Bruno -
Please see inline.
From: Lsr <lsr-boun...@ietf.org> On Behalf Of bruno.decra...@orange.com
Sent: Tuesday, August 04, 2020 5:45 AM
To: lsr@ietf.org
Subject: [Lsr] draft-ietf-lsr-isis-area-proxy-02
Hi,
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<https://tools.ietf.org/html/rfc8667#section-2.4..4> 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<https://tools.ietf.org/html/rfc8667#section-2.4.5> 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
TLV."
https://tools.ietf.org/html/draft-ietf-lsr-isis-area-proxy-02#page-14
By definition, legacy L2 external node will support vanilla RFC 8667, which
says:
"The Prefix-SID sub-TLV MUST be present in the SID/Label Binding TLV when the
M-Flag is clear."
https://www.rfc-editor.org/rfc/rfc8667.html#name-sid-label-binding-tlv
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 https://www.rfc-editor.org/rfc/rfc8667.html#section-2.1 and the SID/Label
sub-TLV defined in https://www.rfc-editor.org/rfc/rfc8667.html#SIDLABELSUBTLV
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 =
4).
I see no issue here.
I would also point out that the new Area Proxy SID sub-TLV (
https://tools.ietf.org/html/draft-ietf-lsr-isis-area-proxy-02#section-4.3.2 )
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
SID.
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.
Les
Thanks,
Regards,
--Bruno
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