Network Working Group S. Venaas Internet-Draft M. Sivakumar Intended status: Experimental IJ. Wijnands Expires: September 6, 2018 L. Ginsberg Cisco Systems, Inc. March 5, 2018 BIER MTU Discovery draft-venaas-bier-mtud-00 Abstract This document defines an IGP based mechanism for discovering the MTU of a BIER sub-domain. This document defines extensions to OSPF and IS-IS, but other protocols could potentially be extended. MTU discovery is usually done for a given path, while this document defines it for a sub-domain. This allows the computed MTU to be independent of the set of receivers. Also, the MTU is independent of rerouting events within the sub-domain. 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 September 6, 2018. Copyright Notice Copyright (c) 2018 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 Venaas, et al. Expires September 6, 2018 [Page 1] Internet-Draft BIER MTU Discovery March 2018 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. Conventions used in this document . . . . . . . . . . . . . . 3 3. IS-IS BIER MTU Sub-sub-TLV . . . . . . . . . . . . . . . . . 3 4. OSPF BIER MTU Sub-TLV . . . . . . . . . . . . . . . . . . . . 4 5. IANA considerations . . . . . . . . . . . . . . . . . . . . . 4 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 6.1. Normative References . . . . . . . . . . . . . . . . . . 5 6.2. Informative References . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction This document defines an IGP based mechanism for discovering the MTU of a BIER sub-domain. The discovered MTU indicates the largest possible BIER payload, such as an IP packet, that can be sent across any link in a BIER sub-domain. This is different from [I-D.ietf-bier-path-mtu-discovery] which performs Path MTU Discovery (PMTUD) for a set of receivers. PMTUD is based on probing, and when there are routing changes, e.g., a link going down, the actual MTU for a path may become less than was previously discovered, and there will be some delay until the next probe is performed. Also, the set of receivers for a flow may change at any time, which may cause the MTU to change. This document instead discovers a BIER sub-domain MTU, which is independent of paths and receivers within the sub- domain. For convenience we will refer to an interface on a router as a BIER interface if the router has a BIER neighbor on the interface. That is, there is a directly connected router on that interface that is announcing a BIER prefix. We say that it is a BIER interface in a given sub-domain if the router itself announces a prefix tagged with the sub-domain, and there is BIER neighbor on the interface also announcing a prefix tagged with the sub-domain. In order to allow MTU discovery in a BIER sub-domain, the procedure is as follows. Every BIER router, for each sub-domain with at least one local BIER interface in the sub-domain, per the above definition of a BIER interface, determines the largest payload that can be sent BIER encapsulated out of any of its BIER interfaces in the sub- domain. That is, for each local BIER interface in the sub-domain, it needs to determine the size of the largest BIER encapsulated payload Venaas, et al. Expires September 6, 2018 [Page 2] Internet-Draft BIER MTU Discovery March 2018 that can be sent out of that interface. We define the local sub- domain MTU of a router to be the minimum of the per BIER interface maximum payload size. A BIER router announces a BIER prefix in either IS-IS or OSPF as specified in [I-D.ietf-bier-isis-extensions] and [I-D.ietf-bier-ospf-bier-extensions]. They both define a BIER Sub- TLV to be included with the prefix. There is one BIER Sub-TLV included for each sub-domain. This document defines how a router includes its local sub-domain MTU in each of the BIER Sub-TLVs it advertizes. A router can discover the MTU of a BIER sub-domain by identifying all the prefixes that have a BIER Sub-TLV for the sub-domain. It then computes the minimum of the advertised MTU values for that sub- domain. This includes its local sub-domain MTU. This allows all the routers in the sub-domain to discover the same sub-domain wide MTU. Note that a router should announce a new local MTU for a sub-domain immediately if the value becomes smaller than what it currently announces. This would happen if the MTU of an interface is configured to a smaller value, or the first BIER neighbor for a sub- domain is detected on an interface, and the MTU of the interface is less than all the other local BIER interfaces in the sub-domain. However, if BIER neighbors go away, or if an interface goes down, so that the local MTU becomes larger, a router SHOULD NOT immediately announce the larger value. A router MAY after some delay announce the new larger MTU. The intention is that dynamic events such as a quick link flap should not cause the announced MTU to be increased. 2. Conventions used in this document 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. IS-IS BIER MTU Sub-sub-TLV A router uses the BIER MTU Sub-sub-TLV to announce the minimum BIER MTU of all its BIER enabled interfaces. The Sub-sub-TLV MUST be ignored if it is included multiple times. Venaas, et al. Expires September 6, 2018 [Page 3] Internet-Draft BIER MTU Discovery March 2018 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 | MTU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type: TBD Length: 2 MTU: MTU in octets 4. OSPF BIER MTU Sub-TLV A router uses the BIER MTU Sub-TLV to annonce the minimum BIER MTU of all its BIER enabled interfaces. It is a Sub-TLV of the BIER Sub- TLV, and SHOULD be included exactly once within each of the advertised BIER Sub-TLVs. The Sub-TLV MUST be ignored if it is included multiple times. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MTU | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type: TBD2 Length: 4 MTU: MTU in octets 5. IANA considerations An allocation from the "sub-sub-TLVs for BIER Info sub-TLV" registry as defined in [I-D.ietf-bier-isis-extensions] is requested for the IS-IS BIER MTU Sub-sub-TLV. Please replace the string TBD in this document with the appropriate value. An allocation from the "OSPF Extended Prefix sub-TLV" registry as defined in [RFC7684] is requested for the OSPF BIER MTU Sub-TLV. Please replace the string TBD2 in this document with the appropriate value. Venaas, et al. Expires September 6, 2018 [Page 4] Internet-Draft BIER MTU Discovery March 2018 6. References 6.1. Normative References [I-D.ietf-bier-isis-extensions] Ginsberg, L., Przygienda, T., Aldrin, S., and Z. Zhang, "BIER support via ISIS", draft-ietf-bier-isis- extensions-09 (work in progress), February 2018. [I-D.ietf-bier-ospf-bier-extensions] Psenak, P., Kumar, N., Wijnands, I., Dolganow, A., Przygienda, T., Zhang, Z., and S. Aldrin, "OSPF Extensions for BIER", draft-ietf-bier-ospf-bier-extensions-15 (work in progress), February 2018. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W., Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute Advertisement", RFC 7684, DOI 10.17487/RFC7684, November 2015, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 6.2. Informative References [I-D.ietf-bier-path-mtu-discovery] Mirsky, G., Przygienda, T., and A. Dolganow, "Path Maximum Transmission Unit Discovery (PMTUD) for Bit Index Explicit Replication (BIER) Layer", draft-ietf-bier-path-mtu- discovery-03 (work in progress), January 2018. Authors' Addresses Stig Venaas Cisco Systems, Inc. Tasman Drive San Jose CA 95134 USA Email: stig@cisco.com Venaas, et al. Expires September 6, 2018 [Page 5] Internet-Draft BIER MTU Discovery March 2018 Mahesh Sivakumar Cisco Systems, Inc. Tasman Drive San Jose CA 95134 USA Email: masivaku@cisco.com IJsbrand Wijnands Cisco Systems, Inc. De kleetlaan 6a Diegem 1831 Belgium Email: ice@cisco.com Les Ginsberg Cisco Systems, Inc. Tasman Drive San Jose CA 95134 USA Email: ginsberg@cisco.com Venaas, et al. Expires September 6, 2018 [Page 6]