The MPLS WG Archive

[Kireeti Kompella <kireeti@juniper.net>]

Hi Curtis,

> I am surprised to see you take such a hard stance on this.
> 
> (1) There should be no requirement that the midpoints of an LSR do special
> processing for a particular L3PID.  (2) There should also be no
> requirement that an LSR be completely ignorant of the L3PID.

I can live with (2) if you grant me (1).  Note that the L3PID is
only valid (to my thinking) when the stack depth is 1.

> If the MTU is passed back to the ingress of a TE tunnel, then the
> ingress can fragment or generate the ICMP for packets with "don't
> fragment" set.

We're on the same page so far.

> If the L3PID is IPv4, it is perfectly legitimate to send TTL expired
> ICMP if TTL expires.  It is equally legitimate to ignore the L3PID and
> not generate ICMP, or disable TTL decrement at all but the egress for
> TE tunnels (where no loop is ever possible).  LSR that don't generate
> ICMP are simply traceroute unfriendly and the path will pick up again
> as soon as the TTL is large enough to reach the egress of the LSP.

Note that many SPs want tunnels not to show up in traceroute.

> (3) If the L3PID is not IPv4 (for example MPLS) then an LSR should not try
> to guess the type of payload.  Providers who want to retain
> functionality that is only available if the L3PID is known will have
> to set up TE hierarchical tunnels for IPv4 only and other hierarchical
> tunnels to carry "anything else".  This would be true of hierarchical
> tunnels for specific diff-serv CT values so this is no surprise.

Cool, agreed to (3).  The rest is actually a bit tricky.

> The text is therefore fine as is.  Perhaps some minor clarification
> can be made, but it does not make sense to remove this.

Stating (1), (2) and (3) above, and stating that the L3PID is only valid
when the stack depth is 1 would do it for me.

> I also think the MPLS ICMP work is fine as is as long as only the MPLS
> stack is exposed.  For those wishing to hide topology, either ICMP can
> be disabled entirely or access lists can be applied to the IP source
> address field before TTL expired is considered for generation of ICMP,
> yielding better control than most implementation of ICMP for IP TTL.

True.  Different topic, however.  Take a look at the tunnel trace
work (so far, only requirements) -- should be out shortly
(draft-bonica-tunneltrace-00.txt).

Note too for IP VPNs and for BGP-free cores, traceroute will not work
(easily) -- intermediate LSRs don't know whom to send the ICMP error
to.  Yes, one may be able to hack around this, but it ain't pretty.

Kireeti.