Subject: [AVT] RE: draft-ietf-avt-hc-over-mpls-protocol-05.txt

From: "Gray, Eric" <[email-masked]>
Date: Thu, 18 May 2006 15:13:03 -0400

Andy,

	I've "snipped" the points on which we appear to agree, or 
where no further comment seems necessary. 

--- [SNIP] ---
--> >COMMENTS:
--> >========
--> >
--> >On page 8, next to last paragraph, it provides the following
--> >example:
--> >
--> >   "For example, if two different compressors, HCa and HCb, both
--> >    assign the same CID to each of 2 separate flows destined to
--> >    decompressor HDc, HDc can still differentiate the flows and
--> >    locate the proper decompression context for each, because the
--> >    tuples <PWlabel-HCa, CID> and <PWlabel-HCb, CID> are still
--> >    unique."
--> >
--> >This is an example, yet I cannot find the explicit statement in
--> >the protocol portion of this specification (nor in the base PWE
--> >signaling specification RFC 4447) that requires the LSR (which
--> >will be performing de-compression) to ensure that the label it
--> >distributes via LDP for a channel is unique.  The example assumes
--> >perfectly reasonable behavior that does not appear to be required.
--> 
--> This draft uses LDP PW signaling to acquire a PW label for each 
--> compression context. Because RFC 4447 requires that PW labels be 
--> allocated from the per-platform label space, every compression 
--> context between a pair of LSRs will have a unique label.
--> 

It is conceivable, in some cases, that an LSR _might_ re-use the 
same label for multiple pseudo wires (even though the platform
wide label space is used) in - for example - cases where a strict
point-to-point connection is not what the distributing LSR is 
interested in setting up.  For example, this _may_ be something 
an LSR could do for Ethernet pseudo wires.

The issue is that I do not know that this is prohibited in RFC
4447, and I am pretty sure it is not explicitly forbidden in
your draft.  Yes, there is an example indicating why this will
not be a good idea.  However, not everybody reads the examples
and - as the saying goes - what is not explicitly forbidden is
allowed.

(My concern is for implementers that might miss the example -
 the problem scenario identified there is not something that
 is likely to arise frequently in testing and verification.)

--- [SNIP] --- 
--> 
--> >MINOR COMMENTS:
--> >==============
--> >
--> >In Figure 3, the delimination of "header" should be removed from
--> >both the Link Layer and PHY portions of the diagram.  It is more
--> >accurate simply to show that the PDU at that level is larger by
--> >some unspecified amount.  The actual formats for a variety of L2
--> >and L1 PDUs is nowhere near as simple as this figure indicates.
--> >
--> >Figure 3 is on page 12 in version "05".
--> 
--> I can agree for the physical layer.  However, I'm not aware 
--> of a link layer defined  for MPLS packets that doesn't have 
--> a header.
--> 

See 

http://www.usc.edu/dept/engineering/eleceng/Adv_Network_Tech/Html/datacom/sl
d024.htm

for an example description of the Ethernet Frame format.  Note
that - in addition to a header - this format includes a trailer
(CRC + Postamble).

See

http://mia.ece.uic.edu/~papers/volans/ppp-configuration.html

for an example description of the PPP frame format.  Note that 
it also includes a trailer (FCS + Flags).

I do not know if you're concerned about ATM or Frame-Relay as
an L2 for this specification.

In addition, as you note in the document itself, padding may
be included in some of these formats as well.

My point is that - by explicitly delimiting "header" - you're
being less accurate than if you did not do so (bad information
is worse than no information).

--- [SNIP] ---

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: "Andrew G. Malis" <[email-masked]>
Date: Fri, 19 May 2006 09:30:00 -0400

Eric,

>  is likely to arise frequently in testing and verification.)

OK - I see you point.  We'll add to section 5.1 that unique PW labels 
MUST be allocated for each context.

>--> >MINOR COMMENTS:

How about if we add a trailer at the link layer?  All of the link 
layers have trailers (even ATM, since AAL5 encapsulation is being 
used).  But really, I think people get the point without the picture 
being exact.

Cheers,
Andy

---

From: "Andrew G. Malis" <[email-masked]>
Date: Fri, 19 May 2006 09:30:00 -0400

Eric,

Cheers,
Andy