As Dino pointed out in a private message to me, the fact that the ALT network
is an overlay on the Internet means that it is unlikely that an ALT Router
would not have "native" connectivity. Here's a slightly simplified section
5.3 that eliminates the reference to such an unlikely situation.
--Vince
--------------------
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 would send this ICMP message using "native" Internet
connectivity, not via the ALT overlay.
o A non-LISP-capable ALT Router may also choose to silently drop the
non-forwardable ALT Datagram.
[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.
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