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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: April 22, 2011 Cisco Systems 6 Jay Kumarasamy 7 October 22, 2010 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) 2010 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. DHCPv6 Virtual Subnet Selection Option..................... 6 80 3.4. Virtual Subnet Selection Type and Information.............. 7 81 4. Overview of Virtual Subnet Selection Usage................... 8 82 4.1. VPN assignment by the DHCP relay agent..................... 9 83 4.2. VPN assignment by the DHCP server.......................... 12 84 4.3. Required Support........................................... 14 85 4.4. Alternative VPN assignment approaches...................... 14 86 5. Relay Agent Behavior......................................... 14 87 5.1. VPN assignment by the DHCP server.......................... 16 88 5.2. DHCP Leasequery............................................ 17 89 6. Client Behavior.............................................. 17 90 7. Server Behavior.............................................. 18 91 7.1. Returning the DHCPv4 or DHCPv6 Option...................... 19 92 7.2. Returning the DHCPv4 Sub-Option............................ 20 93 7.3. Making sense of conflicting VSS information................ 21 94 8. Security..................................................... 21 95 9. IANA Considerations.......................................... 22 96 10. Acknowledgments............................................. 23 97 11. References.................................................. 23 98 11.1. Normative References...................................... 23 99 11.2. Informative References.................................... 24 101 1. Introduction 103 There is a growing use of Virtual Private Network (VPN) 104 configurations. The growth comes from many areas; individual client 105 systems needing to appear to be on the home corporate network even 106 when traveling, ISPs providing extranet connectivity for customer 107 companies, etc. In some of these cases there is a need for the DHCP 108 server to know the VPN (hereafter called a "Virtual Subnet Selector" 109 or "VSS") from which an address, and other resources, should be 110 allocated. 112 This memo defines a Virtual Subnet Selection (VSS) option for each of 113 DHCPv4 and DHCPv6, and a VSS sub-option carried in the DHCPv4 relay- 114 agent-information option. These are intended for use by DHCP 115 clients, relay agents, and proxy clients in situations where VSS 116 information needs to be passed to the DHCP server for proper address 117 or prefix allocation to take place. If the receiving DHCP server 118 understands the VSS option or sub-option, this information may be 119 used in conjunction with other information in determining the subnet 120 on which to select an address as well as other information such as 121 DNS server, default router, etc. 123 If the allocation is being done through a DHCPv4 relay, then the 124 relay sub-option defined here should be included. In some cases, 125 however, an IP address is being sought by a DHCPv4 proxy on behalf of 126 a client (which may be assigned the address via a different 127 protocol). In this case, there is a need to include VSS information 128 relating to the client as a DHCPv4 option. 130 If the allocation is being done through a DHCPv6 relay, then the 131 DHCPv6 VSS option defined in this document should be included in the 132 Relay-forward and Relay-reply message going between the DHCPv6 relay 133 and server. In some cases, addresses or prefixes are being sought by 134 a DHCPv6 proxy on behalf of a client. In this case, there is a need 135 for the client itself to supply the VSS information using the DHCPv6 136 VSS option in the messages that it sends to the DHCPv6 server. 138 In the remaining text of this document, when a DHCPv6 address is 139 indicated the same information applies to DHCPv6 Prefix Delegation 140 [RFC3633] as well. 142 In the remaining text of this document, when the term VSS sub-option 143 is used, it refers to the VSS sub-option carried in the DHCPv4 144 relay-agent-information option. 146 2. Terminology 148 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 149 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 150 document are to be interpreted as described in RFC 2119 [RFC2119]. 152 This document uses the following terms: 154 o "DHCP client" 156 A DHCP client is a host using DHCP to obtain configuration 157 parameters such as a network address. 159 o "DHCP proxy" 161 A DHCP proxy is a DHCP client which acquires IP addresses not 162 for its own use, but rather on behalf of another entity. There 163 are a variety of ways that a DHCP proxy can supply the addresses 164 it acquires to other entities that need them. 166 o "DHCP relay agent" 168 A DHCP relay agent is an agent that transfers BOOTP and DHCP 169 messages between clients and servers residing on different 170 subnets, per [RFC951], [RFC1542], and [RFC3315]. 172 o "DHCP server" 174 A DHCP server is a host that returns configuration parameters to 175 DHCP clients. 177 o "DHCPv4 option" 179 An option used to implement a capability defined by the DHCPv4 180 RFCs [RFC2131][RFC2132]. These options have one-octet code and 181 size fields. 183 o "DHCPv4 sub-option" 185 As used in this document, a DHCPv4 sub-option refers to a sub- 186 option of the relay-agent-information option [RFC3046]. These 187 sub-options have one-octet code and size fields. 189 o "DHCPv6 option" 191 An option used to implement a capability defined by the DHCPv6 192 RFC [RFC3315]. These options have two-octet code and size 193 fields. 195 o "Global VPN" 197 Indicates that the address being described belongs to the set of 198 addresses not part of any VPN. In other words, the normal 199 address space operated on by DHCP. This includes private 200 addresses, for example the 10.x.x.x addresses as well as the 201 other private subnets that are not routed on the open internet. 203 o "VSS information" 205 Information about a VPN necessary to allocate an address to a 206 DHCP client on that VPN and necessary to forward a DHCP reply 207 packet to a DHCP client on that VPN. 209 o "VPN" 211 Virtual private network. A network which appears to the client 212 to be a private network. 214 o "VPN Identifier" 216 The VPN-ID is defined by [RFC2685] to be a sequence of 7 octets. 218 3. Virtual Subnet Selection Option and Sub-Options Definitions 220 The Virtual Subnet Selection options and sub-options contain a 221 generalized way to specify the VSS information about a VPN. There 222 are two options and one sub-option defined in this section. The 223 actual VSS information is identical in each. 225 3.1. DHCPv4 Virtual Subnet Selection Option 227 The format of the option is: 229 0 1 2 3 230 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 1 231 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 232 | Code | Length | Type | VSS Info ... 233 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 235 Code The option code (221). 237 Length The option length, minimum 1 octets. 239 Type and VSS Information -- see Section 3.4 241 3.2. DHCPv4 Virtual Subnet Selection Sub-Option 243 This is a sub-option of the relay-agent-information option [RFC3046]. 244 The format of the sub-option is: 246 0 1 2 3 247 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 1 248 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 249 | Code | Length | Type | VSS Info. ... 250 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 252 Code The sub-option code (151). 254 Length The option length, minimum 1 octets. 256 Type and VSS Information -- see Section 3.4 258 3.3. DHCPv6 Virtual Subnet Selection Option 260 The format of the DHCPv6 Virtual Subnet Selection option is shown 261 below. This option may be included by a client or relay-agent (or 262 both). 264 0 1 2 3 265 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 1 266 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 267 | OPTION_VSS | option-len | 268 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 269 | Type | VSS Information ... | 270 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 272 option-code OPTION_VSS (TBD). 274 option-len The number of octets in the option, minimum 1. 276 Type and VSS Information -- see Section 3.4 278 3.4. Virtual Subnet Selection Type and Information 280 All of the (sub)options defined above carry identical payloads, 281 consisting of a type and additional VSS information as follows: 283 Type VSS Information format: 285 0 NVT ASCII VPN identifier 286 1 RFC2685 VPN-ID 287 2-252 Reserved 288 253 CONTROL (DHCPv4 VSS sub-option only) 289 254 Reserved 290 255 Global, default VPN. 292 o Type 0 -- NVT ASCII VPN identifier 294 Indicates that the VSS information consists of a NVT ASCII 295 string. It MUST NOT be terminated with a zero byte. 297 o Type 1 -- RFC2685 VPN-ID 299 Indicates that the VSS information consists of an RFC2685 VPN-ID 300 [RFC2685], which is defined to be 7 octets in length. 302 o Type 253 -- CONTROL 304 This is only valid for the DHCPv4 relay-agent-information option 305 sub-option. It indicates that another DHCPV4 VSS sub-option is 306 present in the relay-agent-information option. The sub-option 307 with type CONTROL MUST be removed by any DHCPv4 server which 308 successfully processes the information in the other DHCPv4 sub- 309 option with valid VSS information. In this case, there MUST NOT 310 be any VSS Information included in the sub-option, and the 311 length of the VSS sub-option MUST be 1. 313 o Type 255 -- Global, default VPN 315 Indicates that there is no explicit, non-default VSS information 316 but rather that this option references the normal, global, 317 default address space. In this case, there MUST NOT be any VSS 318 Information included in the VSS option or sub-option and the 319 length of the MUST be 1. 321 All other values of the Type field are reserved. 323 4. Overview of Virtual Subnet Selection Usage 325 At the highest level, the VSS option or sub-option determines the VPN 326 on which a DHCP client is supposed to receive an IP address. How the 327 option or sub-option is entered and processed is discussed below, but 328 the point of all of the discussion is to determine the VPN on which 329 the DHCP client resides. This will affect a relay agent, in that it 330 will have to ensure that DHCP packets sent to and received from the 331 DHCP client flow over the correct VPN. This will affect the DHCP 332 server in that it determines the IP address space used for the IP 333 address allocation. 335 A DHCP server has as part of its configuration some IP address space 336 from which it allocates IP addresses to DHCP clients. These 337 allocations are typically for a limited time, and thus the DHCP 338 client gets a lease on the IP address. In the absence of any VPN 339 information, the IP address space is in the global or default VPN 340 used throughout the Internet. When a DHCP server deals with VPN 341 information, each VPN defines a new address space inside the server, 342 one distinct from the global or default IP address space. A server 343 which supports the VSS option or sub-option thereby supports 344 allocation of IP addresses from multiple different VPNs. Supporting 345 IP address allocation from multiple different VPNs means that the 346 DHCP server must be prepared to configure multiple different address 347 spaces (one per distinct VPN) and allocate IP addresses from these 348 different address spaces. 350 These address spaces are typically independent, so that the same IP 351 address (consisting of the same string of bytes) could be allocated 352 to one client in the global, default VPN, and to a different client 353 residing in a different VPN. There is no conflict in this 354 allocation, since the clients have essentially different addresses, 355 even though these addresses consist of the same string of bytes, 356 because the IPv4 or IPv6 address is qualified by the VPN. 358 Thus a VSS option or sub-option is a way of signaling the use of a 359 VPN other than the global or default VPN. The next question is: who 360 decides what VPN a DHCP client should be using? 362 There are three entities which can either insert a VSS option or 363 sub-option into a DHCPv4 packet or DHCPv6 message; a DHCP client, a 364 relay agent, or a DHCPv4 or DHCPv6 server. While all of these 365 entities could include a different VSS option or sub-option in every 366 request or response, this situation is neither typical nor useful. 367 There are two known paradigms for use of the VSS option or sub- 368 option, which are discussed below. 370 4.1. VPN assignment by the DHCP relay agent 372 The typical use of the VSS option or sub-option is for the relay 373 agent to know the VPN on which the DHCP client is operating. The 374 DHCP client itself does not, in this approach, know the VPN on which 375 it resides. The relay agent is responsible for mediating the access 376 between the VPN on which the DHCP client resides and the DHCP server. 377 In this situation, the relay agent will insert two DHCPv4 VSS sub- 378 options (one with valid VSS information, and one with type CONTROL) 379 into the relay-agent-information option or a DHCPv6 VSS option into 380 the Relay-forward message of every request it forwards from the DHCP 381 client. The server will use the VSS option or sub-option to 382 determine the VPN on which the client resides, and use that VPN 383 information to select the address space within its configuration from 384 which to allocate an IP address to the DHCP client. 386 When, using this approach, a DHCPv4 relay agent inserts a VSS sub- 387 option with containing VSS information it MUST also insert a VSS 388 sub-option containing type CONTROL, no additional VSS information, 389 and a length of 1. This is to allow determination of whether or not 390 the DHCPv4 server actually processes the VSS information provided by 391 the DHCPv4 relay agent. If the DHCPv4 server supports the VSS 392 capabilities described in this document, it will remove the VSS sub- 393 option with type CONTROL from the relay-agent-information option that 394 it returns to the DHCPv4 relay agent. See Section 5 for more 395 information. 397 In this approach, the relay agent might also send a VSS option or 398 sub-option in either a DHCPv4 or DHCPv6 Leasequery request, but in 399 this case, it would use the VSS option in the Leasequery request to 400 select the correct address space for the Leasequery. In this 401 approach, the relay agent would be acting as a DHCP client from a 402 Leasequery standpoint, but it would not be as if a DHCP client were 403 sending in a VSS option in a standard DHCP address allocation 404 request, say a DHCPDISCOVER. 406 In this approach, only one relay agent would mediate the VPN access 407 for the DHCP client to the DHCP server, and it would be the relay 408 agent which inserts the VSS information into the request packet and 409 would remove it prior to forwarding the response packet on. 411 In the diagram below is an example of a DHCPv4 client, DHCPv4 relay 412 agent, and DHCPv4 server. The DHCPv6 situation is similar, but uses 413 the DHCPv6 VSS option. 415 DHCPv4 416 DHCPv4 Relay DHCPv4 417 Client Agent Server 419 | | | 420 | >--DHCPDISCOVER--> | | 421 | on VRF "abc" | | 422 | | >--DHCPDISCOVER----> | 423 | | relay-agent-info: | 424 | | VSS type VRF:"abc"| 425 | | VSS type CONTROL | 426 | | | 427 | | <----DHCPOFFER-----< | 428 | | relay-agent-info: | 429 | | VSS type VRF:"abc"| 430 | | | 431 | <---DHCPOFFER----< | | 432 | on VRF "abc" | | 433 | | | 434 | >--DHCPREQUEST---> | | 435 | on VRF "abc" | | 436 | | >--DHCPREQUEST-----> | 437 | | relay-agent-info: | 438 | | VSS type VRF:"abc"| 439 | | VSS type CONTROL | 440 | | | 441 | | <----DHCPACK-------< | 442 | | relay-agent-info: | 443 | | VSS type VRF:"abc"| 444 | | | 445 | <---DHCPACK------< | | 446 | on VRF "abc" | | 447 | | | 448 ... ... ... 450 Figure 4.1-1: DHCPv4 - Relay Agent knows VPN 452 The DHCP server would know that it should respond to VPN information 453 specified in a VSS option or sub-option, and it would be configured 454 with appropriate VPN address spaces to service the projected client 455 requirements. Thus, in this common approach, the DHCP client knows 456 nothing of any VPN access, the relay agent has been configured in 457 some way that allows it to determine the VPN of the DHCP client and 458 transmit that using a VSS option or sub-option to the DHCP server, 459 and the DHCP server responds to the VPN specified by the relay agent. 460 There is no conflict between different entities trying to specify 461 different VSS information -- each entity knows its role through 462 policy or configuration external to this document. 464 If any mis-configuration exists, it SHOULD result in a DHCP client 465 being unable to acquire an IP address. For instance, a relay agent 466 which supports VPN access SHOULD couple transmission of VSS options 467 or sub-options to the configuration of VPN support, and not allow one 468 without the other. 470 It is important to ensure that the relay agent and DHCP server both 471 support the VSS option and sub-option (for DHCPv4) or the VSS option 472 (for DHCPv6). Deploying DHCPv4 relay agents which support and emit 473 VSS sub-options in concert with DHCPv4 servers which do not support 474 the VSS option or sub-option as defined in this document SHOULD NOT 475 be done, as such an ensemble will not operate correctly. Should this 476 situation occur, however, the relay agent can detect the problem 477 (since the VSS sub-option with type CONTROL will appear in the 478 packets it receives from the DHCPv4 server), and it can issue 479 appropriate diagnostic messages. 481 4.2. VPN assignment by the DHCP server 483 In this approach, the DHCP server would be configured in some way to 484 know the VPN on which a particular DHCP client should be given 485 access. The DHCP server would in this case include the VSS sub- 486 option in the relay-agent-information option for DHCPv4 or the VSS 487 option in the Relay-reply message for DHCPv6. The relay agent 488 responsible for mediating VPN access would use this information to 489 select the correct VPN for the DHCP client. In the unusal event that 490 there were more than one relay agent involved in this transaction, 491 some external configuration or policy would be needed to inform the 492 DHCPv6 server into which Relay-reply message the VSS option should 493 go. 495 Once the relay agent has placed the DHCP client into the proper VPN, 496 it SHOULD begin including VSS information in requests that it 497 forwards to the DHCP server. Since this information does not 498 conflict with the DHCP server's idea of the proper VPN for the 499 client, everything works correctly. 501 The diagram below shows this approach using DHCPv4. The DHCPv6 502 situation is similar, but uses the DHCPv6 VSS option instead. 504 DHCPv4 505 DHCPv4 Relay DHCPv4 506 Client Agent Server 508 | | | 509 | >--DHCPDISCOVER--> | | 510 | on unknown VPN | | 511 | | >--DHCPDISCOVER----> | 512 | | | 513 | | <----DHCPOFFER-----< | 514 | | relay-agent-info: | 515 | | VSS type VRF:"abc"| 516 | | | 517 | <---DHCPOFFER----< | | 518 | on VRF "abc" | | 519 | | | 520 | >--DHCPREQUEST---> | | 521 | on VRF "abc" | | 522 | | >--DHCPREQUEST-----> | 523 | | relay-agent-info: | 524 | | VSS type VRF:"abc"| 525 | | VSS type CONTROL | 526 | | | 527 | | <----DHCPACK-------< | 528 | | relay-agent-info: | 529 | | VSS type VRF:"abc"| 530 | | | 531 | <---DHCPACK------< | | 532 | on VRF "abc" | | 533 | | | 534 | | | 535 ... ... ... 537 Figure 4.2-1: DHCPv4 - DHCPv4 Server knows VPN 539 In this approach, the DHCP client is again unaware of any VPN 540 activity. In this case, however, the DHCP server knows the VPN for 541 the client, and the relay agent responds to the VSS information 542 specified by the DHCP server. Similar to the previous approach, each 543 entity knows its role through a means external to this document and 544 no two entities try to specify VSS information in conflict. 546 It is important that both the relay agent as well as the DHCP server 547 both support the VSS option and sub-option (for DHCPv4) and the VSS 548 option (for DHCPv6). Deploying and configuring VPN support in one 549 element and not in the other is not a practical approach. 551 4.3. Required Support 553 DHCP relay agents and servers MUST support the approach discussed in 554 Section 4.1. DHCP relay agents and server SHOULD support the 555 approach discussed in Section 4.2. DHCP relay agents and servers 556 SHOULD NOT be configured to operate with both approaches 557 simultaneously. 559 4.4. Alternative VPN assignment approaches 561 There are many other approaches which can be created with multiple 562 relay agents each inserting VSS information into different Relay- 563 forward messages, relay agent VSS information conflicting with client 564 VSS information, or DHCP server VSS information conflicting with 565 relay agent and client VSS information. Since these approaches do 566 not describe situations that are useful today, specifying precisely 567 how to resolve all of these conflicts is unlikely to be valuable in 568 the event that these approaches actually become practical in the 569 future. 571 The current use of the VSS option and sub-option require that each 572 entity knows the part that it plays in dealing with VPN data. Each 573 entity -- client, relay agent or agents, and server -- SHOULD know 574 through some policy or configuration beyond the scope of this 575 document whether it is responsible for specifying VPN information 576 using the VSS option or sub-option or responsible for responding to 577 VSS information specified by another entity, or simply ignoring any 578 VSS information which it might see. 580 Some simple conflict resolution approaches are discussed below, in 581 the hopes that they will cover simple cases that may arise from 582 situations beyond those envisioned today. However, for more complex 583 situations, or simple situations where appropriate conflict 584 resolution strategies differ from those discussed in this document, a 585 document detailing the usage situations and appropriate conflict 586 resolution strategies SHOULD be created and submitted for discussion 587 and approval. 589 5. Relay Agent Behavior 591 A relay agent which receives a DHCP request from a DHCP client on a 592 VPN SHOULD include Virtual Subnet Selection information in the DHCP 593 packet prior to forwarding the packet on to the DHCP server unless 594 inhibited from doing so by configuration information or policy to the 595 contrary. 597 In this situation, a DHCPv4 relay agent MUST include a DHCPv4 VSS 598 sub-option in a relay-agent-information option [RFC3046], while a 599 DHCPv6 relay agent MUST include a DHCPv6 VSS option in the Relay- 600 forward message. 602 The value placed in the Virtual Subnet Selection sub-option or option 603 would typically be sufficient for the relay agent to properly route 604 any DHCP reply packet returned from the DHCP server to the DHCP 605 client for which it is destined. In some cases, the information in 606 the VSS sub-option or option might be an index into some internal 607 table held in the relay agent, though this document places no 608 requirement on a relay agent to have any such internal state. 610 A DHCPv4 relay agent SHOULD, in addition, include a DHCPv4 VSS sub- 611 option with a type of CONTROL, no additional VSS information, and a 612 length of one, in the relay-agent-information option [RFC3046]. The 613 inclusion of two VSS sub-options in the relay-agent-information 614 option, one with valid VSS information, and one with a type of 615 CONTROL, will allow the DHCPv4 relay agent to determine whether the 616 DHCPv4 server actually processed the information in the VSS sub- 617 option containing valid VSS information. 619 The reason to include this additional VSS DHCPv4 sub-option is that 620 [RFC3046] specifies (essentially) that a DHCPv4 server should copy 621 all sub-options that it receives in a relay-agent-information option 622 in a request into a corresponding relay-agent-information option in 623 the response. Thus, a server that didn't support the DHCPv4 VSS 624 sub-option would normally just copy it to the response packet, 625 leaving the relay agent to wonder if in fact the DHCPv4 server 626 actually used the VSS information when processing the request. 628 To alleviate this potential confusion, a DHCPvr4 relay agent instead 629 sends in two VSS sub-options, one with valid VSS information, and one 630 with a VSS type of CONTROL. If both sub-options appear in the 631 response from the DHCPv4 server, then the DHCPv4 relay agent MUST 632 assume that the DHCPv4 server did not act on the valid VSS 633 information in one of the sub-options. If only the VSS sub-option 634 with the valid information appears in the response from the DHCPv4 635 server and no VSS sub-option with type CONTROL appears in the 636 response from the DHCPv4 server, then the relay agent SHOULD assume 637 that the DHCPv4 server acted successfully on the VSS sub-option with 638 the valid VSS information. 640 Anytime a relay agent places a VSS option or sub-option in a DHCP 641 request, it SHOULD send it only to a DHCP server which supports the 642 VSS option or sub-option, and it MUST check the response to determine 643 if the DHCP server actually honored the requested VSS information. 645 In the DHCPv6 case, the appearance of the option in the Relay-reply 646 packet indicates that the DHCPv6 server understood and acted upon the 647 contents of the VSS option in the Relay-forward packet. In the 648 DHCPv4 case, as discussed above, the appearance of the VSS sub-option 649 containing valid VSS information without the appearance of a VSS 650 sub-option of type CONTROL indicates that the DHCPv4 server 651 successfully acted upon the VSS sub-option that was returned 652 containing valid VSS information. 654 This document does not create a requirement that a relay agent 655 remember the contents of a VSS DHCPv4 sub-option or VSS DHCPv6 option 656 sent to a DHCP server. In many cases, the relay agent may simply use 657 the value of the VSS returned by the DHCP server to forward the 658 response to the DHCP client. If the VSS information, the IP address 659 allocated, and the VPN capabilities of the relay agent all 660 interoperate correctly, then the DHCP client will receive a working 661 IP address. Alternatively, if any of these items don't interoperate 662 with the others, the DHCP client will not receive a working address. 664 Note that in some environments a relay agent may choose to always 665 place a VSS option or sub-option into packets and messages that it 666 forwards in order to forestall any attempt by a relay agent closer to 667 the client or the client itself to specify VSS information. In this 668 case, a type field of 255 is used to denote the global, default VPN. 669 When the type field of 255 is used, there MUST NOT be any additional 670 VSS information in the VSS option or sub-option. In the DHCPv4 case, 671 an additional VSS sub-option with type CONTROL should be used, as 672 discussed above. 674 5.1. VPN assignment by the DHCP server 676 In some cases, a DHCP server may use the Virtual Subnet Selection 677 sub-option or option to inform a relay agent that a particular DHCP 678 client is associated with a particular VPN. It does this by sending 679 the Virtual Subnet Selection sub-option or option with the 680 appropriate information to the relay agent in the relay-agent- 681 information option for DHCPv4 or the Relay-reply message in DHCPv6. 682 If the relay agent cannot respond correctly to the DHCP server's 683 requirement to place the DHCP client into that VPN (perhaps because 684 it has not been configured with a VPN that matches the VSS 685 information received from the DHCP server) it MUST drop the packet 686 and not send it to the DHCP client. 688 In this situation, once the relay agent has placed the DHCP client 689 into the VPN specified by the DHCP server, it will insert a VSS 690 option or sub-option when forwarding packets from the client. The 691 DHCP server in normal operation will echo this VSS information into 692 the outgoing replies. 694 In the event that the relay agent doesn't include VSS information on 695 subsequent requests after the DHCP server has included VSS 696 information in a reply to the relay agent, the DHCP server can 697 conclude that the relay agent doesn't support VSS processing, and the 698 DHCP server SHOULD stop processing this transaction and not respond 699 to the request. 701 5.2. DHCP Leasequery 703 Sometimes a relay-agent needs to submit a DHCP Leasequery [RFC4388] 704 [RFC5007] packet to the DHCP server in order to recover information 705 about existing DHCP allocated IP addresses on other than the normal, 706 global VPN. In the context of a DHCP Leasequery the relay agent is a 707 direct client of the DHCP server and is not relaying a packet for 708 another DHCP client. Thus, the instructions in Section 6 on Client 709 Behavior should be followed to include the necessary VSS information. 711 6. Client Behavior 713 Typically, DHCPv4 and DHCPv6 clients have no interaction with VSS 714 options or sub-options. The VSS information is handled by exchanges 715 between a DHCPv4 or DHCPv6 relay agent and the corresponding DHCPv4 716 or DHCPv6 server. 718 However, there are times when an entity is acting as a DHCPv4 or 719 DHCPv6 client in that it is communicating directly with a DHCPv4 or 720 DHCPv6 server. In these instances -- where communications is 721 occurring without employing the DHCPv4 relay-agent-information option 722 or the DHCPv6 Relay-forward or Relay-reply messages, the entity is 723 acting as a DHCPv4 or DHCPv6 client with regard to its communication 724 with the DHCPv4 or DHCPv6 server, but not necessarily as a DHCP 725 client who is requesting a DHCPv4 or DHCPv6 address for its own use. 727 The client, in this context, may be requesting an IP address for 728 another entity, thus acting as a DHCP proxy. The client may be 729 requesting information about another client-to-address binding, using 730 the DHCPv4 [RFC4388] or DHCPv6 [RFC5007] Leasequery protocol. 732 In the rest of this section, the term "client" refers to an entity 733 communicating VSS information directly to a DHCPv4 or DHCPv6 server 734 without using the DHCPv4 relay-agent-information option or the DHCPv6 735 Relay-forward or Relay-reply messages, and there is no requirement 736 that such a client is a traditional DHCPv4 or DHCPv6 client 737 requesting an IP address binding for itself. 739 A DHCPv4 or DHCPv6 client will employ the VSS option to communicate 740 VSS information to their respective servers. This information MUST 741 be included in every message concerning any IP address on a different 742 VPN than the global or default VPN. A DHCPv4 client will place the 743 DHCPv4 VSS option in its packets, and a DHCPv6 client will place the 744 DHCPv6 VSS option in its messages. 746 A DHCPv6 client that needs to place a VSS option into a DHCPv6 747 message SHOULD place a single VSS option into the DHCPv6 message at 748 the same level as the Client Identifier option. A DHCPv6 client MUST 749 NOT include different VSS options in the same DHCPv6 message. 751 Note that, as mentioned in Section 1, throughout this document when a 752 DHCPv6 address is indicated the same information applies to DHCPv6 753 Prefix Delegation [RFC3633] as well. 755 Since this option is placed in the packet in order to change the VPN 756 on which an IP address is allocated for a particular DHCP client, one 757 presumes that an allocation on that VPN is necessary for correct 758 operation. Thus, a client which places this option in a packet and 759 doesn't receive it or receives a different value in a returning 760 packet SHOULD drop the packet since the IP address that was allocated 761 will not be in the requested VPN. 763 Clients should be aware that some DHCP servers will return a VSS 764 option with different values than that which was sent in. In 765 addition, a client may receive a response from a DHCP server with a 766 VSS option when none was sent in by the Client. 768 Note that when sending a DHCP Leasequery request, a relay agent is 769 acting as a DHCP client and so it SHOULD include the respective 770 DHCPv4 or DHCPv6 VSS option in its DHCPv4 or DHCPv6 Leasequery packet 771 if the DHCP Leasequery request is generated for other than the 772 default, global VPN. It SHOULD NOT include a DHCPv4 sub-option in 773 this case. 775 7. Server Behavior 777 A DHCP server receiving the VSS option or sub-option SHOULD allocate 778 an IP address (or use the VSS information to access an already 779 allocated IP address) from the VPN specified by the included VSS 780 information. 782 In the case where the type field of the VSS option or sub-option is 783 255, the VSS option denotes the global, default VPN. In this case, 784 there is no explicit VSS information beyond the type field. 786 This document does not prescribe any particular address allocation 787 policy. A DHCP server may choose to attempt to allocate an address 788 using the VSS information and, if this is impossible, to not allocate 789 an address. Alternatively, a DHCP server may choose to attempt 790 address allocation based on the VSS information and, if that is not 791 possible, it may fall back to allocating an address on the global or 792 default VPN. This, of course, is also the apparent behavior of any 793 DHCP server which doesn't implement support for the VSS option and 794 sub-option. Thus, DHCP clients and relay agents SHOULD be prepared 795 for either of these alternatives. 797 In some cases, a DHCP server may use the Virtual Subnet Selection 798 sub-option or option to inform a relay agent that a particular DHCP 799 client is associated with a particular VPN. It does this by sending 800 the Virtual Subnet Selection sub-option or option with the 801 appropriate information to the relay agent in the relay-agent- 802 information option for DHCPv4 or the Relay-reply message in DHCPv6. 804 In this situation, the relay agent will place the client in the 805 proper VPN, and then it will insert a VSS option or sub-option in 806 subsequent forwarded requests. The DHCP server will see this VSS 807 information and since it doesn't conflict in any way with the 808 server's notion of the VPN on which the client is supposed to reside, 809 it will process the requests based on the VPN specified in the VSS 810 option or sub-option, and echo the same VSS information in the 811 outgoing replies. 813 The relay agent receiving a reply containing a VSS option should 814 support the VSS option. Otherwise the relay agent will end up 815 attempting to use the address as though it were a global address. 816 Should this happen, the subsequent DHCPREQUEST will not contain any 817 VSS information, in which case the DHCP server SHOULD NOT respond 818 with a DHCPACK. 820 If a server uses a different VPN than what was specified in the VSS 821 option or sub-option, it SHOULD send back the VPN information using 822 the same type as the received type. It MAY send back a different type 823 if it is not possible to use the same type (such as the RFC2685 VPN- 824 ID if no ASCII VPN identifier exists). 826 7.1. Returning the DHCPv4 or DHCPv6 Option 828 DHCPv4 or DHCPv6 servers receiving a VSS option (for sub-option 829 processing, see below) MUST return an instance of this option in the 830 reply packet or message if the server successfully uses this option 831 to allocate an IP address, and it MUST NOT include an instance of 832 this option if the server is unable to support, is not configured to 833 support, or does not implement support for VSS information in general 834 or the requested VPN in particular. 836 If they echo the option (based on the criteria above), servers SHOULD 837 return an exact copy of the option unless they desire to change the 838 VPN on which a client was configured. 840 The appearance of the DHCPv4 VSS option code in the DHCPv4 Parameter 841 Request List option [RFC2132] should not change the processing or 842 decision to return or not return the VSS option as specified in this 843 document. The appearance of the DHCPv6 VSS option in the OPTION_ORO 844 [RFC3315] or the OPTION_ERO [RFC4994] should not change the 845 processing or decision to return (or not to return) the VSS option as 846 specified in this document. 848 7.2. Returning the DHCPv4 Sub-Option 850 The case of the DHCPv4 sub-option is a bit more complicated. Note 851 that [RFC3046] specifies that a DHCPv4 server which supports the 852 relay-agent-information option SHALL copy all sub-options received in 853 a relay-agent-information option into any outgoing relay-agent- 854 information option. Thus, the default behavior for any DHCPv4 server 855 is to return any VSS sub-option received to the relay agent whether 856 or not the DHCPv4 server understands the VSS sub-option. 858 In order to distinguish a DHCPv4 server which is simply copying 859 relay-agent-information option sub-options from an incoming to an 860 outgoing relay-agent-informaion option from one which successfully 861 acted upon the information in the VSS sub-option, DHCPv4 relay agents 862 MUST include two VSS sub-options in the relay-agent-information in 863 the request. One of these VSS sub-options contains valid VSS 864 information, and one of these VSS sub-options has a type of CONTROL, 865 no additional VSS information, and a length of one. 867 A DHCPv4 server which does not support the VSS sub-option will copy 868 both sub-options into the outgoing relay-agent-information option, 869 thus signalling to the DHCPv4 relay agent that it did not understand 870 the VSS sub-option. 872 A DHCPv4 server which supports the VSS sub-option and acts upon the 873 VSS sub-option with valid VSS information in it: 875 o MUST copy the VSS sub-option containing the valid VSS 876 information into the outgoing relay-agent-information option 878 o MUST NOT copy the VSS sub-option with the type of CONTROL into 879 the outgoing relay-agent-information option 881 Moreover, if a server uses different VSS information to allocate an 882 IP address than it receives in a particular DHCPv4 sub-option, it 883 MUST include that alternative VSS information in the VSS sub-option 884 that it returns to the DHCPv4 relay agent instead of the origian VSS 885 information it was given. 887 If a DHCPv4 server supports this sub-option and for some reason 888 (perhaps administrative control) does not honor this sub-option from 889 the request then it MUST NOT echo either sub-option into the outgoing 890 relay-agent-information option. 892 7.3. Making sense of conflicting VSS information 894 It is possible for a DHCPv4 server to receive both a VSS option and 895 VSS sub-options in the same packet. Likewise, a DHCPv6 server can 896 receive multiple VSS options in nested Relay-forward messages as well 897 as in the client message itself. In either of these cases, the VSS 898 information from the relay agent closest to the DHCP server SHOULD be 899 used in preference to all other VSS information received. In the 900 DHCPv4 case, this means that the VSS sub-option takes precedence over 901 the VSS option, and in the DHCPv6 case, this means that the VSS 902 option from the outer-most Relay-forward message in which a VSS 903 option appears takes precedence. 905 The reasoning behind this approach is that the relay-agent closer to 906 the DHCP server is almost certainly more trusted than the DHCP client 907 or more distant relay agents, and therefore information in the 908 relay-agent-information option or the Relay-forward message is more 909 likely to be correct. 911 In general, relay agents SHOULD be aware through configuration or 912 policy external to this document whether or not they should be 913 including VSS information in packets that they forward and so there 914 should not be conflicts among relay agent specified VSS information. 916 In these situations where multiple VSS option or sub-options appear 917 in the incoming packet or message, when the DHCP server constructs 918 the response to be sent to the DHCP client or relay agent, all 919 existing VSS options or sub-options MUST be replicated in the 920 appropriate places in the response and MUST contain only the VSS 921 information that was used by the DHCP server to allocate the IP 922 address (with, of course, the exception of a DHCPv4 relay-agent- 923 information VSS sub-option with a type of CONTROL). 925 8. Security 927 Message authentication in DHCPv4 for intradomain use where the out- 928 of-band exchange of a shared secret is feasible is defined in 929 [RFC3118]. Potential exposures to attack are discussed in section 7 930 of the DHCP protocol specification in [RFC2131]. 932 Implementations should consider using the DHCPv4 Authentication 933 option [RFC3118] to protect DHCPv4 client access in order to provide 934 a higher level of security if it is deemed necessary in their 935 environment. 937 Message authentication in DHCPv4 relay agents as defined in [RFC4030] 938 should be considered for DHCPv4 relay agents employing this sub- 939 option. Potential exposures to attack are discussed in section 7 of 940 the DHCP protocol specification in [RFC2131]. 942 For DHCPv6 use of the VSS option, the "Security Considerations" 943 section of [RFC3315] details the general threats to DHCPv6, and thus 944 to messages using the VSS option. The "Authentication of DHCP 945 Messages" section of [RFC3315] describes securing communication 946 between relay agents and servers, as well as clients and servers. 948 The VSS option could be used by a client in order to obtain an IP 949 address from any VPN. This option would allow a client to perform a 950 more complete address-pool exhaustion attack since the client would 951 no longer be restricted to attacking address-pools on just its local 952 subnet. 954 A DHCP server that implements these options and sub-option should be 955 aware of this possibility and use whatever techniques that can be 956 devised to prevent such an attack. Information such as the giaddr in 957 DHCPv4 or link address in the Relay-forward DHCPv6 message might be 958 used to detect and prevent this sort of attack. 960 One possible defense would be for the DHCP relay to insert a VSS 961 option or sub-option to override the DHCP client's VSS option. 963 Servers that implement the VSS option and sub-option MUST by default 964 disable use of the feature; it must specifically be enabled through 965 configuration. Moreover, a server SHOULD provide the ability to 966 selectively enable use of the feature under restricted conditions, 967 e.g., by enabling use of the option only from explicitly configured 968 client-ids, enabling its use only by clients on a particular subnet, 969 or restricting the VSSs from which addresses may be requested. 971 9. IANA Considerations 973 IANA is requested to assign DHCPv4 option number 221 for the DHCPv4 974 VSS option defined in Section 3.1, in accordance with [RFC3942]. 976 IANA is requested to assign sub-option number 151 for the DHCPv4 977 sub-option defined in Section 3.2 from the DHCP Relay Agent Sub- 978 options space [RFC3046], in accordance with the spirit of [RFC3942]. 979 While [RFC3942] doesn't explicitly mention the sub-option space for 980 the DHCP Relay Agent Information option [RFC3046], sub-option 151 is 981 already in use by existing implementations of this sub-option and the 982 current draft is essentially compatible with these current 983 implementations. 985 IANA is requested to assign the value of TBD for the DHCPv6 VSS 986 option defined in Section 3.3 from the DHCPv6 option registry. 988 The type byte defined in Section 3.4 defines a number space for which 989 IANA is to create and maintain a new sub-registry entitled "VSS Type 990 values". This sub-registry needs to be related to both the DHCPv4 991 and DHCPv6 VSS options and the DHCPv4 relay-agent-information option 992 sub-option (all defined by this document), since the type byte in 993 these two options and one sub-option MUST have identical definitions. 995 New values for the type byte may only be defined by IETF Consensus, 996 as described in [RFC5226]. Basically, this means that they are 997 defined by RFCs approved by the IESG. 999 10. Acknowledgments 1001 Bernie Volz recommended consolidation of the DHCPv4 option and sub- 1002 option drafts after extensive review of the former drafts, and 1003 provided valuable assistance in structuring and reviewing this 1004 document. Alper Yegin expressed interest in the DHCPv6 VSS option, 1005 resulting in this combined draft covering all three areas. Alfred 1006 Hoenes provided assistance with editorial review as well as raising 1007 substantive protocol issues. David Hankins and Bernie Volz each 1008 raised important protocol issues which resulted in a clarified 1009 document. Josh Littlefield provided editorial assistance. Several 1010 IESG reviewers took the time to substantially review this document, 1011 resulting in much increased clarity. 1013 11. References 1015 11.1. Normative References 1017 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1018 Requirement Levels", RFC 2119, March 1997. 1020 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, 1021 March 1997. 1023 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor 1024 Extensions", RFC 2132, March 1997. 1026 [RFC2685] Fox, B., Gleeson, B., "Virtual Private Networks 1027 Identifier", RFC 2685, September 1999. 1029 [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 1030 3046, January 2001. 1032 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and 1033 M. Carney, "Dynamic Host Configuration Protocol for IPv6 1034 (DHCPv6)", RFC 3315, July 2003. 1036 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 1037 Host Configuration Protocol (DHCP) version 6", RFC 3633, December 1038 2003. 1040 [RFC4994] Zeng, S., Volz, B., Kinnear, K. and J. Brzozowski, "DHCPv6 1041 Relay Agent Echo Request Option", RFC 4994, September 2007. 1043 11.2. Informative References 1045 [RFC951] Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951, 1046 September 1985. 1048 [RFC1542] Wimer, W., "Clarifications and Extensions for the Bootstrap 1049 Protocol", RFC 1542, October 1993. 1051 [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP 1052 Messages", RFC 3118, June 2001. 1054 [RFC3942] Volz, B., "Reclassifying Dynamic Host Configuration 1055 Protocol version 4 (DHCPv4) Options", RFC 3942, November 2004. 1057 [RFC4030] Stapp, M. and T. Lemon, "The Authentication Suboption for 1058 the Dynamic Host Configuration Protocol (DHCP) Relay Agent 1059 Option", RFC 4030, March 2005. 1061 [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration 1062 Protocol (DHCP) Leasequery", RFC 4388, February 2006. 1064 [RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng, "DHCPv6 1065 Leasequery", RFC 5007, September 2007. 1067 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1068 IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 1070 Authors' Addresses 1072 Kim Kinnear 1073 Cisco Systems 1074 1414 Massachusetts Ave. 1075 Boxborough, Massachusetts 01719 1077 Phone: (978) 936-0000 1079 EMail: kkinnear@cisco.com 1081 Richard Johnson 1082 Cisco Systems 1083 170 W. Tasman Dr. 1084 San Jose, CA 95134 1086 Phone: (408) 526-4000 1088 EMail: raj@cisco.com 1090 Mark Stapp 1091 Cisco Systems 1092 1414 Massachusetts Ave. 1093 Boxborough, Massachusetts 01719 1095 Phone: (978) 936-0000 1097 EMail: mjs@cisco.com 1099 Jay Kumarasamy