IP over SONET considered harmful?

Fri Mar 20 17:33:17 UTC 1998

Alan,

> Subject: IP over SONET considered harmful?

[clipped...]

>   I'd hoped that MPLS would solve this problem, but from reviewing
>   the drafts I believe that the LSRs _WILL_ decrement the TTL.

To be more precise, the issue is that an ingress LSR is required to
copy IP TTL into Tag-TTL, *and* the egress LSR is *required* to copy
Tag-TTL into IP TTL. The problem you mentioned in your message would be
solved if the egress LSR would just decrement IP TTL by 1, rather than
copying Tag-TTL into IP TTL. However, doing this introduces another
problem - it breaks traceroute. And there are enough folks in the MPLS
WG who think that the ability to traceroute through all the LSRs is an
"unalienated right".

In view of the above here are some of the possible avenues:

(a) try to get "rough consensus" with the MPLS WG to allow
    decrement IP TTL by 1 on egress (rather than copy Tag TTL
    into IP TTL), or

(b) talk to your favorite vendor(s), and ask the vendor(s) to put
    a "knob" that would decrement IP TTL by 1 on egress (rather
    than copying Tag TTL into IP TTL).

Yakov.