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Summary: 5 errors (**), 0 flaws (~~), 2 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Naiming Shen 3 Internet Draft Acee Lindem 4 Expiration Date: March 2003 Jenny Yuan 5 File name: draft-ietf-isis-igp-p2p-over-lan-01.txt Redback Networks 6 Alex Zinin 7 Alcatel 8 Russ White 9 Stefano Previdi 10 Cisco Systems 11 September 2002 13 Point-to-point operation over LAN 14 in link-state routing protocols 16 draft-ietf-isis-igp-p2p-over-lan-01.txt 18 Status of this Memo 20 This document is an Internet-Draft and is in full conformance with 21 all provisions of Section 10 of RFC2026. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF), its areas, and its working groups. Note that 25 other groups may also distribute working documents as 26 Internet-Drafts. 28 Internet-Drafts are draft documents valid for a maximum of six 29 months and may be updated, replaced, or obsoleted by other 30 documents at any time. It is inappropriate to use Internet- 31 Drafts as reference material or to cite them other than as 32 "work in progress." 34 The list of current Internet-Drafts can be accessed at 35 http://www.ietf.org/1id-abstracts.html 37 The list of Internet-Draft Shadow Directories can be accessed at 38 http://www.ietf.org/shadow.html 40 Abstract 42 The two predominant circuit types used by link state routing 43 protocols are point-to-point and broadcast. It is important to 44 identify the correct circuit type when forming adjacencies, 45 flooding link state database packets, and representing the circuit 46 topologically. This document describes a simple mechanism to treat 47 the broadcast network as a point-to-point connection from the 48 standpoint of IP routing. 50 1. Introduction 52 Point-to-point and broadcast are the two predominant circuit 53 types used by link state routing protocols such as IS-IS [ref1] 54 [ref2] and OSPF [ref3]. They are treated differently with respect 55 to establishing neighbor adjacencies, flooding of link-state 56 information, representation of the topology, SPF calculation and 57 protocol packets. The most important differences are that broadcast 58 circuits utilize the concept of a designated router and are 59 represented topologically as virtual nodes in the network topology 60 graph. 62 Compared with broadcast circuits, point-to-point circuits 63 afford more straightforward IGP operation. There is no designated 64 router involved and there is no representation of the pseudo-node 65 or network LSA in the link state database. For ISIS, there also is 66 no periodic database synchronization. Conversely, if there are more 67 than two routers on the LAN media, the traditional view of the 68 broadcast circuit will reduce the routing information in the network. 70 When there are only two routers on the LAN, it makes more sense to 71 treat the connection between the two routers as a point-to-point 72 circuit. This document describes the mechanism to allow link state 73 routing protocols to operate using point-to-point connections over 74 a LAN under this condition. Some implications related to forwarding 75 IP packets on this type of circuit are also discussed. We will refer 76 to this as a p2p-over-lan circuit in this document. 78 2. Motivation 80 Even though a broadcast circuit is meant to handle more than two 81 devices, there are cases where only two routers are connected 82 over either the physical or logical LAN segment: 84 1. The media itself is being used for point-to-point 85 operation between two routers. This is mainly for 86 long-haul operation. 87 2. There are only two routers on the physical LAN. 88 3. There are only two routers on a virtual LAN (vLAN). 90 In any of the above cases, the link state routing protocols will 91 normally still treat the media as a broadcast circuit. Hence, they 92 will have the overhead involved with protocol LAN operation without 93 the benefits of reducing routing information and optimized flooding. 95 Being able to treat a LAN as a point-to-point circuit provides the 96 benefit of reduction in the amount of information routing 97 protocols must carry and manage. DR/DIS election can be omitted. 98 Flooding can be done as in p2p links without the need of using 99 "LSA reflection" by the DR in OSPF or periodic CSNPs in ISIS. 101 Also, if a broadcast segment wired as a point-to-point link 102 can be treated as a point-to-point link, only the connection between 103 the two routers would need to be advertised as a topological entity. 105 Even when there are multiple routers on the LAN an ISP may want 106 to sub-group the routers into multiple vLANs since this allows 107 them to assign different costs to IGP neighbors. When there are 108 only two routers in some of the vLANs, this LAN can be viewed by 109 the IGP as a mesh of point-to-point connections. 111 As a side benefit, unnumbered interface can also be applied over 112 p2p-over-lan circuits. The advantages of unnumbered point-to-point 113 links are obvious in the current IP addressing environment where 114 addresses are a scarce resource. Separating the concept of network 115 type from media type will allow LANs, e.g. ethernet, to be 116 unnumbered and realize the IP address space savings. Another 117 advantage is in simpler network management and configuration. 119 3. IP multi-access subnets 121 When an IP network includes multi-access segments, each segment is 122 usually assigned a separate subnet and each router connected to it is 123 assigned a distinct IP address within that subnet. The role of the 124 IP address assigned to a multi-access interface can be outlined as 125 follows: 127 1. Source IP address - The interface address can be used by 128 the router as the source IP address in locally originated 129 IP packets destined for that subnet or having a best path 130 next hop on that subnet. 132 2. Destination IP address - The interface address can be used by 133 other devices in the network as a destination address for 134 packets to router applications (examples include telnet, SMTP, 135 TFTP, OSPF, BGP, etc). 137 3. Next-hop identifier - If other routers connected to the same 138 segment need to forward traffic through the router, the 139 corresponding routes in their routing tables will include the 140 router's interface IP address. This address will be used to 141 find the router's MAC address using the ARP protocol. 142 Effectively, the interface IP addresses help other routers 143 find the data-link layer details that are required to specify 144 the destination of the encapsulating data-link frame when it 145 is sent on the segment. 147 The IP addressing scheme includes an option that allows the 148 administrators to not assign any subnets to point-to-point links 149 (links connecting only two devices and using protocols like PPP, SLIP 150 or HDLC for IP encapsulation). This is possible, because the routers 151 do not need next-hop identifiers on point-to-point links (there is 152 only one destination for any transmission), and an interface 153 independent IP address can be used as the source and destination. 154 Using the unnumbered option for a point-to-point link essentially 155 makes it a purely topological entity used only to reach other 156 destinations. 158 4. Point-to-point connection over LAN media 160 The idea is very simple: provide a configuration mechanism to 161 inform the IGP that the circuit is type point-to-point 162 irrespective of the physical media type. For the IGP, this implies 163 that it will send protocol packets with the appropriate 164 point-to-point information and expects to receive protocol packets 165 as they would be received on a point-to-point circuit. Over LAN 166 media, the MAC header must contain the correct multicast MAC address 167 to be received by the other side of the connection. For vLAN 168 environments, the MAC header must also contain the proper vLAN ID. 170 In order to allow LAN links used to connect only two routers to be 171 treated as unnumbered point-to-point interfaces, the MAC address 172 resolution and nexthop IP address issues need to be addressed. 174 4.1 Operation of IS-IS 176 This p2p-over-lan circuit extension for IS-IS is only concerned 177 in pure IP routing and forwarding operation. 179 Since the physically circuit is a broadcast one, the IS-IS protocol 180 packets need to have MAC addresses for this p2p-over-lan circuit. 181 From link layer point of view, those packets are IS-IS LAN packets. 182 The Multi-destination address including AllISs, AllL1ISs and AllL2ISs 183 defined in [ref1] can be used for link layer encapsulation, the 184 use of AllISs is recommended. 186 The circuit needs to have IP address(es) and the p2p IIH over this 187 circuit MUST include the IP interface address(es) as defined in 188 [ref2]. The IP address(es) can be numbered or unnumbered. 190 4.2 Operation of OSPF 192 OSPF routers supporting the capabilities described herein should 193 support an additional interface configuration parameter specifying 194 the interface topology type. For a LAN (i.e., broadcast capable) 195 interface, the interface may be viewed as a point-to-point interface. 196 Both routers on the LAN will simply join the AllSPFRouters 197 (224.0.0.5) multicast group and send all OSPF packets to 224.0.0.5. 198 This is identical to operation over a physical point-to-point link 199 as described in sections 8.1 and 8.2 of [ref3]. 201 4.3 IP forwarding and ARP 203 Unlike normal point-to-point IGP circuit, the IP nexthop for the 204 routes using this p2p-over-lan circuit as an outbound interface is 205 not optional. The IP nexthop address has to be a valid interface 206 or internal address on the adjacent router. This address is used by 207 local router to obtain the MAC address for IP packet forwarding. 208 Proxy ARP has to be enabled if the address is not the adjacent 209 interface IP address. 211 In the case where unnumbered IP addresses are used for p2p-over-lan 212 circuit, the source IP address of ARP request and the target 213 interface IP address are usually on different subnets. The ARP 214 should reply only if this is a p2p-over-lan circuit and the source 215 IP address of the ARP request is the same as the neighbor's 216 interface IP address at the other end. The neighbor's address is 217 learned from IGP hello exchanges over this circuit. 219 4.4 Other MAC address resolution mechanisms 221 In more general cases while p2p-over-lan circuit is used as an 222 unnumbered link, other MAC address resolution mechanisms are needed 223 for IP packet forwarding. For example, if link-state IGP is not 224 configured over this p2p-over-lan link, or Proxy ARP is not enabled 225 on the circuit. The following techniques can be used to acquire the 226 MAC address and/or the next-hop IP address of the remote device on 227 an unnumbered point-to-point LAN link. 229 1. Static configuration. A router can be statically configured 230 with the MAC address that should be used as the destination 231 MAC address when sending data out of the interface. 233 2. MAC address gleaning. If a dynamic routing protocol is running 234 between the routers connected to the link, the MAC address of 235 the remote device can be taken from a data-link frame carrying 236 a packet of the corresponding routing protocol. 238 3. ARP for reference IP address. When a point-to-point link is 239 configured as unnumbered, the router usually associates with 240 it a "reference IP address", that is used as the source IP 241 address in the packets originated for the unnumbered 242 interface. When such an address is known to a router, the 243 router may announce its MAC address by sending a gratuitous 244 ARP message. This solution will also help in the situations 245 where routers calculate the next-hop addresses for the routes 246 through point-to-point interfaces. Since the source IP address 247 in the received routing protocol packet is used as the next- 248 hop address in the route, forwarding an IP packet along such 249 a route will lead to an ARP request submission on the LAN 250 link that will be answered by the remote device. 252 4. Broadcast/multicast/proprietary. 254 4.5 Detection of mis-configuration 256 With this p2p-over-lan extension, the difference between a LAN and 257 a point-to-point circuit can be made purely by configuration. It is 258 important to implement the mechanisms for early detection of 259 mis-configuration. 261 If the circuit is configured as point-to-point type and receives 262 LAN hello packets, the router MUST discard the incoming packets; If 263 the circuit is a LAN type and receive point-to-point hello packets, 264 it MUST discard the incoming packets. If the system ID or the 265 router ID of incoming hello packet does not match the system ID or 266 the router ID of already established adjacency over this p2p-over-lan 267 circuit, it MUST discard the packet. The implementation should offer 268 logging and debugging information of the above events. 270 5. Compatibility considerations 272 Both routers on a LAN must support the p2p-over-lan extension 273 and both must have the LAN segment configured as a p2p-over-lan 274 circuit for successful operation. Both routers MAY also support 275 one of the above listed methods for mapping ip addresses on the 276 link to MAC address, and MUST support proxy ARP on the link. If 277 a proprietary method of IP address to MAC address resolution is 278 used by one router, both routers must be capable of using the 279 same method. Otherwise, the link should be configured as a 280 standard LAN link, with traditional IGP LAN models used. 282 6. Scalability and deployment considerations 284 There is obvious advantage to use this extension on the LANs 285 that are connected back-to-back or only contain two routers. 286 However, there are tradeoffs when modeling a LAN as multiple vLANs 287 and using this extension since one does sacrifice the inherent 288 scalability benefits of multi-access networks. In general, 289 it will increase the link-state database size, the amount of 290 packets flooded and the route calculation overhead. Network design 291 engineers should carefully balance between the associated 292 overhead. The scalability impact is less of a concern if all the 293 vLANs are within a single OSPF area or ISIS level. 295 Deployment of the described technique brings noticeable benefits from 296 the perspective of IP address usage, the network management and the 297 router configuration. Note, however, that use of the IP unnumbered 298 option for point-to-point LAN links inherits the same problems as 299 those present for serial links, i.e., not being able to ping or 300 monitor a specific interface between routers. 302 7. Security Issues 304 This document does not introduce any new security issues to ISIS or 305 OSPF. For ARP to support unnumbered IP interface addresses, it needs 306 to verify the p2p-over-lan circuit type described in this document 307 and to verify the ARP packet source interface address to match the 308 IGP adjacency interface IP address. This is due to normal ARP sanity 309 check for common subnet can not be applied in this case. 311 8. Acknowledgments 313 The authors would like to acknowledge the following individuals: 314 (in last name alphabetical order) Pedro Marques, Christian Martin, 315 Danny McPherson, Ajay Patel, Tony Przygienda and Alvaro Retana. 317 9. References 319 [ref1] ISO. Information Technology - Telecommunications and 320 Information Exchange between Systems - Intermediate System 321 to Intermediate System Routing Exchange Protocol for 322 Use in Conjunction with the Protocol for Providing the 323 Connectionless-Mode Network Service. ISO, 1990. 325 [ref2] R. Callon. Use of OSI ISIS for Routing in TCP/IP and Dual 326 Environments. INTERNET-RFC, Internet Engineering Task Force, 327 December 1990. 329 [ref3] J. Moy. OSPF Version 2. Technical Report RFC2328 Internet 330 Engineering Task Force, 1998. 332 10. Authors' Addresses 334 Naiming Shen 335 Redback Networks 336 350 Holger Way 337 San Jose, CA, 95134 USA 338 naiming@redback.com 340 Acee Lindem 341 Redback Networks 342 102 Carric Bend Court 343 Cary, NC 27519 USA 344 acee@redback.com 345 Jenny Yuan 346 Redback Networks 347 350 Holger Way 348 San Jose, CA, 95134 USA 349 jenny@redback.com 351 Alex Zinin 352 Alcatel 353 Sunnyvale, CA, USA 354 e-mail: zinin@psg.com 356 Russ White 357 Cisco Systems, Inc. 358 7025 Kit Creek Rd. 359 Research Triangle Park, NC 27709 360 e-mail: riw@cisco.com 362 Stefano Previdi 363 Cisco Systems, Inc. 364 De Kleetlaan 6A 365 1831 Diegem - Belgium 366 email: sprevidi@cisco.com