The MPLS WG Archive

[David Charlap <david.charlap@marconi.com>]

Punj, Arun wrote:
> Matt Squire wrote:
>>
>> What prevents a packet with a router alert flag set from being
>> sent over an LSP?  I don't recall anything in the specs that
>> specifies router alter packets can't be LSP'd.
> 
> Interesting.. Should it not be in the spec? If not, than are you
> not changing the meaning of router alert. If yes, Eric's solution
> seems perfectly OK, although it would require some sort of
> additional support on the intermediate nodes.

Logically, it would not make sense to forward a router-alerted packet
into an LSP.

The purpose of router alert is so that intermediate nodes will closely
examine the packet, and possibly intercept it for processing.  The
purpose of an LSP is to quickly forward a packet with a minimum of
processing by the transit routers.  These are two diametrically opposed
goals.

Nevertheless, I don't think there is anything forbidding a router from
sending a router-alert packet into an LSP.

> Presumably router alert is the reason why the RSVP messages
> are finding the way back in reverse direction!!

RSVP Resv messages are targetted at the PHOP router, so even if they
would be sent into an LSP, they'd still arrive at the same place.

Forward-going messages (Path, PathTear, ResvErr and ResvConf) are sent
with router alert and have the egress router's address in the IP
header.  In theory, they could be forwarded into an LSP, but any router
doing this wouldn't be able to process the messages and would therefore
not be a useful participant in the signaling process.  (Unless that LSP
terminates at the next-hop router.)

-- David