RFC 1889
RTP: A Transport Protocol for Real-Time Applications
6. RTP Control Protocol -- RTCP The RTP control protocol (RTCP) is based on the periodic transmission of control packets to all participants in the session, using the same distribution mechanism as the data packets. The underlying protocol must provide multiplexing of the data and control packets, for example using separate port numbers with UDP. RTCP performs four functions: 1. The primary function is to provide feedback on the quality of the data distribution. This is an integral part of the RTP's role as a transport protocol and is related to the flow and congestion control functions of other transport protocols. The feedback may be directly useful for control of adaptive encodings [8,9], but experiments with IP
multicasting have shown that it is also critical to get
feedback from the receivers to diagnose faults in the
distribution. Sending reception feedback reports to all
participants allows one who is observing problems to
evaluate whether those problems are local or global. With a
distribution mechanism like IP multicast, it is also
possible for an entity such as a network service provider
who is not otherwise involved in the session to receive the
feedback information and act as a third-party monitor to
diagnose network problems. This feedback function is
performed by the RTCP sender and receiver reports,
described below in Section 6.3.
2. RTCP carries a persistent transport-level identifier for an
RTP source called the canonical name or CNAME, Section
6.4.1. Since the SSRC identifier may change if a conflict
is discovered or a program is restarted, receivers require
the CNAME to keep track of each participant. Receivers also
require the CNAME to associate multiple data streams from a
given participant in a set of related RTP sessions, for
example to synchronize audio and video.
3. The first two functions require that all participants send
RTCP packets, therefore the rate must be controlled in
order for RTP to scale up to a large number of
participants. By having each participant send its control
packets to all the others, each can independently observe
the number of participants. This number is used to
calculate the rate at which the packets are sent, as
explained in Section 6.2.
4. A fourth, optional function is to convey minimal session
control information, for example participant identification
to be displayed in the user interface. This is most likely
to be useful in "loosely controlled" sessions where
participants enter and leave without membership control or
parameter negotiation. RTCP serves as a convenient channel
to reach all the participants, but it is not necessarily
expected to support all the control communication
requirements of an application. A higher-level session
control protocol, which is beyond the scope of this
document, may be needed.
Functions 1-3 are mandatory when RTP is used in the IP multicast
environment, and are recommended for all environments. RTP
application designers are advised to avoid mechanisms that can only
work in unicast mode and will not scale to larger numbers.
6.1 RTCP Packet Format This specification defines several RTCP packet types to carry a variety of control information: SR: Sender report, for transmission and reception statistics from participants that are active senders RR: Receiver report, for reception statistics from participants that are not active senders SDES: Source description items, including CNAME BYE: Indicates end of participation APP: Application specific functions Each RTCP packet begins with a fixed part similar to that of RTP data packets, followed by structured elements that may be of variable length according to the packet type but always end on a 32-bit boundary. The alignment requirement and a length field in the fixed part are included to make RTCP packets "stackable". Multiple RTCP packets may be concatenated without any intervening separators to form a compound RTCP packet that is sent in a single packet of the lower layer protocol, for example UDP. There is no explicit count of individual RTCP packets in the compound packet since the lower layer protocols are expected to provide an overall length to determine the end of the compound packet. Each individual RTCP packet in the compound packet may be processed independently with no requirements upon the order or combination of packets. However, in order to perform the functions of the protocol, the following constraints are imposed: o Reception statistics (in SR or RR) should be sent as often as bandwidth constraints will allow to maximize the resolution of the statistics, therefore each periodically transmitted compound RTCP packet should include a report packet. o New receivers need to receive the CNAME for a source as soon as possible to identify the source and to begin associating media for purposes such as lip-sync, so each compound RTCP packet should also include the SDES CNAME. o The number of packet types that may appear first in the compound packet should be limited to increase the number of constant bits in the first word and the probability of successfully validating RTCP packets against misaddressed RTP
data packets or other unrelated packets.
Thus, all RTCP packets must be sent in a compound packet of at least
two individual packets, with the following format recommended:
Encryption prefix: If and only if the compound packet is to be
encrypted, it is prefixed by a random 32-bit quantity redrawn
for every compound packet transmitted.
SR or RR: The first RTCP packet in the compound packet must always
be a report packet to facilitate header validation as described
in Appendix A.2. This is true even if no data has been sent nor
received, in which case an empty RR is sent, and even if the
only other RTCP packet in the compound packet is a BYE.
Additional RRs: If the number of sources for which reception
statistics are being reported exceeds 31, the number that will
fit into one SR or RR packet, then additional RR packets should
follow the initial report packet.
SDES: An SDES packet containing a CNAME item must be included in
each compound RTCP packet. Other source description items may
optionally be included if required by a particular application,
subject to bandwidth constraints (see Section 6.2.2).
BYE or APP: Other RTCP packet types, including those yet to be
defined, may follow in any order, except that BYE should be the
last packet sent with a given SSRC/CSRC. Packet types may appear
more than once.
It is advisable for translators and mixers to combine individual RTCP
packets from the multiple sources they are forwarding into one
compound packet whenever feasible in order to amortize the packet
overhead (see Section 7). An example RTCP compound packet as might be
produced by a mixer is shown in Fig. 1. If the overall length of a
compound packet would exceed the maximum transmission unit (MTU) of
the network path, it may be segmented into multiple shorter compound
packets to be transmitted in separate packets of the underlying
protocol. Note that each of the compound packets must begin with an
SR or RR packet.
An implementation may ignore incoming RTCP packets with types unknown
to it. Additional RTCP packet types may be registered with the
Internet Assigned Numbers Authority (IANA).
6.2 RTCP Transmission Interval if encrypted: random 32-bit integer | |[------- packet -------][----------- packet -----------][-packet-] | | receiver reports chunk chunk V item item item item -------------------------------------------------------------------- |R[SR|# sender #site#site][SDES|# CNAME PHONE |#CNAME LOC][BYE##why] |R[ |# report # 1 # 2 ][ |# |# ][ ## ] |R[ |# # # ][ |# |# ][ ## ] |R[ |# # # ][ |# |# ][ ## ] -------------------------------------------------------------------- |<------------------ UDP packet (compound packet) --------------->| #: SSRC/CSRC Figure 1: Example of an RTCP compound packet RTP is designed to allow an application to scale automatically over session sizes ranging from a few participants to thousands. For example, in an audio conference the data traffic is inherently self- limiting because only one or two people will speak at a time, so with multicast distribution the data rate on any given link remains relatively constant independent of the number of participants. However, the control traffic is not self-limiting. If the reception reports from each participant were sent at a constant rate, the control traffic would grow linearly with the number of participants. Therefore, the rate must be scaled down. For each session, it is assumed that the data traffic is subject to an aggregate limit called the "session bandwidth" to be divided among the participants. This bandwidth might be reserved and the limit enforced by the network, or it might just be a reasonable share. The session bandwidth may be chosen based or some cost or a priori knowledge of the available network bandwidth for the session. It is somewhat independent of the media encoding, but the encoding choice may be limited by the session bandwidth. The session bandwidth parameter is expected to be supplied by a session management application when it invokes a media application, but media applications may also set a default based on the single-sender data bandwidth for the encoding selected for the session. The application may also enforce bandwidth limits based on multicast scope rules or other criteria.
Bandwidth calculations for control and data traffic include lower-
layer transport and network protocols (e.g., UDP and IP) since that
is what the resource reservation system would need to know. The
application can also be expected to know which of these protocols are
in use. Link level headers are not included in the calculation since
the packet will be encapsulated with different link level headers as
it travels.
The control traffic should be limited to a small and known fraction
of the session bandwidth: small so that the primary function of the
transport protocol to carry data is not impaired; known so that the
control traffic can be included in the bandwidth specification given
to a resource reservation protocol, and so that each participant can
independently calculate its share. It is suggested that the fraction
of the session bandwidth allocated to RTCP be fixed at 5%. While the
value of this and other constants in the interval calculation is not
critical, all participants in the session must use the same values so
the same interval will be calculated. Therefore, these constants
should be fixed for a particular profile.
The algorithm described in Appendix A.7 was designed to meet the
goals outlined above. It calculates the interval between sending
compound RTCP packets to divide the allowed control traffic bandwidth
among the participants. This allows an application to provide fast
response for small sessions where, for example, identification of all
participants is important, yet automatically adapt to large sessions.
The algorithm incorporates the following characteristics:
o Senders are collectively allocated at least 1/4 of the control
traffic bandwidth so that in sessions with a large number of
receivers but a small number of senders, newly joining
participants will more quickly receive the CNAME for the
sending sites.
o The calculated interval between RTCP packets is required to be
greater than a minimum of 5 seconds to avoid having bursts of
RTCP packets exceed the allowed bandwidth when the number of
participants is small and the traffic isn't smoothed according
to the law of large numbers.
o The interval between RTCP packets is varied randomly over the
range [0.5,1.5] times the calculated interval to avoid
unintended synchronization of all participants [10]. The first
RTCP packet sent after joining a session is also delayed by a
random variation of half the minimum RTCP interval in case the
application is started at multiple sites simultaneously, for
example as initiated by a session announcement.
o A dynamic estimate of the average compound RTCP packet size is
calculated, including all those received and sent, to
automatically adapt to changes in the amount of control
information carried.
This algorithm may be used for sessions in which all participants are
allowed to send. In that case, the session bandwidth parameter is the
product of the individual sender's bandwidth times the number of
participants, and the RTCP bandwidth is 5% of that.
6.2.1 Maintaining the number of session members
Calculation of the RTCP packet interval depends upon an estimate of
the number of sites participating in the session. New sites are added
to the count when they are heard, and an entry for each is created in
a table indexed by the SSRC or CSRC identifier (see Section 8.2) to
keep track of them. New entries may not be considered valid until
multiple packets carrying the new SSRC have been received (see
Appendix A.1). Entries may be deleted from the table when an RTCP BYE
packet with the corresponding SSRC identifier is received.
A participant may mark another site inactive, or delete it if not yet
valid, if no RTP or RTCP packet has been received for a small number
of RTCP report intervals (5 is suggested). This provides some
robustness against packet loss. All sites must calculate roughly the
same value for the RTCP report interval in order for this timeout to
work properly.
Once a site has been validated, then if it is later marked inactive
the state for that site should still be retained and the site should
continue to be counted in the total number of sites sharing RTCP
bandwidth for a period long enough to span typical network
partitions. This is to avoid excessive traffic, when the partition
heals, due to an RTCP report interval that is too small. A timeout of
30 minutes is suggested. Note that this is still larger than 5 times
the largest value to which the RTCP report interval is expected to
usefully scale, about 2 to 5 minutes.
6.2.2 Allocation of source description bandwidth
This specification defines several source description (SDES) items in
addition to the mandatory CNAME item, such as NAME (personal name)
and EMAIL (email address). It also provides a means to define new
application-specific RTCP packet types. Applications should exercise
caution in allocating control bandwidth to this additional
information because it will slow down the rate at which reception
reports and CNAME are sent, thus impairing the performance of the
protocol. It is recommended that no more than 20% of the RTCP
bandwidth allocated to a single participant be used to carry the additional information. Furthermore, it is not intended that all SDES items should be included in every application. Those that are included should be assigned a fraction of the bandwidth according to their utility. Rather than estimate these fractions dynamically, it is recommended that the percentages be translated statically into report interval counts based on the typical length of an item. For example, an application may be designed to send only CNAME, NAME and EMAIL and not any others. NAME might be given much higher priority than EMAIL because the NAME would be displayed continuously in the application's user interface, whereas EMAIL would be displayed only when requested. At every RTCP interval, an RR packet and an SDES packet with the CNAME item would be sent. For a small session operating at the minimum interval, that would be every 5 seconds on the average. Every third interval (15 seconds), one extra item would be included in the SDES packet. Seven out of eight times this would be the NAME item, and every eighth time (2 minutes) it would be the EMAIL item. When multiple applications operate in concert using cross-application binding through a common CNAME for each participant, for example in a multimedia conference composed of an RTP session for each medium, the additional SDES information might be sent in only one RTP session. The other sessions would carry only the CNAME item. 6.3 Sender and Receiver Reports RTP receivers provide reception quality feedback using RTCP report packets which may take one of two forms depending upon whether or not the receiver is also a sender. The only difference between the sender report (SR) and receiver report (RR) forms, besides the packet type code, is that the sender report includes a 20-byte sender information section for use by active senders. The SR is issued if a site has sent any data packets during the interval since issuing the last report or the previous one, otherwise the RR is issued. Both the SR and RR forms include zero or more reception report blocks, one for each of the synchronization sources from which this receiver has received RTP data packets since the last report. Reports are not issued for contributing sources listed in the CSRC list. Each reception report block provides statistics about the data received from the particular source indicated in that block. Since a maximum of 31 reception report blocks will fit in an SR or RR packet, additional RR packets may be stacked after the initial SR or RR packet as needed to contain the reception reports for all sources heard during the interval since the last report.
The next sections define the formats of the two reports, how they may be extended in a profile-specific manner if an application requires additional feedback information, and how the reports may be used. Details of reception reporting by translators and mixers is given in Section 7. 6.3.1 SR: Sender report RTCP packet 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| RC | PT=SR=200 | length | header +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of sender | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | NTP timestamp, most significant word | sender +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ info | NTP timestamp, least significant word | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sender's packet count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sender's octet count | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | SSRC_1 (SSRC of first source) | report +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block | fraction lost | cumulative number of packets lost | 1 -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | extended highest sequence number received | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | interarrival jitter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | last SR (LSR) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | delay since last SR (DLSR) | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | SSRC_2 (SSRC of second source) | report +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block : ... : 2 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | profile-specific extensions | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The sender report packet consists of three sections, possibly followed by a fourth profile-specific extension section if defined. The first section, the header, is 8 octets long. The fields have the following meaning:
version (V): 2 bits
Identifies the version of RTP, which is the same in RTCP packets
as in RTP data packets. The version defined by this
specification is two (2).
padding (P): 1 bit
If the padding bit is set, this RTCP packet contains some
additional padding octets at the end which are not part of the
control information. The last octet of the padding is a count of
how many padding octets should be ignored. Padding may be needed
by some encryption algorithms with fixed block sizes. In a
compound RTCP packet, padding should only be required on the
last individual packet because the compound packet is encrypted
as a whole.
reception report count (RC): 5 bits
The number of reception report blocks contained in this packet.
A value of zero is valid.
packet type (PT): 8 bits
Contains the constant 200 to identify this as an RTCP SR packet.
length: 16 bits
The length of this RTCP packet in 32-bit words minus one,
including the header and any padding. (The offset of one makes
zero a valid length and avoids a possible infinite loop in
scanning a compound RTCP packet, while counting 32-bit words
avoids a validity check for a multiple of 4.)
SSRC: 32 bits
The synchronization source identifier for the originator of this
SR packet.
The second section, the sender information, is 20 octets long and is
present in every sender report packet. It summarizes the data
transmissions from this sender. The fields have the following
meaning:
NTP timestamp: 64 bits
Indicates the wallclock time when this report was sent so that
it may be used in combination with timestamps returned in
reception reports from other receivers to measure round-trip
propagation to those receivers. Receivers should expect that the
measurement accuracy of the timestamp may be limited to far less
than the resolution of the NTP timestamp. The measurement
uncertainty of the timestamp is not indicated as it may not be
known. A sender that can keep track of elapsed time but has no
notion of wallclock time may use the elapsed time since joining
the session instead. This is assumed to be less than 68 years,
so the high bit will be zero. It is permissible to use the
sampling clock to estimate elapsed wallclock time. A sender that
has no notion of wallclock or elapsed time may set the NTP
timestamp to zero.
RTP timestamp: 32 bits
Corresponds to the same time as the NTP timestamp (above), but
in the same units and with the same random offset as the RTP
timestamps in data packets. This correspondence may be used for
intra- and inter-media synchronization for sources whose NTP
timestamps are synchronized, and may be used by media-
independent receivers to estimate the nominal RTP clock
frequency. Note that in most cases this timestamp will not be
equal to the RTP timestamp in any adjacent data packet. Rather,
it is calculated from the corresponding NTP timestamp using the
relationship between the RTP timestamp counter and real time as
maintained by periodically checking the wallclock time at a
sampling instant.
sender's packet count: 32 bits
The total number of RTP data packets transmitted by the sender
since starting transmission up until the time this SR packet was
generated. The count is reset if the sender changes its SSRC
identifier.
sender's octet count: 32 bits
The total number of payload octets (i.e., not including header
or padding) transmitted in RTP data packets by the sender since
starting transmission up until the time this SR packet was
generated. The count is reset if the sender changes its SSRC
identifier. This field can be used to estimate the average
payload data rate.
The third section contains zero or more reception report blocks
depending on the number of other sources heard by this sender since
the last report. Each reception report block conveys statistics on
the reception of RTP packets from a single synchronization source.
Receivers do not carry over statistics when a source changes its SSRC
identifier due to a collision. These statistics are:
SSRC_n (source identifier): 32 bits
The SSRC identifier of the source to which the information in
this reception report block pertains.
fraction lost: 8 bits
The fraction of RTP data packets from source SSRC_n lost since
the previous SR or RR packet was sent, expressed as a fixed
point number with the binary point at the left edge of the
field. (That is equivalent to taking the integer part after
multiplying the loss fraction by 256.) This fraction is defined
to be the number of packets lost divided by the number of
packets expected, as defined in the next paragraph. An
implementation is shown in Appendix A.3. If the loss is negative
due to duplicates, the fraction lost is set to zero. Note that a
receiver cannot tell whether any packets were lost after the
last one received, and that there will be no reception report
block issued for a source if all packets from that source sent
during the last reporting interval have been lost.
cumulative number of packets lost: 24 bits
The total number of RTP data packets from source SSRC_n that
have been lost since the beginning of reception. This number is
defined to be the number of packets expected less the number of
packets actually received, where the number of packets received
includes any which are late or duplicates. Thus packets that
arrive late are not counted as lost, and the loss may be
negative if there are duplicates. The number of packets
expected is defined to be the extended last sequence number
received, as defined next, less the initial sequence number
received. This may be calculated as shown in Appendix A.3.
extended highest sequence number received: 32 bits
The low 16 bits contain the highest sequence number received in
an RTP data packet from source SSRC_n, and the most significant
16 bits extend that sequence number with the corresponding count
of sequence number cycles, which may be maintained according to
the algorithm in Appendix A.1. Note that different receivers
within the same session will generate different extensions to
the sequence number if their start times differ significantly.
interarrival jitter: 32 bits
An estimate of the statistical variance of the RTP data packet
interarrival time, measured in timestamp units and expressed as
an unsigned integer. The interarrival jitter J is defined to be
the mean deviation (smoothed absolute value) of the difference D
in packet spacing at the receiver compared to the sender for a
pair of packets. As shown in the equation below, this is
equivalent to the difference in the "relative transit time" for
the two packets; the relative transit time is the difference
between a packet's RTP timestamp and the receiver's clock at the
time of arrival, measured in the same units.
If Si is the RTP timestamp from packet i, and Ri is the time of
arrival in RTP timestamp units for packet i, then for two packets i
and j, D may be expressed as
D(i,j)=(Rj-Ri)-(Sj-Si)=(Rj-Sj)-(Ri-Si)
The interarrival jitter is calculated continuously as each data
packet i is received from source SSRC_n, using this difference D for
that packet and the previous packet i-1 in order of arrival (not
necessarily in sequence), according to the formula
J=J+(|D(i-1,i)|-J)/16
Whenever a reception report is issued, the current value of J is
sampled.
The jitter calculation is prescribed here to allow profile-
independent monitors to make valid interpretations of reports coming
from different implementations. This algorithm is the optimal first-
order estimator and the gain parameter 1/16 gives a good noise
reduction ratio while maintaining a reasonable rate of convergence
[11]. A sample implementation is shown in Appendix A.8.
last SR timestamp (LSR): 32 bits
The middle 32 bits out of 64 in the NTP timestamp (as explained
in Section 4) received as part of the most recent RTCP sender
report (SR) packet from source SSRC_n. If no SR has been
received yet, the field is set to zero.
delay since last SR (DLSR): 32 bits
The delay, expressed in units of 1/65536 seconds, between
receiving the last SR packet from source SSRC_n and sending this
reception report block. If no SR packet has been received yet
from SSRC_n, the DLSR field is set to zero.
Let SSRC_r denote the receiver issuing this receiver report. Source
SSRC_n can compute the round propagation delay to SSRC_r by recording
the time A when this reception report block is received. It
calculates the total round-trip time A-LSR using the last SR
timestamp (LSR) field, and then subtracting this field to leave the
round-trip propagation delay as (A- LSR - DLSR). This is illustrated
in Fig. 2.
This may be used as an approximate measure of distance to cluster
receivers, although some links have very asymmetric delays.
6.3.2 RR: Receiver report RTCP packet [10 Nov 1995 11:33:25.125] [10 Nov 1995 11:33:36.5] n SR(n) A=b710:8000 (46864.500 s) ----------------------------------------------------------------> v ^ ntp_sec =0xb44db705 v ^ dlsr=0x0005.4000 ( 5.250s) ntp_frac=0x20000000 v ^ lsr =0xb705:2000 (46853.125s) (3024992016.125 s) v ^ r v ^ RR(n) ----------------------------------------------------------------> |<-DLSR->| (5.250 s) A 0xb710:8000 (46864.500 s) DLSR -0x0005:4000 ( 5.250 s) LSR -0xb705:2000 (46853.125 s) ------------------------------- delay 0x 6:2000 ( 6.125 s) Figure 2: Example for round-trip time computation 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| RC | PT=RR=201 | length | header +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of packet sender | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | SSRC_1 (SSRC of first source) | report +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block | fraction lost | cumulative number of packets lost | 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | extended highest sequence number received | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | interarrival jitter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | last SR (LSR) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | delay since last SR (DLSR) | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | SSRC_2 (SSRC of second source) | report +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block : ... : 2 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | profile-specific extensions | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the receiver report (RR) packet is the same as that of the SR packet except that the packet type field contains the constant 201 and the five words of sender information are omitted (these are the NTP and RTP timestamps and sender's packet and octet counts). The remaining fields have the same meaning as for the SR packet. An empty RR packet (RC = 0) is put at the head of a compound RTCP packet when there is no data transmission or reception to report. 6.3.3 Extending the sender and receiver reports A profile should define profile- or application-specific extensions to the sender report and receiver if there is additional information that should be reported regularly about the sender or receivers. This method should be used in preference to defining another RTCP packet type because it requires less overhead: o fewer octets in the packet (no RTCP header or SSRC field); o simpler and faster parsing because applications running under that profile would be programmed to always expect the extension fields in the directly accessible location after the reception reports. If additional sender information is required, it should be included first in the extension for sender reports, but would not be present in receiver reports. If information about receivers is to be included, that data may be structured as an array of blocks parallel to the existing array of reception report blocks; that is, the number of blocks would be indicated by the RC field. 6.3.4 Analyzing sender and receiver reports It is expected that reception quality feedback will be useful not only for the sender but also for other receivers and third-party monitors. The sender may modify its transmissions based on the feedback; receivers can determine whether problems are local, regional or global; network managers may use profile-independent monitors that receive only the RTCP packets and not the corresponding RTP data packets to evaluate the performance of their networks for multicast distribution. Cumulative counts are used in both the sender information and receiver report blocks so that differences may be calculated between any two reports to make measurements over both short and long time periods, and to provide resilience against the loss of a report. The difference between the last two reports received can be used to estimate the recent quality of the distribution. The NTP timestamp is
included so that rates may be calculated from these differences over the interval between two reports. Since that timestamp is independent of the clock rate for the data encoding, it is possible to implement encoding- and profile-independent quality monitors. An example calculation is the packet loss rate over the interval between two reception reports. The difference in the cumulative number of packets lost gives the number lost during that interval. The difference in the extended last sequence numbers received gives the number of packets expected during the interval. The ratio of these two is the packet loss fraction over the interval. This ratio should equal the fraction lost field if the two reports are consecutive, but otherwise not. The loss rate per second can be obtained by dividing the loss fraction by the difference in NTP timestamps, expressed in seconds. The number of packets received is the number of packets expected minus the number lost. The number of packets expected may also be used to judge the statistical validity of any loss estimates. For example, 1 out of 5 packets lost has a lower significance than 200 out of 1000. From the sender information, a third-party monitor can calculate the average payload data rate and the average packet rate over an interval without receiving the data. Taking the ratio of the two gives the average payload size. If it can be assumed that packet loss is independent of packet size, then the number of packets received by a particular receiver times the average payload size (or the corresponding packet size) gives the apparent throughput available to that receiver. In addition to the cumulative counts which allow long-term packet loss measurements using differences between reports, the fraction lost field provides a short-term measurement from a single report. This becomes more important as the size of a session scales up enough that reception state information might not be kept for all receivers or the interval between reports becomes long enough that only one report might have been received from a particular receiver. The interarrival jitter field provides a second short-term measure of network congestion. Packet loss tracks persistent congestion while the jitter measure tracks transient congestion. The jitter measure may indicate congestion before it leads to packet loss. Since the interarrival jitter field is only a snapshot of the jitter at the time of a report, it may be necessary to analyze a number of reports from one receiver over time or from multiple receivers, e.g., within a single network.
6.4 SDES: Source description RTCP packet 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| SC | PT=SDES=202 | length | header +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | SSRC/CSRC_1 | chunk +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1 | SDES items | | ... | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | SSRC/CSRC_2 | chunk +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2 | SDES items | | ... | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ The SDES packet is a three-level structure composed of a header and zero or more chunks, each of of which is composed of items describing the source identified in that chunk. The items are described individually in subsequent sections. version (V), padding (P), length: As described for the SR packet (see Section 6.3.1). packet type (PT): 8 bits Contains the constant 202 to identify this as an RTCP SDES packet. source count (SC): 5 bits The number of SSRC/CSRC chunks contained in this SDES packet. A value of zero is valid but useless. Each chunk consists of an SSRC/CSRC identifier followed by a list of zero or more items, which carry information about the SSRC/CSRC. Each chunk starts on a 32-bit boundary. Each item consists of an 8-bit type field, an 8-bit octet count describing the length of the text (thus, not including this two-octet header), and the text itself. Note that the text can be no longer than 255 octets, but this is consistent with the need to limit RTCP bandwidth consumption. The text is encoded according to the UTF-2 encoding specified in Annex F of ISO standard 10646 [12,13]. This encoding is also known as UTF-8 or UTF-FSS. It is described in "File System Safe UCS Transformation Format (FSS_UTF)", X/Open Preliminary Specification, Document Number P316 and Unicode Technical Report #4. US-ASCII is a subset of this encoding and requires no additional encoding. The
presence of multi-octet encodings is indicated by setting the most significant bit of a character to a value of one. Items are contiguous, i.e., items are not individually padded to a 32-bit boundary. Text is not null terminated because some multi-octet encodings include null octets. The list of items in each chunk is terminated by one or more null octets, the first of which is interpreted as an item type of zero to denote the end of the list, and the remainder as needed to pad until the next 32-bit boundary. A chunk with zero items (four null octets) is valid but useless. End systems send one SDES packet containing their own source identifier (the same as the SSRC in the fixed RTP header). A mixer sends one SDES packet containing a chunk for each contributing source from which it is receiving SDES information, or multiple complete SDES packets in the format above if there are more than 31 such sources (see Section 7). The SDES items currently defined are described in the next sections. Only the CNAME item is mandatory. Some items shown here may be useful only for particular profiles, but the item types are all assigned from one common space to promote shared use and to simplify profile- independent applications. Additional items may be defined in a profile by registering the type numbers with IANA. 6.4.1 CNAME: Canonical end-point identifier SDES item 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CNAME=1 | length | user and domain name ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The CNAME identifier has the following properties: o Because the randomly allocated SSRC identifier may change if a conflict is discovered or if a program is restarted, the CNAME item is required to provide the binding from the SSRC identifier to an identifier for the source that remains constant. o Like the SSRC identifier, the CNAME identifier should also be unique among all participants within one RTP session. o To provide a binding across multiple media tools used by one participant in a set of related RTP sessions, the CNAME should be fixed for that participant.
o To facilitate third-party monitoring, the CNAME should be
suitable for either a program or a person to locate the source.
Therefore, the CNAME should be derived algorithmically and not
entered manually, when possible. To meet these requirements, the
following format should be used unless a profile specifies an
alternate syntax or semantics. The CNAME item should have the format
"user@host", or "host" if a user name is not available as on single-
user systems. For both formats, "host" is either the fully qualified
domain name of the host from which the real-time data originates,
formatted according to the rules specified in RFC 1034 [14], RFC 1035
[15] and Section 2.1 of RFC 1123 [16]; or the standard ASCII
representation of the host's numeric address on the interface used
for the RTP communication. For example, the standard ASCII
representation of an IP Version 4 address is "dotted decimal", also
known as dotted quad. Other address types are expected to have ASCII
representations that are mutually unique. The fully qualified domain
name is more convenient for a human observer and may avoid the need
to send a NAME item in addition, but it may be difficult or
impossible to obtain reliably in some operating environments.
Applications that may be run in such environments should use the
ASCII representation of the address instead.
Examples are "doe@sleepy.megacorp.com" or "doe@192.0.2.89" for a
multi-user system. On a system with no user name, examples would be
"sleepy.megacorp.com" or "192.0.2.89".
The user name should be in a form that a program such as "finger" or
"talk" could use, i.e., it typically is the login name rather than
the personal name. The host name is not necessarily identical to the
one in the participant's electronic mail address.
This syntax will not provide unique identifiers for each source if an
application permits a user to generate multiple sources from one
host. Such an application would have to rely on the SSRC to further
identify the source, or the profile for that application would have
to specify additional syntax for the CNAME identifier.
If each application creates its CNAME independently, the resulting
CNAMEs may not be identical as would be required to provide a binding
across multiple media tools belonging to one participant in a set of
related RTP sessions. If cross-media binding is required, it may be
necessary for the CNAME of each tool to be externally configured with
the same value by a coordination tool.
Application writers should be aware that private network address
assignments such as the Net-10 assignment proposed in RFC 1597 [17]
may create network addresses that are not globally unique. This would
lead to non-unique CNAMEs if hosts with private addresses and no direct IP connectivity to the public Internet have their RTP packets forwarded to the public Internet through an RTP-level translator. (See also RFC 1627 [18].) To handle this case, applications may provide a means to configure a unique CNAME, but the burden is on the translator to translate CNAMEs from private addresses to public addresses if necessary to keep private addresses from being exposed. 6.4.2 NAME: User name SDES item 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NAME=2 | length | common name of source ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This is the real name used to describe the source, e.g., "John Doe, Bit Recycler, Megacorp". It may be in any form desired by the user. For applications such as conferencing, this form of name may be the most desirable for display in participant lists, and therefore might be sent most frequently of those items other than CNAME. Profiles may establish such priorities. The NAME value is expected to remain constant at least for the duration of a session. It should not be relied upon to be unique among all participants in the session. 6.4.3 EMAIL: Electronic mail address SDES item 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | EMAIL=3 | length | email address of source ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The email address is formatted according to RFC 822 [19], for example, "John.Doe@megacorp.com". The EMAIL value is expected to remain constant for the duration of a session. 6.4.4 PHONE: Phone number SDES item 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PHONE=4 | length | phone number of source ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The phone number should be formatted with the plus sign replacing the international access code. For example, "+1 908 555 1212" for a number in the United States.
6.4.5 LOC: Geographic user location SDES item 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LOC=5 | length | geographic location of site ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Depending on the application, different degrees of detail are appropriate for this item. For conference applications, a string like "Murray Hill, New Jersey" may be sufficient, while, for an active badge system, strings like "Room 2A244, AT&T BL MH" might be appropriate. The degree of detail is left to the implementation and/or user, but format and content may be prescribed by a profile. The LOC value is expected to remain constant for the duration of a session, except for mobile hosts. 6.4.6 TOOL: Application or tool name SDES item 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TOOL=6 | length | name/version of source appl. ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A string giving the name and possibly version of the application generating the stream, e.g., "videotool 1.2". This information may be useful for debugging purposes and is similar to the Mailer or Mail- System-Version SMTP headers. The TOOL value is expected to remain constant for the duration of the session. 6.4.7 NOTE: Notice/status SDES item 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NOTE=7 | length | note about the source ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The following semantics are suggested for this item, but these or other semantics may be explicitly defined by a profile. The NOTE item is intended for transient messages describing the current state of the source, e.g., "on the phone, can't talk". Or, during a seminar, this item might be used to convey the title of the talk. It should be used only to carry exceptional information and should not be included routinely by all participants because this would slow down the rate at which reception reports and CNAME are sent, thus impairing the performance of the protocol. In particular, it should not be included
as an item in a user's configuration file nor automatically generated as in a quote-of-the-day. Since the NOTE item may be important to display while it is active, the rate at which other non-CNAME items such as NAME are transmitted might be reduced so that the NOTE item can take that part of the RTCP bandwidth. When the transient message becomes inactive, the NOTE item should continue to be transmitted a few times at the same repetition rate but with a string of length zero to signal the receivers. However, receivers should also consider the NOTE item inactive if it is not received for a small multiple of the repetition rate, or perhaps 20-30 RTCP intervals. 6.4.8 PRIV: Private extensions SDES item 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PRIV=8 | length | prefix length | prefix string... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | value string ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This item is used to define experimental or application-specific SDES extensions. The item contains a prefix consisting of a length-string pair, followed by the value string filling the remainder of the item and carrying the desired information. The prefix length field is 8 bits long. The prefix string is a name chosen by the person defining the PRIV item to be unique with respect to other PRIV items this application might receive. The application creator might choose to use the application name plus an additional subtype identification if needed. Alternatively, it is recommended that others choose a name based on the entity they represent, then coordinate the use of the name within that entity. Note that the prefix consumes some space within the item's total length of 255 octets, so the prefix should be kept as short as possible. This facility and the constrained RTCP bandwidth should not be overloaded; it is not intended to satisfy all the control communication requirements of all applications. SDES PRIV prefixes will not be registered by IANA. If some form of the PRIV item proves to be of general utility, it should instead be assigned a regular SDES item type registered with IANA so that no prefix is required. This simplifies use and increases transmission efficiency.
6.5 BYE: Goodbye RTCP packet 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| SC | PT=BYE=203 | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC/CSRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : ... : +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | length | reason for leaving ... (opt) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The BYE packet indicates that one or more sources are no longer active. version (V), padding (P), length: As described for the SR packet (see Section 6.3.1). packet type (PT): 8 bits Contains the constant 203 to identify this as an RTCP BYE packet. source count (SC): 5 bits The number of SSRC/CSRC identifiers included in this BYE packet. A count value of zero is valid, but useless. If a BYE packet is received by a mixer, the mixer forwards the BYE packet with the SSRC/CSRC identifier(s) unchanged. If a mixer shuts down, it should send a BYE packet listing all contributing sources it handles, as well as its own SSRC identifier. Optionally, the BYE packet may include an 8-bit octet count followed by that many octets of text indicating the reason for leaving, e.g., "camera malfunction" or "RTP loop detected". The string has the same encoding as that described for SDES. If the string fills the packet to the next 32-bit boundary, the string is not null terminated. If not, the BYE packet is padded with null octets.
6.6 APP: Application-defined RTCP packet 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| subtype | PT=APP=204 | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC/CSRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | name (ASCII) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | application-dependent data ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The APP packet is intended for experimental use as new applications and new features are developed, without requiring packet type value registration. APP packets with unrecognized names should be ignored. After testing and if wider use is justified, it is recommended that each APP packet be redefined without the subtype and name fields and registered with the Internet Assigned Numbers Authority using an RTCP packet type. version (V), padding (P), length: As described for the SR packet (see Section 6.3.1). subtype: 5 bits May be used as a subtype to allow a set of APP packets to be defined under one unique name, or for any application-dependent data. packet type (PT): 8 bits Contains the constant 204 to identify this as an RTCP APP packet. name: 4 octets A name chosen by the person defining the set of APP packets to be unique with respect to other APP packets this application might receive. The application creator might choose to use the application name, and then coordinate the allocation of subtype values to others who want to define new packet types for the application. Alternatively, it is recommended that others choose a name based on the entity they represent, then coordinate the use of the name within that entity. The name is interpreted as a sequence of four ASCII characters, with uppercase and lowercase characters treated as distinct.
application-dependent data: variable length
Application-dependent data may or may not appear in an APP
packet. It is interpreted by the application and not RTP itself.
It must be a multiple of 32 bits long.
7. RTP Translators and Mixers
In addition to end systems, RTP supports the notion of "translators"
and "mixers", which could be considered as "intermediate systems" at
the RTP level. Although this support adds some complexity to the
protocol, the need for these functions has been clearly established
by experiments with multicast audio and video applications in the
Internet. Example uses of translators and mixers given in Section 2.3
stem from the presence of firewalls and low bandwidth connections,
both of which are likely to remain.
7.1 General Description
An RTP translator/mixer connects two or more transport-level
"clouds". Typically, each cloud is defined by a common network and
transport protocol (e.g., IP/UDP), multicast address or pair of
unicast addresses, and transport level destination port. (Network-
level protocol translators, such as IP version 4 to IP version 6, may
be present within a cloud invisibly to RTP.) One system may serve as
a translator or mixer for a number of RTP sessions, but each is
considered a logically separate entity.
In order to avoid creating a loop when a translator or mixer is
installed, the following rules must be observed:
o Each of the clouds connected by translators and mixers
participating in one RTP session either must be distinct from
all the others in at least one of these parameters (protocol,
address, port), or must be isolated at the network level from
the others.
o A derivative of the first rule is that there must not be
multiple translators or mixers connected in parallel unless by
some arrangement they partition the set of sources to be
forwarded.
Similarly, all RTP end systems that can communicate through one or
more RTP translators or mixers share the same SSRC space, that is,
the SSRC identifiers must be unique among all these end systems.
Section 8.2 describes the collision resolution algorithm by which
SSRC identifiers are kept unique and loops are detected.
There may be many varieties of translators and mixers designed for
different purposes and applications. Some examples are to add or
remove encryption, change the encoding of the data or the underlying
protocols, or replicate between a multicast address and one or more
unicast addresses. The distinction between translators and mixers is
that a translator passes through the data streams from different
sources separately, whereas a mixer combines them to form one new
stream:
Translator: Forwards RTP packets with their SSRC identifier intact;
this makes it possible for receivers to identify individual
sources even though packets from all the sources pass through
the same translator and carry the translator's network source
address. Some kinds of translators will pass through the data
untouched, but others may change the encoding of the data and
thus the RTP data payload type and timestamp. If multiple data
packets are re-encoded into one, or vice versa, a translator
must assign new sequence numbers to the outgoing packets. Losses
in the incoming packet stream may induce corresponding gaps in
the outgoing sequence numbers. Receivers cannot detect the
presence of a translator unless they know by some other means
what payload type or transport address was used by the original
source.
Mixer: Receives streams of RTP data packets from one or more sources,
possibly changes the data format, combines the streams in some
manner and then forwards the combined stream. 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, so it is the
synchronization source. Thus, all data packets forwarded by a
mixer will be marked with the mixer's own SSRC identifier. In
order to preserve the identity of the original sources
contributing to the mixed packet, the mixer should insert their
SSRC identifiers into the CSRC identifier list following the
fixed RTP header of the packet. A mixer that is also itself a
contributing source for some packet should explicitly include
its own SSRC identifier in the CSRC list for that packet.
For some applications, it may be acceptable for a mixer not to
identify sources in the CSRC list. However, this introduces the
danger that loops involving those sources could not be detected.
The advantage of a mixer over a translator for applications like
audio is that the output bandwidth is limited to that of one source
even when multiple sources are active on the input side. This may be
important for low-bandwidth links. The disadvantage is that receivers
on the output side don't have any control over which sources are
passed through or muted, unless some mechanism is implemented for
remote control of the mixer. The regeneration of synchronization
information by mixers also means that receivers can't do inter-media
synchronization of the original streams. A multi-media mixer could do
it.
[E1] [E6]
| |
E1:17 | E6:15 |
| | E6:15
V M1:48 (1,17) M1:48 (1,17) V M1:48 (1,17)
(M1)-------------><T1>-----------------><T2>-------------->[E7]
^ ^ E4:47 ^ E4:47
E2:1 | E4:47 | | M3:89 (64,45)
| | |
[E2] [E4] M3:89 (64,45) |
| legend:
[E3] --------->(M2)----------->(M3)------------| [End system]
E3:64 M2:12 (64) ^ (Mixer)
| E5:45 <Translator>
|
[E5] source: SSRC (CSRCs)
------------------->
Figure 3: Sample RTP network with end systems, mixers and translators
A collection of mixers and translators is shown in Figure 3 to
illustrate their effect on SSRC and CSRC identifiers. In the figure,
end systems are shown as rectangles (named E), translators as
triangles (named T) and mixers as ovals (named M). The notation "M1:
48(1,17)" designates a packet originating a mixer M1, identified with
M1's (random) SSRC value of 48 and two CSRC identifiers, 1 and 17,
copied from the SSRC identifiers of packets from E1 and E2.
7.2 RTCP Processing in Translators
In addition to forwarding data packets, perhaps modified, translators
and mixers must also process RTCP packets. In many cases, they will
take apart the compound RTCP packets received from end systems to
aggregate SDES information and to modify the SR or RR packets.
Retransmission of this information may be triggered by the packet
arrival or by the RTCP interval timer of the translator or mixer
itself.
A translator that does not modify the data packets, for example one
that just replicates between a multicast address and a unicast
address, may simply forward RTCP packets unmodified as well. A
translator that transforms the payload in some way must make
corresponding transformations in the SR and RR information so that it
still reflects the characteristics of the data and the reception
quality. These translators must not simply forward RTCP packets. In
general, a translator should not aggregate SR and RR packets from
different sources into one packet since that would reduce the
accuracy of the propagation delay measurements based on the LSR and
DLSR fields.
SR sender information: A translator does not generate its own sender
information, but forwards the SR packets received from one cloud
to the others. The SSRC is left intact but the sender
information must be modified if required by the translation. If
a translator changes the data encoding, it must change the
"sender's byte count" field. If it also combines several data
packets into one output packet, it must change the "sender's
packet count" field. If it changes the timestamp frequency, it
must change the "RTP timestamp" field in the SR packet.
SR/RR reception report blocks: A translator forwards reception
reports received from one cloud to the others. Note that these
flow in the direction opposite to the data. The SSRC is left
intact. If a translator combines several data packets into one
output packet, and therefore changes the sequence numbers, it
must make the inverse manipulation for the packet loss fields
and the "extended last sequence number" field. This may be
complex. In the extreme case, there may be no meaningful way to
translate the reception reports, so the translator may pass on
no reception report at all or a synthetic report based on its
own reception. The general rule is to do what makes sense for a
particular translation.
A translator does not require an SSRC identifier of its own, but may
choose to allocate one for the purpose of sending reports about what
it has received. These would be sent to all the connected clouds,
each corresponding to the translation of the data stream as sent to
that cloud, since reception reports are normally multicast to all
participants.
SDES: Translators typically forward without change the SDES
information they receive from one cloud to the others, but may,
for example, decide to filter non-CNAME SDES information if
bandwidth is limited. The CNAMEs must be forwarded to allow SSRC
identifier collision detection to work. A translator that
generates its own RR packets must send SDES CNAME information
about itself to the same clouds that it sends those RR packets.
BYE: Translators forward BYE packets unchanged. Translators with
their own SSRC should generate BYE packets with that SSRC
identifier if they are about to cease forwarding packets.
APP: Translators forward APP packets unchanged.
7.3 RTCP Processing in Mixers
Since a mixer generates a new data stream of its own, it does not
pass through SR or RR packets at all and instead generates new
information for both sides.
SR sender information: A mixer does not pass through sender
information from the sources it mixes because the
characteristics of the source streams are lost in the mix. As a
synchronization source, the mixer generates its own SR packets
with sender information about the mixed data stream and sends
them in the same direction as the mixed stream.
SR/RR reception report blocks: A mixer generates its own reception
reports for sources in each cloud and sends them out only to the
same cloud. It does not send these reception reports to the
other clouds and does not forward reception reports from one
cloud to the others because the sources would not be SSRCs there
(only CSRCs).
SDES: Mixers typically forward without change the SDES information
they receive from one cloud to the others, but may, for example,
decide to filter non-CNAME SDES information if bandwidth is
limited. The CNAMEs must be forwarded to allow SSRC identifier
collision detection to work. (An identifier in a CSRC list
generated by a mixer might collide with an SSRC identifier
generated by an end system.) A mixer must send SDES CNAME
information about itself to the same clouds that it sends SR or
RR packets.
Since mixers do not forward SR or RR packets, they will typically be
extracting SDES packets from a compound RTCP packet. To minimize
overhead, chunks from the SDES packets may be aggregated into a
single SDES packet which is then stacked on an SR or RR packet
originating from the mixer. The RTCP packet rate may be different on
each side of the mixer.
A mixer that does not insert CSRC identifiers may also refrain from
forwarding SDES CNAMEs. In this case, the SSRC identifier spaces in
the two clouds are independent. As mentioned earlier, this mode of
operation creates a danger that loops can't be detected.
BYE: Mixers need to forward BYE packets. They should generate BYE
packets with their own SSRC identifiers if they are about to
cease forwarding packets.
APP: The treatment of APP packets by mixers is application-specific.
7.4 Cascaded Mixers
An RTP session may involve a collection of mixers and translators as
shown in Figure 3. If two mixers are cascaded, such as M2 and M3 in
the figure, packets received by a mixer may already have been mixed
and may include a CSRC list with multiple identifiers. The second
mixer should build the CSRC list for the outgoing packet using the
CSRC identifiers from already-mixed input packets and the SSRC
identifiers from unmixed input packets. This is shown in the output
arc from mixer M3 labeled M3:89(64,45) in the figure. As in the case
of mixers that are not cascaded, if the resulting CSRC list has more
than 15 identifiers, the remainder cannot be included.
(page 44 continued on part 3)
