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

<> Mon, 20 April 2015 15:13 UTC

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To: "Stefano Previdi (sprevidi)" <>, " list" <>
Thread-Topic: Proposed Changes in draft-ietf-isis-segment-routing-extensions
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Date: Mon, 20 Apr 2015 15:13:47 +0000
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Subject: Re: [Isis-wg] Proposed Changes in draft-ietf-isis-segment-routing-extensions
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I'm fine with 1)
About 2), it's more a question on implementations, ... so the only point that I want to raise is if we go to this proposal, we need to ensure that we will never need more than 27 SRGBs ... Thinking now, it sounds that it would never happen but it's just the beginning of SR ...

-----Original Message-----
From: Stefano Previdi (sprevidi) [] 
Sent: Wednesday, March 25, 2015 12:42
To: list
Subject: Proposed Changes in draft-ietf-isis-segment-routing-extensions


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 [...]"


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


  "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."


  "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.



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