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Boers 4 Expires: August 5, 2006 E. Rosen 5 Cisco Systems, Inc. 6 february 2006 8 The RPF Vector TLV 9 draft-ietf-pim-rpf-vector-02 11 Status of this Memo 13 By submitting this Internet-Draft, each author represents that any 14 applicable patent or other IPR claims of which he or she is aware 15 have been or will be disclosed, and any of which he or she becomes 16 aware will be disclosed, in accordance with Section 6 of BCP 79. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt. 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html. 34 This Internet-Draft will expire on August 5, 2006. 36 Copyright Notice 38 Copyright (C) The Internet Society (2006). 40 Abstract 42 This document describes a use of the PIM Join Attribute as defined in 43 draft-ietf-pim-join-attributes[I-D.ietf-pim-join-attributes] which 44 enables PIM to build multicast trees through an MPLS-enabled network, 45 even if that network's IGP does not have a route to the source of the 46 tree. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 51 2. Use of the RPF Vector TLV . . . . . . . . . . . . . . . . . . 4 52 2.1. Attribute and shared tree joins . . . . . . . . . . . . . 4 53 2.2. Attribute and Bootstrap messages . . . . . . . . . . . . . 5 54 2.3. The Vector Attribute . . . . . . . . . . . . . . . . . . . 5 55 2.3.1. Inserting a Vector Attribute in a Join . . . . . . . . 5 56 2.3.2. Processing a Received Vector Attribute . . . . . . . . 5 57 2.3.3. Vector Attribute and Asserts . . . . . . . . . . . . . 5 58 3. Vector Attribute TLV Format . . . . . . . . . . . . . . . . . 7 59 4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7 60 5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 61 5.1. Normative References . . . . . . . . . . . . . . . . . . . 7 62 5.2. Informative References . . . . . . . . . . . . . . . . . . 8 63 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 64 Intellectual Property and Copyright Statements . . . . . . . . . . 10 66 1. Introduction 68 It is sometimes convenient to distinguish the routers of a particular 69 network into two categories: "edge routers" and "core routers". The 70 edge routers attach directly to users or to other networks, but the 71 core routers attach only to other routers of the same network. If 72 the network is MPLS-enabled, then any unicast packet which needs to 73 travel outside the network can be "tunneled" via MPLS from one edge 74 router to another. To handle a unicast packet which must travel 75 outside the network, an edge router needs to know which of the other 76 edge routers is the best exit point from the network for that 77 packet's destination IP address. The core routers, however, do not 78 need to have any knowledge of routes which lead outside the network; 79 as they handle only tunneled packets, they only need to know how to 80 reach the edge routers and the other core routers. 82 Consider, for example, the case where the network is an Autonomous 83 System (AS), the edge routers are EBGP speakers, the core routers may 84 be said to constitute a "BGP-free core". The edge routers must 85 distribute BGP routes to each other, but not to the core routers. 87 However, when multicast packets are considered, the strategy of 88 keeping the core routers free of "external" routes is more 89 problematic. When using PIM-SM[I-D.ietf-pim-sm-v2-new], PIM-SSM[I- 90 D.ietf-ssm-arch] or PIM-BIDIR[I-D.ietf-pim-bidir] to create a 91 multicast distribution tree for a particular multicast group, one 92 wants the core routers to be full participants in the PIM protocol, 93 so that multicasting can be done efficiently in the core.This means 94 that the core routers must be able to correctly process PIM Join 95 messages for the group, which in turn means that the core routes must 96 be able to send the Join messages towards the root of the 97 distribution tree. If the root of the tree lies outside the 98 network's borders (e.g., is in a different AS), and the core routers 99 do not maintain routes to external destinations, then the PIM Join 100 messages cannot be processed, and the multicast distribution tree 101 cannot be created. 103 In order to allow PIM to work properly in an environment where the 104 core routers do not maintain external routes, a PIM extension is 105 needed. When an edge router sends a PIM Join message into the core, 106 it must include in that message a "Vector" which specifies the IP 107 address of the next edge router along the path to the root of the 108 multicast distribution tree. The core routers can then process the 109 Join message by sending it towards the specified edge router (i.e., 110 toward the Vector). In effect, the Vector serves as an attribute, 111 within a particular network, for the root of the tree. 113 This document defines a new TLV in the PIM Join Attribute message[I- 114 D.ietf-pim-join-attributes]. It consists of a single Vector which 115 identifies the exit point of the network. 117 2. Use of the RPF Vector TLV 119 Before we can start forwarding multicast packets we need to build a 120 forwarding tree by sending PIM Joins hop by hop. Each router in the 121 path creates a forwarding state and propagates the Join towards the 122 root of the forwarding tree. The building of this tree is receiver 123 driven. See Figure 1. 125 ------------------ BGP ----------------- 126 | | 127 [S]---( Edge 1)--(Core 1)---( Core )--(Core 2)---( Edge 2 )---[R] 128 <--- (S,G) Join 130 Figure 1 132 In this example, the 2 edge routers are BGP speakers. The core 133 routers are not BGP speakers and do not have any BGP distributed 134 routes. The route to S is a BGP distributed route, hence is known to 135 the edge but not to the core. The Edge 2 router determines the 136 interface leading to S, and sends a PIM Join to the upstream router. 137 In this example, though, the upstream router is a core router, with 138 no route to S. Without the PIM extensions specified in this document, 139 the core router cannot determine where the send the Join, so the tree 140 cannot be constructed. 142 To allow the core router to participate in the construction of the 143 tree, the Edge 2 router will include an attribute field in the PIM 144 Join. In this example, the Attribute field will contain the IP 145 address of Edge 1. Edge 2 then forwards the PIM Join towards Edge 1. 146 The intermediate core router do their RPF check on the Attribute (IP 147 address of Edge 1) rather than the Source, this allows the tree to be 148 constructed. 150 2.1. Attribute and shared tree joins 152 In the example above we build a source tree to illustrate the 153 attribute behavior. The attribute is however not restricted to 154 source tree only. The tree may also be constructed towards a 155 Rendezvous Point (RP) IP address. The RP IP address is used in a 156 similar way as the Source in the example above. PIM Attribute 157 procedures defined for sources are equally applicable to (*,G) and 158 (*,*,RP) joins unless otherwise noted. 160 2.2. Attribute and Bootstrap messages 162 The RPF vector does not apply to BSR bootstrap messages. To allow 163 BSR messages to be forwarded across a core where the RP IP address is 164 not routable in the core a solution has the developed in BSR. 166 2.3. The Vector Attribute 168 2.3.1. Inserting a Vector Attribute in a Join 170 In the example of Figure 1, when the Edge 2 router looks up the route 171 to the source of the multicast distribution tree, it will find a BGP- 172 distributed route whose "BGP next-hop" is Edge 1. Edge 2 then looks 173 up the route to Edge 1 to find interface and PIM adjacency which is 174 the next hop to the source, namely Core 2. 176 When Edge 2 sends a PIM Join to Core 2, it includes a Vector 177 Attribute specifying the address of Edge 1. Core 2, and subsequent 178 core routers, will forwarding the Join along the Vector (i.e, towards 179 Edge 1) instead of trying to forward it towards S. 181 Whether an ttribute is actually needed depends on whether the Core 182 routers have a route to the source of the multicast tree. How the 183 Edge router knows whether or not this is the case (and thus how the 184 Edge router determines whether or not to insert an attribute field) 185 is outside the scope of this document. 187 2.3.2. Processing a Received Vector Attribute 189 When processing a received PIM Join which contains a Vector 190 Attribute, a router must first check to see if the Vector IP address 191 is one of its own IP addresses. If so, the Vector Attribute is 192 discarded, and not passed further upstream. Otherwise, the Vector 193 Attribute is used to find the route to the source, and is passed 194 along when a PIM Join is sent upstream. Note that a router which 195 receives a Vector Attribute must use it, even if that router happens 196 to have a route to the source. A router which discards a Vector 197 Attribute may of course insert a new Vector Attribute. This would 198 typically happen if a PIM Join needed to pass through a sequence of 199 Edge routers, each pair of which is separated by a core which does 200 not have external routes. In the absence of periodic refreshment, 201 Vectors expire along with the corresponding (S,G) state. 203 2.3.3. Vector Attribute and Asserts 205 In a PIM Assert message we include the routing protocol's "metric" to 206 the source of the tree. This information is used in the selection of 207 the assert winner. If a PIM Join is being sent towards a Vector, 208 rather than towards the source, the Assert message must have the 209 metric to the Vector instead of the metric to the source. The Assert 210 message however does not have an attribute field and does not mention 211 the Vector. 213 A router may change its upstream neighbor on a particular multicast 214 tree as the result of receiving Assert messages. However a Vector 215 Attribute should not be sent in a PIM Join to an upstream neighbor 216 which is chosen as the result of processing the Assert messages. 217 Reachability of the Vector is only guaranteed by the router that 218 advertises reachability to the Vector in it's IGP. If the assert 219 winner upstream is not our real preferred next-hop, we can't be sure 220 this router knows the path to the Vector. In the worst case the 221 assert winner has a route to the Vector that is on the same interface 222 where the assert was won. That will point the RPF interface to that 223 interface and will result in a O-list being NULL. The Vector 224 attribute is not inserted if the RPF neighbor was chosen via an 225 assert process and the RPF neighbor is different from the RPF 226 neighbor that would have been selected via the local routing table. 227 In all other cases the Vector has to be included in the Join message. 229 3. Vector Attribute TLV Format 231 0 1 2 3 232 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 1 233 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 234 |F|S| Type | Length | Encoded-Unicast address 235 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-....... 237 F bit 238 ----- 239 Forward Unknown TLV. If this bit is set the TLV is forwarded 240 regardless if the router understands the Type. 242 S bit 243 ----- 244 Bottom of Stack. If this bit is set then this is the last 245 TLV in the stack. 247 Type 248 ---- 249 The Vector Attribute type is 0. 251 Length 252 ------ 253 Length depending on Address Family of Encoded-Unicast address. 255 Value 256 ----- 257 Encoded-Unicast address, see PIM-SM 258 [I-D.ietf-pim-sm-v2-new] 260 4. Acknowledgments 262 The authors would like to thank Yakov Rekhter and Dino Farinacci for 263 their initial ideas on this topic. 265 5. References 267 5.1. Normative References 269 [I-D.ietf-pim-sm-v2-new] 270 Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas, 271 "Protocol Independent Multicast - Sparse Mode PIM-SM): 272 Protocol Specification (Revised)", 273 draft-ietf-pim-sm-v2-new-11 (work in progress), 274 October 2004. 276 [I-D.ietf-pim-bidir] 277 Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano, 278 "Bi-directional Protocol Independent Multicast (BIDIR- 279 PIM)", draft-ietf-pim-bidir-07 (work in progress), 280 March 2005. 282 [I-D.ietf-pim-join-attributes] 283 Boers, A., "Format for using TLVs in PIM messages", 284 draft-ietf-pim-join-attributes-00 (work in progress), 285 October 2005. 287 [I-D.ietf-ssm-arch] 288 Holbrook, H. and B. Cain, "Source-Specific Multicast for 289 IP", draft-ietf-ssm-arch-06 (work in progress), 290 September 2004. 292 5.2. Informative References 293 Authors' Addresses 295 IJsbrand Wijnands 296 Cisco Systems, Inc. 297 De kleetlaan 6a 298 Diegem 1831 299 Belgium 301 Email: ice@cisco.com 303 Arjen Boers 304 Cisco Systems, Inc. 305 Avda. Diagonal, 682 306 Barcelona 08034 307 Spain 309 Email: aboers@cisco.com 311 Eric Rosen 312 Cisco Systems, Inc. 313 1414 Massachusetts Avenue 314 Boxborough, Ma 01719 316 Email: erosen@cisco.com 318 Intellectual Property Statement 320 The IETF takes no position regarding the validity or scope of any 321 Intellectual Property Rights or other rights that might be claimed to 322 pertain to the implementation or use of the technology described in 323 this document or the extent to which any license under such rights 324 might or might not be available; nor does it represent that it has 325 made any independent effort to identify any such rights. 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