Re: [Isis-wg] Proposed Changes in draft-ietf-isis-segment-routing-extensions

All,

Since the changes being proposed to the ISIS SR extensions will break backwards compatibility, I would like to suggest that that working group consider taking advantage of this opportunity to improve the way that SR extensions support forwarding based on algorithms other than SPF.

Currently, in order to establish forwarding next-hops based on another algorithm, each node must be configured with an additional node-SID, each unique in the IGP domain.    The configuration and management of unique node-SIDs on a per-algorithm basis can be avoided by having each node assign a label block for each algorithm and advertise label blocks on a per-algorithm basis.  In this way, a given node only needs to have a single unique node-SID configured, while still supporting forwarding next-hops computed by different algorithms.

As far as I can tell, the main drawback of this approach is that it would break backwards compatibility with existing implementations since the current extensions do not support the association of an algorithm with a label block.  However, if we group this change together with other non-backwards compatible changes, that drawback is minimized or eliminated.

It may also make sense to take this opportunity to improve support for multi-topology routing in SR by introducing a mechanism to allow the SR-related sub-TLVs carried in the Router Capability TLV to be associated with a given MT-ID.

Chris

-----Original Message-----
From: Isis-wg [mailto:isis-wg-bounces@ietf.org] On Behalf Of Stefano Previdi (sprevidi)
Sent: Wednesday, March 25, 2015 6:42 AM
To: isis-wg@ietf.org list
Cc: draft-ietf-isis-segment-routing-extensions@tools.ietf.org
Subject: [Isis-wg] Proposed Changes in draft-ietf-isis-segment-routing-extensions

All,

The authors of draft-ietf-isis-segment-routing-extensions would like to expose the following proposed changes to SRGB advertisement which are being considered.

1. Single Vs. Multiple SRGB ranges
  Currently, section 3.1.  SR-Capabilities Sub-TLV defines that:

  "A router not supporting multiple occurrences of the SR-Capability
   sub-TLV MUST take into consideration the first occurrence in the
   received set."

  The authors would like to remove above text so that a compliant
  implementation MUST support the receiving of multiple ranges.

2. Encoding the SR-Cap in a single LSP Fragment Vs. Single TLV
  Currently, section 3.1.  SR-Capabilities Sub-TLV defines that:

  "The SR Capabilities sub-TLV (Type: TBD, suggested value 2) MAY
   appear multiple times inside the Router Capability TLV and has
   following format [...]"

  and

  "Only the Flags in the first occurrence of the sub-TLV are to be
   taken into account"

  and

  "The originating router MUST encode ranges each into a different
   SR-Capability sub-TLV and all SR-Capability TLVs MUST be encoded
   within the same LSP fragment."

  and

  "The order of the ranges (i.e.: SR-Capability sub-TLVs) in the
   LSP fragment is decided by the originating router and hence the
   receiving routers MUST NOT re-order the received ranges. This
   is required for avoiding label churn when for example a
   numerical lower Segment/Label Block gets added to an already
   advertised Segment/Label Block."

  Authors agreed that:
  . the encoding scheme is suboptimal and doesn't make best use of
    the TLV/LSP space (e.g.: flags field is replicated and unused).
  . we want to preserve the requirement of NOT sorting the received
    srgb ranges in order to avoid churns and downtime when a change
    is advertised (typically when the srgb is extended).

  Therefore a possible option is to restrict the advertisement of
  multiple srgb's into the SAME SR-Cap SubTLV where flags get
  defined once and srgb ranges encoded within the same (unique)
  SR-Cap SubTLV (btw, we still have room for up to 27 srgb ranges).

  Now, doing this will improve the encoding and clarity of the spec
  but introduces a backward compatibility issue with current
  version of the draft. Therefore it is important that all
  implementors make themselves known and tell the authors how
  difficult this change is from an implementation perspective.

  Among the authors we have 4 implementors for which the change
  seems not to be a problem but other implementations of ISIS,
  Segment Routing extension may exists and so it is necessary to
  check whether anyone has a problem with the proposed change.

Thanks.
s.

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