Zap Sourcecode Documentation

Overview

The zap source code consists of a number of components which integrate into the Mozilla build to form a custom version of XULRunner, xulrunner-zapi. Graphical client applications, such as the Zap SIP client or the Zap Sandbox are built on top of xulrunner-zapi.

The zap modules are loosly organized into layers. At the bottom, base provides shared utility code, such as a JavaScript class framework and facilities for asynchronous code execution.

On the next layer, the netutils module implements basic networking facilities not provided natively by Mozilla, such as UDP socket support and advanced DNS queries (SRV/NAPTR). The STUN module implements NAT-traversal mechanisms (STUN, TURN, ICE, draft-ietf-behave-rfc3489bis-18, draft-ietf-mmusic-ice-19). ZMK, the Zap Media Kit, provides a framework for dataflow-oriented media processing.

The next layer up provides support for the higher level protocols. The SIP module contains an implementation of the Session Initiation Protocol (RFC3261 and many others). The SDP module implements the Session Description Protocol (RFC4566), media negotiation protocols (RFC3264) and the media session logic.

At the top level, the zappshell, or Zap Application Shell, provides shell-like access to the lower level APIs. The idea is that user-level applications communicate to the Zap APIs only through the zappshell. (Note: This component is still under development and not checked into Mercurial yet).

base module

Common shared (mostly JS) utility code.

STUN module

NAT traversal facilities: STUN, TURN , ICE (draft-ietf-behave-rfc3489bis-18, draft-ietf-mmusic-ice-19).

netutils module

• UDP socket support
• SRV/NAPTR DNS queries

The Zap Media Kit (ZMK)

The ZMK is a media processing framework featuring a "mediagraph" component which runs with its own thread and event queue and manages a set of media processing nodes and the data connections between them.

Some features include:

• new node classes (codecs, filters, etc) can be implemented as xpcom plugins by implementing the zapIMediaNode interface.
• nodes can be composed to form larger nodes.
• the media graph itself is media-type agnostic. Data packets moved between nodes implement the zapIMediaFrame interface.
• the media graph only handles media dataflow, control protocols are implemented externally.
• the media graph can be dynamically reconfigured at any time (by adding/removing nodes and/or connections).
• both 'push' and 'pull' dataflow. Sinks and sources can be 'active', 'passive' or 'neutral'.
• control interfaces for media nodes (e.g. for changing codec parameters or for UI feedback) are proxied onto the media thread using the xpcom proxy mechanism.
• built-in media node types include: audioin, audioout, rtp-session, speex-encoder, speex-decoder, speex-rtp-packetizer, speex-rtp-depacketizer, buffer, udp-socket, udp-socket-pair.

See also slides 11ff of this FOSDEM presentation.

SIP module

Implementation of the Session Initiation Protocol. Further split into submodules:

• syntax layer
• transport layer
• transaction layer
• UA layer

SDP module

Implements the Session Description Protocol (RFC4566), media negotiation protocol (RFC3264) and the media session logic.

The Zap Application Shell

Zap SIP client

Zap SIP client application chrome.

• Overview of Zap's inbound media path