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Status: Rejected (1)

RFC 8287, "Label Switched Path (LSP) Ping/Traceroute for Segment Routing (SR) IGP-Prefix and IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data Planes", December 2017

Source of RFC: mpls (rtg)

Errata ID: 5290
Status: Rejected
Type: Technical

Reported By: Alexander ("Sasha") Vainshtein
Date Reported: 2018-03-20
Rejected by: Deborah Brungard
Date Rejected: 2018-06-29

Section 5.1 says:

   The IPv4 IGP-Prefix Segment ID is defined in [SR].  The format is as
   specified below:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         IPv4 Prefix                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Prefix Length  |    Protocol   |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IPv4 Prefix

      This field carries the IPv4 Prefix to which the Segment ID is
      assigned.  In case of an Anycast Segment ID, this field will carry
      the IPv4 Anycast address.  If the prefix is shorter than 32 bits,
      trailing bits SHOULD be set to zero.

   Prefix Length

      The Prefix Length field is one octet.  It gives the length of the
      prefix in bits (values can be 1-32).

It should say:

   The IPv4 IGP-Prefix Segment ID is defined in [SR]. 
   The sub-TLV length MUST be set to 8, and its format is 
   as specified below:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         IPv4 Prefix                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Prefix Length  |    Protocol   |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IPv4 Prefix

      This field carries the IPv4 Prefix to which the Segment ID is
      assigned.  In case of an Anycast Segment ID, this field will carry
      the IPv4 Anycast address.  If the prefix is shorter than 32 bits,
      trailing bits SHOULD be set to zero.

   Prefix Length

      The Prefix Length field is one octet.  
      It gives the length of the
      prefix in bits (values can be 1-32).

Notes:

The RFC in its current form does not explicitly specify the length of the sub-TLV for the IPv4 IGP-Prefix Segment ID, while the format includes a reserved (MBZ) field at the end.
the implementers therefore must guess whether the reserved bits are or are not included in the sub-TLV guess. Such guesses have already caused interoperabilty issues with some implementations including these bits and some not including them.

For comparison, RFC 8029 explicitly specifies length of every sub-TLV it defines. It also never includes MBZ fields at the end of sub-TLVs in the sub-TLV length.

The proposed text is aligned with majority of implementations known to me.

Note also that sub-TLV length is also omitted in section 6.1. However, I am not aware of any actual interoperability issues with this sub-TLV.
--VERIFIER NOTES--
This change requires an update to the RFC, requires consensus.

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