Re: [spring] Updating the SPRING WG Charter

[Alexander Vainshtein <[e-mail]>]

Bruno, Stewart and all,
I have looked up Section 5 “Binding Segment” of the Segment Routing Architecture<https://tools.ietf.org/html/draft-ietf-spring-segment-routing-15> draft, and it says:

   In order to provide greater scalability, network opacity, and service
   independence, SR utilizes a Binding SID (BSID).  The BSID is bound to
   an SR policy, instantiation of which may involve a list of SIDs.  Any
   packets received with active segment = BSID are steered onto the
   bound SR Policy.

   A BSID may either be a local or a global SID.  If local, a BSID
   SHOULD be allocated from the SRLB.  If global, a BSID MUST be
   allocated from the SRGB.

   Use of a BSID allows the instantiation of the policy (the SID list)
   to be stored only on the node(s) which need to impose the policy.
   Direction of traffic to a node supporting the policy then only
   requires imposition of the BSID.  If the policy changes, this also
   means that only the nodes imposing the policy need to be updated.
   Users of the policy are not impacted.

From my POV this definition is fully aligned both with the Bruno’s last email and with my earlier argument why a BSID does not add a per-path state in the transit nodes.


Regards,
Sasha

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From: spring [mailto:[e-mail]] On Behalf Of [e-mail]
Sent: Tuesday, June 19, 2018 3:38 PM
To: Stewart Bryant <[e-mail]>
Cc: SPRING WG List <[e-mail]>; Rob Shakir <robjs=[e-mail]>
Subject: Re: [spring] Updating the SPRING WG Charter

Hi Stewart,

Speaking as individual contributor, please see inline [Bruno]

From: spring [mailto:[e-mail]] On Behalf Of Stewart Bryant
Sent: Tuesday, June 19, 2018 1:19 PM






On 01/06/2018 17:05, Rob Shakir wrote:
The SPRING WG defines procedures that allow a node to steer a packet through an
SR Policy instantiated as an ordered list of instructions called segments and
without the need for per-path state information to be held at transit nodes.

I am not sure where the line gets drawn with the per-path state statement. If I introduce a
binding-SID to allow the creation of a path, have I introduced per-path state or not? In practise
a management entity will choose between the infinity of possible binding-SIDs by considering
the need to create specific paths and I would imagine that many will be instantiated just-in-time.

I think that the key point is that the ingress creates the path by using SIDs to create
a concatenation of paths, policies and resources.
[Bruno] Agreed. And it can be argued that this creates a state on the ingress. An “outgoing” state, very roughly comparable to Next Hop Label Forwarding Entry (NHLFE))
However it could be argued that
as soon as we introduced Binding SIDs we introduced per-path state.
[Bruno] Adding a Binding SID to this SR policy introduces an “incoming” state, very roughly comparable to Incoming Label Map (ILM). But this gets additional benefit as it allows others SR nodes to use this state/SR policy.
I’m not sure that in such high level text, we need to make such distinction.
I think we might
be best served by deleting the text I have highlighted.



[Bruno] This text is mostly a copy/past from existing charter so is not really new:

 “allow a node to steer a packet along an explicit
  route using information attached to the packet and
  without the need for per-path state information to be
  held at transit nodes. ”

At high level, I believe that this is a key distinction of spring/segment routing hence worth keeping in the high level introduction of the WG.
Now do we need to add text about more specific details, such as BSID? I’d rather not, as I don’t see what this would bring. I also don’t think that this sentence prohibits the creation of states when required/useful.
--Bruno

- Stewart






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