| Internet-Draft | BGP SR Policy Extensions for template | May 2024 |
| Zhang, et al. | Expires 15 November 2024 | [Page] |
Segment Routing(SR) Policies can be advertised using BGP. An SR Policy may has lots of attributes, and as the application and features evolve, the SR Policy may need have more and more attribute attributes. To avoid modifying BGP when attributes are added to an SR Policy, we can define a template. The identifier and content of the template are defined by the receiver of the SR Policy. The advertiser of an SR policy only needs to know the ID of the template. When advertising SR policy, the advertiser carries the template ID in the tunnel encapsulation information of the SR policy. After receiving the SR Policy information, the receiver obtains the corresponding template and content according to the template ID, thereby obtaining abundant constraint configuration information.¶
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[I-D.ietf-idr-segment-routing-te-policy]defines some attributes encoding of the SR Policy path. However, in actual applications, there are many other attributes of SR Policy path. These attributes are valid only on the device where the SR Policy path is installed. Such attributes may include backup protection, Bidirectional Forwarding Detection information, traffic statistics collection, or in-situ Flow Information Telemetry detection information, etc. If these attributes are directly delivered through BGP, the BGP SR Policy protocol may change frequently. This document defines a general method to carry the path attributes of SR Policies.¶
SR Policy: An ordered list of segments.¶
Candidate Path: the unit for signaling of an SR Policy to a headend via protocol extensions like Path Computation Element (PCE) Communication Protocol (PCEP) [RFC8664] [I-D.ietf-pce-segment-routing-policy-cp] or BGP SR Policy [I-D.ietf-idr-segment-routing-te-policy].¶
SRPM: SR Policy Module.¶
Template: A collection of attributes sets.¶
Template ID: The identifier of a template.¶
To support the attributes extension of SR Policies, this document defines a constraint template identifier. The constraint template ID is valid only for the recipient. The SR policy publisher only needs to carry the template ID when publishing the SR policy. The receiver of the SR Policy may create a template corresponding to the template identifier in advance before receiving the SR Policy, or may define a corresponding template after receiving the template definition of the SR Policy. The template can contain any attributes on the SR Policy path, including but not limited to backup protection, Bidirectional Forwarding Detection information, traffic statistics collection, or in-situ Flow Information Telemetry detection information, etc. After receiving the SR Policy information, the receiver matches the template information based on the template ID and adds attributes to the SR Policy based on the attributes defined in the template.¶
Template ID is an local identifier, just to use on the headend of the SR Policy. And it is a local configured identifier, need to be unique only on the headend device. We need no further process to coordinate the template ID between multiple routers.¶
A template represents a configuration plan intent. In a network managed by a controller, the template can be configured by controller and template ID is specified during the configuration. Therefore, the template ID is planned on the entire network. All policies can use the same template as long as they require the same configuration attributes, regardless of whether they are on the same device.¶
For example, a policy has two candidate paths with preference 200 and 100. The path with preference 200 has a higher preference and is used as the primary path. The BFD detection time is 10 ms x 3. The candidate path with preference 100 is used as the backup path, uses BFD detection, and the BFD detection time is 50 ms x 3. Then we can define two templates:¶
Template 1:
Hotstandby backup Enable
BFD Seamless Enable
BFD min-tx-interval 10
BFD detect-multiplier 3
Template 2:
BFD Seamless Enable
BFD min-tx-interval 50
BFD detect-multiplier 3
CP1<E1, C1>:
<preference = 200,
SL(Weight=W1): <S1, S2, S3>,
SL(Weight=W2): <S4, S5, S6>,
TemplateID = 1>
CP2<E1, C1>:
<preference = 100,
SL(Weight=W1): <S7, S8, S9>,
SL(Weight=W2): <S10, S11, S12>,
TemplateID = 2>
¶
We can see that the template defines some common attributes and can be used by different SR Policies.¶
As the template ID is defined, the tunnel attribute encapsulation of the BGP SR Policy needs to be updated.¶
The SR Policy Encoding structure is as follows:¶
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>¶
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
Preference
Priority
Policy Name
Policy Candidate Path Name
Explicit NULL Label Policy (ENLP)
Template ID
Segment List
Weight
Segment
Segment
....
....
¶
Where Tempate ID indicates the template ID for the SR Policy candidate path.¶
A new sub-TLV called Template ID sub-TLV is defined. Template ID sub-TLV specifies the template ID of an SR policy candidate path. Each sub-TLV is encoded as shown in Figure 1.¶
0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- | Type | Length | Flags |N| RESERVED | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- | Template ID(4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- // Template Name (optional) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Type: Template ID, 1 octet, TBD.¶
Length: 6.¶
Flags: 1 octet of flags.¶
Where:
N-Flag: This flag, when set, indicates the presence of the Template
Name encoding.
¶
RESERVED: 1 octet of reserved bits. SHOULD be set to zero on transmission and MUST be ignored on receipt.¶
Template ID: a 4-octet value.¶
Template Name: Template MAY also be assigned with a template name, such template name MUST NOT be considered as identifiers for a template. The size of the tempalte name for the template is limited to 255 bytes.¶
When BGP advertises an SR Policy, different candidate paths of the same SR Policy may have different template IDs or the same template ID, depending on the attributes required by the candidate paths of the SR Policy.¶
Reflectors just need to advertise the route of SR, no need to process it.¶
SR Policy is only to be processed on the SR Policy headend, reflectors just need to reflect the route of SR Policy, no need to process it. To make this possible, an attribute needs to be attached to the advertisement that enables a BGP speaker to determine whether it is intended to be a headend for the advertised policy. This is done by attaching one or more Route Target Extended Communities to the advertisement [RFC4360]. This process is defined in [I-D.ietf-idr-segment-routing-te-policy]. This draft does not add any extra process in this process.¶
Once BGP on the receiving node has determined that the SR Policy NLRI is usable, it passes the SR Policy candidate path to the SRPM. The SRPM then determine how to use the template ID in SR Policy. The SRPM find the local configured template by template ID, and get all the attributes that is configured in the template, and then process the candidate path with these attributes. For example, if the template configure seamless bfd, then the SRPM can create sbfd sessions for each Segment List in the candidate path. If there is no template find, the SRPM should ignore the template ID and use the candidate path as there is no template ID.¶
TBD.¶
This document requests that IANA allocates a new sub-TLV type as defined in Section 4.1 from the "Sub-TLVs for SR Policy" registry as specified.¶
Value Description Reference ---------------------- ---------------------------- -------------- TBD SR Policy Template ID This document
These extensions to BGP SR Policy do not add any new security issues to the existing protocol.¶