S-BFD over SRv6China MobileBeijing100053Chinalizhiqiangyjy@chinamobile.comChina MobileBeijing100053Chinasuntao@chinamobile.comCentecSuzhou215000Chinachengw@centecnetworks.comCentecSuzhou21500Chinawangjj@centecnetworks.com
Networking
Network Working GroupBFD,S-BFD;SRv6Bidirectional Forwarding Detection (BFD) can be used to monitor
paths between node. 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. This draft describes a method
to simplify the implementation of S-BFD over SRv6 by using SRH.flag to
instruct the S-BFD peer node to do reverse operation of SRv6 SID list.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .With the increasing adoption of segment routing (SR) technology,
ISPs have upgraded their networks seamlessly from MPLS to SR MPLS,
and their next goal might be the overall upgrading of the IPv6
underlay network forwarding plane.We hope to implement BFD over SRv6 while retaining the
bidirectional detection capabilities of traditional BFD,
rather than using asymmetrical path detection only. Another
problem relates to the bidirectional detection mechanism
in BFD over SRv6, Using SR Policy or using TLV to carry the
return path brings extra load to the message parsing depth
on existing SRv6 device.In order to accelerate applying BFD in SRv6 networks,
this paper proposed a S-BFD over SRv6 implementation solution.As shown in the figure below, the BFD initiator is A and the
peer node is D, while bfd packets forwarding from A to D via
the path: A->B->C->D, and return via the path: D->C->B->A.
+-----B-------C-----+
/ \
A-----------E-----------D
\ /
+-----F-------G-----+
Forward Paths: A-B-C-D
Return Paths: D-C-B-A
SRv6 SID operations on the initial node A: The SRv6 SID
list {A, B, C, D} is pushed into Node A.SRv6 SID operations on the terminal node D: The SRv6 SID
list {A, B, C, D} is swapped in Node D, and the updated SRv6
SID list is : {D, C, B, A}, and the Last Entry, Segment Left,
and other fields are updated.Return Path: D->C->B->A.As shown in the figure below, the length of the flags
field in the SRH header is 8-bit. This draft uses the left third
bit (0|0|R|0|0|0|0|0) to represent the reverse operation of
the SRv6 SID list. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hdr=144 | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last Entry | Flags | Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[0] (128-bit IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
...
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[n] (128-bit IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| SRv6 SPAN Header |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Origin Packet |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+BFD peer node D check if SRH.Flags[5] == 1, it means that
this device requires the reverse operation of the SRv6 SID list.This solution does not need to use the SRv6 Policy to add length
of the SID list or to carry the SID list of the return path by TLV.
It only needs to support reverse SRv6 SID in the reflector node to
solve the issue of S-BFD over SRv6 described in the previous.In future versions of this paper, we will also consider the
compatibility of using compressed IDs in SRv6, such as seamlessly
merging S-BFD over G-SRv6. Furthermore, there will be no effect on
intermediate nodes within the SRv6 network and it only requires
S-BFD reflector support the SID reverse operation.TBD.TBD.