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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 Updates: 3046 Mark Stapp 5 Intended Status: Standards Track Cisco Systems 6 Expires: May 15, 2012 Jay Kumarasamy 7 November 15, 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]. This memo 71 updates RFC 3046 [RFC3046] regarding details relating to copying of 72 sub-options (see Section 8). 74 Table of Contents 76 1. Introduction................................................. 3 77 2. Terminology.................................................. 4 78 3. Virtual Subnet Selection Option and Sub-Options Definitions.. 5 79 3.1. DHCPv4 Virtual Subnet Selection Option..................... 5 80 3.2. DHCPv4 Virtual Subnet Selection Sub-Option................. 6 81 3.3. DHCPv4 Virtual Subnet Selection Control Sub-Option......... 6 82 3.4. DHCPv6 Virtual Subnet Selection Option..................... 7 83 3.5. Virtual Subnet Selection Type and Information.............. 7 84 4. Overview of Virtual Subnet Selection Usage................... 8 85 4.1. VPN assignment by the DHCP relay agent..................... 9 86 4.2. VPN assignment by the DHCP server.......................... 12 87 4.3. Required Support........................................... 14 88 4.4. Alternative VPN assignment approaches...................... 14 89 5. Relay Agent Behavior......................................... 14 90 5.1. VPN assignment by the DHCP server.......................... 16 91 5.2. DHCP Leasequery............................................ 17 92 6. Client Behavior.............................................. 17 93 7. Server Behavior.............................................. 18 94 7.1. Returning the DHCPv4 or DHCPv6 Option...................... 19 95 7.2. Returning the DHCPv4 Sub-Option............................ 20 96 7.3. Making sense of conflicting VSS information................ 21 97 8. Updates to RFC 3046.......................................... 21 98 9. Security..................................................... 22 99 10. IANA Considerations......................................... 23 100 11. Acknowledgments............................................. 23 101 12. References.................................................. 24 102 12.1. Normative References...................................... 24 103 12.2. Informative References.................................... 24 105 1. Introduction 107 There is a growing use of Virtual Private Network (VPN) 108 configurations. The growth comes from many areas; individual client 109 systems needing to appear to be on the home corporate network even 110 when traveling, ISPs providing extranet connectivity for customer 111 companies, etc. In some of these cases there is a need for the DHCP 112 server to know the VPN (hereafter called a "Virtual Subnet Selector" 113 or "VSS") from which an address, and other resources, should be 114 allocated. 116 This memo defines a Virtual Subnet Selection (VSS) option for each of 117 DHCPv4 and DHCPv6, and a VSS sub-option carried in the DHCPv4 relay- 118 agent-information option. These are intended for use by DHCP 119 clients, relay agents, and proxy clients in situations where VSS 120 information needs to be passed to the DHCP server for proper address 121 or prefix allocation to take place. If the receiving DHCP server 122 understands the VSS option or sub-option, this information may be 123 used in conjunction with other information in determining the subnet 124 on which to select an address as well as other information such as 125 DNS server, default router, etc. 127 If the allocation is being done through a DHCPv4 relay, then the 128 relay sub-option defined here should be included. In some cases, 129 however, an IP address is being sought by a DHCPv4 proxy on behalf of 130 a client (which may be assigned the address via a different 131 protocol). In this case, there is a need to include VSS information 132 relating to the client as a DHCPv4 option. 134 If the allocation is being done through a DHCPv6 relay, then the 135 DHCPv6 VSS option defined in this document should be included in the 136 Relay-forward and Relay-reply message going between the DHCPv6 relay 137 and server. In some cases, addresses or prefixes are being sought by 138 a DHCPv6 proxy on behalf of a client. In this case, there is a need 139 for the client itself to supply the VSS information using the DHCPv6 140 VSS option in the messages that it sends to the DHCPv6 server. 142 In the remaining text of this document, when a DHCPv6 address is 143 indicated the same information applies to DHCPv6 Prefix Delegation 144 [RFC3633] as well. 146 In the remaining text of this document, when the term VSS sub-option 147 is used, it refers to the VSS sub-option carried in the DHCPv4 148 relay-agent-information option. 150 2. Terminology 152 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 153 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 154 document are to be interpreted as described in RFC 2119 [RFC2119]. 156 This document uses the following terms: 158 o "DHCP client" 160 A DHCP client is a host using DHCP to obtain configuration 161 parameters such as a network address. 163 o "DHCP proxy" 165 A DHCP proxy is a DHCP client which acquires IP addresses not 166 for its own use, but rather on behalf of another entity. There 167 are a variety of ways that a DHCP proxy can supply the addresses 168 it acquires to other entities that need them. 170 o "DHCP relay agent" 172 A DHCP relay agent is an agent that transfers BOOTP and DHCP 173 messages between clients and servers residing on different 174 subnets, per [RFC951], [RFC1542], and [RFC3315]. 176 o "DHCP server" 178 A DHCP server is a host that returns configuration parameters to 179 DHCP clients. 181 o "DHCPv4 option" 183 An option used to implement a capability defined by the DHCPv4 184 RFCs [RFC2131][RFC2132]. These options have one-octet code and 185 size fields. 187 o "DHCPv4 sub-option" 188 As used in this document, a DHCPv4 sub-option refers to a sub- 189 option of the relay-agent-information option [RFC3046]. These 190 sub-options have one-octet code and size fields. 192 o "DHCPv6 option" 194 An option used to implement a capability defined by the DHCPv6 195 RFC [RFC3315]. These options have two-octet code and size 196 fields. 198 o "Global VPN" 200 Indicates that the address being described belongs to the set of 201 addresses not part of any VPN. In other words, the normal 202 address space operated on by DHCP. This includes private 203 addresses, for example the 10.x.x.x addresses as well as the 204 other private subnets that are not routed on the open internet. 206 o "VSS information" 208 Information about a VPN necessary to allocate an address to a 209 DHCP client on that VPN and necessary to forward a DHCP reply 210 packet to a DHCP client on that VPN. 212 o "VPN" 214 Virtual private network. A network which appears to the client 215 to be a private network. 217 o "VPN Identifier" 219 The VPN-ID is defined by [RFC2685] to be a sequence of 7 octets. 221 3. Virtual Subnet Selection Option and Sub-Options Definitions 223 The Virtual Subnet Selection options and sub-options contain a 224 generalized way to specify the VSS information about a VPN. There 225 are two options and two sub-options defined in this section. The 226 actual VSS information is identical both options and one of the two 227 sub-options. 229 3.1. DHCPv4 Virtual Subnet Selection Option 231 The format of the option is: 233 0 1 2 3 234 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 1 235 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 236 | Code | Length | Type | VSS Info ... 237 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 239 Code The option code (221). 241 Length The option length, minimum 1 octets. 243 Type and VSS Information -- see Section 3.5 245 3.2. DHCPv4 Virtual Subnet Selection Sub-Option 247 This is a sub-option of the relay-agent-information option [RFC3046]. 248 The format of the sub-option is: 250 0 1 2 3 251 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 1 252 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 253 | Code | Length | Type | VSS Info. ... 254 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 256 Code The sub-option code (151). 258 Length The sub-option length, minimum 1 octets. 260 Type and VSS Information -- see Section 35. 262 3.3. DHCPv4 Virtual Subnet Selection Control Sub-Option 264 This is a sub-option of the relay-agent-information option [RFC3046]. 265 The format of the sub-option is: 267 0 1 268 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 269 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 270 | Code | Length | 271 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 273 Code The sub-option code (TBD). 275 Length The sub-option length, 0. 277 This sub-option only only appears in the DHCPv4 relay-agent- 278 information option. In a DHCP request, it indicates that a DHCPv4 279 VSS sub-option is also present in the relay-agent-information option. 280 In a DHCP reply, if it appears in the relay-agent-information option, 281 it indicates that the DHCP server did not understand any DHCPv4 VSS 282 sub-option that also appears in the relay-agent-information option. 284 3.4. DHCPv6 Virtual Subnet Selection Option 286 The format of the DHCPv6 Virtual Subnet Selection option is shown 287 below. This option may be included by a client or relay-agent (or 288 both). 290 0 1 2 3 291 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 1 292 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 293 | OPTION_VSS | option-len | 294 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 295 | Type | VSS Information ... | 296 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 298 option-code OPTION_VSS (TBD). 300 option-len The number of octets in the option, minimum 1. 302 Type and VSS Information -- see Section 3.5 304 3.5. Virtual Subnet Selection Type and Information 306 All of the (sub)options defined above carry identical payloads, 307 consisting of a type and additional VSS information as follows: 309 Type VSS Information format: 311 0 NVT ASCII VPN identifier 312 1 RFC2685 VPN-ID 313 2-254 Reserved 314 255 Global, default VPN. 316 o Type 0 -- NVT ASCII VPN identifier 318 Indicates that the VSS information consists of a NVT ASCII 319 string. It MUST NOT be terminated with a zero byte. 321 o Type 1 -- RFC2685 VPN-ID 323 Indicates that the VSS information consists of an RFC2685 VPN-ID 324 [RFC2685], which is defined to be 7 octets in length. 326 o Type 255 -- Global, default VPN 328 Indicates that there is no explicit, non-default VSS information 329 but rather that this option references the normal, global, 330 default address space. In this case, there MUST NOT be any VSS 331 Information included in the VSS option or sub-option and the 332 length of the MUST be 1. 334 All other values of the Type field are reserved. 336 4. Overview of Virtual Subnet Selection Usage 338 At the highest level, the VSS option or sub-option determines the VPN 339 on which a DHCP client is supposed to receive an IP address. How the 340 option or sub-option is entered and processed is discussed below, but 341 the point of all of the discussion is to determine the VPN on which 342 the DHCP client resides. This will affect a relay agent, in that it 343 will have to ensure that DHCP packets sent to and received from the 344 DHCP client flow over the correct VPN. This will affect the DHCP 345 server in that it determines the IP address space used for the IP 346 address allocation. 348 A DHCP server has as part of its configuration some IP address space 349 from which it allocates IP addresses to DHCP clients. These 350 allocations are typically for a limited time, and thus the DHCP 351 client gets a lease on the IP address. In the absence of any VPN 352 information, the IP address space is in the global or default VPN 353 used throughout the Internet. When a DHCP server deals with VPN 354 information, each VPN defines a new address space inside the server, 355 one distinct from the global or default IP address space. A server 356 which supports the VSS option or sub-option thereby supports 357 allocation of IP addresses from multiple different VPNs. Supporting 358 IP address allocation from multiple different VPNs means that the 359 DHCP server must be prepared to configure multiple different address 360 spaces (one per distinct VPN) and allocate IP addresses from these 361 different address spaces. 363 These address spaces are typically independent, so that the same IP 364 address (consisting of the same string of bytes) could be allocated 365 to one client in the global, default VPN, and to a different client 366 residing in a different VPN. There is no conflict in this 367 allocation, since the clients have essentially different addresses, 368 even though these addresses consist of the same string of bytes, 369 because the IPv4 or IPv6 address is qualified by the VPN. 371 Thus a VSS option or sub-option is a way of signaling the use of a 372 VPN other than the global or default VPN. The next question is: who 373 decides what VPN a DHCP client should be using? 375 There are three entities which can either insert a VSS option or 376 sub-option into a DHCPv4 packet or DHCPv6 message; a DHCP client, a 377 relay agent, or a DHCPv4 or DHCPv6 server. While all of these 378 entities could include a different VSS option or sub-option in every 379 request or response, this situation is neither typical nor useful. 380 There are two known paradigms for use of the VSS option or sub- 381 option, which are discussed below. 383 4.1. VPN assignment by the DHCP relay agent 385 The typical use of the VSS option or sub-option is for the relay 386 agent to know the VPN on which the DHCP client is operating. The 387 DHCP client itself does not, in this approach, know the VPN on which 388 it resides. The relay agent is responsible for mediating the access 389 between the VPN on which the DHCP client resides and the DHCP server. 390 In this situation, the relay agent will insert two DHCPv4 relay- 391 agent-information sub-options (one VSS sub-option, and one VSS- 392 Control sub-option) into the relay-agent-information option or a 393 DHCPv6 VSS option into the Relay-forward message of every request it 394 forwards from the DHCP client. The server will use the DHCPv6 VSS 395 option or DHCPv4 VSS sub-option to determine the VPN on which the 396 client resides, and use that VPN information to select the address 397 space within its configuration from which to allocate an IP address 398 to the DHCP client. 400 When, using this approach, a DHCPv4 relay agent inserts a VSS sub- 401 option into the relay-agent-information option it MUST also insert a 402 VSS-Control sub-option into the relay-agent-information-option. This 403 is to allow determination of whether or not the DHCPv4 server 404 actually processes the VSS information provided by the DHCPv4 relay 405 agent. If the DHCPv4 server supports the VSS capabilities described 406 in this document, it will remove the VSS-Control sub-option from the 407 relay-agent-information option that it returns to the DHCPv4 relay 408 agent. See Section 5 for more information. 410 In this approach, the relay agent might also send a VSS option or 411 sub-option in either a DHCPv4 or DHCPv6 Leasequery request, but in 412 this case, it would use the VSS option in the Leasequery request to 413 select the correct address space for the Leasequery. In this 414 approach, the relay agent would be acting as a DHCP client from a 415 Leasequery standpoint, but it would not be as if a DHCP client were 416 sending in a VSS option in a standard DHCP address allocation 417 request, say a DHCPDISCOVER. 419 In this approach, only one relay agent would mediate the VPN access 420 for the DHCP client to the DHCP server, and it would be the relay 421 agent which inserts the VSS information into the request packet and 422 would remove it prior to forwarding the response packet on. 424 In the diagram below is an example of a DHCPv4 client, DHCPv4 relay 425 agent, and DHCPv4 server. The DHCPv6 situation is similar, but uses 426 the DHCPv6 VSS option. 428 DHCPv4 429 DHCPv4 Relay DHCPv4 430 Client Agent Server 432 | | | 433 | >--DHCPDISCOVER--> | | 434 | on VRF "abc" | | 435 | | >--DHCPDISCOVER----> | 436 | | relay-agent-info: | 437 | | VSS type VRF:"abc"| 438 | | VSS-Control | 439 | | | 440 | | <----DHCPOFFER-----< | 441 | | relay-agent-info: | 442 | | VSS type VRF:"abc"| 443 | | | 444 | <---DHCPOFFER----< | | 445 | on VRF "abc" | | 446 | | | 447 | >--DHCPREQUEST---> | | 448 | on VRF "abc" | | 449 | | >--DHCPREQUEST-----> | 450 | | relay-agent-info: | 451 | | VSS type VRF:"abc"| 452 | | VSS-Control | 453 | | | 454 | | <----DHCPACK-------< | 455 | | relay-agent-info: | 456 | | VSS type VRF:"abc"| 457 | | | 458 | <---DHCPACK------< | | 459 | on VRF "abc" | | 460 | | | 461 ... ... ... 463 Figure 4.1-1: DHCPv4 - Relay Agent knows VPN 465 The DHCP server would know that it should respond to VPN information 466 specified in a VSS option or sub-option, and it would be configured 467 with appropriate VPN address spaces to service the projected client 468 requirements. Thus, in this common approach, the DHCP client knows 469 nothing of any VPN access, the relay agent has been configured in 470 some way that allows it to determine the VPN of the DHCP client and 471 transmit that using a VSS option or sub-option to the DHCP server, 472 and the DHCP server responds to the VPN specified by the relay agent. 473 There is no conflict between different entities trying to specify 474 different VSS information -- each entity knows its role through 475 policy or configuration external to this document. 477 If any mis-configuration exists, it SHOULD result in a DHCP client 478 being unable to acquire an IP address. For instance, a relay agent 479 which supports VPN access SHOULD couple transmission of VSS options 480 or sub-options to the configuration of VPN support, and not allow one 481 without the other. 483 It is important to ensure that the relay agent and DHCP server both 484 support the VSS option and sub-option (for DHCPv4) or the VSS option 485 (for DHCPv6). Deploying DHCPv4 relay agents which support and emit 486 VSS sub-options in concert with DHCPv4 servers which do not support 487 the VSS option or sub-option as defined in this document SHOULD NOT 488 be done, as such an ensemble will not operate correctly. Should this 489 situation occur, however, the relay agent can detect the problem 490 (since the VSS-Control sub-option will appear in the packets it 491 receives from the DHCPv4 server, indicating the server did not 492 effectively process the VSS sub-option), and it can issue appropriate 493 diagnostic messages. 495 4.2. VPN assignment by the DHCP server 497 In this approach, the DHCP server would be configured in some way to 498 know the VPN on which a particular DHCP client should be given 499 access. The DHCP server would in this case include the VSS sub- 500 option in the relay-agent-information option for DHCPv4 or the VSS 501 option in the Relay-reply message for DHCPv6. The relay agent 502 responsible for mediating VPN access would use this information to 503 select the correct VPN for the DHCP client. In the unusal event that 504 there were more than one relay agent involved in this transaction, 505 some external configuration or policy would be needed to inform the 506 DHCPv6 server into which Relay-reply message the VSS option should 507 go. 509 Once the relay agent has placed the DHCP client into the proper VPN, 510 it SHOULD begin including VSS information in requests that it 511 forwards to the DHCP server. Since this information does not 512 conflict with the DHCP server's idea of the proper VPN for the 513 client, everything works correctly. 515 The diagram below shows this approach using DHCPv4. The DHCPv6 516 situation is similar, but uses the DHCPv6 VSS option instead. 518 DHCPv4 519 DHCPv4 Relay DHCPv4 520 Client Agent Server 522 | | | 523 | >--DHCPDISCOVER--> | | 524 | on unknown VPN | | 525 | | >--DHCPDISCOVER----> | 526 | | | 527 | | <----DHCPOFFER-----< | 528 | | relay-agent-info: | 529 | | VSS type VRF:"abc"| 530 | | | 531 | <---DHCPOFFER----< | | 532 | on VRF "abc" | | 533 | | | 534 | >--DHCPREQUEST---> | | 535 | on VRF "abc" | | 536 | | >--DHCPREQUEST-----> | 537 | | relay-agent-info: | 538 | | VSS type VRF:"abc"| 539 | | VSS-Control | 540 | | | 541 | | <----DHCPACK-------< | 542 | | relay-agent-info: | 543 | | VSS type VRF:"abc"| 544 | | | 545 | <---DHCPACK------< | | 546 | on VRF "abc" | | 547 | | | 548 | | | 549 ... ... ... 551 Figure 4.2-1: DHCPv4 - DHCPv4 Server knows VPN 553 In this approach, the DHCP client is again unaware of any VPN 554 activity. In this case, however, the DHCP server knows the VPN for 555 the client, and the relay agent responds to the VSS information 556 specified by the DHCP server. Similar to the previous approach, each 557 entity knows its role through a means external to this document and 558 no two entities try to specify VSS information in conflict. 560 It is important that both the relay agent as well as the DHCP server 561 both support the VSS option and sub-option (for DHCPv4) and the VSS 562 option (for DHCPv6). Deploying and configuring VPN support in one 563 element and not in the other is not a practical approach. 565 4.3. Required Support 567 DHCP relay agents and servers MUST support the approach discussed in 568 Section 4.1. DHCP relay agents and server SHOULD support the 569 approach discussed in Section 4.2. DHCP relay agents and servers 570 SHOULD NOT be configured to operate with both approaches 571 simultaneously. 573 4.4. Alternative VPN assignment approaches 575 There are many other approaches which can be created with multiple 576 relay agents each inserting VSS information into different Relay- 577 forward messages, relay agent VSS information conflicting with client 578 VSS information, or DHCP server VSS information conflicting with 579 relay agent and client VSS information. Since these approaches do 580 not describe situations that are useful today, specifying precisely 581 how to resolve all of these conflicts is unlikely to be valuable in 582 the event that these approaches actually become practical in the 583 future. 585 The current use of the VSS option and sub-option require that each 586 entity knows the part that it plays in dealing with VPN data. Each 587 entity -- client, relay agent or agents, and server -- SHOULD know 588 through some policy or configuration beyond the scope of this 589 document whether it is responsible for specifying VPN information 590 using the VSS option or sub-option or responsible for responding to 591 VSS information specified by another entity, or simply ignoring any 592 VSS information which it might see. 594 Some simple conflict resolution approaches are discussed below, in 595 the hopes that they will cover simple cases that may arise from 596 situations beyond those envisioned today. However, for more complex 597 situations, or simple situations where appropriate conflict 598 resolution strategies differ from those discussed in this document, a 599 document detailing the usage situations and appropriate conflict 600 resolution strategies SHOULD be created and submitted for discussion 601 and approval. 603 5. Relay Agent Behavior 605 A relay agent which receives a DHCP request from a DHCP client on a 606 VPN SHOULD include Virtual Subnet Selection information in the DHCP 607 packet prior to forwarding the packet on to the DHCP server unless 608 inhibited from doing so by configuration information or policy to the 609 contrary. 611 In this situation, a DHCPv4 relay agent MUST include a DHCPv4 VSS 612 sub-option in a relay-agent-information option [RFC3046], while a 613 DHCPv6 relay agent MUST include a DHCPv6 VSS option in the Relay- 614 forward message. 616 The value placed in the Virtual Subnet Selection sub-option or option 617 would typically be sufficient for the relay agent to properly route 618 any DHCP reply packet returned from the DHCP server to the DHCP 619 client for which it is destined. In some cases, the information in 620 the VSS sub-option or option might be an index into some internal 621 table held in the relay agent, though this document places no 622 requirement on a relay agent to have any such internal state. 624 A DHCPv4 relay agent MUST, in addition, include a DHCPv4 VSS-Control 625 sub-option (which has a length of zero) in the relay-agent- 626 information option [RFC3046] whenever it includes a VSS sub-option in 627 the relay-agent-information option. The inclusion of the VSS sub- 628 option and the VSS-Control sub-option in the relay-agent-information 629 option will allow the DHCPv4 relay agent to determine whether the 630 DHCPv4 server actually processed the information in the VSS sub- 631 option when it receives the relay-agent-information option in the 632 reply from the DHCPv4 server. 634 The reason to include this additional VSS DHCPv4 sub-option is that 635 [RFC3046] specifies (essentially) that a DHCPv4 server should copy 636 all sub-options that it receives in a relay-agent-information option 637 in a request into a corresponding relay-agent-information option in 638 the response. Thus, a server that didn't support the DHCPv4 VSS 639 sub-option would normally just copy it to the response packet, 640 leaving the relay agent to wonder if in fact the DHCPv4 server 641 actually used the VSS information when processing the request. 643 To alleviate this potential confusion, a DHCPvr4 relay agent instead 644 sends in two sub-options: one VSS sub-option, and one VSS-Control 645 sub-option. If both sub-options appear in the response from the 646 DHCPv4 server, then the DHCPv4 relay agent MUST assume that the 647 DHCPv4 server did not act on the VSS information in the VSS sub- 648 option. If only the VSS sub-option appears in the response from the 649 DHCPv4 server and no VSS-Control sub-option appears in the response 650 from the DHCPv4 server, then the relay agent SHOULD assume that the 651 DHCPv4 server acted successfully on the VSS sub-option. 653 Anytime a relay agent places a VSS option or sub-option in a DHCP 654 request, it SHOULD send it only to a DHCP server which supports the 655 VSS option or sub-option, and it MUST check the response to determine 656 if the DHCP server actually honored the requested VSS information. 658 In the DHCPv6 case, the appearance of the option in the Relay-reply 659 packet indicates that the DHCPv6 server understood and acted upon the 660 contents of the VSS option in the Relay-forward packet. In the 661 DHCPv4 case, as discussed above, the appearance of the VSS sub-option 662 without the appearance of a VSS-Control sub-option indicates that the 663 DHCPv4 server successfully acted upon the VSS sub-option. 665 This document does not create a requirement that a relay agent 666 remember the contents of a VSS DHCPv4 sub-option or VSS DHCPv6 option 667 sent to a DHCP server. In many cases, the relay agent may simply use 668 the value of the VSS returned by the DHCP server to forward the 669 response to the DHCP client. If the VSS information, the IP address 670 allocated, and the VPN capabilities of the relay agent all 671 interoperate correctly, then the DHCP client will receive a working 672 IP address. Alternatively, if any of these items don't interoperate 673 with the others, the DHCP client will not receive a working address. 675 Note that in some environments a relay agent may choose to always 676 place a VSS option or sub-option into packets and messages that it 677 forwards in order to forestall any attempt by a relay agent closer to 678 the client or the client itself to specify VSS information. In this 679 case, a type field of 255 is used to denote the global, default VPN. 680 When the type field of 255 is used, there MUST NOT be any additional 681 VSS information in the VSS option or sub-option. In the DHCPv4 case, 682 an additional VSS-Control sub-option, as discussed above. 684 5.1. VPN assignment by the DHCP server 686 In some cases, a DHCP server may use the Virtual Subnet Selection 687 sub-option or option to inform a relay agent that a particular DHCP 688 client is associated with a particular VPN. It does this by sending 689 the Virtual Subnet Selection sub-option or option with the 690 appropriate information to the relay agent in the relay-agent- 691 information option for DHCPv4 or the Relay-reply message in DHCPv6. 692 If the relay agent cannot respond correctly to the DHCP server's 693 requirement to place the DHCP client into that VPN (perhaps because 694 it has not been configured with a VPN that matches the VSS 695 information received from the DHCP server) it MUST drop the packet 696 and not send it to the DHCP client. 698 In this situation, once the relay agent has placed the DHCP client 699 into the VPN specified by the DHCP server, it will insert a VSS 700 option or sub-option when forwarding packets from the client. The 701 DHCP server in normal operation will echo this VSS information into 702 the outgoing replies. 704 In the event that the relay agent doesn't include VSS information on 705 subsequent requests after the DHCP server has included VSS 706 information in a reply to the relay agent, the DHCP server can 707 conclude that the relay agent doesn't support VSS processing, and the 708 DHCP server SHOULD stop processing this transaction and not respond 709 to the request. 711 5.2. DHCP Leasequery 713 Sometimes a relay-agent needs to submit a DHCP Leasequery [RFC4388] 714 [RFC5007] packet to the DHCP server in order to recover information 715 about existing DHCP allocated IP addresses on other than the normal, 716 global VPN. In the context of a DHCP Leasequery the relay agent is a 717 direct client of the DHCP server and is not relaying a packet for 718 another DHCP client. Thus, the instructions in Section 6 on Client 719 Behavior should be followed to include the necessary VSS information. 721 6. Client Behavior 723 Typically, DHCPv4 and DHCPv6 clients have no interaction with VSS 724 options or sub-options. The VSS information is handled by exchanges 725 between a DHCPv4 or DHCPv6 relay agent and the corresponding DHCPv4 726 or DHCPv6 server. 728 However, there are times when an entity is acting as a DHCPv4 or 729 DHCPv6 client in that it is communicating directly with a DHCPv4 or 730 DHCPv6 server. In these instances -- where communications is 731 occurring without employing the DHCPv4 relay-agent-information option 732 or the DHCPv6 Relay-forward or Relay-reply messages, the entity is 733 acting as a DHCPv4 or DHCPv6 client with regard to its communication 734 with the DHCPv4 or DHCPv6 server, but not necessarily as a DHCP 735 client who is requesting a DHCPv4 or DHCPv6 address for its own use. 737 The client, in this context, may be requesting an IP address for 738 another entity, thus acting as a DHCP proxy. The client may be 739 requesting information about another client-to-address binding, using 740 the DHCPv4 [RFC4388] or DHCPv6 [RFC5007] Leasequery protocol. 742 In the rest of this section, the term "client" refers to an entity 743 communicating VSS information directly to a DHCPv4 or DHCPv6 server 744 without using the DHCPv4 relay-agent-information option or the DHCPv6 745 Relay-forward or Relay-reply messages, and there is no requirement 746 that such a client is a traditional DHCPv4 or DHCPv6 client 747 requesting an IP address binding for itself. 749 A DHCPv4 or DHCPv6 client will employ the VSS option to communicate 750 VSS information to their respective servers. This information MUST 751 be included in every message concerning any IP address on a different 752 VPN than the global or default VPN. A DHCPv4 client will place the 753 DHCPv4 VSS option in its packets, and a DHCPv6 client will place the 754 DHCPv6 VSS option in its messages. 756 A DHCPv6 client that needs to place a VSS option into a DHCPv6 757 message SHOULD place a single VSS option into the DHCPv6 message at 758 the same level as the Client Identifier option. A DHCPv6 client MUST 759 NOT include different VSS options in the same DHCPv6 message. 761 Note that, as mentioned in Section 1, throughout this document when a 762 DHCPv6 address is indicated the same information applies to DHCPv6 763 Prefix Delegation [RFC3633] as well. 765 Since this option is placed in the packet in order to change the VPN 766 on which an IP address is allocated for a particular DHCP client, one 767 presumes that an allocation on that VPN is necessary for correct 768 operation. Thus, a client which places this option in a packet and 769 doesn't receive it or receives a different value in a returning 770 packet SHOULD drop the packet since the IP address that was allocated 771 will not be in the requested VPN. 773 Clients should be aware that some DHCP servers will return a VSS 774 option with different values than that which was sent in. In 775 addition, a client may receive a response from a DHCP server with a 776 VSS option when none was sent in by the Client. 778 Note that when sending a DHCP Leasequery request, a relay agent is 779 acting as a DHCP client and so it SHOULD include the respective 780 DHCPv4 or DHCPv6 VSS option in its DHCPv4 or DHCPv6 Leasequery packet 781 if the DHCP Leasequery request is generated for other than the 782 default, global VPN. It SHOULD NOT include a DHCPv4 sub-option in 783 this case. 785 7. Server Behavior 787 A DHCP server receiving the VSS option or sub-option SHOULD allocate 788 an IP address (or use the VSS information to access an already 789 allocated IP address) from the VPN specified by the included VSS 790 information. 792 In the case where the type field of the VSS option or sub-option is 793 255, the VSS option denotes the global, default VPN. In this case, 794 there is no explicit VSS information beyond the type field. 796 This document does not prescribe any particular address allocation 797 policy. A DHCP server may choose to attempt to allocate an address 798 using the VSS information and, if this is impossible, to not allocate 799 an address. Alternatively, a DHCP server may choose to attempt 800 address allocation based on the VSS information and, if that is not 801 possible, it may fall back to allocating an address on the global or 802 default VPN. This, of course, is also the apparent behavior of any 803 DHCP server which doesn't implement support for the VSS option and 804 sub-option. Thus, DHCP clients and relay agents SHOULD be prepared 805 for either of these alternatives. 807 In some cases, a DHCP server may use the Virtual Subnet Selection 808 sub-option or option to inform a relay agent that a particular DHCP 809 client is associated with a particular VPN. It does this by sending 810 the Virtual Subnet Selection sub-option or option with the 811 appropriate information to the relay agent in the relay-agent- 812 information option for DHCPv4 or the Relay-reply message in DHCPv6. 814 In this situation, the relay agent will place the client in the 815 proper VPN, and then it will insert a VSS option or sub-option in 816 subsequent forwarded requests. The DHCP server will see this VSS 817 information and since it doesn't conflict in any way with the 818 server's notion of the VPN on which the client is supposed to reside, 819 it will process the requests based on the VPN specified in the VSS 820 option or sub-option, and echo the same VSS information in the 821 outgoing replies. 823 The relay agent receiving a reply containing a VSS option should 824 support the VSS option. Otherwise the relay agent will end up 825 attempting to use the address as though it were a global address. 826 Should this happen, the subsequent DHCPREQUEST will not contain any 827 VSS information, in which case the DHCP server SHOULD NOT respond 828 with a DHCPACK. 830 If a server uses a different VPN than what was specified in the VSS 831 option or sub-option, it SHOULD send back the VPN information using 832 the same type as the received type. It MAY send back a different type 833 if it is not possible to use the same type (such as the RFC2685 VPN- 834 ID if no ASCII VPN identifier exists). 836 A server which receives a VSS sub-option in the DHCPv4 relay-agent- 837 information option and does not receive a VSS-Control sub-option in 838 the relay-agent-information option MUST process the information 839 specified in the VSS sub-option in the same fashion as it would have 840 if it received both sub-options. 842 7.1. Returning the DHCPv4 or DHCPv6 Option 844 DHCPv4 or DHCPv6 servers receiving a VSS option (for sub-option 845 processing, see below) MUST return an instance of this option in the 846 reply packet or message if the server successfully uses this option 847 to allocate an IP address, and it MUST NOT include an instance of 848 this option if the server is unable to support, is not configured to 849 support, or does not implement support for VSS information in general 850 or the requested VPN in particular. 852 If they echo the option (based on the criteria above), servers SHOULD 853 return an exact copy of the option unless they desire to change the 854 VPN on which a client was configured. 856 The appearance of the DHCPv4 VSS option code in the DHCPv4 Parameter 857 Request List option [RFC2132] should not change the processing or 858 decision to return or not return the VSS option as specified in this 859 document. The appearance of the DHCPv6 VSS option in the OPTION_ORO 860 [RFC3315] or the OPTION_ERO [RFC4994] should not change the 861 processing or decision to return (or not to return) the VSS option as 862 specified in this document. 864 7.2. Returning the DHCPv4 Sub-Option 866 The case of the DHCPv4 sub-option is a bit more complicated. Note 867 that [RFC3046] specifies that a DHCPv4 server which supports the 868 relay-agent-information option SHALL copy all sub-options received in 869 a relay-agent-information option into any outgoing relay-agent- 870 information option. Thus, the default behavior for any DHCPv4 server 871 is to return any VSS sub-option received to the relay agent whether 872 or not the DHCPv4 server understands the VSS sub-option. 874 In order to distinguish a DHCPv4 server which is simply copying 875 relay-agent-information option sub-options from an incoming to an 876 outgoing relay-agent-informaion option from one which successfully 877 acted upon the information in the VSS sub-option, DHCPv4 relay agents 878 MUST include a VSS-Control sub-option in the relay-agent-information 879 any time that it includes a VSS sub-option in the relay-agent- 880 information option. 882 A DHCPv4 server which does not support the VSS sub-option will copy 883 both sub-options into the outgoing relay-agent-information option, 884 thus signalling to the DHCPv4 relay agent that it did not understand 885 the VSS sub-option. 887 A DHCPv4 server which supports the VSS sub-option: 889 o MUST copy the VSS sub-option into the outgoing relay-agent- 890 information option 892 o MUST NOT copy the VSS-Control sub-option into the outgoing 893 relay-agent-information option 895 Moreover, if a server uses different VSS information to allocate an 896 IP address than it receives in a particular DHCPv4 sub-option, it 897 MUST include that alternative VSS information in the VSS sub-option 898 that it returns to the DHCPv4 relay agent instead of the original VSS 899 information it was given. 901 If a DHCPv4 server supports this sub-option and for some reason 902 (perhaps administrative control) does not honor this sub-option from 903 the request then it MUST NOT echo either sub-option into the outgoing 904 relay-agent-information option. 906 7.3. Making sense of conflicting VSS information 908 It is possible for a DHCPv4 server to receive both a VSS option and 909 VSS sub-options in the same packet. Likewise, a DHCPv6 server can 910 receive multiple VSS options in nested Relay-forward messages as well 911 as in the client message itself. In either of these cases, the VSS 912 information from the relay agent closest to the DHCP server SHOULD be 913 used in preference to all other VSS information received. In the 914 DHCPv4 case, this means that the VSS sub-option takes precedence over 915 the VSS option, and in the DHCPv6 case, this means that the VSS 916 option from the outer-most Relay-forward message in which a VSS 917 option appears takes precedence. 919 The reasoning behind this approach is that the relay-agent closer to 920 the DHCP server is almost certainly more trusted than the DHCP client 921 or more distant relay agents, and therefore information in the 922 relay-agent-information option or the Relay-forward message is more 923 likely to be correct. 925 In general, relay agents SHOULD be aware through configuration or 926 policy external to this document whether or not they should be 927 including VSS information in packets that they forward and so there 928 should not be conflicts among relay agent specified VSS information. 930 In these situations where multiple VSS option or sub-options appear 931 in the incoming packet or message, when the DHCP server constructs 932 the response to be sent to the DHCP client or relay agent, all 933 existing VSS options or sub-options MUST be replicated in the 934 appropriate places in the response and MUST contain only the VSS 935 information that was used by the DHCP server to allocate the IP 936 address (with, of course, the exception of a DHCPv4 relay-agent- 937 information sub-option VSS-Control). 939 8. Updates to RFC 3046 941 This document updates the specification of the Relay Agent 942 Information option in RFC 3046 as follows: 944 Change the first sentence, second paragraph, section 2.2 of RFC 3046: 946 o OLD: 948 DHCP servers claiming to support the Relay Agent Information 949 option SHALL echo the entire contents of the Relay Agent 950 Information option in all replies. 952 o NEW: 954 DHCP servers claiming to support the Relay Agent Information 955 option SHALL echo the entire contents of the Relay Agent 956 Information option in all replies, except if otherwise specified 957 in the definition of specific Relay Agent Information sub- 958 options. 960 9. Security 962 Message authentication in DHCPv4 for intradomain use where the out- 963 of-band exchange of a shared secret is feasible is defined in 964 [RFC3118]. Potential exposures to attack are discussed in Section 7 965 of the DHCP protocol specification in [RFC2131]. 967 Implementations should consider using the DHCPv4 Authentication 968 option [RFC3118] to protect DHCPv4 client access in order to provide 969 a higher level of security if it is deemed necessary in their 970 environment. 972 Message authentication in DHCPv4 relay agents as defined in [RFC4030] 973 should be considered for DHCPv4 relay agents employing this sub- 974 option. Potential exposures to attack are discussed in Section 7 of 975 the DHCP protocol specification in [RFC2131]. 977 For DHCPv6 use of the VSS option, the "Security Considerations" 978 Section of [RFC3315] details the general threats to DHCPv6, and thus 979 to messages using the VSS option. The "Authentication of DHCP 980 Messages" Section of [RFC3315] describes securing communication 981 between relay agents and servers, as well as clients and servers. 983 The VSS option could be used by a client in order to obtain an IP 984 address from any VPN. This option would allow a client to perform a 985 more complete address-pool exhaustion attack since the client would 986 no longer be restricted to attacking address-pools on just its local 987 subnet. 989 A DHCP server that implements these options and sub-option should be 990 aware of this possibility and use whatever techniques that can be 991 devised to prevent such an attack. Information such as the giaddr in 992 DHCPv4 or link address in the Relay-forward DHCPv6 message might be 993 used to detect and prevent this sort of attack. 995 One possible defense would be for the DHCP relay to insert a VSS 996 option or sub-option to override the DHCP client's VSS option. 998 Servers that implement the VSS option and sub-option MUST by default 999 disable use of the feature; it must specifically be enabled through 1000 configuration. Moreover, a server SHOULD provide the ability to 1001 selectively enable use of the feature under restricted conditions, 1002 e.g., by enabling use of the option only from explicitly configured 1003 client-ids, enabling its use only by clients on a particular subnet, 1004 or restricting the VSSs from which addresses may be requested. 1006 10. IANA Considerations 1008 IANA is requested to assign DHCPv4 option number 221 for the DHCPv4 1009 VSS option defined in Section 3.1, in accordance with [RFC3942]. 1011 IANA is requested to assign sub-option number 151 for the DHCPv4 VSS 1012 sub-option defined in Section 3.2 from the DHCP Relay Agent Sub- 1013 options space [RFC3046], in accordance with the spirit of [RFC3942]. 1014 While [RFC3942] doesn't explicitly mention the sub-option space for 1015 the DHCP Relay Agent Information option [RFC3046], sub-option 151 is 1016 already in use by existing implementations of this sub-option and the 1017 current draft is essentially upward compatible with these current 1018 implementations. 1020 IANA is requested to assign the value of TBD for the DHCPv4 VSS- 1021 Control sub-option defined in Section 3.3. 1023 IANA is requested to assign the value of TBD for the DHCPv6 VSS 1024 option defined in Section 3.4 from the DHCPv6 option registry. 1026 The type byte defined in Section 3.5 defines a number space for which 1027 IANA is to create and maintain a new sub-registry entitled "VSS Type 1028 values". This sub-registry needs to be related to both the DHCPv4 1029 and DHCPv6 VSS options and the DHCPv4 relay-agent-information option 1030 sub-option (all defined by this document), since the type byte in 1031 these two options and one sub-option MUST have identical definitions. 1033 New values for the type byte may only be defined by IETF Consensus, 1034 as described in [RFC5226]. Basically, this means that they are 1035 defined by RFCs approved by the IESG. 1037 11. Acknowledgments 1039 Bernie Volz recommended consolidation of the DHCPv4 option and sub- 1040 option drafts after extensive review of the former drafts, and 1041 provided valuable assistance in structuring and reviewing this 1042 document. Alper Yegin expressed interest in the DHCPv6 VSS option, 1043 resulting in this combined draft covering all three areas. Alfred 1044 Hoenes provided assistance with editorial review as well as raising 1045 substantive protocol issues. David Hankins and Bernie Volz each 1046 raised important protocol issues which resulted in a clarified 1047 document. Josh Littlefield provided editorial assistance. Several 1048 IESG reviewers took the time to substantially review this document, 1049 resulting in much increased clarity. 1051 12. References 1053 12.1. Normative References 1055 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1056 Requirement Levels", RFC 2119, March 1997. 1058 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, 1059 March 1997. 1061 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor 1062 Extensions", RFC 2132, March 1997. 1064 [RFC2685] Fox, B., Gleeson, B., "Virtual Private Networks 1065 Identifier", RFC 2685, September 1999. 1067 [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 1068 3046, January 2001. 1070 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and 1071 M. Carney, "Dynamic Host Configuration Protocol for IPv6 1072 (DHCPv6)", RFC 3315, July 2003. 1074 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 1075 Host Configuration Protocol (DHCP) version 6", RFC 3633, December 1076 2003. 1078 [RFC4994] Zeng, S., Volz, B., Kinnear, K. and J. Brzozowski, "DHCPv6 1079 Relay Agent Echo Request Option", RFC 4994, September 2007. 1081 12.2. Informative References 1083 [RFC951] Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951, 1084 September 1985. 1086 [RFC1542] Wimer, W., "Clarifications and Extensions for the Bootstrap 1087 Protocol", RFC 1542, October 1993. 1089 [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP 1090 Messages", RFC 3118, June 2001. 1092 [RFC3942] Volz, B., "Reclassifying Dynamic Host Configuration 1093 Protocol version 4 (DHCPv4) Options", RFC 3942, November 2004. 1095 [RFC4030] Stapp, M. and T. Lemon, "The Authentication Suboption for 1096 the Dynamic Host Configuration Protocol (DHCP) Relay Agent 1097 Option", RFC 4030, March 2005. 1099 [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration 1100 Protocol (DHCP) Leasequery", RFC 4388, February 2006. 1102 [RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng, "DHCPv6 1103 Leasequery", RFC 5007, September 2007. 1105 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1106 IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 1108 Authors' Addresses 1110 Kim Kinnear 1111 Cisco Systems 1112 1414 Massachusetts Ave. 1113 Boxborough, Massachusetts 01719 1115 Phone: (978) 936-0000 1117 EMail: kkinnear@cisco.com 1119 Richard Johnson 1120 Cisco Systems 1121 170 W. Tasman Dr. 1122 San Jose, CA 95134 1124 Phone: (408) 526-4000 1126 EMail: raj@cisco.com 1128 Mark Stapp 1129 Cisco Systems 1130 1414 Massachusetts Ave. 1131 Boxborough, Massachusetts 01719 1133 Phone: (978) 936-0000 1134 EMail: mjs@cisco.com 1136 Jay Kumarasamy