Segment Routing Path MTU in BGP

Segment routing (SR) is a source routing paradigm that explicitly indicates the forwarding path for packets at the ingress node. The ingress node steers packets into a specific path according to the Segment Routing Policy ( SR Policy) as defined in . In order to distribute SR policies to the headend, specifies a BGP mechanism to pass SR Policies and Candidate SR Policies in BGP UPDATE message. Each SR Candidate Path is passed as combination of a specific type of NLRI and BGP Tunnel Encapsulation Attribute (Tunnel-Encaps) with SR Policy Tunnel type tunnel. The NLRI must contain either be the IPv4 Unicast AFI with SR Policy SAFI (AFI=1/SAFI=73), the IPv6 Unicast AFI with the SR Policy SAFI (AFI=2/SAFI=73). The maximum transmission unit (MTU) is the largest size packet or frame, in bytes, that can be sent in a network. An MTU that is too large might cause retransmissions. Too small an MTU might cause the router to send and handle relatively more header overhead and acknowledgments. When an LSP is created across a set of links with different MTU sizes, the ingress router needs to know what the smallest MTU is on the LSP path. If this MTU is larger than the MTU of one of the intermediate links, traffic might be dropped, because MPLS packets cannot be fragmented. Also, the ingress router may not be aware of this type of traffic loss, because the control plane for the LSP would still function normally. specifies the mechanism of MTU signaling in RSVP. Similarly, the SRv6 packets will be dropped if the packet size is larger than the path MTU, since IPv6 packet cannot be fragmented on transmission . The host may discover the PMTU by Path MTU Discovery (PMTUD) or other mechanisms. But the ingress router still needs to examine the packet size for dropping too large packets to avoid malicious traffic or error traffic. Also, the packet size may exceeds the PMTU because of the new encapsulation of SR-MPLS or SRv6 packet at the ingress router. In order to check whether the Packet size exceeds the PMTU or not, the ingress node needs to know the Path MTU associated to the forwarding path. However, the path maximum transmission unit (MTU) information for SR path is not currently distributed in the BGP Tunnel-Encaps attribute TLV for the SR Policy Tunnel. This document defines a new sub-TLV for the BGP Tunnel-Encaps attribute for the SR Policy Tunnel type to specify Maximum Path MTU for a Segment list (Sub-TLV). The Maximum Path MTU can be calculated as the maximum of individual Link MTU information. The Link MTU information can be obtained via BGP-LS or some other means. based on all Link MTUs, the controller can compute the PMTU and convey the information via the BGP SR policy.

This memo makes use of the terms defined in and .

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 when, and only when, they appear in all capitals, as shown here.

As defined in , the SR policy encoding structure is as follows:

Attributes: Tunnel Encaps Attribute (23) Tunnel Type: SR Policy Binding SID Preference Priority Policy Name Explicit NULL Label Policy (ENLP) Segment List Weight Segment Segment ... ... ]]> As introduced in Section 1, each SR path has it's path MTU. SR policy with SR path MTU information is expressed as below:

Attributes: Tunnel Encaps Attribute (23) Tunnel Type: SR Policy Binding SID Preference Priority Policy Name Explicit NULL Label Policy (ENLP) Segment List Weight Path MTU Segment Segment ... ... ]]>

A Path MTU sub-TLV is an Optional sub-TLV. When it appears, it must appear only once at most within a Segment List sub-TLV. If multiple Path MTU sub-TLVs appear within a Segment List sub-TLV, the NLRI MUST be treated as a malformed NLRI. As per , when the error determined allows for the router to skip the malformed NLRI(s) and continue processing of the rest of the update message, then it MUST handle such malformed NLRIs as 'Treat-as-withdraw'. This document does not define new error handling rules for Path MTU sub-TLV, and the error handling rules defined in apply to this document. A Path MTU sub-TLV is associated with an SR path specified by a segment list sub-TLV or a path segment . The Path MTU sub-TLV has the following format:

Where: Type: TBD1 to be assigned by IANA. Length: the total length in octets the value field not including Type and Length fields. The value must be 6. Reserved: 16 bits reserved and MUST be set to 0 on transmission and MUST be ignored on receipt. Path MTU: 4 bytes value of path MTU in octets. The value can be calculated by a central controller or other devices based on the information that learned via IGP of BGP-LS or other means. Whenever the path MTU of a physical or logical interface is changed, a new SR policy with new path MTU information should be updated accordingly by BGP.

The document does not bring new operation beyond the description of operations defined in . The existing operations defined in can apply to this document directly. Typically but not limit to, the SR policies carrying path MTU infomation are configured by a controller. After configuration, the SR policies carrying path MTU infomation will be advertised by BGP update messages. The operation of advertisement is the same as defined in , as well as the receiption. The consumer of the SR policies is not the BGP process. The operation of sending information to consumers is out of scope of this document.

[Note to the RFC Editor - remove this section before publication, as well as remove the reference to . This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to , "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".

The feature has been implemented on Huawei VRP8. Organization: Huawei Implementation: Huawei's Commercial Delivery implementation based on VRP8. Description: The implementation has been done. Maturity Level: Product Contact: guokeqiang@huawei.com

This document defines a new Sub-TLV in registries "SR Policy Segment List Sub-TLVs" :

Currently, there is no standardized YANG data model that explicitly exposes the Path MTU value carried in the BGP SR Policy Path MTU Sub-TLV to network management systems or operators. Similarly, while BGP-LS can distribute link-level MTU information (as defined in ), it does not natively convey end-to-end Path MTU values associated with specific SR Policies. Standardized manageability support would significantly improve operational visibility and automation. Therefore, the authors recommend a YANG Module Extension: Extend the YANG data model for SR Policies (e.g., ietf-sr-policy or vendor-specific models) to include a read-only leaf for path-mtu, reflecting the value received via BGP or configured locally. This would enable NETCONF/RESTCONF-based retrieval and integration with network assurance systems.

This document defines the extension to BGP to distribute path MTU information within SR policies. Therefore, the security mechanisms of the base BGP security model and the security considereations in apply to this document. The path MTU extension is included in the SR Policy extension , so it does not introduce extra security problems comparing the existing SR policy entension. The path MTU information is critical to the path, and a wrong path MTU may cause packet dropping in the forwarding. An implementation needs to make sure that the value of the link MTU is correctly collected from some means, such as BGP-LS. It also must ensure the processing and calculation of path MTU is correct to avoid packet dropping in forwarding. In addition, the path MTU distribution from a controller to an ingress router has to be protected. The security considereations in apply to this distribution procedure.

Jun Qiu Huawei Technologies China Email: qiujun8@huawei.com

Authors would like to thank Ketan Talaulikar, Aijun Wang, Weiqiang Cheng, Huanan Chen, Chongfeng Xie, Stefano Previdi, Taishan Tang, Keqiang Guo, Chen Zhang, Susan Hares, Weiguo Hao, Gong Xia, Bing Yang, Linda Dunbar, Shunwan Zhuang, Huaimo Chen, Mach Chen, Jingring Xie, Zhibo Hu, Jimmy Dong and Jianwei Mao for their proprefessional comments and help.