Inter-Domain Routing Z. Li, Ed.
Internet-Draft L. Song, Ed.
Updates: 4271 (if approved) China Mobile
Intended status: Standards Track 3 March 2025
Expires: 4 September 2025
BGP SR Policy Extensions for Performance-Aware Path Selection
draft-li-idr-sr-policy-metric-00
Abstract
To enable the headend node to do performance-aware path selection,
this document proposes an extension to the BGP SR Policy protocol by
defining a new optional Metric Sub-TLV within the BGP Tunnel
Encapsulation Attribute [RFC9012]. The introduced Metric Sub-TLV
encodes performance parameters (such as latency, bandwidth,
reliability, etc.) for SR Policy paths. This specification also
updates the BGP route selection procedures in [RFC4271], modifying
the Breaking Ties (Phase 2) logic to prioritize the metrics for SR
Policy paths.
Key contributions include:
* Introduce Metric Sub-TLV in BGP SR Policy
* Update the tie-breaking procedure for BGP route selection
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 4 September 2025.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Use Case . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Metric Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . 4
4. Policy Computation and Provisioning System Behavior . . . . . 6
5. Headend Behavior . . . . . . . . . . . . . . . . . . . . . . 7
6. Updated Tie-Breaking Procedure for BGP . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Segment Routing (SR) [RFC8402] allows a headend node to steer a
packet flow along a specific path. [RFC9256] further details the
concepts of SR Policy and steering into an SR Policy.
[I-D.ietf-idr-sr-policy-safi] specifies the use of BGP to distribute
one or more of the candidate paths of an SR Policy to the headend of
that policy.Currently [I-D.ietf-idr-sr-policy-safi] lacks the
capability to propagate performance metrics such as path latency,
bandwidth, or reliability. This limitation prevents headends from
implementing policy selection based on path metrics when there are
multiple paths reaching the same destination. Consequently, the
headends cannot dynamically elect performance-optimal path among
multiple SR Policies. To address this limitation, this document
extends the BGP SR Policy protocol to carry performance metrics in SR
Policy advertisements.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Use Case
As shown in Figure 1,the SR Policy Computation and Provisioning
System, such as a SDN controller, collects real-time network state
information (e.g., topology, link utilization) and performance
metrics (e.g., link latency, jitter, packet loss rate). Based on
service or customer requirements (e.g., minimum latency), it computes
SR Policy paths between designated endpoints and distributes them to
headend nodes via the BGP SR Policy protocol
[I-D.ietf-idr-sr-policy-safi].
For example:
The system provisions two low-latency policies to headend node PE3:
Policy1: Path via P1-->PE1, with a measured latency of 20 ms.
Policy2: Path via P2-->PE2, with a measured latency of 12 ms.
However, the current BGP SR Policy protocol
[I-D.ietf-idr-sr-policy-safi] only propagates path definitions (e.g.,
segment lists) without embedding performance metrics. This forces
headend nodes to select paths based solely on static criteria (e.g.,
administrative preferences), potentially leading to suboptimal
traffic engineering decisions.
To address this limitation, this proposal extends the BGP SR Policy
protocol by introducing a new Performance Metrics Sub-TLV within the
BGP Tunnel Encapsulation Attribute [RFC9012]. This Sub-TLV encodes
key performance indicators (KPIs) such as latency, bandwidth, and
reliability (see Section 3 for details). With this extension: The SR
Policy Computation and Provisioning System can advertise SR Policies
alongside their associated KPIs. Headend nodes leverage the enhanced
BGP route selection logic (Section 6) to prioritize paths that meet
dynamic performance requirements.
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+---------------------------------------------+
|SR Policy Computation and Provisioning System|
+---------------------------------------------+
* * * *
* * * *Extended BGP
* * * *SR Policy
* * * *
+---+ 15ms* +---+ 5ms * +---+
/|PE1| ----*----| P1|-------------*-----|PE3|\
/ +---+ * +---+----\ /--*----+---+ \
/ | * | \ / * | \
+---+/ | * | ----\ * | \+---+
|CE1|0.5ms| * |0.5ms / \ * |0.5ms |CE2|
+---+\ | * | ---- ---\ * | /+---+
\ | * | /1ms 5ms\ * | /
\ +---+ +---+ --+---+ /
\|PE2|----------|P2 |-------------------|PE4|/
+---+ 11ms +---+ 1ms +---+
Figure 1: Use Case for Performance-Aware SR Policy Selection
3. Metric Sub-TLV
This document extends the BGP SR Policy protocol by introducing a new
sub-TLV, Metric Sub-TLV, within the BGP Tunnel Encapsulation
Attribute. The Extended BGP SR Policy Encoding structure is as
follows:
SR Policy SAFI NLRl: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encapsulation Attribute(23)
Tunnel Type: SR Policy(15)
Binding SID
SRv6 Binding SID
Preference
Priority
Metric
Policy Name
Policy Candidate Path Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Segment
Segment
...
...
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Figure 2: Extended BGP SR Policy Encoding
Metric Sub-TLV is used to carry performance metrics such as latency,
bandwidth, and reliability. The format of Metric Sub-TLV is as
follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delay(8 octets,optinal) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth(4 octets,optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reliability(4 octets,optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Metric Sub-TLV
Where:
Type (1 octet): Indicates this sub-TLV is Metric, Specific values
need to be assigned by IANA.
Length (1 octet): Indicates the length of the Metric sub-TLV in
bytes.
Flags (1 octet): Indicates the presence of specific performance
metrics. Its definition is shown in Figure 4.
Reserved (1 octet): Reserved for future use. This field MUST be set
to 0 when sending and ignored when receiving.
Delay(8 octets): Carries delay information. Its format depends on
the D flag in Flags Field:
* If D = 01: NTPv4 format delay
* If D = 10: IEEE 1588v2 PTP format delay
Bandwidth (4 octets): Carries bandwidth information in Mbps.
Reliability (4 octets): Carries reliability information, such as the
maximum number of failures that have occurred on all links in the SR
policy path within the past month.
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0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| D |B|R|Reserved|
+-+-+-+-+-+-+-+-+
Figure 4: Flag Field for Metric Sub-TLV
Where, D Flag is for delay, B Flag is for Bandwidth and R Flag is for
reliability, all the other bits are reserved. The detailed encodings
of the three flags defined in this document are as follows:
+------+-------+-------------------------------+
| Flag | Bits | Description |
+------+-------+-------------------------------+
| D | 0-1 | 00: No delay |
| | | 01: NTPv4 delay |
| | | 10: PTP delay |
| | | 11: Reserved |
+------+-------+-------------------------------+
| B | 2 | 0: No bandwidth |
| | | 1: Bandwidth |
+------+-------+-------------------------------+
| R | 3 | 0: No reliability |
| | | 1: Reliability |
+------+-------+-------------------------------+
Figure 5: Flags for Metric Sub-TLV
Implementations SHOULD set only one flag (D, B, or R) at a time, as
these metrics are typically not directly comparable. Network
operators MAY configure which metric to prioritize based on service
requirements.
4. Policy Computation and Provisioning System Behavior
The Policy Computation and Provisioning System is responsible for
calculating Segment Routing (SR) policies based on network state and
business requirements, and provisioning them to headend nodes. When
provisioning SR policies that include performance metrics, the system
should follow these steps:
Collect Network State Information: Gather real-time network topology,
link utilization, and other relevant data.
Compute SR Policies: Calculate SR policy paths that meet performance
requirements based on service needs and network state.
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Encapsulate Performance Metrics: Embed performance metrics such as
latency, bandwidth, and reliability within the Metric Sub-TLV of the
BGP Tunnel Encapsulation Attribute.
Provision BGP Update Messages: Include the SR policies with
performance metrics in BGP update messages and send them to the
appropriate headend nodes.
Process BGP Acknowledgments: Await acknowledgments from headend nodes
to ensure successful reception and processing of the SR policies.
5. Headend Behavior
Upon receiving SR policies with performance metrics, headend nodes
should process them as follows:
Parse BGP Update Messages: Extract SR policies and their associated
performance metrics from the received BGP update messages.
Store Performance Metrics: Save the performance metrics in a local
database for subsequent path selection.
Path Selection: Prioritize paths that meet dynamic performance
requirements when multiple paths are available.
Update Routing Tables: Modify routing tables based on the selected
paths to ensure traffic is forwarded along optimized routes.
6. Updated Tie-Breaking Procedure for BGP
Based on practical use cases and hands-on operational experience, the
Breaking Ties (Phase 2) logic in [RFC4271] SHOULD be updated. This
can be achieved by inserting the comparison of Metric attribute
values between steps e and f. More precisely, when the next-hop
costs are identical, first compare the Metric attribute values of the
SR policy, then compare the BGP IDs of the BGP Speakers. The
detailed comparison algorithm is as follows:
Check if the Route Iterates to an SR Policy Tunnel:
* All Iterations to Tunnels:
1. All Tunnels Carry Metric Attributes: Prioritize the SR policy
with the smallest latency, largest bandwidth, or smallest
reliability.
2. Some Tunnels Carry Metric Attributes: Select from the tunnels
that carry Metric attributes.
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3. No Tunnels Carry Metric Attributes: Proceed to the next step
in [RFC4271], comparing the BGP Speaker's BGP ID.
* Some Iterations to Tunnels:
1. Tunnels Carrying Metric Attributes Exist: Select based on the
smallest latency, largest bandwidth, or smallest reliability.
2. No Tunnels Carry Metric Attributes: Compare the BGP Speaker's
BGP ID.
* No Iterations to Tunnels:
1. Proceed to the next step in [RFC4271], comparing the BGP
Speaker's BGP ID.
This update aims to make route selection more aligned with actual
network requirements, thereby enhancing the accuracy and efficiency
of traffic engineering.
7. IANA Considerations
IANA is requested to assign the following code point from the "BGP
Tunnel Encapsulation Attribute Sub-TLVs" Registry:
+============+================+===============+
| Code Point | Description | Reference |
+============+================+===============+
| TBD | Metric Sub-TLV | This document |
+------------+----------------+---------------+
Table 1: Code Point for Metric Sub-TLV
8. Security Considerations
TBD
9. References
9.1. Normative References
[I-D.ietf-idr-sr-policy-safi]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
D. Jain, "Advertising Segment Routing Policies in BGP",
Work in Progress, Internet-Draft, draft-ietf-idr-sr-
policy-safi-13, 6 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
policy-safi-13>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012,
DOI 10.17487/RFC9012, April 2021,
<https://www.rfc-editor.org/info/rfc9012>.
9.2. Informative References
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.
Acknowledgements
The authors would like to acknowledge the supports from Cheng Chang
and Bo Liu.
Authors' Addresses
Zhenqiang Li (editor)
China Mobile
China
Email: lizhenqiang@chinamobile.com
Liyan Song (editor)
China Mobile
China
Email: songliyan@chinamobile.com
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