[Ietf-behave] Conflicting ICMP payload translation reqs: MUST vs. SHOULD

[Saikat Guha <saikat@xxxxxxxxxxxxxx>]

UDP *RECOMMENDS* that ICMP payloads be translated and forwarded. From
draft-ietf-behave-nat-udp-08.txt:

        The NAT's default configuration SHOULD NOT filter ICMP messages
        based on their source IP address.  Such ICMP messages SHOULD be
        rewritten by the NAT (specifically the IP headers and the ICMP
        payload) and forwarded to the appropriate internal or external
        host.
        
ICMP *REQUIRES* that ICMP payloads be translated and forwarded. From
draft-ietf-behave-nat-icmp-01.txt:

        If the the NAT has active mapping for the embedded payload, then
        the NAT MUST do the following prior to forwarding the packet. 
        a) Revert the IP and transport headers of the embedded IP packet to
           their original form, using the matching mapping; and
    ...


TCP currently aligns itself with ICMP/MUST because receiving the ICMP
messages and being able to link them to the actual connection is
essential for path  MTU discovery.

From my reading of the documents, the requirements in ICMP/TCP and UDP
are in conflict.

Is there any particular UDP has a SHOULD and not a MUST? If so, should
the recommendation be narrowed only to UDP payloads inside ICMP packets?

-- 
Saikat

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