S-BFD Path Consistency over SRv6
draft-lin-sbfd-path-consistency-over-srv6-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Changwang Lin , Weiqiang Cheng , Jiang Wenying | ||
| Last updated | 2022-03-02 | ||
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draft-lin-sbfd-path-consistency-over-srv6-00
BFD Working Group C. Lin
Internet Draft New H3C Technologies
Intended status: Informational W. Cheng
Expires: Sep 2, 2022 W. Jiang
China Mobile
March 2, 2022
S-BFD Path Consistency over SRv6
draft-lin-sbfd-path-consistency-over-srv6-00
Abstract
Bidirectional Forwarding Detection (BFD) can be used to monitor
paths between nodes. Seamless BFD (S-BFD) provides a simplified
mechanism which is suitable for monitoring of paths that are setup
dynamically and on a large scale network. In SRv6, when a headend
use S-BFD to monitor the segment list/CPath of SRv6 Policy, the
forward path of S-BFD packet is indicated by segment list, the
reverse path of BFD packet is via the shortest path from the
reflector back to the initiator (headend) as determined by routing.
The forward path and reverse path of S-BFD packet are likely
inconsistent going through different intermediate nodes or links.
This document describes a method to keep the forward path and
reverse path of S-BFD consistent when detecting SRv6 Policy.
Status of this Memo
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This Internet-Draft will expire on September 2 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction ................................................ 2
1.1. Requirements Language .................................. 3
2. Requirement for S-BFD in SRv6 ............................... 3
3. Correlate bidirectional path using Path Segment ............. 4
4. S-BFD Procedure with Path segment ........................... 6
4.1. S-BFD Initiator procedure .............................. 6
4.2. S-BFD Reflector procedure .............................. 8
5. IANA Considerations ........................................ 10
6. Security Considerations .................................... 10
7. References ................................................. 10
7.1. Normative References .................................. 10
Contributors .................................................. 11
Authors' Addresses ............................................ 12
1. Introduction
Segment Routing (SR) allows a headend node to steer a packet flow
along any path. Per-path states of Intermediate nodes are eliminated
thanks to source routing. The headend node steers a flow into an SR
Policy. The packets steered into an SR Policy carry an ordered list
of segments associated with that SR Policy.
S-BFD is used to monitor different kinds of paths between nodes. In
SRv6, when a headend use S-BFD to monitor the segment list/CPath of
SRv6 Policy, the forward and reverse path of S-BFD packet are
inconsistent with high probability because the reverse path is via
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IPv6 forwarding and forward path is via SRv6 segment list (loose
path or explicit path).
The inconsistency impacts the detecting result. If the forward path
is up and reverse path is down, then the S-BFD session will be down.
If there are multiple path (segment list) in a SRv6 Policy between a
headend (initiator) router and a tailend(reflector) router, multiple
S-BFD session will be created for each path. Each S-BFD session uses
corresponding path to send echo packet, but the reverse path is
identical for all S-BFD sessions. If the reverse path is down, all
sessions will be down. Then the SRv6 Policy is down.
The consistency of forward and reverse path of the same S-BFD
session should be guaranteed. This document describes a method to
keep the forward path and reverse path of S-BFD consistent using
path segment when detecting SRv6 Policy.
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. Requirement for S-BFD in SRv6
Monitor SRv6 Policy using S-BFD is usually based on segment list S-
BFD creates session for each segment list and associates the session
with segment list.
When S-BFD initiator detects the connectivity of an S-BFD session,
it will use the associated segment list to encapsulate IPv6 header
and SRH of the echo packet.
After the reflector receives the S-BFD echo, the response packet
should be able to return along the path to avoid the false detection
of the session caused by the inconsistency of the forward and
reverse paths.
Referring to the following topology, there are two paths between
NodeA and NodeD, and All nodes allocate end.x Segments. NodeA and
NodeD are headend and tailend nodes of each other, and SRv6 policy
is created on A and D respectively.
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SID-B1 SID-B2 SID-C1 SID-C2
+--------B-----------------C-----------+
SID-A1/ \ SID-D1
/ \
A D
\ /SID-D2
SID-A2\ SID-E1 SID-E2 /
+-------------------E-------------------+
Figure 1: reference topology
Assuming that the deployed SRv6 policy has one candidate path and
each path has two segment lists. For ease of description, segment
lists with the same number on NodeA and NodeD are forward and
reverse paths to each other.
NodeA: NodeD:
SRv6 Policy A-D SRv6 Policy D-A
Candidate Path1 Candidate Path1
Segment list1 Segment list1
SID-A1, SID-B2, SID-C2 SID-D1, SID-C1, SID-B1
Segment list2 Segment list2
SID-A2, SID-E2 SID-D2, SID-E1
When nodeA is the S-BFD initiator, S-BFD sessions for segment list1
and segment list2 could be created respectively.
The echo packet of S-BFD session associated with the segment list1
is forwarded to nodeD according to the segment list1 of nodeA. The
response packet of node D needs to be returned to node A according
to the segment list1 of node D. Thus the forward and reverse paths
of S-BFD packets are ensured to be consistent.
3. Correlate bidirectional path using Path Segment
A Path Segment is defined to identify an SR path in [draft-ietf-
spring-srv6-path-segment]. SRv6 Path segments can be used to
correlate the two unidirectional SRv6 paths at both ends of the
paths.
[draft-ietf-idr-sr-policy-path-segment] proposes an extension to BGP
SR Policy distribute SR policies carrying Path Segment and
bidirectional path information.
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Through this extension, when distributing SRv6 policy to the
headend, reverse path information and path segment of segment list
can be carried together.
NodeA NodeD
SRv6 Policy A-D SRv6 Policy D-A
Candidate Path1 Candidate Path1
Segment list1 Segment list1
SID-A1, SID-B2, SID-C2 SID-D1, SID-C1, SID-B1
Path Segment: SID-Path-A1 Path Segment: SID-Path-D1
Reverse Path Segment: Reverse Path Segment:
SID-Path-D1 SID-Path-A1
Segment list2 Segment list2
SID-A2, SID-E2 SID-D2, SID-E1
Path Segment: SID-Path-A2 Path Segment: SID-Path-D2
Reverse Path Segment: Reverse Path Segment:
SID-Path-D2 SID-Path-A2
In this way, on the headend in both directions of the forward and
reverse paths, the path segment of the paths in both directions can
be obtained, and the paths in both directions use the same
intermediate link.
The headend can use path segment in two directions to establish a
mapping table. Using this mapping table, the headend can index the
reverse path through the path segment of the forward path.
The mapping table of NodeA and Node D is shown below:
NodeA:
+-----------------+ +--------------------+
| Path Segment | |Reverse Path Segment|
+-----------------+ +--------------------+
| SID-Path-A1 |-+ | SID-Path-D1 |--+
+-----------------+ | +--------------------+ |
| SID-Path-A2 | | | SID-Path-D2 |--|-+
+-----------------+ | +--------------------+ | |
| | | |
| | +-----------------------+ | |
| | | segment List | | |
| | +-----------------------+ | |
| +->|SID-A1, SID-B2, SID-C2 |<----+ |
| +-----------------------+ |
+-------------->|SID-A2, SID-E2 |<------+
+-----------------------+
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NodeD:
+-----------------+ +--------------------+
| Path Segment | |Reverse Path Segment|
+-----------------+ +--------------------+
| SID-Path-D1 |-+ | SID-Path-A1 |--+
+-----------------+ | +--------------------+ |
| SID-Path-D2 | | | SID-Path-A2 |--|-+
+-----------------+ | +--------------------+ | |
| | | |
| | +-----------------------+ | |
| | | segment List | | |
| | +-----------------------+ | |
| +->|SID-D1, SID-C1, SID-B1 |<----+ |
| +-----------------------+ |
+-------------->|SID-D2, SID-E1 |<------+
+-----------------------+
Figure 2: mapping table
4. S-BFD Procedure with Path segment
This document proposes to forward S-BFD echo and response packets
through the consistent path by path segment.
4.1. S-BFD Initiator procedure
For instance, the S-BFD initiator is Node A in Figure 1, and the S-
BFD session is bounded with Segment List1 of Policy A-D. The
encapsulation format of S-BFD echo packet is as follows:
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+-----------------------------------------------------------+
| IPv6 Header |
. Source IP Address = S-BFD Initiator IPv6 Address .
. Destination IP Address = SegmentList[SL] .
. Next-Header = SRH (43) .
. .
+-----------------------------------------------------------+
| SRH as specified in RFC 8754 |
. Next-Header = IPv6 .
. <PathSegment, Segment List> .
. .
+-----------------------------------------------------------+
| IPv6 Header |
. Source IP Address = S-BFD Initiator IPv6 Address .
. Destination IP Address = S-BFD Reflector IPv6 Address .
. Next-Header = UDP .
. .
+-----------------------------------------------------------+
| UDP Header |
. .
+-----------------------------------------------------------+
| Payload |
. .
+-----------------------------------------------------------+
Figure 3: Encapsulation format of S-BFD echo packet
NodeA Encapsulates the path segment of segment list1 in SRH, and set
SRH.P-Flag.
The S-BFD echo packet is as follows:
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A------------->B------------>C---------->D
+-----------------+ +-----------------+
| SA=A's Ipv6Addr | | SA=A's Ipv6Addr |
+-----------------+ +-----------------+
| DA=SID-A1 | | DA=D's ipv6Addr |
+-----------------+ +-----------------+
| SL=3 | P-Flag=1 | | SL=0 | P-Flag=1 |
+-----------------+ +-----------------+
| D's ipv6Addr | | D's ipv6Addr |
+-----------------+ +-----------------+
| SID-C2 | | SID-C2 |
+-----------------+ +-----------------+
| SID-B2 | | SID-B2 |
+-----------------+ +-----------------+
| SID-A1 | | SID-A1 |
+-----------------+ +-----------------+
| SID-Path-A1 | | SID-Path-A1 |
+-----------------+ +-----------------+
| sbfd-payload | | sbfd-payload |
| | | |
+-----------------+ +-----------------+
Figure 4: Example of S-BFD echo packet
4.2. S-BFD Reflector procedure
S-BFD echo packet is forwarded along the path A->B->C-D. While
packet arrives at node D, RH.SL is 0 and the destination address is
the NodeD IPv6 address. Packet is delivered up to the S-BFD module
in control plane.
S-BFD module detects SRH.P-flag is set, extracts the path segment of
the forward path from SRH, gets the path segment of the reverse path
through the mapping table. When responding to S-BFD echo, S-BFD
module uses the segment list associated with path segment of the
reverse path to encapsulate SRH.
The encapsulation format of S-BFD response packet is as follows:
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+----------------------------------------------------------+
| IPv6 Header |
. Source IP Address = S-BFD Reflector IPv6 Address .
. Destination IP Address = SegmentList[SL] .
. Next-Header = SRH (43) .
. .
+----------------------------------------------------------+
| SRH as specified in RFC 8754 |
. Next-Header = IPv6 .
. <Segment List> .
. .
+----------------------------------------------------------+
| IPv6 Header |
. Source IP Address = S-BFD Reflector IPv6 Address .
. Destination IP Address = S-BFD Sender IPv6 Address .
. Next-Header = UDP .
. .
+----------------------------------------------------------+
| UDP Header |
. .
+----------------------------------------------------------+
| Payload |
. .
+----------------------------------------------------------+
Figure 5: Encapsulation format of S-BFD response packet
The Example of S-BFD response packet is as follows:
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D------------->C------------>B---------->A
+-----------------+ +-----------------+
| SA=D's Ipv6Addr | | SA=D's Ipv6Addr |
+-----------------+ +-----------------+
| DA=SID-D1 | | DA=A's ipv6Addr |
+-----------------+ +-----------------+
| SL=3 | P-Flag=0 | | SL=0 | P-Flag=0 |
+-----------------+ +-----------------+
| A's ipv6Addr | | A's ipv6Addr |
+-----------------+ +-----------------+
| SID-B1 | | SID-B1 |
+-----------------+ +-----------------+
| SID-C1 | | SID-C1 |
+-----------------+ +-----------------+
| SID-D1 | | SID-D1 |
+-----------------+ +-----------------+
| sbfd-payload | | sbfd-payload |
| | | |
+-----------------+ +-----------------+
Figure 6: Example of S-BFD response packet
The S-BFD response packet will be forward along the path D->C->B->A.
In this way, the forward and reverse paths of S-BFD are guaranteed
to be consistent.
5. IANA Considerations
This document has no IANA actions.
6. Security Considerations
The security requirements and mechanisms described in [RFC8402] and
[RFC8754] also apply to this document.
This document does not introduce any new security consideration.
7. References
7.1. Normative References
[I-D.ietf-idr-segment-routing-te-policy] Previdi, S., Filsfils, C.,
Talaulikar, K., Mattes, P.,Rosen, E., Jain, D., and S.
Lin, "Advertising Segment Routing Policies in BGP", draft-
ietf-idr-segment-routing-te-policy-14 (work in progress),
November 2021
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[I-D.ietf-spring-mpls-path-segment] Cheng, W., Li, H., Chen, M.,
Gandhi, R., and R. Zigler, "Path Segment in MPLS Based
Segment Routing Network",draft-ietf-spring-mpls-path-
segment-07 (work in progress), December 2021.
[I-D.ietf-spring-segment-routing-policy] Filsfils, C., Talaulikar,
K., Voyer, D., Bogdanov, A., and P. Mattes, "Segment
Routing Policy Architecture", draft-ietf-spring-segment-
routing-policy-18 (work in progress),February 2022.
[I-D.ietf-spring-srv6-path-segment] Li, C., Cheng, W., Chen, M.,
Dhody, D., and Y. Zhu, "Path Segment for SRv6 (Segment
Routing in IPv6)", draft-ietf-spring-srv6-path-segment-03
(work in progress),November 2021.
[I-D.ietf-idr-sr-policy-path-segment] Li, C., Li, Z., Yin, Y.,
Cheng, W., Talaulikar, K., "SR Policy Extensions for Path
Segment and Bidirectional Path", draft-ietf-idr-sr-policy-
path-segment-05(work in progress), January 2022.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June
2010,<https://www.rfc-editor.org/info/rfc5880>.
[RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
Pallagatti, "Seamless Bidirectional Forwarding Detection
(S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
<https://www.rfc-editor.org/info/rfc7880>.
[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>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy,
J., Matsushima, S., and D. Voyer, "IPv6 Segment Routing
Header (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986, DOI
0.17487/RFC8986, February 2021, <https://www.rfc-
editor.org/info/rfc8986>.
Contributors
Yisong Liu contributed to the content of this document.
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Authors' Addresses
Changwang Lin
New H3C Technologies
Beijing
China
Email: linchangwang.04414@h3c.com
Weiqiang Cheng
China Mobile
Beijing
CN
Email: chengweiqiang@chinamobile.com
Wenying Jiang
China Mobile
Beijing
CN
Email: jiangwenying@chinamobile.com
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