Network Working Group K. Kompella Internet Draft Juniper Networks Updates: 3630 October 2003 Category: Standards Track Expires: April 2004 OSPF v2 Traffic Engineering Extensions for Multi-access Networks draft-kompella-ospf-multiaccess-te-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This memo specifies Traffic Engineering extensions for OSPF version 2 for dealing with multi-access networks. In particular, the bandwidth attributes in the original OSPFv2 TE specification do not accurately model the available bandwidth across a multi-access network; this memo rectifies this shortcoming. Kompella Standards Track [Page 1] Internet Draft OSPF TE for Multi-access Networks October 2003 Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [1]. 1. Introduction RFC 3630 [2] specifies how Traffic Engineering (TE) properties of links can be carried in OSPF v2 Opaque Link State Advertisements (LSAs) [3], but explicits states (in section 1.2) that "The reservation state of multi-access links may not be accurately reflected". This is the case for at least two reasons: (a) the Designated Router (DR) doesn't generate an Opaque LSA to define the reservation state of links _towards_ routers in a multi-access network; (b) even if it did, it would be hard for the DR to keep track of this reservation state. Consider the following multi-access network N: A B C D | | | | | | | | N --------------------------------------------- | | | | | | E F G This is modelled in OSPF (and OSPF-TE) as: B C \ / \ / \ / A ----- X ----- D / | \ / | \ / | \ E F G Kompella Standards Track [Page 2] Internet Draft OSPF TE for Multi-access Networks October 2003 where one of the routers A-G is elected the DR, here called X. In OSPF-TE, each of the routers A through G advertises its available (unreserved) bandwidth towards X by means of a TE LSA. X, on the other hand, does not advertise a TE LSA, instead advertising a regular network LSA per the OSPF v2 protocol [4], section 7.3. Routers computing paths across network N simply assume that there is always sufficient bandwidth from X to each of the routers A through G. Consider the case where N is implemented by a switch such that each router has a full-duplex 100Mbps connection to the switch, and advertises 100Mbps in its TE LSAs. A remote router R computing a 100Mbps path would see that 100Mbps is available from A to X, and ignoring the X->D link, could set up a path via A and D. Subsequently, another remote router Q looking at the B->X->D links would conclude that there was sufficient bandwidth for a 100Mbps path via B->D. However, this would put 200Mbps of traffic on the 100Mbps link from the switch to D. This memo attempts to rectify this shortcoming of [2]. 2. Approach A simple approach would be for the DR to advertise TE LSAs for links from itself to other routers. This is undesirable for at least three reasons: (a) This change is not backward compatible with [2]. (b) The only relevant TE information on the links advertised from the DR to other routers is the available bandwidth. Advertising a metric or administrative groups from X to A, for example, would change the topology of the overall network. (c) It is non-trivial for the DR to track the available bandwidth on other routers' links. In the above example, how would the DR know how much traffic has been reserved on the link from N to D? The approach taken here is for each router (say A) to advertise in its TE LSA for its link to the DR (i.e., A->X) the available "reverse bandwidth" (i.e., X to A). Going back to the example above, each router A-G would advertise its available forward bandwidth (100Mbps), as well as its available reverse bandwidth (100Mbps). When router R set up the path via A->X->D, D would advertise that its available reverse bandwith (D<-X) was now zero. Hence router Q would not attempt to use the X->D link. Kompella Standards Track [Page 3] Internet Draft OSPF TE for Multi-access Networks October 2003 3. Encoding in OSPF v2 TE LSAs The Reverse Bandwidth sub-TLV of the TE Link TLV is encoded in OSPF v2 as follows: 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 (TBD by IANA) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Media Type | Reserved (sbz) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reverse Available Bandwidth at Index 0 | . . . . . . | Reverse Available Bandwidth at Index k | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ "sbz" means SHOULD be zero when sending, SHOULD be ignored on receipt. The Type is [TBD]. The Length is equal to 4 + 4*(k+1) octets. 3.1. Types of Multiaccess Media Media Type values are given in the following table: Value Media Type ----- ---------- 0 Unknown 1 Shared 2 Switched Half Duplex 3 Switched Full Duplex 4-255 Reserved An example of a "Shared" link is a CSMA/CD Ethernet connection ([5]). An example of a "Switched Half Duplex" link is a half duplex port on a switched Ethernet ([6]). An example of a "Switched Full Duplex" link is a full duplex port on a switched Ethernet. 3.2. Reverse Available Bandwidth The semantics of the "Reverse Available Bandwidth at Index i" parallels exactly the available bandwidth at index i as advertised in the "Unreserved Bandwidth" field, except that it is in the reverse direction (DR to router, rather than router to DR). The format and units are identical to that of the "Unreserved Bandwidth" field. Kompella Standards Track [Page 4] Internet Draft OSPF TE for Multi-access Networks October 2003 3.3. Consistency Checks A Reverse Bandwidth sub-TLV within a TE Link TLV is ignored in its entirety if the link's Link Type is not 'Multiaccess' (value 2). Consider the set of links belonging to a given multi-access network. Not all the corresponding TE Link TLVs need have a Reverse Bandwidth sub-TLV. However those that do MUST have compatible Media Types, i.e., they MUST all be "Shared"; or they MUST all be either "Switched Half Duplex" or "Switched Full Duplex". If this test fails, all Reverse Bandwidth sub-TLVs for the set of links connected to this DR SHOULD be ignored. If a new Media Type is defined, so must corresponding consistency checks for the new type. For multi-access links with Media Type "Shared" or "Switched Half Duplex", the Reverse Available Bandwidths SHOULD NOT be advertised, and SHOULD be ignored on receipt (i.e., k = -1, and the Reverse Bandwidth TLV Length SHOULD be 4). For multi-access links with Media Type "Switched Full Duplex", if the Unreserved Bandwidth sub-TLV (type 8) of the Link TLV contains priority levels 0 through n, then k SHOULD be equal to n. Values of Reverse Available Bandwidth at index i > n (if any) SHOULD be ignored. If k < n, then computation of available bandwidth for index i, k < i < n, will perforce be inaccurate. 4. Computing Available Bandwidth This section illustrates how the information in the Reverse Bandwidth TLV can be used to compute the available bandwidth in the direction DR->router. It is however not meant as a primer on Constraint-based Routing. 4.1. Shared Media For each link l in multi-access network N of type Shared, let Ml be the Maximum Reservable Bandwidth (sub-TLV 7 of the Link TLV) for l, and let Ul be the Unreserved Bandwidth at index i for l. Let M be min (Ml) over all links l in N. The available bandwidth at index i Ai across N (independent of which pair of routers in N) is given by Ai = M - sum (Ml - Ul) (sum taken over all links l in N) Kompella Standards Track [Page 5] Internet Draft OSPF TE for Multi-access Networks October 2003 4.2. Switched Media Consider two routers A and B and a designated router X in a multi- access network N. Call the link from A to X 'a' and the link from B to X 'b'. The available bandwidth at index i from A to B when the Media Type of b is "Switched Half Duplex" is given by; min (Ua, Ub) where Ua is the Unreserved Bandwidth at index i for link a, and Ub is the Unreserved Bandwidth at index i for link b. If link b is of Media Type "Switched Full Duplex", the available bandwidth at index i from A to B is given by: min (Ua, Rb) where Ua is the Unreserved Bandwidth at index i for link a, and Rb is the Reverse Available Bandwidth at index i for link b. If link b doesn't have a Reverse Bandwidth sub-TLV, it is treated as a link of type "Switched Half Duplex". Note 1: if a flow of bandwidth B at index i in any direction is successfully admitted on a link of Media Type "Switched Half Duplex", then the Unreserved Bandwidth at index i on that link is decremented by B. Note 2: if a flow of bandwidth B at index i in the direction towards the DR is successfully admitted on a link of Media Type "Switched Full Duplex", then the Unreserved Bandwidth at index i on that link is decremented by B. If the flow is away from the DR, the Reverse Available Bandwidth at index i on that link is decremented by B. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 [2] Katz, D., Kompella, K. and D. Yeung, "Traffic Engineering Extensions to OSPF Version 2", RFC 3630, September 2003 [3] Coltun, R., "The OSPF Opaque LSA Option," RFC 2370, July 1998. [4] Moy, J., "OSPF Version 2", RFC 2328, April 1998. Kompella Standards Track [Page 6] Internet Draft OSPF TE for Multi-access Networks October 2003 Informative References [5] Metcalfe, R. M. and D. R. Boggs, "Ethernet: Distributed Packet Switching for Local Computer Networks", Communications of the ACM, Vol. 19, No. 5, July 1976, pp. 395 - 404 [6] Spurgeon, C. E., "Ethernet: The Definitive Guide", O'Reilly & Associates, February 2000 Security Considerations No new security vulnerabilities are introduced by this document. IANA Considerations IANA must assign a new sub-TLV of the Link TLV for the Reverse Bandwidth sub-TLV from the space allocated for Standards Action. Authors' Addresses Kireeti Kompella Juniper Networks 1194 N. 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Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Kompella Standards Track [Page 8]