Re: [Idr] 2 Week WG LC for draft-ietf-idr-bgp-prefix-sid-04.txt - 3/6 to 3/20/2017

"Susan Hares" <> Wed, 08 March 2017 16:57 UTC

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Date: Wed, 8 Mar 2017 11:52:14 -0500
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Subject: Re: [Idr] 2 Week WG LC for draft-ietf-idr-bgp-prefix-sid-04.txt - 3/6 to 3/20/2017
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Thank you for cross-posing this to the spring WG. 




From: [] 
Sent: Wednesday, March 8, 2017 11:51 AM
To: Susan Hares
Cc: 'Hannes Gredler';;
Subject: RE: [Idr] 2 Week WG LC for draft-ietf-idr-bgp-prefix-sid-04.txt -
3/6 to 3/20/2017


Cross-posting to SPRING WG as this document is highly related to
draft-ietf-spring-segment-routing-msdc which is under WG Last Call in the


I’ve read the document and have the following comments.




‘A BGP-Prefix-SID is always global within the SR/BGP domain’


Do you imply that this extension may only be used within one SR/BGP domain?

IOW all involved ASes must coordinate their BGP SID allocation?


This would fit the MSDC use case discussed in
draft-ietf-spring-segment-routing-msdc, but may be a restriction for other
uses cases (e.g. Inter AS VPN option C requiring an inter-AS MPLS LSP).

Could the document clarify/elaborate on this?

One option may be for the ASBR to be configured to shit the received index
by an offset (e.g. +1000). (this possibility could be referred to in section



‘A BGP-Prefix-SID is always global within the SR/BGP domain’


On a side note,

- on the BGP side, IMHO, a BGP domain probably means an AS (looking back up
to RFC 1771 I’m not seeing the definition of a BGP domain)

- on the SR side, I think you mean that all ASes in the DC are in the same
SR domain


Hence SR domain does not equal a BGP domain.



§2: “the newly proposed BGP Prefix-SID attribute can be attached to prefixes
from AFI/SAFI: »

§3: “The BGP-Prefix-SID attached to a BGP prefix P“ 
§3.1: “this TLV can be used to advertise the label index of a given prefix”


IINM, BGP Attributes are not attached to (IP) prefixes but shared by all
prefixes of a given BGP UPDATE.

In addition to the terminology point, this has consequences as:

- [RFC3107] and [RFC4760] allow the advertisement of multiple IP prefixes

- draft-ietf-idr-bgp-prefix-sid would add the new requirement that each IP
prefix be advertised in a dedicated BGP UPDATE. (unless multiple Label-Index
TLB must be advertised)


If this extension requires the use of one BGP update message per IP prefix,
I think this point should be highlighted and discussed in both
draft-ietf-idr-bgp-prefix-sid and draft-ietf-spring-segment-routing-msdc




“the newly proposed BGP Prefix-SID attribute can be attached to prefixes
from AFI/SAFI:

      Multiprotocol BGP labeled IPv4/IPv6 Unicast ([
<> RFC3107]).

      Multiprotocol BGP ([ <> RFC4760])
unlabeled IPv6 Unicast.”


If this attribute is found on routes of different AFI/SAFI, how is this
handled? Is this considered an error (hence attribute is discarded as per

If so, this does not seem to work in Carrier’s carrier VPN case (where the
CE use AFI/SAFI 1/4 routes, may attach the Prefix-SID attribute; then the PE
will transform this route into a AFI/SAFI 1/128 with the same Prefix-SID
attribute attached).

If not, it must be clear that the BGP-Prefix-SID Index must not be used on
the VPN route.


Can this also be clarified in section 7 (error handling)?



“ The BGP Prefix SID attribute is an optional, transitive BGP path
attribute. »
Hence this attribute may spread very far, e.g. received over Internet
IPv6 Internet routes may be converted in AFI/SAFI 2/4 routes (labeled IPv6),
in particular for AS running 6PE (RFC 4798). Hence this AS would interpret
the Prefix SID and the received index would likely (or even deliberately)
collision with Index provisioned by this AS.
I think the use of a transitive attribute is debatable given that this
attribute is not required for the service/network to work. i.e. it’s only
“nice to have” in order for each node to use the same label (and assuming
they can be configured with the same SRGB). So the document is trading
security risk for a nice to have feature.
“   A BGP Prefix-SID attribute is called "unacceptable" for a speaker M
   if the derived label value L lies outside the SRGB configured on M.
   Otherwise the Label Index attribute is called "acceptable" to speaker  M.


The case where two different IP Prefixes are received with the same Index
(but with different labels) does not seem to be correctly handled. As per
current text, both Prefix would incorrectly be merged into the same FEC
(incoming label).

Also, the index in the BGP Prefix-SID may collision with the same index,
received from a different protocol (e.g. IS-IS) with a different prefix. How
is this handled?

This seems specifically an issue when combined with the previous point


§5.2 (IPv6 SID)


“      *  S flag: if set then it means that the BGP speaker attaching the
         Prefix-SID Attribute to a prefix is capable of processing the
         IPv6 Segment Routing Header (SRH,
man-segment-routing-header> I-D.ietf-6man-segment-routing-header]) for the
         corresponding to the originated IPv6 prefix”


A similar text has just been removed in the latest version of the SR ISIS
extension published yesterday
(draft-ietf-isis-segment-routing-extensions-11). (cf “H-Flag”).

Hence, are you sure that this SRv6 specific part is still applicable/up to




“9.  Security Considerations


   This document introduces no new security considerations above and beyond
those already specified in [RFC4271] and [RFC3107].”


Some of the comments above may probably have security considerations.





OLD: None are defined at this stage of the document.
NEW: None are defined by this document.



OLD: the newly proposed BGP Prefix-SID attribute
NEW: the BGP Prefix-SID attribute defined in this document
“11.  Change Log
   Initial Version:  Sep 21 2014”
This section does not seem that useful and could either be removed or
completed with the change log of all versions.

Name of the attribute is not consistently spelled (e.g. BGP-Prefix-SID, BGP
Prefix SID, BGP Prefix-SID) 






From: Idr [] On Behalf Of Susan Hares
Sent: Tuesday, March 07, 2017 4:27 AM
Cc: 'Hannes Gredler'
Subject: [Idr] 2 Week WG LC for draft-ietf-idr-bgp-prefix-sid-04.txt - 3/6
to 3/20/2017


This message begins a 2 week WG LC for draft-ietf-idr-bgp-prefix-sid-04.txt
(3/6 to 3/20).   


This draft proposed a new optional, transitive BGP path  attribute, the BGP
Prefix Segment attribute to carry BGP Prefix Segment Identifier (BGP Prefix
SID) information for:   Label-Index, IPv6 SID, and Originator SRGB.  This
draft is linked to work on DC segment routing describe


We have one remaining author (Arjun Sreekantiah) has not responded to the
IPR call, but it is likely he will respond shortly.  If Arun does not
respond to the IPR call during the first week, this WG LC will be extended
for 1 additional week.  


Three implementations exist and the details are on the web site, and in the



Sue Hares 

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