Subject: Expanded ERO draft

From: Vach Kompella <[email-masked]>
Date: Tue, 27 Feb 2001 09:34:49 -0800

I apologize if this has been discussed ad nauseam.  I looked through the
archives before venturing out on the subject, and couldn't find an
(explicit) answer to the question: why isn't the router ID in the ERO, and
how come 6.3 of the rsvp-unnum draft says:
	"If, after processing and removing all initial subobjects in the ERO
that refer to itself, the receiving node find a subobject of type Unnumbered
Interface ID, it determines the next hop as follows..."

I can understand the router ID not being there makes it more concise, at the
expense of being a little less readable, and requiring a little more
processing at the receiving end.  In the interest of clarity and easier
processing, I'd have eaten the 4 byte hit.  But that's something the WG can
vote (has voted?) on.

The other aspect is the first clause of the quoted sentence above from the
rsvp-unnum draft: "all initial subobjects that refer to itself."  Suppose
the ERO is made up of Unnum links (1, 1, 1, 1, 1), i.e., a series of hops,
each of which used their Interface #1 outbound.  Processing according to
6.2, a receiving node would strip out the first subobject, after validating
that it received the ERO over its PHOP's interface #1.  Then, by the
statement above, it looks at the next one which coincidentally appears to
refer to itself also.  If I'm not missing something here, this first hop
removes all the subobjects before it is done, which is not what is was
intended.  With a router ID in each subobject, the ERO would look like ((r1,
1), (r2, 1), (r3, 1), (r4, 1), (r5, 1)), which would remove the ambiguity.

Can someone clarify?  Thanks.

-Vach

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From: Yangguang Xu <[email-masked]>
Date: Tue, 27 Feb 2001 13:35:44 -0500

ERO should contain the router ID for several reason:

1) Simplify receiving node operation and avoid the ambiguity as Vach and Bora
mentioned

2) Keep ERO operation consistent. (A node typically strip off all information of
itself before sending to next hop, so the outbound interface ID in ERO should be
stripped off)

3) Consistent format of unnumbered FA and unnumbered link. These two types of
links don't need to be differentiated especially if the unnumbered FA is through
an circuit switched transport network.

4) For CR-LDP, we don't need to invent another PHOP TLV

Yangguang

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From: Yakov Rekhter <[email-masked]>
Date: Tue, 27 Feb 2001 11:46:29 -0800

Vach,

> Can someone clarify?  Thanks.

The text needs to be clarified to say something like "all initial subobjects
of type other than Unnumbered Interface ID"...".

Yakov.

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From: Yakov Rekhter <[email-masked]>
Date: Tue, 27 Feb 2001 11:51:38 -0800

Yangguang,

> 1) Simplify receiving node operation and avoid the ambiguity as Vach and Bora

The ambiguity pointed by Vach is the spec ambiguity, and will be addressed
in the next revision of the Internet Draft.
> 
> 2) Keep ERO operation consistent. (A node typically strip off all information
> of itself before sending to next hop, so the outbound interface ID in ERO 
> should be stripped off)
> 
> links don't need to be differentiated especially if the unnumbered FA is 
> through an circuit switched transport network.

What makes you think that the format is different ?

Yakov.

---

From: Kireeti Kompella <[email-masked]>
Date: Tue, 27 Feb 2001 12:40:59 -0800 (PST)

Vach, Yakov,

> > rsvp-unnum draft: "all initial subobjects that refer to itself."
> 

While the change in text is possible, it is clear that, unless the
interface is physically looped back to itself, the second subobject
does *not* refer to "itself".

             1       1       1
         A ----- B ----- C ----- D

B receives an ERO of (1, 1, 1) from A (PHOP).  B checks that (A, 1)
refers to itself, removes it.  Now it sees unnum interface 1, which
clearly points at C.  Why would B think that this "1" refers to B?

Kireeti.

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From: Eric Gray <[email-masked]>
Date: Tue, 27 Feb 2001 15:47:33 -0500

Kireeti,

    Out of curiosity, how many un-numbered interfaces can
B have that are known to it as interface 1?

--
Eric Gray

---

From: Kireeti Kompella <[email-masked]>
Date: Tue, 27 Feb 2001 13:11:49 -0800 (PST)

Hi Eric,

> B have that are known to it as interface 1?

One *outgoing* interface.  To clarify, in the picture below,
the LSP goes left to right, and the interface indices are
assigned by the LSR to the left.  So, the index on the AB
link was assigned by A.  B of course also could have assigned
an index to the BA link; if so, the value should be distinct
from any other index assigned by B.  So, the BA link could not
(in this instance) get an index of 1.  However, B could assign
1 to the BA link if it assigns 2 to the BC link.

             1        1        1
         A -----> B -----> C -----> D

Kireeti.

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From: Eric Gray <[email-masked]>
Date: Tue, 27 Feb 2001 16:34:33 -0500

Kireeti,

    Thanks, and thanks to Vach as well.

    I guess the bottom line is - what Yakov said applies:
i.e. (paraphrasing) all self references refers to references
other than un-numbered interface IDs.  This might be a
point that would be covered in the BCP you mentioned...

--
Eric Gray

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From: Bora Akyol <[email-masked]>
Date: Tue, 27 Feb 2001 13:40:08 -0800

Or alternately, one can add the router ID to the unnumbered ERO to 
remove any ambiguities and also simplify fast reroute.

Bora

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From: Yangguang Xu <[email-masked]>
Date: Tue, 27 Feb 2001 16:51:21 -0500

This is what I proposed in draft-xu-ccamp-gmpls-sig-reorg-00.txt  Make it simple
and put related pieces of information into one meaningful unit. I don't know why
we have to design something complicate intentionally.

Yangguang

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From: Kireeti Kompella <[email-masked]>
Date: Tue, 27 Feb 2001 14:24:29 -0800 (PST)

> remove any ambiguities and also simplify fast reroute.

As Yakov said, no concrete reason for adding the router ID has been
given.  However, we've wasted enough time and bandwidth.  Let's put
this to vote:

Who all think that adding a router ID to the unnumbered ERO subobject
would be beneficial?  A simple yes, no or don't care, please.

Kireeti.

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From: Yangguang Xu <[email-masked]>
Date: Tue, 27 Feb 2001 18:28:04 -0500

Kireeti,

Would you please tell me which one is simpler and more straight forward?

Isn't being simpler and more straight forward a concrete reason?

Yangguang

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From: Bora Akyol <[email-masked]>
Date: Tue, 27 Feb 2001 16:08:39 -0800

You know my answer already, I think adding the router ID simplifies 
code, and is much clearer.

It is 4 bytes.

Bora