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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DHC Working Group Kim Kinnear 3 Internet Draft Richard Johnson 4 Intended Status: Standards Track Mark Stapp 5 Expires: September 3, 2009 Jay Kumarasamy 6 Cisco Systems 7 March 3, 2009 9 Virtual Subnet Selection Options for DHCPv4 and DHCPv6 10 12 Status of this Memo 14 This Internet-Draft is submitted to IETF in full conformance with the 15 provisions of BCP 78 and BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on September 3, 2009 35 Copyright Notice 37 Copyright (c) 2009 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents in effect on the date of 42 publication of this document (http://trustee.ietf.org/license-info). 43 Please review these documents carefully, as they describe your rights 44 and restrictions with respect to this document. 46 Abstract 48 This memo defines a Virtual Subnet Selection (VSS) option for DHCPv4 49 and DHCPv6, and a DHCPv4 relay-agent-information sub-option. These 50 are intended for use by DHCP clients, relay agents, and proxy clients 51 in situations where VSS information needs to be passed to the DHCP 52 server for proper address or prefix allocation to take place. 54 For the DHCPv4 option and relay-agent-information sub-option, this 55 memo documents existing usage as per RFC 3942 [RFC3942]. 57 Table of Contents 59 1. Introduction................................................. 2 60 2. Terminology.................................................. 3 61 3. Virtual Subnet Selection Option and Sub-Option Definitions... 5 62 3.1. DHCPv4 Virtual Subnet Selection Option..................... 5 63 3.2. DHCPv4 Virtual Subnet Selection Sub-Option................. 5 64 3.3. DHCPv6 Virtual Subnet Selection Option..................... 6 65 3.4. Virtual Subnet Selection Type and Information.............. 6 66 4. Overview of Virtual Subnet Selection Usage................... 7 67 5. Relay Agent Behavior......................................... 10 68 5.1. VPN assignment by the DHCP server.......................... 12 69 5.2. DHCP Leasequery............................................ 12 70 6. Client Behavior.............................................. 12 71 7. Server Behavior.............................................. 13 72 7.1. Returning the DHCPv4 or DHCPv6 Option...................... 14 73 7.2. Returning the DHCPv4 Sub-Option............................ 15 74 7.3. Making sense of conflicting VSS information................ 15 75 8. Security..................................................... 16 76 9. IANA Considerations.......................................... 17 77 10. Acknowledgments............................................. 17 78 11. References.................................................. 18 79 11.1. Normative References...................................... 18 80 11.2. Informative References.................................... 18 81 12. Authors' Addresses.......................................... 19 83 1. Introduction 85 There is a growing use of Virtual Private Network (VPN) 86 configurations. The growth comes from many areas; individual client 87 systems needing to appear to be on the home corporate network even 88 when traveling, ISPs providing extranet connectivity for customer 89 companies, etc. In some of these cases there is a need for the DHCP 90 server to know the VPN (hereafter called a "Virtual Subnet Selector" 91 or "VSS") from which an address, and other resources, should be 92 allocated. 94 This memo defines a Virtual Subnet Selection (VSS) option for DHCPv4 95 and DHCPv6, and a DHCPv4 relay-agent-information sub-option. These 96 are intended for use by DHCP clients, relay agents, and proxy clients 97 in situations where VSS information needs to be passed to the DHCP 98 server for proper address or prefix allocation to take place. If the 99 receiving DHCP server understands the VSS option or sub-option, this 100 information may be used in conjunction with other information in 101 determining the subnet on which to select an address as well as other 102 information such as DNS server, default router, etc. 104 If the allocation is being done through a DHCPv4 relay, then the 105 relay sub-option defined here should be included. In some cases, 106 however an IP address is being sought by a DHCPv4 proxy on behalf of 107 a client (which may be assigned the address via a different 108 protocol). In this case, there is a need to include VSS information 109 relating to the client as a DHCPv4 option. 111 If the allocation is being done through a DHCPv6 relay, then the 112 DHCPv6 VSS option defined in this document should be included in the 113 Relay-forward and Relay-reply message going between the DHCPv6 relay 114 and server. In some cases, addresses or prefixes are being sought by 115 a DHCPv6 proxy on behalf of a client. In this case, there is a need 116 for the client itself to supply the VSS information using the DHCPv6 117 VSS option in the messages that it sends to the DHCPv6 server. 119 In the remaining text of this document, when a DHCPv6 address is 120 indicated the same information applies to DHCPv6 Prefix Delegation 121 [RFC3633] as well. 123 2. Terminology 125 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 126 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 127 document are to be interpreted as described in RFC 2119 [RFC2119]. 129 This document uses the following terms: 131 o "DHCP client" 133 A DHCP client is a host using DHCP to obtain configuration 134 parameters such as a network address. 136 o "DHCP relay agent" 138 A DHCP relay agent is a third-party agent that transfers BOOTP 139 and DHCP messages between clients and servers residing on 140 different subnets, per [RFC951] and [RFC1542]. 142 o "DHCP server" 144 A DHCP server is a host that returns configuration parameters to 145 DHCP clients. 147 o "DHCPv4 option" 149 An option used to implement a capability defined by the DHCPv4 150 RFCs [RFC2131][RFC2132]. These options have one-octet code and 151 size fields. 153 o "DHCPv4 sub-option" 155 As used in this document, a DHCPv4 sub-option refers to a sub- 156 option of the relay-agent-information option [RFC3046]. These 157 sub-options have one-octet code and size fields. 159 o "DHCPv6 option" 161 An option used to implement a capability defined by the DHCPv6 162 RFC [RFC3315]. These options have two-octet code and size 163 fields. 165 o "downstream" 167 Downstream is the direction from the access concentrator towards 168 the subscriber. 170 o "upstream" 172 Upstream is the direction from the subscriber towards the access 173 concentrator. 175 o "VSS information" 177 Information about a VPN necessary to allocate an address to a 178 DHCP client on that VPN and necessary to forward a DHCP reply 179 packet to a DHCP client on that VPN. 181 o "VPN" 183 Virtual private network. A network which appears to the client 184 to be a private network. 186 o "VPN Identifier" 187 The VPN-ID is defined by [RFC2685] to be a sequence of 7 octets. 189 3. Virtual Subnet Selection Option and Sub-Option Definitions 191 The Virtual Subnet Selection options and sub-option contain a 192 generalized way to specify the VSS information about a VPN. There 193 are two options and one sub-option defined in this section. The 194 actual VSS information is identical in each. 196 3.1. DHCPv4 Virtual Subnet Selection Option 198 The format of the option is: 200 0 1 2 3 201 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 1 202 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 203 | Code | Length | Type | VSS Info ... 204 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 206 Code The option code (221). 208 Length The option length, minimum 1 octets. 210 Type and VSS Information -- see Section 3.4 212 3.2. DHCPv4 Virtual Subnet Selection Sub-Option 214 This is a sub-option of the relay-agent-information option [RFC3046]. 215 The format of the sub-option is: 217 0 1 2 3 218 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 1 219 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 220 | Code | Length | Type | VSS Info. ... 221 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 223 Code The sub-option code (151). 225 Length The option length, minimum 1 octets. 227 Type and VSS Information -- see Section 3.4 229 3.3. DHCPv6 Virtual Subnet Selection Option 231 The format of the DHCPv6 Virtual Subnet Selection option is shown 232 below. This option may be included by a client or relay-agent (or 233 both). 235 0 1 2 3 236 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 1 237 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 238 | OPTION_VSS | option-len | 239 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 240 | Type | VSS Information ... | 241 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 243 option-code OPTION_VSS (TBD). 245 option-len The number of octets in the option, minimum 1. 247 Type and VSS Information -- see Section 3.4 249 3.4. Virtual Subnet Selection Type and Information 251 All of the (sub)options defined above carry identical payloads, 252 consisting of a type and additional VSS information as follows: 254 Type VSS Information format: 256 0 NVT ASCII VPN identifier 257 1 RFC2685 VPN-ID 258 2-254 Not Allowed 259 255 Global, default VPN. 261 o Type 0 -- NVT ASCII VPN identifier 263 Indicates that the VSS information consists of a NVT ASCII 264 string. It MUST NOT be terminated with a zero byte. 266 o Type 1 -- RFC2685 VPN-ID 268 Indicates that the VSS information consists of an RFC2685 VPN-ID 269 [RFC2685], which is defined to be 7 octets in length. 271 o Type 255 -- Global, default VPN 273 Indicates that there is no explicit, non-default VSS information 274 but rather that this option references the normal, global, 275 default address space. In this case, there MUST NOT be any VSS 276 Information and the length of the VSS option MUST be 1. 278 All other values of the Type field are invalid as of this memo and 279 a VSS option with a Type field containing any value other than 280 zero (0), one (1), or 255 SHOULD be ignored. 282 4. Overview of Virtual Subnet Selection Usage 284 At the highest level, the VSS option or sub-option determines the VPN 285 on which a DHCP client is supposed to receive an IP address. How the 286 option or sub-option is entered and processed is discussed below, but 287 the point of all of the discussion is to determine the VPN on which 288 the DHCP client resides. This will affect a relay agent, in that it 289 will have to ensure that the packets sent to and received from the 290 DHCP client flow over the correct VPN. This will affect the DHCP 291 server in that it determines the IP address space used for the IP 292 address allocation. 294 A DHCP server has as part of its configuration some IP address space 295 from which it allocates IP addresses to DHCP clients. These 296 allocations are typically for a limited time, and thus the DHCP 297 client gets a lease on the IP address. In the absence of any VPN 298 information, the IP address space is in the global or default VPN 299 used throughout the Internet. When a DHCP server deals with VPN 300 information, each VPN defines a new address space inside the server, 301 one distinct from the global or default IP address space. A server 302 which supports the VSS option or sub-option thereby supports 303 allocation of IP addresses from multiple different VPNs. Supporting 304 IP address allocation from multiple different VPNs means that the 305 DHCP server must be prepared to configure multiple different address 306 spaces (one per distinct VPN) and allocate IP addresses from these 307 different address spaces. 309 These address spaces are typically independent, so that the same IP 310 address could be allocated to one client in the global, default VPN, 311 and to a different client residing in a different VPN. There is no 312 conflict in this allocation, since the clients have essentially 313 different IP addresses. The IPv4 or IPv6 address is qualified by the 314 VPN. 316 Thus a VSS option or sub-option is a way of signaling the use of a 317 VPN other than the global or default VPN. The next question is: who 318 decides what VPN a DHCP client should be using? 320 There are three entities which can either insert a VSS option or 321 sub-option into a DHCPv4 packet or DHCPv6 message; a DHCP client, a 322 relay agent, or a DHCPv4 or DHCPv6 server. While all of these 323 entities could include a different VSS option or sub-option in every 324 request or response, this situation is neither typical nor useful. 325 There are two known paradigms for use of the VSS option or sub- 326 option, which are discussed below. 328 The typical use of the VSS option or sub-option is for the relay 329 agent to know the VPN on which the DHCP client is operating. The 330 DHCP client itself does not, in this scenario, know the VPN on which 331 it resides. The relay agent is responsible for mediating the access 332 between the VPN on which the DHCP client resides and the DHCP server. 333 In this situation, the relay agent will insert a VSS sub-option into 334 the relay-agent-information option (for DHCPv4) or a VSS option into 335 the Relay-forward message (for DHCPv6) of every request it forwards 336 from the DHCP client. The server will use the VSS option or sub- 337 option to determine the VPN on which the client resides, and use that 338 VPN information to select the address space within its configuration 339 from which to allocate an IP address to the DHCP client. 341 In this scenario, the relay agent might also send a VSS option or 342 sub-option in either a DHCPv4 or DHCPv6 Leasequery request, but in 343 this case, it would use the VSS option in the Leasequery request to 344 select the correct address space for the Leasequery. In this 345 scenario, the relay agent would be acting as a DHCP client from a 346 Leasequery standpoint, but it would not be as if a DHCP client were 347 sending in a VSS option in a standard DHCP address allocation 348 request, say a DHCPDISCOVER. 350 In this scenario, only one relay agent would mediate the VPN access 351 for the DHCP client to the DHCP server, and it would be the relay 352 agent which inserts the VSS information into the request packet and 353 would remove it prior to forwarding the response packet on. 355 The DHCP server would know that it should respond to VPN information 356 specified in a VSS option or sub-option, and it would be configured 357 with appropriate VPN address spaces to service the projected client 358 requirements. Thus, in this common scenario, the DHCP client knows 359 nothing of any VPN access, the relay agent has been configured in 360 some way that allows it to determine the VPN of the DHCP client and 361 transmit that using a VSS option or sub-option to the DHCP server, 362 and the DHCP server responds to the VPN specified by the relay agent. 363 There is no conflict between different entities trying to specify 364 different VSS information -- each entity knows its role through 365 policy or configuration external to this document. 367 It is important to ensure that each entity in this scenario both 368 supports the VSS option and sub-option (for DHCPv4) or the VSS option 369 (for DHCPv6), and that it is configured correctly. Deploying relay 370 agents which support and emit VSS sub-options in concert with DHCPv4 371 servers which do not support the VSS option or sub-option as defined 372 in this document SHOULD NOT be done, as such an ensemble will not 373 operate correctly together because all of the IP addresses will be 374 allocated from the global or default VPN regardless of the VPN on 375 which the client's reside. 377 In the second scenario, the DHCP server would be configured in some 378 way to know the VPN on which a particular DHCP client should be given 379 access. The DHCP server would in this case include the VSS sub- 380 option in the relay-agent-information option for DHCPv4 or the VSS 381 option in the Relay-reply message for DHCPv6. The relay agent 382 responsible for mediating VPN access would use this information to 383 select the correct VPN for the DHCP client. In the event that there 384 were more than one relay agent involved in this transaction, some 385 external configuration or policy would be needed to inform the DHCPv6 386 server into which Relay-reply message the VSS option should go. 388 Once the relay agent has placed the DHCP client into the proper VPN, 389 it SHOULD begin including VSS information in requests that it 390 forwards to the DHCP server. Since this information does not 391 conflict with the DHCP server's idea of the proper VPN for the 392 client, everything works correctly. 394 In this second scenario, the DHCP client is again unaware of any VPN 395 activity. In this case, however, the DHCP server knows the VPN for 396 the client, and the relay agent responds to the VSS information 397 specified by the DHCP server. Similar to the first scenario, each 398 entity knows its role through a means external to this document and 399 no two entities try to specify VSS information in conflict. 401 Again, in this scenario, it is important that both the relay agent as 402 well as the DHCP server both support the VSS option and sub-option 403 (for DHCPv4) and the VSS option (for DHCPv6). Deploying and 404 configuring VPN support in one element and not in the other is not a 405 practical approach. 407 There are many other scenarios which can be created with multiple 408 relay agents each inserting VSS information into different Relay- 409 forward messages, relay agent VSS information conflicting with client 410 VSS information, or DHCP server VSS information conflicting with 411 relay agent and client VSS information. Since these scenarios do not 412 describe situations that are useful today, specifying precisely how 413 to resolve all of these conflicts is unlikely to be valuable in the 414 event that these scenarios actually become practical in the future. 416 The current use of the VSS option and sub-option require that each 417 entity knows the part that it plays in dealing with VPN data. Each 418 entity -- client, relay agent or agents, and server -- SHOULD know 419 through some policy or configuration beyond the scope of this 420 document whether it is responsible for specifying VPN information 421 using the VSS option or sub-option or responsible for responding to 422 VSS information specified by another entity, or simply ignoring any 423 VSS information which it might see. 425 Some simple conflict resolution approaches are discussed below, in 426 the hopes that they will cover simple cases that may arise from 427 scenarios beyond those envisioned today. However, for more complex 428 scenarios, or simple scenarios where appropriate conflict resolution 429 strategies differ from those discussed in this document, a document 430 detailing the usage scenarios and appropriate conflict resolution 431 strategies SHOULD be created and submitted for discussion and 432 approval. 434 5. Relay Agent Behavior 436 A relay agent which receives a DHCP request from a DHCP client on a 437 VPN SHOULD include Virtual Subnet Selection information in the DHCP 438 packet prior to forwarding the packet on to the DHCP server unless 439 inhibited from doing so by configuration information or policy to the 440 contrary. 442 A DHCPv4 relay agent SHOULD include a DHCPv4 VSS sub-option in a 443 relay-agent-information option [RFC3046], while a DHCPv6 relay agent 444 SHOULD include a DHCPv6 VSS option in the Relay-forward message. 446 The value placed in the Virtual Subnet Selection sub-option or option 447 SHOULD be sufficient for the relay agent to properly route any DHCP 448 reply packet returned from the DHCP server to the DHCP client for 449 which it is destined. 451 Anytime a relay agent places a VSS option or sub-option in a DHCP 452 request, it MUST send it only to a DHCP server which supports the VSS 453 option or sub-option. 455 Since this option or sub-option is placed in the packet in order to 456 specify the VPN on which an IP address is allocated for a particular 457 DHCP client, one presumes that an allocation on that VPN is necessary 458 for correct operation. If this presumption is correct, then a relay 459 agent which places this option in a packet and doesn't receive it (or 460 receives a different value than that sent to the server) in the 461 returning packet should drop the packet since the IP address that was 462 allocated will not be in the correct VPN. If an IP address that is 463 on the requested VPN is not required, then the relay agent is free to 464 accept the IP address that is not on the VPN that was requested. 466 The converse, however, is more complicated. In the DHCPv6 case, the 467 appearance of the option in the Relay-reply packet does indeed 468 indicate that the DHCPv6 server understood and acted upon the 469 contents of the VSS option in the Relay-forward packet. In the 470 DHCPv4 case, however, the appearance of the sub-option in the relay- 471 agent-information option received by the relay agent does not 472 necessarily indicate that the DHCPv4 server even understood, let 473 alone acted correctly upon, the VSS sub-option that it received. 475 The reason is that [RFC3046] specifies that a DHCPv4 server which 476 supports the relay-agent-information option SHALL copy all sub- 477 options received in a relay-agent-information option into any 478 outgoing relay-agent-information option. Because of these 479 requirements, even a DHCPv4 server which doesn't implement support 480 for the Virtual Subnet Selection sub-option will almost certainly 481 copy it into the outgoing relay-agent-information option. This means 482 that the appearance of the Virtual Subnet Selection sub-option in a 483 relay-agent-information option doesn't indicate support for the 484 Virtual Subnet Selection sub-option. 486 There are only two pieces of information which can be determined from 487 the appearance or lack of appearance of the DHCPv4 Virtual Subnet 488 Selection sub-option in a relay-agent-information option received by 489 a relay agent from a DHCPv4 server. First, if the Virtual Subnet 490 Selection sub-option does not appear, then the server was able to 491 support this sub-option but chose not to do so. Second, if the 492 Virtual Subnet Selection sub-option appears and has a different value 493 than the one originally included in the relay-agent-information 494 option, then the DHCP server was able to support this sub-option and 495 allocated an address using different VSS information than was 496 originally provided by the relay agent. 498 Thus, if a DHCPv4 relay agent has a requirement to determine if the 499 address allocated by a DHCPv4 server is on a particular VPN, it must 500 use some other approach than the appearance of the VSS sub-option in 501 the reply packet to make this determination. 503 This document does not create a requirement that a relay agent 504 remember the contents of a VSS DHCPv4 sub-option or VSS DHCPv6 option 505 sent to a DHCP server. In many cases, the relay agent may simply use 506 the value of the VSS returned by the DHCP server to forward the 507 response to the DHCP client. If the VSS information, the IP address 508 allocated, and the VPN capabilities of the relay agent all 509 interoperate correctly, then the DHCP client will receive a working 510 IP address. Alternatively, if any of these items don't interoperate 511 with the others, the DHCP client will not receive a working address. 513 Note that in some environments a relay agent may choose to always 514 place a VSS option or sub-option into packets and messages that it 515 forwards in order to forestall any attempt by a downstream relay 516 agent or client to specify VSS information. In this case, a type 517 field of 255 is used to denote the global, default VPN. When the 518 type field of 255 is used, there MUST NOT be any additional VSS 519 Information in the VSS option. 521 5.1. VPN assignment by the DHCP server 523 In some cases, a DHCP server may use the Virtual Subnet Selection 524 sub-option or option to inform a relay agent that a particular DHCP 525 client is associated with a particular VPN. It does this by sending 526 the Virtual Subnet Selection sub-option or option with the 527 appropriate information to the relay agent in the relay-agent- 528 information option for DHCPv4 or the Relay-reply message in DHCPv6. 529 If the relay agent is unable to honor the DHCP server's requirement 530 to place the DHCP client into that VPN it MUST drop the packet and 531 not send it to the DHCP client. 533 The DHCP server MUST NOT place VSS information in an outgoing packet 534 if the relay agent or DHCP client is unprepared to properly interpret 535 the VSS information. 537 In this situation, once the relay agent has placed the DHCP client 538 into the VPN specified by the DHCP server, it will send in a VSS 539 option or sub-option when forwarding packets from the client. The 540 DHCP server in normal operation will echo this VSS information into 541 the outgoing replies. 543 5.2. DHCP Leasequery 545 Sometimes a relay-agent needs to submit a DHCP Leasequery [RFC4388] 546 [RFC5007] packet to the DHCP server in order to recover information 547 about existing DHCP allocated IP addresses on other than the normal, 548 global VPN. In the context of a DHCP Leasequery the relay agent is a 549 direct client of the DHCP server and is not relaying a packet for 550 another DHCP client. Thus, the instructions in Section 6 on Client 551 Behavior should be followed to include the necessary VSS information. 553 6. Client Behavior 555 A DHCPv4 or DHCPv6 client will employ the VSS option to communicate 556 VSS information to their respective servers. This information MUST 557 be included in every message concerning any IP address on a different 558 VPN than the global or default VPN. A DHCPv4 client will place the 559 DHCPv4 VSS option in its packets, and a DHCPv6 client will place the 560 DHCPv6 VSS option in its messages. 562 A DHCPv6 client that needs to place a VSS option into a DHCPv6 563 message SHOULD place a single VSS option into the DHCPv6 message at 564 the same level as the Client Identifier option. A DHCPv6 client MUST 565 NOT include different VSS options in the same DHCPv6 message. 567 Note that, as mentioned in Section 1, throughout this document when a 568 DHCPv6 address is indicated the same information applies to DHCPv6 569 Prefix Delegation [RFC3633] as well. 571 Since this option is placed in the packet in order to change the VPN 572 on which an IP address is allocated for a particular DHCP client, one 573 presumes that an allocation on that VPN is necessary for correct 574 operation. If this presumption is correct, then a client which 575 places this option in a packet and doesn't receive it or receives a 576 different value in the returning packet should drop the packet since 577 the IP address that was allocated will not be in the correct VPN. If 578 an IP address that is on the requested VPN is not required, then the 579 client is free to accept the IP address that is not on the VPN that 580 the was requested. 582 Clients should be aware that some DHCP servers will return a VSS 583 option with different values than that which was sent in. In 584 addition, a client may receive a response from a DHCP server with a 585 VSS option when none was sent in by the Client. 587 Note that when sending a DHCP Leasequery request, a relay agent is 588 acting as a DHCP client and so it should include the respective 589 DHCPv4 or DHCPv6 VSS option in its DHCPv4 or DHCPv6 Leasequery packet 590 if the DHCP Leasequery request is generated for other than the 591 default, global VPN. It should not include a DHCPv4 sub-option in 592 this case. 594 7. Server Behavior 596 A DHCP server receiving the VSS option or sub-option SHOULD allocate 597 an IP address (or use the VSS information to access an already 598 allocated IP address) from the VPN specified by the included VSS 599 information. 601 In the case where the type field of the VSS option or sub-option is 602 255, the VSS option denotes the global, default VPN. In this case, 603 there is no explicit VSS information beyond the type field. 605 This document does not prescribe any particular address allocation 606 policy. A DHCP server may choose to attempt to allocate an address 607 using the VSS information and, if this is impossible, to not allocate 608 an address. Alternatively, a DHCP server may choose to attempt 609 address allocation based on the VSS information and, if that is not 610 possible, it may fall back to allocating an address on the global or 611 default VPN. This, of course, is also the apparent behavior of any 612 DHCP server which doesn't implement support for the VSS option and 613 sub-option. Thus, DHCP clients and relay agents SHOULD be prepared 614 for either of these alternatives. 616 In some cases, a DHCP server may use the Virtual Subnet Selection 617 sub-option or option to inform a relay agent that a particular DHCP 618 client is associated with a particular VPN. It does this by sending 619 the Virtual Subnet Selection sub-option or option with the 620 appropriate information to the relay agent in the relay-agent- 621 information option for DHCPv4 or the Relay-reply message in DHCPv6. 623 In this situation, the relay agent will place the client in the 624 proper VPN, and then it will send in a VSS option or sub-option in 625 subsequent forwarded requests. The DHCP server will see this VSS 626 information and since it doesn't conflict in any way with the 627 server's notion of the VPN on which the client is supposed to reside, 628 it will process the requests based on the VPN specified in the VSS 629 option or sub-option, and echo the same VSS information in the 630 outgoing replies. 632 In a similar manner, a DHCP server may use the Virtual Subnet 633 Selection option to inform a DHCP client that the address (or 634 addresses) it allocated for the client is on a particular VPN. 636 In either case above, care should be taken to ensure that a client or 637 relay agent receiving a reply containing a VSS option will correctly 638 understand the VSS option. Otherwise, the client or relay agent will 639 end up using the address as though it were a global address. 641 7.1. Returning the DHCPv4 or DHCPv6 Option 643 DHCPv4 or DHCPv6 servers receiving a VSS option (for sub-option 644 processing, see below) MUST return an instance of this option in the 645 reply packet or message if the server successfully uses this option 646 to allocate an IP address, and it MUST NOT include an instance of 647 this option if the server is unable to support, is not configured to 648 support, or does not implement support for VSS information in general 649 or the requested VPN in particular. 651 If they echo the option (based on the criteria above), servers SHOULD 652 return an exact copy of the option unless they desire to change the 653 VPN on which a client was configured. 655 The appearance of the DHCPv6 VSS option in the OPTION_ORO [RFC3315] 656 or the OPTION_ERO [RFC4994] should not change the processing or 657 decision to return (or not to return) the VSS option as specified in 658 this document. 660 7.2. Returning the DHCPv4 Sub-Option 662 The case of the DHCPv4 sub-option is a bit more complicated. Note 663 that [RFC3046] specifies that a DHCPv4 server which supports the 664 relay-agent-information option SHALL copy all sub-options received in 665 a relay-agent-information option into any outgoing relay-agent- 666 information option. Thus, the default behavior for any DHCPv4 server 667 is to return any VSS sub-option received to the relay agent whether 668 or not the DHCPv4 server understand the VSS sub-option. A server 669 which implements the VSS sub-option MUST include the VSS sub-option 670 in the relay-agent-information option in the reply packet if it 671 successfully acted upon the VSS information in the incoming VSS sub- 672 option. 674 Moreover, if a server uses different VSS information to allocate an 675 IP address than it receives in a particular DHCPv4 sub-option, it 676 MUST include that alternative VSS information in a sub-option that it 677 returns to the DHCPv4 relay agent. 679 If a DHCPv4 server supports this sub-option and for some reason 680 (perhaps administrative control) does not honor this sub-option from 681 the request then it MUST NOT echo this sub-option in the outgoing 682 relay-agent-information option. 684 Note that the appearance of the VSS sub-option in a reply packet from 685 a DHCPv4 server to a relay-agent does not communicate any useful 686 information about whether or not the server used the VSS sub-option 687 in its processing. However, the absence of a VSS sub-option in a 688 reply from a DHCPv4 server when a VSS sub-option was included in a 689 request to the DHCPv4 server is significant, and means that the 690 server did not use the VSS information present in the sub-option in 691 its processing. 693 7.3. Making sense of conflicting VSS information 695 It is possible for a DHCPv4 server to receive both a VSS option and a 696 VSS sub-option in the same packet. Likewise, a DHCPv6 server can 697 receive multiple VSS options in nested Relay-forward messages as well 698 as in the client message itself. In either of these cases, the VSS 699 information from the relay agent closest to the DHCP server SHOULD be 700 used in preference to all other VSS information received. In the 701 DHCPv4 case, this means that the VSS sub-option takes precedence over 702 the VSS option, and in the DHCPv6 case, this means that the VSS 703 option from the outer-most Relay-forward message in which a VSS 704 option appears takes precedence. 706 The reasoning behind this approach is that the relay-agent closer to 707 the DHCP server is almost certainly more trusted than the DHCP client 708 or more distant relay agents, and therefore information in the 709 relay-agent-information option or the Relay-forward message is more 710 likely to be correct. 712 In general, relay agents SHOULD be aware through configuration or 713 policy external to this document whether or not they should be 714 including VSS information in packets that they forward and so there 715 should not be conflicts among relay agent specified VSS information. 717 In these situations where multiple VSS option or sub-options appear 718 in the incoming packet or message, when constructing the response to 719 be sent to the DHCP client or relay agent, all existing VSS options 720 or sub-options MUST be replicated in the appropriate places in the 721 response and MUST contain the VSS information that was used by the 722 DHCP server to allocate the IP address. 724 8. Security 726 Message authentication in DHCPv4 for intradomain use where the out- 727 of-band exchange of a shared secret is feasible is defined in 728 [RFC3118]. Potential exposures to attack are discussed in section 7 729 of the DHCP protocol specification in [RFC2131]. 731 Implementations should consider using the DHCPv4 Authentication 732 option [RFC3118] to protect DHCPv4 client access in order to provide 733 a higher level of security if it is deemed necessary in their 734 environment. 736 Message authentication in DHCPv4 relay agents as defined in [RFC4030] 737 should be considered for DHCPv4 relay agents employing this sub- 738 option. Potential exposures to attack are discussed in section 7 of 739 the DHCP protocol specification in [RFC2131]. 741 For DHCPv6 use of the VSS option, the "Security Considerations" 742 section of [RFC3315] details the general threats to DHCPv6, and thus 743 to messages using the VSS option. The "Authentication of DHCP 744 Messages" section of [RFC3315] describes securing communication 745 between relay agents and servers, as well as clients and servers. 747 The VSS option could be used by a client in order to obtain an IP 748 address from a VPN other than the one where it should. This option 749 would allow a client to perform a more complete address-pool 750 exhaustion attack since the client would no longer be restricted to 751 attacking address-pools on just its local subnet. 753 A DHCP server that implements these options and sub-option should be 754 aware of this possibility and use whatever techniques that can be 755 devised to prevent such an attack. Information such as the giaddr in 756 DHCPv4 or link address in the Relay-forward DHCPv6 message might be 757 used to detect and prevent this sort of attack. 759 One possible defense would be for the DHCP relay to insert a VSS 760 option or sub-option to override the DHCP client's VSS option. 762 Servers that implement the VSS option and sub-option MUST by default 763 disable use of the feature; it must specifically be enabled through 764 configuration. Moreover, a server SHOULD provide the ability to 765 selectively enable use of the feature under restricted conditions, 766 e.g., by enabling use of the option only from explicitly configured 767 client-ids, enabling its use only by clients on a particular subnet, 768 or restricting the VSSs from which addresses may be requested. 770 9. IANA Considerations 772 IANA is requested to assign DHCPv4 option number 221 for the DHCPv4 773 VSS option defined in Section 3.1, in accordance with [RFC3942]. 775 IANA is requested to assign sub-option number 151 for the DHCPv4 776 sub-option defined in Section 3.2 from the DHCP Relay Agent Sub- 777 options space [RFC3046], in accordance with the spirit of [RFC3942]. 778 While [RFC3942] doesn't explicitly mention the sub-option space for 779 the DHCP Relay Agent Information option [RFC3046], sub-option 151 is 780 already in use by existing implementations of this sub-option and the 781 current draft is essentially compatible with these current 782 implementations. 784 IANA has assigned the value of TBD for the DHCPv6 VSS option defined 785 in Section 3.3. 787 While the type byte defined in Section 3.4 defines a number space 788 that could be managed by IANA, expansion of this number space is not 789 anticipated and so creation of a registry of these numbers is not 790 required by this document. In the event that additional values for 791 the type byte are defined in subsequent documents, IANA should at 792 that time create a registry for these type bytes. New values for the 793 type byte may only be defined by IETF Consensus, as described in 794 [RFC5226]. Basically, this means that they are defined by RFCs 795 approved by the IESG. 797 10. Acknowledgments 799 Bernie Volz recommended consolidation of the DHCPv4 option and sub- 800 option drafts after extensive review of the former drafts, and 801 provided valuable assistance in structuring and reviewing this 802 document. Alper Yegin expressed interest in the DHCPv6 VSS option, 803 resulting in this combined draft covering all three areas. Alfred 804 Hoenes provided assistance with editorial review as well as raising 805 substantive protocol issues. David Hankins and Bernie Volz each 806 raised important protocol issues which resulted in a clarified 807 document. Josh Littlefield provided editorial assistance. 809 11. References 811 11.1. Normative References 813 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 814 Requirement Levels", RFC 2119, March 1997. 816 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, 817 March 1997. 819 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor 820 Extensions", RFC 2132, March 1997. 822 [RFC2685] Fox, B., Gleeson, B., "Virtual Private Networks 823 Identifier", RFC 2685, September 1999. 825 [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 826 3046, January 2001. 828 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and 829 M. Carney, "Dynamic Host Configuration Protocol for IPv6 830 (DHCPv6)", RFC 3315, July 2003. 832 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 833 Host Configuration Protocol (DHCP) version 6", RFC 3633, December 834 2003. 836 [RFC4994] Zeng, S., Volz, B., Kinnear, K. and J. Brzozowski, "DHCPv6 837 Relay Agent Echo Request Option", RFC 4994, September 2007. 839 11.2. Informative References 841 [RFC951] Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951, 842 September 1985. 844 [RFC1542] Wimer, W., "Clarifications and Extensions for the Bootstrap 845 Protocol", RFC 1542, October 1993. 847 [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP 848 Messages", RFC 3118, June 2001. 850 [RFC3942] Volz, B., "Reclassifying Dynamic Host Configuration 851 Protocol version 4 (DHCPv4) Options", RFC 3942, November 2004. 853 [RFC4030] Stapp, M. and T. Lemon, "The Authentication Suboption for 854 the Dynamic Host Configuration Protocol (DHCP) Relay Agent 855 Option", RFC 4030, March 2005. 857 [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration 858 Protocol (DHCP) Leasequery", RFC 4388, February 2006. 860 [RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng, "DHCPv6 861 Leasequery", RFC 5007, September 2007. 863 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 864 IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 866 12. Authors' Addresses 868 Kim Kinnear 869 Cisco Systems 870 1414 Massachusetts Ave. 871 Boxborough, Massachusetts 01719 873 Phone: (978) 936-0000 875 EMail: kkinnear@cisco.com 877 Richard Johnson 878 Cisco Systems 879 170 W. Tasman Dr. 880 San Jose, CA 95134 882 Phone: (408) 526-4000 884 EMail: raj@cisco.com 886 Mark Stapp 887 Cisco Systems 888 1414 Massachusetts Ave. 889 Boxborough, Massachusetts 01719 891 Phone: (978) 936-0000 892 EMail: mjs@cisco.com 894 Jay Kumarasamy 895 Cisco Systems 896 170 W. Tasman Dr. 897 San Jose, CA 95134 899 Phone: (408) 526-4000 901 EMail: jayk@cisco.com