Re: Purpose of Port 0.

Joe Touch <touch@xxxxxxx> · Tue, 21 Feb 2017 12:25:20 -0800



    Hi James,

    
    On 2/21/2017 12:05 PM, james woodyatt
      wrote:

    
      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.
    
    
    Per RFC6335, there is one registry for all transport port numbers.
    "RESERVED" in that registry applies to all transports.

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

    
    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.
    Per section 14.5, SCTP uses the existing transport port registry. It
    lists a set of assignments starting at port 9, but does not request
    assignment of 0.

    
    Section 3.1 prohibits the use of 0 as either source or destination
    port, but that's not an assignment. That's a prohibition, and not
    indicated as a reservation in the IANA Considerations (Sec 14).

    
      p4. Port zero is not reserved for DCCP by RFC 4340,
        so port zero remains Unassigned there at this point.
    
    
    It remains RESERVED (as per the ports registry) but potentially
    assignable to DCCP.

    
        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.
      
    
    "RESERVED" would involve IANA Considerations, and that act would
    have resulted in their being recorded in the IANA registry.

    
    Prohibitions within the protocol do not result in IANA actions.

    
      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, 
    
    
    IMO, an RFC that needs an API to pass the value "unspecified" should
    indicate just that. It would be far out of scope for an RFC to
    indicate the *value* passed in an API to indicate that parameter.

    
    Joe