RFC 2873 (rfc2873) - Page 2 of 8

TCP Processing of the IPv4 Precedence Field

RFC 2873           TCP and the IPv4 Precedence Field           June 2000


   and compartment, etc.  Both ends must agree on the setting of the
   precedence and security parameters in order to establish a connection
   and keep it open.

   There is no field in the TCP header that indicates the precedence of
   a segment. Instead, the precedence field in the header of the IP
   packet is used as the indication.  The security level and compartment
   are likewise carried in the IP header, but as IP options rather than
   a fixed header field.  Because of this difference, the problem with
   precedence discussed in this memo does not apply to them.

   TCP requires that the precedence (and security parameters) of a
   connection must remain unchanged during the lifetime of the
   connection. Therefore, for an established TCP connection with
   precedence, the receipt of a segment with different precedence
   indicates an error. The connection must be reset [RFC 793, pp. 36, 37,
   40, 66, 67, 71].

   With the advent of DiffServ, intermediate nodes may modify the
   Differentiated Services Codepoint (DSCP) [RFC 2474] of the IP header
   to indicate the desired Per-hop Behavior (PHB) [RFC 2475, RFC 2597,
   RFC 2598]. The DSCP includes the three bits formerly known as the
   precedence field.  Because any modification to those three bits will
   be considered illegal by endpoints that are precedence-aware, they
   may cause failures in establishing connections, or may cause
   established connections to be reset.

2. Terminology

   Segment: the unit of data that TCP sends to IP

   Precedence Field: the three leftmost bits in the TOS octet of an IPv4
   header. Note that in DiffServ, these three bits may or may not be
   used to denote the precedence of the IP packet. There is no
   precedence field in the traffic class octet in IPv6.

   TOS Field: bits 3-6 in the TOS octet of IPv4 header [RFC 1349].

   MBZ field: Must Be Zero

   The structure of the TOS octet is depicted below:

                   0     1     2     3     4     5     6     7
                +-----+-----+-----+-----+-----+-----+-----+-----+
                |   PRECEDENCE    |          TOS          | MBZ |
                +-----+-----+-----+-----+-----+-----+-----+-----+





Xiao, et al.                Standards Track