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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET-DRAFT 21 February 2000 3 Mark Handley 4 ACIRI 5 Colin Perkins 6 UCL 7 Edmund Whelan 8 UCL 10 Session Announcement Protocol 11 draft-ietf-mmusic-sap-v2-05.txt 13 Status of this memo 15 This document is an Internet-Draft and is in full conformance with 16 all provisions of Section 10 of RFC2026. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet-Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at 24 any time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This document is a product of the Multiparty Multimedia Session Control 34 working group of the Internet Engineering Task Force. Comments are 35 solicited and should be addressed to the working group's mailing list at 36 confctrl@isi.edu and/or the authors. 38 Abstract 40 This document describes version 2 of the multicast session 41 directory announcement protocol, SAP, and the related issues 42 affecting security and scalability that should be taken 43 into account by implementors. 45 1 Introduction 47 In order to assist the advertisement of multicast multimedia conferences 48 and other multicast sessions, and to communicate the relevant session 49 setup information to prospective participants, a distributed session 50 directory may be used. An instance of such a session directory periodically 51 multicasts packets containing a description of the session, and these 52 advertisements are received by potential participants who can use the 53 session description to start the tools required to participate in the 54 session. 56 This memo describes the issues involved in the multicast announcement of 57 session description information and defines an announcement protocol to be 58 used. Sessions are described using the session description protocol which 59 is described in a companion memo [4]. 61 2 Terminology 63 A SAP announcer periodically multicasts an announcement packet to a well 64 known multicast address and port. The announcement is multicast with the 65 same scope as the session it is announcing, ensuring that the recipients of 66 the announcement can also be potential recipients of the session the 67 announcement describes (bandwidth and other such constraints permitting). 68 This is also important for the scalability of the protocol, as it keeps 69 local session announcements local. 71 A SAP listener learns of the multicast scopes it is within (for example, 72 using the Multicast-Scope Zone Announcement Protocol [5]) and listens on 73 the well known SAP address and port for those scopes. In this manner, it 74 will eventually learn of all the sessions being announced, allowing those 75 sessions to be joined. 77 The key words `MUST', `MUST NOT', `REQUIRED', `SHALL', `SHALL NOT', 78 `SHOULD', `SHOULD NOT', `RECOMMENDED', `MAY', and `OPTIONAL' in this 79 document are to be interpreted as described in [1]. 81 3 Session Announcement 83 As noted previously, a SAP announcer periodically sends an announcement 84 packet to a well known multicast address and port. There is no rendezvous 85 mechanism - the SAP announcer is not aware of the presence or absence of 86 any SAP listeners - and no additional reliability is provided over the 87 standard best-effort UDP/IP semantics. 89 That announcement contains a session description and SHOULD contain 90 an authentication header. The session description MAY be encrypted 91 although this is NOT RECOMMENDED (see section 7). 93 A SAP announcement is multicast with the same scope as the session 94 it is announcing, ensuring that the recipients of the announcement 95 can also be potential recipients of the session being advertised. 96 There a number of possiblities: 98 IPv4 global scope sessions use multicast addresses in the range 99 224.2.128.0 - 224.2.255.255 with SAP announcements being sent 100 to 224.2.127.254 (note that 224.2.127.255 is used by the obsolete 101 SAPv0 and MUST NOT be used). 103 IPv4 administrative scope sessions using administratively scoped 104 IP multicast as defined in [7]. The multicast address to be 105 used for announcements is the highest multicast address in the 106 relevant administrative scope zone. For example, if the scope 107 range is 239.16.32.0 - 239.16.33.255, then 239.16.33.255 is used 108 for SAP announcements. 110 IPv6 sessions are announced on the address FF0X:0:0:0:0:0:2:7FFE 111 where X is the 4-bit scope value. For example, an announcement 112 for a link-local session assigned the address FF02:0:0:0:0:0:1234:5678, 113 should be advertised on SAP address FF02:0:0:0:0:0:2:7FFE. 115 SAP announcements MUST be sent on port 9875 and SHOULD be sent with 116 an IP time-to-live of 255 (the use of TTL scoping for multicast is 117 discouraged [7]). 119 If a session uses addresses in multiple administrative scope ranges, 120 it is necessary for the announcer to send identical copies of the 121 announcement to each administrative scope range. It is up to the 122 listeners to parse such multiple announcements as the same session 123 (as identified by the SDP origin field, for example). The announcement 124 rate for each administrative scope range MUST be calculated separately, 125 as if the multiple announcements were separate. 127 Multiple announcers may announce a single session, as an aid to robustness 128 in the face of packet loss and failure of one or more announcers. The rate 129 at which each announcer repeats its announcement MUST be scaled back such 130 that the total announcement rate is equal to that which a single server 131 would choose. Announcements made in this manner MUST be identical. 133 If multiple announcements are being made for a session, then each 134 announcement MUST carry an authentication header signed by the same 135 key, or be treated as a completely separate announcement by listeners. 137 An IPv4 SAP listener SHOULD listen on the IPv4 global scope SAP address and 138 on the SAP addresses for each IPv4 administrative scope zone it is within. 139 The discovery of administrative scope zones is outside the scope of this 140 memo, but it is assumed that each SAP listener within a particular scope 141 zone is aware of that scope zone. A SAP listener which supports IPv6 142 SHOULD also listen to the IPv6 SAP addresses. 144 3.1 Announcement Interval 146 The time period between repetitions of an announcement is chosen 147 such that the total bandwidth used by all announcements on a single 148 SAP group remains below a preconfigured limit. If not otherwise 149 specified, the bandwidth limit SHOULD be assumed to be 4000 bits 150 per second. 152 Each announcer is expected to listen to other announcements in order to 153 determine the total number of sessions being announced on a particular 154 group. Sessions are uniquely identified by the combination of the message 155 identifier hash and originating source fields of the SAP header (note that 156 SAP v0 announcers always set the message identifier hash to zero, and if 157 such an announcement is received the entire message MUST be compared to 158 determine uniqueness). 160 Announcements are made by periodic multicast to the group. The base 161 interval between announcements is derived from the number of announcements 162 being made in that group, the size of the announcement and the configured 163 bandwidth limit. The actual transmission time is derived from this base 164 interval as follows: 166 1.The announcer initialises the variable tp to be the last time 167 a particular announcement was transmitted (or the current time 168 if this is the first time this announcement is to be made). 170 2.Given a configured bandwidth limit in bits/second and an announcement 171 of ad_size bytes, the base announcement interval in seconds is 172 interval = max(300; (8*no_of_ads*ad_size)/limit) 174 3.An offset is calculated based on the base announcement interval 176 offset = rand(interval * 2/3)-(interval/3) 178 4.The next transmission time for an announcement derived as 179 tn = tp + interval + offset 181 The announcer then sets a timer to expire at tn and waits. At time 182 tn the announcer SHOULD recalculate the next transmission time. If 183 the new value of tn is before the current time, the announcement 184 is sent immediately. Otherwise the transmission is rescheduled for 185 the new tn. This reconsideration prevents transient packet bursts 186 on startup and when a network partition heals. 188 4 Session Deletion 190 Sessions may be deleted in one of several ways: 192 Explicit Timeout The session description payload may contain timestamp 193 information specifying the start- and end-times of the session. 194 If the current time is later than the end-time of the session, 195 then the session SHOULD be deleted from the receiver's session 196 cache. 198 Implicit Timeout A session announcement message should be received 199 periodically for each session description in a receiver's session 200 cache. The announcement period can be predicted by the receiver 201 from the set of sessions currently being announced. If a session 202 announcement message has not been received for ten times the 203 announcement period, or one hour, whichever is the greater, then 204 the session is deleted from the receiver's session cache. The 205 one hour minimum is to allow for transient network partitionings. 207 Explicit Deletion A session deletion packet is received specifying 208 the session to be deleted. Session deletion packets SHOULD have 209 a valid authentication header, matching that used to authenticate 210 previous announcement packets. If this authentication is missing, 211 the deletion message SHOULD be ignored. 213 5 Session Modification 215 A pre-announced session can be modified by simply announcing the modified 216 session description. In this case, the version hash in the SAP header MUST 217 be changed to indicate to receivers that the packet contents should be 218 parsed (or decrypted and parsed if it is encrypted). The session itself, 219 as distinct from the session announcement, is uniquely identified by the 220 payload and not by the message identifier hash in the header. 222 The same rules apply for session modification as for session deletion: 224 o Either the modified announcement must contain an authentication 225 header signed by the same key as the cached session announcement 226 it is modifying, or: 228 o The cached session announcement must not contain an authentication 229 header, and the session modification announcement must originate 230 from the same host as the session it is modifying. 232 If an announcement is received containing an authentication header 233 and the cached announcement did not contain an authentication header, 234 or it contained a different authentication header, then the modified 235 announcement MUST be treated as a new and different announcement, 236 and displayed in addition to the un-authenticated announcement. The 237 same should happen if a modified packet without an authentication 238 header is received from a different source than the original announcement. 239 These rules prevent an announcement having an authentication header 240 added by a malicious user and then being deleted using that header, 241 and it also prevents a denial-of-service attack by someone putting 242 out a spoof announcement which, due to packet loss, reaches some 243 participants before the original announcement. Note that under such 244 0 1 2 3 245 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 1 246 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 247 | V=1 |A|R|T|E|C| auth len | msg id hash | 248 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 249 | | 250 : originating source (32 or 128 bits) : 251 : : 252 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 253 | optional authentication data | 254 : .... : 255 *-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-* 256 | optional payload type | 257 + +-+- - - - - - - - - -+ 258 | |0| | 259 + - - - - - - - - - - - - - - - - - - - - +-+ | 260 | | 261 : payload : 262 | | 263 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 265 Figure 1: Packet format 267 circumstances, being able to authenticate the message originator is 268 the only way to discover which session is the correct session. 270 6 Packet Format 272 SAP data packets have the format described in figure 1. 274 V: Version Number. The version number field MUST be set to 1 (SAPv2 275 announcements which use only SAPv1 features are backwards compatible, 276 those which use new features can be detected by other means, 277 so the SAP version number doesn't need to change). 279 A: Address type. If the A bit is 0, the originating source field 280 contains a 32-bit IPv4 address. If the A bit is 1, the originating 281 source contains a 128-bit IPv6 address. 283 R: Reserved. SAP announcers MUST set this to 0, SAP listeners MUST 284 ignore the contents of this field. 286 T: Message Type. If the T field is set to 0 this is a session announcement 287 packet, if 1 this is a session deletion packet. 289 E: Encryption Bit. If the encryption bit is set to 1, the payload 290 of the SAP packet is encrypted. If this bit is 0 the packet 291 is not encrypted. See section 7 for details of the encryption 292 process. 294 C: Compressed bit. If the compressed bit is set to 1, the payload 295 is compressed using the zlib compression algorithm [3]. If the 296 payload is to be compressed and encrypted, the compression MUST 297 be performed first. 299 Authentication Length. An 8 bit unsigned quantity giving the number 300 of 32 bit words following the main SAP header that contain 301 authentication data. If it is zero, no authentication header is 302 present. 304 Authentication data containing a digital signature of the packet, 305 with length as specified by the authentication length header 306 field. See section 8 for details of the authentication process. 308 Message Identifier Hash. A 16 bit quantity that, used in combination 309 with the originating source, provides a globally unique identifier 310 indicating the precise version of this announcement. The choice 311 of value for this field is not specified here, except that it 312 MUST be unique for each session announced by a particular SAP 313 announcer and it MUST be changed if the session description is 314 modified. 316 Earlier versions of SAP used a value of zero to mean that the 317 hash should be ignored and the payload should always be parsed. 318 This had the unfortunate side-effect that SAP announcers had 319 to study the payload data to determine how many unique sessions 320 were being advertised, making the calculation of the announcement 321 interval more complex that necessary. In order to decouple the 322 session announcement process from the contents of those announcements, 323 SAP announcers SHOULD NOT set the message identifier hash to 324 zero. 326 SAP listeners MAY silently discard messages if the message identifier 327 hash is set to zero. 329 Originating Source. This gives the IP address of the original source 330 of the message. This is an IPv4 address if the A field is set 331 to zero, else it is an IPv6 address. The address is stored 332 in network byte order. 334 SAPv0 permitted the originating source to be zero if the message 335 identifier hash was also zero. This practise is no longer legal, 336 and SAP announcers SHOULD NOT set the originating source to zero. 337 SAP listeners MAY silently discard packets with the originating 338 source set to zero. 340 The header is followed by an optional payload type field and the 341 payload data itself. If the E or C bits are set in the header both 342 the payload type and payload are encrypted and/or compressed. 344 The payload type field is a MIME content type specifier, describing the 345 format of the payload. This is a variable length ASCII text string, 346 followed by a single zero byte (ASCII NUL). The payload type SHOULD be 347 included in all packets. If the payload type is `application/sdp' 348 both the payload type and its terminating zero byte MAY be omitted, 349 although this is intended for backwards compatibility with SAP v1 350 listeners only. 352 The absence of a payload type field may be noted since the payload 353 section of such a packet will start with an SDP `v=0' field, which 354 is not a legal MIME content type specifier. 356 All implementations MUST support payloads of type `application/sdp' [4]. 357 Other formats MAY be supported although since there is no negotiation in 358 SAP an announcer which chooses to use a session description format other 359 than SDP cannot know that the listeners are able to understand the 360 announcement. A proliferation of payload types in announcements has the 361 potential to lead to severe interoperability problems, and for this reason, 362 the use of non-SDP payloads is NOT RECOMMENDED. 364 If the packet is an announcement packet, the payload contains a session 365 description. 367 If the packet is a session deletion packet, the payload contains a session 368 deletion message. If the payload format is `application/sdp' the deletion 369 message is a single SDP line consisting of the origin field of the 370 announcement to be deleted. 372 It is desirable for the payload to be sufficiently small that SAP packets 373 do not get fragmented by the underlying network. Fragmentation has a loss 374 multiplier effect, which is known to significantly affect the reliability 375 of announcements. It is RECOMMENDED that SAP packets are smaller than 376 1kByte in length, although if it is known that announcements will use a 377 network with a smaller MTU than this, then that SHOULD be used as the 378 maximum recommended packet size. 380 7 Encrypted Announcements 382 An announcement is received by all listeners in the scope to which 383 it is sent. If an announcement is encrypted, and many of the receivers 384 do not have the encryption key, there is a considerable waste of 385 bandwidth since those receivers cannot use the announcement they have 386 received. For this reason, the use of encrypted SAP announcements 387 is NOT RECOMMENDED on the global scope SAP group or on administrative 388 scope groups which may have many receivers which cannot decrypt those 389 announcements. 391 The opinion of the authors is that encrypted SAP is useful in special 392 cases only, and that the vast majority of scenarios where encrypted 393 SAP has been proposed may be better served by distributing session 394 details using another mechanism. There are, however, certain scenarios 395 where encrypted announcements may be useful. For this reason, the 396 encryption bit is included in the SAP header to allow experimentation 397 with encrypted announcements. 399 This memo does not specify details of the encryption algorithm to 400 be used or the means by which keys are generated and distributed. 401 An additional specification should define these, if it is desired 402 to use encrypted SAP. 404 Note that if an encrypted announcement is being announced via a proxy, then 405 there may be no way for the proxy to discover that the announcement has 406 been superseded, and so it may continue to relay the old announcement in 407 addition to the new announcement. SAP provides no mechanism to chain 408 modified encrypted announcements, so it is advisable to announce the 409 unmodified session as deleted for a short time after the modification has 410 occurred. This does not guarantee that all proxies have deleted the 411 session, and so receivers of encrypted sessions should be prepared to 412 discard old versions of session announcements that they may receive. In 413 most cases however, the only stateful proxy will be local to (and known to) 414 the sender, and an additional (local-area) protocol involving a handshake 415 for such session modifications can be used to avoid this problem. 417 Session announcements that are encrypted with a symmetric algorithm 418 may allow a degree of privacy in the announcement of a session, but 419 it should be recognised that a user in possession of such a key can 420 pass it on to other users who should not be in possession of such 421 a key. Thus announcements to such a group of key holders cannot 422 be assumed to have come from an authorised key holder unless there 423 is an appropriate authentication header signed by an authorised key 424 holder. In addition the recipients of such encrypted announcements 425 cannot be assumed to only be authorised key holders. Such encrypted 426 announcements do not provide any real security unless all of the 427 authorised key holders are trusted to maintain security of such session 428 directory keys. This property is shared by the multicast session 429 tools themselves, where it is possible for an un-trustworthy member 430 of the session to pass on encryption keys to un-authorised users. 431 However it is likely that keys used for the session tools will be 432 more short lived than those used for session directories. 434 Similar considerations should apply when session announcements are 435 encrypted with an asymmetric algorithm, but then it is possible to 436 restrict the possessor(s) of the private key, so that announcements 437 to a key-holder group can not be made, even if one of the untrusted 438 members of the group proves to be un-trustworthy. 440 8 Authenticated Announcements 442 The authentication header can be used for two purposes: 444 o Verification that changes to a session description or deletion 445 of a session are permitted. 447 o Authentication of the identity of the session creator. 449 1 2 3 450 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 1 451 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 452 | V=1 |P| Auth | | 453 +-+-+-+-+-+-+-+-+ | 454 | Format specific authentication subheader | 455 : .................. : 456 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 458 Figure 2: Format of the authentication data in the SAP header 460 In some circumstances only verification is possible because a certificate 461 signed by a mutually trusted person or authority is not available. 462 However, under such circumstances, the session originator may still be 463 authenticated to be the same as the session originator of previous sessions 464 claiming to be from the same person. This may or may not be sufficient 465 depending on the purpose of the session and the people involved. 467 Clearly the key used for the authentication should not be trusted 468 to belong to the session originator unless it has been separately 469 authenticated by some other means, such as being certified by a trusted 470 third party. Such certificates are not normally included in an SAP 471 header because they take more space than can normally be afforded 472 in an SAP packet, and such verification must therefore take place 473 by some other mechanism. However, as certified public keys are normally 474 locally cached, authentication of a particular key only has to take 475 place once, rather than every time the session directory retransmits 476 the announcement. 478 SAP is not tied to any single authentication mechanism. Authentication 479 data in the header is self-describing, but the precise format depends 480 on the authentication mechanism in use. The generic format of the 481 authentication data is given in figure 2. The structure of the format 482 specific authentication subheader, using both the PGP and the CMS 483 formats, is discussed in sections 8.1 and 8.2 respectively. 485 Version Number, V: The version number of the authentication format 486 specified by this memo is 1. 488 Padding Bit, P: If necessary the authentication data is padded 489 to be a multiple of 32 bits and the padding bit is set. In 490 this case the last byte of the authentication data contains the 491 number of padding bytes (including the last byte) that must be 492 discarded. 494 Authentication Type, Auth: The authentication type is a 4 bit encoded 495 field that denotes the authentication infrastructure the sender 496 expects the recipients to use to check the authenticity and integrity 497 of the information. This defines the format of the authentication 498 subheader and can take the values: 0 = PGP format, 1 = CMS 499 format. All other values are undefined and SHOULD be ignored. 501 If a SAP packet is to be compressed or encrypted, this MUST be done 502 before the authentication is added. 504 The digital signature in the authentication data MUST be calculated 505 over the entire packet, including the header. The authentication 506 length MUST be set to zero and the authentication data excluded when 507 calculating the digital signature. 509 It is to be expected that sessions may be announced by a number of 510 different mechanisms, not only SAP. For example, a session description 511 may placed on a web page, sent by email or conveyed in a session 512 initiation protocol. To ease interoperability with these other mechanisms, 513 application level security is employed, rather than using IPsec authentication 514 headers. 516 8.1 PGP Authentication 518 Implementations which support authentication MUST support this format. A 519 full description of the PGP protocol can be found in [2]. When using PGP 520 for SAP authentication the basic format specific authentication subheader 521 comprises a digital signature packet as described in [2]. The signature 522 type MUST be 0x01 which means the signature is that of a canonical text 523 document. 525 8.2 CMS Authentication 527 Support for this format is OPTIONAL. 529 A full description of the Cryptographic Message Syntax can be found 530 in [6]. The format specific authentication subheader will, in the 531 CMS case, have an ASN.1 ContentInfo type with the ContentType being 532 signedData. 534 Use is made of the option available in PKCS#7 to leave the content itself 535 blank as the content which is signed is already present in the packet. 536 Inclusion of it within the SignedData type would duplicate this data and 537 increase the packet length unnecessarily. In addition this allows 538 recipients with either no interest in the authentication, or with no 539 mechanism for checking it, to more easily skip the authentication 540 information. 542 There SHOULD be only one signerInfo and related fields corresponding 543 to the originator of the SAP announcement. The signingTime SHOULD 544 be present as a signedAttribute. However, due to the strict size 545 limitations on the size of SAP packets, certificates and CRLs SHOULD 546 NOT be included in the signedData structure. It is expected that 547 users of the protocol will have other methods for certificate and 548 CRL distribution. 550 9 Scalability and caching 552 SAP is intended to announce the existence of long-lived wide-area 553 multicast sessions. It is not an especially timely protocol: sessions 554 are announced by periodic multicast with a repeat rate on the order 555 of tens of minutes, and no enhanced reliability over UDP. This leads 556 to a long startup delay before a complete set of announcements is 557 heard by a listener. This delay is clearly undesirable for interactive 558 browsing of announced sessions. 560 In order to reduce the delays inherent in SAP, it is recommended 561 that proxy caches are deployed. A SAP proxy cache is expected to 562 listen to all SAP groups in its scope, and to maintain an up-to-date 563 list of all announced sessions along with the time each announcement 564 was last received. When a new SAP listeners starts, it should contact 565 its local proxy to download this information, which is then sufficient 566 for it to process future announcements directly, as if it has been 567 continually listening. 569 The protocol by which a SAP listener contacts its local proxy cache 570 is not specified here. 572 10 Security Considerations 574 SAP contains mechanisms for ensuring integrity of session announcements, 575 for authenticating the origin of an announcement and for encrypting 576 such announcements (sections 7 and 8). 578 As stated in section 5, if a session modification announcement is 579 received that contains a valid authentication header, but which is 580 not signed by the original creator of the session, then the session 581 must be treated as a new session in addition to the original session 582 with the same SDP origin information unless the originator of one 583 of the session descriptions can be authenticated using a certificate 584 signed by a trusted third party. If this were not done, there would 585 be a possible denial of service attack whereby a party listens for 586 new announcements, strips off the original authentication header, 587 modifies the session description, adds a new authentication header 588 and re-announces the session. If a rule was imposed that such spoof 589 announcements were ignored, then if packet loss or late starting 590 of a session directory instance caused the original announcement to 591 fail to arrive at a site, but the spoof announcement did so, this 592 would then prevent the original announcement from being accepted at 593 that site. 595 A similar denial-of-service attack is possible if a session announcement 596 receiver relies completely on the originating source and hash fields 597 to indicate change, and fails to parse the remainder of announcements 598 for which it has seen the origin/hash combination before. 600 A denial of service attack is possible from a malicious site close 601 to a legitimate site which is making a session announcement. This 602 can happen if the malicious site floods the legitimate site with 603 huge numbers of (illegal) low TTL announcements describing high TTL 604 sessions. This may reduce the session announcement rate of the legitimate 605 announcement to below a tenth of the rate expected at remote sites 606 and therefore cause the session to time out. Such an attack is likely 607 to be easily detectable, and we do not provide any mechanism here 608 to prevent it. 610 A Summary of differences between SAPv0 and SAPv1 612 For this purpose SAPv0 is defined as the protocol in use by version 613 2.2 of the session directory tool, sdr. SAPv1 is the protocol described 614 in the 19 November 1996 version of this memo (draft-ietf-mmusic-sap-00.txt). 615 The packet headers of SAP messages are the same in V0 and V1 in 616 that a V1 tool can parse a V0 announcement header but not vice-versa. 618 In SAPv0, the fields have the following values: 620 o Version Number: 0 622 o Message Type: 0 (Announcement) 624 o Authentication Type: 0 (No Authentication) 626 o Encryption Bit: 0 (No Encryption) 628 o Compression Bit: 0 (No compression) 630 o Message Id Hash: 0 (No Hash Specified) 632 o Originating Source: 0 (No source specified, announcement has 633 not been relayed) 635 B Summary of differences between SAPv1 and SAPv2 637 The packet headers of SAP messages are the same in V1 and V2 in 638 that a V2 tool can parse a V1 announcement header but not necessarily 639 vice-versa. 641 o The A bit has been added to the SAP header, replacing one of 642 the bits of the SAPv1 message type field. If set to zero the 643 announcement is of an IPv4 session, and the packet is backwards 644 compatible with SAPv1. If set to one the announcement is of 645 an IPv6 session, and SAPv1 listeners (which do not support IPv6) 646 will see this as an illegal message type (MT) field. 648 o The second bit of the message type field in SAPv1 has been replaced 649 by a reserved, must-be-zero, bit. This bit was unused in SAPv1, 650 so this change just codifies existing usage. 652 o SAPv1 specified encryption of the payload. SAPv2 includes the 653 E bit in the SAP header to indicate that the payload is encrypted, 654 but does not specify any details of the encryption. 656 o SAPv1 allowed the message identifier hash and originating source 657 fields to be set to zero, for backwards compatibility. This 658 is no longer legal. 660 o SAPv1 specified gzip compression. SAPv2 uses zlib (the only 661 known implementation of SAP compression used zlib, and gzip compression 662 was a mistake). 664 o SAPv2 provides a more complete specification for authentication. 666 o SAPv2 allows for non-SDP payloads to be transported. SAPv1 required 667 that the payload was SDP. 669 o SAPv1 included a timeout field for encrypted announcement, SAPv2 670 does not (and relies of explicit deletion messages or implicit 671 timeouts). 673 C Acknowledgments 675 SAP and SDP were originally based on the protocol used by the sd 676 session directory from Van Jacobson at LBNL. Version 1 of SAP was 677 designed by Mark Handley as part of the European Commission MICE 678 (Esprit 7602) and MERCI (Telematics 1007) projects. Version 2 includes 679 authentication features developed by Edmund Whelan, Goli Montasser-Kohsari 680 and Peter Kirstein as part of the European Commission ICE-TEL project 681 (Telematics 1005), and support for IPv6 developed by Maryann P. Maher 682 and Colin Perkins. 684 D Authors' Addresses 686 Mark Handley 687 AT&T Center for Internet Research at ICSI, 688 International Computer Science Institute, 689 1947 Center Street, Suite 600, 690 Berkeley, CA 94704, USA 692 Colin Perkins 693 Department of Computer Science, 694 University College London, 695 Gower Street, 696 London, WC1E 6BT, UK 698 Edmund Whelan 699 Department of Computer Science, 700 University College London, 701 Gower Street, 702 London, WC1E 6BT, UK 704 References 706 [1] S. Bradner. Key words for use in RFCs to indicate requirement levels, 707 March 1997. RFC2119. 709 [2] J. Callas, L. Donnerhacke, H. Finney, and R. Thayer. OpenPGP message 710 format, November 1998. RFC2440. 712 [3] P. Deutsch and J.-L. Gailly. Zlib compressed data format specification 713 version 3.3, May 1996. RFC1950. 715 [4] M. Handley and V. Jacobson. SDP: Session Description Protocol, April 716 1998. RFC2327. 718 [5] M. Handley, D. Thaler, and R. Kermode. Multicast-scope zone 719 announcement protocol (MZAP), February 2000, RFC2776. 721 [6] R. Housley. Cryptographic message syntax, June 1999. RFC2630. 723 [7] D. Mayer. Administratively scoped IP multicast, July 1998. RFC2365.