Resuming this thread…the authors were kind enough to meet with me and educate 
me on how Area Proxy works. The resulting conclusions are:

1)Area Proxy TLV is meant to  be sent ONLY in L1L2 LSPs (NOT Proxy LSPs).

2)Advertisement of Area SID in the Area Proxy TLV is needed so that Inside 
Nodes can install a receive entry for that SID

3)Advertisement of Area SID in Binding TLV is needed in Proxy LSPs so that 
outside nodes can install a forwarding entry towards the Inside Node(s).
This SID MUST be identical to the SID advertised in the Area Proxy TLV in L2 
Use of the Binding TLVs to do this is the current choice.

4)Other uses of an Area SID not related to Area Proxy are outside the scope of 
the Area Proxy draft. However, as the concept seems potentially useful in other 
scenarios, being able to advertise an Area SID in the Binding TLV allows for 
these other uses.
But, any advertisement of Area SID in Binding TLV which appears in L1L2 LSPs is 
unrelated to Area Proxy. Whether that SID is the same value as that used for 
Area Proxy is something future use cases for Area SID will have to decide.

I therefore think it makes sense to allow advertisement of the Area SID in both 
the Area Proxy TLV and the Binding TLV, subject to the constraints above. This 
will NOT result in “duplicate advertisements”.

Further details regarding the encoding will need to be provided in future 
revisions of the draft.

   Les (both as WG member and Designated Expert for IS-IS TLV Codepoints 

From: Lsr <> On Behalf Of Les Ginsberg (ginsberg)
Sent: Wednesday, July 08, 2020 11:19 AM
Subject: Re: [Lsr] New Version Notification for 

Tony –


From: Tony Li <<>> On Behalf 
Sent: Wednesday, July 08, 2020 11:12 AM
To: Les Ginsberg (ginsberg) <<>>
Subject: Re: [Lsr] New Version Notification for 

Hi Les,

Again, the subTLVs of the area proxy TLV are for the coordination of the Inside 
Area. The Area Proxy TLV appears in the Inside Node’s normal LSP.

The Proxy LSP is for informing the Outside Area.

[Les:] Understood – but I do not see why this requires you to advertise the SID 
in two different TLVs. As you allow the Binding SID TLVs to be advertised in 
both standard LSPs and Proxy LSPs, there seems to be no need for two different 
TLVs to include the advertisement.

The semantics are completely different. Advertising it in the Binding TLV is 
permission to use the SID for packet forwarding.

The advertisement in the Area Proxy LSP is to instruct the Inside Nodes to use 
that SID to establish the forwarding state.
[Les:] Sorry, I do not see why this requires you to advertise the SID in two 
different places.
The SID gets advertised in one place (which TLV is TBD for the moment). The 
instruction to enable use of it for Area Proxy comes in the Area Proxy TLV. 
That only requires (at most) a bit – and perhaps nothing other than the 
presence of the Area Proxy TLV. It certainly does not require the SID itself to 
be readvertised.

Let’s see if we can agree that the SID only needs to be advertised in one place 
before proceeding further.


[Les:] Yes – of course – this is pathological. (Probably not hilarious to the 
customer. 😊 )

No doubt, but I’ve found that if one doesn’t regard bugs with some humor, our 
profession would be morose, in the extreme.

I am just saying by having two sources for the advertisement you introduce the 
possibility of inconsistency – and the spec would have to speak to this – even 
if it should not happen.

Ummm…. Not sure what I can say other than “don’t write bugs”.  But ok, I’ll 
happily do so.

We are abusing the definition of the Binding TLV.  2.4 says: "The SID/Label 
Binding TLV may be used to advertise prefixes to SID/Label mappings.”  That’s 
not what we’re doing, so it’s not too surprising that there’s some conflicts.

[Les:] Yes – the Binding TLV has some issues being generalized. There is 
history here as to why the format is the way it is and why it isn’t more easily 
extensible – and that is open for discussion AFAIAC, but we cannot break 
backwards compatibility for SR.

Agreed, trying not to. :-)

But I am also responding (in part) to your desire to make the Area Segment SID 
a more general tool – usable outside of Area Proxy – which seems like a good 

And you’ve requested that we put the Area Segment SID in the Binding TLV.  I’ve 
tried to do so and you don’t like how I’ve done it.  Please tell me what I can 
do that will satisfy you.  From what you’ve said so far, there is no legal way 
to use the Binding TLV.

Catch-22.  Yossarian!


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