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Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Obsolete informational reference (is this intentional?): RFC 3736 (Obsoleted by RFC 8415) ** Obsolete normative reference: RFC 3315 (Obsoleted by RFC 8415) ** Downref: Normative reference to an Informational RFC: RFC 7039 Summary: 2 errors (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SAVI J. Bi 3 Internet-Draft J. Wu 4 Intended status: Standards Track G. Yao 5 Expires: October 11, 2014 Tsinghua Univ. 6 F. Baker 7 Cisco 8 April 9, 2014 10 SAVI Solution for DHCP 11 draft-ietf-savi-dhcp-23 13 Abstract 15 This document specifies the procedure for creating a binding between 16 a DHCPv4/DHCPv6 assigned IP address and a binding anchor on a SAVI 17 (Source Address Validation Improvements) device. The bindings set up 18 by this procedure can be used to filter out packets with forged 19 source IP address in DHCP scenario. This mechanism is proposed as a 20 complement to ingress filtering to provide finer-grained source IP 21 address validation. 23 Status of this Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at http://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on October 11, 2014. 40 Copyright Notice 42 Copyright (c) 2014 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (http://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 This document may contain material from IETF Documents or IETF 56 Contributions published or made publicly available before November 57 10, 2008. The person(s) controlling the copyright in some of this 58 material may not have granted the IETF Trust the right to allow 59 modifications of such material outside the IETF Standards Process. 60 Without obtaining an adequate license from the person(s) controlling 61 the copyright in such materials, this document may not be modified 62 outside the IETF Standards Process, and derivative works of it may 63 not be created outside the IETF Standards Process, except to format 64 it for publication as an RFC or to translate it into languages other 65 than English. 67 Table of Contents 69 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 70 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5 71 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 72 4. Deployment Scenario and Configuration . . . . . . . . . . . . 8 73 4.1. Elements and Scenario . . . . . . . . . . . . . . . . . . 8 74 4.2. Attribute . . . . . . . . . . . . . . . . . . . . . . . . 10 75 4.2.1. Trust Attribute . . . . . . . . . . . . . . . . . . . 11 76 4.2.2. DHCP-Trust Attribute . . . . . . . . . . . . . . . . . 11 77 4.2.3. DHCP-Snooping Attribute . . . . . . . . . . . . . . . 12 78 4.2.4. Data-Snooping Attribute . . . . . . . . . . . . . . . 12 79 4.2.5. Validating Attribute . . . . . . . . . . . . . . . . . 13 80 4.2.6. Table of Mutual Exclusions . . . . . . . . . . . . . . 13 81 4.3. Perimeter . . . . . . . . . . . . . . . . . . . . . . . . 14 82 4.3.1. SAVI-DHCP Perimeter Overview . . . . . . . . . . . . . 14 83 4.3.2. SAVI-DHCP Perimeter Configuration Guideline . . . . . 14 84 4.3.3. On the Placement of DHCP Server/Relay . . . . . . . . 15 85 5. Binding State Table (BST) . . . . . . . . . . . . . . . . . . 16 86 6. DHCP Snooping Process . . . . . . . . . . . . . . . . . . . . 17 87 6.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 17 88 6.2. Binding States Description . . . . . . . . . . . . . . . . 18 89 6.3. Events . . . . . . . . . . . . . . . . . . . . . . . . . . 18 90 6.3.1. Timer Expiration Event . . . . . . . . . . . . . . . . 18 91 6.3.2. Control Message Arriving Events . . . . . . . . . . . 18 92 6.4. The State Machine of DHCP Snooping Process . . . . . . . . 20 93 6.4.1. From NO_BIND to INIT_BIND . . . . . . . . . . . . . . 20 94 6.4.1.1. Trigger Events . . . . . . . . . . . . . . . . . . 20 95 6.4.1.2. Following Actions . . . . . . . . . . . . . . . . 20 96 6.4.2. From INIT_BIND to Other States . . . . . . . . . . . . 21 97 6.4.2.1. Trigger Events . . . . . . . . . . . . . . . . . . 21 98 6.4.2.2. Following Actions . . . . . . . . . . . . . . . . 22 99 6.4.3. From BOUND to Other States . . . . . . . . . . . . . . 24 100 6.4.3.1. Trigger Events . . . . . . . . . . . . . . . . . . 24 101 6.4.3.2. Following Actions . . . . . . . . . . . . . . . . 24 102 6.5. Table of State Machine . . . . . . . . . . . . . . . . . . 25 103 7. Data Snooping Process . . . . . . . . . . . . . . . . . . . . 26 104 7.1. Scenario . . . . . . . . . . . . . . . . . . . . . . . . . 26 105 7.2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 27 106 7.3. Additional Binding States Description . . . . . . . . . . 28 107 7.4. Events . . . . . . . . . . . . . . . . . . . . . . . . . . 28 108 7.5. State Machine of Binding Recovery Process . . . . . . . . 29 109 7.5.1. From NO_BIND to DETECTION . . . . . . . . . . . . . . 29 110 7.5.1.1. Trigger Event . . . . . . . . . . . . . . . . . . 29 111 7.5.1.2. Following Actions . . . . . . . . . . . . . . . . 29 112 7.5.2. From DETECTION to Other States . . . . . . . . . . . . 30 113 7.5.2.1. Trigger Event . . . . . . . . . . . . . . . . . . 30 114 7.5.2.2. Following Actions . . . . . . . . . . . . . . . . 30 116 7.5.3. From RECOVERY to Other States . . . . . . . . . . . . 31 117 7.5.3.1. Trigger Event . . . . . . . . . . . . . . . . . . 31 118 7.5.3.2. Following Actions . . . . . . . . . . . . . . . . 31 119 7.5.4. After BOUND . . . . . . . . . . . . . . . . . . . . . 32 120 7.5.4.1. Trigger Event . . . . . . . . . . . . . . . . . . 33 121 7.5.4.2. Following Action . . . . . . . . . . . . . . . . . 33 122 7.6. Table of State Machine . . . . . . . . . . . . . . . . . . 33 123 8. Filtering Specification . . . . . . . . . . . . . . . . . . . 34 124 8.1. Data Packet Filtering . . . . . . . . . . . . . . . . . . 35 125 8.2. Control Packet Filtering . . . . . . . . . . . . . . . . . 35 126 9. State Restoration . . . . . . . . . . . . . . . . . . . . . . 36 127 9.1. Attribute Configuration Restoration . . . . . . . . . . . 36 128 9.2. Binding State Restoration . . . . . . . . . . . . . . . . 36 129 10. Constants . . . . . . . . . . . . . . . . . . . . . . . . . . 37 130 11. Security Considerations . . . . . . . . . . . . . . . . . . . 37 131 11.1. Security Problems about the Data Snooping Process . . . . 37 132 11.2. Issues about Leaving Clients . . . . . . . . . . . . . . . 37 133 11.3. Duplicate Bindings to the Same Address . . . . . . . . . . 38 134 11.4. Compatibility with DNA (Detecting Network Attachment) . . 39 135 11.5. Authentication in DHCPv6 Leasequery . . . . . . . . . . . 40 136 11.6. Binding Number Limitation . . . . . . . . . . . . . . . . 40 137 11.7. Residual Threats . . . . . . . . . . . . . . . . . . . . . 40 138 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41 139 13. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 41 140 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41 141 14.1. Informative References . . . . . . . . . . . . . . . . . . 41 142 14.2. Normative References . . . . . . . . . . . . . . . . . . . 42 143 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43 145 1. Introduction 147 This document describes a fine-grained source IP address validation 148 mechanism. This mechanism creates bindings between addresses 149 assigned to network attachment points by DHCP and suitable binding 150 anchors (refer to Section 3) of the attachments. Then the bindings 151 are used to identify and filter out packets originated from these 152 attachments with forged source IP addresses. In this way, this 153 mechanism can prevent hosts from spoofing IP addresses assigned to 154 the other attachment points. Compared with [BCP38], which provides 155 prefix granularity source IP address validity, this mechanism can 156 benefit the network with finer-grained validity and traceability of 157 source IP addresses. 159 This mechanism primarily performs DHCP snooping to set up bindings 160 between IP addresses assigned by DHCP and corresponding binding 161 anchors. This binding process is inspired by the work of [BA2007]. 162 Different from [BA2007], which designs specifications about DHCPv4, 163 this mechanism covers the DHCPv6 snooping process, the Data Snooping 164 process(refer to Section 7), as well as a number of other technical 165 details. Specially, the Data Snooping process is a data-triggered 166 procedure which snoops the header of data packet to set up bindings. 167 It is designed to avoid permanent block of valid address in case that 168 DHCP snooping is insufficient to set up all the valid bindings. 170 This mechanism is designed for the stateful DHCP scenario [RFC2131], 171 [RFC3315]. Stateless DHCP [RFC3736] is out of scope for this 172 document, because it has nothing to do with IP address allocation. A 173 client doing stateless DHCP acquires its IP address(es) using some 174 other mechanism. It is through that mechanism the client uses that 175 SAVI must be accomplished. For example, for hosts using Stateless 176 Auto-configuration address, SAVI-FCFS [RFC6620] should be enabled. 177 Besides, this mechanism is primarily designed for pure DHCP scenarios 178 in which only addresses assigned through DHCP are allowed. However, 179 it does not block any link-local address. It is because link-local 180 addresses are used by DHCPv6 clients before the clients are assigned 181 a DHCPv6 address. Considering that link-local addresses are 182 generally self-generated, and the spoofing of link local address may 183 disturb this mechanism, it is RECOMMENDED to enable a SAVI solution 184 for link-local addresses, e.g., the SAVI-FCFS [RFC6620]. 186 2. Requirements Language 188 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 189 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 190 document are to be interpreted as described in RFC 2119 [RFC2119]. 192 3. Terminology 194 Binding anchor: A "binding anchor" is defined to be a link layer 195 property of network attachment in [RFC7039]. A list of proper 196 binding anchors can be found in Section 3.2 of [RFC7039]. 198 Attribute: A configurable property of each network attachment which 199 indicates the actions to be performed on packets received from the 200 network attachment. 202 DHCP address: An IP address assigned via DHCP. 204 SAVI-DHCP: The name of this SAVI function for DHCP address. 206 SAVI device: A network device on which this SAVI function is enabled. 208 Non-SAVI device: A network device on which this SAVI function is not 209 enabled. 211 DHCP Client-Server message: A message that is sent from a DHCP client 212 to a DHCP server or DHCP servers. Such a message is of one of the 213 following types: 215 o DHCPv4 Discover: DHCPDISCOVER [RFC2131] 217 o DHCPv4 Request: DHCPREQUEST generated during SELECTING state 218 [RFC2131] 220 o DHCPv4 Renew: DHCPREQUEST generated during RENEWING state 221 [RFC2131] 223 o DHCPv4 Rebind: DHCPREQUEST generated during REBINDING state 224 [RFC2131] 226 o DHCPv4 Reboot: DHCPREQUEST generated during INIT-REBOOT state 227 [RFC2131] 229 o Note: DHCPv4 Request/Renew/Rebind/Reboot messages can be 230 identified based on the Table 4 of [RFC2131] 232 o DHCPv4 Decline: DHCPDECLINE [RFC2131] 234 o DHCPv4 Release: DHCPRELEASE [RFC2131] 236 o DHCPv4 Inform: DHCPINFORM [RFC2131] 238 o DHCPv6 Request: REQUEST [RFC3315] 239 o DHCPv6 Solicit: SOLICIT [RFC3315] 241 o DHCPv6 Confirm: CONFIRM [RFC3315] 243 o DHCPv6 Decline: DECLINE [RFC3315] 245 o DHCPv6 Release: RELEASE [RFC3315] 247 o DHCPv6 Rebind: REBIND [RFC3315] 249 o DHCPv6 Renew: RENEW [RFC3315] 251 o DHCPv6 Information-Request: INFORMATION-REQUEST [RFC3315] 253 DHCP Server-Client message: A message that is sent from a DHCP server 254 to a DHCP client. Such a message is of one of the following types: 256 o DHCPv4 ACK: DHCPACK [RFC2131] 258 o DHCPv4 NAK: DHCPNAK [RFC2131] 260 o DHCPv4 Offer: DHCPOFFER [RFC2131] 262 o DHCPv6 Reply: REPLY [RFC3315] 264 o DHCPv6 Advertise: ADVERTISE [RFC3315] 266 o DHCPv6 Reconfigure: RECONFIGURE [RFC3315] 268 Lease time: The lease time in IPv4 [RFC2131] or the valid lifetime in 269 IPv6 [RFC3315]. 271 Binding entry: An 'permit' rule that defines a valid association 272 between an IP address and a binding anchor. 274 Binding State Table (BST): The data structure that contains all the 275 binding entries. 277 Binding entry limit: The maximum number of binding entries that may 278 be associated with any one binding anchor. Limiting the number of 279 binding entries per binding anchor prevents a malicious or 280 malfunctioning node from overloading the binding table on a SAVI 281 device. 283 Direct attachment: Ideally, a SAVI device should be an access device 284 which is directly attached by hosts. In such case, the hosts are 285 direct attachments of the SAVI device. 287 Indirect attachment: A SAVI device can be an aggregration device 288 which is connected with a number of access devices, which are 289 attached by hosts. In such case, the hosts are indirect attachments 290 of the SAVI device. Sometimes, it is expressed as "the hosts are 291 indirectly attached to the SAVI device". 293 Upstream link: Upstream links are links connected to non-SAVI devices 294 from which the valid source address space of traffic contains the 295 prefixes of other networks. 297 Upstream device: An upstream device is a non-SAVI device associated 298 with an upstream link. For example, the gateway router of the 299 network. 301 Downstream link: Downstream links are links connected to non-SAVI 302 devices from which the valid source address space of traffic only 303 contains the prefix(es) of the local network. 305 Downstream device: A downstream device is a non-SAVI device 306 associated with an downstream link. For example, an access switch in 307 the network. 309 CUT VERTEX: A cut vertex is 'any vertex whose removal increases the 310 number of connected components'. This is a concept in graph theory. 311 This term is used in Section 6.1 to accurately specify the required 312 deployment location of SAVI devices when they only perform the DHCP 313 snooping process. 315 Identity Association (IA): "A collection of addresses assigned to a 316 client." [RFC3315] 318 Detection message: a DAD or ARP message intended to detect a 319 duplicate address by the Data Snooping Process. 321 4. Deployment Scenario and Configuration 323 4.1. Elements and Scenario 325 A list of essential elements in a SAVI-DHCP deployment scenario is 326 given as follows: 328 (1) DHCP server 330 (2) DHCP client 331 (3) SAVI device 333 And there may be following optional elements in a SAVI-DHCP 334 deployment scenario: 336 (1) DHCP relay 338 (2) Non-SAVI device 340 Figure 1 shows a deployment scenario that contains these elements. 341 Note that a physical device can be multiple elements, e.g, a switch 342 can be both a SAVI device and a DHCP relay. In such cases, the links 343 are logic links rather than physical links. 345 +--------+ +------------+ 346 |DHCP |-----| Non-SAVI | 347 |Server A| | Device 1 | 348 +--------+ +-----|------+ 349 ......................|............................ 350 . | upstream link . 351 . Protection +---|------+ . 352 . Perimeter | SAVI |--------------+ . 353 . | Device C| | . 354 . +---|------+ | . 355 . | | . 356 . +----------+ +---|------+ +------|---+ . 357 downstream . | SAVI | | Non SAVI| | SAVI | . 358 link +----.-| Device A|----| Device 3|-------| Device B| . 359 | . +----|--|--+ +----------+ +-|---|----+ . 360 | . | +----------+ ............ | | . 361 | '.............. | . . | | . 362 | | . | . +--------+ | . 363 +----|-----+ +--|---+ . +----|-+ . +--|---+ . +---|----+ . 364 | Non-SAVI | |Client| . |DHCP | . |Client| . |DHCP | . 365 | Device 2 | |A | . |Relay | . |B | . |Server B| . 366 +----------+ +------+ . +------+ . +------+ . +--------+ . 367 ............ ............... 369 Figure 1: SAVI-DHCP Scenario 371 Note: To distinguish upstream/downstream links is essential for SAVI- 372 DHCP. 374 Networks are not isolated and traffic from other networks, i.e., 375 transit traffic specified in RFC6620, may get into the network with 376 SAVI-DHCP deployed through the upstream links. Since SAVI solutions 377 are limited to check traffic generated from local link, SAVI-DHCP is 378 not to set up bindings for addresses assigned in other networks. 379 Thus, SAVI-DHCP will not set up bindings for addresses appearing on 380 upstream links and will not check data traffic from upstream links. 381 The traffic from upstream links should be checked by a prefix 382 granularity source address validation mechanism to avoid spoofing of 383 local addresses from other networks. How to generate and deploy such 384 a mechanism is out of the scope of this document. 386 However, traffic from downstream links are generated from local 387 network. For example, a hub, which is attached by some DHCP clients, 388 is on the downstream link of a SAVI device. The traffic from 389 downstream links should be checked by SAVI-DHCP if possible. 390 However, because DHCP clients on the downstream links are indirectly 391 attached, a number of security problems Section 11.7 can be 392 introduced. 394 4.2. Attribute 396 As illustrated in Figure 1, an attachment to a SAVI device can be 397 from either a DHCP client, or a DHCP relay/server, or a SAVI device, 398 or a non-SAVI device. Different actions are performed on traffic 399 originated from different elements. To distinguish different types 400 of attachments, an attachment property named 'attribute' is 401 configured on SAVI devices. This section specifies the attributes 402 used by SAVI-DHCP. 404 Before configuration, an attachment is with no attribute. An 405 attachment MAY be configured to have one or more compatible 406 attributes(refer to Section 4.2.6). The attributes of each 407 attachment MUST be configured before this SAVI-DHCP function is 408 enabled on the attachment. The procedure performed by SAVI devices 409 on traffic from each attachment is determined by the attribute(s) set 410 on the attachment. 412 Particularly, if an attachment has no attribute, data traffic from 413 such attachments will not be checked by SAVI-DHCP and will be 414 forwarded directly. This prevents SAVI-DHCP from causing a break in 415 the network when it is turned on without any binding anchors 416 configured. However, if a binding anchor has no attributes, this 417 means that the SAVI-DHCP-Trust attribute is not present. Because of 418 this, DHCP server-client messages from that binding anchor will be 419 discarded. This prevents a host from connecting to an unconfigured 420 binding anchor and acting as a DHCP server. It is SUGGESTED to 421 configure SAVI-DHCP-Trust on necessary binding anchors before turning 422 on the SAVI-DHCP function. 424 Binding anchors associated with upstream links MAY have no attribute 425 after configuration. For example, in Figure 1, the attachment from 426 the Non-SAVI Device 1 to the SAVI Device C should be configured with 427 no attribute. It means 1) SAVI devices will neither set up bindings 428 for upstream hosts nor check traffic from upstream hosts; 2) SAVI 429 devices will drop DHCP server-client messages from upstream devices 430 unless the DHCP-Trust attribute (refer to Section 4.2.2) is set on 431 the corresponding attachment. The reason that DHCP messages from 432 upstream devices are not trusted is discussed in Section 4.3.3. 434 4.2.1. Trust Attribute 436 The "Trust Attribute" indicates the packets from the corresponding 437 attachment are completely trustable. 439 SAVI devices will not set up bindings for attachments with Trust 440 attribute; DHCP messages and data packets from such attachments with 441 this attribute will not be checked. If the DHCP Server-Client 442 messages from attachments with this attribute can trigger the state 443 transitions specified in Section 6 and Section 7, these messages will 444 be handled by the corresponding processes in Section 6 and Section 7. 446 This attribute is generally configured on the attachments from other 447 SAVI devices. For example, in Figure 1, the attachment from the SAVI 448 Device B to the SAVI Device C and the attachment from the SAVI Device 449 C to the SAVI Device B should be configured with this attribute. 450 Besides, it can be configured on attachments from Non-SAVI devices 451 only if the Non-SAVI devices will not introduce unchecked traffic 452 from DHCP clients. For example, the attachments from Non-SAVI device 453 3 to SAVI device A, SAVI device B and SAVI device C can be configured 454 with this attribute, only if Non-SAVI device 3 does not have 455 attachment from DHCP clients. 457 4.2.2. DHCP-Trust Attribute 459 The "DHCP-Trust Attribute" indicates the DHCP Server-Client messages 460 from the corresponding attachment is trustable. 462 SAVI devices will forward DHCP Server-Client messages coming from the 463 attachments with this attribute. If the DHCP Server-Client messages 464 can trigger the state transitions, they will be handled by the 465 binding setup processes specified in Section 6 and Section 7. 467 This attribute is generally used on the direct attachments from the 468 trusted DHCP servers/relays. In Figure 1, the attachment from the 469 DHCP Relay to the SAVI Device A, and the attachment from the DHCP 470 Server B to the SAVI Device B should be configured with this 471 attribute. It is NOT RECOMMENDED to configure this attribute on any 472 indirect attachment point of the non-neighboring DHCP servers and 473 relays, unless all the elements that can be reached through that 474 attachment point can be trusted, i.e., bogus DHCP Server-Client 475 messages will not be generated by these elements. For example, in 476 Figure 1, the attachment from the Non-SAVI Device 1 to the SAVI 477 Device C should not be configured with this attribute. This issue is 478 discussed in Section 4.3.3. 480 4.2.3. DHCP-Snooping Attribute 482 The "DHCP-Snooping Attribute" indicates bindings will be set up based 483 on DHCP snooping. 485 DHCP Client-Server messages from attachments with this attribute will 486 trigger the setup of bindings. SAVI devices will set up bindings on 487 attachments with this attribute based on the DHCP snooping procedure 488 described in Section 6. 490 DHCP-Snooping attribute is configured on the attachments from DHCP 491 clients. This attribute can be also used on the attachments from 492 downstream Non-SAVI devices which are attached by DHCP clients. In 493 Figure 1, the attachment from the Client A to the SAVI Device A, the 494 attachment from the Client B to the SAVI Device B, and the attachment 495 from the Non-SAVI Device 2 to the SAVI Device A can be configured 496 with this attribute. 498 4.2.4. Data-Snooping Attribute 500 The "Data-Snooping Attribute" indicates data packets from the 501 corresponding attachment may trigger binding setup procedure. 503 Data packets from attachments with this attribute may trigger the 504 setup of bindings. SAVI devices will set up bindings on attachments 505 with this attribute based on the data-triggered process described in 506 Section 7. 508 If DHCP-Snooping attribute is configured on an attachment, the 509 bindings on this attachment are set up based on DHCP message 510 snooping. However, in some scenarios, a DHCP address may be used by 511 a DHCP client without DHCP address assignment procedure performed on 512 its current attachment. For such attachments, the Data-Snooping 513 process, which is described in Section 7, is necessary. This 514 attribute is configured on such attachments. The usage of this 515 attribute is further discussed in Section 7. 517 4.2.5. Validating Attribute 519 The "Validating Attribute" indicates packets from the corresponding 520 attachment will be checked based on binding entries on the 521 attachment. 523 Packets coming from attachments with this attribute will be checked 524 based on binding entries on the attachment as specified in Section 8. 526 Validating attribute is configured on the attachments from which the 527 data packets should be checked. For example, the DHCP clients. 529 4.2.6. Table of Mutual Exclusions 531 Different types of attributes may indicate mutually exclusive actions 532 on packet. Mutually exclusive attributes MUST NOT be set on the same 533 attachment. The compatibility of different attributes is listed in 534 Figure 2. Note that although Trust and DHCP-Trust are compatible, 535 there is no need to configure DHCP-Trust on an attachment with Trust 536 attribute. 538 +----------+----------+----------+----------+----------+----------+ 539 | | | | DHCP- | Data- | | 540 | | Trust |DHCP-Trust| Snooping | Snooping |Validating| 541 +----------+----------+----------+----------+----------+----------+ 542 | | | | mutually | mutually | mutually | 543 | Trust | - |compatible| exclusive| exclusive| exclusive| 544 +----------+----------+----------+----------+----------+----------+ 545 | | | | | | | 546 |DHCP-Trust|compatible| - |compatible|compatible|compatible| 547 +----------+----------+----------+----------+----------+----------+ 548 |DHCP- |mutually | | | | | 549 |Snooping |exclusive |compatible| - |compatible|compatible| 550 +----------+----------+----------+----------+----------+----------+ 551 |Data- |mutually | | | | | 552 |Snooping |exclusive |compatible|compatible| - |compatible| 553 +----------+----------+----------+----------+----------+----------+ 554 | |mutually | | | | | 555 |Validating|exclusive |compatible|compatible|compatible| - | 556 +----------+----------+----------+----------+----------+----------+ 558 Figure 2: Table of Mutual Exclusions 560 4.3. Perimeter 562 4.3.1. SAVI-DHCP Perimeter Overview 564 SAVI devices can form a perimeter separating untrusted and trusted 565 areas, similarly to SAVI-FCFS (refer to Section 2.5 of [RFC6620]). 566 Each SAVI device need only establish bindings for a client if it is 567 connected to that client by a link that crosses the perimeter that 568 encloses the SAVI device. 570 The perimeter is primarily designed for scalability. This has two 571 implications. First, SAVI devices only need to establish bindings 572 for directly attached clients, or clients indirectly attached through 573 non-SAVI device, rather than all the clients in the network. Second, 574 each SAVI device only need to check traffic from clients attached to 575 it, without checking all the traffic passing by. 577 Consider the example in Figure 1. The protection perimeter is formed 578 by SAVI Device A, B and C. In this case, SAVI device B doesn't create 579 a binding for client A. However, because SAVI device A filters 580 spoofing traffic from client A, SAVI device B can avoid receiving 581 spoofing traffic from client A. 583 There is three main differences between the SAVI-DHCP protection 584 perimeter and SAVI-FCFS protection perimeter: 586 (1) SAVI-DHCP follows the state announced in DHCP messages, so there 587 is no need to distribute state using Neighbor Solicitation/ 588 Neighbor Advertisement messages. 590 (2) The perimeter in SAVI-DHCP is not only a perimeter for data 591 packets, but also a perimeter for DHCP messages. The placement 592 of DHCP Relay/Server, which is not involved in SAVI-FCFS , is 593 related with the construction of the perimeter. The requirement 594 on the placement and configuration of DHCP Relay/Server are 595 discussed in Section 4.3.3. 597 (3) Downstream/upstream links MUST be distinguished when configuring 598 the perimeter to avoid estabilshing binding for addresses of 599 other networks. 601 4.3.2. SAVI-DHCP Perimeter Configuration Guideline 603 Through configuring attribute of each attachment properly, a 604 perimeter separating untrusted area and trusted area can be formed: 606 (1) Configure Validating and DHCP-Snooping attribute on the direct 607 attachments of all the DHCP clients. 609 (2) Configure Validating and DHCP-Snooping attribute on the indirect 610 attachments of all the DHCP clients(i.e., DHCP clients on the 611 downstream links). 613 (3) Configure Trust attribute on the attachments of other SAVI 614 devices. 616 (4) If a Non-SAVI device, or a number of connected Non-SAVI devices, 617 have only attachments from SAVI devices or upstream devices, set 618 their attachments to SAVI devices with Trust attribute. 620 (5) Configure DHCP-Trust attribute on the direct attachments of 621 trusted DHCP relays/servers. 623 (6) Optional: configure filters on the upstream links to filter out 624 spoofing of local addresses from other networks. If the 625 upstream networks are completely trustable, Trust attribute can 626 be used on the upstream links. 628 In this way, the points of attachments with Validating attribute (and 629 generally together with attachments of upstream devices) on SAVI 630 devices can form a perimeter separating DHCP clients and trusted 631 devices. Data packet check is only performed on the perimeter. The 632 perimeter is also a perimeter for DHCP messages. DHCP-Trust 633 attribute is only configured on the inside links of the perimeter. 634 Only DHCP server-client messages originated in the perimeter is 635 trusted. 637 4.3.3. On the Placement of DHCP Server/Relay 639 Based on the configuration guideline, it can be found that the SAVI 640 devices only trust DHCP Server-Client messages originated inside the 641 perimeter. It means the trusted DHCP relays/servers must be placed 642 in the perimeter. DHCP server-client messages will be filtered on 643 the perimeter (Note: server-relay messages will not be filtered). In 644 this way, DHCP server-client messages from bogus DHCP servers are 645 filtered on the perimeter, and then the SAVI devices can be protected 646 from fabricated DHCP messages. 648 Such a requirement is due to the limitation of this binding based 649 mechanism. This document makes no assumption that the DHCP server- 650 client messages arriving the perimeter from the outside can be 651 trusted. The binding anchor of a trusted remote DHCP server can be 652 shared by a bogus DHCP server. Thus, the SAVI device cannot 653 distinguish bogus and valid DHCP messages only based on the 654 associated binding anchor of DHCP messages in such case. 656 Note that even if a DHCP server is valid, it may be not contained in 657 the perimeter based on the guideline. For example, in Figure 1, DHCP 658 server A is valid, but it is attached to a Non-SAVI device. The Non- 659 SAVI device may be attached by attackers which generate fabricated 660 DHCP messages. This binding based mechanism may not have the ability 661 to distinguish whether a message received from the attachment of the 662 Non-SAVI device 1 is from DHCP server A or the attackers. If the 663 DHCP server A is contained in the perimeter, the Non-SAVI device 1 664 will also be contained in the perimter. However, the Non-SAVI device 665 1 can introduce fabricated DHCP messages into the perimeter. Thus, 666 the DHCP server A cannot be contained in the perimeter. 668 In this case, the SAVI devices can set up bindings for addresses 669 assigned by DHCP server A through snooping the messages relayed by 670 trusted relay in the network. For example, the DHCP relay may relay 671 messages between DHCP server A and the clients in the network, and 672 the SAVI devices can snoop messages from the DHCP relay which is 673 inside the perimeter. The authentication mechanism (i.e., IPSec, as 674 specified in section 21.1 of [RFC3315]) enforced between the DHCP 675 relay and the DHCP server outside the perimeter can compensate this 676 binding based mechanism. It is SUGGESTED to configure IPSec between 677 the DHCP relay and the DHCP server in such case. If source address 678 validation is enforced in the whole network, which makes the source 679 IP address trustable, the DHCP relay and the DHCP server can simply 680 authenticate the messages from each other based on the source IP 681 address without the requriement to deploy IPSec. 683 Another considration on the placement is that if the DHCP server/ 684 relay is not inside the perimeter, the SAVI devices may not be able 685 to set up bindings correctly, because the SAVI devices may not be on 686 the path between the clients and the server/relay, or the DHCP 687 messages are encapsulated (e.g., Relay-reply and Relay-forward). 689 5. Binding State Table (BST) 691 Binding State Table is used to contain the bindings between the IP 692 addresses assigned to the attachments and the corresponding binding 693 anchors of the attachments. Each entry of the table, i.e., binding 694 entry, has 5 fields: 696 o Binding Anchor(Anchor): the binding anchor, i.e., a link-layer 697 property of the attachment. 699 o IP Address(Address): the IP address assigned to the attachment by 700 DHCP. 702 o State: the state of the binding. Possible values of this field 703 are listed in Section 6.2 and Section 7.3. 705 o Lifetime: the remaining seconds of the binding. The Lifetime 706 field counts down automatically. 708 o TID: the Transaction ID (TID) (refer to [RFC2131] [RFC3315]) of 709 the corresponding DHCP transaction. TID field is used to 710 associate DHCP Server-Client messages with corresponding binding 711 entries. 713 IA does not present in the BST. On the one hand, IA is not found to 714 be necessary because the lease of each address in one IA is assigned 715 respectively. On the other hand, when the binding is set up based on 716 data-snooping, IA cannot be recovered from the leasequery protocol. 717 Besides, there is no IA for DHCPv4. 719 An instance of this table is shown in Figure 3. 721 +---------+----------+----------+-----------+-------+ 722 | Anchor | Address | State | Lifetime |TID | 723 +---------+----------+----------+-----------+-------+ 724 | Port_1 | IP_1 | BOUND | 65535 |TID_1 | 725 +---------+----------+----------+-----------+-------+ 726 | Port_1 | IP_2 | BOUND | 10000 |TID_2 | 727 +---------+----------+----------+-----------+-------+ 728 | Port_2 | IP_3 |INIT_BIND | 1 |TID_3 | 729 +---------+----------+----------+-----------+-------+ 731 Figure 3: Instance of BST 733 6. DHCP Snooping Process 735 This section specifies the process of setting up bindings based on 736 DHCP snooping, named DHCP Snooping Process. This process is 737 illustrated making use of a state machine. 739 6.1. Rationale 741 The rationale of the DHCP Snooping Process is that if a DHCP client 742 is legitimate to use a DHCP address, the DHCP address assignment 743 procedure which assigns the IP address to the client must have been 744 performed on the attachment of the client. This basis stands when 745 the SAVI device is always on the path(s) from the DHCP client to the 746 DHCP server(s)/relay(s). Without considering the movement of DHCP 747 clients, the SAVI device should be the CUT VERTEX whose removal will 748 disjoin the DHCP client and the remaining network containing the DHCP 749 server(s)/relay(s). For most of the networks whose topologies are 750 simple, it is possible to deploy this SAVI function at proper devices 751 to meet this requirement. 753 However, a deployment of this SAVI function may not meet the 754 requirement. For example, there are multiple paths from a DHCP 755 client to the DHCP server and the SAVI device is only on one of them. 756 Then the SAVI device may not be able to snoop the DHCP procedure. 757 Host movement may also make this requirement can not be met. For 758 exmaple, when a DHCP client moves from one attachment to another 759 attachment in the same network, it may not reinitialize its interface 760 or send a Confirm message because of imcomplete protocol 761 implementation. Thus, there can be scenarios in which only 762 performing this DHCP snooping process is insufficient to set up 763 bindings for all the valid DHCP addresses. These exceptions and the 764 solutions are discussed in Section 7. 766 6.2. Binding States Description 768 Following binding states present in this process and the 769 corresponding state machine: 771 NO_BIND: The state before a binding has been set up. 773 INIT_BIND: A potential binding has been set up. 775 BOUND: The binding has been set up. 777 6.3. Events 779 This section describes events in this process and the corresponding 780 state machine. 782 6.3.1. Timer Expiration Event 784 EVE_ENTRY_EXPIRE: The lifetime of a binding entry expires. 786 6.3.2. Control Message Arriving Events 788 EVE_DHCP_REQUEST: A DHCPv4 Request or a DHCPv6 Request message is 789 received. 791 EVE_DHCP_CONFIRM: A DHCPv6 Confirm message is received. 793 EVE_DHCP_REBOOT: A DHCPv4 Reboot message is received. 795 EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is 796 received. 798 EVE_DHCP_RENEW: A DHCPv4 Renew or a DHCPv6 Renew message is received. 800 EVE_DHCP_SOLICIT_RC: A DHCPv6 Solicitation message with Rapid Commit 801 option is received. 803 EVE_DHCP_REPLY: A DHCPv4 ACK or a DHCPv6 Reply message is received. 805 EVE_DHCP_DECLINE: A DHCPv4 Decline or a DHCPv6 Decline message is 806 received. 808 EVE_DHCP_RELEASE: A DHCPv4 Release or a DHCPv6 Release message is 809 received. 811 EVE_DCHP_LEASEQUERY: A successful DHCPv6 LEASEQUERY_REPLY (refer to 812 section 4.3.3 of [RFC5007]) is received. 814 Note: the events listed here do not cover all the DHCP messages in 815 section 3. The messages which do not really determine address usage 816 (DHCPv4 Discover, DHCPv4 Inform, DHCPv6 Solicit without Rapid Commit, 817 DHCPv6 Information-Request, DHCPv4 Offer, DHCPv6 Advertise, DHCPv6 818 Reconfigure), and which are not neccessary to snoop (DHCPv4 NAK, 819 refer to section 6.4.2.1), are not included. 821 Moreover, only if a DHCP message can pass the following checks, the 822 corresponding event is regarded as a valid event: 824 o Attribute check: the DHCP Server-Client messages and 825 LEASEQUERY_REPLY should be from attachments with DHCP-Trust 826 attribute; the DHCP Client-Server messages should be from 827 attachments with DHCP-Snooping attribute. 829 o Destination check: the DHCP Server-Client messages should be 830 destined to attachments with DHCP-Snooping attribute. This check 831 is performed to ensure the binding is set up on the SAVI device 832 which is nearest to the destination client. 834 o Binding anchor check: the DHCP Client-Server messages which may 835 trigger modification or removal of an existing binding entry must 836 have matched binding anchor with the corresponding entry. 838 o TID check: the DHCP Server-Client/Client-Server messages which 839 may cause modification on existing binding entries must have 840 matched TID with the corresponding entry. Note that this check 841 is not performed on Leasequery and Leasequery-reply messages as 842 they are exchanged between the SAVI devices and the DHCP servers. 844 o Binding limitation check: the DHCP messages must not cause new 845 binding setup on an attachment whose binding entry limitation has 846 been reached. (refer to Section 11.6). 848 o Address check: the source address of the DHCP messages should 849 pass the check specified in Section 8.2. 851 On receiving a DHCP message without triggering a valid event, the 852 state will not transit and actions will not be performed. Note that 853 if a message does not trigger a valid event but it can pass the 854 checks in Section 8.2, it MUST be forwarded. 856 6.4. The State Machine of DHCP Snooping Process 858 This section specifies the transits of each state and the 859 corresponding actions. 861 6.4.1. From NO_BIND to INIT_BIND 863 6.4.1.1. Trigger Events 865 Trigger events: EVE_DHCP_REQUEST, EVE_DHCP_SOLICIT_RC, 866 EVE_DHCP_CONFIRM, EVE_DHCP_REBOOT. 868 6.4.1.2. Following Actions 870 If the triggering event is EVE_DHCP_REQUEST/EVE_DHCP_SOLICIT_RC/ 871 EVE_DHCP_REBOOT: 873 The SAVI device MUST forward the message. 875 The SAVI device will generate an entry in the BST. The Binding 876 anchor field is set to the binding anchor of the attachment from 877 which the message is received. The State field is set to INIT_BIND. 878 The Lifetime field is set to be MAX_DHCP_RESPONSE_TIME. The TID 879 field is set to the TID of the message. If the message is DHCPv4 880 Request or DHCPv4 Reboot, the Address field can be set to the address 881 to request, i.e., the 'requested IP address'. An example of the 882 entry is illustrated in Figure 4. 884 +---------+-------+---------+-----------------------+-------+ 885 | Anchor |Address| State | Lifetime |TID | 886 +---------+-------+---------+-----------------------+-------+ 887 | Port_1 | |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID | 888 +---------+-------+---------+-----------------------+-------+ 890 Figure 4: Binding entry in BST on Request/Rapid Commit/Reboot 891 triggered initialization 893 If the triggering event is EVE_DHCP_CONFIRM: 895 The SAVI device MUST forward the message. 897 The SAVI device will generate corresponding entries in the BST for 898 all the addresses in each the IA option of the Confirm message. The 899 Binding anchor field is set to the binding anchor of the attachment 900 from which the message is received. The State field is set to 901 INIT_BIND. The Lifetime field is set to be MAX_DHCP_RESPONSE_TIME. 902 The TID field is set to the TID of the message. The Address field is 903 set to the address(es) to confirm. An example of the entries is 904 illustrated in Figure 5. 906 +---------+--------+---------+-----------------------+-------+ 907 | Anchor | Address| State | Lifetime |TID | 908 +---------+--------+---------+-----------------------+-------+ 909 | Port_1 | Addr1 |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID | 910 +---------+--------+---------+-----------------------+-------+ 911 | Port_1 | Addr2 |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID | 912 +---------+--------+---------+-----------------------+-------+ 914 Figure 5: Binding entry in BST on Confirm triggered initialization 916 6.4.2. From INIT_BIND to Other States 918 6.4.2.1. Trigger Events 920 Trigger events: EVE_DHCP_REPLY, EVE_ENTRY_EXPIRE. 922 Note: If no DHCP Server-Client messages which assign addresses or 923 confirm addresses are received, corresponding entries will expire 924 automatically. Thus, other DHCP Server-Client messages (e.g., DHCPv4 925 NAK) are not specially processed. 927 6.4.2.2. Following Actions 929 If the trigger event is EVE_DHCP_REPLY: 931 The message MUST be forwarded to the corresponding client. 933 If the message is DHCPv4 ACK, the Address field of the corresponding 934 entry (i.e., the binding entry whose TID is the same of the message) 935 is set to the address in the message(i.e., 'yiaddr' in DHCPv4 ACK). 936 The Lifetime field is set to the sum of the lease time in ACK message 937 and MAX_DHCP_RESPONSE_TIME. The State field is changed to BOUND. 939 If the message is DHCPv6 Reply, there are following cases: 941 1. If the status code is not "Success", no modification on 942 corresponding entries will be made. Corresponding entries will 943 expire automatically if no "Success" Reply is received during the 944 lifetime. The entries are not removed immediately due to the client 945 may be able to use the addresses whenever a "Success" Reply is 946 received ("If the client receives any Reply messages that do not 947 indicate a NotOnLink status, the client can use the addresses in the 948 IA and ignore any messages that indicate a NotOnLink status." 949 [RFC3315]). 951 2. If the status code is "Success", the SAVI device checks the IA 952 options in the Reply message. 954 2.1 If there are no IA options in the Reply message, the DHCP Reply 955 message is in response to a Confirm message. The state of the 956 binding entries with matched TID is changed to BOUND. Because 957 [RFC3315] does not require lease time of addresses to be contained in 958 the Reply message, the SAVI device MUST send a LEASEQUERY [RFC5007] 959 message querying by IP address to All_DHCP_Servers multicast address 960 [RFC3315] or a list of configured DHCP server addresses. The 961 Leasequery message is generated for each IP address if multiple 962 addresses are confirmed. The Lifetime of corresponding entries is 963 set to 2*MAX_LEASEQUERY_DELAY. If there is no response message after 964 MAX_LEASEQUERY_DELAY, send the LEASEQUERY message again. An example 965 of the entries is illustrated in Figure 6. The related security 966 problem about DHCPv6 LEASEQUERY is discussed in Section 11.5. 968 2.2 If there are IA options in the Reply message, the SAVI device 969 checks each IA option. When the first assigned address is found, the 970 Address field of the binding entry with matched TID is set to the 971 address. The Lifetime field is set to the sum of the lease time in 972 Reply message and MAX_DHCP_RESPONSE_TIME. The State field is changed 973 to BOUND. If there are more than one address assigned in the 974 message, new binding entries are set up for the remaining address 975 assigned in the IA options. An example of the entries is illustrated 976 in Figure 7. SAVI devices do not specially process IA options with 977 NoAddrsAvail status, because there should be no address contained in 978 such IA options. 980 Note: the SAVI devices do not check if the assigned addresses are 981 duplicated because in SAVI-DHCP scenarios, the DHCP servers are the 982 only source of valid addresses. However, the DHCP servers should be 983 configured to make sure no duplicated addresses are assigned. 985 +---------+----------+-------+------------------------+-------+ 986 | Anchor | Address | State | Lifetime |TID | 987 +---------+----------+-------+------------------------+-------+ 988 | Port_1 | Addr1 | BOUND | 2*MAX_LEASEQUERY_DELAY |TID | 989 +---------+----------+-------+------------------------+-------+ 990 | Port_1 | Addr2 | BOUND | 2*MAX_LEASEQUERY_DELAY |TID | 991 +---------+----------+-------+------------------------+-------+ 993 Figure 6: From INIT_BIND to BOUND on DHCP Reply in response to 994 Confirm 996 +---------+----------+-------+------------------------+-------+ 997 | Anchor | Address | State | Lifetime |TID | 998 +---------+----------+-------+------------------------+-------+ 999 | Port_1 | Addr1 | BOUND |Lease time+ |TID | 1000 | | | |MAX_DHCP_RESPONSE_TIME | | 1001 +---------+----------+-------+------------------------+-------+ 1002 | Port_1 | Addr2 | BOUND |Lease time+ |TID | 1003 | | | |MAX_DHCP_RESPONSE_TIME | | 1004 +---------+----------+-------+------------------------+-------+ 1006 Figure 7: From INIT_BIND to BOUND on DHCP Reply in response to 1007 Request 1009 If the trigger event is EVE_ENTRY_EXPIRE: 1011 The entry MUST be deleted from BST. 1013 6.4.3. From BOUND to Other States 1015 6.4.3.1. Trigger Events 1017 Trigger events: EVE_ENTRY_EXPIRE, EVE_DHCP_RELEASE, EVE_DHCP_DECLINE, 1018 EVE_DHCP_REPLY, EVE_DCHP_LEASEQUERY. 1020 6.4.3.2. Following Actions 1022 If the trigger event is EVE_ENTRY_EXPIRE: 1024 Remove the corresponding entry in BST. 1026 If the trigger event is EVE_DHCP_RELEASE/EVE_DHCP_DECLINE: 1028 The message MUST be forwarded. 1030 The SAVI device first gets all the addresses ("Requested IP address" 1031 in DHCPv4 Decline, "ciaddr" in DHCPv4 Release, addresses in all the 1032 IA options of DHCPv6 Decline/Release) to decline/release in the 1033 message. Then the corresponding entries MUST be removed. 1035 If the trigger event is EVE_DHCP_REPLY: 1037 The message MUST be forwarded. 1039 The DHCP Reply messages received in current states should be in 1040 response to DHCP Renew/Rebind. 1042 If the message is DHCPv4 ACK, the SAVI device just simply update the 1043 binding entry with matched TID, with the Lifetime field set to be the 1044 sum of the new lease time and MAX_DHCP_RESPONSE_TIME. 1046 If the message is DHCPv6 Reply, the SAVI device checks each IA 1047 Address option in each IA option. If the valid lifetime of an IA 1048 address option is 0, the binding entry with matched TID and address 1049 is removed. Or else, set the Lifetime field of the binding entry 1050 with matched TID and address to be the sum of the new valid lifetime 1051 and MAX_DHCP_RESPONSE_TIME. 1053 The SAVI device does not specially process IA options in Reply 1054 message with status NoBinding, because no address is contained in 1055 such IA options and no actions will be performed. 1057 If the trigger event is EVE_DCHP_LEASEQUERY: 1059 The message MUST be forwarded. 1061 The message should be in response to the Leasequery message sent in 1062 Section 6.4.2. The related binding entry can be determined based on 1063 the address in the IA Address option in the Leasequery-reply message. 1064 The Lifetime field of the corresponding binding entry is set to the 1065 sum of the lease time in the LEASEQUERY_REPLY message and 1066 MAX_DHCP_RESPONSE_TIME. 1068 6.5. Table of State Machine 1070 The main state transits are listed as follows. Note that not all the 1071 details are specified in the table and the diagram. 1073 State Event Action Next State 1074 NO_BIND RQ/RC/CF/RE Generate entry INIT_BIND 1075 INIT_BIND RPL Record lease time BOUND 1076 (send lease query if no lease) 1077 INIT_BIND Timeout Remove entry NO_BIND 1078 BOUND RLS/DCL Remove entry NO_BIND 1079 BOUND Timeout Remove entry NO_BIND 1080 BOUND RPL Set new lifetime BOUND 1081 BOUND LQR Record lease time BOUND 1083 Figure 8: Table of Transit 1085 RQ: EVE_DHCP_REQUEST 1087 CF: EVE_DHCP_CONFIRM 1089 RC: EVE_DHCP_SOLICIT_RC 1091 RE: EVE_DHCP_REBOOT 1093 RPL: EVE_DHCP_REPLY 1095 DCL: EVE_DHCP_DECLINE 1097 RLS: EVE_DHCP_RELEASE 1099 LQR: EVE_DCHP_LEASEQUERY 1101 Timeout: EVE_ENTRY_EXPIRE 1102 +-------------+ 1103 | | 1104 /---------| NO_BIND |<----------\ 1105 | ------>| | | 1106 | | +-------------+ |EVE_DHCP_RELEASE 1107 EVE_DHCP_REQUEST | | |EVE_DHCP_DECLINE 1108 EVE_DHCP_CONFIRM | |TIMEOUT |TIMEOUT 1109 EVE_DHCP_SOLICIT_RC| | | 1110 EVE_DHCP_REBOOT | | | 1111 | | | 1112 | | | 1113 v | | 1114 +-------------+ +------------+ 1115 | | EVE_DHCP_REPLY | | 1116 | INIT_BIND ------------------------>| BOUND |<-\ 1117 | | | | | 1118 +-------------+ +------------+ | 1119 | | 1120 \--------/ 1121 EVE_DHCP_REPLY 1122 EVE_DCHP_LEASEQUERY 1124 Figure 9: Diagram of Transit 1126 7. Data Snooping Process 1128 7.1. Scenario 1130 The rationale of the DHCP Snooping Process specified in Section 6 is 1131 that if a DHCP client's use of a DHCP address is legitimate, the 1132 corresponding DHCP address assignment procedure must have been 1133 finished on the attachment of the DHCP client. This is the case 1134 stands when the SAVI device is persistently on the path(s) from the 1135 DHCP client to the DHCP server(s)/relay(s). However, there are two 1136 case when this does not work: 1138 o Multiple paths: there is more than one feasible layer-2 paths 1139 from the client to the DHCP server/relay, and the SAVI device is 1140 not on everyone of them. The client may get its address through 1141 one of the paths not passing by the SAVI device, but packets from 1142 the client can travel through paths that pass through the SAVI 1143 device. Because the SAVI device could not snoop the DHCP packet 1144 exchange procedure, the DHCP snooping procedure cannot set up the 1145 corresponding binding. 1147 o Dynamic path: there is only one feasible layer-2 path from the 1148 client to the DHCP server/relay, but the path is dynamic due to 1149 topology change (for example, some link turns broken due to 1150 failure or as planned) or layer-2 path change. This situation 1151 also covers the local-link movement of clients without address 1152 confirm/re-configuration process. For example, a host changes 1153 its attached switch port in a very short time. In such cases, 1154 the DHCP snooping process will not set up the corresponding 1155 binding. 1157 Data Snooping Process prevents permanently blocking legitimate 1158 traffic in case of these two exceptions. This process is performed 1159 on attachments with the Data-Snooping attribute. Data packets 1160 without matching binding entry may trigger this process to set up 1161 bindings. 1163 Snooping data traffic introduces considerable burden on the processor 1164 and ASIC-to-Processor bandwidth of SAVI devices. Because of the 1165 overhead of this process, the implementation of this process is a 1166 conditional SHOULD. This function SHOULD be enabled unless the 1167 implementation is known to be used in the scenarios without the above 1168 exceptions. For example, if the implementation is to be used in 1169 networks with tree topology and without host local-link movement, 1170 there is no need to implement this process in such scenarios. 1172 This process is not intended to set up a binding whenever a data 1173 packet without matched binding entry is received. Instead, unmatched 1174 data packets trigger this process probabilistically and generally a 1175 number of unmatched packets will be discarded before the binding is 1176 set up. 1178 To perform this process, the SAVI device MUST join the Solicited Node 1179 Multicast group of the source address of triggering IPv6 data packet 1180 whenever performing duplicate detection. 1182 7.2. Rationale 1184 This process makes use of DAD/ARP and DHCP Leasequery to set up 1185 bindings. If an address is not used by another client in the 1186 network, and the address has been assigned in the network, the 1187 address can be bound with the binding anchor of the attachment from 1188 which the unmatched packet is received. 1190 The security issues about this process is discussed is Section 11.1. 1192 7.3. Additional Binding States Description 1194 In addition to Section 6.2, new states used in this process are 1195 listed here: 1197 DETECTION: The address in the entry is under local duplication 1198 detection. 1200 RECOVERY: The SAVI device is querying the assignment and lease time 1201 of the address in the entry through DHCP Leasequery. 1203 7.4. Events 1205 Additional events in this process are described here. Also, if an 1206 event will trigger the creation of a new binding entry, the binding 1207 entry limit on the binding anchor MUST NOT be exceeded. 1209 EVE_DATA_UNMATCH: A data packet without matched binding is received. 1211 EVE_DATA_CONFLICT: ARP Reply/Neighbor Advertisement(NA) message 1212 against an address in DETECTION state is received from a host other 1213 than the one for which the entry was added. 1215 EVE_DATA_LEASEQUERY: 1217 IPv4: A DHCPLEASEACTIVE message with IP Address Lease Time option 1218 is received. 1220 IPv6: A successful LEASEQUERY-REPLY is received. 1222 The triggering packet should pass the following checks to trigger a 1223 valid event: 1225 o Attribute check: the data packet should be from attachments with 1226 Data-Snooping attribute; the DHCPLEASEACTIVE/LEASEQUERY_REPLY 1227 messages should be from attachments with DHCP-Snooping attribute. 1229 o Binding limitation check: the DHCP messages must not cause new 1230 binding setup on an attachment whose binding entry limitation has 1231 been reached. (refer to Section 11.6). 1233 o Address check: For EVE_DATA_LEASEQUERY, the source address of the 1234 DHCP Leasequery messages must pass the check specified in 1235 Section 8.2. For EVE_DATA_CONFLICT, the source address and 1236 target address of the ARP or NA messages must pass the check 1237 specified in Section 8.2. 1239 o Interval check: the interval between two successive 1240 EVE_DATA_UNMATCH events triggered by an attachment MUST be no 1241 smaller than DATA_SNOOPING_INTERVAL. 1243 o TID check: the DHCPLEASEACTIVE/LEASEQUERY-REPLY messages must 1244 have matched TID with the corresponding entry. 1246 o Prefix check: the source address of the data packet should be of 1247 a valid local prefix, as specified in section 7 of [RFC7039]. 1249 7.5. State Machine of Binding Recovery Process 1251 Through using additional states, the state machine of this process 1252 doesn't conflict the regular process described in Section 6. Thus, 1253 it can be implemented separately without changing the state machine 1254 in Section 6. 1256 7.5.1. From NO_BIND to DETECTION 1258 7.5.1.1. Trigger Event 1260 Trigger event: EVE_DATA_UNMATCH. 1262 7.5.1.2. Following Actions 1264 Make a probabilistic determination whether to act on this event. The 1265 probability can be configured or calculated based on the state of the 1266 SAVI device. This probability should be low enough to mitigate the 1267 damage from DoS attack against this process. 1269 Create a new entry in the BST. Set the Binding Anchor field to the 1270 corresponding binding anchor of the attachment. Set the Address 1271 field to be source address of the packet. Set the State field to 1272 DETECTION. Set the Lifetime of the created entry to 2*DAD_TIMEOUT. 1274 Check if the address has a local conflict (it violates an address 1275 being used by another node): 1277 (1) IPv4 address: send an Address Resolution Protocol (ARP) Request 1278 [RFC826]or a ARP probe [RFC5227] on the address; if there is no 1279 response message after DAD_TIMEOUT, send another ARP Request or 1280 ARP probe; 1282 (2) IPv6 address: perform Duplicate Address Detection (DAD) 1283 [RFC4862] on the address; if there is no response message after 1284 DAD_TIMEOUT, perform another DAD procedure. 1286 Because the delivery of detection message is unreliable, the 1287 detection message may fail to reach the targeting node. If there is 1288 a node that has the IP address seen in the Data Snooping Process, it 1289 may not get the detection messages. This failure mode enables an 1290 attack against the Data Snooping Process. Thus, the detection is 1291 performed again if there is no response after the first detection. 1293 The messages MUST NOT be sent to the attachment from which the 1294 triggering packet is received. 1296 The packet which triggers this event SHOULD be discarded. 1298 An example of the entry is illustrated in Figure 10. 1300 +---------+-------+---------+-----------------------+-------+ 1301 | Anchor |Address| State | Lifetime |TID | 1302 +---------+-------+---------+-----------------------+-------+ 1303 | Port_1 | Addr1 |DETECTION|2*DAD_TIMEOUT | | 1304 +---------+-------+---------+-----------------------+-------+ 1306 Figure 10: Binding entry in BST on data triggered initialization 1308 7.5.2. From DETECTION to Other States 1310 7.5.2.1. Trigger Event 1312 Trigger events: EVE_ENTRY_EXPIRE, EVE_DATA_CONFLICT. 1314 7.5.2.2. Following Actions 1316 If the trigger event is EVE_ENTRY_EXPIRE: 1318 (1) IPv4 address: Send a DHCPLEASEQUERY [RFC4388] message querying 1319 by IP address to each DHCPv4 server with IP Address Lease Time 1320 option (option 51). A list of authorized DHCP servers are kept 1321 by the SAVI device. The list should be pre-configured or 1322 discovered by sending DHCPv4 Discover messages and parsing the 1323 replied DHCPv4 Offer messages. Change the state of the 1324 corresponding entry to RECOVERY. Change the lifetime of the 1325 entry to be 2*MAX_LEASEQUERY_DELAY. The TID field is set to the 1326 TID used in the DHCPLEASEQUERY message. If there is no response 1327 message after MAX_LEASEQUERY_DELAY, send a DHCPLEASEQUERY to 1328 each DHCPv4 server again. 1330 (2) IPv6 address: Send a LEASEQUERY [RFC5007] message querying by IP 1331 address to All_DHCP_Relay_Agents_and_Servers multicast address 1332 or a list of pre-configured DHCPv6 server addresses. Change the 1333 state of the corresponding entry to RECOVERY. Change the 1334 lifetime of the entry to be 2*MAX_LEASEQUERY_DELAY. The TID 1335 field is set to the TID used in the LEASEQUERY message. If 1336 there is no response message after MAX_LEASEQUERY_DELAY, send 1337 the LEASEQUERY message again. 1339 An example of the entry is illustrated in Figure 11. 1341 +---------+-------+---------+-----------------------+-------+ 1342 | Anchor |Address| State | Lifetime |TID | 1343 +---------+-------+---------+-----------------------+-------+ 1344 | Port_1 | Addr1 |RECOVERY |2*MAX_LEASEQUERY_DELAY |TID | 1345 +---------+-------+---------+-----------------------+-------+ 1347 Figure 11: Binding entry in BST on Lease Query 1349 If the trigger event is EVE_DATA_CONFLICT: 1351 Remove the entry. 1353 7.5.3. From RECOVERY to Other States 1355 7.5.3.1. Trigger Event 1357 Trigger events: EVE_ENTRY_EXPIRE, EVE_DATA_LEASEQUERY. 1359 7.5.3.2. Following Actions 1361 If the trigger event is EVE_DATA_LEASEQUERY: 1363 IPv4 address: 1365 (1) Send an ARP Request with the Target Protocol Address set to the 1366 IP address in the corresponding entry. The ARP Request is only 1367 sent to the attachment which triggers the binding. If there is 1368 no response after DAD_TIMEOUT, send another ARP Request. If 1369 there is still no response, the following actions will not be 1370 performed. If there is only one identical response, get the 1371 sender hardware address. Check if the 'chaddr' field (hardware 1372 address) of the DHCPLEASEACTIVE message matches the sender 1373 hardware address. If the two addresses do not match, the 1374 following actions will not be performed. If there is more than 1375 one response, if any of the sender hardware addresses matches 1376 the 'chaddr' field (hardware address) of the DHCPLEASEACTIVE 1377 message, the following actions are to be performed. 1379 (2) Change the state of the corresponding binding to BOUND. Set 1380 life time to the sum of the value encoded in IP Address Lease 1381 Time option of the DHCPLEASEACTIVE message and 1382 MAX_DHCP_RESPONSE_TIME. Erase the TID field. 1384 IPv6 address: 1386 (1) Send a Neighbor Solicitation message with the target address set 1387 to the IP address in the corresponding entry. The Neighbor 1388 Solicitation is only sent to the attachment which triggers the 1389 binding. If there is no response after DAD_TIMEOUT, send 1390 another Neighbor Solicitation. If there is still no response, 1391 the following actions will not be performed. 1393 (2) Change the state of the corresponding binding to BOUND. Set the 1394 lifetime to the sum of the valid lifetime extracted from 1395 OPTION_CLIENT_DATA option in the LEASEQUERY-REPLY message and 1396 MAX_DHCP_RESPONSE_TIME. Erase the TID field. 1398 (3) After the above checks, if multiple addresses are specified in 1399 the LEASEQUERY-REPLY message and there are no corresponding 1400 binding entries, new entries MUST also be created 1401 correspondingly on the same binding anchor. 1403 If responses are received from multiple DHCP servers, the conflict 1404 resolution mechanisms specified in section 6.8 of [RFC4388] and 1405 section 4.3.4 of [RFC5007] will be used to determine which message 1406 should be used. 1408 If the trigger event is EVE_ENTRY_EXPIRE: 1410 Remove the entry. 1412 7.5.4. After BOUND 1414 Note that the TID field contains no value after the binding state 1415 changes to BOUND. The TID field is recovered from snooping DHCP 1416 Renew/Rebind messages. Because TID is used to associate binding 1417 entries with messages from DHCP servers, it must be recovered; or 1418 else a number of state transits of this mechanism will be not 1419 executed normally. 1421 7.5.4.1. Trigger Event 1423 Trigger events: EVE_DHCP_RENEW, EVE_DHCP_REBIND. 1425 7.5.4.2. Following Action 1427 Set the TID field of the corresponding entry to the TID in the 1428 triggering message. 1430 7.6. Table of State Machine 1432 The main state transits are listed as follows. 1434 State Event Action Next State 1435 NO_BIND EVE_DATA_UNMATCH Duplication detection DETECTION 1436 DETECTION Timeout Send Leasequery RECOVERY 1437 DETECTION EVE_DATA_CONFLICT Remove entry NO_BIND 1438 RECOVERY EVE_DATA_LEASEQUERY Set lease time BOUND or NO_BIND 1439 RECOVERY Timeout Remove entry NO_BIND 1440 BOUND RENEW/REBIND Record TID BOUND 1442 Figure 12: Table of Transit 1444 RENEW: EVE_DHCP_RENEW 1446 REBIND: EVE_DHCP_REBIND 1448 Timeout: EVE_ENTRY_EXPIRE 1449 +-------------+ 1450 | | 1451 /---------| NO_BIND |<--------\ 1452 | ------>| | | TIMEOUT 1453 | | +-------------+ |(2nd LQ_DELAY) 1454 EVE_DATA_UNMATCH | | | 1455 | | | 1456 | | | 1457 1st DAD_TIMEOUT | | | 1st LQ_DELAY 1458 /------\ | | | /---------\ 1459 | | | | EVE_DATA_CONFLICT | | | 1460 | v v | | v | 1461 | +-------------+ TIMEOUT +------------+ | 1462 | | | (2nd DAD_TIMEOUT) | | | 1463 \----| DETECTION ------------------------>| RECOVERY ----/ 1464 | | | | 1465 +-------------+ +------------+ 1466 EVE_DATA_LEASEQUERY| 1467 /----------\ (+ 2x DAD) | 1468 EVE_DHCP_RENEW| | | 1469 EVE_DHCP_REBIND| +-----v-------+ | 1470 | | | | 1471 \----| BOUND |<----------/ 1472 | | 1473 +-------------+ 1475 Figure 13: Diagram of Transit 1477 LQ_DELAY: MAX_LEASEQUERY_DELAY 1479 8. Filtering Specification 1481 This section specifies how to use bindings to filter out spoofing 1482 packets. 1484 Filtering policies are different for data packets and control 1485 packets. DHCP and NDP (Neighbor Discovery Protocol) [RFC4861] 1486 messages that may cause state transit are classified as control 1487 packet. Neighbor Advertisement (NA) and ARP Reply are also included 1488 in control packet because the Target Address of NA and ARP Reply 1489 should be checked to prevent spoofing. All other packets are 1490 classified as data packets. 1492 8.1. Data Packet Filtering 1494 Data packets from attachments with the Validating attribute MUST be 1495 checked. 1497 Packet whose source IP address is a link-local address will not be 1498 checked. Note: as explained in Section 1, a SAVI solution for link- 1499 local addresses, e.g., the SAVI-FCFS [RFC6620], can be enabled to 1500 check packets with link-local source address. 1502 If the source IP address of a packet is not a link-local address, but 1503 there is not a matched entry in BST with state BOUND, this packet 1504 MUST be discarded. However, the packet may trigger Data Snooping 1505 Process Section 7 if Data-Snooping attribute is set on the 1506 attachment. 1508 Data packets from attachments with no attribute will forwarded 1509 without checking. 1511 The SAVI device MAY record any violation. 1513 8.2. Control Packet Filtering 1515 For attachments with the Validating attribute: 1517 Discard DHCPv4 Client-Server message messages whose source IP address 1518 is neither all zeros nor bound with the corresponding binding anchor 1519 in the BST. 1521 Discard DHCPv6 Client-Server message messages whose source IP address 1522 is neither a link-local address nor bound with the corresponding 1523 binding anchor in the BST. 1525 Discard NDP messages whose source IP address is neither a link-local 1526 address nor bound with the corresponding binding anchor. Especially, 1527 discard NA message whose target address is neither a link-local 1528 address nor bound with the corresponding binding anchor. 1530 Discard ARP messages whose protocol is IP and sender protocol address 1531 is neither all zeros address nor bound with the corresponding binding 1532 anchor. Especially, discard ARP Reply messages whose target protocol 1533 address is not bound with the corresponding binding anchor. 1535 For attachments with other attributes: 1537 Discard DHCP Server-Client message not from attachments with the 1538 DHCP-Trust attribute or Trust attribute. 1540 For attachments with no attribute: 1542 Discard DHCP Server-Client message from such attachments. 1544 The SAVI device MAY record any violation. 1546 9. State Restoration 1548 If a SAVI device reboots, the information kept in volatile memory 1549 will be lost. This section specifies the restoration of attribute 1550 configuration and BST. 1552 9.1. Attribute Configuration Restoration 1554 The lost of attribute configuration will not break the network: no 1555 action will be performed on traffic from attachments with no 1556 attribute. However, the lost of attribute configuration makes this 1557 SAVI function unable to work. 1559 To avoid the loss of binding anchor attribute configuration, the 1560 configuration MUST be able to be stored in non-volatile storage. 1561 After the reboot of SAVI device, if the configuration of binding 1562 anchor attribute can be found in non-volatile storage, the 1563 configuration MUST be used. 1565 9.2. Binding State Restoration 1567 The loss of binding state will cause the SAVI devices discard 1568 legitimate traffic. Purely using the Data Snooping Process to 1569 recover a large number of bindings is of heavy overhead and 1570 considerable delay. Thus, to recover bindings from non-volatile 1571 storage, as specified below, is RECOMMENDED. 1573 Binding entries MAY be saved into non-volatile storage whenever a new 1574 binding entry changes to BOUND state. If a binding with BOUND state 1575 is removed, the saved entry MUST be removed correspondingly. The 1576 time when each binding entry is established is also saved. 1578 Immediately after reboot, the SAVI device SHOULD restore binding 1579 states from the non-volatile storage. The system time of save 1580 process MUST be stored. After rebooting, the SAVI device MUST check 1581 whether each entry has been obsolete by comparing the saved lifetime 1582 and the difference between the current time and time when the binding 1583 entry is established. 1585 10. Constants 1587 MAX_DHCP_RESPONSE_TIME 120s 1589 DATA_SNOOPING_INTERVAL 60s and configurable 1591 MAX_LEASEQUERY_DELAY 10s 1593 OFFLINK_DELAY 30s 1595 DAD_TIMEOUT 0.5s 1597 11. Security Considerations 1599 11.1. Security Problems about the Data Snooping Process 1601 There are two security problems about the Data Snooping Process 1602 Section 7: 1604 (1) The Data Snooping Process is costly, but an attacker can trigger 1605 it simply through sending a number of data packets. To avoid 1606 Denial of Services attack against the SAVI device itself, the 1607 Data Snooping Process MUST be rate limited. A constant 1608 DATA_SNOOPING_INTERVAL is used to control the frequency. Two 1609 Data Snooping Processes on one attachment MUST have a minimum 1610 interval time DATA_SNOOPING_INTERVAL. This constant SHOULD be 1611 configured prudently to avoid Denial of Service attacks. 1613 (2) The Data Snooping Process may set up wrong bindings if the 1614 clients do not reply to the detection probes. An attack will 1615 pass the duplicate detection if the client assigned the target 1616 address does not reply to the detection probes. The DHCP 1617 Leasequery procedure performed by the SAVI device just tells 1618 whether the address is assigned in the network or not. However, 1619 the SAVI device cannot determine whether the address is just 1620 assigned to the triggering attachment from the DHCP Leasequery 1621 Reply. 1623 11.2. Issues about Leaving Clients 1625 After a binding is set up, the corresponding client may leave its 1626 attachment point. It may leave temporarily due to link flapping, or 1627 permanently by moving to a new attachment point or leaving the 1628 network. Since the client may return shortly, the binding should be 1629 kept, or legtimate traffic from the client will be blocked. However, 1630 if the client leaves permanently, it may be insecure to keep the 1631 binding. If the binding anchor is a property of the attachment point 1632 rather than the client, e.g., the switch port, an attacker which is 1633 attached to the attachment point of the leaving client can send 1634 spoofing packets with the addresses assigned to the client. Even if 1635 the binding anchor is a property of the client, it is a waste of 1636 binding resources to keep bindings for departed clients. 1638 The following mechanism is designed to handle the leaving of client: 1640 (1) Whenever a client with the Validating attribute leaves, a timer 1641 of duration OFFLINK_DELAY is set on the corresponding binding 1642 entries. 1644 (2) If a DAD Neighbor Solicitation/Gratuitous ARP request is 1645 received that targets the address during OFFLINK_DELAY, the 1646 entry MAY be removed. 1648 (3) If the client returns on-link during OFFLINK_DELAY, cancel the 1649 timer. 1651 In this way, the bindings of a departing client are kept for 1652 OFFLINK_DELAY. In case of link flapping, the client will not be 1653 blocked. If the client leaves permanently, the bindings will be 1654 removed after OFFLINK_DELAY. 1656 11.3. Duplicate Bindings to the Same Address 1658 The same address may be bound to multiple binding anchors only if the 1659 binding setup processes successfully complete for each binding 1660 anchor. This mechanism is designed to address the case where a 1661 client moves on the local link, and the case where a client has 1662 multiple attachments to a SAVI device. 1664 There are two security issues with such a design: 1666 First, by allowing one address to be bound to multiple binding 1667 anchors, the traceability of the address is weakened. An address can 1668 be traced to multiple attachments. 1670 Second, in the local link movement scenario, the former binding may 1671 not be removed and it can be used by an attacker sharing the same 1672 binding anchor. For example, when a switch port is used as binding 1673 anchor and the port is shared by an attacker and a client with a hub, 1674 the attacker can make use of the address assigned to the client after 1675 the client leaves. 1677 11.4. Compatibility with DNA (Detecting Network Attachment) 1679 DNA [RFC4436] [RFC6059] is designed to decrease the handover latency 1680 after re-attachment to the same network. DNA mainly relies on 1681 performing reachability test by sending unicast Neighbor 1682 Solicitation/Router Solicitation/ARP Request message to determine 1683 whether a previously configured address is still valid. 1685 Although DNA provides optimization for clients, there is insufficient 1686 information for this mechanism to migrate the previous binding or 1687 establish a new binding. If a binding is set up only by snooping the 1688 reachability test message, the binding may be invalid. For example, 1689 an attacker can perform reachability test with an address bound to 1690 another client. If binding is migrated to the attacker, the attacker 1691 can successfully obtain the binding from the victim. Because this 1692 mechanism wouldn't set up a binding based on snooping the DNA 1693 procedure, it cannot achieve perfect compatibility with DNA. 1694 However, it only means the re-configuration of the interface is 1695 slowed but not prevented. Details are discussed as follows. 1697 In Simple DNAv6 [RFC6059], the probe is sent with the source address 1698 set to a link-local address, and such messages will not be discarded 1699 by the policy specified in section Section 8.2. If a client is re- 1700 attached to a previous network, the detection will be completed, and 1701 the address will be regarded as valid by the client. However, the 1702 candidate address is not contained in the probe. Thus, the binding 1703 cannot be recovered through snooping the probe. As the client will 1704 perform DHCP exchange at the same time, the binding will be recovered 1705 from the DHCP Snooping Process. The DHCP Request messages will not 1706 be filtered out in this case because they have link-local source 1707 addresses. Before the DHCP procedure is completed, packets will be 1708 filtered out by the SAVI device. In other words, if this SAVI 1709 function is enabled, Simple DNAv6 will not help reduce the handover 1710 latency. If Data-Snooping attribute is configured on the new 1711 attachment of the client, the data triggered procedure may reduce 1712 latency. 1714 In DNAv4 [RFC4436], the ARP probe will be discarded because an 1715 unbound address is used as the sender protocol address. As a result, 1716 the client will regard the address under detection is valid. 1717 However, the data traffic will be filtered. The DHCP Request message 1718 sent by the client will not be discarded, because the source IP 1719 address field should be all zero as required by [RFC2131]. Thus, if 1720 the address is still valid, the binding will be recovered from the 1721 DHCP Snooping Process. 1723 11.5. Authentication in DHCPv6 Leasequery 1725 As required in section 5 of RFC5007, DHCPv6 Leasequery 'Should' use 1726 IPsec-based authentication specified in the section 21.1 of RFC3315. 1727 However, with the deployment of this mechanism, there may be no need 1728 to enforce IPSec to perform DHCP Leasequery. 1730 By containing the DHCP servers in the protection perimeter, the DHCP 1731 servers can be protected from spoofing based attacks. Then by 1732 checking the source IP address of Leasequery messages, the DHCP 1733 server can identify if the messages are from SAVI devices or not. 1734 For the SAVI devices, because the perimeter filters out bogus DHCP 1735 messages, they can trust the DHCP Leasequery responses. Thus, there 1736 is no need to enforce IPSec to validate the DHCP Leasequery messages 1737 in this mechanism. 1739 11.6. Binding Number Limitation 1741 A binding entry will consume a certain high-speed memory resources. 1742 In general, a SAVI device can afford only a quite limited number of 1743 binding entries. In order to prevent an attacker from overloading 1744 the resource of the SAVI device, a binding entry limit is set on each 1745 attachment. The binding entry limit is the maximum number of 1746 bindings supported on each attachment with Validating attribute. No 1747 new binding should be set up after the limit has been reached. If a 1748 DHCP Reply assigns more addresses than the remaining binding entry 1749 quota of each client, the message will be discarded and no binding 1750 will be set up. 1752 11.7. Residual Threats 1754 As described in [RFC7039], this solution cannot strictly prevent 1755 spoofing. There are two scenarios in which spoofing can still 1756 happen: 1758 (1) The binding anchor is spoofable. If the binding anchor is 1759 spoofable, e.g., plain MAC address, an attacker can use forged 1760 binding anchor to send packet which will not be regarded as 1761 spoofing by SAVI device. Indeed, using binding anchor that can 1762 be easily spoofed is more serious than allowing IP spoofing 1763 traffic. For example, an attacker can use the binding anchor of 1764 another client to get a large number of addresses, and the SAVI 1765 device will refuse to set up new binding for the client whenever 1766 the binding number limitation has been reached. Thus, it is 1767 RECOMMENDED to use strong enough binding anchor, e.g., switch 1768 port, secure association in 802.11ae/af and 802.11i. 1770 (2) The binding anchor is shared by more than one clients. If the 1771 binding anchor is shared by more than one clients, the clients 1772 can spoof each other addresses. For example, if a switch port 1773 is used as binding anchor, a number of clients can attach to the 1774 same switch port of a SAVI device through a hub. The SAVI 1775 device cannot distinguish packets from different clients and 1776 thus the spoofing between them will not be detected. A number 1777 of the above security problems are caused by sharing binding 1778 anchors. If binding anchor is shared, TID spoofing based attack 1779 is possible. Thus, it is RECOMMENDED to use exclusive binding 1780 anchor. 1782 12. IANA Considerations 1784 This memo asks the IANA for no new parameters. 1786 Note to RFC Editor: This section will have served its purpose if it 1787 correctly tells IANA that no new assignments or registries are 1788 required, or if those assignments or registries are created during 1789 the RFC publication process. From the authors' perspective, it may 1790 therefore be removed upon publication as an RFC at the RFC Editor's 1791 discretion. 1793 13. Acknowledgment 1795 Special thanks to Jean-Michel Combes, Christian Vogt, Joel M. 1796 Halpern, Eric Levy-Abegnoli, Marcelo Bagnulo Braun, Jari Arkko, Elwyn 1797 Davies, Barry Leiba, Ted Lemon, Leaf Yeh, Ralph Droms and Alberto 1798 Garcia for careful review and valuation comments on the mechanism and 1799 text. 1801 Thanks to Mark Williams, Erik Nordmark, Mikael Abrahamsson, David 1802 Harrington, Pekka Savola, Xing Li, Lixia Zhang, Bingyang Liu, Duanqi 1803 Zhou, Robert Raszuk, Greg Daley, John Kaippallimalil and Tao Lin for 1804 their valuable contributions. 1806 This document was generated using the xml2rfc tool. 1808 14. References 1810 14.1. Informative References 1812 [BA2007] Baker, F., "Cisco IP Version 4 Source Guard", IETF 1813 Internet draft (work in progress), November 2007. 1815 [BCP38] Paul, P. and D. Senie, "Network Ingress Filtering: 1816 Defeating Denial of Service Attacks which employ IP Source 1817 Address Spoofing", RFC 2827, BCP 38, May 2000. 1819 [RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol 1820 (DHCP) Service for IPv6", RFC 3736, April 2004. 1822 14.2. Normative References 1824 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1825 Requirement Levels", RFC 2119, BCP 14, Match 1997. 1827 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 1828 RFC 2131, March 1997. 1830 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 1831 and M. Carney, "Dynamic Host Configuration Protocol for 1832 IPv6 (DHCPv6)", RFC 3315, July 2003. 1834 [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration 1835 Protocol (DHCP) Leasequery", RFC 4388, February 2006. 1837 [RFC4436] Aboba, B., Carlson, J., and S. Cheshire, "Detecting 1838 Network Attachment in IPv4 (DNAv4)", RFC 4436, March 2006. 1840 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1841 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 1842 September 2007. 1844 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 1845 Address Autoconfiguration", RFC 4862, September 2007. 1847 [RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng, 1848 "DHCPv6 Leasequery", RFC 5007, September 2007. 1850 [RFC5227] Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227, 1851 July 2008. 1853 [RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for 1854 Detecting Network Attachment in IPv6", RFC 6059, 1855 November 2010. 1857 [RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS- 1858 SAVI: First-Come First-Serve Source-Address Validation for 1859 Locally Assigned Addresses", RFC 6620, May 2012. 1861 [RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed., 1862 "Source Address Validation Improvement Framework", 1863 RFC 7039, October 2013. 1865 [RFC826] Plummer, D., "Ethernet Address Resolution Protocol: Or 1866 converting network protocol addresses to 48.bit Ethernet 1867 address for transmission on Ethernet hardware", RFC 826, 1868 November 1982. 1870 Authors' Addresses 1872 Jun Bi 1873 Tsinghua University 1874 Network Research Center, Tsinghua University 1875 Beijing 100084 1876 China 1878 Email: junbi@tsinghua.edu.cn 1880 Jianping Wu 1881 Tsinghua University 1882 Computer Science, Tsinghua University 1883 Beijing 100084 1884 China 1886 Email: jianping@cernet.edu.cn 1888 Guang Yao 1889 Tsinghua University 1890 Network Research Center, Tsinghua University 1891 Beijing 100084 1892 China 1894 Email: yaoguang@cernet.edu.cn 1896 Fred Baker 1897 Cisco Systems 1898 Santa Barbara, CA 93117 1899 United States 1901 Email: fred@cisco.com