Hi Uma,

I would like share more context on the concerns that I raised on this proposal 
in LSR WG yesterday where we could not complete our discussion on the mike due 
to time constraints.

IGPs were originally invented for topology computation and then route 
programming based on the SPT computed. We've extended IGPs to carry/flood 
information and this includes information that were meant for various 
applications. IGPs always do distribute topology computation - that is the core 

The PPR proposal takes IGPs into the area of flooding p2p paths and then 
setting up forwarding state along the path - essentially introducing path 
provisioning capabilities into it. Essentially adding a new functionality that 
is NOT distributed topology computation.

For clarity, I could summarize the PPR as follows (please correct if wrong):
- Someone (head-end or controller) computes a SR Path which is expressed as a 
SID List (it's a list of EROs just like in RSVP-TE - loose or strict)
- The head-end floods this SR Path (and its EROs) into the IGP domain so all 
routers in the area get the P2P paths computed by all head-ends
- Each router then must look at every such SR Path flooded by every router and 
examine if it is part of the ERO list; if so then it needs to program the 
forwarding state for that PPR id (aka label)
- The headend can then just look at this like a "tunnel" and do something like 
IGP shortcut to the destination behind it

This is picking EROs from RSVP-TE and putting them into IGPs for flooding p2p 
path state pervasively. Consider the kind of flooding scale and challenges when 
all these SR Paths go to every router irrespective of whether they need/use it. 
Then on top of that, we are proposing IGPs to program a local cross-connect if 
they are on that SR Path.

My question is, why not just use RSVP-TE in this case? RSVP-TE does signalling 
but it does it only on the nodes that matter for a specific LSP. 

This is called SR but it introduces a forwarding state on each of the hops 
(i.e. the PPR label cross-connect) - something different from SR architecture. 
Including SPRING group as well for the review of this proposed SR solution.

Seems like a mishmash of SR and RSVP-TE to me and I am concerned about where 
this takes IGPs.


Lsr mailing list

Reply via email to