Connected: An Internet Encyclopedia
5.1 RTP Fixed Header Fields
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5.1 RTP Fixed Header Fields
5.1 RTP Fixed Header Fields
The RTP header has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first twelve octets are present in every RTP packet, while the
list of CSRC identifiers is present only when inserted by a mixer.
The fields have the following meaning:
- version (V): 2 bits
-
This field identifies the version of RTP. The version defined by
this specification is two (2). (The value 1 is used by the first
draft version of RTP and the value 0 is used by the protocol
initially implemented in the "vat" audio tool.)
- padding (P): 1 bit
-
If the padding bit is set, the packet contains one or more
additional padding octets at the end which are not part of the
payload. The last octet of the padding contains a count of how
many padding octets should be ignored. Padding may be needed by
some encryption algorithms with fixed block sizes or for
carrying several RTP packets in a lower-layer protocol data
unit.
- extension (X): 1 bit
-
If the extension bit is set, the fixed header is followed by
exactly one header extension, with a format defined in Section
5.3.1.
- CSRC count (CC): 4 bits
-
The CSRC count contains the number of CSRC identifiers that
follow the fixed header.
- marker (M): 1 bit
-
The interpretation of the marker is defined by a profile. It is
intended to allow significant events such as frame boundaries to
be marked in the packet stream. A profile may define additional
marker bits or specify that there is no marker bit by changing
the number of bits in the payload type field (see Section 5.3).
- payload type (PT): 7 bits
-
This field identifies the format of the RTP payload and
determines its interpretation by the application. A profile
specifies a default static mapping of payload type codes to
payload formats. Additional payload type codes may be defined
dynamically through non-RTP means (see Section 3). An initial
set of default mappings for audio and video is specified in the
companion profile Internet-Draft draft-ietf-avt-profile, and
may be extended in future editions of the Assigned Numbers RFC
[6]. An RTP sender emits a single RTP payload type at any given
time; this field is not intended for multiplexing separate media
streams (see Section 5.2).
- sequence number: 16 bits
-
The sequence number increments by one for each RTP data packet
sent, and may be used by the receiver to detect packet loss and
to restore packet sequence. The initial value of the sequence
number is random (unpredictable) to make known-plaintext attacks
on encryption more difficult, even if the source itself does not
encrypt, because the packets may flow through a translator that
does. Techniques for choosing unpredictable numbers are
discussed in [7].
- timestamp: 32 bits
-
The timestamp reflects the sampling instant of the first octet
in the RTP data packet. The sampling instant must be derived
from a clock that increments monotonically and linearly in time
to allow synchronization and jitter calculations (see Section
6.3.1). The resolution of the clock must be sufficient for the
desired synchronization accuracy and for measuring packet
arrival jitter (one tick per video frame is typically not
sufficient). The clock frequency is dependent on the format of
data carried as payload and is specified statically in the
profile or payload format specification that defines the format,
or may be specified dynamically for payload formats defined
through non-RTP means. If RTP packets are generated
periodically, the nominal sampling instant as determined from
the sampling clock is to be used, not a reading of the system
clock. As an example, for fixed-rate audio the timestamp clock
would likely increment by one for each sampling period. If an
audio application reads blocks covering 160 sampling periods
from the input device, the timestamp would be increased by 160
for each such block, regardless of whether the block is
transmitted in a packet or dropped as silent.
The initial value of the timestamp is random, as for the sequence
number. Several consecutive RTP packets may have equal timestamps if
they are (logically) generated at once, e.g., belong to the same
video frame. Consecutive RTP packets may contain timestamps that are
not monotonic if the data is not transmitted in the order it was
sampled, as in the case of MPEG interpolated video frames. (The
sequence numbers of the packets as transmitted will still be
monotonic.)
- SSRC: 32 bits
-
The SSRC field identifies the synchronization source. This
identifier is chosen randomly, with the intent that no two
synchronization sources within the same RTP session will have
the same SSRC identifier. An example algorithm for generating a
random identifier is presented in Appendix A.6. Although the
probability of multiple sources choosing the same identifier is
low, all RTP implementations must be prepared to detect and
resolve collisions. Section 8 describes the probability of
collision along with a mechanism for resolving collisions and
detecting RTP-level forwarding loops based on the uniqueness of
the SSRC identifier. If a source changes its source transport
address, it must also choose a new SSRC identifier to avoid
being interpreted as a looped source.
- CSRC list: 0 to 15 items, 32 bits each
-
The CSRC list identifies the contributing sources for the
payload contained in this packet. The number of identifiers is
given by the CC field. If there are more than 15 contributing
sources, only 15 may be identified. CSRC identifiers are
inserted by mixers, using the SSRC identifiers of contributing
sources. For example, for audio packets the SSRC identifiers of
all sources that were mixed together to create a packet are
listed, allowing correct talker indication at the receiver.
Next: 5.2 Multiplexing RTP Sessions
Connected: An Internet Encyclopedia
5.1 RTP Fixed Header Fields