rfc1323.txt

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           otherwise, the TSval is ignored.

      (3)  When a TSopt is sent, its TSecr field is set to the current
           TS.Recent value.

      The following examples illustrate these rules.  Here A, B, C...
      represent data segments occupying successive blocks of sequence
      numbers, and ACK(A),...  represent the corresponding
      acknowledgment segments.  Note that ACK(A) has the same sequence
      number as B.  We show only one direction of timestamp echoing, for
      clarity.


      o    Packets arrive in sequence, and some of the ACKs are delayed.

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

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

      o    Packets arrive out of order, and every packet is
           acknowledged.

           By Case (B), the timestamp from the last segment that
           advanced the left window edge is echoed, until the missing
           segment arrives; it is echoed according to Case (C).  The
           same sequence would occur if segments B and D were lost and
           retransmitted..









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




4.  PAWS: PROTECT AGAINST WRAPPED SEQUENCE NUMBERS

   4.1  Introduction

      Section 4.2 describes a simple mechanism to reject old duplicate
      segments that might corrupt an open TCP connection; we call this
      mechanism PAWS (Protect Against Wrapped Sequence numbers).  PAWS
      operates within a single TCP connection, using state that is saved
      in the connection control block.  Section 4.3 and Appendix C
      discuss the implications of the PAWS mechanism for avoiding old
      duplicates from previous incarnations of the same connection.

   4.2  The PAWS Mechanism

      PAWS uses the same TCP Timestamps option as the RTTM mechanism
      described earlier, and assumes that every received TCP segment
      (including data and ACK segments) contains a timestamp SEG.TSval
      whose values are monotone non-decreasing in time.  The basic idea
      is that a segment can be discarded as an old duplicate if it is
      received with a timestamp SEG.TSval less than some timestamp
      recently received on this connection.

      In both the PAWS and the RTTM mechanism, the "timestamps" are 32-



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      bit unsigned integers in a modular 32-bit space.  Thus, "less
      than" is defined the same way it is for TCP sequence numbers, and
      the same implementation techniques apply.  If s and t are
      timestamp values, s < t if 0 < (t - s) < 2**31, computed in
      unsigned 32-bit arithmetic.

      The choice of incoming timestamps to be saved for this comparison
      must guarantee a value that is monotone increasing.  For example,
      we might save the timestamp from the segment that last advanced
      the left edge of the receive window, i.e., the most recent in-
      sequence segment.  Instead, we choose the value TS.Recent
      introduced in Section 3.4 for the RTTM mechanism, since using a
      common value for both PAWS and RTTM simplifies the implementation
      of both.  As Section 3.4 explained, TS.Recent differs from the
      timestamp from the last in-sequence segment only in the case of
      delayed ACKs, and therefore by less than one window.  Either
      choice will therefore protect against sequence number wrap-around.

      RTTM was specified in a symmetrical manner, so that TSval
      timestamps are carried in both data and ACK segments and are
      echoed in TSecr fields carried in returning ACK or data segments.
      PAWS submits all incoming segments to the same test, and therefore
      protects against duplicate ACK segments as well as data segments.
      (An alternative un-symmetric algorithm would protect against old
      duplicate ACKs: the sender of data would reject incoming ACK
      segments whose TSecr values were less than the TSecr saved from
      the last segment whose ACK field advanced the left edge of the
      send window.  This algorithm was deemed to lack economy of
      mechanism and symmetry.)

      TSval timestamps sent on {SYN} and {SYN,ACK} segments are used to
      initialize PAWS.  PAWS protects against old duplicate non-SYN
      segments, and duplicate SYN segments received while there is a
      synchronized connection.  Duplicate {SYN} and {SYN,ACK} segments
      received when there is no connection will be discarded by the
      normal 3-way handshake and sequence number checks of TCP.

      It is recommended that RST segments NOT carry timestamps, and that
      RST segments be acceptable regardless of their timestamp.  Old
      duplicate RST segments should be exceedingly unlikely, and their
      cleanup function should take precedence over timestamps.

      4.2.1  Basic PAWS Algorithm

         The PAWS algorithm requires the following processing to be
         performed on all incoming segments for a synchronized
         connection:




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

                   Send an acknowledgement in reply as specified in
                   RFC-793 page 69 and drop the segment.

                   Note: it is necessary to send an ACK segment in order
                   to retain TCP's mechanisms for detecting and
                   recovering from half-open connections.  For example,
                   see Figure 10 of RFC-793.

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

         R3)  If an arriving segment satisfies: SEG.SEQ <= Last.ACK.sent
              (see Section 3.4), then record its timestamp in TS.Recent.

         R4)  If an arriving segment is in-sequence (i.e., at the left
              window edge), then accept it normally.

         R5)  Otherwise, treat the segment as a normal in-window, out-
              of-sequence TCP segment (e.g., queue it for later delivery
              to the user).

         Steps R2, R4, and R5 are the normal TCP processing steps
         specified by RFC-793.

         It is important to note that the timestamp is checked only when
         a segment first arrives at the receiver, regardless of whether
         it is in-sequence or it must be queued for later delivery.
         Consider the following example.

              Suppose the segment sequence: A.1, B.1, C.1, ..., Z.1 has
              been sent, where the letter indicates the sequence number
              and the digit represents the timestamp.  Suppose also that
              segment B.1 has been lost.  The timestamp in TS.TStamp is
              1 (from A.1), so C.1, ..., Z.1 are considered acceptable
              and are queued.  When B is retransmitted as segment B.2
              (using the latest timestamp), it fills the hole and causes
              all the segments through Z to be acknowledged and passed
              to the user.  The timestamps of the queued segments are
              *not* inspected again at this time, since they have
              already been accepted.  When B.2 is accepted, TS.Stamp is
              set to 2.

         This rule allows reasonable performance under loss.  A full



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         window of data is in transit at all times, and after a loss a
         full window less one packet will show up out-of-sequence to be
         queued at the receiver (e.g., up to ~2**30 bytes of data); the
         timestamp option must not result in discarding this data.

         In certain unlikely circumstances, the algorithm of rules R1-R4
         could lead to discarding some segments unnecessarily, as shown
         in the following example:

              Suppose again that segments: A.1, B.1, C.1, ..., Z.1 have
              been sent in sequence and that segment B.1 has been lost.
              Furthermore, suppose delivery of some of C.1, ... Z.1 is
              delayed until AFTER the retransmission B.2 arrives at the
              receiver.  These delayed segments will be discarded
              unnecessarily when they do arrive, since their timestamps
              are now out of date.

         This case is very unlikely to occur.  If the retransmission was
         triggered by a timeout, some of the segments C.1, ... Z.1 must
         have been delayed longer than the RTO time.  This is presumably
         an unlikely event, or there would be many spurious timeouts and
         retransmissions.  If B's retransmission was triggered by the
         "fast retransmit" algorithm, i.e., by duplicate ACKs, then the
         queued segments that caused these ACKs must have been received
         already.

         Even if a segment were delayed past the RTO, the Fast
         Retransmit mechanism [Jacobson90c] will cause the delayed
         packets to be retransmitted at the same time as B.2, avoiding
         an extra RTT and therefore causing a very small performance
         penalty.

         We know of no case with a significant probability of occurrence
         in which timestamps will cause performance degradation by
         unnecessarily discarding segments.

      4.2.2  Timestamp Clock

         It is important to understand that the PAWS algorithm does not
         require clock synchronization between sender and receiver.  The
         sender's timestamp clock is used to stamp the segments, and the
         sender uses the echoed timestamp to measure RTT's.  However,
         the receiver treats the timestamp as simply a monotone-
         increasing serial number, without any necessary connection to
         its clock.  From the receiver's viewpoint, the timestamp is
         acting as a logical extension of the high-order bits of the
         sequence number.




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         The receiver algorithm does place some requirements on the
         frequency of the timestamp clock.

         (a)  The timestamp clock must not be "too slow".

              It must tick at least once for each 2**31 bytes sent.  In
              fact, in order to be useful to the sender for round trip
              timing, the clock should tick at least once per window's
              worth of data, and even with the RFC-1072 window
              extension, 2**31 bytes must be at least two windows.