Subject: RE: [PWE3] draft-ietf-pwe3-cw-04.txt

From: "Yaakov Stein" <[email-masked]>
Date: Wed, 8 Jun 2005 17:32:03 +0200

> Add the text:
> 
>  When IP is used as the PSN and L2TPv3 is used as the sesssion
>    management control plane, the PW Associated Channel is 
>    identified by the following header:  
> 
>    0                   1                   2                   3 
>    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
>    |1 0 0 0| FmtID |   Reserved    |         Channel Type          | 
>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  
> 
>        Figure 4: PW Associated Channel Header

Isn't the CW draft ONLY for the MPLS case?

Y(J)S

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From: Carlos Pignataro <[email-masked]>
Date: Tue, 14 Jun 2005 09:41:53 -0400

> Isn't the CW draft ONLY for the MPLS case?

It may seem so from the draft title and the motivation to avoid ECMP
treatment in MPLS PSNs. However, I do not fully understand the reason
cited to out-of-scope the L2TPv3 case. From the paragraph:

   Note that L2TPv3 [RFC3931] has its own mechanisms for providing this
   associated channel, and is therefore out of the scope of this
   document.

RFC3931 Section 4.4 defines a keepalive (Hello) mechanism over the
control connection for dead peer detection, but that's not a per-PW
Associated Channel to run for example OAM. There's no PW-ACH for L2TPv3
other than what's defined in VCCV draft Section 7.

In addition, there are currently 2 "Control Words" (L2-Specific
Sublayer) defined for L2TPv3: Default and ATM-Specific. Both share the
following format (Note the first bit set to zero to differentiate from
PW-ACH):

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|S|Frg| Flags |          Sequence Number                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Figure X: PW Preferred L2TPv3 L2-Specific Sublayer

Adding this preferred format for l2tpv3 CWs in Section 3 would help in
ensuring future L2-Specific Sublayers comply with it and help in the
processing of common fields for different PW Types. At least, a MUST for
a "Generic PW L2TPv3 L2-Specific Sublayer" including the first bit
cleared would allow for defining the PW-ACH for L2TPv3 in Section 4.

And VCCV uses the first bit to identify the PW-ACH for L2TPv3, such that
 the PW-ACH can be identified by it and have the following format as
suggested by Scott:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1 0 0 0| FmtID |   Reserved    |         Channel Type          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Figure X: PW Associated Channel Header for L2TPv3

Because of the above, it seems to me that it would be most valuable to
add a text like the one Scott suggested in Section 4 of pwe3-cw-04.
Additionally, it would also add value to include the "PW Preferred
L2TPv3 L2-Specific Sublayer" (or at least mention the first bit be zero)
in Section 3. The rational would be to ensure future L2-Specific
Sublayers adhere to these 2 formats.

Thanks,

--Carlos.

-- 
--Carlos.
Escalation RTP - cisco Systems