BGP SR Policy
Extensions for templateHuaweiHuawei Bld., No.156 Beiqing Rd.Beijing100095Chinazhangka@huawei.comHuaweiHuawei Bld., No.156 Beiqing Rd.Beijing100095Chinahuzhibo@huawei.comHuaweiHuawei Bld., No.156 Beiqing Rd.Beijing100095Chinajie.dong@huawei.comHuaweiHuawei Bld., No.156 Beiqing Rd.Beijing100095Chinagaoqiangzhou@huawei.comSegment 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.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 when, and only when, they
appear in all capitals, as shown here.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) or BGP SR 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 . This process
is defined in .
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.