The MPLS WG Archive

[Ping Pan <pingpan@juniper.net>]

"Punj, Arun" 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.
> 
> > Eric Rosen wrote:
> >
> > > George> The mechanism is  intentionally tied to the control
> >  plane to ensure
> > > George> that the source  is properly informed even if  the
> > reverse data path
> > > George> may be broken.
> > >
> > > If  the reply is  encapsulated in  a UDP  packet which  is
> > addressed  to the
> > > tunnel head and has router alert  set, isn't this just as
> > effective, without
> > > the dependence on the control plane?
> >
> 
> 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.
> 

Arun,

In fact, this idea of using IP Router Alert option had been discussed
extensively before the submission of the LSP-ping draft. 

The problem with this approach is: there is no guarantee that the
returning path (with or without IP Router Alert) won't go over an LSP.
If that LSP is bad, we are in trouble.

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

No. RSVP messages find the way back in reverse based on PHOP installed
at each LSR by RSVP Path, and has nothing to do with IP Router Alert.

- Ping