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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (March 11, 2014) is 3699 days in the past. 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) Summary: 1 error (**), 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: September 12, 2014 Tsinghua Univ. 6 F. Baker 7 Cisco 8 March 11, 2014 10 SAVI Solution for DHCP 11 draft-ietf-savi-dhcp-20 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 September 12, 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 Other States . . . . . . . . . . . . . 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 Other States . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . 34 125 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 36 130 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) . . 38 135 11.5. Authentication in DHCPv6 Leasequery . . . . . . . . . . . 39 136 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 . . . . . . . . . . . . . . . . . . . 41 143 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42 145 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 [savi-fcfs] 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 [savi-fcfs]. 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 [savi-framework]. A list of proper 196 binding anchors can be found in Section 3.2 of [savi-framework]. 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 B 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 A to the SAVI Device B and the attachment from the SAVI Device 449 B to the SAVI Device A 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 B, 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 [savi-fcfs]). 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. SAVI device A doesn't create a binding for 580 client B. But the SAVI device B is still protected from spoofing from 581 client A and the SAVI device A is still protected from spoofing from 582 client B. 584 There is three main differences between the SAVI-DHCP protection 585 perimeter and SAVI-FCFS protection perimeter: 587 (1) SAVI-DHCP follows the state announced in DHCP messages, so there 588 is no need to distribute state using Neighbor Solicitation/ 589 Neighbor Advertisement messages. 591 (2) The perimeter in SAVI-DHCP is not only a perimeter for data 592 packets, but also a perimeter for DHCP messages. The placement 593 of DHCP Relay/Server, which is not involved in SAVI-FCFS , is 594 related with the construction of the perimeter. The requirement 595 on the placement and configuration of DHCP Relay/Server are 596 discussed in Section 4.3.3. 598 (3) Downstream/upstream links MUST be distinguished when configuring 599 the perimeter to avoid estabilshing binding for addresses of 600 other networks. 602 4.3.2. SAVI-DHCP Perimeter Configuration Guideline 604 Through configuring attribute of each attachment properly, a 605 perimeter separating untrusted area and trusted area can be formed: 607 (1) Configure Validating and DHCP-Snooping attribute on the direct 608 attachments of all the DHCP clients. 610 (2) Configure Validating and DHCP-Snooping attribute on the indirect 611 attachments of all the DHCP clients(i.e., DHCP clients on the 612 downstream links). 614 (3) Configure Trust attribute on the attachments of other SAVI 615 devices. 617 (4) If a Non-SAVI device, or a number of connected Non-SAVI devices, 618 have only attachments from SAVI devices or upstream devices, set 619 their attachments to SAVI devices with Trust attribute. 621 (5) Configure DHCP-Trust attribute on the direct attachments of 622 trusted DHCP relays/servers. 624 (6) Optional: configure filters on the upstream links to filter out 625 spoofing of local addresses from other networks. 627 In this way, the points of attachments with Validating attribute (and 628 generally together with attachments of upstream devices) on SAVI 629 devices can form a perimeter separating DHCP clients and trusted 630 devices. Data packet check is only performed on the perimeter. The 631 perimeter is also a perimeter for DHCP messages. DHCP-Trust 632 attribute is only configured on the inside links of the perimeter. 633 Only DHCP server-client messages originated in the perimeter is 634 trusted. 636 4.3.3. On the Placement of DHCP Server/Relay 638 Based on the configuration guideline, it can be found that the SAVI 639 devices only trust DHCP Server-Client messages originated inside the 640 perimeter. It means the trusted DHCP relays/servers must be placed 641 in the perimeter. DHCP server-client messages will be filtered on 642 the perimeter (Note: server-relay messages will not be filtered). In 643 this way, DHCP server-client messages from bogus DHCP servers are 644 filtered on the perimeter, and then the SAVI devices can be protected 645 from fabricated DHCP messages. 647 Such a requirement is due to the limitation of this binding based 648 mechanism. This document makes no assumption that the DHCP server- 649 client messages arriving the perimeter from the outside can be 650 trusted. The binding anchor of a trusted remote DHCP server can be 651 shared by a bogus DHCP server. Thus, the SAVI device cannot 652 distinguish bogus and valid DHCP messages only based on the 653 associated binding anchor of DHCP messages in such case. 655 Note that even if a DHCP server is valid, it may be not contained in 656 the perimeter based on the guideline. For example, in Figure 1, DHCP 657 server A is valid, but it is attached to a Non-SAVI device. The Non- 658 SAVI device may be attached by attackers which generate fabricated 659 DHCP messages. This binding based mechanism may not have the ability 660 to distinguish whether a message received from the attachment of the 661 Non-SAVI device 1 is from DHCP server A or the attackers. If the 662 DHCP server A is contained in the perimeter, the Non-SAVI device 1 663 will also be contained in the perimter. However, the Non-SAVI device 664 1 can introduce fabricated DHCP messages into the perimeter. Thus, 665 the DHCP server A cannot be contained in the perimeter. 667 In this case, the SAVI devices can set up bindings for addresses 668 assigned by DHCP server A through snooping the messages relayed by 669 trusted relay in the network. For example, the DHCP relay may relay 670 messages between DHCP server A and the clients in the network, and 671 the SAVI devices can snoop messages from the DHCP relay which is 672 inside the perimeter. The authentication mechanism (i.e., IPSec, as 673 specified in section 21.1 of [RFC3315]) enforced between the DHCP 674 relay and the DHCP server outside the perimeter can compensate this 675 binding based mechanism. It is SUGGESTED to configure IPSec between 676 the DHCP relay and the DHCP server in such case. If source address 677 validation is enforced in the whole network, which makes the source 678 IP address trustable, the DHCP relay and the DHCP server can simply 679 authenticate the messages from each other based on the source IP 680 address without the requriement to deploy IPSec. 682 Another considration on the placement is that if the DHCP server/ 683 relay is not inside the perimeter, the SAVI devices may not be able 684 to set up bindings correctly, because the SAVI devices may not be on 685 the path between the clients and the server/relay, or the DHCP 686 messages are encapsulated (e.g., Relay-reply and Relay-forward). 688 5. Binding State Table (BST) 690 Binding State Table is used to contain the bindings between the IP 691 addresses assigned to the attachments and the corresponding binding 692 anchors of the attachments. Each entry of the table, i.e., binding 693 entry, has 5 fields: 695 o Binding Anchor(Anchor): the binding anchor, i.e., a link-layer 696 property of the attachment. 698 o IP Address(Address): the IP address assigned to the attachment by 699 DHCP. 701 o State: the state of the binding. Possible values of this field 702 are listed in Section 6.2 and Section 7.3. 704 o Lifetime: the remaining seconds of the binding. The Lifetime 705 field counts down automatically. 707 o TID: the Transaction ID (TID) (refer to [RFC2131] [RFC3315]) of 708 the corresponding DHCP transaction. TID field is used to 709 associate DHCP Server-Client messages with corresponding binding 710 entries. 712 IA does not present in the BST. On the one hand, IA is not found to 713 be necessary because the lease of each address in one IA is assigned 714 respectively. On the other hand, when the binding is set up based on 715 data-snooping, IA cannot be recovered from the leasequery protocol. 716 Besides, there is no IA for DHCPv4. 718 An instance of this table is shown in Figure 3. 720 +---------+----------+----------+-----------+-------+ 721 | Anchor | Address | State | Lifetime |TID | 722 +---------+----------+----------+-----------+-------+ 723 | A | IP_1 | BOUND | 65535 |TID_1 | 724 +---------+----------+----------+-----------+-------+ 725 | A | IP_2 | BOUND | 10000 |TID_2 | 726 +---------+----------+----------+-----------+-------+ 727 | B | IP_3 |INIT_BIND | 1 |TID_3 | 728 +---------+----------+----------+-----------+-------+ 730 Figure 3: Instance of BST 732 6. DHCP Snooping Process 734 This section specifies the process of setting up bindings based on 735 DHCP snooping, named DHCP Snooping Process. This process is 736 illustrated making use of a state machine. 738 6.1. Rationale 740 The rationale of the DHCP Snooping Process is that if a DHCP client 741 is legitimate to use a DHCP address, the DHCP address assignment 742 procedure which assigns the IP address to the client must have been 743 performed on the attachment of the client. This basis stands when 744 the SAVI device is always on the path(s) from the DHCP client to the 745 DHCP server(s)/relay(s). Without considering the movement of DHCP 746 clients, the SAVI device should be the CUT VERTEX whose removal will 747 disjoin the DHCP client and the remaining network containing the DHCP 748 server(s)/relay(s). For most of the networks whose topologies are 749 simple, it is possible to deploy this SAVI function at proper devices 750 to meet this requirement. 752 However, a deployment of this SAVI function may not meet the 753 requirement. For example, there are multiple paths from a DHCP 754 client to the DHCP server and the SAVI device is only on one of them. 755 Then the SAVI device may not be able to snoop the DHCP procedure. 756 Host movement may also make this requirement can not be met. For 757 exmaple, when a DHCP client moves from one attachment to another 758 attachment in the same network, it may not reinitialize its interface 759 or send a Confirm message because of imcomplete protocol 760 implementation. Thus, there can be scenarios in which only 761 performing this DHCP snooping process is insufficient to set up 762 bindings for all the valid DHCP addresses. These exceptions and the 763 solutions are discussed in Section 7. 765 6.2. Binding States Description 767 Following binding states present in this process and the 768 corresponding state machine: 770 NO_BIND: The state before a binding has been set up. 772 INIT_BIND: A potential binding has been set up. 774 BOUND: The binding has been set up. 776 6.3. Events 778 This section describes events in this process and the corresponding 779 state machine. 781 6.3.1. Timer Expiration Event 783 EVE_ENTRY_EXPIRE: The lifetime of a binding entry expires. 785 6.3.2. Control Message Arriving Events 787 EVE_DHCP_REQUEST: A DHCPv4 Request or a DHCPv6 Request message is 788 received. 790 EVE_DHCP_CONFIRM: A DHCPv6 Confirm message is received. 792 EVE_DHCP_REBOOT: A DHCPv4 Reboot message is received. 794 EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is 795 received. 797 EVE_DHCP_RENEW: A DHCPv4 Renew or a DHCPv6 Renew message is received. 799 EVE_DHCP_OPTION_RC: A DHCPv6 Solicitation message with Rapid Commit 800 option is received. 802 EVE_DHCP_REPLY: A DHCPv4 ACK or a DHCPv6 Reply message is received. 804 EVE_DHCP_DECLINE: A DHCPv4 Decline or a DHCPv6 Decline message is 805 received. 807 EVE_DHCP_RELEASE: A DHCPv4 Release or a DHCPv6 Release message is 808 received. 810 EVE_DCHP_LEASEQUERY: A successful DHCPv6 LEASEQUERY_REPLY (refer to 811 section 4.3.3 of [RFC5007]) is received. 813 Note: the events listed here do not cover all the DHCP messages in 814 section 3. The messages which do not really determine address usage 815 (DHCPv4 Discover, DHCPv4 Inform, DHCPv6 Solicit without Rapid Commit, 816 DHCPv6 Information-Request, DHCPv4 Offer, DHCPv6 Advertise, DHCPv6 817 Reconfigure), and which are not neccessary to snoop (DHCPv4 NAK, 818 refer to section 6.4.2.1), are not included. 820 Moreover, only if a DHCP message can pass the following checks, the 821 corresponding event is regarded as a valid event: 823 o Attribute check: the DHCP Server-Client messages and 824 LEASEQUERY_REPLY should be from attachments with DHCP-Trust 825 attribute; the DHCP Client-Server messages should be from 826 attachments with DHCP-Snooping attribute. 828 o Destination check: the DHCP Server-Client messages should be 829 destined to attachments with DHCP-Snooping attribute. This check 830 is performed to ensure the binding is set up on the SAVI device 831 which is nearest to the destination client. 833 o Binding anchor check: the DHCP Client-Server messages which may 834 trigger modification or removal of an existing binding entry must 835 have matched binding anchor with the corresponding entry. 837 o TID check: the DHCP Server-Client/Client-Server messages which 838 may cause modification on existing binding entries must have 839 matched TID with the corresponding entry. Note that this check 840 is not performed on Leasequery and Leasequery-reply messages as 841 they are exchanged between the SAVI devices and the DHCP servers. 843 o Binding limitation check: the DHCP messages must not cause new 844 binding setup on an attachment whose binding entry limitation has 845 been reached. (refer to Section 11.6). 847 o Address check: the source address of the DHCP messages should 848 pass the check specified in Section 8.2. 850 On receiving a DHCP message without triggering a valid event, the 851 state will not transit and actions will not be performed. Note that 852 if a message does not trigger a valid event but it can pass the 853 checks in Section 8.2, it MUST be forwarded. 855 6.4. The State Machine of DHCP Snooping Process 857 This section specifies the transits of each state and the 858 corresponding actions. 860 6.4.1. From NO_BIND to Other States 862 6.4.1.1. Trigger Events 864 Trigger events: EVE_DHCP_REQUEST, EVE_DHCP_OPTION_RC, 865 EVE_DHCP_CONFIRM, EVE_DHCP_REBOOT. 867 6.4.1.2. Following Actions 869 If the triggering event is EVE_DHCP_REQUEST/EVE_DHCP_OPTION_RC/ 870 EVE_DHCP_REBOOT: 872 The SAVI device MUST forward the message. 874 The SAVI device will generate an entry in the BST. The Binding 875 anchor field is set to the binding anchor of the attachment from 876 which the message is received. The State field is set to INIT_BIND. 877 The Lifetime field is set to be MAX_DHCP_RESPONSE_TIME. The TID 878 field is set to the TID of the message. If the message is DHCPv4 879 Request or DHCPv4 Reboot, the Address field can be set to the address 880 to request, i.e., the 'requested IP address'. An example of the 881 entry is illustrated in Figure 4. 883 +---------+-------+---------+-----------------------+-------+ 884 | Anchor |Address| State | Lifetime |TID | 885 +---------+-------+---------+-----------------------+-------+ 886 | A | |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID | 887 +---------+-------+---------+-----------------------+-------+ 889 Figure 4: Binding entry in BST on Request/Rapid Commit/Reboot 890 triggered initialization 892 If the triggering event is EVE_DHCP_CONFIRM: 894 The SAVI device MUST forward the message. 896 The SAVI device will generate corresponding entries in the BST for 897 all the addresses in each the IA option of the Confirm message. The 898 Binding anchor field is set to the binding anchor of the attachment 899 from which the message is received. The State field is set to 900 INIT_BIND. The Lifetime field is set to be MAX_DHCP_RESPONSE_TIME. 901 The TID field is set to the TID of the message. The Address field is 902 set to the address(es) to confirm. An example of the entries is 903 illustrated in Figure 5. 905 +---------+--------+---------+-----------------------+-------+ 906 | Anchor | Address| State | Lifetime |TID | 907 +---------+--------+---------+-----------------------+-------+ 908 | A | Addr1 |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID | 909 +---------+--------+---------+-----------------------+-------+ 910 | A | Addr2 |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID | 911 +---------+--------+---------+-----------------------+-------+ 913 Figure 5: Binding entry in BST on Confirm triggered initialization 915 6.4.2. From INIT_BIND to Other States 917 6.4.2.1. Trigger Events 919 Trigger events: EVE_DHCP_REPLY, EVE_ENTRY_EXPIRE. 921 Note: If no DHCP Server-Client messages which assign addresses or 922 confirm addresses are received, corresponding entries will expire 923 automatically. Thus, other DHCP Server-Client messages (e.g., DHCPv4 924 NAK) are not specially processed. 926 6.4.2.2. Following Actions 928 If the trigger event is EVE_DHCP_REPLY: 930 The message MUST be forwarded to the corresponding client. 932 If the message is DHCPv4 ACK, the Address field of the corresponding 933 entry (i.e., the binding entry whose TID is the same of the message) 934 is set to the address in the message(i.e., 'yiaddr' in DHCPv4 ACK). 935 The Lifetime field is set to the sum of the lease time in ACK message 936 and MAX_DHCP_RESPONSE_TIME. The State field is changed to BOUND. 938 If the message is DHCPv6 Reply, there are following cases: 940 1. If the status code is not "Success", no modification on 941 corresponding entries will be made. Corresponding entries will 942 expire automatically if no "Success" Reply is received during the 943 lifetime. The entries are not removed immediately due to the client 944 may be able to use the addresses whenever a "Success" Reply is 945 received ("If the client receives any Reply messages that do not 946 indicate a NotOnLink status, the client can use the addresses in the 947 IA and ignore any messages that indicate a NotOnLink status." 948 [RFC3315]). 950 2. If the status code is "Success", the SAVI device checks the IA 951 options in the Reply message. 953 2.1 If there are no IA options in the Reply message, the DHCP Reply 954 message is in response to a Confirm message. The state of the 955 binding entries with matched TID is changed to BOUND. Because 956 [RFC3315] does not require lease time of addresses to be contained in 957 the Reply message, the SAVI device MUST send a LEASEQUERY [RFC5007] 958 message querying by IP address to All_DHCP_Servers multicast address 959 [RFC3315] or a list of configured DHCP server addresses. The 960 Leasequery message is generated for each IP address if multiple 961 addresses are confirmed. The Lifetime of corresponding entries is 962 set to 2*MAX_LEASEQUERY_DELAY. If there is no response message after 963 MAX_LEASEQUERY_DELAY, send the LEASEQUERY message again. An example 964 of the entries is illustrated in Figure 6. The related security 965 problem about DHCPv6 LEASEQUERY is discussed in Section 11.5. 967 2.2 If there are IA options in the Reply message, the SAVI device 968 checks each IA option. When the first assigned address is found, the 969 Address field of the binding entry with matched TID is set to the 970 address. The Lifetime field is set to the sum of the lease time in 971 Reply message and MAX_DHCP_RESPONSE_TIME. The State field is changed 972 to BOUND. If there are more than one address assigned in the 973 message, new binding entries are set up for the remaining address 974 assigned in the IA options. An example of the entries is illustrated 975 in Figure 7. SAVI devices do not specially process IA options with 976 NoAddrsAvail status, because there should be no address contained in 977 such IA options. 979 Note: the SAVI devices do not check if the assigned addresses are 980 duplicated because in SAVI-DHCP scenarios, the DHCP servers are the 981 only source of valid addresses. However, the DHCP servers should be 982 configured to make sure no duplicated addresses are assigned. 984 +---------+----------+-------+------------------------+-------+ 985 | Anchor | Address | State | Lifetime |TID | 986 +---------+----------+-------+------------------------+-------+ 987 | A | Addr1 | BOUND | 2*MAX_LEASEQUERY_DELAY |TID | 988 +---------+----------+-------+------------------------+-------+ 989 | A | Addr2 | BOUND | 2*MAX_LEASEQUERY_DELAY |TID | 990 +---------+----------+-------+------------------------+-------+ 992 Figure 6: From INIT_BIND to BOUND on DHCP Reply in response to 993 Confirm 995 +---------+----------+-------+------------------------+-------+ 996 | Anchor | Address | State | Lifetime |TID | 997 +---------+----------+-------+------------------------+-------+ 998 | A | Addr1 | BOUND |Lease time+ |TID | 999 | | | |MAX_DHCP_RESPONSE_TIME | | 1000 +---------+----------+-------+------------------------+-------+ 1001 | A | Addr2 | BOUND |Lease time+ |TID | 1002 | | | |MAX_DHCP_RESPONSE_TIME | | 1003 +---------+----------+-------+------------------------+-------+ 1005 Figure 7: From INIT_BIND to BOUND on DHCP Reply in response to 1006 Request 1008 If the trigger event is EVE_ENTRY_EXPIRE: 1010 The entry MUST be deleted from BST. 1012 6.4.3. From BOUND to Other States 1014 6.4.3.1. Trigger Events 1016 Trigger events: EVE_ENTRY_EXPIRE, EVE_DHCP_RELEASE, EVE_DHCP_DECLINE, 1017 EVE_DHCP_REPLY, EVE_DCHP_LEASEQUERY. 1019 6.4.3.2. Following Actions 1021 If the trigger event is EVE_ENTRY_EXPIRE: 1023 Remove the corresponding entry in BST. 1025 If the trigger event is EVE_DHCP_RELEASE/EVE_DHCP_DECLINE: 1027 The message MUST be forwarded. 1029 The SAVI device first gets all the addresses ("Requested IP address" 1030 in DHCPv4 Decline, "ciaddr" in DHCPv4 Release, addresses in all the 1031 IA options of DHCPv6 Decline/Release) to decline/release in the 1032 message. Then the corresponding entries MUST be removed. 1034 If the trigger event is EVE_DHCP_REPLY: 1036 The message MUST be forwarded. 1038 The DHCP Reply messages received in current states should be in 1039 response to DHCP Renew/Rebind. 1041 If the message is DHCPv4 ACK, the SAVI device just simply update the 1042 binding entry with matched TID, with the Lifetime field set to be the 1043 sum of the new lease time and MAX_DHCP_RESPONSE_TIME. 1045 If the message is DHCPv6 Reply, the SAVI device checks each IA 1046 Address option in each IA option. If the valid lifetime of an IA 1047 address option is 0, the binding entry with matched TID and address 1048 is removed. Or else, set the Lifetime field of the binding entry 1049 with matched TID and address to be the sum of the new valid lifetime 1050 and MAX_DHCP_RESPONSE_TIME. 1052 The SAVI device does not specially process IA options in Reply 1053 message with status NoBinding, because no address is contained in 1054 such IA options and no actions will be performed. 1056 If the trigger event is EVE_DCHP_LEASEQUERY: 1058 The message MUST be forwarded. 1060 The message should be in response to the Leasequery message sent in 1061 Section 6.4.2. The related binding entry can be determined based on 1062 the address in the IAADDR option in the Leasequery-reply message. 1063 The Lifetime field of the corresponding binding entry is set to the 1064 sum of the lease time in the LEASEQUERY_REPLY message and 1065 MAX_DHCP_RESPONSE_TIME. 1067 6.5. Table of State Machine 1069 The main state transits are listed as follows. Note that not all the 1070 details are specified in the table and the diagram. 1072 State Event Action Next State 1073 NO_BIND RQ/RC/CF/RE Generate entry INIT_BIND 1074 INIT_BIND RPL Record lease time BOUND 1075 (send lease query if no lease) 1076 INIT_BIND Timeout Remove entry NO_BIND 1077 BOUND RLS/DCL Remove entry NO_BIND 1078 BOUND Timeout Remove entry NO_BIND 1079 BOUND RPL Set new lifetime BOUND 1080 BOUND LQR Record lease time BOUND 1082 Figure 8: Table of Transit 1084 RQ: EVE_DHCP_REQUEST 1086 CF: EVE_DHCP_CONFIRM 1088 RC: EVE_DHCP_OPTION_RC 1090 RE: EVE_DHCP_REBOOT 1092 RPL: EVE_DHCP_REPLY 1094 DCL: EVE_DHCP_DECLINE 1096 RLS: EVE_DHCP_RELEASE 1098 LQR: EVE_DCHP_LEASEQUERY 1100 Timeout: EVE_ENTRY_EXPIRE 1101 +-------------+ 1102 | | 1103 /---------| NO_BIND |<----------\ 1104 | ------>| | | 1105 | | +-------------+ |EVE_DHCP_RELEASE 1106 EVE_DHCP_REQUEST | | |EVE_DHCP_DECLINE 1107 EVE_DHCP_CONFIRM | |TIMEOUT |TIMEOUT 1108 EVE_DHCP_OPTION_RC| | | 1109 EVE_DHCP_REBOOT | | | 1110 | | | 1111 | | | 1112 v | | 1113 +-------------+ +------------+ 1114 | | EVE_DHCP_REPLY | | 1115 | INIT_BIND ------------------------>| BOUND |<-\ 1116 | | | | | 1117 +-------------+ +------------+ | 1118 | | 1119 \--------/ 1120 EVE_DHCP_REPLY 1121 EVE_DCHP_LEASEQUERY 1123 Figure 9: Diagram of Transit 1125 7. Data Snooping Process 1127 7.1. Scenario 1129 The rationale of the DHCP Snooping Process specified in Section 6 is 1130 that if a DHCP client's use of a DHCP address is legitimate, the 1131 corresponding DHCP address assignment procedure must have been 1132 finished on the attachment of the DHCP client. This is the case 1133 stands when the SAVI device is persistently on the path(s) from the 1134 DHCP client to the DHCP server(s)/relay(s). However, there are two 1135 case when this does not work: 1137 o Multiple paths: there is more than one feasible layer-2 paths 1138 from the client to the DHCP server/relay, and the SAVI device is 1139 not on everyone of them. The client may get its address through 1140 one of the paths not passing by the SAVI device, but packets from 1141 the client can travel through paths that pass through the SAVI 1142 device. Because the SAVI device could not snoop the DHCP packet 1143 exchange procedure, the DHCP snooping procedure cannot set up the 1144 corresponding binding. 1146 o Dynamic path: there is only one feasible layer-2 path from the 1147 client to the DHCP server/relay, but the path is dynamic due to 1148 topology change (for example, some link turns broken due to 1149 failure or as planned) or layer-2 path change. This situation 1150 also covers the local-link movement of clients without address 1151 confirm/re-configuration process. For example, a host changes 1152 its attached switch port in a very short time. In such cases, 1153 the DHCP snooping process will not set up the corresponding 1154 binding. 1156 Data Snooping Process prevents permanently blocking legitimate 1157 traffic in case of these two exceptions. This process is performed 1158 on attachments with the Data-Snooping attribute. Data packets 1159 without matching binding entry may trigger this process to set up 1160 bindings. 1162 Snooping data traffic introduces considerable burden on the processor 1163 and ASIC-to-Processor bandwidth of SAVI devices. Because of the 1164 overhead of this process, the implementation of this process is a 1165 conditional SHOULD. This function SHOULD be enabled unless the 1166 implementation is known to be used in the scenarios without the above 1167 exceptions. For example, if the implementation is to be used in 1168 networks with tree topology and without host local-link movement, 1169 there is no need to implement this process in such scenarios. 1171 This process is not intended to set up a binding whenever a data 1172 packet without matched binding entry is received. Instead, unmatched 1173 data packets trigger this process probabilistically and generally a 1174 number of unmatched packets will be discarded before the binding is 1175 set up. 1177 To perform this process, the SAVI device MUST join the Solicited Node 1178 Multicast group of the source address of triggering IPv6 data packet 1179 whenever performing duplicate detection. 1181 7.2. Rationale 1183 This process makes use of DAD/ARP and DHCP Leasequery to set up 1184 bindings. If an address is not used by another client in the 1185 network, and the address has been assigned in the network, the 1186 address can be bound with the binding anchor of the attachment from 1187 which the unmatched packet is received. 1189 The security issues about this process is discussed is Section 11.1. 1191 7.3. Additional Binding States Description 1193 In addition to Section 6.2, new states used in this process are 1194 listed here: 1196 DETECTION: The address in the entry is under local duplication 1197 detection. 1199 RECOVERY: The SAVI device is querying the assignment and lease time 1200 of the address in the entry through DHCP Leasequery. 1202 7.4. Events 1204 Additional events in this process are described here. Also, if an 1205 event will trigger the creation of a new binding entry, the binding 1206 entry limit on the binding anchor MUST NOT be exceeded. 1208 EVE_DATA_UNMATCH: A data packet without matched binding is received. 1210 EVE_DATA_CONFLICT: ARP Reply/Neighbor Advertisement(NA) message 1211 against an address in DETECTION state is received from a host other 1212 than the one for which the entry was added. 1214 EVE_DATA_LEASEQUERY: 1216 IPv4: A DHCPLEASEACTIVE message with IP Address Lease Time option 1217 is received. 1219 IPv6: A successful LEASEQUERY-REPLY is received. 1221 The triggering packet should pass the following checks to trigger a 1222 valid event: 1224 o Attribute check: the data packet should be from attachments with 1225 Data-Snooping attribute; the DHCPLEASEACTIVE/LEASEQUERY_REPLY 1226 messages should be from attachments with DHCP-Snooping attribute. 1228 o Binding limitation check: the DHCP messages must not cause new 1229 binding setup on an attachment whose binding entry limitation has 1230 been reached. (refer to Section 11.6). 1232 o Address check: For EVE_DATA_LEASEQUERY, the source address of the 1233 DHCP Leasequery messages must pass the check specified in 1234 Section 8.2. For EVE_DATA_CONFLICT, the source address and 1235 target address of the ARP or NA messages must pass the check 1236 specified in Section 8.2. 1238 o Interval check: the interval between two successive 1239 EVE_DATA_UNMATCH events triggered by an attachment MUST be no 1240 smaller than DATA_SNOOPING_INTERVAL. 1242 o TID check: the DHCPLEASEACTIVE/LEASEQUERY-REPLY messages must 1243 have matched TID with the corresponding entry. 1245 7.5. State Machine of Binding Recovery Process 1247 Through using additional states, the state machine of this process 1248 doesn't conflict the regular process described in Section 6. Thus, 1249 it can be implemented separately without changing the state machine 1250 in Section 6. 1252 7.5.1. From NO_BIND to Other States 1254 7.5.1.1. Trigger Event 1256 Trigger event: EVE_DATA_UNMATCH. 1258 7.5.1.2. Following Actions 1260 Make a probabilistic determination whether to act on this event. The 1261 probability can be configured or calculated based on the state of the 1262 SAVI device. This probability should be low enough to mitigate the 1263 damage from DoS attack against this process. 1265 Create a new entry in the BST. Set the Binding Anchor field to the 1266 corresponding binding anchor of the attachment. Set the Address 1267 field to be source address of the packet. Set the State field to 1268 DETECTION. Set the Lifetime of the created entry to 2*DAD_TIMEOUT. 1270 Check if the address has a local conflict (it violates an address 1271 being used by another node): 1273 (1) IPv4 address: send an Address Resolution Protocol (ARP) Request 1274 [RFC826]or a ARP probe [RFC5227] on the address; if there is no 1275 response message after DAD_TIMEOUT, send another ARP Request or 1276 ARP probe; 1278 (2) IPv6 address: perform Duplicate Address Detection (DAD) 1279 [RFC4862] on the address; if there is no response message after 1280 DAD_TIMEOUT, perform another DAD procedure. 1282 Because the delivery of detection message is unreliable, the 1283 detection message may fail to reach the targeting node. If there is 1284 a node that has the IP address seen in the Data Snooping Process, it 1285 may not get the detection messages. This failure mode enables an 1286 attack against the Data Snooping Process. Thus, the detection is 1287 performed again if there is no response after the first detection. 1289 The messages MUST NOT be sent to the attachment from which the 1290 triggering packet is received. 1292 The packet which triggers this event SHOULD be discarded. 1294 An example of the entry is illustrated in Figure 10. 1296 +---------+-------+---------+-----------------------+-------+ 1297 | Anchor |Address| State | Lifetime |TID | 1298 +---------+-------+---------+-----------------------+-------+ 1299 | A | Addr1 |DETECTION|2*DAD_TIMEOUT | | 1300 +---------+-------+---------+-----------------------+-------+ 1302 Figure 10: Binding entry in BST on data triggered initialization 1304 7.5.2. From DETECTION to Other States 1306 7.5.2.1. Trigger Event 1308 Trigger events: EVE_ENTRY_EXPIRE, EVE_DATA_CONFLICT. 1310 7.5.2.2. Following Actions 1312 If the trigger event is EVE_ENTRY_EXPIRE: 1314 (1) IPv4 address: Send a DHCPLEASEQUERY [RFC4388] message querying 1315 by IP address to each DHCPv4 server with IP Address Lease Time 1316 option (option 51). A list of authorized DHCP servers are kept 1317 by the SAVI device. The list should be pre-configured or 1318 discovered by sending DHCPv4 Discover messages and parsing the 1319 replied DHCPv4 Offer messages. Change the state of the 1320 corresponding entry to RECOVERY. Change the lifetime of the 1321 entry to be 2*MAX_LEASEQUERY_DELAY. The TID field is set to the 1322 TID used in the DHCPLEASEQUERY message. If there is no response 1323 message after MAX_LEASEQUERY_DELAY, send a DHCPLEASEQUERY to 1324 each DHCPv4 server again. 1326 (2) IPv6 address: Send a LEASEQUERY [RFC5007] message querying by IP 1327 address to All_DHCP_Relay_Agents_and_Servers multicast address 1328 or a list of pre-configured DHCPv6 server addresses. Change the 1329 state of the corresponding entry to RECOVERY. Change the 1330 lifetime of the entry to be 2*MAX_LEASEQUERY_DELAY. The TID 1331 field is set to the TID used in the LEASEQUERY message. If 1332 there is no response message after MAX_LEASEQUERY_DELAY, send 1333 the LEASEQUERY message again. 1335 An example of the entry is illustrated in Figure 11. 1337 +---------+-------+---------+-----------------------+-------+ 1338 | Anchor |Address| State | Lifetime |TID | 1339 +---------+-------+---------+-----------------------+-------+ 1340 | A | Addr1 |RECOVERY |2*MAX_LEASEQUERY_DELAY |TID | 1341 +---------+-------+---------+-----------------------+-------+ 1343 Figure 11: Binding entry in BST on Lease Query 1345 If the trigger event is EVE_DATA_CONFLICT: 1347 Remove the entry. 1349 7.5.3. From RECOVERY to Other States 1351 7.5.3.1. Trigger Event 1353 Trigger events: EVE_ENTRY_EXPIRE, EVE_DATA_LEASEQUERY. 1355 7.5.3.2. Following Actions 1357 If the trigger event is EVE_DATA_LEASEQUERY: 1359 IPv4 address: 1361 (1) Send an ARP Request with the Target Protocol Address set to the 1362 IP address in the corresponding entry. The ARP Request is only 1363 sent to the attachment which triggers the binding. If there is 1364 no response after DAD_TIMEOUT, send another ARP Request. If 1365 there is still no response, the following actions will not be 1366 performed. If there is only one identical response, get the 1367 sender hardware address. Check if the 'chaddr' field (hardware 1368 address) of the DHCPLEASEACTIVE message matches the sender 1369 hardware address. If the two addresses do not match, the 1370 following actions will not be performed. If there is more than 1371 one response, if any of the sender hardware addresses matches 1372 the 'chaddr' field (hardware address) of the DHCPLEASEACTIVE 1373 message, the following actions are to be performed. 1375 (2) Change the state of the corresponding binding to BOUND. Set 1376 life time to the sum of the value encoded in IP Address Lease 1377 Time option of the DHCPLEASEACTIVE message and 1378 MAX_DHCP_RESPONSE_TIME. Erase the TID field. 1380 IPv6 address: 1382 (1) Send a Neighbor Solicitation message with the target address set 1383 to the IP address in the corresponding entry. The Neighbor 1384 Solicitation is only sent to the attachment which triggers the 1385 binding. If there is no response after DAD_TIMEOUT, send 1386 another Neighbor Solicitation. If there is still no response, 1387 the following actions will not be performed. If there is only 1388 one identical response, get the source hardware address from the 1389 response. Check if the 'chaddr' field (hardware address) of the 1390 LEASEQUERY-REPLY message matches the source hardware address. 1391 If the two addresses do not match, the following actions will 1392 not be performed. If there is more than one response, if any of 1393 the source hardware addresses matches the 'chaddr' field 1394 (hardware address) of the LEASEQUERY-REPLY message, the 1395 following actions are to be performed. 1397 (2) Change the state of the corresponding binding to BOUND. Set the 1398 lifetime to the sum of the valid lifetime extracted from 1399 OPTION_CLIENT_DATA option in the LEASEQUERY-REPLY message and 1400 MAX_DHCP_RESPONSE_TIME. Erase the TID field. 1402 (3) After the above checks, if multiple addresses are specified in 1403 the LEASEQUERY-REPLY message and there are no corresponding 1404 binding entries, new entries MUST also be created 1405 correspondingly on the same binding anchor. 1407 If responses are received from multiple DHCP servers, the conflict 1408 resolution mechanisms specified in section 6.8 of [RFC4388] and 1409 section 4.3.4 of [RFC5007] will be used to determine which message 1410 should be used. 1412 If the trigger event is EVE_ENTRY_EXPIRE: 1414 Remove the entry. 1416 7.5.4. After BOUND 1418 Note that the TID field contains no value after the binding state 1419 changes to BOUND. The TID field is recovered from snooping DHCP 1420 Renew/Rebind messages. Because TID is used to associate binding 1421 entries with messages from DHCP servers, it must be recovered; or 1422 else a number of state transits of this mechanism will be not 1423 executed normally. 1425 7.5.4.1. Trigger Event 1427 Trigger events: EVE_DHCP_RENEW, EVE_DHCP_REBIND. 1429 7.5.4.2. Following Action 1431 Set the TID field of the corresponding entry to the TID in the 1432 triggering message. 1434 7.6. Table of State Machine 1436 The main state transits are listed as follows. 1438 State Event Action Next State 1439 NO_BIND EVE_DATA_UNMATCH Duplication detection DETECTION 1440 DETECTION Timeout Send Leasequery RECOVERY 1441 DETECTION EVE_DATA_CONFLICT Remove entry NO_BIND 1442 RECOVERY EVE_DATA_LEASEQUERY Set lease time BOUND or NO_BIND 1443 RECOVERY Timeout Remove entry NO_BIND 1444 BOUND RENEW/REBIND Record TID BOUND 1446 Figure 12: Table of Transit 1448 RENEW: EVE_DHCP_RENEW 1450 REBIND: EVE_DHCP_REBIND 1452 Timeout: EVE_ENTRY_EXPIRE 1454 LQ_DELAY: MAX_LEASEQUERY_DELAY 1455 +-------------+ 1456 | | 1457 /---------| NO_BIND |<--------\ 1458 | ------>| | | TIMEOUT 1459 | | +-------------+ |(2nd LQ_DELAY) 1460 EVE_DATA_UNMATCH | | | 1461 | | | 1462 | | | 1463 1st DAD_TIMEOUT | | | 1st LQ_DELAY 1464 /------\ | | | /---------\ 1465 | | | | EVE_DATA_CONFLICT | | | 1466 | v v | | v | 1467 | +-------------+ TIMEOUT +------------+ | 1468 | | | (2nd DAD_TIMEOUT) | | | 1469 \----| DETECTION ------------------------>| RECOVERY ----/ 1470 | | | | 1471 +-------------+ +------------+ 1472 EVE_DATA_LEASEQUERY| 1473 /----------\ | 1474 EVE_DHCP_RENEW| | | 1475 EVE_DHCP_REBIND| +-----v-------+ | 1476 | | | | 1477 \----| BOUND |<----------/ 1478 | | 1479 +-------------+ 1481 Figure 13: Diagram of Transit 1483 8. Filtering Specification 1485 This section specifies how to use bindings to filter out spoofing 1486 packets. 1488 Filtering policies are different for data packets and control 1489 packets. DHCP and NDP (Neighbor Discovery Protocol) [RFC4861] 1490 messages that may cause state transit are classified as control 1491 packet. Neighbor Advertisement (NA) and ARP Reply are also included 1492 in control packet because the Target Address of NA and ARP Reply 1493 should be checked to prevent spoofing. All other packets are 1494 classified as data packets. 1496 8.1. Data Packet Filtering 1498 Data packets from attachments with the Validating attribute MUST be 1499 checked. 1501 Packet whose source IP address is a link-local address will not be 1502 checked. Note: as explained in Section 1, a SAVI solution for link- 1503 local addresses, e.g., the SAVI-FCFS [savi-fcfs], can be enabled to 1504 check packets with link-local source address. 1506 If the source IP address of a packet is not a link-local address, but 1507 there is not a matched entry in BST with state BOUND, this packet 1508 MUST be discarded. However, the packet may trigger Data Snooping 1509 Process Section 7 if Data-Snooping attribute is set on the 1510 attachment. 1512 Data packets from attachments with no attribute will forwarded 1513 without checking. 1515 The SAVI device MAY record any violation. 1517 8.2. Control Packet Filtering 1519 For attachments with the Validating attribute: 1521 Discard DHCPv4 Client-Server message messages whose source IP address 1522 is neither all zeros nor bound with the corresponding binding anchor 1523 in the BST. 1525 Discard DHCPv6 Client-Server message messages whose source IP address 1526 is neither a link-local address nor bound with the corresponding 1527 binding anchor in the BST. 1529 Discard NDP messages whose source IP address is neither a link-local 1530 address nor bound with the corresponding binding anchor. Especially, 1531 discard NA message whose target address is neither a link-local 1532 address nor bound with the corresponding binding anchor. 1534 Discard ARP messages whose protocol is IP and sender protocol address 1535 is neither all zeros address nor bound with the corresponding binding 1536 anchor. Especially, discard ARP Reply messages whose target protocol 1537 address is not bound with the corresponding binding anchor. 1539 For attachments with other attributes: 1541 Discard DHCP Server-Client message not from attachments with the 1542 DHCP-Trust attribute or Trust attribute. 1544 For attachments with no attribute: 1546 Discard DHCP Server-Client message from such attachments. 1548 The SAVI device MAY record any violation. 1550 9. State Restoration 1552 If a SAVI device reboots, the information kept in volatile memory 1553 will be lost. This section specifies the restoration of attribute 1554 configuration and BST. 1556 9.1. Attribute Configuration Restoration 1558 The lost of attribute configuration will not break the network: no 1559 action will be performed on traffic from attachments with no 1560 attribute. However, the lost of attribute configuration makes this 1561 SAVI function unable to work. 1563 To avoid the loss of binding anchor attribute configuration, the 1564 configuration MUST be able to be stored in non-volatile storage. 1565 After the reboot of SAVI device, if the configuration of binding 1566 anchor attribute can be found in non-volatile storage, the 1567 configuration MUST be used. 1569 9.2. Binding State Restoration 1571 The loss of binding state will cause the SAVI devices discard 1572 legitimate traffic. Purely using the Data Snooping Process to 1573 recover a large number of bindings is of heavy overhead and 1574 considerable delay. Thus, to recover bindings from non-volatile 1575 storage, as specified below, is RECOMMENDED. 1577 Binding entries MAY be saved into non-volatile storage whenever a new 1578 binding entry changes to BOUND state. If a binding with BOUND state 1579 is removed, the saved entry MUST be removed correspondingly. The 1580 time when each binding entry is established is also saved. 1582 Immediately after reboot, the SAVI device SHOULD restore binding 1583 states from the non-volatile storage. The system time of save 1584 process MUST be stored. After rebooting, the SAVI device MUST check 1585 whether each entry has been obsolete by comparing the saved lifetime 1586 and the difference between the current time and time when the binding 1587 entry is established. 1589 10. Constants 1591 MAX_DHCP_RESPONSE_TIME 120s 1593 DATA_SNOOPING_INTERVAL 60s and configurable 1595 MAX_LEASEQUERY_DELAY 10s 1596 OFFLINK_DELAY 30s 1598 DAD_TIMEOUT 0.5s 1600 11. Security Considerations 1602 11.1. Security Problems about the Data Snooping Process 1604 There are two security problems about the Data Snooping Process 1605 Section 7: 1607 (1) The Data Snooping Process is costly, but an attacker can trigger 1608 it simply through sending a number of data packets. To avoid 1609 Denial of Services attack against the SAVI device itself, the 1610 Data Snooping Process MUST be rate limited. A constant 1611 DATA_SNOOPING_INTERVAL is used to control the frequency. Two 1612 Data Snooping Processes on one attachment MUST have a minimum 1613 interval time DATA_SNOOPING_INTERVAL. This constant SHOULD be 1614 configured prudently to avoid Denial of Service attacks. 1616 (2) The Data Snooping Process may set up wrong bindings if the 1617 clients do not reply to the detection probes. An attack will 1618 pass the duplicate detection if the client assigned the target 1619 address does not reply to the detection probes. The DHCP 1620 Leasequery procedure performed by the SAVI device just tells 1621 whether the address is assigned in the network or not. However, 1622 the SAVI device cannot determine whether the address is just 1623 assigned to the triggering attachment from the DHCP Leasequery 1624 Reply. 1626 11.2. Issues about Leaving Clients 1628 After a binding is set up, the corresponding client may leave its 1629 attachment point. It may leave temporarily due to link flapping, or 1630 permanently by moving to a new attachment point or leaving the 1631 network. Since the client may return shortly, the binding should be 1632 kept, or legtimate traffic from the client will be blocked. However, 1633 if the client leaves permanently, it may be insecure to keep the 1634 binding. If the binding anchor is a property of the attachment point 1635 rather than the client, e.g., the switch port, an attacker which is 1636 attached to the attachment point of the leaving client can send 1637 spoofing packets with the addresses assigned to the client. Even if 1638 the binding anchor is a property of the client, it is a waste of 1639 binding resources to keep bindings for departed clients. 1641 The following mechanism is designed to handle the leaving of client: 1643 (1) Whenever a client with the Validating attribute leaves, a timer 1644 of duration OFFLINK_DELAY is set on the corresponding binding 1645 entries. 1647 (2) If a DAD Neighbor Solicitation/Gratuitous ARP request is 1648 received that targets the address during OFFLINK_DELAY, the 1649 entry MAY be removed. 1651 (3) If the client returns on-link during OFFLINK_DELAY, cancel the 1652 timer. 1654 In this way, the bindings of a departing client are kept for 1655 OFFLINK_DELAY. In case of link flapping, the client will not be 1656 blocked. If the client leaves permanently, the bindings will be 1657 removed after OFFLINK_DELAY. 1659 11.3. Duplicate Bindings to the Same Address 1661 The same address may be bound to multiple binding anchors only if the 1662 binding setup processes successfully complete for each binding 1663 anchor. This mechanism is designed to address the case where a 1664 client moves on the local link, and the case where a client has 1665 multiple attachments to a SAVI device. 1667 There are two security issues with such a design: 1669 First, by allowing one address to be bound to multiple binding 1670 anchors, the traceability of the address is weakened. An address can 1671 be traced to multiple attachments. 1673 Second, in the local link movement scenario, the former binding may 1674 not be removed and it can be used by an attacker sharing the same 1675 binding anchor. For example, when a switch port is used as binding 1676 anchor and the port is shared by an attacker and a client with a hub, 1677 the attacker can make use of the address assigned to the client after 1678 the client leaves. 1680 11.4. Compatibility with DNA (Detecting Network Attachment) 1682 DNA [RFC4436] [RFC6059] is designed to decrease the handover latency 1683 after re-attachment to the same network. DNA mainly relies on 1684 performing reachability test by sending unicast Neighbor 1685 Solicitation/Router Solicitation/ARP Request message to determine 1686 whether a previously configured address is still valid. 1688 Although DNA provides optimization for clients, there is insufficient 1689 information for this mechanism to migrate the previous binding or 1690 establish a new binding. If a binding is set up only by snooping the 1691 reachability test message, the binding may be invalid. For example, 1692 an attacker can perform reachability test with an address bound to 1693 another client. If binding is migrated to the attacker, the attacker 1694 can successfully obtain the binding from the victim. Because this 1695 mechanism wouldn't set up a binding based on snooping the DNA 1696 procedure, it cannot achieve perfect compatibility with DNA. 1697 However, it only means the re-configuration of the interface is 1698 slowed but not prevented. Details are discussed as follows. 1700 In Simple DNAv6 [RFC6059], the probe is sent with the source address 1701 set to a link-local address, and such messages will not be discarded 1702 by the policy specified in section Section 8.2. If a client is re- 1703 attached to a previous network, the detection will be completed, and 1704 the address will be regarded as valid by the client. However, the 1705 candidate address is not contained in the probe. Thus, the binding 1706 cannot be recovered through snooping the probe. As the client will 1707 perform DHCP exchange at the same time, the binding will be recovered 1708 from the DHCP Snooping Process. The DHCP Request messages will not 1709 be filtered out in this case because they have link-local source 1710 addresses. Before the DHCP procedure is completed, packets will be 1711 filtered out by the SAVI device. In other words, if this SAVI 1712 function is enabled, Simple DNAv6 will not help reduce the handover 1713 latency. If Data-Snooping attribute is configured on the new 1714 attachment of the client, the data triggered procedure may reduce 1715 latency. 1717 In DNAv4 [RFC4436], the ARP probe will be discarded because an 1718 unbound address is used as the sender protocol address. As a result, 1719 the client will regard the address under detection is valid. 1720 However, the data traffic will be filtered. The DHCP Request message 1721 sent by the client will not be discarded, because the source IP 1722 address field should be all zero as required by [RFC2131]. Thus, if 1723 the address is still valid, the binding will be recovered from the 1724 DHCP Snooping Process. 1726 11.5. Authentication in DHCPv6 Leasequery 1728 As required in section 5 of RFC5007, DHCPv6 Leasequery 'Should' use 1729 IPsec-based authentication specified in the section 21.1 of RFC3315. 1730 However, with the deployment of this mechanism, there may be no need 1731 to enforce IPSec to perform DHCP Leasequery. 1733 By containing the DHCP servers in the protection perimeter, the DHCP 1734 servers can be protected from spoofing based attacks. Then by 1735 checking the source IP address of Leasequery messages, the DHCP 1736 server can identify if the messages are from SAVI devices or not. 1737 For the SAVI devices, because the perimeter filters out bogus DHCP 1738 messages, they can trust the DHCP Leasequery responses. Thus, there 1739 is no need to enforce IPSec to validate the DHCP Leasequery messages 1740 in this mechanism. 1742 11.6. Binding Number Limitation 1744 A binding entry will consume a certain high-speed memory resources. 1745 In general, a SAVI device can afford only a quite limited number of 1746 binding entries. In order to prevent an attacker from overloading 1747 the resource of the SAVI device, a binding entry limit is set on each 1748 attachment. The binding entry limit is the maximum number of 1749 bindings supported on each attachment with Validating attribute. No 1750 new binding should be set up after the limit has been reached. If a 1751 DHCP Reply assigns more addresses than the remaining binding entry 1752 quota of each client, the message will be discarded and no binding 1753 will be set up. 1755 11.7. Residual Threats 1757 As described in [savi-framework], this solution cannot strictly 1758 prevent spoofing. There are two scenarios in which spoofing can 1759 still happen: 1761 (1) The binding anchor is spoofable. If the binding anchor is 1762 spoofable, e.g., plain MAC address, an attacker can use forged 1763 binding anchor to send packet which will not be regarded as 1764 spoofing by SAVI device. Indeed, using binding anchor that can 1765 be easily spoofed is more serious than allowing IP spoofing 1766 traffic. For example, an attacker can use the binding anchor of 1767 another client to get a large number of addresses, and the SAVI 1768 device will refuse to set up new binding for the client whenever 1769 the binding number limitation has been reached. Thus, it is 1770 RECOMMENDED to use strong enough binding anchor, e.g., switch 1771 port, secure association in 802.11ae/af and 802.11i. 1773 (2) The binding anchor is shared by more than one clients. If the 1774 binding anchor is shared by more than one clients, the clients 1775 can spoof each other addresses. For example, if a switch port 1776 is used as binding anchor, a number of clients can attach to the 1777 same switch port of a SAVI device through a hub. The SAVI 1778 device cannot distinguish packets from different clients and 1779 thus the spoofing between them will not be detected. A number 1780 of the above security problems are caused by sharing binding 1781 anchors. If binding anchor is shared, TID spoofing based attack 1782 is possible. Thus, it is RECOMMENDED to use exclusive binding 1783 anchor. 1785 12. IANA Considerations 1787 This memo asks the IANA for no new parameters. 1789 Note to RFC Editor: This section will have served its purpose if it 1790 correctly tells IANA that no new assignments or registries are 1791 required, or if those assignments or registries are created during 1792 the RFC publication process. From the authors' perspective, it may 1793 therefore be removed upon publication as an RFC at the RFC Editor's 1794 discretion. 1796 13. Acknowledgment 1798 Special thanks to Jean-Michel Combes, Christian Vogt, Joel M. 1799 Halpern, Eric Levy-Abegnoli, Marcelo Bagnulo Braun, Jari Arkko, Elwyn 1800 Davies, Barry Leiba, Ted Lemon, Ralph Droms and Alberto Garcia for 1801 careful review and valuation comments on the mechanism and text. 1803 Thanks to Mark Williams, Erik Nordmark, Mikael Abrahamsson, David 1804 Harrington, Pekka Savola, Xing Li, Lixia Zhang, Bingyang Liu, Duanqi 1805 Zhou, Robert Raszuk, Greg Daley, John Kaippallimalil and Tao Lin for 1806 their valuable contributions. 1808 This document was generated using the xml2rfc tool. 1810 14. References 1812 14.1. Informative References 1814 [BA2007] Baker, F., "Cisco IP Version 4 Source Guard", IETF 1815 Internet draft (work in progress), November 2007. 1817 [BCP38] Paul, P. and D. Senie, "Network Ingress Filtering: 1818 Defeating Denial of Service Attacks which employ IP Source 1819 Address Spoofing", RFC 2827, BCP 38, May 2000. 1821 [RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol 1822 (DHCP) Service for IPv6", RFC 3736, April 2004. 1824 14.2. Normative References 1826 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1827 Requirement Levels", RFC 2119, BCP 14, Match 1997. 1829 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 1830 RFC 2131, March 1997. 1832 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 1833 and M. Carney, "Dynamic Host Configuration Protocol for 1834 IPv6 (DHCPv6)", RFC 3315, July 2003. 1836 [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration 1837 Protocol (DHCP) Leasequery", RFC 4388, February 2006. 1839 [RFC4436] Aboba, B., Carlson, J., and S. Cheshire, "Detecting 1840 Network Attachment in IPv4 (DNAv4)", RFC 4436, March 2006. 1842 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1843 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 1844 September 2007. 1846 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 1847 Address Autoconfiguration", RFC 4862, September 2007. 1849 [RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng, 1850 "DHCPv6 Leasequery", RFC 5007, September 2007. 1852 [RFC5227] Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227, 1853 July 2008. 1855 [RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for 1856 Detecting Network Attachment in IPv6", RFC 6059, 1857 November 2010. 1859 [RFC826] Plummer, D., "Ethernet Address Resolution Protocol: Or 1860 converting network protocol addresses to 48.bit Ethernet 1861 address for transmission on Ethernet hardware", RFC 826, 1862 November 1982. 1864 [savi-fcfs] 1865 Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS- 1866 SAVI: First-Come First-Serve Source-Address Validation for 1867 Locally Assigned Addresses", RFC 6620, May 2012. 1869 [savi-framework] 1870 Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed., 1871 "Source Address Validation Improvement Framework", 1872 draft-ietf-savi-framework-06 (work in progress), 1873 December 2011. 1875 Authors' Addresses 1877 Jun Bi 1878 Tsinghua University 1879 Network Research Center, Tsinghua University 1880 Beijing 100084 1881 China 1883 Email: junbi@tsinghua.edu.cn 1885 Jianping Wu 1886 Tsinghua University 1887 Computer Science, Tsinghua University 1888 Beijing 100084 1889 China 1891 Email: jianping@cernet.edu.cn 1893 Guang Yao 1894 Tsinghua University 1895 Network Research Center, Tsinghua University 1896 Beijing 100084 1897 China 1899 Email: yaoguang@cernet.edu.cn 1901 Fred Baker 1902 Cisco Systems 1903 Santa Barbara, CA 93117 1904 United States 1906 Email: fred@cisco.com