RFC 5456
IAX: Inter-Asterisk eXchange Version 2
Independent Submission M. Spencer Request for Comments: 5456 Digium, Inc. Category: Informational B. Capouch ISSN: 2070-1721 Saint Joseph's College E. Guy, Ed. Truphone F. Miller Cornfed Systems, LLC K. Shumard February 2010 IAX: Inter-Asterisk eXchange Version 2Abstract
This document describes IAX, the Inter-Asterisk eXchange protocol, an application-layer control and media protocol for creating, modifying, and terminating multimedia sessions over Internet Protocol (IP) networks. IAX was developed by the open source community for the Asterisk Private Branch Exchange (PBX) and is targeted primarily at Voice over Internet Protocol (VoIP) call control, but it can be used with streaming video or any other type of multimedia. IAX is an "all in one" protocol for handling multimedia in IP networks. It combines both control and media services in the same protocol. In addition, IAX uses a single UDP data stream on a static port greatly simplifying Network Address Translation (NAT) gateway traversal, eliminating the need for other protocols to work around NAT, and simplifying network and firewall management. IAX employs a compact encoding that decreases bandwidth usage and is well suited for Internet telephony service. In addition, its open nature permits new payload type additions needed to support additional services. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5456. IESG Note This RFC is not a candidate for any level of Internet Standard. The IETF disclaims any knowledge of the fitness of this RFC for any purpose and in particular notes that the decision to publish is not based on IETF review for such things as security, congestion control, or inappropriate interaction with deployed protocols. The RFC Editor has chosen to publish this document at its discretion. Readers of this document should exercise caution in evaluating its value for implementation and deployment. See RFC 3932 for more information. The IESG thinks that this work is related to IETF work done in SIP, MMUSIC, and AVT WGs, but this does not prevent publishing. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.
Table of Contents
1. Introduction ....................................................4 1.1. Basic Properties ...........................................4 1.2. Drawbacks ..................................................5 2. IAX Terminology .................................................6 3. Overview of IAX Protocol ........................................6 4. Naming Conventions ..............................................8 5. IAX Uniform Resource Identifiers ................................8 5.1. IAX URI Scheme Registration ................................8 5.2. URI Comparison ............................................11 6. Peer Behavior and Related Messages .............................11 6.1. Registration (OPTIONAL) ...................................12 6.2. Call Leg Management .......................................18 6.3. Call Control ..............................................24 6.4. Mid-Call Link Operations ..................................26 6.5. Call Path Optimization ....................................28 6.6. Call Tear Down ............................................33 6.7. Network Monitoring ........................................33 6.8. Digit Dialing .............................................34 6.9. Miscellaneous .............................................36 6.10. Media Messages ...........................................38 7. Message Transport ..............................................39 7.1. Trunking ..................................................40 7.2. Timers ....................................................41 7.3. NAT Considerations ........................................41 7.4. Encryption ................................................42 8. Message Encoding ...............................................42 8.1. Frame Structure ...........................................42 8.2. Frame Types ...............................................52 8.3. Control Frames Subclasses .................................55 8.4. IAX Frames ................................................56 8.5. HTML Command Subclasses ...................................58 8.6. Information Elements ......................................58 8.7. Media Formats .............................................86 9. Example Message Flows ..........................................87 9.1. Ping/Pong .................................................88 9.2. Lagrq/Lagrp ...............................................88 9.3. Registration ..............................................89 9.4. Registration Release ......................................89 9.5. Call Path Optimization ....................................90 9.6. IAX Media Call ............................................91 9.7. IAX Media Call via an IAX Device ..........................93 10. Security Considerations .......................................94 11. IANA Considerations ...........................................96 12. Implementation Notes ..........................................96 13. Acknowledgments ...............................................97
14. References ....................................................97 14.1. Normative References .....................................97 14.2. Informative References ...................................991. Introduction
Numerous protocols have been specified by the Internet community to support control or signaling of multimedia sessions, for instance, SIP [RFC3261], Media Gateway Control Protocol (MGCP) [RFC3435], and MEGACO/H.248 [RFC3525] (which has been obsoleted and made historic by [RFC5125]). In general, these protocols are designed to offer full support for many types of media transmission. This flexible approach adds some overhead to the protocol headers, but allows for the protocol use well beyond the current application. Typically, these protocols reference, but do not specify, the media transmission protocol used to carry the actual stream. SIP commonly uses Session Description Protocol (SDP) [RFC4566] to specify Real-Time Transport Protocol (RTP) [RFC3550] streams. This method allows for great flexibility, but again leads to more overhead. Furthermore, multimedia solutions that use different, perhaps dynamic, network addresses for signaling and media transmission frequently suffer from Network Address Translation (NAT) traversal and security challenges. IAX is the Inter-Asterisk eXchange protocol, which facilitates VoIP connections between servers, and between servers and clients that also use the IAX protocol. IAX was created through an open source methodology rather than through a traditional, standards-based methodology. It is an open protocol originally used by Asterisk, a dual-licensed open source and commercial PBX server from Digium. Independent IAX implementations may be open, proprietary, or licensed in anyway the author seems fit without royalty to the protocol creators.1.1. Basic Properties
IAX is a robust and full-featured, yet, simple protocol. It is general enough that it can handle most common types of media streams. However, the protocol is highly optimized for VoIP calls where low- overhead and low-bandwidth consumption are priorities. This pragmatic aspect makes IAX more efficient for VoIP than protocols that consider possibilities far beyond current needs and specify many more details than are strictly necessary to describe or transport a point-to-point call. Furthermore, because IAX is designed to be lightweight and VoIP-friendly, it consumes less bandwidth than more general approaches. IAX is a binary protocol, designed to reduce overhead, especially in regards to voice streams. Bandwidth efficiency, in some places, is sacrificed in exchange for bandwidth efficiency for individual voice calls. For example, when
transmitting a voice stream compressed to 8 kbit/s with a 20 ms packetization, each data packet consists of 20 bytes. IAX adds 20% overhead, 4 bytes, on the majority of voice packets while RTP adds 60% overhead with 12 additional bytes per voice packet. In addition to efficiency, IAX's single static UDP port approach makes IAX traffic easy for network managers to shape, prioritize, and pass through firewalls. IAX's basic structure is that it multiplexes signaling and multiple media streams over a single UDP stream between two computers. IAX also uses the same UDP port for both its signaling and media messages, and because all communications regarding a call are done over a the same point-to-point path, NAT traversal is much simpler for IAX than for other commonly deployed protocols.1.2. Drawbacks
While IAX is very effective, addressing many of today's communications needs, it does have a few limitations. For instance, IAX uses a point-to-point codec negotiation mechanism that limits extensibility because every IAX node in a call path must support every used codec to some degree. In addition, the codec definition is controlled by an internally defined 32-bit mask, so the codecs must be defined in the protocol, and the maximum number of simultaneous codecs is, therefore, limited. One of IAX's design strengths also presents a potential problem. The use of a single, well-known, port makes the protocol an easier target for denial-of-service attacks. Real-time systems like VoIP are particularly sensitive to these attacks. The protocol is typically deployed with all signaling and media going to a centralized server. While this combined path approach provides a great deal of control, it limits the overall system scalability. IAX now provides the ability to split the media from the signaling stream, which overcomes this limitation of earlier IAX versions. Most IAX drawbacks are due to implementation issues rather than protocol issues. Threading presents a series of problems. Many implementations have a limited number of threads available to process IAX traffic and can become overwhelmed by high use or denial-of- service attacks. Newer implementations have additional controls to minimize the impact of these challenges.
2. IAX Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Additionally, this document uses the following terminology: Peer: A host or device that implements the IAX protocol. Call: A call is a relationship between two or more parties (i.e., resources such as devices, user agents, or programs) that exists for some time for the purpose of exchanging real-time media. In the context of this document, a call is an end-to-end relationship where at least the one leg of call path is implemented using the IAX protocol. Calling Party: A device or program that initiates a call. Called Party: A device or program to which a call is directed. Context: A context is a named partition of a Dialplan. Dialplan: A Dialplan is a set of rules for associating provided names and numbers with a particular called party. Frame: The atomic communication unit between two IAX peers. All IAX messages are carried within frames. Information Element (IE): A discrete data unit appended to an IAX frame that specifies user- or call-specific data. Registrant: A registrant is a peer that makes REGISTER requests in order to advertise the address of a resource, i.e., a device or program to which a call may be directed. Registrar: A registrar is a peer that processes REGISTER requests and places the information it receives in those requests into the location service. [RFC3261].3. Overview of IAX Protocol
IAX is a peer-to-peer, VoIP-oriented protocol. IAX includes both control and media functions. It can register locations, create, modify, terminate multimedia sessions, and carry the actual media streams specified by the sessions it manages. The protocol is designed and optimized for describing and transporting multimedia
calls using Internet Protocol. This document describes Version 2 of IAX; Version 1, although somewhat similar in design, utilized a different port and was not widely deployed. The basic design approach for IAX multiplexes signaling and multiple media streams over a single UDP association between two hosts. This is accomplished by using the same "well-known" UDP port, 4569, for all types of IAX traffic. IAX's unified signaling and media paths achieve NAT transparency, which is an advantage of IAX over alternative media transport protocols such as SIP [RFC3261]. IAX is coded as a binary protocol. One major benefit of using a binary protocol is bandwidth efficiency because the quality of voice calls is frequently related to the amount of bandwidth consumed. This is one way the protocol is specifically optimized to make efficient use of bandwidth for individual voice calls. The bandwidth efficiency for other stream types is sacrificed for the sake of individual voice calls. Other benefits of a binary protocol are robustness against buffer-overrun attacks, and compact implementation capability, which reduces interoperability issues related to parsing. The atomic communication unit in IAX is the "Frame". There are multiple classes of Frames, each of which is described below. In general, "Full Frames" carry signaling/control data, while "Mini Frames" carry media stream data. Full Frames enclose optional 'Information Elements' (IEs). IEs describe various types of user- or call-specific data. "Meta Frames" are used for call trunking or video stream transmission. An IAX-based call may consist of many call legs, or segments. Each call leg may be implemented using different protocols, e.g., SIP to IAX to ISDN (Integrated Services Digital Network). IAX is responsible for setting up one or more legs of a complete call path, not necessarily the end-to-end call. IAX is an optimized peer-to-peer protocol. If two adjacent call legs utilize the IAX protocol and if the intermediate peer determines that it does not need to remain in the call path, it can supervise a calling path change such that it removes itself from the path. This supervision is complete, a call path is not changed until all peers in the optimized call path confirm they can properly communicate. IAX supports security features by allowing multiple methods of user authentication and authorization, as well as allowing multiple security methods for peer registration. IAX also specifies a generic framework for native encryption.
4. Naming Conventions
Call Identifier: A call leg is marked with two unique integers, one assigned by each peer involved in creating the call leg. Number: The Calling and Called Numbers are a set of digits and letters identifying a call originator and the desired terminating resource. The term 'Number' is historic and has been expanded to include letters. A peer is responsible for defining its own dialplan. A peer MAY define its dialplan according to ITU-T Recommendation E.164 [E164]. However, this is not required. Username: A username is a string used for identification purposes.5. IAX Uniform Resource Identifiers
5.1. IAX URI Scheme Registration
This section registers IAX according to the guidelines in [RFC4395]. URI scheme name: iax. Status: Permanent. URI scheme syntax: The "iax:" scheme follows the guidelines in [RFC3986]. The general form is as follows: iax:[username@]host[:port][/number[?context]] where these tokens have the following meanings: iax: The literal 'iax:'. username: A string used for identification purposes. host: The domain of the resource. The host part contains either a fully-qualified domain name or numeric IPv4 or IPv6 address. An IPv6 address must be enclosed within brackets (i.e., '[2001:db8::1]') as defined in [RFC3986]. Using the fully-qualified domain name form is RECOMMENDED whenever possible.
port: The numeric UDP port number.
number: The name or number identifying the resource on that
host.
context: The name of the host partition in which the service
is identified or processed.
Examples
iax:example.com/alice
iax:example.com:4569/alice
iax:example.com:4570/alice?friends
iax:192.0.2.4:4569/alice?friends
iax:[2001:db8::1]:4569/alice?friends
iax:example.com/12022561414
iax:johnQ@example.com/12022561414
ABNF
Formal syntax is defined using ABNF [RFC5234]. Certain values
are included by reference from [RFC3986]:
iax-uri = "iax:" [ userinfo "@" ] host [ ":" port ]
[ "/" number [ "?" context ] ]
userinfo = <as specified in RFC 3986>
host = <as specified in RFC 3986>
port = <as specified in RFC 3986>
number = *(unreserved / sub-delims / pct-encoded )
context = *(unreserved / sub-delims / pct-encoded )
unreserved = <as specified in RFC 3986>
sub-delims = <as specified in RFC 3986>
pct-encoded = <as specified in RFC 3986>
URI Scheme Semantics:
An IAX URI identifies a communications resource capable of
communicating using the IAX Version 2 protocol defined in this
document. Within this document, we refer to IAX Version 2
protocol URI as IAX. An IAX URI contains enough information to
initiate an IAX-based call with that resource.
IAX URIs are associated with server resources to which calls may
be routed. For instance, an IAX URI may represent an appearance
on a phone, a voice-mail box on a messaging service, an
interactive program, a Public Switched Telephone Network (PSTN)
address or gateway, or any group of the above.
The IAX URI scheme translates into a location that may be used by
the IAX protocol to establish a new call using the URI scheme
components described in the previous section. This new call
function is the only defined operation.
Encoding considerations:
IAX URI scheme encoding conforms to the encoding rules established
for URIs in [RFC3986].
Applications/protocols that use this URI scheme name:
The scheme is used by ENUM Dynamic Delegation Discovery System
(DDDS) services to specify resources that support the IAX
protocol. The IAX protocol provides application-layer control and
media protocol for creating, modifying, and terminating multimedia
sessions over Internet Protocol (IP) networks.
Interoperability considerations:
None.
Security considerations:
The IAX URI Scheme does not introduce any new security concerns
except that it provides a uniform syntax for describing IAX
resources and that, when published, these addresses are subject to
various denial-of-service attacks.
Contact:
Ed Guy, edguy@emcsw.com, +1.973.437.4519.
Author/Change controller
Not Applicable.
References:
RFC 5456 (this document)
5.2. URI Comparison
Some operations in this specification require determining whether two IAX URIs are equivalent. IAX URIs are compared for equality according to the following rules: All components of the URI MUST be identical except: The port, if omitted, is considered to be the same as the default, 4569. All URI components, except the username field, are case insensitive, and MUST be normalized to lower case as per Section 6.2.2.1 of [RFC3986] before comparison. The URIs within each of the following sets are equivalent: iax:atlanta.com/alice iax:AtLaNtA.com/ALicE iax:atlanta.com:4569/alice iax:alice@atlanta.com/alice iax:alice@AtLaNtA.com:4569/ALicE The URIs within the following set are not equivalent: iax:ALICE@atlanta.com/alice iax:alice@atlanta.com/alice NOTE: A host in domain form and in IP address form are NOT considered identical even if the host name resolves to an address record that matches the given IP address.
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