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PS:
Joe Touch wrote:
> I had a question about the current state of draft-ietf-mpls-icmp-03:
>
> I'm concerned about the issue of backward compatibility, notably the
> ways in which the use of the extensions proposed will cause existing
> ICMP processing to break, notably binding the length of the final field
> to 128 bytes, esp. considering RFC1812 recommends:
>
> Therefore, the ICMP
> datagram SHOULD contain as much of the original datagram as possible
> without the length of the ICMP datagram exceeding 576 bytes
>
> Given that, it seems that this new variant of ICMP message (which
> includes headers before IP, rather than IP and thereafter - which is
> curious enough in itself) ought to demand a new message type, which
> necessatates use of the "Parameter Problem" code, or the definition of
> other new codes. Inside those, the new format that uses a list of
> pointers to include MPLS information might be appropriate.
One possible solution to the use of new codes, and the need for
compatibility with old programs:
Send two ICMP error messages
I.e., for a particular message, send
ICMP type 3 code 0 net unreachable
with regular IP packet as payload
ICMP type 16 (e.g., to indicate MPLS)
with internal fields that equate to the
conventional type (3) and code (0),
with a payload that includes the MPLS
header and pointer where the error occurred
as well as the IP packet as payload
The general rules would be:
a) any program that interprets MPLS-enabled messages
MUST NOT also act on or interpret the equivalent
non-MPLS messages
b) routers MUST decrease the rate at which they send
errors, as an aggregate, by at least half, to
compensate for doubling the number of messages
i.e., an MPLS-enabled router MUST NOT send more of
the dual-message errors than an equivalent
non-MPLS router would send
Does that help?
Joe
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