Interdomain Routing Working Group C. Li Internet-Draft Huawei Technologies Intended status: Standards Track Y. Zhu Expires: January 2, 2020 China Telecom Z. Li Huawei Technologies July 1, 2019 Segment Routing Path MTU in BGP draft-li-idr-sr-policy-path-mtu-02 Abstract Segment Routing is a source routing paradigm that explicitly indicates the forwarding path for packets at the ingress node. An SR policy is a set of candidate SR paths consisting of one or more segment lists with necessary path attributes. However, the path maximum transmission unit (MTU) information for SR path is not available in the SR policy since the SR does not require signaling. This document defines extensions to BGP to distribute path MTU information within SR policies. 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 January 2, 2020. Copyright Notice Copyright (c) 2019 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 Li, et al. Expires January 2, 2020 [Page 1] Internet-Draft SR Path MTU in BGP July 2019 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 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 3. SR Policy for Path MTU . . . . . . . . . . . . . . . . . . . 3 3.1. SR Path MTU Sub-TLV . . . . . . . . . . . . . . . . . . . 4 4. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 8.1. Normative References . . . . . . . . . . . . . . . . . . 6 8.2. Informative References . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction Segment routing (SR) [RFC8402] 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 [I-D.ietf-spring-segment-routing-policy]. In order to distribute SR policies to the headend, [I-D.ietf-idr-segment-routing-te-policy] 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. [RFC3209] 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 [RFC8200] . Li, et al. Expires January 2, 2020 [Page 2] Internet-Draft SR Path MTU in BGP July 2019 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 MTU information can be obtained via IGP [I-D.hu-lsr-isis-path-mtu], BGP-LS [I-D.zhu-idr-bgp-ls-path-mtu] or some other means. 2. Terminology This memo makes use of the terms defined in [RFC8402] and [RFC3209]. 2.1. 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. 3. SR Policy for Path MTU As defined in [I-D.ietf-idr-segment-routing-te-policy] , the SR policy encoding structure is as follows: SR Policy SAFI NLRI: 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: Li, et al. Expires January 2, 2020 [Page 3] Internet-Draft SR Path MTU in BGP July 2019 SR Policy SAFI NLRI: 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 ... ... 3.1. SR Path MTU Sub-TLV 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 [I-D.ietf-spring-mpls-path-segment] and [I-D.li-spring-srv6-path-segment], and it MUST appear only once within a Segment List sub-TLV. It has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | RESERVED | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Path MTU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1. Path MTU sub-TLV 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. Li, et al. Expires January 2, 2020 [Page 4] Internet-Draft SR Path MTU in BGP July 2019 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. 4. Operations The document does not bring new operation beyong the description of operations defined in [I-D.ietf-idr-segment-routing-te-policy]. The existing operations defined in [I-D.ietf-idr-segment-routing-te-policy] 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 [I-D.ietf-idr-segment-routing-te-policy], 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. 5. IANA Considerations This document defines a new Sub-TLV in registries "SR Policy List Sub- TLVs" [I-D.ietf-idr-segment-routing-te-policy]: Value Description Reference --------------------------------------------------------------------- TBA Path MTU sub-TLV This document 6. Security Considerations TBA 7. Acknowledgements TBA 8. References Li, et al. Expires January 2, 2020 [Page 5] Internet-Draft SR Path MTU in BGP July 2019 8.1. Normative References [I-D.ietf-idr-segment-routing-te-policy] Previdi, S., Filsfils, C., Jain, D., Mattes, P., Rosen, E., and S. Lin, "Advertising Segment Routing Policies in BGP", draft-ietf-idr-segment-routing-te-policy-06 (work in progress), May 2019. [I-D.ietf-spring-segment-routing-policy] Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d., bogdanov@google.com, b., and P. Mattes, "Segment Routing Policy Architecture", draft-ietf-spring-segment-routing- policy-03 (work in progress), May 2019. [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, . 8.2. Informative References [I-D.hu-lsr-isis-path-mtu] Hu, Z., Zhu, Y., Li, Z., and L. Dai, "IS-IS Extensions for Path MTU", draft-hu-lsr-isis-path-mtu-00 (work in progress), June 2018. [I-D.ietf-spring-mpls-path-segment] Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler, "Path Segment in MPLS Based Segment Routing Network", draft-ietf-spring-mpls-path-segment-00 (work in progress), March 2019. [I-D.li-spring-srv6-path-segment] Li, C., Cheng, W., Chen, M., Dhody, D., Li, Z., Dong, J., and R. Gandhi, "Path Segment for SRv6 (Segment Routing in IPv6)", draft-li-spring-srv6-path-segment-01 (work in progress), June 2019. Li, et al. Expires January 2, 2020 [Page 6] Internet-Draft SR Path MTU in BGP July 2019 [I-D.zhu-idr-bgp-ls-path-mtu] Zhu, Y., Hu, Z., Yan, G., and J. Yao, "BGP-LS Extensions for Advertising Path MTU", draft-zhu-idr-bgp-ls-path- mtu-00 (work in progress), June 2018. [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001, . [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017, . Authors' Addresses Cheng Li Huawei Technologies Huawei Campus, No. 156 Beiqing Rd. Beijing 100095 China Email: chengli13@huawei.com YongQing Zhu China Telecom 109, West Zhongshan Road, Tianhe District. Guangzhou China Email: zhuyq.gd@chinatelecom.cn Zhenbin Li Huawei Technologies Huawei Campus, No. 156 Beiqing Rd. Beijing 100095 China Email: lizhenbin@huawei.com Li, et al. Expires January 2, 2020 [Page 7]