Bruno -

Please see inline.

From: Lsr <> On Behalf Of
Sent: Tuesday, August 04, 2020 5:45 AM
Subject: [Lsr] draft-ietf-lsr-isis-area-proxy-02


I may be missing something but the SR Binding SID TLV extension  is not clear 
to me.

  1.  It does not seem compliant with RFC 8667

Draft says that the advertisement has: T-flag set, M & A flags cleared, 
SID/Label sub-TLV present, Prefix-SID sub-TLV NOT present

The following extensions to the Binding TLV are defined in order to

   support Area SID:

      A new flag is defined:

         T-flag: The SID directs traffic to an area.  (Bit 5)

         When T-flag is set:

            M and A flag MUST be clear

            Range and Prefix are ignored

      Section 2.4.4 of RFC 
8667<> is altered to say:

         "The Prefix-SID sub-TLV MUST be present in the SID/Label

         Binding TLV when the M-Flag and T-flag are both clear.  The

         Prefix-SID sub-TLV MUST NOT be present when either the M-Flag

         or T-flag are set."

      Regarding the SID/Label sub-TLV Section 2.4.5 of RFC 
8667<> is

      altered to say:

         "It MUST be present in the SID/Label Binding TLV when either

         the M-Flag or T-flag is set in the Flags field of the parent


By definition, legacy L2 external  node will support vanilla RFC 8667, which 
"The Prefix-SID sub-TLV MUST be present in the SID/Label Binding TLV when the 
M-Flag is clear."

So it seems that the extension violates the above MUST in RFC8667, as regarding 
the Prefix-SID sub-TLV

  *   Area proxy says "MUST NOT be present" (as T-flag is set)
  *   RFC 8667 says "MUST be present" (as M-flag is cleared)

In addition to the above, legacy node _will_ interpret the 'Range' and 'Prefix' 
fields. So there is probably a need to specify which values need to be 
advertised for those legacy nodes. A priori range would be one as a single SID 
is advertised. Prefix seems more problematic as you need to find an IP prefix 
to advertise. And please let's avoid SID conflict and Prefix conflict...
[Les:] Format of the Binding TLV when the new T-bit is set is similar to the 
format when the M-bit is set in that Prefix-SID sub-TLV is NOT present.
A legacy node parsing the Binding TLV would be looking for the Prefix-SID 
sub-TLV (M-bit NOT set) and would not find it. The contents of the Binding TLV 
would therefore be unusable to a legacy node.
The correct behaviour for a legacy node would be to (optionally) report an 
"invalid TLV" and to ignore the TLV.

  1.  It's not clear to me whether the segment/SID is global or local.
As per my understanding of the draft-ietf-lsr-isis-area-proxy use case, the 
area-proxy SID would be global (in the external L2): "Area SID which will 
direct traffic to any of the Inside Edge Routers."

But the SID/Label Sub-TLV used by area-proxy has no flag (L-flag) indicating 
whether the SID is global or local. One could argue that if it carries a label 
it's a local SID and if it carries and index it's a global SID. But this has 
not been specified.
It has also no "algorithm" indicating how it needs to be routed global, so at 
minimum would not work with different routing algo/flex algo.
I'm not seeing in RFC 8402 or 8667 any text saying that such SID would be 
global hence globally routed in the L2 domain. (To me, this IS-IS SID was 
local, but arguably also can't find text stating this).

[Les:] There is a subtle difference between the Prefix-SID sub-TLV as defined 
in and the SID/Label 
sub-TLV defined in

The Prefix-SID sub-TLV has a flags field which includes V-bit/L-bit to indicate 
whether the variable length field which follows is a 3 byte label (both bits 
set to 1) or a 4 byte index (both bits set to 0).

The SID/Label sub-TLV has no flags field. The length of the sub-TLV indicates 
whether the advertised value has is a label (length = 3)  or an index (length = 

I see no issue here.

I would also point out that the new Area Proxy SID sub-TLV ( ) 
does include V/L bits - similar to the Prefix-SID sub-TLV.

At minimum, area-proxy would need to specify whether the SID is global and 
local. And if global, with which hard coded algorithm it is routed. (I would 
assume "0")

[Les:]Just as the Mirror SID has no algorithm associated, neither does the Area 
If you feel that is an issue, please expand on how you intend to use an 
algorithm specific Area SID.
Thus far you seem more inclined to use an anycast  Prefix-SID, so I am not 
clear on what you think is needed here and why.
I would agree that if algorithm is required it is currently not available - but 
it is not yet clear that algorithm is required.




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