BFD Working Group C. Lin
Internet Draft New H3C Technologies
Intended status: Informational W. Cheng
Expires: Oct 12, 2022 W. Jiang
China Mobile
April 12, 2022
S-BFD Path Consistency over SRv6
draft-lin-sbfd-path-consistency-over-srv6-01
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 control packet is indicated by segment list, the
reverse path of response control 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 control 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 October 12 2022.
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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 control 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 continuity of an S-BFD session, it
will use the associated segment list to encapsulate IPv6 header and
SRH of the control packet.
After the reflector receives the S-BFD control packet, the response
control 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
Node A and D, and All nodes allocate end.x Segments. Node A and D
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 Node A and D are forward and reverse
paths to each other.
Node A: Node D:
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 node A is the S-BFD initiator, S-BFD sessions for segment list1
and segment list2 could be created respectively.
The control packet of S-BFD session associated with the segment
list1 is forwarded to node D according to the segment list1 of node
A. The response control 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.
Node A Node D
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-1 Path Segment: SID-Path-2
Reverse Path Segment: Reverse Path Segment:
SID-Path-2 SID-Path-1
Segment list2 Segment list2
SID-A2, SID-E2 SID-D2, SID-E1
Path Segment: SID-Path-3 Path Segment: SID-Path-4
Reverse Path Segment: Reverse Path Segment:
SID-Path-4 SID-Path-3
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 Node A and Node D is shown below:
Node A:
+-----------------+ +--------------------+
| Path Segment | |Reverse Path Segment|
+-----------------+ +--------------------+
| SID-Path-1 |-+ | SID-Path-2 |--+
+-----------------+ | +--------------------+ |
| SID-Path-3 | | | SID-Path-4 |--|-+
+-----------------+ | +--------------------+ | |
| | | |
| | +-----------------------+ | |
| | | segment List | | |
| | +-----------------------+ | |
| +->|SID-A1, SID-B2, SID-C2 |<----+ |
| +-----------------------+ |
+-------------->|SID-A2, SID-E2 |<------+
+-----------------------+
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Node D:
+-----------------+ +--------------------+
| Path Segment | |Reverse Path Segment|
+-----------------+ +--------------------+
| SID-Path-2 |-+ | SID-Path-1 |--+
+-----------------+ | +--------------------+ |
| SID-Path-4 | | | SID-Path-3 |--|-+
+-----------------+ | +--------------------+ | |
| | | |
| | +-----------------------+ | |
| | | 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 control packets and response
control 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 control 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 control packet
NodeA Encapsulates the path segment of segment list1 in SRH, and set
SRH.P-Flag.
The S-BFD control 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 control packet
4.2. S-BFD Reflector procedure
S-BFD control 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
IPv6 address of Node D. 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 control packet,
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 control 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 control packet
The Example of S-BFD response control 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 control packet
The S-BFD response control 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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