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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: October 29, 2011 Cisco Systems 6 Jay Kumarasamy 7 April 29, 2011 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 Copyright Notice 35 Copyright (c) 2011 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (http://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 This document may contain material from IETF Documents or IETF 49 Contributions published or made publicly available before November 50 10, 2008. The person(s) controlling the copyright in some of this 51 material may not have granted the IETF Trust the right to allow 52 modifications of such material outside the IETF Standards Process. 53 Without obtaining an adequate license from the person(s) controlling 54 the copyright in such materials, this document may not be modified 55 outside the IETF Standards Process, and derivative works of it may 56 not be created outside the IETF Standards Process, except to format 57 it for publication as an RFC or to translate it into languages other 58 than English. 60 Abstract 62 This memo defines a Virtual Subnet Selection (VSS) option for each of 63 DHCPv4 and DHCPv6, and a VSS sub-option carried in the DHCPv4 relay- 64 agent-information option. These are intended for use by DHCP 65 clients, relay agents, and proxy clients in situations where VSS 66 information needs to be passed to the DHCP server for proper address 67 or prefix allocation to take place. 69 For the DHCPv4 option and relay-agent-information sub-option, this 70 memo documents existing usage as per RFC 3942 [RFC3942]. 72 Table of Contents 74 1. Introduction................................................. 3 75 2. Terminology.................................................. 4 76 3. Virtual Subnet Selection Option and Sub-Options Definitions.. 5 77 3.1. DHCPv4 Virtual Subnet Selection Option..................... 5 78 3.2. DHCPv4 Virtual Subnet Selection Sub-Option................. 6 79 3.3. DHCPv4 Virtual Subnet Selection Control Sub-Option......... 6 80 3.4. DHCPv6 Virtual Subnet Selection Option..................... 7 81 3.5. Virtual Subnet Selection Type and Information.............. 7 82 4. Overview of Virtual Subnet Selection Usage................... 8 83 4.1. VPN assignment by the DHCP relay agent..................... 9 84 4.2. VPN assignment by the DHCP server.......................... 12 85 4.3. Required Support........................................... 14 86 4.4. Alternative VPN assignment approaches...................... 14 87 5. Relay Agent Behavior......................................... 14 88 5.1. VPN assignment by the DHCP server.......................... 16 89 5.2. DHCP Leasequery............................................ 17 90 6. Client Behavior.............................................. 17 91 7. Server Behavior.............................................. 18 92 7.1. Returning the DHCPv4 or DHCPv6 Option...................... 19 93 7.2. Returning the DHCPv4 Sub-Option............................ 20 94 7.3. Making sense of conflicting VSS information................ 21 95 8. Security..................................................... 21 96 9. IANA Considerations.......................................... 22 97 10. Acknowledgments............................................. 23 98 11. References.................................................. 23 99 11.1. Normative References...................................... 23 100 11.2. Informative References.................................... 24 102 1. Introduction 104 There is a growing use of Virtual Private Network (VPN) 105 configurations. The growth comes from many areas; individual client 106 systems needing to appear to be on the home corporate network even 107 when traveling, ISPs providing extranet connectivity for customer 108 companies, etc. In some of these cases there is a need for the DHCP 109 server to know the VPN (hereafter called a "Virtual Subnet Selector" 110 or "VSS") from which an address, and other resources, should be 111 allocated. 113 This memo defines a Virtual Subnet Selection (VSS) option for each of 114 DHCPv4 and DHCPv6, and a VSS sub-option carried in the DHCPv4 relay- 115 agent-information option. These are intended for use by DHCP 116 clients, relay agents, and proxy clients in situations where VSS 117 information needs to be passed to the DHCP server for proper address 118 or prefix allocation to take place. If the receiving DHCP server 119 understands the VSS option or sub-option, this information may be 120 used in conjunction with other information in determining the subnet 121 on which to select an address as well as other information such as 122 DNS server, default router, etc. 124 If the allocation is being done through a DHCPv4 relay, then the 125 relay sub-option defined here should be included. In some cases, 126 however, an IP address is being sought by a DHCPv4 proxy on behalf of 127 a client (which may be assigned the address via a different 128 protocol). In this case, there is a need to include VSS information 129 relating to the client as a DHCPv4 option. 131 If the allocation is being done through a DHCPv6 relay, then the 132 DHCPv6 VSS option defined in this document should be included in the 133 Relay-forward and Relay-reply message going between the DHCPv6 relay 134 and server. In some cases, addresses or prefixes are being sought by 135 a DHCPv6 proxy on behalf of a client. In this case, there is a need 136 for the client itself to supply the VSS information using the DHCPv6 137 VSS option in the messages that it sends to the DHCPv6 server. 139 In the remaining text of this document, when a DHCPv6 address is 140 indicated the same information applies to DHCPv6 Prefix Delegation 141 [RFC3633] as well. 143 In the remaining text of this document, when the term VSS sub-option 144 is used, it refers to the VSS sub-option carried in the DHCPv4 145 relay-agent-information option. 147 2. Terminology 149 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 150 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 151 document are to be interpreted as described in RFC 2119 [RFC2119]. 153 This document uses the following terms: 155 o "DHCP client" 157 A DHCP client is a host using DHCP to obtain configuration 158 parameters such as a network address. 160 o "DHCP proxy" 162 A DHCP proxy is a DHCP client which acquires IP addresses not 163 for its own use, but rather on behalf of another entity. There 164 are a variety of ways that a DHCP proxy can supply the addresses 165 it acquires to other entities that need them. 167 o "DHCP relay agent" 169 A DHCP relay agent is an agent that transfers BOOTP and DHCP 170 messages between clients and servers residing on different 171 subnets, per [RFC951], [RFC1542], and [RFC3315]. 173 o "DHCP server" 175 A DHCP server is a host that returns configuration parameters to 176 DHCP clients. 178 o "DHCPv4 option" 180 An option used to implement a capability defined by the DHCPv4 181 RFCs [RFC2131][RFC2132]. These options have one-octet code and 182 size fields. 184 o "DHCPv4 sub-option" 186 As used in this document, a DHCPv4 sub-option refers to a sub- 187 option of the relay-agent-information option [RFC3046]. These 188 sub-options have one-octet code and size fields. 190 o "DHCPv6 option" 192 An option used to implement a capability defined by the DHCPv6 193 RFC [RFC3315]. These options have two-octet code and size 194 fields. 196 o "Global VPN" 198 Indicates that the address being described belongs to the set of 199 addresses not part of any VPN. In other words, the normal 200 address space operated on by DHCP. This includes private 201 addresses, for example the 10.x.x.x addresses as well as the 202 other private subnets that are not routed on the open internet. 204 o "VSS information" 206 Information about a VPN necessary to allocate an address to a 207 DHCP client on that VPN and necessary to forward a DHCP reply 208 packet to a DHCP client on that VPN. 210 o "VPN" 212 Virtual private network. A network which appears to the client 213 to be a private network. 215 o "VPN Identifier" 217 The VPN-ID is defined by [RFC2685] to be a sequence of 7 octets. 219 3. Virtual Subnet Selection Option and Sub-Options Definitions 221 The Virtual Subnet Selection options and sub-options contain a 222 generalized way to specify the VSS information about a VPN. There 223 are two options and two sub-options defined in this section. The 224 actual VSS information is identical both options and one of the two 225 sub-options. 227 3.1. DHCPv4 Virtual Subnet Selection Option 229 The format of the option is: 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 | Code | Length | Type | VSS Info ... 235 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 237 Code The option code (221). 239 Length The option length, minimum 1 octets. 241 Type and VSS Information -- see Section 3.5 243 3.2. DHCPv4 Virtual Subnet Selection Sub-Option 245 This is a sub-option of the relay-agent-information option [RFC3046]. 246 The format of the sub-option is: 248 0 1 2 3 249 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 1 250 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 251 | Code | Length | Type | VSS Info. ... 252 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 254 Code The sub-option code (151). 256 Length The sub-option length, minimum 1 octets. 258 Type and VSS Information -- see Section 35. 260 3.3. DHCPv4 Virtual Subnet Selection Control Sub-Option 262 This is a sub-option of the relay-agent-information option [RFC3046]. 263 The format of the sub-option is: 265 0 1 266 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 267 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 268 | Code | Length | 269 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 271 Code The sub-option code (TBD). 273 Length The sub-option length, 0. 275 This sub-option only only appears in the DHCPv4 relay-agent- 276 information option. In a DHCP request, it indicates that a DHCPv4 277 VSS sub-option is also present in the relay-agent-information option. 278 In a DHCP reply, if it appears in the relay-agent-information option, 279 it indicates that the DHCP server did not understand any DHCPv4 VSS 280 sub-option that also appears in the relay-agent-information option. 282 3.4. DHCPv6 Virtual Subnet Selection Option 284 The format of the DHCPv6 Virtual Subnet Selection option is shown 285 below. This option may be included by a client or relay-agent (or 286 both). 288 0 1 2 3 289 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 1 290 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 291 | OPTION_VSS | option-len | 292 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 293 | Type | VSS Information ... | 294 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 296 option-code OPTION_VSS (TBD). 298 option-len The number of octets in the option, minimum 1. 300 Type and VSS Information -- see Section 3.5 302 3.5. Virtual Subnet Selection Type and Information 304 All of the (sub)options defined above carry identical payloads, 305 consisting of a type and additional VSS information as follows: 307 Type VSS Information format: 309 0 NVT ASCII VPN identifier 310 1 RFC2685 VPN-ID 311 2-254 Reserved 312 255 Global, default VPN. 314 o Type 0 -- NVT ASCII VPN identifier 316 Indicates that the VSS information consists of a NVT ASCII 317 string. It MUST NOT be terminated with a zero byte. 319 o Type 1 -- RFC2685 VPN-ID 321 Indicates that the VSS information consists of an RFC2685 VPN-ID 322 [RFC2685], which is defined to be 7 octets in length. 324 o Type 255 -- Global, default VPN 326 Indicates that there is no explicit, non-default VSS information 327 but rather that this option references the normal, global, 328 default address space. In this case, there MUST NOT be any VSS 329 Information included in the VSS option or sub-option and the 330 length of the MUST be 1. 332 All other values of the Type field are reserved. 334 4. Overview of Virtual Subnet Selection Usage 336 At the highest level, the VSS option or sub-option determines the VPN 337 on which a DHCP client is supposed to receive an IP address. How the 338 option or sub-option is entered and processed is discussed below, but 339 the point of all of the discussion is to determine the VPN on which 340 the DHCP client resides. This will affect a relay agent, in that it 341 will have to ensure that DHCP packets sent to and received from the 342 DHCP client flow over the correct VPN. This will affect the DHCP 343 server in that it determines the IP address space used for the IP 344 address allocation. 346 A DHCP server has as part of its configuration some IP address space 347 from which it allocates IP addresses to DHCP clients. These 348 allocations are typically for a limited time, and thus the DHCP 349 client gets a lease on the IP address. In the absence of any VPN 350 information, the IP address space is in the global or default VPN 351 used throughout the Internet. When a DHCP server deals with VPN 352 information, each VPN defines a new address space inside the server, 353 one distinct from the global or default IP address space. A server 354 which supports the VSS option or sub-option thereby supports 355 allocation of IP addresses from multiple different VPNs. Supporting 356 IP address allocation from multiple different VPNs means that the 357 DHCP server must be prepared to configure multiple different address 358 spaces (one per distinct VPN) and allocate IP addresses from these 359 different address spaces. 361 These address spaces are typically independent, so that the same IP 362 address (consisting of the same string of bytes) could be allocated 363 to one client in the global, default VPN, and to a different client 364 residing in a different VPN. There is no conflict in this 365 allocation, since the clients have essentially different addresses, 366 even though these addresses consist of the same string of bytes, 367 because the IPv4 or IPv6 address is qualified by the VPN. 369 Thus a VSS option or sub-option is a way of signaling the use of a 370 VPN other than the global or default VPN. The next question is: who 371 decides what VPN a DHCP client should be using? 373 There are three entities which can either insert a VSS option or 374 sub-option into a DHCPv4 packet or DHCPv6 message; a DHCP client, a 375 relay agent, or a DHCPv4 or DHCPv6 server. While all of these 376 entities could include a different VSS option or sub-option in every 377 request or response, this situation is neither typical nor useful. 378 There are two known paradigms for use of the VSS option or sub- 379 option, which are discussed below. 381 4.1. VPN assignment by the DHCP relay agent 383 The typical use of the VSS option or sub-option is for the relay 384 agent to know the VPN on which the DHCP client is operating. The 385 DHCP client itself does not, in this approach, know the VPN on which 386 it resides. The relay agent is responsible for mediating the access 387 between the VPN on which the DHCP client resides and the DHCP server. 388 In this situation, the relay agent will insert two DHCPv4 relay- 389 agent-information sub-options (one VSS sub-option, and one VSS- 390 Control sub-option) into the relay-agent-information option or a 391 DHCPv6 VSS option into the Relay-forward message of every request it 392 forwards from the DHCP client. The server will use the DHCPv6 VSS 393 option or DHCPv4 VSS sub-option to determine the VPN on which the 394 client resides, and use that VPN information to select the address 395 space within its configuration from which to allocate an IP address 396 to the DHCP client. 398 When, using this approach, a DHCPv4 relay agent inserts a VSS sub- 399 option into the relay-agent-information option it MUST also insert a 400 VSS-Control sub-option into the relay-agent-information-option. This 401 is to allow determination of whether or not the DHCPv4 server 402 actually processes the VSS information provided by the DHCPv4 relay 403 agent. If the DHCPv4 server supports the VSS capabilities described 404 in this document, it will remove the VSS-Control sub-option from the 405 relay-agent-information option that it returns to the DHCPv4 relay 406 agent. See Section 5 for more information. 408 In this approach, the relay agent might also send a VSS option or 409 sub-option in either a DHCPv4 or DHCPv6 Leasequery request, but in 410 this case, it would use the VSS option in the Leasequery request to 411 select the correct address space for the Leasequery. In this 412 approach, the relay agent would be acting as a DHCP client from a 413 Leasequery standpoint, but it would not be as if a DHCP client were 414 sending in a VSS option in a standard DHCP address allocation 415 request, say a DHCPDISCOVER. 417 In this approach, only one relay agent would mediate the VPN access 418 for the DHCP client to the DHCP server, and it would be the relay 419 agent which inserts the VSS information into the request packet and 420 would remove it prior to forwarding the response packet on. 422 In the diagram below is an example of a DHCPv4 client, DHCPv4 relay 423 agent, and DHCPv4 server. The DHCPv6 situation is similar, but uses 424 the DHCPv6 VSS option. 426 DHCPv4 427 DHCPv4 Relay DHCPv4 428 Client Agent Server 430 | | | 431 | >--DHCPDISCOVER--> | | 432 | on VRF "abc" | | 433 | | >--DHCPDISCOVER----> | 434 | | relay-agent-info: | 435 | | VSS type VRF:"abc"| 436 | | VSS-Control | 437 | | | 438 | | <----DHCPOFFER-----< | 439 | | relay-agent-info: | 440 | | VSS type VRF:"abc"| 441 | | | 442 | <---DHCPOFFER----< | | 443 | on VRF "abc" | | 444 | | | 445 | >--DHCPREQUEST---> | | 446 | on VRF "abc" | | 447 | | >--DHCPREQUEST-----> | 448 | | relay-agent-info: | 449 | | VSS type VRF:"abc"| 450 | | VSS-Control | 451 | | | 452 | | <----DHCPACK-------< | 453 | | relay-agent-info: | 454 | | VSS type VRF:"abc"| 455 | | | 456 | <---DHCPACK------< | | 457 | on VRF "abc" | | 458 | | | 459 ... ... ... 461 Figure 4.1-1: DHCPv4 - Relay Agent knows VPN 463 The DHCP server would know that it should respond to VPN information 464 specified in a VSS option or sub-option, and it would be configured 465 with appropriate VPN address spaces to service the projected client 466 requirements. Thus, in this common approach, the DHCP client knows 467 nothing of any VPN access, the relay agent has been configured in 468 some way that allows it to determine the VPN of the DHCP client and 469 transmit that using a VSS option or sub-option to the DHCP server, 470 and the DHCP server responds to the VPN specified by the relay agent. 471 There is no conflict between different entities trying to specify 472 different VSS information -- each entity knows its role through 473 policy or configuration external to this document. 475 If any mis-configuration exists, it SHOULD result in a DHCP client 476 being unable to acquire an IP address. For instance, a relay agent 477 which supports VPN access SHOULD couple transmission of VSS options 478 or sub-options to the configuration of VPN support, and not allow one 479 without the other. 481 It is important to ensure that the relay agent and DHCP server both 482 support the VSS option and sub-option (for DHCPv4) or the VSS option 483 (for DHCPv6). Deploying DHCPv4 relay agents which support and emit 484 VSS sub-options in concert with DHCPv4 servers which do not support 485 the VSS option or sub-option as defined in this document SHOULD NOT 486 be done, as such an ensemble will not operate correctly. Should this 487 situation occur, however, the relay agent can detect the problem 488 (since the VSS-Control sub-option will appear in the packets it 489 receives from the DHCPv4 server, indicating the server did not 490 effectively process the VSS sub-option), and it can issue appropriate 491 diagnostic messages. 493 4.2. VPN assignment by the DHCP server 495 In this approach, the DHCP server would be configured in some way to 496 know the VPN on which a particular DHCP client should be given 497 access. The DHCP server would in this case include the VSS sub- 498 option in the relay-agent-information option for DHCPv4 or the VSS 499 option in the Relay-reply message for DHCPv6. The relay agent 500 responsible for mediating VPN access would use this information to 501 select the correct VPN for the DHCP client. In the unusal event that 502 there were more than one relay agent involved in this transaction, 503 some external configuration or policy would be needed to inform the 504 DHCPv6 server into which Relay-reply message the VSS option should 505 go. 507 Once the relay agent has placed the DHCP client into the proper VPN, 508 it SHOULD begin including VSS information in requests that it 509 forwards to the DHCP server. Since this information does not 510 conflict with the DHCP server's idea of the proper VPN for the 511 client, everything works correctly. 513 The diagram below shows this approach using DHCPv4. The DHCPv6 514 situation is similar, but uses the DHCPv6 VSS option instead. 516 DHCPv4 517 DHCPv4 Relay DHCPv4 518 Client Agent Server 520 | | | 521 | >--DHCPDISCOVER--> | | 522 | on unknown VPN | | 523 | | >--DHCPDISCOVER----> | 524 | | | 525 | | <----DHCPOFFER-----< | 526 | | relay-agent-info: | 527 | | VSS type VRF:"abc"| 528 | | | 529 | <---DHCPOFFER----< | | 530 | on VRF "abc" | | 531 | | | 532 | >--DHCPREQUEST---> | | 533 | on VRF "abc" | | 534 | | >--DHCPREQUEST-----> | 535 | | relay-agent-info: | 536 | | VSS type VRF:"abc"| 537 | | VSS-Control | 538 | | | 539 | | <----DHCPACK-------< | 540 | | relay-agent-info: | 541 | | VSS type VRF:"abc"| 542 | | | 543 | <---DHCPACK------< | | 544 | on VRF "abc" | | 545 | | | 546 | | | 547 ... ... ... 549 Figure 4.2-1: DHCPv4 - DHCPv4 Server knows VPN 551 In this approach, the DHCP client is again unaware of any VPN 552 activity. In this case, however, the DHCP server knows the VPN for 553 the client, and the relay agent responds to the VSS information 554 specified by the DHCP server. Similar to the previous approach, each 555 entity knows its role through a means external to this document and 556 no two entities try to specify VSS information in conflict. 558 It is important that both the relay agent as well as the DHCP server 559 both support the VSS option and sub-option (for DHCPv4) and the VSS 560 option (for DHCPv6). Deploying and configuring VPN support in one 561 element and not in the other is not a practical approach. 563 4.3. Required Support 565 DHCP relay agents and servers MUST support the approach discussed in 566 Section 4.1. DHCP relay agents and server SHOULD support the 567 approach discussed in Section 4.2. DHCP relay agents and servers 568 SHOULD NOT be configured to operate with both approaches 569 simultaneously. 571 4.4. Alternative VPN assignment approaches 573 There are many other approaches which can be created with multiple 574 relay agents each inserting VSS information into different Relay- 575 forward messages, relay agent VSS information conflicting with client 576 VSS information, or DHCP server VSS information conflicting with 577 relay agent and client VSS information. Since these approaches do 578 not describe situations that are useful today, specifying precisely 579 how to resolve all of these conflicts is unlikely to be valuable in 580 the event that these approaches actually become practical in the 581 future. 583 The current use of the VSS option and sub-option require that each 584 entity knows the part that it plays in dealing with VPN data. Each 585 entity -- client, relay agent or agents, and server -- SHOULD know 586 through some policy or configuration beyond the scope of this 587 document whether it is responsible for specifying VPN information 588 using the VSS option or sub-option or responsible for responding to 589 VSS information specified by another entity, or simply ignoring any 590 VSS information which it might see. 592 Some simple conflict resolution approaches are discussed below, in 593 the hopes that they will cover simple cases that may arise from 594 situations beyond those envisioned today. However, for more complex 595 situations, or simple situations where appropriate conflict 596 resolution strategies differ from those discussed in this document, a 597 document detailing the usage situations and appropriate conflict 598 resolution strategies SHOULD be created and submitted for discussion 599 and approval. 601 5. Relay Agent Behavior 603 A relay agent which receives a DHCP request from a DHCP client on a 604 VPN SHOULD include Virtual Subnet Selection information in the DHCP 605 packet prior to forwarding the packet on to the DHCP server unless 606 inhibited from doing so by configuration information or policy to the 607 contrary. 609 In this situation, a DHCPv4 relay agent MUST include a DHCPv4 VSS 610 sub-option in a relay-agent-information option [RFC3046], while a 611 DHCPv6 relay agent MUST include a DHCPv6 VSS option in the Relay- 612 forward message. 614 The value placed in the Virtual Subnet Selection sub-option or option 615 would typically be sufficient for the relay agent to properly route 616 any DHCP reply packet returned from the DHCP server to the DHCP 617 client for which it is destined. In some cases, the information in 618 the VSS sub-option or option might be an index into some internal 619 table held in the relay agent, though this document places no 620 requirement on a relay agent to have any such internal state. 622 A DHCPv4 relay agent MUST, in addition, include a DHCPv4 VSS-Control 623 sub-option (which has a length of zero) in the relay-agent- 624 information option [RFC3046] whenever it includes a VSS sub-option in 625 the relay-agent-information option. The inclusion of the VSS sub- 626 option and the VSS-Control sub-option in the relay-agent-information 627 option will allow the DHCPv4 relay agent to determine whether the 628 DHCPv4 server actually processed the information in the VSS sub- 629 option when it receives the relay-agent-information option in the 630 reply from the DHCPv4 server. 632 The reason to include this additional VSS DHCPv4 sub-option is that 633 [RFC3046] specifies (essentially) that a DHCPv4 server should copy 634 all sub-options that it receives in a relay-agent-information option 635 in a request into a corresponding relay-agent-information option in 636 the response. Thus, a server that didn't support the DHCPv4 VSS 637 sub-option would normally just copy it to the response packet, 638 leaving the relay agent to wonder if in fact the DHCPv4 server 639 actually used the VSS information when processing the request. 641 To alleviate this potential confusion, a DHCPvr4 relay agent instead 642 sends in two sub-options: one VSS sub-option, and one VSS-Control 643 sub-option. If both sub-options appear in the response from the 644 DHCPv4 server, then the DHCPv4 relay agent MUST assume that the 645 DHCPv4 server did not act on the VSS information in the VSS sub- 646 option. If only the VSS sub-option appears in the response from the 647 DHCPv4 server and no VSS-Control sub-option appears in the response 648 from the DHCPv4 server, then the relay agent SHOULD assume that the 649 DHCPv4 server acted successfully on the VSS sub-option. 651 Anytime a relay agent places a VSS option or sub-option in a DHCP 652 request, it SHOULD send it only to a DHCP server which supports the 653 VSS option or sub-option, and it MUST check the response to determine 654 if the DHCP server actually honored the requested VSS information. 656 In the DHCPv6 case, the appearance of the option in the Relay-reply 657 packet indicates that the DHCPv6 server understood and acted upon the 658 contents of the VSS option in the Relay-forward packet. In the 659 DHCPv4 case, as discussed above, the appearance of the VSS sub-option 660 without the appearance of a VSS-Control sub-option indicates that the 661 DHCPv4 server successfully acted upon the VSS sub-option. 663 This document does not create a requirement that a relay agent 664 remember the contents of a VSS DHCPv4 sub-option or VSS DHCPv6 option 665 sent to a DHCP server. In many cases, the relay agent may simply use 666 the value of the VSS returned by the DHCP server to forward the 667 response to the DHCP client. If the VSS information, the IP address 668 allocated, and the VPN capabilities of the relay agent all 669 interoperate correctly, then the DHCP client will receive a working 670 IP address. Alternatively, if any of these items don't interoperate 671 with the others, the DHCP client will not receive a working address. 673 Note that in some environments a relay agent may choose to always 674 place a VSS option or sub-option into packets and messages that it 675 forwards in order to forestall any attempt by a relay agent closer to 676 the client or the client itself to specify VSS information. In this 677 case, a type field of 255 is used to denote the global, default VPN. 678 When the type field of 255 is used, there MUST NOT be any additional 679 VSS information in the VSS option or sub-option. In the DHCPv4 case, 680 an additional VSS-Control sub-option, as discussed above. 682 5.1. VPN assignment by the DHCP server 684 In some cases, a DHCP server may use the Virtual Subnet Selection 685 sub-option or option to inform a relay agent that a particular DHCP 686 client is associated with a particular VPN. It does this by sending 687 the Virtual Subnet Selection sub-option or option with the 688 appropriate information to the relay agent in the relay-agent- 689 information option for DHCPv4 or the Relay-reply message in DHCPv6. 690 If the relay agent cannot respond correctly to the DHCP server's 691 requirement to place the DHCP client into that VPN (perhaps because 692 it has not been configured with a VPN that matches the VSS 693 information received from the DHCP server) it MUST drop the packet 694 and not send it to the DHCP client. 696 In this situation, once the relay agent has placed the DHCP client 697 into the VPN specified by the DHCP server, it will insert a VSS 698 option or sub-option when forwarding packets from the client. The 699 DHCP server in normal operation will echo this VSS information into 700 the outgoing replies. 702 In the event that the relay agent doesn't include VSS information on 703 subsequent requests after the DHCP server has included VSS 704 information in a reply to the relay agent, the DHCP server can 705 conclude that the relay agent doesn't support VSS processing, and the 706 DHCP server SHOULD stop processing this transaction and not respond 707 to the request. 709 5.2. DHCP Leasequery 711 Sometimes a relay-agent needs to submit a DHCP Leasequery [RFC4388] 712 [RFC5007] packet to the DHCP server in order to recover information 713 about existing DHCP allocated IP addresses on other than the normal, 714 global VPN. In the context of a DHCP Leasequery the relay agent is a 715 direct client of the DHCP server and is not relaying a packet for 716 another DHCP client. Thus, the instructions in Section 6 on Client 717 Behavior should be followed to include the necessary VSS information. 719 6. Client Behavior 721 Typically, DHCPv4 and DHCPv6 clients have no interaction with VSS 722 options or sub-options. The VSS information is handled by exchanges 723 between a DHCPv4 or DHCPv6 relay agent and the corresponding DHCPv4 724 or DHCPv6 server. 726 However, there are times when an entity is acting as a DHCPv4 or 727 DHCPv6 client in that it is communicating directly with a DHCPv4 or 728 DHCPv6 server. In these instances -- where communications is 729 occurring without employing the DHCPv4 relay-agent-information option 730 or the DHCPv6 Relay-forward or Relay-reply messages, the entity is 731 acting as a DHCPv4 or DHCPv6 client with regard to its communication 732 with the DHCPv4 or DHCPv6 server, but not necessarily as a DHCP 733 client who is requesting a DHCPv4 or DHCPv6 address for its own use. 735 The client, in this context, may be requesting an IP address for 736 another entity, thus acting as a DHCP proxy. The client may be 737 requesting information about another client-to-address binding, using 738 the DHCPv4 [RFC4388] or DHCPv6 [RFC5007] Leasequery protocol. 740 In the rest of this section, the term "client" refers to an entity 741 communicating VSS information directly to a DHCPv4 or DHCPv6 server 742 without using the DHCPv4 relay-agent-information option or the DHCPv6 743 Relay-forward or Relay-reply messages, and there is no requirement 744 that such a client is a traditional DHCPv4 or DHCPv6 client 745 requesting an IP address binding for itself. 747 A DHCPv4 or DHCPv6 client will employ the VSS option to communicate 748 VSS information to their respective servers. This information MUST 749 be included in every message concerning any IP address on a different 750 VPN than the global or default VPN. A DHCPv4 client will place the 751 DHCPv4 VSS option in its packets, and a DHCPv6 client will place the 752 DHCPv6 VSS option in its messages. 754 A DHCPv6 client that needs to place a VSS option into a DHCPv6 755 message SHOULD place a single VSS option into the DHCPv6 message at 756 the same level as the Client Identifier option. A DHCPv6 client MUST 757 NOT include different VSS options in the same DHCPv6 message. 759 Note that, as mentioned in Section 1, throughout this document when a 760 DHCPv6 address is indicated the same information applies to DHCPv6 761 Prefix Delegation [RFC3633] as well. 763 Since this option is placed in the packet in order to change the VPN 764 on which an IP address is allocated for a particular DHCP client, one 765 presumes that an allocation on that VPN is necessary for correct 766 operation. Thus, a client which places this option in a packet and 767 doesn't receive it or receives a different value in a returning 768 packet SHOULD drop the packet since the IP address that was allocated 769 will not be in the requested VPN. 771 Clients should be aware that some DHCP servers will return a VSS 772 option with different values than that which was sent in. In 773 addition, a client may receive a response from a DHCP server with a 774 VSS option when none was sent in by the Client. 776 Note that when sending a DHCP Leasequery request, a relay agent is 777 acting as a DHCP client and so it SHOULD include the respective 778 DHCPv4 or DHCPv6 VSS option in its DHCPv4 or DHCPv6 Leasequery packet 779 if the DHCP Leasequery request is generated for other than the 780 default, global VPN. It SHOULD NOT include a DHCPv4 sub-option in 781 this case. 783 7. Server Behavior 785 A DHCP server receiving the VSS option or sub-option SHOULD allocate 786 an IP address (or use the VSS information to access an already 787 allocated IP address) from the VPN specified by the included VSS 788 information. 790 In the case where the type field of the VSS option or sub-option is 791 255, the VSS option denotes the global, default VPN. In this case, 792 there is no explicit VSS information beyond the type field. 794 This document does not prescribe any particular address allocation 795 policy. A DHCP server may choose to attempt to allocate an address 796 using the VSS information and, if this is impossible, to not allocate 797 an address. Alternatively, a DHCP server may choose to attempt 798 address allocation based on the VSS information and, if that is not 799 possible, it may fall back to allocating an address on the global or 800 default VPN. This, of course, is also the apparent behavior of any 801 DHCP server which doesn't implement support for the VSS option and 802 sub-option. Thus, DHCP clients and relay agents SHOULD be prepared 803 for either of these alternatives. 805 In some cases, a DHCP server may use the Virtual Subnet Selection 806 sub-option or option to inform a relay agent that a particular DHCP 807 client is associated with a particular VPN. It does this by sending 808 the Virtual Subnet Selection sub-option or option with the 809 appropriate information to the relay agent in the relay-agent- 810 information option for DHCPv4 or the Relay-reply message in DHCPv6. 812 In this situation, the relay agent will place the client in the 813 proper VPN, and then it will insert a VSS option or sub-option in 814 subsequent forwarded requests. The DHCP server will see this VSS 815 information and since it doesn't conflict in any way with the 816 server's notion of the VPN on which the client is supposed to reside, 817 it will process the requests based on the VPN specified in the VSS 818 option or sub-option, and echo the same VSS information in the 819 outgoing replies. 821 The relay agent receiving a reply containing a VSS option should 822 support the VSS option. Otherwise the relay agent will end up 823 attempting to use the address as though it were a global address. 824 Should this happen, the subsequent DHCPREQUEST will not contain any 825 VSS information, in which case the DHCP server SHOULD NOT respond 826 with a DHCPACK. 828 If a server uses a different VPN than what was specified in the VSS 829 option or sub-option, it SHOULD send back the VPN information using 830 the same type as the received type. It MAY send back a different type 831 if it is not possible to use the same type (such as the RFC2685 VPN- 832 ID if no ASCII VPN identifier exists). 834 A server which receives a VSS sub-option in the DHCPv4 relay-agent- 835 information option and does not receive a VSS-Control sub-option in 836 the relay-agent-information option MUST process the information 837 specified in the VSS sub-option in the same fashion as it would have 838 if it received both sub-options. 840 7.1. Returning the DHCPv4 or DHCPv6 Option 842 DHCPv4 or DHCPv6 servers receiving a VSS option (for sub-option 843 processing, see below) MUST return an instance of this option in the 844 reply packet or message if the server successfully uses this option 845 to allocate an IP address, and it MUST NOT include an instance of 846 this option if the server is unable to support, is not configured to 847 support, or does not implement support for VSS information in general 848 or the requested VPN in particular. 850 If they echo the option (based on the criteria above), servers SHOULD 851 return an exact copy of the option unless they desire to change the 852 VPN on which a client was configured. 854 The appearance of the DHCPv4 VSS option code in the DHCPv4 Parameter 855 Request List option [RFC2132] should not change the processing or 856 decision to return or not return the VSS option as specified in this 857 document. The appearance of the DHCPv6 VSS option in the OPTION_ORO 858 [RFC3315] or the OPTION_ERO [RFC4994] should not change the 859 processing or decision to return (or not to return) the VSS option as 860 specified in this document. 862 7.2. Returning the DHCPv4 Sub-Option 864 The case of the DHCPv4 sub-option is a bit more complicated. Note 865 that [RFC3046] specifies that a DHCPv4 server which supports the 866 relay-agent-information option SHALL copy all sub-options received in 867 a relay-agent-information option into any outgoing relay-agent- 868 information option. Thus, the default behavior for any DHCPv4 server 869 is to return any VSS sub-option received to the relay agent whether 870 or not the DHCPv4 server understands the VSS sub-option. 872 In order to distinguish a DHCPv4 server which is simply copying 873 relay-agent-information option sub-options from an incoming to an 874 outgoing relay-agent-informaion option from one which successfully 875 acted upon the information in the VSS sub-option, DHCPv4 relay agents 876 MUST include a VSS-Control sub-option in the relay-agent-information 877 any time that it includes a VSS sub-option in the relay-agent- 878 information option. 880 A DHCPv4 server which does not support the VSS sub-option will copy 881 both sub-options into the outgoing relay-agent-information option, 882 thus signalling to the DHCPv4 relay agent that it did not understand 883 the VSS sub-option. 885 A DHCPv4 server which supports the VSS sub-option: 887 o MUST copy the VSS sub-option into the outgoing relay-agent- 888 information option 890 o MUST NOT copy the VSS-Control sub-option into the outgoing 891 relay-agent-information option 893 Moreover, if a server uses different VSS information to allocate an 894 IP address than it receives in a particular DHCPv4 sub-option, it 895 MUST include that alternative VSS information in the VSS sub-option 896 that it returns to the DHCPv4 relay agent instead of the original VSS 897 information it was given. 899 If a DHCPv4 server supports this sub-option and for some reason 900 (perhaps administrative control) does not honor this sub-option from 901 the request then it MUST NOT echo either sub-option into the outgoing 902 relay-agent-information option. 904 7.3. Making sense of conflicting VSS information 906 It is possible for a DHCPv4 server to receive both a VSS option and 907 VSS sub-options in the same packet. Likewise, a DHCPv6 server can 908 receive multiple VSS options in nested Relay-forward messages as well 909 as in the client message itself. In either of these cases, the VSS 910 information from the relay agent closest to the DHCP server SHOULD be 911 used in preference to all other VSS information received. In the 912 DHCPv4 case, this means that the VSS sub-option takes precedence over 913 the VSS option, and in the DHCPv6 case, this means that the VSS 914 option from the outer-most Relay-forward message in which a VSS 915 option appears takes precedence. 917 The reasoning behind this approach is that the relay-agent closer to 918 the DHCP server is almost certainly more trusted than the DHCP client 919 or more distant relay agents, and therefore information in the 920 relay-agent-information option or the Relay-forward message is more 921 likely to be correct. 923 In general, relay agents SHOULD be aware through configuration or 924 policy external to this document whether or not they should be 925 including VSS information in packets that they forward and so there 926 should not be conflicts among relay agent specified VSS information. 928 In these situations where multiple VSS option or sub-options appear 929 in the incoming packet or message, when the DHCP server constructs 930 the response to be sent to the DHCP client or relay agent, all 931 existing VSS options or sub-options MUST be replicated in the 932 appropriate places in the response and MUST contain only the VSS 933 information that was used by the DHCP server to allocate the IP 934 address (with, of course, the exception of a DHCPv4 relay-agent- 935 information sub-option VSS-Control). 937 8. Security 939 Message authentication in DHCPv4 for intradomain use where the out- 940 of-band exchange of a shared secret is feasible is defined in 941 [RFC3118]. Potential exposures to attack are discussed in Section 7 942 of the DHCP protocol specification in [RFC2131]. 944 Implementations should consider using the DHCPv4 Authentication 945 option [RFC3118] to protect DHCPv4 client access in order to provide 946 a higher level of security if it is deemed necessary in their 947 environment. 949 Message authentication in DHCPv4 relay agents as defined in [RFC4030] 950 should be considered for DHCPv4 relay agents employing this sub- 951 option. Potential exposures to attack are discussed in Section 7 of 952 the DHCP protocol specification in [RFC2131]. 954 For DHCPv6 use of the VSS option, the "Security Considerations" 955 Section of [RFC3315] details the general threats to DHCPv6, and thus 956 to messages using the VSS option. The "Authentication of DHCP 957 Messages" Section of [RFC3315] describes securing communication 958 between relay agents and servers, as well as clients and servers. 960 The VSS option could be used by a client in order to obtain an IP 961 address from any VPN. This option would allow a client to perform a 962 more complete address-pool exhaustion attack since the client would 963 no longer be restricted to attacking address-pools on just its local 964 subnet. 966 A DHCP server that implements these options and sub-option should be 967 aware of this possibility and use whatever techniques that can be 968 devised to prevent such an attack. Information such as the giaddr in 969 DHCPv4 or link address in the Relay-forward DHCPv6 message might be 970 used to detect and prevent this sort of attack. 972 One possible defense would be for the DHCP relay to insert a VSS 973 option or sub-option to override the DHCP client's VSS option. 975 Servers that implement the VSS option and sub-option MUST by default 976 disable use of the feature; it must specifically be enabled through 977 configuration. Moreover, a server SHOULD provide the ability to 978 selectively enable use of the feature under restricted conditions, 979 e.g., by enabling use of the option only from explicitly configured 980 client-ids, enabling its use only by clients on a particular subnet, 981 or restricting the VSSs from which addresses may be requested. 983 9. IANA Considerations 985 IANA is requested to assign DHCPv4 option number 221 for the DHCPv4 986 VSS option defined in Section 3.1, in accordance with [RFC3942]. 988 IANA is requested to assign sub-option number 151 for the DHCPv4 VSS 989 sub-option defined in Section 3.2 from the DHCP Relay Agent Sub- 990 options space [RFC3046], in accordance with the spirit of [RFC3942]. 991 While [RFC3942] doesn't explicitly mention the sub-option space for 992 the DHCP Relay Agent Information option [RFC3046], sub-option 151 is 993 already in use by existing implementations of this sub-option and the 994 current draft is essentially upward compatible with these current 995 implementations. 997 IANA is requested to assign the value of TBD for the DHCPv4 VSS- 998 Control sub-option defined in Section 3.3. 1000 IANA is requested to assign the value of TBD for the DHCPv6 VSS 1001 option defined in Section 3.4 from the DHCPv6 option registry. 1003 The type byte defined in Section 3.5 defines a number space for which 1004 IANA is to create and maintain a new sub-registry entitled "VSS Type 1005 values". This sub-registry needs to be related to both the DHCPv4 1006 and DHCPv6 VSS options and the DHCPv4 relay-agent-information option 1007 sub-option (all defined by this document), since the type byte in 1008 these two options and one sub-option MUST have identical definitions. 1010 New values for the type byte may only be defined by IETF Consensus, 1011 as described in [RFC5226]. Basically, this means that they are 1012 defined by RFCs approved by the IESG. 1014 10. Acknowledgments 1016 Bernie Volz recommended consolidation of the DHCPv4 option and sub- 1017 option drafts after extensive review of the former drafts, and 1018 provided valuable assistance in structuring and reviewing this 1019 document. Alper Yegin expressed interest in the DHCPv6 VSS option, 1020 resulting in this combined draft covering all three areas. Alfred 1021 Hoenes provided assistance with editorial review as well as raising 1022 substantive protocol issues. David Hankins and Bernie Volz each 1023 raised important protocol issues which resulted in a clarified 1024 document. Josh Littlefield provided editorial assistance. Several 1025 IESG reviewers took the time to substantially review this document, 1026 resulting in much increased clarity. 1028 11. References 1030 11.1. Normative References 1032 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1033 Requirement Levels", RFC 2119, March 1997. 1035 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, 1036 March 1997. 1038 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor 1039 Extensions", RFC 2132, March 1997. 1041 [RFC2685] Fox, B., Gleeson, B., "Virtual Private Networks 1042 Identifier", RFC 2685, September 1999. 1044 [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 1045 3046, January 2001. 1047 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and 1048 M. Carney, "Dynamic Host Configuration Protocol for IPv6 1049 (DHCPv6)", RFC 3315, July 2003. 1051 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 1052 Host Configuration Protocol (DHCP) version 6", RFC 3633, December 1053 2003. 1055 [RFC4994] Zeng, S., Volz, B., Kinnear, K. and J. Brzozowski, "DHCPv6 1056 Relay Agent Echo Request Option", RFC 4994, September 2007. 1058 11.2. Informative References 1060 [RFC951] Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951, 1061 September 1985. 1063 [RFC1542] Wimer, W., "Clarifications and Extensions for the Bootstrap 1064 Protocol", RFC 1542, October 1993. 1066 [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP 1067 Messages", RFC 3118, June 2001. 1069 [RFC3942] Volz, B., "Reclassifying Dynamic Host Configuration 1070 Protocol version 4 (DHCPv4) Options", RFC 3942, November 2004. 1072 [RFC4030] Stapp, M. and T. Lemon, "The Authentication Suboption for 1073 the Dynamic Host Configuration Protocol (DHCP) Relay Agent 1074 Option", RFC 4030, March 2005. 1076 [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration 1077 Protocol (DHCP) Leasequery", RFC 4388, February 2006. 1079 [RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng, "DHCPv6 1080 Leasequery", RFC 5007, September 2007. 1082 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1083 IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 1085 Authors' Addresses 1087 Kim Kinnear 1088 Cisco Systems 1089 1414 Massachusetts Ave. 1090 Boxborough, Massachusetts 01719 1092 Phone: (978) 936-0000 1094 EMail: kkinnear@cisco.com 1096 Richard Johnson 1097 Cisco Systems 1098 170 W. Tasman Dr. 1099 San Jose, CA 95134 1101 Phone: (408) 526-4000 1103 EMail: raj@cisco.com 1105 Mark Stapp 1106 Cisco Systems 1107 1414 Massachusetts Ave. 1108 Boxborough, Massachusetts 01719 1110 Phone: (978) 936-0000 1112 EMail: mjs@cisco.com 1114 Jay Kumarasamy