state information.
   SEG.TSecr MAY be used to provide stricter <RST> acceptance checks.

5.3.  Basic PAWS Algorithm

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

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

           Send an acknowledgment in reply as specified in Section 3.9
           of [RFC0793], page 69, and drop the segment.

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           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 an example, see Figure 10 of

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

   R3)  If an arriving segment satisfies SEG.TSval >= TS.Recent and
        SEG.SEQ <= Last.ACK.sent (see Section 4.3), 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

   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.

   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 Timestamps option must not
   result in discarding this data.

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   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.

5.4.  Timestamp Clock

   It is important to understand that the PAWS algorithm does not
   require clock synchronization between the sender and receiver.  The
   sender's timestamp clock is used as a source of monotonic non-
   decreasing values to stamp the segments.  The receiver treats the
   timestamp value as simply a monotonically non-decreasing serial
   number, without any connection to time.  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.

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   (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 5.5), 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
   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.

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   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).

   o  A random offset may be added to the timestamp clock on a per-
      connection basis.  See [RFC6528], Section 3, on randomizing the
      initial sequence number (ISN).  The same function with a different
      secret key can be used to generate the per-connection timestamp

5.5.  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 wrap-
   around 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 update a
   clock or timestamp value associated with the connection whenever
   TS.Recent is updated, for example.  The details will be
   implementation dependent.

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5.6.  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 the 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
   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

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      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.  [Note: the notation for exponentiation
      has been changed from how it appeared in RFC 1185.]

   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.

5.7.  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.

5.8.  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 an MSL by the Internet (IP)
   layer (see the 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.

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   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

6.  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 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 and PAWS.

   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 others 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.

7.  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
   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.

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   See [RFC5961] for mitigation strategies to blind in-window attacks.

   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.  It is therefore
   RECOMMENDED to generate a random, per-connection offset to be used
   with the clock source when generating the Timestamps option value
   (see Section 5.4).  By carefully choosing this random offset, further
   improvements as described in [RFC6191] are possible.

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

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

   Middleboxes and TCP options:

      Some middleboxes have been known to remove the TCP options
      described in this document from TCP segments [Honda11].
      Middleboxes that remove TCP options described in this document
      from the <SYN> segment interfere with the selection of parameters
      appropriate for the session.  Removing any of these options in a
      <SYN,ACK> segment will leave the end hosts in a state that
      destroys the proper operation of the protocol.

      *  If a Window Scale option is removed from a <SYN,ACK> segment,
         the end hosts will not negotiate the window scaling factor
         correctly.  Middleboxes must not remove or modify the Window
         Scale option from <SYN,ACK> segments.

      *  If a stateful firewall uses the window field to detect whether
         a received segment is inside the current window, and does not
         support the Window Scale option, it will not be able to
         correctly determine whether or not a packet is in the window.
         These middle boxes must also support the Window Scale option
         and apply the scale factor when processing segments.  If the
         window scale factor cannot be determined, it must not do
         window-based processing.

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      *  If the Timestamps option is removed from the <SYN> or <SYN,ACK>
         segments, high speed connections that need PAWS would not have
         that protection.  Successful negotiation of the Timestamps
         option enforces a stricter verification of incoming segments at
         the receiver.  If the Timestamps option was removed from a
         subsequent data segment after a successful negotiation (e.g.,
         as part of resegmentation), the segment is discarded by the
         receiver without further processing.  Middleboxes should not
         remove the Timestamps option.

      *  It must be noted that [RFC1323] doesn't address the case of the
         Timestamps option being dropped or selectively omitted after
         being negotiated, and that the update in this document may
         cause some broken middlebox behavior to be detected
         (potentially unresponsive TCP sessions).

   Implementations that depend on PAWS could provide a mechanism for the
   application to determine whether or not PAWS is in use on the
   connection and choose to terminate the connection if that protection
   doesn't exist.  This is not just to protect the connection against
   middleboxes that might remove the Timestamps option, but also against
   remote hosts that do not have Timestamp support.

7.1.  Privacy Considerations

   The TCP options described in this document do not expose individual
   user's data.  However, a naive implementation simply using the system
   clock as a source for the Timestamps option will reveal
   characteristics of the TCP, potentially allowing more targeted
   attacks.  It is therefore RECOMMENDED to generate a random, per-
   connection offset to be used with the clock source when generating
   the Timestamps option value (see Section 5.4).

   Furthermore, the combination, relative ordering, and padding of the
   TCP options described in Sections 2.2 and 3.2 will reveal additional
   clues to allow the fingerprinting of the system.

8.  IANA Considerations

   The described TCP options are well known from the superceded
   [RFC1323].  IANA has updated the "TCP Option Kind Numbers" table
   under "TCP Parameters" to list this document (RFC 7323) as the
   reference for "Window Scale" and "Timestamps".

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9.  References

9.1.  Normative References

   [RFC793]   Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

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

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

9.2.  Informative References

   [Allman99] Allman, M. and V. Paxson, "On Estimating End-to-End
              Network Path Properties", Proceedings of the ACM SIGCOMM
              Technical Symposium, Cambridge, MA, September 1999,

   [Floyd05]  Floyd, S., "Subject: Re: [tcpm] RFC 1323: Timestamps
              option", message to the TCPM mailing list, 26 January
              2007, <http://www.ietf.org/mail-archive/web/tcpm/current/

              Garlick, L., Rom, R., and J. Postel, "Issues in Reliable
              Host-to-Host Protocols", Proceedings of the Second
              Berkeley Workshop on Distributed Data Management and
              Computer Networks, March 1977,

   [Honda11]  Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
              Handley, M., and H. Tokuda, "Is it Still Possible to
              Extend TCP?", Proceedings of the ACM Internet Measurement
              Conference (IMC) '11, November 2011.

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

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

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              Jacobson, V., "Subject: modified TCP congestion avoidance
              algorithm", message to the End2End-Interest mailing list,
              30 April 1990, <ftp://ftp.isi.edu/end2end/

   [Karn87]   Karn, P. and C. Partridge, "Estimating Round-Trip Times in
              Reliable Transport Protocols", Proceedings of SIGCOMM '87,
              August 1987.

              Kuehlewind, M. and B. Briscoe, "Chirping for Congestion
              Control - Implementation Feasibility", November 2010,

              Kuzmanovic, A. and E. Knightly, "TCP-LP: Low-Priority
              Service via End-Point Congestion Control", 2003,

   [Ludwig00] Ludwig, R. and K. Sklower, "The Eifel Retransmission
              Timer", ACM SIGCOMM Computer Communication Review Volume
              30 Issue 3, July 2000,

   [Martin03] Martin, D., "Subject: [Tsvwg] RFC 1323.bis", message to
              the TSVWG mailing list, 30 September 2003,

   [Medina04] Medina, A., Allman, M., and S. Floyd, "Measuring
              Interactions Between Transport Protocols and Middleboxes",
              Proceedings of the ACM SIGCOMM/USENIX Internet Measurement
              Conference, October 2004,

   [Medina05] Medina, A., Allman, M., and S. Floyd, "Measuring the
              Evolution of Transport Protocols in the Internet", ACM
              Computer Communication Review Volume 35, No. 2, April

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              Oppermann, A., "Subject: Re: [tcpm] I-D Action: draft-
              ietf.tcpm-1323bis-13.txt", message to the TCPM mailing
              list, 01 June 2013, <http://www.ietf.org/

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

   [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.

   [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.

   [RFC4015]  Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
              for TCP", RFC 4015, February 2005.

   [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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   [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks", RFC 5961, August

   [RFC6191]  Gont, F., "Reducing the TIME-WAIT State Using TCP
              Timestamps", BCP 159, RFC 6191, April 2011.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298, June

   [RFC6528]  Gont, F. and S. Bellovin, "Defending against Sequence
              Number Attacks", RFC 6528, February 2012.

   [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.

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              December 2012.

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Appendix A.  Implementation Suggestions

   TCP Option Layout

      The following layout is 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 (64 KiB - 1 - size of
           fixed IP and TCP headers).

      (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 steps are described in Section 5.2 of [RFC2675].

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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, "Knowing 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 Timestamps 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/
   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 resynchronized 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:

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   (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 (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

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        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 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:       System-wide source of 32-bit timestamp values
      my.TSclock.rate:  Period of my.TSclock (1 ms to 1 sec)
      Snd.TSoffset:     An 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.Shift:   Receive window scale exponent
      Snd.Wind.Shift:   Send window scale exponent
      Start.Time:       Snd.TSclock value when the segment being timed
                        was sent (used by code from before RFC 1323).


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

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   Send Sequence Variables

      SND.UNA:          Send unacknowledged
      SND.NXT:          Send next
      SND.WND:          Send window
      ISS:              Initial send sequence number

   Receive Sequence Variables

      RCV.NXT:          Receive next
      RCV.WND:          Receive window
      IRS:              Initial receive sequence number

Appendix D.  Event Processing Summary

   This appendix attempts to specify the algorithms unambiguously by
   presenting modifications to the Event Processing rules in Section 3.9
   of RFC 793.  The change bars ("|") indicate lines that are different
   from RFC 793.

   OPEN Call


      An initial send sequence number (ISS) is selected.  Send a <SYN>
 |    segment of the form:
 |      <SEQ=ISS><CTL=SYN><TSval=Snd.TSclock><WSopt=Rcv.Wind.Shift>


   SEND Call

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



         If active and 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.Shift>.  Set SND.UNA to ISS, SND.NXT to ISS+1.
         Enter SYN-SENT state.  ...


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         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=Snd.TSclock,TSecr=TS.Recent> in each data
 |       segment.
 |       Scale the receive window for transmission in the segment
 |       header:
 |               SEG.WND = (RCV.WND >> Rcv.Wind.Shift).



      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.Shift and set Snd.WS.OK flag on.
 |          Otherwise, set both Snd.Wind.Shift and Rcv.Wind.Shift 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.

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            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
 |           <WSopt=Rcv.Wind.Shift> 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


            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.  SND.UNA should be advanced to equal
            SEG.ACK (if there is an ACK), and any segments 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.Shift; otherwise, set both
 |          Snd.Wind.Shift and Rcv.Wind.Shift to zero.

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 |          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.TS.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 that 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.TS.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.Shift> 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.


      first check the sequence number

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

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            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.Shift,
 |          and use "TrueWindow" in place of SEG.WND in the following
 |          steps.
 |          Check whether the segment contains a Timestamps option and
 |          if bit Snd.TS.OK is on.  If so:
 |             If SEG.TSval < TS.Recent and the RST bit is off:
 |                If the connection has been idle more than 24 days,
 |                save SEG.TSval in variable TS.Recent, else the segment
 |                is not acceptable; follow the steps below for an
 |                unacceptable segment.
 |             If SEG.TSval >= TS.Recent and SEG.SEQ <= 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):


 |          Last.ACK.sent is set to SEG.ACK of the acknowledgment.  If
 |          the Snd.TS.OK bit is on, include the Timestamps 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.


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      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 that 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 the Timestamps 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 piggybacked 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 acknowledges 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 an established TCP connection using a scale factor of 128,
   Snd.Wind.Shift=7 and Rcv.Wind.Shift=7, that is running with a very
   small window 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

   The sender writes 40 bytes and receiver ACKs:

   1040     2       1296               1300

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   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 is a general problem and can happen any time the sender does a
   write, which is smaller than the window scale factor.

   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 an integral number of segments,
   rounded up to the next scale factor.  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 the window, the BEYOND BUFFER case above,
   rather than retracting it.

Appendix G.  RTO Calculation Modification

   Taking multiple RTT samples per window would shorten the history
   calculated by the RTO mechanism in [RFC6298], and the below algorithm
   aims to maintain a similar history as originally intended by

   It is roughly known how many samples a congestion window worth of
   data will yield, not accounting for ACK compression, and ACK losses.
   Such events will result in more history of the path being reflected
   in the final value for RTO, and are uncritical.  This modification
   will ensure that a similar amount of time is taken into account for
   the RTO estimation, regardless of how many samples are taken per

      ExpectedSamples = ceiling(FlightSize / (SMSS * 2))

      alpha' = alpha / ExpectedSamples

      beta' = beta / ExpectedSamples

   Note that the factor 2 in ExpectedSamples is due to "Delayed ACKs".

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   Instead of using alpha and beta in the algorithm of [RFC6298], use
   alpha' and beta' instead:

      RTTVAR <- (1 - beta') * RTTVAR + beta' * |SRTT - R'|

      SRTT <- (1 - alpha') * SRTT + alpha' * R'

      (for each sample R')

Appendix H.  Changes from RFC 1323

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

   (a)  A wrong reference to SND.WND was corrected to SEG.WND in
        Section 2.3.

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

   (c)  In Section 3.2, the wording how the Timestamps 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 RFC 1323 left room for interpretation -- e.g., potential late
        enablement of TSopt.

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

   (e)  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

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   (f)  In RFC 1323, 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.

   (g)  It is now recommended that the Timestamps option is included in
        <RST> segments if the incoming segment contained a Timestamps

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

   (i)  Added text to clarify the precedence between regular TCP
        [RFC0793] and this document's Timestamps option / PAWS
        processing.  Discussion about combined acceptability checks are

   (j)  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].  Text was added to guide
        implementers to the proper selection of these offsets, as
        entirely random offsets for each new connection will conflict
        with PAWS.

   (k)  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].

   (l)  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.

   (m)  Appendix G was added to exemplify how an RTO calculation might
        be updated to properly take the much higher RTT sampling
        frequency enabled by the Timestamps option into account.

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

   (a)  Removed much of the discussion in Section 1 to streamline the
        document.  However, detailed examples and discussions in
        Sections 2, 3, and 5 are kept as guidelines for implementers.

   (b)  Added short text that the use of WS increases the chances of
        sequence number wrap, thus the PAWS mechanism is required in
        certain environments.

   (c)  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.

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

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

   (f)  Updated the text in Section 3 to take into account what has been
        learned since [RFC1323].

   (g)  Removed some unused references.

   (h)  Removed the list of changes between [RFC1323] and prior
        versions.  These changes are mentioned in Appendix C of

   (i)  Moved "Changes from RFC 1323" to 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.

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Authors' Addresses

   David Borman
   Quantum Corporation
   Mendota Heights, MN  55120

   EMail: david.borman@quantum.com

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

   EMail: braden@isi.edu

   Van Jacobson
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   EMail: vanj@google.com

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

   EMail: rs@netapp.com

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