Network Working Group Y. Liu Internet Draft W. Cheng Intended status: Standards Track China Mobile Expires: October 24, 2023 C. Lin M. Chen New H3C Technologies X. Min ZTE April 24, 2023 Encapsulation of BFD for SRv6 Policy draft-liu-spring-bfd-srv6-policy-encap-01 Abstract Bidirectional Forwarding Detection (BFD) mechanisms can be used for fast detection of failures in the forwarding path of SR Policy. This document describes the encapsulation of BFD for SRv6 Policy, which can be applied for both S-BFD and U-BFD. The BFD packets may be encapsulated in transport mode or tunnel mode. 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). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on October 24, 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 Liu, et al. Expire October, 2023 [Page 1] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 (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...................................................2 1.1. Requirements Language.....................................3 2. Encapsulation of BFD Packet for SRv6 Policy....................3 2.1. Control Packet in Transport Mode..........................4 2.2. Echo Packet in Transport Mode.............................6 2.3. Control Packet in Tunnel Mode.............................7 2.4. Echo Packet in Tunnel Mode................................8 3. Choice of Headend and Tail-end IPv6 Addresses.................10 3.1. Control Packet...........................................10 3.2. Echo Packet..............................................10 4. Checksum in UDP Header........................................11 5. Control of Inserting Additional IPv6 Address in SRH...........11 6. Example.......................................................11 7. Security Considerations.......................................13 8. IANA Considerations...........................................13 9. References....................................................14 9.1. Normative References.....................................14 9.2. Informative References...................................15 Authors' Addresses...............................................16 1. Introduction Segment Routing (SR) [RFC8402] allows a headend node to steer a packet flow along any path. Per-path states of Intermediate nodes are eliminated thanks to source routing. A Segment Routing Policy (SR Policy) [RFC9256] is an ordered list of segments (i.e., instructions) that represent a source-routed policy. The packets steered into an SR Policy carry an ordered list of segments associated with that SR Policy. The SRv6 Policy is the instantiation of SR Policy for SR over IPv6 (SRv6) data-plane. In order to provide end-to-end protection, the headend node need to rapidly detect any failures in the forwarding path of SR Policy, so that it could switch from the active candidate path to another backup candidate path within the same SR Policy or switch from the active SR Policy to another backup SR Policy. Bidirectional Liu, et al. Expires October, 2023 [Page 2] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 Forwarding Detection (BFD) mechanisms [RFC5880] [RFC7880] can be used for fast detection of such failures. As described in [I-D.ali-spring-bfd-sr-policy], the BFD mechanism to be used for monitoring SRv6 Policies is expected to be simple and lightweight, which can be setup and deleted dynamically and on- demand. Seamless Bidirectional Forwarding Detection (S-BFD) [RFC7880] simplifies the mechanism for using BFD with a large proportion of negotiation aspects eliminated. [I-D.ali-spring-bfd- sr-policy] describes the use of S-BFD for monitoring of SR Policy paths, and specifies the S-BFD Discriminator selection, the S-BFD session initiation and the return path control related to S-BFD use for SR Policy. Unaffiliated BFD Echo Function (U-BFD) [I-D.ietf-bfd-unaffiliated- echo] simplifies the BFD Echo Function procedure, by which the remote system does not need to support BFD or maintain any BFD session states. U-BFD may also be used to monitor SR Policies. This document describes the encapsulation of BFD for SRv6 Policy, which can be applied for both S-BFD and U-BFD. The BFD packets may be encapsulated in transport mode or tunnel mode. 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. Encapsulation of BFD Packet for SRv6 Policy On SRv6 data-plane, a BFD packet for SRv6 Policy carries a Segment Routing Header (SRH) [RFC8754] containing a list of SRv6 SIDs associated with that SRv6 Policy. The BFD packets may be encapsulated in transport mode or tunnel mode. In transport mode, the SRH is inserted after the IPv6 header. In tunnel mode, an outer IPv6 header with an SRH is encapsulated, which looks like an BFD packet for plain IPv6 is steered into an SRv6 Policy. Liu, et al. Expires October, 2023 [Page 3] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 +-------------+---------+-------------+------------+ | IPv6 header | SRH | UDP Header | Payload | +-------------+---------+-------------+------------+ Figure 1: Transport Mode Encapsulation +-------------+---------+-------------+------------+------------+ | IPv6 header | SRH | IPv6 header | UDP Header | Payload | +-------------+---------+-------------+------------+------------+ Figure 2: Tunnel Mode Encapsulation An SRv6 Policy may consist of multiple candidate paths, and each candidate path may consist of multiple Segment-Lists. To monitor a candidate path, an implementation may setup multiple sessions for each Segment-List associated with that path. If some of the Segment- Lists fail, the forwarding will be weighted load-balancing among the other Segment-Lists. If all of the Segment-Lists fail, the candidate path is deemed to be failed. An implementation may monitor each candidate path belonging to the SRv6 Policy respectively. If the active candidate path fails, the switchover to another backup candidate path will be triggered. If all the candidate paths fails, the SRv6 Policy is deemed to be failed. How to setup sessions for the candidate paths and Segment-Lists associated with an SRv6 Policy is out of the scope of this document. 2.1. Control Packet in Transport Mode In transport mode, the encapsulation format of BFD control packet is as follows: Liu, et al. Expires October, 2023 [Page 4] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 +-----------------------------------------------------------+ | IPv6 Header | . Source IP Address = Headend IPv6 Address . . Destination IP Address = Segment List[SL] . . Next-Header = SRH . . . +-----------------------------------------------------------+ | SRH | . Segment List[0] = Tail-end IPv6 Address, or . . Last Segment of SRv6 Policy Segment-List . . Segment List[1] . . Segment List[2] . . ... . . Next-Header = UDP . . . +-----------------------------------------------------------+ | UDP Header | . . +-----------------------------------------------------------+ | Payload | . . +-----------------------------------------------------------+ Figure 3: Format of Control Packet in Transport Mode In the SRH, the first element of the Segment List (Segment List[0]) contains the SRv6 SID or IPv6 address of the tail-end node. If the last segment of the SRv6 Policy Segment-List does not belong to the tail-end node, an IPv6 address of tail-end should be added as Segment List[0], while Segment List[1] contains the last segment of the SRv6 Policy Segment-List. The typical scenarios are as follows: o The last segment of the SRv6 Policy Segment-List may be an End.X SID of the penultimate hop. If it is used as Segment List[0], the final destination for the BFD packet is missing. o The last segment of the SRv6 Policy Segment-List may be a Binding SID, for example, the application of SRv6 Policy for L3VPN service across multiple domains. If it is used as Segment List[0], according to [RFC8986], the node which instantiates the BSID will not perform the encapsulation behavior of the associated SRv6 Policy, but stop processing the SRH and proceed to process the next header in the packet. Else, the additional tail-end IPv6 address is not necessary, and it can be omitted in order to reduce the SRH size. Liu, et al. Expires October, 2023 [Page 5] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 After tail-end receives the control packet, it will send response packet back to the headend. The response packet is IP routed based on the IPv6 SA of the control packet from headend. Additional measures may be taken to control the forwarding path of response packet, which is out of the scope of this document. 2.2. Echo Packet in Transport Mode In transport mode, the encapsulation format of BFD echo packet is as follows: +-----------------------------------------------------------+ | IPv6 Header | . Source IP Address = Headend IPv6 Address . . Destination IP Address = Segment List[SL] . . Next-Header = SRH . . . +-----------------------------------------------------------+ | SRH | . Segment List[0] = Headend IPv6 Address . . Segment List[1] . . Segment List[2] . . ... . . Next-Header = UDP . . . +-----------------------------------------------------------+ | UDP Header | . . +-----------------------------------------------------------+ | Payload | . . +-----------------------------------------------------------+ Figure 4: Format of Echo Packet in Transport Mode The BFD echo packet u-turns on the tail-end node and returns to the headend node. The difference from the control packet is that the final destination of the echo packet is the headend itself. So, Segment List[0] in the SRH should contain the IPv6 address of the headend, in order to indicate the tail-end to forward the echo packet back to headend. The return path is IP routed. In both S-BFD and U-BFD for SRv6 Policy, the echo packet may be used to control the return path. After the echo packet reaches the tail- end along the forwarding path of SRv6 Policy Segment-List, additional segments will indicate the packet to be forwarded along specific path back to the headend. Liu, et al. Expires October, 2023 [Page 6] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 If segments of the return path is included in the SRH of echo packet and the last segment of the return path belongs to the headend, the additional headend IPv6 address is not necessary to be added as Segment List[0]. How to identify corresponding segment stack for the return paths is outside the scope of this document. 2.3. Control Packet in Tunnel Mode In tunnel mode, the encapsulation format of BFD control packet is as follows: +-----------------------------------------------------------+ | IPv6 Header | . Source IP Address = Headend IPv6 Address . . Destination IP Address = Segment List[SL] . . Next-Header = SRH . . . +-----------------------------------------------------------+ | SRH | . Segment List[0] = Tail-end IPv6 Address, or . . Last Segment of SRv6 Policy Segment-List . . Segment List[1] . . Segment List[2] . . ... . . Next-Header = IPv6 . . . +-----------------------------------------------------------+ | IPv6 Header | . Source IP Address = Headend IPv6 Address . . Destination IP Address = Tail-end IPv6 Address . . Next-Header = UDP . . . +-----------------------------------------------------------+ | UDP Header | . . +-----------------------------------------------------------+ | Payload | . . +-----------------------------------------------------------+ Figure 5: Format of Control Packet in Tunnel Mode In the SRH, the first element of the Segment List (Segment List[0]) contains the SRv6 SID or IPv6 address of the tail-end node. If the last segment of the SRv6 Policy Segment-List does not belong to the tail-end node and its function does not include decapsulation of the outer IPv6 header, an IPv6 address of tail-end should be Liu, et al. Expires October, 2023 [Page 7] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 added as Segment List[0], while Segment List[1] contains the last segment of the SRv6 Policy Segment-List. The typical scenarios are as follows: o The last segment of the SRv6 Policy may be an End.X SID of the penultimate hop. If it is used as Segment List[0], the penultimate hop needs to remove the outer IPv6 header with all SRH, and forwards the inner IPv6 packet to reflector. If the last segment is with Ultimate Segment Decapsulation (USD) flavor, the penultimate SR endpoint node will perform such decapsulation as defined in [RFC8986]. Otherwise, how to process the packet when the upper-layer header type is IPv6, is not clearly defined in [RFC8986]. It depends on implementation, and may not work well for BFD. o The last segment of the SRv6 Policy may be a Binding SID, which is the same with the Binding SID case in section 2.1. Else, the additional tail-end IPv6 address is not necessary, and it can be omitted in order to reduce the SRH size. After tail-end receives the control packet, it will send response packet back to the headend. The response packet is IP routed based on the inner IPv6 SA of the control packet from headend. Additional measures may be taken to control the forwarding path of response packet, which is out of the scope of this document. 2.4. Echo Packet in Tunnel Mode In tunnel mode, the encapsulation format of BFD echo packet is as follows: Liu, et al. Expires October, 2023 [Page 8] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 +-----------------------------------------------------------+ | IPv6 Header | . Source IP Address = Headend IPv6 Address . . Destination IP Address = Segment List[SL] . . Next-Header = SRH . . . +-----------------------------------------------------------+ | SRH | . Segment List[0] = Tail-end IPv6 Address, or . . Last Segment of SRv6 Policy Segment-List . . Segment List[1] . . Segment List[2] . . ... . . Next-Header = IPv6 . . . +-----------------------------------------------------------+ | IPv6 Header | . Source IP Address = Headend IPv6 Address . . Destination IP Address = Headend IPv6 Address . . Next-Header = UDP . . . +-----------------------------------------------------------+ | UDP Header | . . +-----------------------------------------------------------+ | Payload | . . +-----------------------------------------------------------+ Figure 6: Format of Echo Packet in Tunnel Mode If the last segment of the SRv6 Policy Segment-List does not belong to the tail-end node, an IPv6 address of tail-end should be added as Segment List[0], in order to guarantee that the packet can reach the tail-end. After the tail-end receives the echo packet, it decapsulates the outer IPv6 header with SRH, and then forwards the inner IPv6 packet back to the headend based on the IPv6 DA. If additional segments of the return path are included in the SRH of echo packet, the tail-end IPv6 address should not be included in the SRH. The segment stack should guarantee that the packet can reach the tail-end and then goes back to the headend. How to identify corresponding segment stack for the return paths are outside the scope of this document. Liu, et al. Expires October, 2023 [Page 9] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 3. Choice of Headend and Tail-end IPv6 Addresses 3.1. Control Packet When traffics are steered into an SRv6 Policy, the headend encapsulates the received packets in an outer IPv6 header along with an SRH. The Source Address of the outer IPv6 header is an IPv6 Address of the headend itself which can be routed. It may be a local interface address of the headend used for all SRv6 Policies. Or, different source addresses may be allocated per SRv6 Policy by local configuration. For the BFD control packet, the headend IPv6 address in the Source Address of IPv6 header may use the source address associated with the SRv6 Policy. An SRv6 Policy is identified through the tuple . The endpoint indicates the destination of the policy, and is usually specified as an IPv6 address of the tail-end node. For the BFD control packet, the headend may choose endpoint of the SRv6 Policy to be the tail-end IPv6 address which appears in the first segment of SRH or DA of inner IPv6 header, without additional knowledge of the tail-end. However, in some scenarios, the endpoint of SRv6 Policy can be the unspecified address (:: for IPv6), and the tail-end IPv6 Address may be specified by local configuration or network controller. 3.2. Echo Packet For the BFD echo packet, the headend IPv6 address in the Source Address of IPv6 header may use the source address associated with the SRv6 Policy, which is similar with the control packet. Because the echo packet u-turns on the tail-end, the tail-end does not need to parse the packet or use the source address as the destination address to send back, which is different from the control packet. So, the SA of echo packet is not necessary to be routable. An implementation may use unreachable address as the headend IPv6 address in the SA, which would prevent icmpv6 messages from flooding into the headend in failure cases. For the headend IPv6 address which appears in the first segment of SRH or DA of inner IPv6 header of the echo packet, it should be an IPv6 address belonging to the headend and can be routed. An implementation may use the source address associated with the SRv6 Policy. Or, particular addresses may be allocated per SRv6 Policy by Liu, et al. Expires October, 2023 [Page 10] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 local configuration, in order to distinguish the BFD echo packets for different SRv6 Policies. 4. Checksum in UDP Header The computation of Checksum in UDP header includes the Destination Address of IPv6 header. In the encapsulation of transport mode, the IPv6 DA may change along the SRv6 forwarding path. When computing the UDP Checksum, the headend should use the first segment in the SRH as the IPv6 DA. It is consistent with the packet received by the final destination, the tail-end node for control packet or the headend node for echo packet. So, when the final destination processes the UDP header, the verification of Checksum will be passed. In the encapsulation of tunnel mode, the computation of UDP Checksum only involves the inner IPv6 header, which does not change en route. No additional action needs to be taken. 5. Control of Inserting Additional IPv6 Address in SRH An implementation may have local configurations to control whether to insert a headend or tail-end IPv6 address as the first segment in the SRH. Or, an implementation may always insert additional IPv6 address. 6. Example In the following network, the headend A installs an SRv6 Policy to tail-end D with the segment list . SID-A1, SID-B1 and SID-C1 are all SRv6 End.X SIDs. A--------------B-------------C-----------D Figure 7: example network Assume that A uses SBFD to monitor that SRv6 Policy. A may send S- BFD control packet in transport mode, as shown in Figure 8. Liu, et al. Expires October, 2023 [Page 11] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 +=================+ +=================+ | IPv6 Header | | IPv6 Header | +-----------------+ +-----------------+ | SA=A's Addr | | SA=A's Addr | | DA=SID-B1 | | DA=D's Addr | +=================+ +=================+ | SRH | | SRH | +-----------------+ +-----------------+ | SL=2 | | SL=0 | | Seg[0]=D's Addr | | Seg[0]=D's Addr | | Seg[1]=SID-C1 | | Seg[1]=SID-C1 | | Seg[2]=SID-B1 | | Seg[2]=SID-B1 | | Seg[3]=SID-A1 | | Seg[3]=SID-A1 | +=================+ +=================+ | UDP-header | | UDP-header | +=================+ +=================+ | Payload | | Payload | +=================+ +=================+ A------------->B------------>C---------->D Figure 8: Example of S-BFD Control Packet in Transport Mode Assume that A uses U-BFD to monitor that SRv6 Policy. A may send U- BFD echo packet in tunnel mode, as shown in Figure 9. Liu, et al. Expires October, 2023 [Page 12] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 +=================+ +=================+ | IPv6 Header | | IPv6 Header | +-----------------+ +-----------------+ | SA=A's Addr | | SA=A's Addr | | DA=SID-B1 | | DA=D's Addr | +=================+ +=================+ | SRH | | SRH | +-----------------+ +-----------------+ | SL=2 | | SL=0 | | Seg[0]=D's Addr | | Seg[0]=D's Addr | | Seg[1]=SID-C1 | | Seg[1]=SID-C1 | | Seg[2]=SID-B1 | | Seg[2]=SID-B1 | | Seg[3]=SID-A1 | | Seg[3]=SID-A1 | +=================+ +=================+ | IPv6 Header | | IPv6 Header | +-----------------+ +-----------------+ | SA=A's Addr | | SA=A's Addr | | DA=A's Addr | | DA=A's Addr | +=================+ +=================+ | UDP-header | | UDP-header | +=================+ +=================+ | Payload | | Payload | +=================+ +=================+ -------------> ------------> ----------> A B C D <------------- <------------ <---------- +=================+ +=================+ | IPv6 Header | | IPv6 Header | +-----------------+ +-----------------+ | SA=A's Addr | | SA=A's Addr | | DA=A's Addr | | DA=A's Addr | +=================+ +=================+ | UDP-header | | UDP-header | +=================+ +=================+ | Payload | | Payload | +=================+ +=================+ Figure 9: Example of U-BFD Echo Packet in Tunnel Mode 7. Security Considerations TBD. 8. IANA Considerations This document has no IANA actions. Liu, et al. Expires October, 2023 [Page 13] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . [RFC9256] Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", RFC9256, DOI 10.17487/RFC9256, July 2022, . [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, . [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, . [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, . [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, . Liu, et al. Expires October, 2023 [Page 14] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 9.2. Informative References [I-D.ali-spring-bfd-sr-policy] Ali, Z., Talaulikar, K., Filsfils, C., Nainar, N., and C. Pignataro, "Bidirectional Forwarding Detection (BFD) for Segment Routing Policies for Traffic Engineering", Work in Progress, Internet- Draft, draft-ali-spring-bfd-sr-policy-10, November 16 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-07, 23 April 2023, . Liu, et al. Expires October, 2023 [Page 15] Internet-Draft Encapsulation of BFD for SRv6 Policy April 2023 Authors' Addresses Yisong Liu China Mobile China Email: liuyisong@chinamobile.com Weiqiang Cheng China Mobile China Email: chengweiqiang@chinamobile.com Changwang Lin New H3C Technologies China Email: linchangwang.04414@h3c.com Mengxiao Chen New H3C Technologies China Email: chen.mengxiao@h3c.com Xiao Min ZTE Corp. China Email: xiao.min2@zte.com.cn Liu, et al. Expires October, 2023 [Page 16]