RFC 793, "Transmission Control Protocol", September 1981

Note: This RFC has been obsoleted by RFC 9293

Note: This RFC has been updated by RFC 1122, RFC 3168, RFC 6093, RFC 6528

Errata ID: 6282
Status: Held for Document Update
Type: Editorial
Publication Format(s) : TEXT
Reported By: Merlin Büge
Date Reported: 2020-09-06
Held for Document Update by: Martin Duke
Date Held: 2020-10-12

Section 3.3 says:

  To avoid confusion we must prevent segments from one incarnation of a
  connection from being used while the same sequence numbers may still
  be present in the network from an earlier incarnation.  We want to
  assure this, even if a TCP crashes and loses all knowledge of the
  sequence numbers it has been using.  When new connections are created,
  an initial sequence number (ISN) generator is employed which selects a
  new 32 bit ISN.  The generator is bound to a (possibly fictitious) 32
  bit clock whose low order bit is incremented roughly every 4
  microseconds.  Thus, the ISN cycles approximately every 4.55 hours.
  Since we assume that segments will stay in the network no more than
  the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55
  hours we can reasonably assume that ISN's will be unique.

  For each connection there is a send sequence number and a receive
  sequence number.  The initial send sequence number (ISS) is chosen by
  the data sending TCP, and the initial receive sequence number (IRS) is
  learned during the connection establishing procedure.

  For a connection to be established or initialized, the two TCPs must
  synchronize on each other's initial sequence numbers.  This is done in
  an exchange of connection establishing segments carrying a control bit
  called "SYN" (for synchronize) and the initial sequence numbers.  As a
  shorthand, segments carrying the SYN bit are also called "SYNs".
  Hence, the solution requires a suitable mechanism for picking an
  initial sequence number and a slightly involved handshake to exchange
  the ISN's.

  The synchronization requires each side to send it's own initial
  sequence number and to receive a confirmation of it in acknowledgment
  from the other side.  Each side must also receive the other side's
  initial sequence number and send a confirming acknowledgment.

    1) A --> B  SYN my sequence number is X
    2) A <-- B  ACK your sequence number is X
    3) A <-- B  SYN my sequence number is Y
    4) A --> B  ACK your sequence number is Y

  Because steps 2 and 3 can be combined in a single message this is
  called the three way (or three message) handshake.

  A three way handshake is necessary because sequence numbers are not
  tied to a global clock in the network, and TCPs may have different
  mechanisms for picking the ISN's.  The receiver of the first SYN has
  no way of knowing whether the segment was an old delayed one or not,
  unless it remembers the last sequence number used on the connection
  (which is not always possible), and so it must ask the sender to
  verify this SYN.  The three way handshake and the advantages of a
  clock-driven scheme are discussed in [3].

It should say:

  To avoid confusion we must prevent segments from one incarnation of a
  connection from being used while the same sequence numbers may still
  be present in the network from an earlier incarnation.  We want to
  assure this, even if a TCP crashes and loses all knowledge of the
  sequence numbers it has been using.  When new connections are created,
  an initial sequence number (ISN) generator is employed which selects a
  new 32 bit ISN.  The generator is bound to a (possibly fictitious) 32
  bit clock whose low order bit is incremented roughly every 4
  microseconds.  Thus, the ISN cycles approximately every 4.55 hours.
  Since we assume that segments will stay in the network no more than
  the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55
  hours we can reasonably assume that ISNs will be unique.

  For each connection there is a send sequence number and a receive
  sequence number.  The initial send sequence number (ISS) is chosen by
  the data sending TCP, and the initial receive sequence number (IRS) is
  learned during the connection establishing procedure.

  For a connection to be established or initialized, the two TCPs must
  synchronize on each other's initial sequence numbers.  This is done in
  an exchange of connection establishing segments carrying a control bit
  called "SYN" (for synchronize) and the initial sequence numbers.  As a
  shorthand, segments carrying the SYN bit are also called "SYNs".
  Hence, the solution requires a suitable mechanism for picking an
  initial sequence number and a slightly involved handshake to exchange
  the ISNs.

  The synchronization requires each side to send it's own initial
  sequence number and to receive a confirmation of it in acknowledgment
  from the other side.  Each side must also receive the other side's
  initial sequence number and send a confirming acknowledgment.

    1) A --> B  SYN my sequence number is X
    2) A <-- B  ACK your sequence number is X
    3) A <-- B  SYN my sequence number is Y
    4) A --> B  ACK your sequence number is Y

  Because steps 2 and 3 can be combined in a single message this is
  called the three way (or three message) handshake.

  A three way handshake is necessary because sequence numbers are not
  tied to a global clock in the network, and TCPs may have different
  mechanisms for picking the ISNs.  The receiver of the first SYN has
  no way of knowing whether the segment was an old delayed one or not,
  unless it remembers the last sequence number used on the connection
  (which is not always possible), and so it must ask the sender to
  verify this SYN.  The three way handshake and the advantages of a
  clock-driven scheme are discussed in [3].

Notes:

The only change: s/ISN's/ISNs/g
"ISN's" has three matches in the whole RFC, all of them in this section, and all matches refer to the plural form of ISN.
ISN stands for "initial sequence number".
Sorry for all the bulk text.

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