1. RTP/RTCP(RFC 1889)
Real-time transport protocol (RTP) is the de facto standard media transport protocol in the Internet
Media transport: audio, vedio, etc. How is it different from data transport?
RTP: media transport.
RTCP: end-to-end monitoring and data delivery and QoS.

2. RTP and RTCP are commonly built on top on UDP.
RTP provides some functionality for media transport. RTP does not:
Guarantee QoS
address resource reservation
Perform signaling (negotiate the media format)
RTP/RTCP are usually implemented within applications --- not as clean cut as TCP.

3. RTP use scenarios
A working group obtains an IP multicast address and a pair of ports through some allocation mechanism.
One port for audio, one for control
The address and the ports are distributed to intended participants.
The participants send audio data in small chucks, 20ms duration. Each chuck of audio data is preceded with a RTP header, which indicates the type of encoding.
The Internet may occasionally lose or reorder packets or delay the packet in variable amount time. To help the receiver reconstruct the timing produced by the sender, RTP header also include timing and sequence number information.
Since the member may join and leave a group dynamically, it is useful to know who is participating and the audio quality at a given time. This task is done through RTCP.

4. RTP packet format: V=2 p x cc m PT sequence number timestamp
synchronization source identifier (SSRC)
contributing source identifiers (CSRC)
p: Padding (1 bit), data content is less than the size of the packet.
x: extension(1 bit), fixed header is followed by 1 variable sized header extension.
cc: CSRC count: number of CSRC identifiers.
m: marker, identify significant events.
PT: payload type (PCM, ADPCM, etc)

5. RTP packet format: V=2 p x cc m PT sequence number timestamp
synchronization source identifier (SSRC)
contributing source identifiers (CSRC)
Timestamp: sampling instant of the first octet in the RTP data packet
SSRC: sender
CSRC list: contributing sources for the payload contained in the packet. (needed for mixers)

6. RTCP A companion control protocol to RTP.
RTCP is not a signaling protocol
RTCP is used to collect end-to-end information about the quality of the session to each participant.
RTCP packet types:
SR: sender report
RR: receiver report
SDES: Source DEScription
BYE: Hangs up from a session
APP: Application-Sprcific packet

7. SR: sender report Carry statistics from the active senders.
V=2 p rc PT=SR= length
SSRC of sender
NTP timestamp, MSW
NTP timestamp, LSW
RTP timestamp
Sender’s packet count
Sender’s octet count
SSRC_1 (SSRC of first source), block 1
fraction lost cumulative number of packets lost
extended highest sequence number received
interarrival jitter
last SR (LSR)
delay since last SR (DLSR)
SSRC_2 (SSRC of second source), begin block 2 ….
profile-specific extensions

8. RR: receiver report Statistics from active receivers
V=2 p rc PR=RR= length
SSRC of sender
SSRC of first source, begin block 1
Fraction lost (FL) cumlative number of packets lost
extended highest sequence number received
interarrival jitter
last SR (LSR)
delay since last SR (DLSR)
SSRC of second source, begin block 2
…….
profile-specific extensions

9. SDES: Source Description packet
Allow the binding of SSRC value with an actual identification of the user.
End points need to send an SDES packet at the beginning of the session
BYE: ends a user’s participation in a call.
APP: Application-specific RTP packet.

10. Some issues in RTP and RTCP:
How often should RTCP reports to exchanges.
Must be able to provide real time and with enough resolution for QoS interpretation.
Fix bandwidth
Proportion to channel bandwidth
How to use the RTCP reports?
Reroute
End-to-end QoS information may not be sufficient.
Application adaptation.