Re: Purpose of Port 0.

james woodyatt <jhw@xxxxxxxxxx> · Tue, 21 Feb 2017 12:05:19 -0800

On Feb 20, 2017, at 08:10, Joe Touch <touch@xxxxxxx> wrote:

There is no IANA registry for source ports, nor should there be. Ports in the incoming first-contact are swapped, as indicated in RFCs 768 and 793. Source ports indicate "who to call back", and "0" is indicated in UDP (RFC768) as "nobody" and TCP (RFC793)

p1. Port zero is currently Unassigned for the SCTP and DCCP standard transports and all the other non-standard transports listed in the Protocol Numbers registry.

  <https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml>

p2. Port zero is a System Port, so the process for assigning its IANA registration is RFC 6335, section 8.1.2, which says:

  o  Ports in the System Ports range (0-1023) are also available for
      assignment through IANA.  Because the System Ports range is both
      the smallest and the most densely assigned, the requirements for
      new assignments are more strict than those for the User Ports
      range, and will only be granted under the "IETF Review" or "IESG
      Approval" procedures [RFC5226].  A request for a System Port
      number MUST document *both* why using a port number from the
      Dynamic Ports range is unsuitable *and* why using a port number
      from the User Ports range is unsuitable for that application.

p3. Port zero is already reserved for SCTP by RFC 4960. IANA just needs to correct its registry to show that port zero is Assigned.

p4. Port zero is not reserved for DCCP by RFC 4340, so port zero remains Unassigned there at this point.

p5. Port zero is already reserved in their respective RFCs for a few of the non-standard transports listed in the Protocol Numbers registry, but being non-standard transports, there’s no way to assign them for those protocols under the procedure in RFC 6335.

I’m guessing the POSIX community could attempt to write an Internet Draft that expressly assigned port zero across all known and unknown transport protocols to the “unspecified service number” semantic that it uses, but my hunch is the need for that draft isn’t very strong (considering the intent declared in RFC 2553 to keep POSIX as the standard API for IPv6 applications) and that getting the language right would not be worth the hassle.

--james woodyatt <jhw@xxxxxxxxxx>