Re: [Idr] Few questions about Segment Routing extensions for BGP LS (draft-ietf-idr-bgp-ls-segment-routing-ext)

Boris Hassanov <bhassanov@yahoo.com> Wed, 21 July 2021 12:23 UTC

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Date: Wed, 21 Jul 2021 12:22:46 +0000 (UTC)
From: Boris Hassanov <bhassanov@yahoo.com>
To: Robert Raszuk <robert@raszuk.net>
Cc: IDR List <idr@ietf.org>
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Subject: Re: [Idr] Few questions about Segment Routing extensions for BGP LS (draft-ietf-idr-bgp-ls-segment-routing-ext)
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 Hi Robert,Glad to hear you. I do agree that Anycast SID is a subset of Prefix SID. I got your point that indirectly controller can realize that this is an Anycast SID  if he gets several identical Prefix SIDs  which do belong to many routers.But such approach creates additional burden on controller side (parsing and comparison those Prefix SID TLVs in many updates etc.) and might create wrong results in such interpretation.I would offer more direct and simpler way: one additional Sub-TLV for Prefix SID TLV , it could be called like Prefix SID differentiation sub-tlv, and will have some special values for Anycast SID, Prefix SID and may be others for future SIDs. Alternative way could be  extension of existing  Prefix Attribute Flags TLV by adding special Anycast flag there.
Regarding 2nd question, I checked  4 major vendors BGP-LS implementations in our lab with SR-MPLS and all of them do use only Prefix SID TLV for sending their Node SIDs. That was a reason for  my confusion about SID/Label TLV usage.
Thank you.
SY,Boris 

 
    On Wednesday, July 21, 2021, 02:20:24 PM GMT+3, Robert Raszuk <robert@raszuk.net> wrote:  
 
 Boris,
IMO Anycast SID is a Prefix SID. Just like anycast address is an IPv4 or IPv6 address. What makes it an anycast is the fact that it is advertised from more then one node. So anycast in IP or SR is a deployment construct.
In other words node advertising it may have no clue that it is anycast. And the moment other nodes from the anycast group stop advertising it - is it no longer anycast. 
To your other question it is my understanding that you use SID/Label TLV when you have SR-MPLS, and for SRv6 you use Prefix SID TLV. 
Best,
R.
On Wed, Jul 21, 2021 at 11:57 AM Boris Hassanov <bhassanov=40yahoo.com@dmarc.ietf.org> wrote:

Hi all,
1) Need an advice, I might be missing something but I see that Anycasi SID (3333 in the example below)  is in BGP-LS update as regular Prefix SID TLV: Path Attribute - BGP-LS Attribute
            Flags: 0x80, Optional, Non-transitive, Complete
                1... .... = Optional: Set
                .0.. .... = Transitive: Not set
                ..0. .... = Partial: Not set
                ...0 .... = Extended-Length: Not set
                .... 0000 = Unused: 0x0
            Type Code: BGP-LS Attribute (29)
            Length: 12
            Link State
                Prefix SID TLV
                    Type: 1158
                    Length: 8
                    Flags: 0x40, Node-SID (N)
                        0... .... = Re-advertisement (R): Not set
                        .1.. .... = Node-SID (N): Set
                        ..0. .... = No-PHP (P): Not set
                        ...0 .... = Explicit-Null (E): Not set
                        .... 0... = Value (V): Not set
                        .... .0.. = Local (L): Not set
                    Algorithm: 0
                    SID/Index: 3333
Here is confusion, how, let's say, a controller, which receives such BGP-LS update, could  distinguish it from regular Prefix/Node SID?
Unfortunately draft-ietf-idr-bgp-ls-segment-routing-ext-18 does not clarify this issue because it only describes Prefix SID TLV type 1158. Any suggestions?
2)  draft-ietf-idr-bgp-ls-segment-routing-ext-18 has two types of labels TLV: SID/Label TLV type 1161 and Prefix SID  TLV type 1158.  I checked several vendors implementations of BGP-LS, all of them use only Prefix SID TLV.Why the draft does not have some conditions or clarifications when to use SID/Label TLV and when Prefix SID TLV?
Thank you.
SY,Boris

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