BESS Yisong Liu
Internet Draft China Mobile
Intended status: Standards Track C. Lin
Expires: September 18, 2025 New H3C Technologies
Y. Liu
ZTE
March 18, 2025
SRv6 Service SID Anycast Flag
draft-liu-bess-srv6-service-sid-anycast-flag-02
Abstract
In some multihoming SRv6 L3VPN and EVPN scenarios, there are
requirements for the egress PE to advertise both unicast and anycast
SRv6 Service SIDs for the same service. This document defines the
Anycast-flag for SRv6 Service SIDs carried in BGP messages.
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Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction...................................................2
1.1. Requirements Language.....................................2
2. Anycast Service SID............................................2
2.1. Use Case 1................................................3
2.2. Use Case 2................................................5
3. Extensions for BGP.............................................6
4. Backward Compatibility.........................................6
5. Security Considerations........................................7
6. IANA Considerations............................................7
7. References.....................................................7
7.1. Normative References......................................7
Authors' Addresses................................................7
1. Introduction
[RFC9252] defines procedures and messages for SRv6-based BGP
services, including Layer 3 Virtual Private Network (L3VPN),
Ethernet VPN (EVPN), and Internet services. In some multihoming
scenarios, there are requirements for the egress PE to advertise
both unicast and anycast SRv6 Service SIDs for the same service. And
those anycast SIDs need to be identified in the BGP messages.
This document defines the Anycast-flag for SRv6 Service SIDs carried
in BGP messages.
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. Anycast Service SID
In some multihoming scenarios, there are requirements for the egress
PE to advertise both unicast and anycast SRv6 Service SIDs for the
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same service. It is required to identify which Service SID is
anycast and which Service SID is unicast, when both two SIDs are
advertised in BGP messages.
IGP has Anycast-flag for SRv6 locator, but the IGP Anycast-flag can
be lost due to summarization. This document defines the Anycast-flag
for SRv6 Service SIDs carried in BGP messages. Below are two
application scenarios where both anycast service SID and unicast
service SID are advertised simultaneously.
2.1. Use Case 1
In the multihoming SRv6 L3VPN and EVPN scenarios, anycast Service
SID may be used to advertise the same service at different egress
PEs, which can improve service reliability and load balancing.
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+-----+ +-----+
| CE1 | | CE2 |
+-----+ +-----+
| |
+-----+ +-----+
---------- | PE1 | | PE2 |
^ +-----+ +-----+
| * *
| * *
SRv6 +-------+
L3VPN/EVPN |BGP-RR |
| +-------+
| * *
| * *
v +-----+ +-----+
---------- | PE3 | | PE4 |
+-----+ +-----+
1. Anycast \ / 1. Anycast
Service SID \ / Service SID
2. Unicast \ / 2. Unicast
Service SID-1 +-----+ Service SID-2
| CE3 |
+-----+
PE1:
VPN Traffic Policy:
PE3 & PE4 Load Balancing
FIB Entry for VPN Traffic:
Next-hop: Anycast Service SID
PE2:
VPN Traffic Policy:
PE3 Active, PE4 Backup
FIB Entry for VPN Traffic:
Primary Next-hop: Unicast Service SID-1
Backup Next-hop: Unicast Service SID-2
Figure 1
As shown in Figure 1, PE3 and PE4 use the same anycast SRv6 Service
SID for the VPN service of CE3. The ingress PE1 encapsulates the
payload in an outer IPv6 header where the destination address is
that anycast SRv6 Service SID. The packets from CE1 can reach CE3
through either PE3 or PE4. Assume that the path from PE1 to PE3 and
the path from PE1 to PE4 have the same cost. The traffic flows will
be load balanced between PE3 and PE4.
PE3 and PE4 also have unicast SRv6 Service SIDs, which are SID-1 and
SID-2, for the VPN service of CE3. The ingress PE2 uses SID-1 as the
primary SRv6 Service SID, and SID-2 as backup. The packets from CE2
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will be forwarded to CE3 through PE3. If any failure occurs on the
path to PE3, service will be switched to PE4.
Since ingress PE1 and PE2 have different strategies for the control
of VPN traffics, egress PE3 and PE4 each need to advertise two SRv6
Service SIDs, an anycast SID for ingress PE1 and a unicast SID for
ingress PE2. Local export policy may be used by egress PE3 and PE4
to control which SID is advertised to ingress PE1 and which is
advertised to ingress PE2. However, if BGP Route Reflector is
deployed, both the anycast Service SID and the unicast Service SID
will be advertised to RR and reflected to ingress PEs, and the
receiver has to choose which Service SID to use.
2.2. Use Case 2
=============================== ===============================
+ Group1 1. Anycast + + Group2 1. Anycast +
+ VPN SID-1 + + VPN SID-2 +
+ 2. Unicast + + 2. Unicast +
+ VPN SID-1 + + VPN SID-3 +
+ +------+ + + +------+ +
+ | PE1 | + + | PE3 | +
+ +------+ + + +------+ +
+ * * + + * * +
+ * * + + * * +
+ +-----+ +-----+ + + +-----+ +-----+ +
+ | CE1 | SRv6 BE | RR1 +----------+ RR2 | SRv6 BE | CE2 | +
+ +-----+ +-----+ + + +-----+ +-----+ +
+ \ / + + \ / +
+ \ / + + \ / +
+ \ / + + \ / +
+ +-----+ + + +-----+ +
+ | PE2 | + + | PE4 | +
+ +-----+ + + +-----+ +
+ 1. Anycast + + 1. Anycast +
+ VPN SID-1 + + VPN SID-2 +
+ 2. Unicast + + 2. Unicast +
+ VPN SID-2 + + VPN SID-4 +
+ + + +
=============================== ===============================
EVPN Over
|<------ SRv6 TE Policy ----->|
Figure 2
PE1 and PE2 belong to Group 1 and use the same Anycast IP 1. PE3 and
PE4 belong to Group 2 and use the same Anycast IP 2. PEs from
different groups use Anycast IP 1 and Anycast IP 2 as tunnel head
nodes to deploy SRv6 TE policies, reducing the number of SRv6 TE
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policies. Each PE device assigns two SRv6 VPN SIDs for the same VPN
service: Anycast VPN SID and Unicast VPN SID. Anycast Service SIDs
are used for forwarding between different groups. Within the same
group, Unicast Service SIDs are used for forwarding between Multi-
homed PE devices.
3. Extensions for BGP
This document defines a new flag in the SRv6 Service SID Flags field
of the SRv6 SID Information Sub-TLV [RFC9252]:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRv6 Service | SRv6 Service | |
| Sub-TLV | Sub-TLV | |
| Type=1 | Length | RESERVED1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRv6 SID Value (16 octets) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Svc SID Flags | SRv6 Endpoint Behavior | RESERVED2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRv6 Service Data Sub-Sub-TLVs //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Svc SID Flags:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| |A| |
+-+-+-+-+-+-+-+-+
o A-flag: Anycast flag. When set, the associated SID is anycast.
The new-defined flag can be used for the SRv6 Service SIDs of L3 and
L2 services, such as End.DX4, End.DT4, End.DX6, End.DT6, End.DT46.
End.DX2, End.DX2V, End.DT2U, etc.
4. Backward Compatibility
According to [RFC9252],
o Any unknown flags in the SRv6 Service SID Flags field MUST be
ignored by the receiver.
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o When multiple SRv6 SID Information Sub-TLVs are present, the
ingress PE SHOULD use the SRv6 SID from the first instance of the
Sub-TLV.
When the egress PE advertises multiple service SIDs, the unicast
service SID SHOULD be carried in the first instance of Sub-TLV. If
there are PE routers not supporting the flag defined in this
document, the egress PE MAY expect those routers to use the first
SID and ignore the new-defined flag.
5. Security Considerations
TBD.
6. IANA Considerations
This document defines the following bit in the SRv6 Service SID
Flags field of SRv6 SID Information Sub-TLV:
TLV Code Point Value
--------------------------------------------------------
TBD A-flag
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017
[RFC9252] Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene,
B., Zhuang, S., and J. Rabadan, "BGP Overlay Services
Based on Segment Routing over IPv6 (SRv6)", RFC 9252, DOI
10.17487/RFC9252, July 2022, <https://www.rfc-
editor.org/info/rfc9252>.
Authors' Addresses
Yisong Liu
China Mobile
China
Email: liuyisong@chinamobile.com
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Changwang Lin
New H3C Technologies
China
Email: linchangwang.04414@h3c.com
Yao Liu
ZTE
China
Email: liu.yao71@zte.com.cn
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