> Yes, and this is good. But FWIW -- and maybe this is one reason why the
> issue has been unclear to me -- Section 4.2 seemed to talk about this only
> in the context of aggregate prefixes and their holes. And it does talk
> about only ETRs, not ALT routers. Would it be possible to add something
> from your above description to the text?
It turns out that there was a placeholder for a section 5.3 that was
intended to describe special processing by intermediate ALT routers. Here's
what I propose to put there to describe forwarding failures:
| 5.3. ALT Datagram forwarding falure
|
| Intermediate ALT routers, forward ALT Datagrams using normal, hop-by-
| hop routing on the ALT overlay network. Should an ALT router not be
| able to forward an ALT Datagram, whether due to an unreachable next-
| hop, TTL exceeded, or other problem, it has several choices:
|
| o If the ALT router understands the LISP protocol, as is the case
| for a Map Resolver or Map Server, it may respond to a forwarding
| failure by returning a negative Map-Reply, as described in
| Section 4.2 and [LISP-MS].
|
| o If the ALT router does not understand LISP, it may attempt to
| return an ICMP message to the source IP address of the packet that
| cannot be forwarded. Since the source address is an RLOC, an ALT
| router can only do this if it has "native" Internet connectivity
| in addition to its ALT overlay network connectivity.
|
| o A non-LISP-capable ALT router that does not have "native" Internet
| connectivity will drop the packet.
|
| [LISP] and [LISP-MS] define how the source of an ALT Datagram should
| handle each of these cases. The last case, where an ALT Datagram is
| silently discarded, will generally result in several retransmissions
| by the source, followed by treating the destination as unreachable
| via LISP when no Map-Reply is received. If a problem on the ALT is
| severe enough to prevent ALT Datagrams from being delivered to a
| specific EID, this is probably the only sensible way to handle this
| case.
|
| Note that the use of GRE tunnels should prevent MTU problems from
| ever occurring on the ALT; an ALT Datagram that exceeds an
| intermediate MTU will be fragmented at that point and will be
| reassembled by the target of the GRE tunnel.
Does this address your concerns?
--Vince
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