Hi authors,
There may be conflicting description in
draft-ietf-bess-srv6-services-15#section-6.1.1 on the usage of Transposition
Length.
<draft-ietf-bess-srv6-services-15#section-6.1.1>
The 24-bit ESI label field of the ESI label extended community
carries the whole or a portion of the Argument part of the SRv6 SID
when the ESI filtering approach is used along with the Transposition
Scheme of encoding (Section 4) and otherwise set to Implicit NULL
value. In either case, the value is set in the high order 20 bits
(e.g., as 0x000030 in the case of Implicit NULL). When using the
Transposition Scheme, the Transposition Length MUST be less than or
equal to 24 and less than or equal to the Argument Length.
</draft-ietf-bess-srv6-services-15>
I think that "The value" which is set in "the high order 20 bits" should be the
same thing as "the whole or a portion of the Argument part of the SRv6 SID",
If this is true, the length of "The value" (which is also the Transposition
Length) can only be as much as 20, it can't be equal to 24.
Otherwise, "The value" may have to be carried in the high X bits of the ESI
label field (wherein X is the argument length), not always the "the high order
20 bits".
I also noted that in some other sections(e.g. Section 5.1), it is clearly
described that the Transposition Length MUST be no more 20.
So I think may be the transposition length for EVPN routes is not necessary to
be greater than 20.
If it is greater than 20, the TC/S field of the ESI label may have to be used,
and the bit order may be not the same "In either case".
Because that if the Transposition Length is less than 21, it MUST be cairried
in the high order 20 bits (along with some padding bits) according to the
history of this draft, not just the high X bits.
Is my understanding correct?
Glad to receive any clarifications.
Thanks,
Yubao
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