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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group Ted Lemon 2 Internet Draft Nominum, Inc. 3 Stuart Cheshire 4 Apple Computer, Inc. 5 Bernie Volz 6 Ericsson 8 Obsoletes: draft-ietf-dhc-csr-05.txt October, 2001 9 Expires April, 2002 11 The Classless Static Route Option for DHCP 12 14 Status of this Memo 16 This document is an Internet-Draft and is in full conformance with 17 all provisions of Section 10 of RFC2026. 19 This document is an Internet-Draft. Internet-Drafts are working 20 documents of the Internet Engineering Task Force (IETF), its areas, 21 and its working groups. Note that other groups may also distribute 22 working documents as Internet-Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress". 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 Abstract 37 This document defines a new DHCP option which is passed from the 38 DHCP Server to the DHCP Client to configure a list of static routes 39 in the client. This option supersedes the Static Route option 40 (option 33) defined in RFC2132 [2]. 42 Introduction 44 The IP protocol [4] uses routers to transmit packets from hosts 45 connected to one IP subnet to hosts connected to a different IP 46 subnet. When an IP host (the source host) wishes to transmit a 47 packet to another IP host (the destination), it consults its 48 routing table to determine the IP address of the router that should 49 be used to forward the packet to the destination host. 51 The routing table on an IP host can be maintained in a variety of 52 ways - using a routing information protocol such as RIP [5], ICMP 53 router discovery [6,7] or using the DHCP Router option, defined in 54 RFC2132 [2]. 56 In a network that already provides DHCP service, using DHCP to 57 update the routing table on a DHCP client has several virtues. It 58 is efficient, since it makes use of messages that would have been 59 sent anyway. It is convenient - the DHCP server configuration 60 is already being maintained, so maintaining routing information, at 61 least on a relatively stable network, requires little extra work. 62 If DHCP service is already in use, no additional infrastructure 63 need be deployed. 65 The DHCP protocol as defined in RFC2131 [1] and the options defined 66 in RFC2132 [2] only provide a mechanism for installing a default 67 route or installing a table of classed routes. Classed routes are 68 routes whose subnet mask is implicit in the subnet number - see 69 section 3.2 of RFC791 [4] for details on classed routing. 71 Classed routing is no longer in common use, so the DHCP Static 72 Route option is no longer useful. Currently, classless routing, 73 described in [8] and [9], is the most commonly-deployed form of 74 routing on the Internet. In classless routing, IP addresses 75 consist of a network number (the combination of the network number 76 and subnet number described in [8]) and a host number. 78 In classed IP, the network number and host number are derived from 79 the IP address using a bitmask whose value is determined by the first 80 few bits of the IP address. In classless IP, the network number 81 and host number are derived from the IP address using a seperate 82 quantity, the subnet mask. In order to determine the network to 83 which a given route applies, an IP host must know both the network 84 number AND the subnet mask for that network. 86 The Static Routes option (option 33) does not provide a subnet mask 87 for each route - it is assumed that the subnet mask is implicit in 88 whatever network number is specified in each route entry. The 89 Classless Static Routes option does provide a subnet mask for each 90 entry, so that the subnet mask can be other than what would be 91 determined using the algorithm specified in RFC791 [4] and RFC950 92 [8]. 94 Definitions 96 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 97 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this 98 document are to be interpreted as described in RFC 2119 [3]. 100 This document also uses the following terms: 102 "DHCP client" 104 DHCP client or "client" is an Internet host using DHCP to 105 obtain configuration parameters such as a network address. 107 "DHCP server" 109 A DHCP server or "server" is an Internet host that returns 110 configuration parameters to DHCP clients. 112 "link" 114 Any set of all network attachment points that will recieve 115 a link-layer broadcast sent on any one of the attachment 116 points. This term is used in DHCP because in some cases 117 more than one IP subnet may be configured on a link. DHCP 118 uses a local-network (all-ones) broadcast, which is not 119 subnet-specific, and will therefore reach all nodes 120 connected to the link, regardless of the IP subnet or 121 subnets on which they are configured. 123 A "link" is sometimes referred to as a broadcast domain or 124 physical network segment. 126 Classless Route Option Format 128 The code for this option is TBD, and its minimum length is 5 bytes. 129 This option can contain one or more static routes, each of which 130 consists of a destination descriptor and the IP address of the 131 router that should be used to reach that destination. 133 Code Len Destination 1 Router 1 134 +-----+---+----+-----+----+----+----+----+----+ 135 | TBD | n | d1 | ... | dN | r1 | r2 | r3 | r4 | 136 +-----+---+----+-----+----+----+----+----+----+ 138 Destination 2 Router 2 139 +----+-----+----+----+----+----+----+ 140 | d1 | ... | dN | r1 | r2 | r3 | r4 | 141 +----+-----+----+----+----+----+----+ 143 In the above example, two static routes are specified. 145 Destination descriptors describe the IP subnet number and subnet 146 mask of a particular destination using a compact encoding. This 147 encoding consists of one octet describing the width of the subnet 148 mask, followed by all the significant octets of the subnet number. 150 The width of the subnet mask describes the number of one bits in 151 the mask, so for example a subnet with a subnet number of 152 10.0.127.0 and a netmask of 255.255.255.0 would have a subnet mask 153 width of 24. 155 The significant portion of the subnet number is simply all of the 156 octets of the subnet number where the corresponding octet in the 157 subnet mask is non-zero. The number of significant octets is the 158 width of the subnet mask divided by eight, rounding up, as shown 159 in the following table: 161 Width of subnet mask Number of significant octets 162 0 0 163 1- 8 1 164 9-16 2 165 17-24 3 166 25-32 4 167 The following table contains some examples of how various subnet 168 number/mask combinations can be encoded: 170 Subnet number Subnet mask Destination descriptor 171 0 0 0 172 10.0.0.0 255.0.0.0 8.10 173 10.0.0.0 255.255.255.0 24.10.0.0 174 10.17.0.0 255.255.0.0 16.10.17 175 10.27.129.0 255.255.255.0 24.10.27.129 176 10.229.0.128 255.255.255.128 25.10.229.0.128 177 10.198.122.47 255.255.255.255 32.10.198.122.47 179 Local Subnet Routes 181 In some cases more than one IP subnet may be configured on a link. 182 In such cases, a host whose IP address is in one IP subnet in the 183 link could communicate directly with a host whose IP address is in 184 a different IP subnet on the same link. In cases where a client is 185 being assigned an IP address on an IP subnet on such a link, 186 for each IP subnet in the link other than the IP subnet on which 187 the client has been assigned the DHCP server MAY be configured to 188 specify a router IP address of 0.0.0.0. 190 For example, consider the case where there are three IP subnets 191 configured on a link: 10.0.0/24, 192.168.0/24, 10.0.21/24. If the 192 client is assigned an IP address of 10.0.21.17, then the server 193 could include a route with a destination of 10.0.0/24 and a router 194 address of 0.0.0.0, and also a route with a destination of 195 192.168.0/24 and a router address of 0.0.0.0. 197 A DHCP client whose underlying TCP/IP stack does not provide this 198 capability MUST ignore routes in the Classless Static Routes option 199 whose router IP address is 0.0.0.0. Please note that the behavior 200 described here only applies to the Classless Static Routes option, 201 not to the Static Routes option nor the Router option. 203 DHCP Client Behavior 205 DHCP clients that do not support this option MUST ignore it if it 206 is received from a DHCP server. DHCP clients that support this 207 option MUST install the routes specified in the option, except as 208 specified in the Local Subnet Routes section. DHCP clients that 209 support this option MUST NOT install the routes specified in the 210 Static Routes option (option code 33) if both a Static Routes 211 option and the Classless Static Routes option are provided. 213 DHCP clients that support this option and that send a DHCP 214 Parameter Request List option MUST request both this option and the 215 Router option [2] in the DHCP Parameter Request List. 217 DHCP clients that support this option and send a parameter request 218 list MAY also request the Static Routes option, for compatibility 219 with older servers that don't support Classless Static Routes. 221 The Classless Static Routes option code MUST appear in the 222 parameter request list prior to both the Router option code and the 223 Static Routes option code, if present. 225 If the DHCP server returns both a Router option and a Classless 226 Static Routes option, the DHCP client MUST ignore the Router 227 option. 229 After deriving a subnet number and subnet mask from each 230 destination descriptor, the DHCP client MUST set any bits in the 231 subnet number that are zero in the subnet mask to zero. For 232 example, if the server sends a route with a destination of 233 129.210.177.132 (hexadecimal 81D4B184) and a subnet mask of 234 255.255.255.128 (hexadecimal FFFFFF80), the client will install a 235 route with a destination of 129.210.177.128 (hexadecimal 236 81D4B180). 238 Requirements to avoid sizing constraints 240 Because a full routing table can be quite large, the standard 576 241 octet maximum size for a DHCP message may be too short to contain 242 some legitimate Classless Static Route options. Because of this, 243 clients implementing the Classless Static Route option SHOULD send 244 a Maximum DHCP Message Size [2] option if the DHCP client's TCP/IP 245 stack is capable of reassembling fragmented IP datagrams. In this 246 case, the client SHOULD set the value of this option to at least 247 the MTU of the interface that the client is configuring. The 248 client MAY set the value of this option higher, up to the size of 249 the largest UDP packet it is prepared to accept. (Note that the 250 value specified in the Maximum DHCP Message Size option is the 251 total maximum packet size, including IP and UDP headers.) 253 DHCP servers sending this option MUST use the technique described 254 in [10] for sending options larger than 255 bytes when storing this 255 option in outgoing DHCP packets. DHCP clients supporting this 256 option MUST support the technique described in [10] when reading 257 this option from incoming DHCP packets. 259 DHCP Server administrator responsibilities 261 Many clients may not implement the Classless Static Routes option. 262 DHCP server administrators should therefore configure their DHCP 263 servers to send both a Router option and a Classless Static Routes 264 option, and should specify the default router(s) both in the 265 Router option and in the Classless Static Routes option. 267 DHCP Server Considerations 269 When a DHCP client requests the Classless Static Routes option and 270 also requests either or both of the Router option and the Static 271 Routes option, and the DHCP server is sending Classless Static 272 Routes options to that client, the server SHOULD NOT include the 273 Router or Static Routes options. 275 Security Considerations 277 DHCP currently provides no authentication or security mechanisms. 278 Potential exposures to attack are discussed in section 7 of the DHCP 279 protocol specification [1]. The Classless Static Routes option can 280 be used to misdirect network traffic by providing incorrect IP 281 addresses for routers. 283 IANA Considerations 285 This DHCP option will require the allocation of an option code in 286 the list DHCP option codes that the IANA maintains. 288 References 290 [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, 291 Bucknell University, March 1997. 292 [2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor 293 Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell 294 University, March 1997. 295 [3] Bradner, S., "Key words for use in RFCs to indicate requirement 296 levels", RFC 2119, Harvard University, March 1997. 297 [4] Postel, J., "Internet Protocol", RFC 791, USC/Information 298 Sciences Institute, September 1981. 299 [5] Hedrick, C.L., "Routing Information Protocol", RFC 1058, 300 Rutgers University, June 1, 1988. 301 [6] Deering, S., "ICMP Router Discovery Messages", RFC 1256, 302 Xerox PARC, September 1991. 303 [7] Postel, J., "Internet Control Message Protocol", RFC 792, 304 USC/Information Sciences Institute, September 1981. 305 [8] Mogul, J., Postel, J., "Internet Standard Subnetting 306 Procedure", RFC950, Stanford University, USC/Information 307 Sciences Institute, August 1985. 308 [9] Pummill, T., Manning, B., "Variable Length Subnet Table For 309 IPv4", RFC1878, Alantec, USC/Information Sciences Institute, 310 December, 1995. 311 [10] Lemon, T., Cheshire, S., "Encoding Long DHCP Options", 312 draft-ietf-dhc-concat-02.txt, Nominum, Inc., October, 2001. 314 Author Information 316 Ted Lemon 317 Nominum, Inc. 318 950 Charter Street 319 Redwood City, CA 94043 320 email: Ted.Lemon@nominum.com 322 Stuart Cheshire 323 Apple Computer, Inc. 324 1 Infinite Loop 325 Cupertino 326 California 95014 327 USA 328 Phone: +1 408 974 3207 329 EMail: rfc@stuartcheshire.org 330 Bernie Volz 331 Ericsson 332 959 Concord Street 333 Framingham, MA, 01701 334 Phone: +1 508 875 3162 335 EMail: bernie.volz@ericsson.com 337 Expiration 339 This document will expire on April 31, 2002. 341 Full Copyright Statement 343 Copyright (C) The Internet Society (2000-2001). All Rights 344 Reserved. 346 This document and translations of it may be copied and furnished to 347 others, and derivative works that comment on or otherwise explain it 348 or assist in its implementation may be prepared, copied, published 349 and distributed, in whole or in part, without restriction of any 350 kind, provided that the above copyright notice and this paragraph are 351 included on all such copies and derivative works. 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