Hi Ketan,
Thanks for your clarifications.
But I still don't notice that RFC7432 has said that the label value is 24 bits,
I just found these paragraphs in RFC7432 or rfc7432bis:
<rfc7432#section9.2.1>
The MPLS Label1 field is encoded as 3 octets, where the high-order
20 bits contain the label value. The MPLS Label1 MUST be downstream
assigned, and it is associated with the MAC address being advertised
by the advertising PE. The advertising PE uses this label when it
receives an MPLS-encapsulated packet to perform forwarding based on
the destination MAC address toward the CE. The forwarding procedures
are specified in Sections 13 and 14.
...
The MPLS Label2 field is an optional field. If it is present, then
it is encoded as 3 octets, where the high-order 20 bits contain the
label value.
</rfc7432>
<rfc7432bis>
7.5. ESI Label Extended Community
This Extended Community is a new transitive Extended Community having
a Type field value of 0x06 and the Sub-Type 0x01. It may be
advertised along with Ethernet Auto-discovery routes, and it enables
split-horizon procedures for multihomed sites as described in
Section 8.3 ("Split Horizon"). The ESI Label field represents an ES
by the advertising PE, and it is used in split-horizon filtering by
other PEs that are connected to the same multihomed Ethernet segment.
The ESI Label field is encoded as 3 octets, where the high-order
20 bits contain the label value.
</rfc7432bis>
In these paragraphs, It is clear that the MPLS Label/ESI label can only carry a
label value which is no more than 20 bits.
Have I missed something important?
When the ESI label field of RFC7432 carries a 20 bits ESI label 17, the 24 bits
ESI label field should be 0x000110 or 0x000011?
When the ESI label field of draft-ietf-bess-srv6-services-15 carries a 20 bits
Arg.FE2 17, the 24 bits ESI label field should be 0x000110 or 0x000011?
When the ESI label field of draft-ietf-bess-srv6-services-15 carries a 16 bits
Arg.FE2 17, the 24 bits ESI label field should be 0x000110 or 0x001100 or
0x000011?
Thanks,
Yubao
原始邮件
发件人：KetanTalaulikar
收件人：王玉保10045807;
抄送人：draft-ietf-bess-srv6-servi...@ietf.org;BESS;
日 期 ：2022年06月16日 22:04
主 题 ：Re: Whether the TC/S field of the ESI label field can be used to carry a
portion of Arg.FE2 according to draft-ietf-bess-srv6-services-15 ?
Hi Yubao,
Sorry for the delay in response and thanks for catching this issue. We'll try
to correct this during the AUTH48 process.
Please check inline below.
On Thu, May 26, 2022 at 8:20 AM <wang.yub...@zte.com.cn> wrote:
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".
KT> This is an error in the draft. Since the field is 24-bit, we should just
say "set in the 24 bits" instead of the existing text "set in the high order 20
bits". This applies to all EVPN encodings in the draft.
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.
KT> This depends on the encoding for the specific AFI/SAFI. For those that use
the RFC8277 label encoding, the transposition length is limited to 20. That is
why the "high order" clarification came in but it does not apply to EVPN.
Please also see the following text in Sec 3.2.1
The size of the MPLS label field limits the bits transposed from the SRv6 SID
value into it. E.g., the size of MPLS label field in [RFC4364] [RFC8277] is 20
bits while in [RFC7432] is 24 bits.
So I think may be the transposition length for EVPN routes is not necessary to
be greater than 20.
KT> For EVPN encodings the field is 24-bit and so we can transpose up to 24
bits.
If it is greater than 20, the TC/S field of the ESI label may have to be used,
KT> There is no TC/S encoding in the ESI label field.
and the bit order may be not the same "In either case".
KT> I am not sure what you mean by bit order. If you are referring to byte
ordering then everything is in network byte order.
Thanks,
Ketan
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_______________________________________________
BESS mailing list
BESS@ietf.org
https://www.ietf.org/mailman/listinfo/bess
