Interactive Connectivity Establishment
Interactive Connectivity Establishment (ICE) is a technique used in computer networking involving network address translators (NATs) in Internet applications of Voice over Internet Protocol (VoIP), peer-to-peer communications, video, instant messaging and other interactive media. In such applications, NAT traversal is an important component to facilitate communications involving hosts on private network installations, often located behind firewalls.
Since the number of IPv4 addresses are limited to their 32-bit representation, not every network enabled device can have a unique public IP with which to be visible on the Internet. Network Address Translators (NAT) work by changing a private address into a public one when an outbound request passes through them. When a client establishes TCP connections through SYN packets, the NAT updates an internal table with each entry creating a mapping between an internal, private IP to a public one. Many applications run into problems when put in this situation with one example being VoIP traffic where a client needs to register with a unique address to a SIP proxy. Another problem relates to firewalls which might block VoIP traffic completely. ICE provides a framework for dealing with these problems.
Session Traversal Utilities for NAT (STUN) is a client server protocol returning the public IP to a client together with information from which the client can infer the type of NAT it sits behind while Traversal Using Relays around NAT (TURN) places a third party server to relay messages between two clients where peer to peer media traffic is not allowed by a firewall.
IETF Specifications 
- RFC 5389: Session Traversal Utilities for NAT (STUN).
- RFC 5766: Traversal Using Relays around NAT (TURN): Relay Extensions to STUN.
- RFC 5245: Interactive Connectivity Establishment (ICE): A Protocol for NAT Traversal for Offer/Answer Protocols.
See also 
- Realm-Specific IP (RSIP)
- Middlebox Middlebox Communications (MIDCOM)
- Universal Plug and Play (UPnP)
- NAT Port Mapping Protocol (NAT-PMP)
- RFC 5245, Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols, J. Rosenberg (April 2010)
- RFC 4091, The Alternative Network Address Types (ANAT) Semantics for the Session Description Protocol (SDP) Grouping Framework, G. Camarillo, J. Rosenberg (June 2005)
- Müller A, Carl (2008) Behavior and Classification of NAT Devices and Implications for NAT Traversal IEEE Network September/October 2008. Available from: http://ieeexplore.ieee.org.ezproxy.liv.ac.uk/stamp/stamp.jsp?tp=&arnumber=4626227 [Accessed at: 2 April 2011]
- IETF Journal article on ICE - read first
- ICE Tutorial
- MMUSIC working group
- BEHAVE working group
- PJNATH - Open Source ICE, STUN, and TURN Library
- libnice: GLib ICE library
- free library supports ICE
- TURN Server - High-Performance Open Source TURN/STUN server (BSD license) and client library (C)
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