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 mechanism by using BGP. 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. specify the mechanism of MTU signaling in RSVP. Likewise, SRv6 pakcets will be dropped if the packet size is larger than path MTU, since IPv6 packet can not be fragmented on transmission . The host may discover the PMTU by Path MTU Discovery (PMTUD) or other mechanisms. But the ingress 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. 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 available since the SR does not require signaling. This document defines extensions to BGP to distribute path MTU information within SR policies. The Link MTU information can be obtained via BGP-LS or some other means. With the Link MTU, 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 ... ... ]]>

An SR 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 first one will be processed, and the rest will be ignored. An SR Path MTU sub-TLV is associated with an SR path specified by a segment list sub-TLV or path segment as defined in and . It has the following format:

Where: Type: to be assigned by IANA. Length: the total length of the value field not including Type and Length fields. 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 beyong 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.

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

TBA

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.