Comments on WIMP and MTU Handling

[Margaret Wasserman <margaret@thingmagic.com>]

As I just discussed at the meeting mic, some of the multi6 proposals 
that involve a mid-Internet layer wedge (including WIMP and NOID) 
have an issue with IP fragmentation and reassembly.  If an ID/Loc 
layer will exist conceptually below the fragmentation/reassembly 
function, it is problematic for it to add optional bytes to the 
packet (such as a header or IP option that may or may not be 
included).  There are some possibilities for dealing with this:

    - Run the ID/Loc separation between the Network and Transport 
layers instead.
    - Reduce the MTU presented to upper layers, so that there is 
always room for the optional header.

If you choose the second option, though, there is no advantage to 
mechanisms that try to avoid the need to add a header (such as the 
special next header values used in NOID).

WIMP also adds another twist on this problem by forbidding 
fragmentation of packets that contain WIMP headers.  There is no 
indication of how/if the upper layers will cooperate to make this 
possible, or what impacts this requirement might have on Path MTU 
discovery.

Margaret