3 ccRTP Overview

At its highest level, ccRTP provides classes for the real-time transport of data through RTP sessions, as well as the control functions of RTCP.

The main concept in the ccRTP implementation of RTP sessions is the use of packet queues to handle transmission and reception of RTP data packets/application data units. In ccRTP, a data block is transmitted by putting it into the transmission (outgoing packets) queue, and received by getting it from the reception (incoming packets) queue.

Starting with 1.0 releases, ccRTP fully supports RTP and RTCP. Here is a brief list of some features of ccRTP:

• Highly extensible to specialized stacks.
• Supports unicast, multi-unicast and multicast. Handles multiple sources (including synchronization sources and contributing sources) and destinations. Also supports symmetric RTP.
• Automatic RTCP functions handling, such as association of synchronization sources from the same participant or NTP-RTP timestamp mapping.
• Genericity as for underlying network and transport protocols through templates.
• It is threadsafe and supports almost any threading model.
• Generic and extensible RTP and RTCP header validity checks.
• Handles source states and information as well as statistics recording.

• Automatically handles SSRC collisions and performs loop detection.

• Implements timer reconsideration and reverse reconsideration.

• Provides good random numbers, based on /dev/urandom or, alternatively, on MD5.

There are several levels of interface (public interface, public or protected inheritance, etc) in ccRTP. For instance, the rtphello demo program distributed with ccRTP just uses the public interface of the RTPSession class and does not redefine the virtual method onGotSR, thus what this program knows about SR reports is the information conveyed in the last sender report from any source, which can be retrieved via the getMRSenderInfo method of the SyncSource class. On the contrary, the rtplisten demo program redefines onGotSR by means of inheritance and could do specialized processing of these RTCP packets. Generally, both data and control packets are not directly accessible through the most external interface.

All this functions are performed through a few essential classes and types. The most basic ones are the enumerated type StaticPayloadType, and the classes StaticPayloadFormat and DynamicPayloadFormat. The most important ones are the classes RTPSession, SyncSource, Participant and AppDataUnit, that represent RTP sessions, synchronization sources, participants in an RTP application, and application data units conveyed in RTP data packets, respectively.

In the next sections, two basic examples are presented. These examples are intended to provide a concrete idea of what the code of a ccRTP based application looks like, they are not complete or compilable programs. You can find some real examples under the demo directory in the source distribution of ccRTP.

• Simple Transmitter: Skeleton of a simple transmitter.
• Simple Receiver: Skeleton of a simple receiver.