Internet DRAFT - draft-liu-bess-multihome-srv6-service-sid-flag
draft-liu-bess-multihome-srv6-service-sid-flag
BESS Yisong Liu
Internet Draft China Mobile
Intended status: Standards Track C. Lin
Expires: April 5, 2024 M. Chen
New H3C Technologies
Y. Liu
ZTE
October 8, 2023
SRv6 Service SID Flag Extension for Multi-homed SRv6 BGP Services
draft-liu-bess-multihome-srv6-service-sid-flag-02
Abstract
In some multihoming SRv6 L3VPN and EVPN scenarios, there are
requirements for the egress PE to advertise multiple SRv6 Service
SIDs for the same service, such as egress fast reroute and anycast
load balancing. This document defines No-Further-FRR-flag and
Anycast-flag for SRv6 Service SIDs carried in BGP messages.
Status of this Memo
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction...................................................2
1.1. Requirements Language.....................................2
2. No-Further-FRR Service SID.....................................3
2.1. Use Case..................................................3
2.1.1. SRv6 L3VPN Multihoming...............................3
2.1.2. SRv6 EVPN Multihoming................................5
2.2. Solution..................................................5
2.2.1. Consideration for EVPN Single-Active Mode............7
3. Anycast Service SID............................................7
4. Extensions for BGP.............................................9
5. Backward Compatibility........................................10
6. Security Considerations.......................................10
7. IANA Considerations...........................................10
8. References....................................................10
8.1. Normative References.....................................10
Authors' Addresses...............................................11
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
multiple SRv6 Service SIDs for the same service, such as egress fast
reroute and anycast load balancing. And those SIDs need to be
identified in the BGP messages.
This document defines No-Further-FRR-flag and 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
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BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. No-Further-FRR Service SID
2.1. Use Case
2.1.1. SRv6 L3VPN Multihoming
In the multihoming SRv6 L3VPN scenarios, two egress PEs may
establish a backup path between them and use it as the protection of
PE-CE link failure.
Take the network in Figure 1 as an example. When traffic goes from
CE1 to CE2, it may be load-balanced between PE2 and PE3 or only
forwarded to the main egress PE. If the link PE2-CE2 fails, PE2 can
still forward the traffic for CE2 by sending it over the backup path
to PE3 (and similarly for PE3 if link2 fails).
+-----+
| CE1 |
+-----+
|
|
+-----+
------------------- | PE1 |***************
^ +-----+ *
| / \ *
| / \ *
| P1 P2 *
| . . +------+
SRv6 VPN . *************.*******|BGP-RR|
| . * . +------+
| P3 * P4 *
| | * | *
| | * | *
v +-----+ Backup +-----+ *
--------- | PE2 |#############| PE3 |*****
+-----+ Path +-----+
\ /
\ /
+-----+
| CE2 |
+-----+
Figure 1
Examples of BGP routes advertised by PE2 and PE3 are as following:
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BGP Route by PE2:
VPN Prefix of CE2:
BGP Prefix SID Attr:
SRv6 L3 Service TLV:
SRv6 SID Information sub-TLV:
SID: SID-2
Behavior: End.DT46
BGP Route by PE3:
VPN Prefix of CE2:
BGP Prefix SID Attr:
SRv6 L3 Service TLV:
SRv6 SID Information sub-TLV:
SID: SID-3
Behavior: End.DT46
Examples of FIB entries for L3VPN service SID on PE2 and PE3 are as
following:
FIB on PE2:
SID-2:
Primary Next-hop: CE2
Backup Next-hop: Service SRv6 SID-3
FIB on PE3:
SID-3:
Primary Next-hop: CE2
Backup Next-hop: Service SRv6 SID-2
However, suppose CE2 is down. PE2 will think PE2-CE2 link is down
and send traffic to PE3 over the backup path. PE3 will also think
PE3-CE3 link is down and send the traffic back to PE2 over the
backup path. So, traffic will loop between PE2 and PE3 until BGP
convergence.
The traffic forwarding when CE2 fails is as following:
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+======+=============+=======+==============+
| Node | Packet | Next | Comment |
+======+=============+=======+==============+
| PE1 | <SID-2> pkt | PE2 | |
+------+-------------+-------+--------------+
| PE2 | pkt | CE2 | PE2-CE2 down |
+------+-------------+-------+--------------+
| PE2 | <SID-3> pkt | PE3 | FRR |
+------+-------------+-------+--------------+
| PE3 | pkt | CE2 | PE3-CE2 down |
+------+-------------+-------+--------------+
| PE3 | <SID-3> pkt | PE2 | FRR |
+------+-------------+-------+--------------+
| PE2 | -- | CE2 | PE2-CE2 down |
+------+-------------+-------+--------------+
| PE2 | <SID-3> pkt | PE3 | FRR |
+------+-------------+-------+--------------+
| ... | | | Loop! |
+------+-------------+-------+--------------+
2.1.2. SRv6 EVPN Multihoming
The EVPN services include Designated Forwarder (DF) election
procedure.
In All-Active mode, all PEs are allowed to forward unicast traffic,
which is similar with the L3VPN case in Section 2.1.
In Single-Active mode, only DF is allowed to forward unicast
traffic, and it requires additional considerations in FRR.
2.2. Solution
Each egress PE advertises an additional SRv6 Service SID in BGP
routes which is called No-Further-FRR SID.
The owner of No-Further-FRR SID will not provide local FRR for it.
When the next-hop of No-Further-FRR SID is down, like PE-CE link
failure or CE node failure, the PE will drop packets rather than
apply FRR.
The No-Further-FRR SID can used by other PE as the protection of
local PE-CE link failure, without worrying about the looping
problem.
To support backwards compatibility and BGP RR deployment, both the
normal SRv6 Service SID and the No-Further-FRR SID MAY be advertised
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together. A No-Further-FRR flag is used to indicate the No-Further-
FRR SID.
Detailed BGP extensions will be described in Section 4.
Still taking the network in Figure 1 as an example, the BGP routes
advertised by PE2 and PE3 are as following:
BGP Route by PE2:
VPN Prefix of CE2:
BGP Prefix SID Attr:
SRv6 L3 Service TLV:
SRv6 SID Information sub-TLV:
SID: SID-21
Behavior: End.DT46(L3VPN) or End.DX2/End.DT2U(EVPN)
SRv6 SID Information sub-TLV:
SID: SID-22
Behavior: End.DT46(L3VPN) or End.DX2/End.DT2U(EVPN)
Flag: No-Further-FRR
BGP Route by PE3:
VPN Prefix of CE2:
BGP Prefix SID Attr:
SRv6 L3 Service TLV:
SRv6 SID Information sub-TLV:
SID: SID-31
Behavior: End.DT46(L3VPN) or End.DX2/End.DT2U(EVPN)
SRv6 SID Information sub-TLV:
SID: SID-32
Behavior: End.DT46(L3VPN) or End.DX2/End.DT2U(EVPN)
Flag: No-Further-FRR
The FIB entries for L3VPN service SID on PE2 and PE3 are as
following:
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FIB on PE2:
SID-21:
Primary Next-hop: CE2
Backup Next-hop: Service SRv6 SID-32
SID-22 (No-Further-FRR):
Primary Next-hop: CE2
FIB on PE3:
SID-31:
Primary Next-hop: CE2
Backup Next-hop: Service SRv6 SID-22
SID-32 (No-Further-FRR):
Primary Next-hop: CE2
After adopting the proposed solution, if CE fails, PE2 will think
PE2-CE2 link is down and send traffic to PE3 by using the No-
Further-FRR SID-32. PE3 will also think PE3-CE3 link is down, but
PE3 will drop the packets rather than apply FRR.
The traffic forwarding when CE2 fails is as following:
+======+==============+=======+==============+
| Node | Packet | Next | Comment |
+======+==============+=======+==============+
| PE1 | <SID-21> pkt | PE2 | |
+------+--------------+-------+--------------+
| PE2 | pkt | CE2 | PE2-CE2 down |
+------+--------------+-------+--------------+
| PE2 | <SID-32> pkt | PE3 | FRR |
+------+--------------+-------+--------------+
| PE3 | pkt | CE2 | PE3-CE2 down |
+------+--------------+-------+--------------+
| PE3 | - | - | Drop |
+------+--------------+-------+--------------+
2.2.1. Consideration for EVPN Single-Active Mode
The processing of the No-Further-FRR SID should apply an override to
EVPN DF-Election and bypass the local blocking state on the AC,
until EVPN control plane reconverges.
3. Anycast Service SID
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 2
As shown in Figure 2, 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.
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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
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. So, 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.
4. Extensions for BGP
This document defines two new flags 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
+-+-+-+-+-+-+-+-+
|N|A| |
+-+-+-+-+-+-+-+-+
o N-flag: No-Further-FRR flag. When set, the associated SID has no
fast reroute protection.
o A-flag: Anycast flag. When set, the associated SID is anycast.
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The new-defined flags 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.
5. Backward Compatibility
According to [RFC9252],
o Any unknown flags in the SRv6 Service SID Flags field MUST be
ignored by the receiver.
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.
If there are PE routers not supporting the flags defined in this
document, the egress PE may expect those routers to use the first
SID and ignore the new-defined flags.
6. Security Considerations
TBD.
7. IANA Considerations
This document defines the following bits in the SRv6 Service SID
Flags field of SRv6 SID Information Sub-TLV:
TLV Code Point Value
--------------------------------------------------------
TBD N-flag
TBD A-flag
8. References
8.1. 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>.
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Authors' Addresses
Yisong Liu
China Mobile
China
Email: liuyisong@chinamobile.com
Changwang Lin
New H3C Technologies
China
Email: linchangwang.04414@h3c.com
Mengxiao Chen
New H3C Technologies
China
Email: chen.mengxiao@h3c.com
Yao Liu
ZTE
China
Email: liu.yao71@zte.com.cn
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