Subject: [6lowpan] post-IETF-last-call fix: Fixing the value of P

From: Carsten Bormann <[email-masked]>
Date: Mon, 19 Mar 2007 14:10:14 +0100

Section 10.2 of the format document defines the value of P as "TBD",  
waiting for IANA assignment, and section 12 accordingly states:

    This document requests an IANA assignment of a port number P, for  
use
    with UDP header compression (Section 10.2).  This port number P is
    used as the base to which the "short_port" 4-bit values are added in
    order to obtain the actual UDP port used.

This has raised a DISCUSS from the IESG, and from the discussion the  
the 6lowpan meeting today, it is clear that this is incorrect.

There is no intention to actually allocate a port for a specific  
protocol (and if there were, we should be allocating 16 ports in  
sequence, which would require some justification).

Instead, the UDP header compression mechanism just causes port  
numbers from the range [P..P+15] to be compressed better than other  
UDP port nunbers.

Experience with managing small number spaces (cf. Payload Types in  
RTP) suggests this space would not be allocated one-by-one to  
applications, but it should be a dynamic space.  Indeed, there is no  
need to "reserve" the port numbers in the [P..P+15] range --  
receivers are free to put their listeners there and senders are free  
to use these ports as the source ports.

The IANA allocated port number space already has already allocated  
16384 ports for dynamic use, we don't need a new allocation.
The result of the meeting was that the [P..P+15] space should simply  
be somewhere in this space.  We just need to pick a P between 49152  
and 65520.
A highly scientific process run in the meeting room after the close  
of the meeting resulted in the choice of 61616 (0xF0B0) as a good  
value for P.

As a result, I propose the fix below as an expression of the meeting  
consensus.

We now need to know quickly whether there is any problem with this  
minor change.

Gruesse, Carsten

Change the text in 10.2 as follows:

OLD:
       UDP source port (bit 0):
          0: Not compressed, carried "in-line" (Section 10.3.2)
          1: Compressed to 4 bits.  The actual 16-bit source port is
             obtained by calculating: P + short_port value.  P is a
             predetermined port number with value TBD.  The  
short_port is
             expressed as a 4-bit value which is carried "in-line"
             (Section 10.3.2)

       UDP destination port (bit 1):
          0: Not compressed, carried "in-line" (Section 10.3.2)
          1: Compressed to 4 bits.  The actual 16-bit destination  
port is
             obtained by calculating: P + short_port value.  P is a
             predetermined port number with value TBD.  The  
short_port is
             expressed as a 4-bit value which is carried "in-line"
             (Section 10.3.2)

NEW:
       UDP source port (bit 0):
          0: Not compressed, carried "in-line" (Section 10.3.2)
          1: Compressed to 4 bits.  The actual 16-bit source port is
             obtained by calculating: P + short_port value.  The value
             of P is the number 61616 (0xF0B0).  The short_port is
             expressed as a 4-bit value which is carried "in-line"
             (Section 10.3.2)

       UDP destination port (bit 1):
          0: Not compressed, carried "in-line" (Section 10.3.2)
          1: Compressed to 4 bits.  The actual 16-bit destination  
port is
             obtained by calculating: P + short_port value.  The value
             of P is the number 61616 (0xF0B0).  The short_port is
             expressed as a 4-bit value which is carried "in-line"
             (Section 10.3.2)

In 12, strike:

OLD:
    This document requests an IANA assignment of a port number P, for  
use
    with UDP header compression (Section 10.2).  This port number P is
    used as the base to which the "short_port" 4-bit values are added in
    order to obtain the actual UDP port used.

NEW: (no replacement text).

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