Subject: Re: MPLS PID

From: Eric Rosen <[email-masked]>
Date: Thu, 27 Mar 2003 11:13:23 -0500

Let's try to focus a little. 

1. The  only  reason for  a  PID  field in  MPLS  and/or  PWE3  is to  allow
   intermediate routers to determine whether a particular MPLS payload is an
   IP packet or not. 

   There are a number  of reasons why this is useful, and  no one has argued
   that this is not useful. 

   No one  has provided  a reason  why it might  be useful  for intermediate
   routers to know, if the payload is not IP, what kind of packet it is.  

2. The reason there is  no PID field in the MPLS header  is that there was a
   consensus to keep the header as short as possible.

3. The reason the PWE3 control word is optional is that there is a consensus
   (apparently a continuing one) to  allow service providers to specify that
   the header be kept as short as possible. 

4. My conclusion from 2 and 3 is that there isn't much hope for any solution
   which includes both a PWE3 control word AND a 4-byte MPLS PID field. 

5. There is anyway  no way for the IETF to change  the basic MPLS forwarding
   logic, at least  not in the time  frame over which one would  like to see
   PWE3 deployments.

5. My further  conclusion is that, as Mark has  suggested, the only possible
   outcome  of a  discussion in  the MPLS  WG is  a recommendation  that any
   non-IP application using  MPLS specify a control word  whose first nibble
   is as Stewart has proposed for PWE3. 

6. No one has given  a reason why the first nibble of  any such control word
   should not be as Stewart has proposed.

7. The PWE3 control word can be specified as a mandatory-to-implement option
   which a SP  can disable if his environment is such  he doesn't care about
   the absence of the PID. 

So I really think there is only one possible outcome.  The only issue is how
much time  and breath we want  to waste until we  get there.  A  SP is faced
with the choice of either:

a. Requiring the use of a Martini-style PID, or 

b. Ensuring that his pseudowires only carry non-order-sensitive traffic, or

c. Ensuring that  his PWE3 packets do not pass  through any MPLS environment
   in which load balancing is done. 

Encapsulations which do not use the Martini-style control word are therefore
at a  disadvantage.  We  should make sure  therefore that  any encapsulation
which actually solves a problem for the industry has a control word with the
first nibble as Stewart has proposed.