RFC 9252, "BGP Overlay Services Based on Segment Routing over IPv6 (SRv6)", July 2022

Errata ID: 7817
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Ketan Talaulikar
Date Reported: 2024-02-20

Section 3.2.1 says:

   As defined in [RFC8986], the sum of the Locator Block Length (LBL),
   Locator Node Length (LNL), Function Length (FL), and Argument Length
   (AL) fields MUST be less than or equal to 128 and greater than the
   sum of Transposition Offset and Transposition Length.

It should say:

   As defined in [RFC8986], the sum of the Locator Block Length (LBL),
   Locator Node Length (LNL), Function Length (FL), and Argument Length
   (AL) fields MUST be less than or equal to 128 and greater than or
   equal to the sum of Transposition Offset and Transposition Length.

Notes:

The sum of Trans Off and Trans Length can be equal to the LBL+LNL+FL+AL when the last part of the SID (function or argument) is getting transposed.

This is clear also from the example below in the next paragraph of the same section:

As an example, consider that the sum of the Locator Block and the
Locator Node parts is 64. For an SRv6 SID where the entire Function
part of size 16 bits is transposed, the transposition offset is set
to 64 and the transposition length is set to 16. While for an SRv6
SID for which the FL is 24 bits and only the lower order 20 bits are
transposed (e.g., due to the limit of the MPLS Label field size), the
transposition offset is set to 68 and the transposition length is set
to 20.

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