BFD Path Consistency over SR
draft-lin-bfd-path-consistency-over-sr-02
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Changwang Lin , Weiqiang Cheng , Jiang Wenying , Ran Chen | ||
| Last updated | 2023-11-08 | ||
| Replaces | draft-lin-sbfd-path-consistency-over-srv6 | ||
| 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 | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-lin-bfd-path-consistency-over-sr-02
BFD Working Group C. Lin
Internet Draft New H3C Technologies
Intended status: Informational W. Cheng
Expires: May 8, 2024 W. Jiang
China Mobile
R. Chen
ZTE Corporation
November 8, 2023
BFD Path Consistency over SR
draft-lin-bfd-path-consistency-over-sr-02
Abstract
Bidirectional Forwarding Detection (BFD) can be used to monitor
paths between nodes.
U-BFD defined in [I-D.ietf-bfd-unaffiliated-echo] can effectively
reduce the device equipment.
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 SR network, BFD can also be used to monitor SR paths. When a
headend use BFD to monitor the segment list/CPath of SR 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 consistent when using S-BFD or U-BFD to detect SR
Policy
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Lin, et al. Expire May, 2024 [Page 1]
Internet-Draft BFD Path Consistency November 2023
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on October 27 2023.
Copyright Notice
Copyright (c) 2023 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
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
respect to this document. Code Components extracted from this
document must include Simplified BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction ................................................ 3
1.1. Requirements Language .................................. 4
2. Path consistency for BFD in SR network ...................... 4
2.1. S-BFD .................................................. 5
2.2. U-BFD .................................................. 6
3. Path consistency for S-BFD .................................. 6
3.1. Correlate bidirectional path using Path Segment ........ 6
3.2. Procedure of S-BFD ..................................... 8
3.2.1. S-BFD in SRv6 ..................................... 8
3.2.2. S-BFD in SR-MPLS ................................. 11
4. Path consistency for U-BFD ................................. 14
4.1. Getting reverse segment list .......................... 14
4.2. Procedure of U-BFD .................................... 15
4.2.1. U-BFD in SRv6 .................................... 15
4.2.2. U-BFD in SR-MPLS ................................. 18
Lin, et al. Expires May, 2024 [Page 2]
Internet-Draft BFD Path Consistency November 2023
5. IANA Considerations ........................................ 20
6. Security Considerations .................................... 20
7. References ................................................. 20
7.1. Normative References .................................. 20
Contributors .................................................. 22
Authors' Addresses ............................................ 23
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.
SR can be instantiated on the MPLS data plane (MPLS-SR) and the IPv6
data plane (SRv6). On the MPLS-SR data plane, a segment is encoded
as an MPLS label, and an ordered list of segments is encoded as a
stack of labels. On the SRv6 data plane, a segment is encoded as an
IPv6 address (SRv6 SID) [RFC8986], and an ordered list of segments
is encoded as an ordered list of SRv6 SIDs in the SR header (SRH)
[RFC8754].
BFD Echo function was originally defined in [RFC5880] and [RFC5881],
where the remote system is required to loop the BFD Echo packets
back to the local system. To support BFD Echo Function, some
negotiations between the local system and remote system are needed,
and both the local and remote system need to maintain the BFD
session state.
Unaffiliated BFD Echo Function (U-BFD) is defined in [I-D.ietf-bfd-
unaffiliated-echo].Where the destination IP address of the BFD Echo
packets is set to one of the IP addresses of the local system.
Therefore, the Echo packets can be automatically looped back
(through normal IP forwarding) by the remote system to the local
system. With U-BFD, the remote system does not need to support any
BFD related functions and maintain any session states. This further
simplifies the BFD Echo Function process at the remote system hence
greatly increases scalability.
Seamless BFD (S-BFD) defined in [RFC7880] provides a simplified
mechanism which is suitable for monitoring of paths that are setup
dynamically and on a large scale network.
In the SR network, the headend node could use BFD(S-BFD or U-BFD) to
monitor the connectivity of the SR path to implement path switching.
When a headend use BFD to monitor the segment list/CPath of SR
Policy, the forward path of control packet is indicated by segment
Lin, et al. Expires May, 2024 [Page 3]
Internet-Draft BFD Path Consistency November 2023
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.
The inconsistency impacts the detecting result. If the forward path
is up and reverse path is down, then the BFD session will be down.
If there are multiple path (segment list) in a SR Policy between a
headend (local system) node and a tailend(remote system) node,
multiple BFD session will be created for each path. Each BFD session
uses corresponding path to send control packet, but the reverse path
is identical for all BFD sessions. If the reverse path is down, all
sessions will be down. Then the SR Policy is down.
The consistency of forward and reverse path of the same BFD session
should be guaranteed.
This document describes how to ensure the consistency of the forward
path and the reverse path when using BFD to detect SR 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. Path consistency for BFD in SR network
Monitor SR Policy using BFD is usually based on segment list. BFD
session is created for each segment list and associated with the
segment list.
Referring to the following topology, there are two paths between
Node A and D, and All nodes allocate end.x Segments on SRv6 data
plane or adjacency SIDs on SR-MPLS data plane. Node A and D are
headend and tailend nodes of each other, and SR policy is created on
A and D respectively.
Lin, et al. Expires May, 2024 [Page 4]
Internet-Draft BFD Path Consistency November 2023
SID-B1 SID-B2 SID-C1 SID-C2
+--------B-----------------C-----------+
SID-A1/ \ SID-D1
/ \
A D
\ /
SID-A2\ SID-E1 SID-E2 /SID-D2
+-------------------E-------------------+
Figure 1: reference topology
Assuming that the deployed SR 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:
SR Policy A-D SR 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
Both Node A and Node D serve as head nodes and need to detect the
connectivity of the segment list of a SR Policy. Regardless of
whether S-BFD or U-BFD is used, there is a requirement for BFD
packet path consistency.
2.1. S-BFD
When node A is the S-BFD initiator, S-BFD sessions for segment list1
and segment list2 could be created respectively. Node A will use the
associated segment list to encapsulate IPv6 header and SRH of the
control packet.
As the S-BFD reflector, after Node D receives the S-BFD control
packet, the response control packet should be able to return along
the same path to avoid the false detection of the session caused by
the inconsistency of the forward and reverse paths.
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
Lin, et al. Expires May, 2024 [Page 5]
Internet-Draft BFD Path Consistency November 2023
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.
2.2. U-BFD
The working mechanism of U-BFD is that the local system sends a bfd
echo packet, and the destination address is the IP address of the
local system. After the bfd echo packet reaches the remote system,
the remote system returns the bfd echo packet to the local system in
the data plane. So U-BFD usually works when there is only one hop
between the local and remote systems.
When deploying U-BFD in SR network, local system could creates a U-
BFD session for each segment list under the SR Policy, and uses the
segment list to encapsulate BFD echo packets. For SR-MPLS
encapsulation is the label stack, for SRv6 it is the segment list in
SRH. In this way, the U-BFD echo packet can reach the remote system
through multiple hops.
When the U-BFD echo packet reaches the remote system, the
destination address of the packet has been updated to the IP address
of the local system, so the remote system sends the U-BFD echo
packet back to the local system on the data plane.
The U-BFD echo packet returned from remote system to local system
should follow the same path from local system to remote system.
3. Path consistency for S-BFD
This draft proposes to forward S-BFD control packets and response
control packets through the consistent path by path segment.
3.1. Correlate bidirectional path using Path Segment
A Path Segment is defined to identify an SR path. In SR for MPLS
data plane (SR-MPLS), Path Segment is defined in [draft-ietf-spring-
mpls-path-segment]. In SR for IPv6 data plane (SRv6), Path Segment
is defined in [I-D.ietf-spring-srv6-path-segment].
SR(SR-MPLS or SRv6) Path segments can be used to correlate the two
unidirectional SR paths at both ends of the paths.
[I-D.ietf-idr-sr-policy-path-segment] proposes an extension to BGP
SR Policy distribute SR policies carrying Path Segment and
bidirectional path information.
Lin, et al. Expires May, 2024 [Page 6]
Internet-Draft BFD Path Consistency November 2023
Through this extension, when distributing SR policy to the headend
node, reverse path information and path segment of segment list can
be carried together.
Node A Node D
SR Policy A-D SR 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 node 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 links.
The headend node can use path segment in two directions to establish
a mapping table. Using this mapping table, the headend node can get
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 |<------+
+-----------------------+
Lin, et al. Expires May, 2024 [Page 7]
Internet-Draft BFD Path Consistency November 2023
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
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
following sub-section describes the processing of S-BFD in SR-MPLS
and SRv6 networks respectively.
3.2. Procedure of S-BFD
3.2.1. S-BFD in SRv6
o S-BFD Initiator procedure
Refer to [I-D.draft-liu-spring-bfd-srv6-policy-encap] for the
description of how to encapsulate S-BFD packet with SRv6 Policy.
When path segment is used, the encapsulation format of S-BFD control
packet is as follows:
Lin, et al. Expires May, 2024 [Page 8]
Internet-Draft BFD Path Consistency November 2023
+-----------------------------------------------+
| IPv6 Header |
. Source IP Address = Initiator's IPv6 Address .
. Destination IP Address = SegmentList[SL] .
. Next-Header = SRH (43) .
. .
+-----------------------------------------------+
| SRH as specified in RFC 8754 |
. Next-Header = IPv6 .
. <P-Flag=1, PathSegment, Segment List> .
. .
+-----------------------------------------------+
| |
. sbfd-payload .
| |
+-----------------------------------------------+
NodeA Encapsulates the path segment of segment list1 in SRH, and set
SRH.P-Flag.
The S-BFD control packet is encapsulated and forwarded as follows:
A------------->B------------>C---------->D
+-----------------+ +-----------------+
| SA=A's Ipv6Addr | | SA=A's Ipv6Addr |
+-----------------+ +-----------------+
| DA=SID-B1 | | DA=D's ipv6Addr |
+-----------------+ +-----------------+
| SL=2 | 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-1 | | SID-Path-1 |
+-----------------+ +-----------------+
| sbfd-payload | | sbfd-payload |
| | | |
+-----------------+ +-----------------+
Figure 3: Example of S-BFD control packet
Lin, et al. Expires May, 2024 [Page 9]
Internet-Draft BFD Path Consistency November 2023
o S-BFD Reflector procedure
S-BFD control packet is forwarded along the path A->B->C-D. While
packet arrives at Node D, SRH.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:
+-----------------------------------------------+
| IPv6 Header |
. Source IP Address = Reflector's IPv6 Address .
. Destination IP Address = SegmentList[SL] .
. Next-Header = SRH (43) .
. .
+-----------------------------------------------+
| SRH as specified in RFC 8754 |
. Next-Header = IPv6 .
. <Segment List> .
. .
+-----------------------------------------------+
| |
. sbfd-payload .
| |
+-----------------------------------------------+
The S-BFD response packet is encapsulated and forwarded as follows:
Lin, et al. Expires May, 2024 [Page 10]
Internet-Draft BFD Path Consistency November 2023
D------------->C------------>B---------->A
+-----------------+ +-----------------+
| SA=D's Ipv6Addr | | SA=D's Ipv6Addr |
+-----------------+ +-----------------+
| DA=SID-C1 | | DA=A's ipv6Addr |
+-----------------+ +-----------------+
| SL=2 | 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 |
| | | |
+-----------------+ +-----------------+
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.
3.2.2. S-BFD in SR-MPLS
o S-BFD Initiator procedure
The encapsulation format using SR Policy with path segment of S-BFD
control packet is as follows:
Lin, et al. Expires May, 2024 [Page 11]
Internet-Draft BFD Path Consistency November 2023
+--------------------+
| ... |
+--------------------+
| Label 1 |
+--------------------+
| Label 2 |
+--------------------+
| ... |
+--------------------+
| Label n |
+--------------------+
| Path Segment |
+--------------------+
| IPv6 Header: |
| Source IP |
| Destination IP |
~ ~
+--------------------+
~ sbfd-Payload ~
+--------------------+
Node A Encapsulates the segment list1 and path segment in label
stack. The source IP is the IPv6 address of Node A, and the
destination IP is the IPv6 address of Node D.
The S-BFD control packet is encapsulated and forwarded as follows:
A------------->B------------>C---------->D
+-----------------+
| SID-A1 |
+-----------------+
| SID-B2 |
+-----------------+
| SID-C2 |
+-----------------+ +-----------------+
| SID-Path-1 | | SID-Path-1 |
+-----------------+ +-----------------+
| SA=A's Ipv6Addr | | SA=A's Ipv6Addr |
+-----------------+ +-----------------+
| DA=D's ipv6Addr | | DA=D's ipv6Addr |
+-----------------+ +-----------------+
| sbfd-payload | | sbfd-payload |
| | | |
+-----------------+ +-----------------+
o S-BFD Reflector procedure
Lin, et al. Expires May, 2024 [Page 12]
Internet-Draft BFD Path Consistency November 2023
S-BFD control packet is forwarded along the path A->B->C-D. When the
packet arrives at node D, the top-level label is path segment.
Packet with path segment is delivered up to the S-BFD module in
control plane.
When responding to S-BFD control packet, the S-BFD module uses the
mapping table to find the label stack of the reverse path through
the path segment to encapsulate the response control packet.
The encapsulation format of S-BFD response control packet is as
follows:
The source IP is the IPv6 address of Node D, and the destination IP
is the IPv6 address of Node A.
+--------------------+
| ... |
+--------------------+
| Label 1 |
+--------------------+
| Label 2 |
+--------------------+
| ... |
+--------------------+
| Label n |
+--------------------+
| IPv6 Header: |
| Source IP |
| Destination IP |
~ ~
+--------------------+
~ sbfd-Payload ~
+--------------------+
The S-BFD response packet is encapsulated and forwarded as follows:
Lin, et al. Expires May, 2024 [Page 13]
Internet-Draft BFD Path Consistency November 2023
D------------->C------------>B---------->A
+-----------------+
| SID-D1 |
+-----------------+
| SID-C1 |
+-----------------+
| SID-B1 |
+-----------------+ +-----------------+
| SA=D's Ipv6Addr | | SA=D's Ipv6Addr |
+-----------------+ +-----------------+
| DA=A's Ipv6Addr | | DA=A's Ipv6Addr |
+-----------------+ +-----------------+
| sbfd-payload | | sbfd-payload |
| | | |
+-----------------+ +-----------------+
4. Path consistency for U-BFD
This document proposes to encapsulate the segment list of the return
path in the U-BFD echo packet to guide the packet from the remote
system to the local system along the same path.
4.1. Getting reverse segment list
[I-D.ietf-idr-sr-policy-path-segment] proposes an extension to BGP
SR Policy distribute SR policies carrying reverse path information.
The reverse path information includes reverse segment list and
reverse path segment. The reverse path segment can be used for S-BFD
path consistency, and the reverse segment list can be used for U-BFD
path consistency.
Referring to the example topology, the SR Policy on nodes A and D
that contains complete reverse path information is as follows
Node A Node D
SR Policy A-D SR 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 segment list Reverse Segment list
SID-D1, SID-C1, SID-B1 SID-A1, SID-B2, SID-C2
Reverse Path Segment: Reverse Path Segment:
Lin, et al. Expires May, 2024 [Page 14]
Internet-Draft BFD Path Consistency November 2023
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 segment list Reverse segment list
SID-D2, SID-E1 SID-A2, SID-E2
Reverse Path Segment: Reverse Path Segment:
SID-Path-4 SID-Path-3
4.2. Procedure of U-BFD
The headend node uses U-BFD to detect a segment list of SR-Policy.
In order to achieve path consistency, the reverse segment list can
be encapsulated in the U-BFD echo packet at the same time. When the
U-BFD echo packet reaches the tailend node of SR-Policy, it will be
looped back to the headend node according to the path specified by
the reverse segment list.
According to this method, when the segment list in the SR-Policy and
its corresponding reverse segment list are planned to pass through
the same intermediate link, the U-BFD echo packet's round-trip path
will be consistent.
4.2.1. U-BFD in SRv6
In SRv6, the reverse segment list can be encapsulated in one SRH
with the forward segment list, or it can be encapsulated in an
independent SRH
When the forward and reverse segment lists are in the same SRH, the
encapsulation is as follows
Lin, et al. Expires May, 2024 [Page 15]
Internet-Draft BFD Path Consistency November 2023
+--------------------------------------------+
| IPv6 Header |
. Source IP Address = Node A's IPv6 Address .
. Destination IP Address = SegmentList[SL] .
. Next-Header = SRH (43) .
. .
+--------------------------------------------+
| SRH as specified in RFC 8754 |
. Next-Header = IPv6 .
. Node A's IPv6 Address .
. <ReverseSegment List> .
. <Segment List> .
. .
+--------------------------------------------+
| |
. ubfd-payload .
| |
+--------------------------------------------+
When the forward and reverse segment lists are in different SRHs,
the encapsulation is as follows
+--------------------------------------------+
| IPv6 Header |
. Source IP Address = Node A's IPv6 Address .
. Destination IP Address = SegmentList[SL] .
. Next-Header = SRH (43) .
. .
+--------------------------------------------+
| SRH as specified in RFC 8754 |
. Next-Header = SRH (43) .
. <Segment List> .
. .
+--------------------------------------------+
| SRH as specified in RFC 8754 |
. Next-Header = IPv6 .
. Node A's IPv6 Address .
. <ReverseSegment List> .
. .
+--------------------------------------------+
| |
. ubfd-payload .
| |
+--------------------------------------------+
Lin, et al. Expires May, 2024 [Page 16]
Internet-Draft BFD Path Consistency November 2023
Referring to the sample topology, take node A as the head node and D
as the tail node as an example. Node A uses U-BFD to detect segment
list1. The forward segment list and reverse segment list and the
address of node A are encapsulated in one SRH.
The U-BFD echo packet is encapsulated and forwarded as follows:
A------------->B------------>C---------->D
+-----------------+ +-----------------+
| SA=A's Ipv6Addr | | SA=A's Ipv6Addr |
+-----------------+ +-----------------+
| DA=SID-B2 | | DA=SID-D1 |
+-----------------+ +-----------------+
| SL=5 | | SL=3 |
+-----------------+ +-----------------+
| A's ipv6Addr | | A's ipv6Addr |
+-----------------+ +-----------------+
| SID-B1 | | SID-B1 |
+-----------------+ +-----------------+
| SID-C1 | | SID-C1 |
+-----------------+ +-----------------+
| SID-D1 | | SID-D1 |
+-----------------+ +-----------------+
| SID-C2 | | SID-C2 |
+-----------------+ +-----------------+
| SID-B2 | | SID-B2 |
+-----------------+ +-----------------+
| SID-A1 | | SID-A1 |
+-----------------+ +-----------------+
| ubfd-payload | | ubfd-payload |
| | | |
+-----------------+ +-----------------+
Figure 7: Example of U-BFD echo packet in SRv6
After the u-BFD packet reaches Node D, Node D continues to forward
the u-BFD packet according to the SRH, and returns the U-BFD echo
packet to Node A.
Lin, et al. Expires May, 2024 [Page 17]
Internet-Draft BFD Path Consistency November 2023
D------------->C------------>B---------->A
+-----------------+ +-----------------+
| SA=A's Ipv6Addr | | SA=A's Ipv6Addr |
+-----------------+ +-----------------+
| DA=SID-C1 | | DA=A's ipv6Addr |
+-----------------+ +-----------------+
| SL=2 | | SL=0 |
+-----------------+ +-----------------+
| A's ipv6Addr | | A's ipv6Addr |
+-----------------+ +-----------------+
| SID-B1 | | SID-B1 |
+-----------------+ +-----------------+
| SID-C1 | | SID-C1 |
+-----------------+ +-----------------+
| SID-D1 | | SID-D1 |
+-----------------+ +-----------------+
| SID-C2 | | SID-C2 |
+-----------------+ +-----------------+
| SID-B2 | | SID-B2 |
+-----------------+ +-----------------+
| SID-A1 | | SID-A1 |
+-----------------+ +-----------------+
| sbfd-payload | | sbfd-payload |
| | | |
+-----------------+ +-----------------+
The U-BFD echo packet will be forward along the path D->C->B->A. In
this way, the forward and reverse paths of U-BFD are guaranteed to
be consistent.
4.2.2. U-BFD in SR-MPLS
In SR-MPLS, The segment list and the reverse segment list can be
encapsulated in the label stack at the same time.
Lin, et al. Expires May, 2024 [Page 18]
Internet-Draft BFD Path Consistency November 2023
+--------------------+
| ... |
+--------------------+
| |
. label stack .
| |
+--------------------+
| |
. reverse label stack.
| |
+--------------------+
| IPv6 Header: |
| Source IP |
| Destination IP |
~ ~
+--------------------+
~ ubfd-Payload ~
+--------------------+
Take node A as the headend node and D as the tailend node as an
example, Node A uses U-BFD to detect segment list1. In order to
achieve consistent paths, the encapsulation and processing of U-BFD
echo packets are as follows
A------------->B------------>C---------->D
+-----------------+
| SID-A1 |
+-----------------+
| SID-B2 |
+-----------------+
| SID-C2 |
+-----------------+ +-----------------+
| SID-D1 | | SID-D1 |
+-----------------+ +-----------------+
| SID-C1 | | SID-C1 |
+-----------------+ +-----------------+
| SID-B1 | | SID-B1 |
+-----------------+ +-----------------+
| SA=A's Ipv6Addr | | SA=A's Ipv6Addr |
+-----------------+ +-----------------+
| DA=A's ipv6Addr | | DA=A's ipv6Addr |
+-----------------+ +-----------------+
| ubfd-payload | | ubfd-payload |
| | | |
+-----------------+ +-----------------+
Lin, et al. Expires May, 2024 [Page 19]
Internet-Draft BFD Path Consistency November 2023
After the u-BFD packet reaches Node D, Node D continues to forward
the u-BFD packet according to the label stack, and returns the U-BFD
echo packet to Node A.
D------------->C------------>B---------->A
+-----------------+
| SID-D1 |
+-----------------+
| SID-C1 |
+-----------------+
| SID-B1 |
+-----------------+ +-----------------+
| SA=A's Ipv6Addr | | SA=A's Ipv6Addr |
+-----------------+ +-----------------+
| DA=A's Ipv6Addr | | DA=A's Ipv6Addr |
+-----------------+ +-----------------+
| ubfd-payload | | ubfd-payload |
| | | |
+-----------------+ +-----------------+
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.draft-liu-spring-bfd-srv6-policy-encap] Liu, Y., Cheng, W.,
Lin, C., Chen, M., "Encapsulation of BFD for SRv6 Policy",
draft-liu-spring-bfd-srv6-policy-encap-00(work in
progress), October 2022
[I-D.ietf-bfd-unaffiliated-echo] Cheng, W., Wang, R., Min, X.,
Rahman, R., and R. C. Boddireddy, "Unaffiliated BFD Echo",
Work in Progress, Internet-Draft, draft-ietf-bfd-
unaffiliated-echo-06, 25 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-bfd-
unaffiliated-echo-06>.
Lin, et al. Expires May, 2024 [Page 20]
Internet-Draft BFD Path Consistency November 2023
[I-D.ietf-idr-segment-routing-te-policy] Previdi, S., Filsfils, C.,
Talaulikar, K., Mattes, P., Jain, D., and S. Lin,
"Advertising Segment Routing Policies in BGP", draft-ietf-
idr-segment-routing-te-policy-20 (work in progress), July
2022
[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-08 (work in progress), September 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-05
(work in progress),October 2022.
[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-07(work in progress), February 2023.
[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>.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, DOI
10.17487/RFC5881, June 2010, <https://www.rfc-
editor.org/info/rfc5881>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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>.
Lin, et al. Expires May, 2024 [Page 21]
Internet-Draft BFD Path Consistency November 2023
[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>.
[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>.
Contributors
Yisong Liu contributed to the content of this document.
Lin, et al. Expires May, 2024 [Page 22]
Internet-Draft BFD Path Consistency November 2023
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
Ran Chen
ZTE Corporation
China
Email: chen.ran@zte.com.cn
Lin, et al. Expires May, 2024 [Page 23]