Subject: [mpls-tp] Does draft-ietf-mpls-gach-gal need the A bit?

From: Stewart Bryant <[email-masked]>
Date: Fri, 16 Jan 2009 18:29:39 +0000

Matthew and I have been taking a look at the relationship
between

draft-ietf-mpls-gach-gal and draft-bryant-mpls-tp-tlv.

and the A bit. This is in response to a last call
comment that an existing implementation (which would
ignore) the A bit would be confused by the presence
of a TLV.

Now there is no formal issue of backward compatibility
because the only two ACH values in the registry are

Registry:
Value   Description                                 Reference
-----   ------------------------------------------  ---------
0x21    Associated Channel carries an IPv4 packet   [RFC4385]
0x57    Associated Channel carries an IPv6 packet   [RFC4385]

Neither of which, by definition, would have a TLV.

The only other case where a clash could occur is with
draft-ietf-pwe3-vccv-bfd-02 on codepoint 7 (technically
a prototype). So we need to decide whether a A bit in
the ACH would upset any existing implementation of
this functionality.

If we continue on this path (keep the A bit) we
need to specify that the presence of one or more of
flag bits is part of the definition of each ACH
codepoint and that the ACH type must be checked
before checking the flag (reserved) bits.

However there is also concern about the mutual dependence
between the two drafts since draft-ietf-mpls-gach-gal
needs to normatively reference draft-bryant-mpls-tp-tlv

There is an alternative way of way of implementing this
function. That we need to consider.

We could require each definition of an ACH protocol
to decide whether it may wish to use a TLV or not.

If the ACH is defined to never need a TLV (0x21 for example)
a TLV is never in the packet.

If the ACH is defined as may need a TLV then a TLV
is aways present. However when there is nothing to
put in a TLV the TLV is the NULL TLV (4 bytes
of zero). I suspect that the use of the NULL TLV
will turn out to be quite rare.

So the question is do we really need the A bit?

If we do not we should remove it and thereby remove the
draft dependency?

If we do need it we need to either move these drafts in
parallel (which could include letting the gach draft sit in
the editors queue for a while) or we need to create the
TLV registry in the draft-ietf-mpls-gach-gal but
only populate it in draft-bryant-mpls-tp-tlv.

So the key question is do we need to define the A bit
in draft-ietf-mpls-gach-gal or will its state always
be the same for any given ACH type.

Stewart

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---

From: Carlos Pignataro <[email-masked]>
Date: Thu, 29 Jan 2009 22:00:52 -0500

Stewart,

>From this description it appears that the A-bit might not be needed.
Another potential alternative could be to assign a Channel Type to the
ACH TLV structure, like an extension shim header, and provide a Channel
Type' for the ACH Payload that follows the TLV structure in the ACH TLV
Header.

Thanks,

-- Carlos.