TCP-RTTM: ROUND-TRIP TIME MEASUREMENT

3.  RTTM: ROUND-TRIP TIME MEASUREMENT
往返时间衡量

   3.1  Introduction

      Accurate and current RTT estimates are necessary to adapt to
      changing traffic conditions and to avoid an instability known as
      "congestion collapse" [Nagle84] in a busy network.  However,
      accurate measurement of RTT may be difficult both in theory and in
      implementation.
     准确和当前的往返时间估计对于应对不断变化的网络情况和避免在繁忙网络时拥塞崩溃。
      Many TCP implementations base their RTT measurements upon a sample
      of only one packet per window.  While this yields an adequate
      approximation to the RTT for small windows, it results in an
      unacceptably poor RTT estimate for an LFN.  If we look at RTT
      estimation as a signal processing problem (which it is), a data
      signal at some frequency, the packet rate, is being sampled at a
      lower frequency, the window rate.  This lower sampling frequency
      violates Nyquist's criteria and may therefore introduce "aliasing"
      artifacts into the estimated RTT [Hamming77].
      一些TCP的实现基于他们的RTT计算，但是一个window只有一个pack作为代表。
      A good RTT estimator with a conservative retransmission timeout
      calculation can tolerate aliasing when the sampling frequency is
      "close" to the data frequency.   For example, with a window of 8
      packets, the sample rate is 1/8 the data frequency -- less than an
      order of magnitude different.  However, when the window is tens or
      hundreds of packets, the RTT estimator may be seriously in error,
      resulting in spurious retransmissions.
      一个好的方案是一个保守的重传评估可以在采样频率‘close’原始数据频率的情况下容忍失真，比如，具有8个包得数据窗口，采样频率是1/8，小于大小差值的顺序。并且，如果一个窗口有几百个包，RTT的估算可能会有错误，导致伪重传的问题。

      If there are dropped packets, the problem becomes worse.  Zhang



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RFC 1323          TCP Extensions for High Performance           May 1992


      [Zhang86], Jain [Jain86] and Karn [Karn87] have shown that it is
      not possible to accumulate reliable RTT estimates if retransmitted
      segments are included in the estimate.  Since a full window of
      data will have been transmitted prior to a retransmission, all of
      the segments in that window will have to be ACKed before the next
      RTT sample can be taken.  This means at least an additional
      window's worth of time between RTT measurements and, as the error
      rate approaches one per window of data (e.g., 10**-6 errors per
      bit for the Wideband satellite network), it becomes effectively
      impossible to obtain a valid RTT measurement.

      A solution to these problems, which actually simplifies the sender
      substantially, is as follows: using TCP options, the sender places
      a timestamp in each data segment, and the receiver reflects these
      timestamps back in ACK segments.  Then a single subtract gives the
      sender an accurate RTT measurement for every ACK segment (which
      will correspond to every other data segment, with a sensible
      receiver).  We call this the RTTM (Round-Trip Time Measurement)
      mechanism.

      It is vitally important to use the RTTM mechanism with big
      windows; otherwise, the door is opened to some dangerous
      instabilities due to aliasing.  Furthermore, the option is
      probably useful for all TCP's, since it simplifies the sender.

   3.2  TCP Timestamps Option

      TCP is a symmetric protocol, allowing data to be sent at any time
      in either direction, and therefore timestamp echoing may occur in
      either direction.  For simplicity and symmetry, we specify that
      timestamps always be sent and echoed in both directions.  For
      efficiency, we combine the timestamp and timestamp reply fields
      into a single TCP Timestamps Option.
      TCP为对称协议，数据可以在任何时间任何方向传递，并且两个方向都可以有时间戳。组合时间戳和应答时间戳作为一个单独的TCP时间戳选项。


















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RFC 1323          TCP Extensions for High Performance           May 1992


      TCP Timestamps Option (TSopt):

         Kind: 8

         Length: 10 bytes

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

         The Timestamps option carries two four-byte timestamp fields.
         The Timestamp Value field (TSval) contains the current value of
         the timestamp clock of the TCP sending the option.
TSval：当前TCP发送请求包的时间戳
TSecr：

         The Timestamp Echo Reply field (TSecr) is only valid if the ACK
         bit is set in the TCP header; if it is valid, it echos a times-
         tamp value that was sent by the remote TCP in the TSval field
         of a Timestamps option.  When TSecr is not valid, its value
         must be zero.  The TSecr value will generally be from the most
         recent Timestamp option that was received; however, there are
         exceptions that are explained below.

         A TCP may send the Timestamps option (TSopt) in an initial
         <SYN> segment (i.e., segment containing a SYN bit and no ACK
         bit), and may send a TSopt in other segments only if it re-
         ceived a TSopt in the initial <SYN> segment for the connection.

   3.3 The RTTM Mechanism

      The timestamp value to be sent in TSval is to be obtained from a
      (virtual) clock that we call the "timestamp clock".  Its values
      must be at least approximately proportional to real time, in order
      to measure actual RTT.
      TSval中的时间戳是来自虚拟时钟的，我们可以称之为时间戳时钟，它的值必须大致的正比于实际时间。

      The following example illustrates a one-way data flow with
      segments arriving in sequence without loss.  Here A, B, C...
      represent data blocks occupying successive blocks of sequence
      numbers, and ACK(A),...  represent the corresponding cumulative
      acknowledgments.  The two timestamp fields of the Timestamps
      option are shown symbolically as <TSval= x,TSecr=y>.  Each TSecr
      field contains the value most recently received in a TSval field.









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RFC 1323          TCP Extensions for High Performance           May 1992



         TCP  A                                          TCP B

                        <A,TSval=1,TSecr=120> ------>

             <---- <ACK(A),TSval=127,TSecr=1>

                        <B,TSval=5,TSecr=127> ------>

             <---- <ACK(B),TSval=131,TSecr=5>

             . . . . . . . . . . . . . . . . . . . . . .

                        <C,TSval=65,TSecr=131> ------>

             <---- <ACK(C),TSval=191,TSecr=65>

                        (etc)


      The dotted line marks a pause (60 time units long) in which A had
      nothing to send.  Note that this pause inflates the RTT which B
      could infer from receiving TSecr=131 in data segment C.  Thus, in
      one-way data flows, RTTM in the reverse direction measures a value
      that is inflated by gaps in sending data.  However, the following
      rule prevents a resulting inflation of the measured RTT:

           A TSecr value received in a segment is used to update the
           averaged RTT measurement only if the segment acknowledges
           some new data, i.e., only if it advances the left edge of the
           send window.

      Since TCP B is not sending data, the data segment C does not
      acknowledge any new data when it arrives at B.  Thus, the inflated
      RTTM measurement is not used to update B's RTTM measurement.

   3.4  Which Timestamp to Echo

      If more than one Timestamps option is received before a reply
      segment is sent, the TCP must choose only one of the TSvals to
      echo, ignoring the others.  To minimize the state kept in the
      receiver (i.e., the number of unprocessed TSvals), the receiver
      should be required to retain at most one timestamp in the
      connection control block.







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RFC 1323          TCP Extensions for High Performance           May 1992


      There are three situations to consider:

      (A)  Delayed ACKs.

           Many TCP's acknowledge only every Kth segment out of a group
           of segments arriving within a short time interval; this
           policy is known generally as "delayed ACKs".  The data-sender
           TCP must measure the effective RTT, including the additional
           time due to delayed ACKs, or else it will retransmit
           unnecessarily.  Thus, when delayed ACKs are in use, the
           receiver should reply with the TSval field from the earliest
           unacknowledged segment.

      (B)  A hole in the sequence space (segment(s) have been lost).

           The sender will continue sending until the window is filled,
           and the receiver may be generating ACKs as these out-of-order
           segments arrive (e.g., to aid "fast retransmit").

           The lost segment is probably a sign of congestion, and in
           that situation the sender should be conservative about
           retransmission.  Furthermore, it is better to overestimate
           than underestimate the RTT.  An ACK for an out-of-order
           segment should therefore contain the timestamp from the most
           recent segment that advanced the window.

           The same situation occurs if segments are re-ordered by the
           network.

      (C)  A filled hole in the sequence space.

           The segment that fills the hole represents the most recent
           measurement of the network characteristics.  On the other
           hand, an RTT computed from an earlier segment would probably
           include the sender's retransmit time-out, badly biasing the
           sender's average RTT estimate.  Thus, the timestamp from the
           latest segment (which filled the hole) must be echoed.

      An algorithm that covers all three cases is described in the
      following rules for Timestamps option processing on a synchronized
      connection:

      (1)  The connection state is augmented with two 32-bit slots:
           TS.Recent holds a timestamp to be echoed in TSecr whenever a
           segment is sent, and Last.ACK.sent holds the ACK field from
           the last segment sent.  Last.ACK.sent will equal RCV.NXT
           except when ACKs have been delayed.




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RFC 1323          TCP Extensions for High Performance           May 1992


      (2)  If Last.ACK.sent falls within the range of sequence numbers
           of an incoming segment:

              SEG.SEQ <= Last.ACK.sent < SEG.SEQ + SEG.LEN

           then the TSval from the segment is copied to TS.Recent;
           otherwise, the TSval is ignored.

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

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


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

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

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

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

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









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RFC 1323          TCP Extensions for High Performance           May 1992


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