Connected: An Internet Encyclopedia
3. Definitions
Up:
Connected: An Internet Encyclopedia
Up:
Requests For Comments
Up:
RFC 1889
Prev: 2.3 Mixers and Translators
Next: 4. Byte Order, Alignment, and Time Format
3. Definitions
3. Definitions
- RTP payload
- The data transported by RTP in a packet, for example
audio samples or compressed video data. The payload format and
interpretation are beyond the scope of this document.
- RTP packet
- A data packet consisting of the fixed RTP header, a
possibly empty list of contributing sources (see below), and the
payload data. Some underlying protocols may require an
encapsulation of the RTP packet to be defined. Typically one
packet of the underlying protocol contains a single RTP packet,
but several RTP packets may be contained if permitted by the
encapsulation method (see Section 10).
- RTCP packet
- A control packet consisting of a fixed header part
similar to that of RTP data packets, followed by structured
elements that vary depending upon the RTCP packet type. The
formats are defined in Section 6. Typically, multiple RTCP
packets are sent together as a compound RTCP packet in a single
packet of the underlying protocol; this is enabled by the length
field in the fixed header of each RTCP packet.
- Port
- The "abstraction that transport protocols use to distinguish
among multiple destinations within a given host computer. TCP/IP
protocols identify ports using small positive integers." [3] The
transport selectors (TSEL) used by the OSI transport layer are
equivalent to ports. RTP depends upon the lower-layer protocol
to provide some mechanism such as ports to multiplex the RTP and
RTCP packets of a session.
- Transport address
- The combination of a network address and port that
identifies a transport-level endpoint, for example an IP address
and a UDP port. Packets are transmitted from a source transport
address to a destination transport address.
- RTP session
- The association among a set of participants
communicating with RTP. For each participant, the session is
defined by a particular pair of destination transport addresses
(one network address plus a port pair for RTP and RTCP). The
destination transport address pair may be common for all
participants, as in the case of IP multicast, or may be
different for each, as in the case of individual unicast network
addresses plus a common port pair. In a multimedia session,
each medium is carried in a separate RTP session with its own
RTCP packets. The multiple RTP sessions are distinguished by
different port number pairs and/or different multicast
addresses.
- Synchronization source (SSRC)
- The source of a stream of RTP packets,
identified by a 32-bit numeric SSRC identifier carried in the
RTP header so as not to be dependent upon the network address.
All packets from a synchronization source form part of the same
timing and sequence number space, so a receiver groups packets
by synchronization source for playback. Examples of
synchronization sources include the sender of a stream of
packets derived from a signal source such as a microphone or a
camera, or an RTP mixer (see below). A synchronization source
may change its data format, e.g., audio encoding, over time. The
SSRC identifier is a randomly chosen value meant to be globally
unique within a particular RTP session (see Section 8). A
participant need not use the same SSRC identifier for all the
RTP sessions in a multimedia session; the binding of the SSRC
identifiers is provided through RTCP (see Section 6.4.1). If a
participant generates multiple streams in one RTP session, for
example from separate video cameras, each must be identified as
a different SSRC.
- Contributing source (CSRC)
- A source of a stream of RTP packets that
has contributed to the combined stream produced by an RTP mixer
(see below). The mixer inserts a list of the SSRC identifiers of
the sources that contributed to the generation of a particular
packet into the RTP header of that packet. This list is called
the CSRC list. An example application is audio conferencing
where a mixer indicates all the talkers whose speech was
combined to produce the outgoing packet, allowing the receiver
to indicate the current talker, even though all the audio
packets contain the same SSRC identifier (that of the mixer).
- End system
- An application that generates the content to be sent in
RTP packets and/or consumes the content of received RTP packets.
An end system can act as one or more synchronization sources in
a particular RTP session, but typically only one.
- Mixer
- An intermediate system that receives RTP packets from one or
more sources, possibly changes the data format, combines the
packets in some manner and then forwards a new RTP packet. Since
the timing among multiple input sources will not generally be
synchronized, the mixer will make timing adjustments among the
streams and generate its own timing for the combined stream.
Thus, all data packets originating from a mixer will be
identified as having the mixer as their synchronization source.
- Translator
- An intermediate system that forwards RTP packets with
their synchronization source identifier intact. Examples of
translators include devices that convert encodings without
mixing, replicators from multicast to unicast, and application-level
filters in firewalls.
- Monitor
- An application that receives RTCP packets sent by
participants in an RTP session, in particular the reception
reports, and estimates the current quality of service for
distribution monitoring, fault diagnosis and long-term
statistics. The monitor function is likely to be built into the
application(s) participating in the session, but may also be a
separate application that does not otherwise participate and
does not send or receive the RTP data packets. These are called
third party monitors.
- Non-RTP means
- Protocols and mechanisms that may be needed in
addition to RTP to provide a usable service. In particular, for
multimedia conferences, a conference control application may
distribute multicast addresses and keys for encryption,
negotiate the encryption algorithm to be used, and define
dynamic mappings between RTP payload type values and the payload
formats they represent for formats that do not have a predefined
payload type value. For simple applications, electronic mail or
a conference database may also be used. The specification of
such protocols and mechanisms is outside the scope of this
document.
Next: 4. Byte Order, Alignment, and Time Format
Connected: An Internet Encyclopedia
3. Definitions