No Further Fast Reroute for SRv6 Service SID
draft-liu-bess-srv6-service-sid-nffrr-flag-00
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Yisong Liu , Changwang Lin , Mengxiao Chen , Yao Liu | ||
| Last updated | 2024-03-01 | ||
| Replaces | draft-liu-bess-multihome-srv6-service-sid-flag | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
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| Send notices to | (None) |
draft-liu-bess-srv6-service-sid-nffrr-flag-00
BESS Yisong Liu
Internet Draft China Mobile
Intended status: Standards Track C. Lin
Expires: August 28, 2024 M. Chen
New H3C Technologies
Y. Liu
ZTE
March 1, 2024
No Further Fast Reroute for SRv6 Service SID
draft-liu-bess-srv6-service-sid-nffrr-flag-00
Abstract
In some multihoming SRv6 L3VPN and EVPN scenarios, once fast reroute
has taken place, a second fast reroute is undesirable and may cause
looping. This document proposes a mechanism to prevent further fast
reroutes by advertising No-Further-FRR flags for SRv6 Service SIDs
in BGP messages.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 28, 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
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Table of Contents
1. Introduction...................................................2
1.1. Requirements Language.....................................2
2. Use Case.......................................................3
2.1. SRv6 L3VPN Multihoming....................................3
2.2. SRv6 EVPN Multihoming.....................................5
3. Solution.......................................................5
3.1. Consideration for EVPN Single-Active Mode.................7
4. Extensions for BGP.............................................7
5. Backward Compatibility.........................................8
6. Security Considerations........................................8
7. IANA Considerations............................................9
8. References.....................................................9
8.1. Normative References......................................9
Authors' Addresses................................................9
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, two egress PEs may establish a backup path between them
and use it as the protection of PE-CE link failure. Once fast
reroute (FRR) has taken place, a second fast reroute is undesirable
and may cause looping.
This document defines the No-Further-FRR flag for SRv6 Service SIDs
carried in BGP messages and proposes a mechanism using the No-
Further-FRR flag to prevent further fast reroutes.
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. Use Case
2.1. 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! |
+------+-------------+-------+--------------+
2.2. 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.
3. 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 |
+------+--------------+-------+--------------+
3.1. 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.
4. Extensions for BGP
This document defines a new flag in the SRv6 Service SID Flags field
of the SRv6 SID Information Sub-TLV [RFC9252]:
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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| |
+-+-+-+-+-+-+-+-+
o N-flag: No-Further-FRR flag. When set, the associated SID has no
fast reroute protection.
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.
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.
When the egress PE advertises multiple service SIDs, the normal
service SID SHOULD be carried in the first instance of Sub-TLV. If
there are some other 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.
6. Security Considerations
TBD.
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7. 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 N-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>.
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
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Yao Liu
ZTE
China
Email: liu.yao71@zte.com.cn
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