Re: [Idr] draft-ietf-idr-bgp-prefix-sid review - Text clarifying removal of SRv6 case

["Ketan Talaulikar (ketant)" <[e-mail]>]

Hi Sue,

I would like to clarify on SRv6 and BGP extensions for it.

Draft-ietf-idr-bgp-prefix-sid associates the Prefix SID attribute to MP BGP Labelled IPv4/IPv6 Unicast AFI/SAFI and is hence applicable to SR/MPLS forwarding plane.

SRv6 architecture is described in draft-filsfils-spring-srv6-network-programming and it provides an overview of the control plane implications for various protocols in https://tools.ietf.org/html/draft-filsfils-spring-srv6-network-programming-03#section-8.

The draft-dawra-idr-srv6-vpn covers the VPN signalling aspects as described in sec 8.3. The sec 8.1 (though marked as IGP) explains the prefix reachability considerations for routing. The routing protocols signal reachability for the locator prefixes.

The advertisement of all SRv6 SIDs of a node is done via BGP-LS and is covered in draft-dawra-idr-bgpls-srv6-ext (as explained in Sec 8.2 of SRv6 net. prg. draft). The SRv6 END SID is functionality equivalent to the SR/MPLS Prefix SID – i.e. instruction to go to the node advertising the prefix via the best routing path. The SRv6 END SID advertisement via BGP-LS for a BGP node is covered in https://tools.ietf.org/html/draft-dawra-idr-bgpls-srv6-ext-03#section-2.1.2

Thanks,
Ketan

From: Idr <[e-mail]> On Behalf Of Susan Hares
Sent: 09 May 2018 01:41
To: Acee Lindem (acee) <[e-mail]>; [e-mail]
Cc: [e-mail]; [e-mail]
Subject: Re: [Idr] draft-ietf-idr-bgp-prefix-sid review - Text clarifying removal of SRv6 case

Acee:

This is important fact to know.  You had mentioned in person, but this
point  not evident in the set of drafts I had.   I’ll switch that set of
questions over to a different thread.

Sue

From: Idr [mailto:[e-mail]] On Behalf Of Acee Lindem (acee)
Sent: Tuesday, May 8, 2018 4:07 PM
To: Susan Hares; [e-mail]<mailto:[e-mail]>
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: Re: [Idr] draft-ietf-idr-bgp-prefix-sid review - Text clarifying removal of SRv6 case

Hi Sue,

From: Susan Hares <[e-mail]<mailto:[e-mail]>>
Date: Tuesday, May 8, 2018 at 4:01 PM
To: Acee Lindem <[e-mail]<mailto:[e-mail]>>, IDR List <[e-mail]<mailto:[e-mail]>>
Cc: Alvaro Retana <[e-mail]<mailto:[e-mail]>>, John Scudder <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Subject: RE: draft-ietf-idr-bgp-prefix-sid review - Text clarifying removal of SRv6 case

Acee:

draft-dawra-idr-bgpls-srv6-ext-03.txt covers the NLRI
and not the BGP attribute (defined in draft-ietf-bgp-prefix-sid).

Right – for the initial use cases, the BGP-LS AF will be used to advertise the SRv6 SID corresponding to a unicast prefix.

Did you mean section 2 of draft-dawra-idr-srv6-vpn-03?

2<https://tools.ietf.org/html/draft-dawra-idr-srv6-vpn-03#section-2>.  SRv6-VPN SID TLV


   The SRv6-VPN SID TLV is defined as another TLV for BGP-Prefix-SID
   Attribute [I-D.ietf-idr-bgp-prefix-sid<https://tools.ietf.org/html/draft-dawra-idr-srv6-vpn-03#ref-I-D.ietf-idr-bgp-prefix-sid>].

This is another use case for SRv6 and VPN prefixes. You won’t find the exact use case that was previously in the BGP Prefix-SID draft. As discussed, it didn’t satisfy any use cases and wasn’t ever implemented.

Thanks,
Acee

Sue


From: Acee Lindem (acee) [mailto:[e-mail]]
Sent: Tuesday, May 8, 2018 3:52 PM
To: Susan Hares; [e-mail]<mailto:[e-mail]>
Cc: 'Alvaro Retana'; 'John Scudder'; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: Re: draft-ietf-idr-bgp-prefix-sid review - Text clarifying removal of SRv6 case

Hi Sue,

From: Susan Hares <[e-mail]<mailto:[e-mail]>>
Date: Tuesday, May 8, 2018 at 3:38 PM
To: Acee Lindem <[e-mail]<mailto:[e-mail]>>, IDR List <[e-mail]<mailto:[e-mail]>>
Cc: Alvaro Retana <[e-mail]<mailto:[e-mail]>>, John Scudder <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Subject: RE: draft-ietf-idr-bgp-prefix-sid review - Text clarifying removal of SRv6 case

Acee:

Thank you for the quick response.  Yes – I did mean the SRv6 (not IPv6).

Just to make sure, I’ve been clear, the scenario is  SR domain is running the SRv6 forwarding.

BGP speaking forwarding BGP-LS with SRV6 information (per draft-dawra-idr-bgpls-srv6-ext-03.txt)
adds  BGP attribute with a valid BGP Prefix SID (which only works with a MPLS forwarding).
What should happen in the attribute processing?

The answer to this question could be placed in

1)      draft-ietf-bgp-prefix-sid,

2)      draft-dawra-idr-bgpls-srv6-ext-03.txt, or

3)      draft-dawra-idr-srv6-vpn-03?

Do I understand correctly from your that this problem should be addressed in the draft-dawra-idr-srv6-vpn-03?

If this is your suggestion, it is a good one – but the draft-dawra-idr-srv6-vpn covers too much.
The issues regarding the processing of the BGP attribute need to go to IDR.

Yes – this is described in https://tools.ietf.org/html/draft-dawra-idr-bgpls-srv6-ext-03#section-2.1.2

The BGP based Ethernet VPN over SRV6 needs to go to the Bess WG which has dealt
with the EVPN creation.   I’ve copied the authors on this message, and I’ll see if they can send
just the BGP Attribute processing and material similar to draft-ietf-bgp-prefix-sid to IDR.

I believe this draft is still evolving. You probably want to wait to review the next revision.

This is not to say that there will not be other SRv6 BGP use cases in the future and I wouldn’t completely rule out future extension of the BGP Prefix-SID attribute.

Thanks,
Acee


Sue

From: Acee Lindem (acee) [mailto:[e-mail]]
Sent: Tuesday, May 8, 2018 1:06 PM
To: Susan Hares; [e-mail]<mailto:[e-mail]>
Cc: 'Alvaro Retana'; 'John Scudder'; [e-mail]<mailto:[e-mail]>
Subject: Re: draft-ietf-idr-bgp-prefix-sid review - Text clarifying removal of SRv6 case

Hi Sue,

From: Susan Hares <[e-mail]<mailto:[e-mail]>>
Date: Tuesday, May 8, 2018 at 12:09 PM
To: IDR List <[e-mail]<mailto:[e-mail]>>
Cc: Alvaro Retana <[e-mail]<mailto:[e-mail]>>, John Scudder <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Subject: draft-ietf-idr-bgp-prefix-sid review - Text clarifying removal of SRv6 case
Resent-From: <[e-mail]<mailto:[e-mail]>>
Resent-To: Stefano Previdi <[e-mail]<mailto:[e-mail]>>, <[e-mail]<mailto:[e-mail]>>, Acee Lindem <[e-mail]<mailto:[e-mail]>>, Arjun Sreekantiah <[e-mail]<mailto:[e-mail]>>, <[e-mail]<mailto:[e-mail]>>
Resent-Date: Tuesday, May 8, 2018 at 12:08 PM

Acee:

I agree with not specifying the IPv6in draft-ietf-idr-bgp-prefix-sid.
If it is not working, it does not belong in an IDR draft going to the IESG for publication.

I’m glad you agree with removing SRv6 (as opposed to IPv6) from the document.

I’ve suggested some text below in the introduction that identifies the SRv6 case and
Indicates that BGP Prefix-SID attribute should not be associated with
SRv6 SIDs (IPv6 addresses).

However, there should also be some text in section 6.
Section 6 states:

/  When a BGP Speaker receives a BGP Update message containing a

   malformed or invalid BGP Prefix-SID attribute attached to a Labeled

   IPv4/IPv6 unicast prefix [RFC8277<https://tools.ietf.org/html/rfc8277>], it MUST ignore the received BGP

   Prefix-SID attributes and not advertise it to other BGP peers.  This

   is equivalent to the "Attribute discard" action specified in

   [RFC7606<https://tools.ietf.org/html/rfc7606>].  When discarding an attribute, a BGP speaker SHOULD log an

   error for further analysis.
/

In section 4.1, you identify invalid as “not having a “Label-Index TLV, and
“conflicting as”

   If multiple different prefixes are received with the same label

   index, all of the different prefixes MUST have their BGP Prefix-SID

   attribute considered as "conflicting".

You do not define malformed in any section.  Normally, a malformed attribute is an attribute that
does not abide by the defined attribute code, length, and value (that is the TLVs within the attribute].
Is this the intent?  If so, you might indicate what malformed is someplace in the draft.

Yes – I’ll add text to section 6.

       In this context, a malformed BGP Prefix-SID attribute
      is one that cannot be parsed due to not meeting the minimum length
      requirement or containing a TLV length that doesn't conform to the
       length constraints for the TLV or would exceed the attribute length.



I do not perceive how section 6’s text handles the valid BGP Prefix SID attribute that is associated
with an SRv6 SID.   The attribute is not malformed, invalid, or conflicting.
Perhaps you could call it “illegal usage”.    My suggested text below uses that term.
IMHO, the case of a valid BGP Prefix SID attribute being associated with SRv6 SID
case needs to be addressed in the error handling section of the draft.

Since SRv6 has been removed from the document, there is no need to describe error handling behavior (see below).

Susan Hares


Suggested textual changes to introduction to clearly state SRv6 is not supported.
 =============
Here’s my suggested
Change 1:
Current/

   As described in [I-D.ietf-spring-segment-routing<https://tools.ietf.org/html/draft-ietf-idr-bgp-prefix-sid-19#ref-I-D.ietf-spring-segment-routing>], when SR is applied

   to the MPLS dataplane ([I-D.ietf-spring-segment-routing-mpls<https://tools.ietf.org/html/draft-ietf-idr-bgp-prefix-sid-19#ref-I-D.ietf-spring-segment-routing-mpls>]), the

   SID consists of a label.

/

Here’s a proposed text that you might add:



New/As described in [I-D.ietf-spring-segment-routing<https://tools.ietf.org/html/draft-ietf-idr-bgp-prefix-sid-19#ref-I-D.ietf-spring-segment-routing>], when SR is applied

    to the MPLS dataplane ([I-D.ietf-spring-segment-routing-mpls<https://tools.ietf.org/html/draft-ietf-idr-bgp-prefix-sid-19#ref-I-D.ietf-spring-segment-routing-mpls>]), the

    SID consists of a label.



    [I-D.ietf-spring-segment-routing] also

    indicates that SR routing can be applied to the IPv6 dataplane

    [I-D.ietf-6man-segment-routing-header] in a new v6 routing header with

    the SID encoded as an IPv6 address.  The SR segment that uses this header

    is denoted as an SR SID.

/



Ok. I’d also add:



           The applicability and support for Segment Routing over IPv6 is beyond

           the scope of this document.



Change 2 is then not necessary.



Thanks,

Acee


Change 2:
Further down in the paragraph I suggest changing:
Old/

   This document describes the BGP extension to signal the BGP Prefix-

   SID.  Specifically, this document defines a BGP attribute known as

   the BGP Prefix-SID attribute and specifies the rules to originate,

   receive, and handle error conditions for the attribute.
/
to
/

   This document describes the BGP extension to signal the BGP Prefix-

   SID.  Specifically, this document defines a BGP attribute known as

   the BGP Prefix-SID attribute and specifies the rules to originate,

   receive, and handle error conditions for the attribute when

   SR is applied to the MPLS dataplane.



   This document does not specify how the BGP Prefix-SID attribute can

   associated used with an SRv6 SID.  A BGP Prefix-SID attribute

   is considered illegal use of this attribute and

   the attribute will be discarded (see section 6 for

   for details on error handling ).

/