Subject: Comment on draft-martini-pwe3-mh-pw-requirements-01.txt

From: David McDysan <[email-masked]>
Date: Wed, 09 Mar 2005 10:48:21 -0500

>From section 6.1 of the cited draft:

     " -vii. MS-PWs are composed of SS-PW, and SS-PW are bi-directional,
            therefore both directions of a PW segment MUST terminate on
            the same S-PE/U-PE."

This requirement can be read in at least the following two ways:
	1) both directions of a PW segment must terminate on the same S-PE
connected to a U-PE at each end of the pseudowire
	2) both directions of every PW segment must terminate on the same
S-PE to S-PE AND on the same S-PE connected to a U-PE at each end of the
pseudowire

I propose that the requirement be clarified to state that there must be a
means to force either of the above conditions. However, there are cases
where this would be suboptimal and more expensive than an approach that did
not impose such a restriction and would like to see at least an option for
routing that does not force traversal of the same set of S-PEs in each
direction such that higher utilization results.

As motivation, below is a simple example that illustrates this point. From
section 6.3, consider the case of pseudowires with asymmetric Quantity of
Service:

         "-v. The PW signaling MUST enable separate traffic parameter
            values to be specified for the forward and reverse
            directions of the PW."

Assume a large metro access network.  A U-PE is connected to two S-PEs via
100 Mbps links. This U-PE only has an MH PW LDP session to these S-PEs for
scalability reasons as described in a use case in the draft. There are three
PW's to be provisioned:
	A) 95 Mbps to this U-PE from some other U-PE 
	   10 Mbps from this U-PE to some other U-PE
	B) 10 Mbps to this U-PE from some other U-PE 
	   95 Mbps from this U-PE to some other U-PE
	C) 90 Mbps to this U-PE from some other U-PE 
         90 Mbps from this U-PE to some other U-PE 

If the same S-PE must be traversed in each direction for each PW, then only
two of the PWs can be established. 

If a different S-PE can be traversed in each direction for at least one PW,
then all three PW's can be established.

Similar situations may also occur between intermediate S-PEs when asymmetric
capacity is required with the result that suboptimal utilization could
result. There may be other considerations (e.g., OAM) that require that the
same set of S-PEs be traversed for certain PW types that may offset the cost
such suboptimality. I expect that TDM and ATM PWs may require traversing the
same set of S-PEs in each direction for OAM segment style processing, but
other PW types such as FR, Ethernet, PPP, and HDLC may not have an OAM
requirement to traverse the same set of S-PEs in each direction.

Dave