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Re: IP over SONET considered harmful?

  • From: Paul Ferguson
  • Date: Fri Mar 20 15:43:37 1998

At 09:33 AM 3/20/98 PST, Yakov Rekhter wrote:

>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".

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.


- paul

ps. In fact, there may even be some ISP's that would prefer that
their internal L2 infrastructure remain invisible.