RFC 3639 (rfc3639) - Page 2 of 8

Considerations on the use of a Service Identifier in Packet Headers

RFC 3639          Service Identifier in Packet Headers      October 2003


2.  Service Identifiers

   Although not necessarily by design, certain conventions have evolved
   with respect to the IP protocol suite relative to the identification
   of services within an IP traffic flow:

   o  Within the IP protocol suite, end point identifiers (e.g.,
      TCP/UDP/SCTP port numbers, IP protocol numbers) are designed to
      identify services to end points.  In particular, TCP, UDP or SCTP
      (Stream Control Transmission Protocol) port numbers are intended
      to identify the source service location and the destination
      service entity to the destination end point.

   o  The IP [2] datagram header contains the source and destination
      address of the datagram as well as an indication of the upper-
      level protocol (ULP) carried within the datagram.  If the ULP is
      either TCP [3], UDP [1], or SCTP [8] the payload will contain both
      source and destination port numbers which allows differentiation
      between services (e.g., TELNET, HTTP) and between multiple
      instances of the same service between the pair of hosts described
      by the source and destination address.

   o  By convention, for at least TCP and UDP, certain port numbers are
      used as rendezvous points and are considered "well known" on the
      source or destination side of the communication.  Such rendezvous
      points are maintained in an IANA registry currently located at
      [11].  Specific registries for protocol and port numbers are at
      [12] and [13].

   o  Notwithstanding the "well knownness" of any given port, port
      numbers are only guaranteed to be meaningful to the end systems.
      An intermediate system should generally not impute specific
      meaning to any given port number, unless specifically indicated by
      an end system (e.g., via the Resource Reservation Protocol (RSVP)
      [4]) or agreed to by convention among the end systems and one or
      more specific intermediate systems (e.g., firewall traversal for
      the IP Security Protocol (IPSEC) [5]).

   o  Some services make use of protocol interactions to dynamically
      allocate service identifiers (i.e., port numbers) to specific
      communications.  One specific example of this is the Session
      Initiation Protocol (SIP) [9].  The implication of this is that
      intermediate systems cannot relate the service identifiers to the
      actual service unless they participate in the protocols which
      allocate the service identifiers, or are explicitly notified of
      the outcome of the allocation.





St. Johns & Huston           Informational