Internet-Draft | SR-MPLS/SRv6 Interworking | June 2024 |
Hegde, et al. | Expires 1 January 2025 | [Page] |
- Workgroup:
- SPRING Working Group
- Internet-Draft:
- draft-bonica-spring-srv6-end-dtm-12
- Published:
- Intended Status:
- Standards Track
- Expires:
SR-MPLS / SRv6 Transport Interworking
Abstract
This document describes procedures for interworking between an SR-MPLS transit domain and an SRv6 transit domain. Each domain contains Provider Edge (PE) and Provider (P) routers. Area Border Routers (ABR) provide connectivity between domains.¶
The procedures described in this document require the ABR to carry a route to each PE router. However, they do not required the ABR to carry service (i.e., customer) routes. In that respect, these procedures resemble L3VPN Interprovider Option C.¶
Procedures described in this document support interworking for global IPv4 and IPv6 service prefixes. They do not support interworking for VPN services prefixes where the SR-MPLS domain uses MPLS service labels.¶
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 1 January 2025.¶
Copyright Notice
Copyright (c) 2024 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 (https://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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
1. Overview
Segment Routing (SR) [RFC8402] allows source nodes to steer packets through SR paths. It can be implemented over IPv6 [RFC8200] or MPLS [RFC3031]. When SR is implemented over IPv6, it is called SRv6 [RFC8986]. When SR is implemented over MPLS, it is called SR-MPLS [RFC8660].¶
This document describes procedures for interworking between an SR-MPLS transit domain and an SRv6 transit domain. Each domain contains Provider Edge (PE) and Provider (P) routers. Area Border Routers (ABR) provide connectivity between domains.¶
The procedures described in this document require the ABR to carry a route to each PE router. However, they do not required the ABR to carry service (i.e., customer) routes. In that respect, these procedures resemble L3VPN Interprovider Option C [RFC4364].¶
Procedures described in this document support interworking for global IPv4 and IPv6 service prefixes. They do not support interworking for VPN services prefixes where the SR-MPLS domain uses MPLS service labels.¶
2. 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.¶
3. Reference Topology
Figure 1 depicts interworking between an SR-MPLS domain and an SRv6 domain. The SRv6 domain contains PE Node 1 and P Node 2. The SR-MPLS domain contains P Node 4 and PE node 5. Both domains contain ABR Node 3.¶
Nodes 1 and 2 MUST support SRv6 but are NOT REQUIRED to support SR-MPLS. Nodes 4 and 5 MUST support SR-MPLS but are NOT required to support SRv6. Node 3 MUST support both SRv6 and SR-MPLS. It must also support interworking procedures.¶
Network operators configure a loopback interface on Nodes 1 through 5. These are called Loopback1 through Loopback5. They also configure 2 additional loopback interfaces on PE Node 5. These are called Loopback5.IPv4 and Loopback5.IPv6.¶
Each node instantiates an SR Segment (i.e., Segment A through Segment E). SR Path 1 begins on PE Node 1 and ends on PE Node 5. It visits Nodes 2, 3, 4, and 5, executing the instructions associated with Segments B, C, and D. SR Path 2 begins on PE Node 5 and ends on PE Node 1. It visits Nodes 4, 3, 2, and 1, executing the instructions associated with Segments D, C, B and A.¶
4. Forwarding Plane
Figure 2 depicts the forwarding plane as a packet traverses SR Path 1, from Node 1 to Node 5. In this example, PE Node 1 receives an IPv4 packet.¶
PE Node 1 encapsulates the IPv4 packet in an SRv6 header. The SRv6 header contains an IPv6 header and a Segment Routing Header (SRH) [RFC8754]. The Destination Address in the IPv6 header is a Segment Identifier (SID) that represents Segment B. Segment B is an END instruction instantiated on P Node 2. The SRH contains a Segments Left field and one SID. The Segments Left field is equal to 1 and the SID represents Segment C, and END.DM (Section 4.1) instruction instantiated on ABR Node 3.¶
PE Node 1 forwards the packet to P Node 2. When P Node 2 receives the packet, it processed the END instruction. It decrements the Segments Left field in the SRH and copies the SID from the SRH to the Destination Address field of the IPv6 header. It then forwards the packet to ABR Node 3.¶
When ABR Node 3 receives the packet, it processes the END.DM instruction. It removes the SRv6 header and replaces it with an SR-MPLS label stack that contains two entries. The top entry represents a prefix SID instantiated on P Node 4. The bottom entry is an Explicit Null instruction (i.e., MPLS label 0), instantiated on PE Node 5.¶
ABR Node 3 then forwards the packet to P Node 4. P Node 4 processes the prefix SID, removing the top entry from the SR-MPLS label stack and forwarding the packet to PE Node 5. PE Node 5 processes the Explicit Null instruction, removing the remaining SR-MPLS label stack entry and processing the payload.¶
Figure 3 depicts the forwarding plane as a packet traverses SR Path 2, from Node 5 to Node 1. In this example, PE Node 5 receives an IPv4 packet.¶
PE Node 5 encapsulates the IPv4 packet in an SR-MPLS label stack that contains two entries. The top entry represents a prefix SID instantiated on P Node 4. The bottom entry is a binding SID instantiated on ABR Node 3.¶
PE Node 5 then forwards the packet to P Node 4. P Node 4 processes the prefix SID, removing the top entry from the SR-MPLS label stack and forwarding the packet to ABR Node 3. ABR Node 3 processes binding SID, removing the remaining SR-MPLS label stack entry and replacing it with an SRv6 header. The SRv6 header contains an IPv6 header and an SRH. The Destination Address in the IPv6 header is a Segment Identifier (SID) that represents Segment B. Segment B is an END instruction instantiated on P Node 2. The SRH contains a Segments Left field and one SID. The Segments Left field is equal to 1 and the SID represents Segment A, an END.DT46 instruction instantiated on PE Node 1. That instruction causes the packet to be forwarded using the main IP forwarding table, not a VPN forwarding table.¶
ABR Node 3 forwards the packet to P Node 2. When P Node 2 receives the packet, it processed the END instruction. It decrements the Segments Left field in the SRH and copies the SID from the SRH to the Destination Address field of the IPv6 header. It then forwards the packet to PE Node 1. PE Node 1 processes its END.DT46 instruction, removing the SRv6 header and processing the payload.¶
4.1. END.DM Processing
The End.DM SID MUST be the last segment in a SR Policy. Its arguments are associated with an SR-MPLS label stack.¶
When Node N receives a packet destined to S and S is a locally instantiated End.DM SID, Node N executes the following procedure:¶
S01. When an IPv6 Routing Header is processed { S02. If (Segments Left != 0) { S03. Send an ICMP Parameter Problem to the Source Address, Code 0 (Erroneous header field encountered), Pointer set to the Segments Left field, interrupt packet processing and discard the packet. S04. } S05. Proceed to process the next header in the packet S06. } When processing the Upper-layer header of a packet matching a FIB entry locally instantiated as an End.DM SID, N executes the following procedure: S01. Decapsulate the packet (i.e., remove the outer IPv6 Header and all its extension headers) S02. Push the SR-MPLS label stack that is associated with the END.DM arguments. Set the MPLS Traffic Class and TTL values to reflect the Traffic Class and Hop count values received in the IPv6 header. S03. Submit the packet to the MPLS FIB lookup for transmission to the new destination¶
5. Control Plane
In the Figure 4, PE Node 1 and PE Node 5 exchange customer Network Layer Reachability Information (NLRI) [RFC4271] using either a direct BGP session or a route reflector [RFC4456]. All customer routes exchanged between PE Node 1 and PE Node 5 belong to the general routing instance. They cannot belong to a VPN.¶
PE Node 1 exchanges loopback routes with ABR Node 3, using either a direct BGP session or a route reflector. Likewise, ABR Node 3 exchanges loopback with PE Node 5, using either a direct BGP session or a route reflector.¶
PE Node 1 and ABR Node 3 bind SIDs to the loopback routes that they exchange, as described in [I-D.ietf-bess-srv6-services]. PE Node 5 and ABR Node 3 bind labels to the loopback routes that they exchange, as described in [RFC8277].¶
Both domains use an IGP to distribute link state information and establish connectivity within the domain.¶
5.1. Signaling SR Paths That Originate In The SRv6 Domain
PE Node 5 advertises an IPv4 customer route to PE Node 1 using BGP as follows:¶
This causes PE Node 1 to resolve the customer route through its route to Loopback5.IPv4. The following paragraphs describe how PE Node 1 acquires a route to Loopback5.IPv4.¶
PE Node 5 advertises Loopback5.IPv4 to ABR Node 3 using BGP Labeled Unicast (BGP-LU) as follows:¶
-
Prefix: Loopback5.IPv4¶
-
Next-hop: Loopback5¶
-
Color Community: Color to distinguish between paths between ABR Node 3 and PE Node 5¶
-
MPLS Label: Explicit Null (0)¶
Now, ABR Node 3 resolves its route to Loopback5.IPv4 through its IGP route to Loopback5. Therefore, when forwarding traffic bound for Loopback5.IPv4, it imposes:¶
-
An SR-MPLS label stack associated with the IGP route to Loopback5¶
-
An additional Explicit Null label¶
ABR Node 3 advertises Loopback5.IPv4 to PE Node 1 using BGP as follows:¶
-
Prefix: Loopback5.IPv4¶
-
Next-hop: Loopback3¶
-
Color Community: Color to distinguish between paths between ABR Node 3 and PE Node 1¶
-
SID: SID C (i.e., an END.DM SID instantiated on ABR Node 3)¶
Now, PE Node 1 resolves its route to Loopback5.IPv4 through its IGP route to Loopback3. Therefore, when forwarding traffic bound for Loopback5.IPv4, it imposes an SRv6 header that includes the following SIDs:¶
5.2. Signaling SR Paths That Originate In The SR-MPLS Domain
PE Node 1 advertises an IPv4 customer route to PE Node 5 using BGP as follows:¶
This causes PE Node 5 to resolve the customer route through its route to Loopback1. The following paragraphs describe how PE Node 5 acquires a route to Loopback1.¶
PE Node 1 advertises Loopback1 to ABR Node 3 using BGP as follows:¶
-
Prefix: Loopback1¶
-
Next-hop: Loopback1¶
-
Color Community: Color to distinguish between paths between ABR Node 3 and PE Node 1¶
-
SID: SID A (i.e., An END.DT46 SID instantiation on PE Node 1. This instruction causes a packet to be forwarded using the main IP forwarding table, not a VPN forwarding table.)¶
Now, ABR Node 3 resolves its route to Loopback1 through its IGP route to Loopback1. Therefore, when forwarding traffic bound for Loopback1, it imposes an SRv6 header that includes:¶
ABR Node 3 advertises Loopback1 to PE Node 5 using BGP-LU as follows:¶
-
Prefix: Loopback1¶
-
Next-hop: Loopback3¶
-
Color Community: Color to distinguish between paths between ABR Node 3 and PE Node 5¶
-
MPLS Label: A binding label that represents the SRv6 path between ABR Node 3 and PE Node 5¶
Now, PE Node 5 resolves its route to Loopback1 through its IGP route to Loopback3. Therefore, when forwarding traffic bound for Loopback1, it imposes:¶
6. IANA Considerations
The authors will request an early allocation from the "SRv6 Endpoint Behaviors" sub-registry of the "Segment Routing Parameters" registry.¶
7. Security Considerations
Because SR inter-working requires co-operation between inter-working domains, this document introduces no security consideration beyond those addressed in [RFC8402], [RFC8754] and [RFC8986].¶
8. Contributors
1.Ketan Talaulikar¶
Cisco Systems¶
ketant.ietf@gmail.com¶
9. Acknowledgements
Thanks to Melchior Aelmans, Takuya Miyasaka and Jeff Tantsura for their comments.¶
10. References
10.1. Normative References
- [I-D.ietf-bess-srv6-services]
- Dawra, G., Talaulikar, K., Raszuk, R., Decraene, B., Zhuang, S., and J. Rabadan, "BGP Overlay Services Based on Segment Routing over IPv6 (SRv6)", Work in Progress, Internet-Draft, draft-ietf-bess-srv6-services-15, , <https://datatracker.ietf.org/doc/html/draft-ietf-bess-srv6-services-15>.
- [RFC2119]
- Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
- [RFC4271]
- Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, , <https://www.rfc-editor.org/info/rfc4271>.
- [RFC4364]
- Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, , <https://www.rfc-editor.org/info/rfc4364>.
- [RFC4456]
- Bates, T., Chen, E., and R. Chandra, "BGP Route Reflection: An Alternative to Full Mesh Internal BGP (IBGP)", RFC 4456, DOI 10.17487/RFC4456, , <https://www.rfc-editor.org/info/rfc4456>.
- [RFC8174]
- Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
- [RFC8200]
- Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, , <https://www.rfc-editor.org/info/rfc8200>.
- [RFC8277]
- Rosen, E., "Using BGP to Bind MPLS Labels to Address Prefixes", RFC 8277, DOI 10.17487/RFC8277, , <https://www.rfc-editor.org/info/rfc8277>.
- [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, , <https://www.rfc-editor.org/info/rfc8402>.
- [RFC8660]
- Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing with the MPLS Data Plane", RFC 8660, DOI 10.17487/RFC8660, , <https://www.rfc-editor.org/info/rfc8660>.
- [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, , <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 10.17487/RFC8986, , <https://www.rfc-editor.org/info/rfc8986>.
10.2. Informative References
- [I-D.hegde-spring-mpls-seamless-sr]
- Hegde, S., Bowers, C., Xu, X., Gulko, A., Bogdanov, A., Uttaro, J., Jalil, L., Khaddam, M., Alston, A., and L. M. Contreras, "Seamless SR Problem Statement", Work in Progress, Internet-Draft, draft-hegde-spring-mpls-seamless-sr-07, , <https://datatracker.ietf.org/doc/html/draft-hegde-spring-mpls-seamless-sr-07>.
- [RFC3031]
- Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, DOI 10.17487/RFC3031, , <https://www.rfc-editor.org/info/rfc3031>.