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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 TRAM P. Patil 3 Internet-Draft T. Reddy 4 Intended status: Standards Track D. Wing 5 Expires: November 14, 2015 Cisco 6 May 13, 2015 8 TURN Server Auto Discovery 9 draft-ietf-tram-turn-server-discovery-02 11 Abstract 13 Current Traversal Using Relays around NAT (TURN) server discovery 14 mechanisms are relatively static and limited to explicit 15 configuration. These are usually under the administrative control of 16 the application or TURN service provider, and not the enterprise, 17 ISP, or the network in which the client is located. Enterprises and 18 ISPs wishing to provide their own TURN servers need auto discovery 19 mechanisms that a TURN client could use with no or minimal 20 configuration. This document describes three such mechanisms for 21 TURN server discovery. 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 November 14, 2015. 40 Copyright Notice 42 Copyright (c) 2015 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 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 58 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 3. Discovery Procedure . . . . . . . . . . . . . . . . . . . . . 3 60 4. Discovery using Service Resolution . . . . . . . . . . . . . 4 61 4.1. Retrieving Domain Name . . . . . . . . . . . . . . . . . 4 62 4.1.1. DHCP . . . . . . . . . . . . . . . . . . . . . . . . 5 63 4.1.2. From own Identity . . . . . . . . . . . . . . . . . . 5 64 4.2. Resolution . . . . . . . . . . . . . . . . . . . . . . . 5 65 5. DNS Service Discovery . . . . . . . . . . . . . . . . . . . . 6 66 5.1. mDNS . . . . . . . . . . . . . . . . . . . . . . . . . . 7 67 6. Discovery using Anycast . . . . . . . . . . . . . . . . . . . 8 68 7. Deployment Considerations . . . . . . . . . . . . . . . . . . 8 69 7.1. Mobility and Changing IP addresses . . . . . . . . . . . 9 70 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 71 8.1. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . 9 72 9. Security Considerations . . . . . . . . . . . . . . . . . . . 9 73 9.1. Service Resolution . . . . . . . . . . . . . . . . . . . 9 74 9.2. DNS Service Discovery . . . . . . . . . . . . . . . . . . 10 75 9.3. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . 10 76 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 77 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 78 11.1. Normative References . . . . . . . . . . . . . . . . . . 11 79 11.2. Informative References . . . . . . . . . . . . . . . . . 12 80 Appendix A. Change History . . . . . . . . . . . . . . . . . . . 12 81 A.1. Change from draft-patil-tram-serv-disc-00 to -01 . . . . 12 82 A.2. Change from draft-ietf-tram-turn-server-discovery-01 to 83 02 . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 84 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 86 1. Introduction 88 TURN [RFC5766] is a protocol that is often used to improve the 89 connectivity of Peer-to-Peer (P2P) applications (as defined in 90 section 2.7 of [RFC5128]). TURN allows a connection to be 91 established when one or both sides are incapable of a direct P2P 92 connection. It is an important building block for interactive, real- 93 time communication using audio, video, collaboration etc. While TURN 94 services are extensively used today, the means to auto discover TURN 95 servers do not exist. TURN clients are usually explicitly configured 96 with a well known TURN server. To allow TURN applications operate 97 seamlessly across different types of networks and encourage the use 98 of TURN without the need for manual configuration, it is important 99 that there exists an auto discovery mechanism for TURN services. Web 100 Real-Time Communication (WebRTC) [I-D.ietf-rtcweb-overview] usages 101 and related extensions, which are mostly based on web applications, 102 need this immediately. 104 This document describes three discovery mechanisms. The reason for 105 providing multiple mechanisms is to maximize the opportunity for 106 discovery, based on the network in the which the TURN client finds 107 itself. 109 o A resolution mechanism based on straightforward Naming Authority 110 Pointer (S-NAPTR) resource records in the Domain Name System 111 (DNS). [RFC5928] describes details on retrieving a list of server 112 transport addresses from DNS that can be used to create a TURN 113 allocation. 115 o DNS Service Discovery 117 o A mechanism based on anycast address for TURN. 119 In general, if a client wishes to communicate using one of its 120 interfaces using a specific IP address family, it SHOULD query the 121 TURN server(s) that has been discovered for that specific interface 122 and address family. How to select an interface and IP address 123 family, is out of the scope of this document. 125 2. Terminology 127 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 128 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 129 document are to be interpreted as described in [RFC2119]. 131 3. Discovery Procedure 133 A TURN client that implements the auto discovery algorithm uses the 134 following mechanisms for discovery: 136 1. Local Configuration : Local or manual configuration should be 137 tried first, as it may be an explicit preferred choice of a user. 138 An implementation MAY give the user an opportunity (e.g., by 139 means of configuration file options or menu items) to specify a 140 TURN server for every address family. 142 2. Service Resolution : Service Resolution : The TURN client 143 attempts to perform TURN service resolution using the host's DNS 144 domain. 146 3. DNS Service Discovery (DNS SD) 148 4. Anycast : Send TURN allocate request to the assigned TURN anycast 149 request for each combination of interface and address family. 151 Not all TURN servers may be discovered using NAPTR records or DNS SD; 152 Similarly, not all TURN servers may support anycast. For best 153 results, a client MUST implement all discovery mechanisms described 154 above. 156 The document does not prescribe a strict order that a client must 157 follow for discovery. An implementation may choose to perform steps 158 2,3 and 4 in parallel for discovery OR choose to follow any desired 159 order and stop the discovery procedure if a mechanism succeeds. 161 On hosts with more than one interface or address family (IPv4/v6), 162 the TURN server discovery procedure has to be performed for each 163 combination of interface and address family. A client MAY optionaly 164 choose to perform the discovery procedure only for a desired 165 interface/address combination, if the client does not wish to 166 discover a TURN server for all combinations of interface and address 167 family. 169 4. Discovery using Service Resolution 171 This mechanism is performed in two steps: 173 1. A DNS domain name is retrieved for each combination of interface 174 and address family. 176 2. Retrieved DNS domain names are then used for S-NAPTR lookups as 177 per [RFC5928]. Further DNS lookups may be necessary to determine 178 TURN server IP address(es). 180 4.1. Retrieving Domain Name 182 A client has to determine the domain in which it is located. The 183 following sections provide two possible mechanisms to learn the 184 domain name, but other means of retrieving domain names may be used, 185 which are outside the scope of this document e.g. local 186 configuration. 188 Implementations may allow the user to specify a default name that is 189 used if no specific name has been configured. 191 4.1.1. DHCP 193 DHCP can be used to determine the domain name related to an 194 interface's point of network attachment. Network operators may 195 provide the domain name to be used for service discovery within an 196 access network using DHCP. [RFC5986] defines DHCP IPv4 and IPv6 197 access network domain name options to identify a domain name that is 198 suitable for service discovery within the access network. [RFC2132] 199 defines the DHCP IPv4 domain name option. While this option is less 200 suitable, it still may be useful if the option defined in [RFC5986] 201 is not available. 203 For IPv6, the TURN server discovery procedure MUST try to retrieve 204 DHCP option 57 (OPTION_V6_ACCESS_DOMAIN). If no such option can be 205 retrieved, the procedure fails for this interface. For IPv4, the 206 TURN server discovery procedure MUST try to retrieve DHCP option 213 207 (OPTION_V4_ACCESS_DOMAIN). If no such option can be retrieved, the 208 procedure SHOULD try to retrieve option 15 (Domain Name). If neither 209 option can be retrieved the procedure fails for this interface. If a 210 result can be retrieved it will be used as an input for S-NAPTR 211 resolution. 213 4.1.2. From own Identity 215 A TURN client could also wish to extract the domain name from its own 216 identity i.e canonical identifier used to reach the user. 218 Example 220 SIP : 'sip:alice@example.com' 221 JID : 'alice@example.com' 222 email : 'alice@example.com' 224 'example.com' is retrieved from the above examples. 226 The means to extract the domain name may be different based on the 227 type of identifier and is outside the scope of this document. 229 4.2. Resolution 231 Once the TURN discovery procedure has retrieved domain names, the 232 resolution mechanism described in [RFC5928] is followed. An S-NAPTR 233 lookup with 'RELAY' application service and the desired protocol tag 234 is made to obtain information necessary to connect to the 235 authoritative TURN server within the given domain. 237 In the example below, for domain 'example.net', the resolution 238 algorithm will result in IP address, port, and protocol tuples as 239 follows: 241 example.net. 242 IN NAPTR 100 10 "" RELAY:turn.udp "" example.net. 244 example.net. 245 IN NAPTR 100 10 S RELAY:turn.udp "" _turn._udp.example.net. 247 _turn._udp.example.net. 248 IN SRV 0 0 3478 a.example.net. 250 a.example.net. 251 IN A 192.0.2.1 253 +-------+----------+------------+------+ 254 | Order | Protocol | IP address | Port | 255 +-------+----------+------------+------+ 256 | 1 | UDP | 192.0.2.1 | 3478 | 257 +-------+----------+------------+------+ 259 If no TURN-specific S-NAPTR records can be retrieved, the discovery 260 procedure fails for this domain name (and the corresponding interface 261 and IP protocol version). If more domain names are known, the 262 discovery procedure may perform the corresponding S-NAPTR lookups 263 immediately. However, before retrying a lookup that has failed, a 264 client MUST wait a time period that is appropriate for the 265 encountered error (NXDOMAIN, timeout, etc.). 267 5. DNS Service Discovery 269 DNS-based Service Discovery (DNS-SD) [RFC6763] and Multicast DNS 270 (mDNS) [RFC6762] provide generic solutions for discovering services 271 available in a local network. DNS-SD/ mDNS define a set of naming 272 rules for certain DNS record types that they use for advertising and 273 discovering services. PTR records are used to enumerate service 274 instances of a given service type. A service instance name is mapped 275 to a host name and a port number using a SRV record. If a service 276 instance has more information to advertise than the host name and 277 port number contained in its SRV record, the additional information 278 is carried in a TXT record. 280 Section 4.1 of [RFC6763] specifies that a service instance name in 281 DNS-SD has the following structure: 283 . . 284 The portion specifies the DNS sub-domain where the service 285 instance is registered. It may be "local.", indicating the mDNS 286 local domain, or it may be a conventional domain name such as 287 "example.com.". The portion of the TURN service instance 288 name MUST be "_turnserver._udp", "_turnserver._tcp". 290 The portion is a DNS label, containing UTF-8-encoded text, 291 limited to 63 octets in length. It is meant to be a user-friendly 292 description of the service instance, suitable for a menu-like user 293 interface display. Thus it can contain any characters including 294 spaces, punctuation, and non-Latin characters as long as they can be 295 encoded in UTF-8. 297 For example, TURN server advertises the following DNS records : 299 _turnserver._udp.local. PTR example.com._turnserver._udp.local. 301 example.com._turnserver._udp.local. SRV 0 0 5030 example-turn- 302 server.local. 304 example-turn-server.local. A 192.168.1.2 306 In addition to the service instance name, IP address and the port 307 number, DNS-SD provides a way to publish other information pertinent 308 to the service being advertised. The additional data can be stored 309 as name/value attributes in a TXT record with the same name as the 310 SRV record for the service. Each name/value pair within the TXT 311 record is preceded by a single length byte, thereby limiting the 312 length of the pair to 255 bytes (See Section 6 of [RFC6763] and 313 Section 3.3.14 of [RFC1035] for details). 315 5.1. mDNS 317 A TURN client tries to discover the TURN servers being advertised in 318 the site by multicasting a PTR query "_turnserver._udp.local." or 319 "_turnserver._tcp.local" or the TURN server can send out gratuitous 320 multicast DNS answer packets whenever it starts up, wakes from sleep, 321 or detects a chance in network configuration. TURN clients receive 322 these gratuitous packet and cache the information contained in it. 324 +------+ +-------------+ 325 | TURN | | TURN Server | 326 |Client| | | 327 +------+ +-------------+ 328 | | 329 | PTR query "_turnserver._udp.local." | 330 |--------------------------------------------->| 331 | PTR reply | 332 |<---------------------------------------------| 333 | SRV query | 334 |--------------------------------------------->| 335 | SRV reply | 336 |<---------------------------------------------| 337 | A/AAAA query reply | 338 |--------------------------------------------->| 339 | TURN Request | 340 |--------------------------------------------->| 341 | TURN Response | 342 |<---------------------------------------------| 344 Figure 1: TURN Server Discovery using mDNS 346 6. Discovery using Anycast 348 IP anycast is an elegant solution for TURN service discovery. A 349 packet sent to an anycast address is delivered to the "topologically 350 nearest" network interface with the anycast address. Using the TURN 351 anycast address, the only two things that need to be deployed in the 352 network are the two things that actually use TURN. 354 When a client requires TURN services, it sends a TURN allocate 355 request to the assigned anycast address. The TURN anycast server 356 responds with a 300 (Try Alternate) error as described in [RFC5766]; 357 The response contains the TURN unicast address in the ALTERNATE- 358 SERVER attribute. For subsequent communication with the TURN server, 359 the client uses the responding server's unicast address. This has to 360 be done because two packets addressed to an anycast address may reach 361 two different anycast servers. The client, thus, also needs to 362 ensure that the initial request fits in a single packet. An 363 implementation may choose to send out every new request to the 364 anycast address to learn the closest TURN server each time. 366 7. Deployment Considerations 367 7.1. Mobility and Changing IP addresses 369 A change of IP address on an interface may invalidate the result of 370 the TURN server discovery procedure. For instance, if the IP address 371 assigned to a mobile host changes due to host mobility, it may be 372 required to re-run the TURN server discovery procedure without 373 relying on earlier gained information. New requests should be made 374 to the newly learned TURN servers learned after TURN discovery re- 375 run. However, if an earlier learned TURN server is still accessible 376 using the new IP address, procedures described for mobility using 377 TURN defined in [I-D.wing-tram-turn-mobility] can be used for ongoing 378 streams. 380 8. IANA Considerations 382 8.1. Anycast 384 IANA should allocate an IPv4 and an IPv6 well-known TURN anycast 385 address. 192.0.0.0/24 and 2001:0000::/48 are reserved for IETF 386 Protocol Assignments, as listed at 388 and 390 392 9. Security Considerations 394 In general, it is recommended that a TURN client authenticate with 395 the TURN server to identify a rouge server. [RFC7350] can be 396 potentially used by a client to validate a previously unknown server. 398 9.1. Service Resolution 400 The primary attack against the methods described in this document is 401 one that would lead to impersonation of a TURN server. An attacker 402 could attempt to compromise the S-NAPTR resolution. Security 403 considerations described in [RFC5928] are applicable here as well. 405 In addition to considerations related to S-NAPTR, it is important to 406 recognize that the output of this is entirely dependent on its input. 407 An attacker who can control the domain name can also control the 408 final result. Because more than one method can be used to determine 409 the domain name, a host implementation needs to consider attacks 410 against each of the methods that are used. 412 If DHCP is used, the integrity of DHCP options is limited by the 413 security of the channel over which they are provided. Physical 414 security and separation of DHCP messages from other packets are 415 commonplace methods that can reduce the possibility of attack within 416 an access network; alternatively, DHCP authentication [RFC3188] can 417 provide a degree of protection against modification. When using DHCP 418 discovery, clients are encouraged to use unicast DHCP INFORM queries 419 instead of broadcast queries which are more easily spoofed in 420 insecure networks. 422 9.2. DNS Service Discovery 424 Since DNS-SD is just a specification for how to name and use records 425 in the existing DNS system, it has no specific additional security 426 requirements over and above those that already apply to DNS queries 427 and DNS updates. For DNS queries, DNS Security Extensions (DNSSEC) 428 [RFC4033] should be used where the authenticity of information is 429 important. For DNS updates, secure updates [RFC2136][RFC3007] should 430 generally be used to control which clients have permission to update 431 DNS records. 433 For mDNS, in addition to what has been described above, a principal 434 security threat is a security threat inherent to IP multicast routing 435 and any application that runs on it. A rogue system can advertise 436 that it is a TURN server. Discovery of such rogue systems as TURN 437 servers, in itself, is not a security threat if there is a means for 438 the TURN client to authenticate and authorize the discovered TURN 439 servers. 441 9.3. Anycast 443 In a network without any TURN server that is aware of the TURN 444 anycast address, outgoing TURN requests could leak out onto the 445 external Internet, possibly revealing information. 447 Using an IANA-assigned well-known TURN anycast address enables border 448 gateways to block such outgoing packets. In the default-free zone, 449 routers should be configured to drop such packets. Such 450 configuration can occur naturally via BGP messages advertising that 451 no route exists to said address. 453 Sensitive clients that do not wish to leak information about their 454 presence can set an IP TTL on their TURN requests that limits how far 455 they can travel into the public Internet. 457 10. Acknowledgements 459 The authors would like to thank Simon Perrault, Paul Kyzivat and Troy 460 Shields for their review and valuable comments. Thanks to Adam Roach 461 for his detailed review and suggesting DNS Service Discovery as an 462 additional discovery mechanism. 464 11. References 466 11.1. Normative References 468 [RFC1035] Mockapetris, P., "Domain names - implementation and 469 specification", STD 13, RFC 1035, November 1987. 471 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 472 Requirement Levels", BCP 14, RFC 2119, March 1997. 474 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor 475 Extensions", RFC 2132, March 1997. 477 [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, 478 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 479 RFC 2136, April 1997. 481 [RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic 482 Update", RFC 3007, November 2000. 484 [RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, 485 "DNS Extensions to Support IP Version 6", RFC 3596, 486 October 2003. 488 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 489 Rose, "DNS Security Introduction and Requirements", RFC 490 4033, March 2005. 492 [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using 493 Relays around NAT (TURN): Relay Extensions to Session 494 Traversal Utilities for NAT (STUN)", RFC 5766, April 2010. 496 [RFC5928] Petit-Huguenin, M., "Traversal Using Relays around NAT 497 (TURN) Resolution Mechanism", RFC 5928, August 2010. 499 [RFC5986] Thomson, M. and J. Winterbottom, "Discovering the Local 500 Location Information Server (LIS)", RFC 5986, September 501 2010. 503 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 504 February 2013. 506 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 507 Discovery", RFC 6763, February 2013. 509 [RFC7216] Thomson, M. and R. Bellis, "Location Information Server 510 (LIS) Discovery Using IP Addresses and Reverse DNS", RFC 511 7216, April 2014. 513 [RFC7350] Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport 514 Layer Security (DTLS) as Transport for Session Traversal 515 Utilities for NAT (STUN)", RFC 7350, August 2014. 517 11.2. Informative References 519 [I-D.ietf-rtcweb-overview] 520 Alvestrand, H., "Overview: Real Time Protocols for 521 Browser-based Applications", draft-ietf-rtcweb-overview-13 522 (work in progress), November 2014. 524 [I-D.kist-alto-3pdisc] 525 Kiesel, S., Krause, K., and M. Stiemerling, "Third-Party 526 ALTO Server Discovery (3pdisc)", draft-kist-alto-3pdisc-05 527 (work in progress), January 2014. 529 [I-D.wing-tram-turn-mobility] 530 Wing, D., Patil, P., Reddy, T., and P. Martinsen, 531 "Mobility with TURN", draft-wing-tram-turn-mobility-03 532 (work in progress), May 2015. 534 [RFC3188] Hakala, J., "Using National Bibliography Numbers as 535 Uniform Resource Names", RFC 3188, October 2001. 537 [RFC5128] Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to- 538 Peer (P2P) Communication across Network Address 539 Translators (NATs)", RFC 5128, March 2008. 541 [RFC7286] Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M., and 542 H. Song, "Application-Layer Traffic Optimization (ALTO) 543 Server Discovery", RFC 7286, November 2014. 545 Appendix A. Change History 547 [Note to RFC Editor: Please remove this section prior to 548 publication.] 550 A.1. Change from draft-patil-tram-serv-disc-00 to -01 552 o Added IP address (Section 4.1.2) and Own identity (4.1.3) as new 553 means to obtain domain names 555 o New Section 4.2.1 SOA (inspired by draft-kist-alto-3pdisc) 557 o 300 (Try Alternate) response for Anycast 559 A.2. Change from draft-ietf-tram-turn-server-discovery-01 to 02 561 o Removed sections that describe reverse IP lookup 563 o Added DNS Service Discovery as an additional discovery mechanism 565 Authors' Addresses 567 Prashanth Patil 568 Cisco Systems, Inc. 569 Bangalore 570 India 572 Email: praspati@cisco.com 574 Tirumaleswar Reddy 575 Cisco Systems, Inc. 576 Cessna Business Park, Varthur Hobli 577 Sarjapur Marathalli Outer Ring Road 578 Bangalore, Karnataka 560103 579 India 581 Email: tireddy@cisco.com 583 Dan Wing 584 Cisco Systems, Inc. 585 170 West Tasman Drive 586 San Jose, California 95134 587 USA 589 Email: dwing@cisco.com