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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group G. Zorn 3 Internet-Draft Microsoft Corporation 4 Updates: RFC 2138 D. Leifer 5 Category: Standards Track A. Rubens 6 Ascend Communications 7 J. Shriver 8 Shiva Corporation 9 November 1997 11 RADIUS Attributes for Tunnel Protocol Support 13 1. Status of this Memo 15 This document is an Internet-Draft. Internet-Drafts are working docu- 16 ments of the Internet Engineering Task Force (IETF), its areas, and its 17 working groups. Note that other groups may also distribute working doc- 18 uments as Internet-Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference material 23 or to cite them other than as work in progress.'' 25 To learn the current status of any Internet-Draft, please check the 26 ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow 27 Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), 28 ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). 30 The distribution of this memo is unlimited. It is filed as , and expires May 19, 1997. Please send com- 32 ments to the RADIUS Working Group mailing list (ietf-radius@liv- 33 ingston.com) or to the authors (leifer@del.com, acr@del.com, 34 jas@shiva.com and glennz@microsoft.com). 36 2. Abstract 38 This document defines a set of RADIUS attributes designed to support the 39 provision of compulsory tunneling in dial-up networks. 41 3. Motivation 43 Many applications of tunneling protocols such as PPTP and L2TP involve 44 dial-up network access. Some, such as the provision of secure access to 45 corporate intranets via the Internet, are characterized by voluntary 46 tunneling: the tunnel is created at the request of the user for a spe- 47 cific purpose. Other applications involve compulsory tunneling: the 48 tunnel is created without any action from the user and without allowing 49 the user any choice in the matter. Examples of applications that might 50 be implemented using compulsory tunnels are Internet software upgrade 51 servers, software registration servers and banking services. These are 52 all services which, without compulsory tunneling, would probably be pro- 53 vided using dedicated networks or at least dedicated network access 54 servers (NAS), since they are characterized by the need to limit user 55 access to specific hosts. Given the existence of widespread support for 56 compulsory tunneling, however, these types of services could be accessed 57 via any Internet service provider (ISP). The most popular means of 58 authorizing dial-up network users today is through the RADIUS protocol. 59 The use of RADIUS allows the dial-up users' authorization and authenti- 60 cation data to be maintained in a central location, rather than on each 61 NAS. It makes sense to use RADIUS to centrally administer compulsory 62 tunneling, since RADIUS is widely deployed and was designed to carry 63 this type of information. In order to provide this functionality, new 64 RADIUS attributes are needed to carry the tunneling information from the 65 RADIUS server to the tunnel initiator; this document defines those 66 attributes. Specific recommendations for, and examples of, the applica- 67 tion of these attributes for the L2TP and PPTP protocols can be found in 68 draft-ietf-radius-tunnel-imp-XX.txt. 70 4. Specification of Requirements 72 In this document, the key words "MAY", "MUST, "MUST NOT", "optional", 73 "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as 74 described in [14]. 76 5. Attributes 78 Multiple instances of each of the attributes defined below may be 79 included in a single RADIUS packet. In this case, the attributes to be 80 applied to any given tunnel SHOULD all contain the same value in their 81 respective Tag fields; otherwise, the Tag field SHOULD NOT be used. 83 If the RADIUS server returns attributes describing multiple tunnels then 84 the tunnels SHOULD be interpreted by the tunnel initiator as alterna- 85 tives and the server SHOULD include an instance of the Tunnel-Preference 86 Attribute in the set of Attributes pertaining to each alternative 87 tunnel. Similarly, if the RADIUS client includes multiple sets of tun- 88 nel Attributes in an Access-Request packet, all the Attributes pertain- 89 ing to a given tunnel SHOULD contain the same value in their respective 90 Tag fiels and each set SHOULD include an appropriately valued instance 91 of the Tunnel-Preference Attribute. 93 5.1. Tunnel-Type 95 Description 97 This Attribute indicates the tunneling protocol(s) to be used. It 98 MAY be included in Access-Request, Access-Accept and Accounting- 99 Request packets. If the Tunnel-Type Attribute is present in an 100 Access-Request packet, it SHOULD be taken as a hint to the RADIUS 101 server as to the tunnelling protocols supported by the tunnel ini- 102 tiator; the RADIUS server MAY ignore the hint, however. A tunnel 103 initiator is not required to implement any of these tunnel types; 104 if a tunnel initiator receives an Access-Accept packet which con- 105 tains only unknown or unsupported Tunnel-Types, the tunnel initia- 106 tor MUST behave as though an Access-Reject had been received 107 instead. 109 A summary of the Tunnel-Type Attribute format is shown below. The 110 fields are transmitted from left to right. 112 0 1 2 3 113 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 114 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 115 | Type | Length | Tag | Value 116 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 117 Value (cont) | 118 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 120 Type 121 64 for Tunnel-Type 123 Length 124 Always 6. 126 Tag 127 The Tag field is one octet in length and is intended to provide a 128 means of grouping attributes in the same packet which refer to the 129 same tunnel. Valid values for this field are 0x01 through 0x1F, 130 inclusive. If the Tag field is unused, it MUST be zero. 132 Value 133 The Value field is three octets and contains one of the following 134 values, indicating the type of tunnel to be started. 136 1 Point-to-Point Tunneling Protocol (PPTP) [1] 137 2 Layer Two Forwarding (L2F) [2] 138 3 Layer Two Tunneling Protocol (L2TP) [3] 139 4 Ascend Tunnel Management Protocol (ATMP) [4] 140 5 Virtual Tunneling Protocol (VTP) [5] 141 6 IP Authentication Header in the Tunnel-mode (AH) [6] 142 7 IP-in-IP Encapsulation (IP-IP) [7] 143 8 Minimal IP-in-IP Encapsulation (MIN-IP-IP) [8] 144 9 IP Encapsulating Security Payload in the Tunnel-mode (ESP) [9] 145 10 Generic Route Encapsulation (GRE) [10] 146 11 Bay Dial Virtual Services (DVS) 147 12 IP-in-IP Tunneling [11] 149 5.2. Tunnel-Medium-Type 151 Description 153 The Tunnel-Medium-Type Attribute indicates which transport medium 154 to use when creating a tunnel for those protocols (such as L2TP) 155 that can operate over multiple transports. It MAY be included in 156 both Access-Request and Access-Accept packets; if it is present in 157 an Access-Request packet, it SHOULD be taken as a hint to the 158 RADIUS server as to the tunnel mediums supported by the tunnel 159 initiator. The RADIUS server MAY ignore the hint, however. 161 A summary of the Tunnel-Medium-Type Attribute format is given below. 162 The fields are transmitted left to right. 164 0 1 2 3 165 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 166 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 167 | Type | Length | Tag | Value | 168 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 169 Value (cont) | 170 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 172 Type 173 65 for Tunnel-Medium-Type 175 Length 176 6 178 Tag 179 The Tag field is one octet in length and is intended to provide a 180 means of grouping attributes in the same packet which refer to the 181 same tunnel. Valid values for this field are 0x01 through 0x1F, 182 inclusive. If the Tag field is unused, it MUST be zero (0x0000). 184 Value 185 The Value field is three octets and contains one of the values 186 listed under "Address Family Numbers" in [15]. For the sake of 187 convenience, a relevant excerpt of this list is reproduced below. 189 1 IP (IP version 4) 190 2 IP6 (IP version 6) 191 3 NSAP 192 4 HDLC (8-bit multidrop) 193 5 BBN 1822 194 6 802 (includes all 802 media plus Ethernet "canonical format") 195 7 E.163 196 8 E.164 (SMDS, Frame Relay, ATM) 197 9 F.69 (Telex) 198 10 X.121 (X.25, Frame Relay) 199 11 IPX 200 12 Appletalk 201 13 Decnet IV 202 14 Banyan Vines 203 15 E.164 with NSAP format subaddress 205 5.3. Tunnel-Client-Endpoint 207 Description 209 This Attribute contains the address of the initiator end of the 210 tunnel. It MAY be included in both Access-Request and Access- 211 Accept packets to indicate the address from which a new tunnel is 212 to be initiated. If the Tunnel-Client-Endpoint Attribute is 213 included in an Access-Request packet, the RADIUS server should 214 take the value as a hint; the server is not obligated to honor the 215 hint, however. It SHOULD be included in Accounting-Request pack- 216 ets which contain Acct-Status-Type attributes with values of 217 either Start or Stop, in which case it indicates the address from 218 which the tunnel was initiated. This Attribute, along with the 219 Tunnel-Server-Endpoint and Acct-Tunnel-Connection-ID attributes, 220 may be used to provide a globally unique means to identify a tun- 221 nel for accounting and auditing purposes. 223 A summary of the Tunnel-Client-Endpoint Attribute format is shown 224 below. The fields are transmitted from left to right. 226 0 1 2 3 227 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 228 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 229 | Type | Length | Tag | String ... 230 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 231 Type 232 66 for Tunnel-Client-Endpoint. 234 Length 235 >= 2 237 Tag 238 The Tag field is one octet in length and is intended to provide a 239 means of grouping attributes in the same packet which refer to the 240 same tunnel. Valid values for this field are 0x01 through 0x1F, 241 inclusive. If the value of the Tag field is less than or equal to 242 0x1F, it SHOULD be interpreted as indicating which tunnel (of sev- 243 eral alternatives) this attribute pertains; otherwise, it SHOULD 244 be interpreted as the first byte of the following String field. 246 String 247 The format of the address represented by the String field depends 248 upon the value of the Tunnel-Medium-Type attribute. 250 If Tunnel-Medium-Type is IP (1) or IP6 (2), then this string is 251 either the fully qualified domain name of the tunnel client 252 machine, or it is a "dotted-decimal" IP address. Conformant 253 implementations MUST support the dotted-decimal format and SHOULD 254 support the FQDN format for IP addresses. 256 If Tunnel-Medium-Type is not IP or IP6, this string is a tag 257 referring to configuration data local to the RADIUS client that 258 describes the interface and medium-specific address to use. 260 5.4. Tunnel-Server-Endpoint 262 Description 264 This Attribute indicates the address of the server end of the tun- 265 nel. The Tunnel-Server-Endpoint Attribute MAY be included (as a 266 hint to the RADIUS server) in the Access-Request packet and MUST 267 be included in the Access-Accept packet if the initiation of a 268 tunnel is desired. It SHOULD be included in Accounting-Request 269 packets which contain Acct-Status-Type attributes with values of 270 either Start or Stop and which pertain to a tunneled session. 271 This Attribute, along with the Tunnel-Client-Endpoint and Acct- 272 Tunnel-Connection-ID Attributes [12], may be used to provide a 273 globally unique means to identify a tunnel for accounting and 274 auditing purposes. 276 A summary of the Tunnel-Server-Endpoint Attribute format is shown 277 below. The fields are transmitted from left to right. 279 0 1 2 3 280 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 281 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 282 | Type | Length | Tag | String ... 283 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 285 Type 286 67 for Tunnel-Server-Endpoint. 288 Length 289 >= 3 291 Tag 292 The Tag field is one octet in length and is intended to provide a 293 means of grouping attributes in the same packet which refer to the 294 same tunnel. Valid values for this field are 0x01 through 0x1F, 295 inclusive. If the value of the Tag field is less than or equal to 296 0x1F, it SHOULD be interpreted as indicating which tunnel (of sev- 297 eral alternatives) this attribute pertains; otherwise, it SHOULD 298 be interpreted as the first byte of the following String field. 300 String 301 The format of the address represented by the String field depends 302 upon the value of the Tunnel-Medium-Type attribute. 304 If Tunnel-Medium-Type is IP (1) or IP6 (2), then this string is 305 either the fully qualified domain name of the tunnel client 306 machine, or it is a "dotted-decimal" IP address. Conformant 307 implementations MUST support the dotted-decimal format and SHOULD 308 support the FQDN format for IP addresses. 310 If Tunnel-Medium-Type is not IP or IP6, this string is a tag 311 referring to configuration data local to the RADIUS client that 312 describes the interface and medium-specific address to use. 314 5.5. Tunnel-Password 316 Description 318 This Attribute may contain a key or password. It may only be 319 included in an Access-Accept packet. 321 A summary of the Tunnel-Password Attribute format is shown below. 322 The fields are transmitted from left to right. 324 0 1 2 3 325 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 327 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 328 | Type | Length | Tag | String ... 329 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 331 Type 332 69 for Tunnel-Password 334 Length 335 >= 3 337 Tag 338 The Tag field is one octet in length and is intended to provide a 339 means of grouping attributes in the same packet which refer to the 340 same tunnel. Valid values for this field are 0x01 through 0x1F, 341 inclusive. If the value of the Tag field is less than or equal to 342 0x1F, it SHOULD be interpreted as indicating which tunnel (of sev- 343 eral alternatives) this attribute pertains; otherwise, it SHOULD 344 be ignored. 346 Salt 347 The Salt field is two octets in length and is used to ensure the 348 uniqueness of the encryption key used to encrypt each instance of 349 the Tunnel-Password attribute occurring in a given Access-Accept 350 packet. The most significant bit (leftmost) of the Salt field 351 MUST be set (1). The contents of each Salt field in a given 352 Access-Accept packet MUST be unique. 354 String 355 The plaintext String field consists of three logical sub-fields: 356 the Data-Length and Password sub-fields (both of which are 357 required), and the optional Padding sub-field. The Data-Length 358 sub-field is one octet in length and contains the length of the 359 unencrypted Password sub-field. The Password sub-field contains 360 the actual tunnel password. If the combined length (in octets) of 361 the unencrypted Data-Length and Password sub-fields is not an even 362 multiple of 16, then the Padding sub-field MUST be present. If it 363 is present, the length of the Padding sub-field is variable, 364 between 1 and 15 octets. The String field MUST be encrypted as 365 follows, prior to transmission: 367 Construct a plaintext version of the String field by concate- 368 nating the Data-Length and Password sub-fields. If necessary, 369 pad the resulting string until its length (in octets) is an 370 even multiple of 16. It is recommended that zero octets (0x00) 371 be used for padding. Call this plaintext P. 373 Call the shared secret S, the pseudo-random 128-bit Request 374 Authenticator (from the corresponding Access-Request packet) R, 375 and the contents of the Salt field A. Break P into 16 octet 376 chunks p(1), p(2)...p(i), where i = len(P)/16. Call the 377 ciphertext blocks c(1), c(2)...c(i) and the final ciphertext C. 378 Intermediate values b(1), b(2)...c(i) are required. Encryption 379 is performed in the following manner ('+' indicates concatena- 380 tion): 382 b(1) = MD5(S + R + A) c(1) = p(1) xor b(1) C = c(1) 383 b(2) = MD5(S + c(1)) c(2) = p(2) xor b(2) C = C + c(2) 384 . . 385 . . 386 . . 387 b(i) = MD5(S + c(i-1)) c(i) = p(i) xor b(i) C = C + c(i) 389 The resulting encrypted String field will contain 390 c(1)+c(2)+...+c(i). 392 On receipt, the process is reversed to yield the plaintext String. 394 5.6. Tunnel-Private-Group-ID 396 Description 398 This Attribute indicates the group ID for a particular tunneled 399 session. The Tunnel-Private-Group-ID Attribute MAY be included in 400 the Access-Request packet if the tunnel initiator can pre-deter- 401 mine the group resulting from a particular connection and SHOULD 402 be included in the Access-Reply packet if this tunnel session is 403 to be treated as belonging to a particular private group. Private 404 groups may be used to associate a tunneled session with a particu- 405 lar group of users. For example, it may be used to facilitate 406 routing of unregistered IP addresses through a particular inter- 407 face. It SHOULD be included in Accounting-Request packets which 408 contain Acct-Status-Type attributes with values of either Start or 409 Stop and which pertain to a tunneled session. 411 A summary of the Tunnel-Private-Group-ID Attribute format is shown 412 below. The fields are transmitted from left to right. 414 0 1 2 3 415 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 416 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 417 | Type | Length | Tag | String ... 418 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 420 Type 421 81 for Tunnel-Private-Group-ID. 423 Length 424 >= 3 426 Tag 427 The Tag field is one octet in length and is intended to provide a 428 means of grouping attributes in the same packet which refer to the 429 same tunnel. Valid values for this field are 0x01 through 0x1F, 430 inclusive. If the value of the Tag field is less than or equal to 431 0x1F, it SHOULD be interpreted as indicating which tunnel (of sev- 432 eral alternatives) this attribute pertains; otherwise, it SHOULD 433 be interpreted as the first byte of the following String field. 435 String 436 This field must be present. The group is represented by the 437 String field. There is no restriction on the format of group IDs. 439 5.7. Tunnel-Assignment-ID 441 Description 443 This Attribute is used to indicate to the tunnel initiator the 444 particular tunnel to which a session is to be assigned. Some tun- 445 neling protocols, such as PPTP and L2TP, allow for sessions 446 between the same two tunnel endpoints to be multiplexed over the 447 same tunnel and also for a given session to utilize its own dedi- 448 cated tunnel. This attribute provides a mechanism for RADIUS to 449 be used to inform the tunnel initiator (e.g. PAC, LAC) whether to 450 assign the session to a multiplexed tunnel or to a separate tun- 451 nel. Furthermore, it allows for sessions sharing multiplexed tun- 452 nels to be assigned to different multiplexed tunnels. 454 A particular tunneling implementation may assign differing charac- 455 teristics to particular tunnels. For example, different tunnels 456 may be assigned different QOS parameters. Such tunnels may be 457 used to carry either individual or multiple sessions. The Tunnel- 458 Assignment-ID attribute thus allows the RADIUS server to indicate 459 that a particular session is to be assigned to a tunnel that pro- 460 vides an appropriate level of service. It is expected that any 461 QOS-related RADIUS tunneling attributes defined in the future that 462 accompany this attribute will be associated by the tunnel initia- 463 tor with the ID given by this attribute. In the meantime, any 464 semantic given to a particular ID string is a matter left to local 465 configuration in the tunnel initiator. 467 The Tunnel-Assignment-ID attribute is of significance only to 468 RADIUS and the tunnel initiator. The ID it specifies is intended 469 to be of only local use to RADIUS and the tunnel initiator. The 470 ID assigned by the tunnel initiator is not conveyed to the tunnel 471 peer. 473 This attribute MAY be included in the Access-Accept. The tunnel 474 initiator receiving this attribute MAY choose to ignore it and 475 assign the session to an arbitrary multiplexed or non-multiplexed 476 tunnel between the desired endpoints. This attribute SHOULD also 477 be included in Accounting-Request packets which contain Acct-Sta- 478 tus-Type attributes with values of either Start or Stop and which 479 pertain to a tunneled session. 481 If a tunnel initiator supports the Tunnel-Assignment-ID Attribute, 482 then it should assign a session to a tunnel in the following man- 483 ner: 485 If this attribute is present and a tunnel exists between the 486 specified endpoints with the specified ID, then the session 487 should be assigned to that tunnel. 489 If this attribute is present and no tunnel exists between the 490 specified endpoints with the specified ID, then a new tunnel 491 should be established for the session and the specified ID 492 should be associated with the new tunnel. 494 If this attribute is not present, then the session is assigned 495 to an unnamed tunnel. If an unnamed tunnel does not yet exist 496 between the specified endpoints then it is established and used 497 for this and subsequent sessions established without the Tun- 498 nel-Assignment-ID attribute. A tunnel initiator MUST NOT 499 assign a session for which a Tunnel-Assignment-ID Attribute was 500 not specified to a named tunnel (i.e. one that was initiated by 501 a session specifying this attribute). 503 Note that the same ID may be used to name different tunnels if 504 such tunnels are between different endpoints. 506 A summary of the Tunnel-Assignment-ID Attribute format is shown 507 below. The fields are transmitted from left to right. 509 0 1 2 3 510 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 511 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 512 | Type | Length | Tag | String ... 513 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 515 Type 516 ?? for Tunnel-Assignment-ID. 518 Length 519 > 3 521 Tag 522 The Tag field is one octet in length and is intended to provide a 523 means of grouping attributes in the same packet which refer to the 524 same tunnel. Valid values for this field are 0x01 through 0x1F, 525 inclusive. If the value of the Tag field is less than or equal to 526 0x1F, it SHOULD be interpreted as indicating which tunnel (of sev- 527 eral alternatives) this attribute pertains; otherwise, it SHOULD 528 be interpreted as the first byte of the following String field. 530 String 531 This field must be present. The tunnel ID is represented by the 532 String field. There is no restriction on the format of the ID. 534 5.8. Tunnel-Preference 536 Description 538 If more than one set of tunneling attributes is returned by the 539 RADIUS server to the tunnel initiator, this Attribute SHOULD be 540 included in each set to indicate the relative preference assigned 541 to each tunnel. For example, suppose that Attributes describing 542 two tunnels are returned by the server, one with a Tunnel-Type of 543 PPTP and the other with a Tunnel-Type of L2TP. If the tunnel ini- 544 tiator supports only one of the Tunnel-Types returned, it will 545 initiate a tunnel of that type. If, however, it supports both 546 tunnel protocols, it SHOULD use the value of the Tunnel-Preference 547 Attribute to decide which tunnel should be started. The tunnel 548 having the numerically lowest value in the Value field of this 549 Attribute SHOULD be given the highest preference. The values 550 assigned to two or more instances of the Tunnel-Preference 551 Attribute within a given Access-Accept packet MAY be identical. 552 In this case, the tunnel initiator SHOULD use locally configured 553 metrics to decide which set of attributes to use. This Attribute 554 MAY be included (as a hint to the server) in Access-Request pack- 555 ets, but the RADIUS server is not required to honor this hint. 557 A summary of the Tunnel-Preference Attribute format is shown below. 558 The fields are transmitted from left to right. 560 0 1 2 3 561 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 562 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 563 | Type | Length | Tag | Value 564 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 565 Value (cont) | 566 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 568 Type 569 ?? for Tunnel-Preference 571 Length 572 Always 6. 574 Tag 575 The Tag field is one octet in length and is intended to provide a 576 means of grouping attributes in the same packet which refer to the 577 same tunnel. Valid values for this field are 0x01 through 0x1F, 578 inclusive. If the Tag field is unused, it MUST be zero. 580 Value 581 The Value field is three octets in length and indicates the pref- 582 erence to be given to the tunnel to which it refers; higher pref- 583 erence is given to lower values, with 0x000000 being most pre- 584 ferred and 0xFFFFFF least preferred. 586 6. Table of Attributes 588 The following table provides a guide to which of the above attributes 589 may be found in which kinds of packets, and in what quantity. 591 Request Accept Reject Challenge Acct-Request # Attribute 592 0+ 0+ 0 0 0-1 64 Tunnel-Type 593 0+ 0+ 0 0 0-1 65 Tunnel-Medium-Type 594 0+ 0+ 0 0 0-1 66 Tunnel-Client-Endpoint 595 0+ 0+ 0 0 0-1 67 Tunnel-Server-Endpoint 596 0 0+ 0 0 0 69 Tunnel-Password 597 0+ 0+ 0 0 0-1 81 Tunnel-Private-Group-ID 598 0 0+ 0 0 0-1 ?? Tunnel-Assignment-ID 599 0+ 0+ 0 0 0 ?? Tunnel-Preference 601 The following table defines the meaning of the above table entries. 603 0 This attribute MUST NOT be present in packet. 604 0+ Zero or more instances of this attribute MAY be present in packet. 605 0-1 Zero or one instance of this attribute MAY be present in packet. 607 7. Security Considerations 609 The Tunnel-Password Attribute may contain information which should only 610 be known to a tunnel endpoint. However, the method used to hide the 611 value of the attribute is such that intervening RADIUS proxies will have 612 knowledge of the contents. For this reason, the Tunnel-Password 613 Attribute SHOULD NOT be included in Access-Accept packets which may pass 614 through (relatively) untrusted RADIUS proxies. 616 8. Acknowledgements 618 Thanks to Dave Mitton (dmitton@baynetworks.com) for pointing out a nasty 619 circular dependency in the original Tunnel-Password attribute definition 620 and (in no particular order) to Kory Hamzeh (kory@ascend.com), Bertrand 621 Buclin (Bertrand.Buclin@att.ch), Dave Mitton (dmitton@baynetworks.com), 622 Andy Valencia (vandys@cisco.com), Bill Westfield (billw@cisco.com), Kris 623 Michielsen (kmichiel@cisco.com), Gurdeep Singh Pall (gur- 624 deep@microsoft.com), Ran Atkinson (rja@home.net), Aydin Edguer 625 (edguer@MorningStar.com) and Bernard Aboba (aboba@internaut.com) for 626 useful input and review. 628 9. Chair's Address 630 The RADIUS Working Group can be contacted via the current chair: 632 Carl Rigney 633 Livingston Enterprises 634 4464 Willow Road 635 Pleasanton, California 94588 637 Phone: +1 510 426 0770 638 E-Mail: cdr@livingston.com 640 10. Authors' Addresses 642 Questions about this memo can also be directed to: 644 Glen Zorn 645 Microsoft Corporation 646 One Microsoft Way 647 Redmond, Washington 98052 649 Phone: +1 206 703 1559 650 E-Mail: glennz@microsoft.com 651 Dory Leifer 652 Ascend Communications 653 1678 Broadway 654 Ann Arbor, MI 48105 656 Phone: +1 313 747 6152 657 E-Mail: leifer@ascend.com 659 John Shriver 660 Shiva Corporation 661 28 Crosby Drive 662 Bedford, MA 01730 664 Phone: +1 781 687 1329 665 E-Mail: jas@shiva.com 667 Allan Rubens 668 Ascend Communications 669 1678 Broadway 670 Ann Arbor, MI 48105 672 Phone: +1 313 761 6025 673 E-Mail: acr@del.com 675 11. References 677 [1] Hamzeh, et al, "Point-to-Point Tunneling Protocol -- PPTP", draft- 678 ietf-pppext-pptp-02.txt (work in progress), July 1997 680 [2] Valencia, Littlewood and Kolar, "Layer Two Forwarding (Protocol) 681 'L2F'", draft-valencia-l2f-00.txt (work in progress), October 1997 683 [3] Hamzeh, et al, "Layer Two Tunnelling Protocol (L2TP)", work in 684 progress, draft-ietf-pppext-l2tp-06.txt, August 1997 686 [4] Hamzeh, "Ascend Tunnel Management Protocol - ATMP", RFC 2107, 687 February 1997 689 [5] Calhoun and Wong, "Virtual Tunneling Protocol (VTP)", draft-cal- 690 houn-vtp-protocol-00.txt (work in progress), July 1996 (expired) 692 [6] Kent and Atkinson, "IP Authentication Header", draft-ietf-ipsec- 693 auth-header-02.txt (work in progress), October 1997 695 [7] Perkins, "IP Encapsulation within IP", RFC 2003, October 1996 697 [8] Perkins, "Minimal Encapsulation within IP", RFC 2004, October 1996 699 [9] Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 1827, 700 August 1995 702 [10] Hanks, et al, "Generic Routing Encapsulation (GRE)", RFC 1701, 703 October 1994 705 [11] Simpson, "IP in IP Tunneling", RFC 1853, October 1995 707 [12] Zorn, draft-ietf-radius-tunnel-acct-XX.txt (work in progress), 708 September 1997 710 [13] Rigney, et al, "Remote Authentication Dialin User Service 711 (RADIUS)", Section 5.2, RFC 2138, April 1997 713 [14] Bradner, "Key words for use in RFCs to Indicate Requirement Lev- 714 els", RFC 2119, March 1997 716 [15] Reynolds and Postel, "Assigned Numbers", STD 2, RFC 1700, October 717 1994