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.
Yakov Rekhter writes:
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".
And somehow it is different that ATM and frame relay also "break"
traceroute just as much, if by "break" it is meant that one cannot see
the "physical" (not they they are really seeing that) topology?
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
It would be nice were that an option at least.
(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).
Quite.
I won't launch into a dissertation on this topic, but the issue
of decrementing the IP TTL by 1 at the egress LSR/TSR has been
discussed to death (and I argued in favor of a knob to both allow
and disallow this on the MPLS list until I was blue in the face)
with no clear consensus, IMO.
I personally believe that this issue needs to be revisited within
the MPLS wg.
Not decrementing the IP TTL at each LSR hop no more breaks the
IP TTL mechanisms than does frame-relay or ATM.
$.02,
- paul
ps. In fact, there may even be some ISP's that would prefer that
their internal L2 infrastructure remain invisible.