Subject: QoS for the CE "heartbeat" frames in HDLC and PPP PWs

From: Sasha Vainshtein <[email-masked]>
Date: Mon, 8 Sep 2003 11:39:07 +0200

Luca, Fred and all,
I would like to raise the following potential issue 
with HDLC and/or PPP PWs as specified in 
draft-ietf-pwe3-hdlc-ppp-encap-mpls-00.txt 
and in conjunction with application of principles 
stated in draft-baker-diffserv-basic-classes-00.txt.

Let's consider an HDLC or PPP PW as shown below:

                           {-------}                          
            |------\      {DiffServ-}     /-------|           
|-----|     |       \    {  enabled }    /        |     |-----|
| CE-1|<--->|  PE-1 |==={ IP or MPLS }===|  PE-2  |<--->| CE-2|
|-----|     |       /    { network  }    \        |     |-----|
      |     |------/      {--------}      \-------|     |     
      |            |                       |            |     
      |            |<=== A PPP or HDLC ===>|            |     
      |                        PW                       |
      |                                                 |
      |<------ Emulated service (application) --------->     
         across a 'link' containing an HDLC or PPP PW

I consider the following cases of emulated services:
- Cisco HDLC with its keep-alive messages
- A FR service using some kind of LMI
- A PPP service using LCP.

In all these cases some  "heartbeat" frames are exchanged between the CEs
in addition to the CE user traffic. If these frames are treated exactly
as the CE user traffic by the PEs and the PSN experiences a congestion,
they frames may be dropped and, as a consequence, the CEs will
recognize a false "link failure" condition. From my experience,
it is worth preventing such situations.

Please note that, at least in the case of a PPP PW, the PEs can identify
the CE "heartbeat" frames. In case of an HDLC PW such a capability could
be also provided.

If the "CE heartbeat" frames can be identified by the PEs, and if the
PSN supports multiple DiffServ classes , it is possible
to improve availability of the emulated service by assigning a 
DiffServ class with higher reliability properties to the PW
packets encapsulating the "CE heartbeat" frames. However, it is not
clear to me if such a behavior is consistent with the basic PWE3 
principles. (To the best of my knowledge, nothing like this have ever 
been mentioned in any of the PWE3 encapsulation drafts).

I would highly appreciate your opinion on the following questions:

1. Is it acceptable to assign the HDLC or PPP PW packets encapsulating
   the CE "heartbeat" frames to a DiffServ class that is different from
   the class(es) to which the PW packets encapsulating the CE user frames
   are assigned?

2. If the answer to the 1st question is positive, then:
   a) Which DiffServ class (among the classes specified in draft-baker) 
      seems the most appropriate, and what kind of admission rule(s) 
      should be applied to these frames?
    b) What kind of a transport LSP should be used to carry an 
       HDLC or PPP PW across a DiffServ-enabled MPLS network?

With best regards,
                                   Sasha Vainshtein
email:     [email-masked]
tel:       +972-3-7659993 (office)
           +972-8-9254948 (res.)
           +972-58-674833 (cell.)