[Idr] Re: draft-jiang-idr-sr-policy-composite-path-03 - Question on precedence of Color setting

[linchangwang <[e-mail]>]

Hi Sue,

Thank you for your detailed explanation of the issue. Below is our understanding and clarification regarding the color setting:

Color Extended Community and Tunnel Encapsulation Attribute are at the same level.
The attributes of SR Policy are under Tunnel Encapsulation Attribute.
The color of SR Policy is in NLRI.
The color in Color Extended Community is used to steer traffic to an SR Policy, not to describe SR Policy attributes.
RFC9830 explicitly states that the Color sub-TLV is not used for SR Policy encoding.

For details, refer to RFC9830: https://www.rfc-editor.org/rfc/rfc9830.html#name-introduction and RFC9830: https://www.rfc-editor.org/rfc/rfc9830.html#section-2.3 and https://www.rfc-editor.org/rfc/rfc9830.html#name-color-extended-community



RFC9830: https://www.rfc-editor.org/rfc/rfc9830.html#name-introduction
The Color Extended Community (as defined in [RFC9012<https://www.rfc-editor.org/rfc/rfc9830.html#RFC9012>] is used to steer traffic into an SR Policy, as described in Section 8.8<https://rfc-editor.org/rfc/rfc9256#section-8.8> of [RFC9256<https://www.rfc-editor.org/rfc/rfc9830.html#RFC9256>]
Section 3<https://www.rfc-editor.org/rfc/rfc9830.html#EXTCOLOR> of this document updates [RFC9012<https://www.rfc-editor.org/rfc/rfc9830.html#RFC9012>] with modifications to the format of the Flags field of the Color Extended Community by using the two leftmost bits of that field.


RFC9830：https://www.rfc-editor.org/rfc/rfc9830.html#section-2.3

The Tunnel Egress Endpoint and Color sub-TLVs of the Tunnel Encapsulation Attribute, as defined in [RFC9012<https://www.rfc-editor.org/rfc/rfc9830.html#RFC9012>] are not utilized for SR Policy encodings. Consequently, their values are not relevant within the context of the SR Policy SAFI NLRI.

RFC9830：https://www.rfc-editor.org/rfc/rfc9830.html#name-color-extended-community
The Color Extended Community [RFC9012<https://www.rfc-editor.org/rfc/rfc9830.html#RFC9012>] is used to steer traffic corresponding to BGP routes into an SR Policy with matching Color value.
The Color Extended Community MAY be carried in any BGP UPDATE message whose AFI/SAFI is 1/1 (IPv4 Unicast), 2/1 (IPv6 Unicast), 1/4 (IPv4 Labeled Unicast), 2/4 (IPv6 Labeled Unicast), 1/128 (VPN-IPv4 Labeled Unicast), 2/128 (VPN-IPv6 Labeled Unicast), or 25/70 (Ethernet VPN, usually known as EVPN).


Therefore, the Color Extended Community is unrelated to the encodings of BGP SR Policy.

The draft document draft-jiang-idr-sr-policy-composite-path-03 introduces a Color field under the Tunnel Encapsulation Attribute when the Tunnel Type is set to SR Policy.
This Color field has no association with the Color Extended Community.
The newly added Color field serves to associate sub SR Policies.


Thanks,
Changwang

发件人: Susan Hares <[e-mail]>
发送时间: 2025年11月8日 1:45
收件人: idr <[e-mail]>
主题: [Idr] draft-jiang-idr-sr-policy-composite-path-03 - Question on precedence of Color setting


Jeffrey Haas12:04 (chat)- statement
I think this is a broader concern with sr policy as a
mechanism rather than this extension in particular?

Sue’s specific Question:
What is the precedence (and interaction) of possible ways
To specify color in the Tunnel Encapsulation Attribute.

See the different ways you can specify color that could
Impact a segment that is part of a segment list.


References
==============
Way 1: draft-jiang-idr-sr-policy-composite-path-03
defines Constituent SR Policy with Color Sub-TLV in the following manner:

      SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
          Attributes:
              Tunnel Encaps Attribute (23)
                  Tunnel Type: SR Policy
                      Binding SID
                      SRv6 Binding SID
                      Preference
                      Priority
                      Policy Name
                      Policy Candidate Path Name
                      Explicit NULL Label Policy (ENLP)
                      Segment List
                          Weight
                          Segment
                          Segment
                          ...
                      Constituent SR Policy
                          Color
                          Weight
                          Forwarding Class


Way 2: RFC 9012 – Tunnel Encapsulation Attribute carries:

3.4.2.  Color Sub-TLV (Type Code 4)

   The Color sub-TLV MAY be used as a way to "color" the corresponding
   Tunnel TLV.  The Value field of the sub-TLV is eight octets long and
   consists of a Color Extended Community, as defined in Section 4.3.
   For the use of this sub-TLV and extended community, please see
   Section 8.

   The format of the Value field is depicted in Figure 15.

   If the Length field of a Color sub-TLV has a value other than 8, or
   the first two octets of its Value field are not 0x030b, the sub-TLV
   MUST be treated as if it were an unrecognized sub-TLV (see
   Section 13).

Way 3: In addition, RFC9012 defines interactions with the Color Extended Community.

4.3.  Color Extended Community

   The Color Extended Community is a Transitive Opaque Extended
   Community with the encoding shown in Figure 15.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | 0x03 (1 octet)| 0x0b (1 octet)|        Flags (2 octets)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Color Value (4 octets)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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