RTPRTCPRFC 1889

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
• V2 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
• V2 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.
• V2 p rc PTSR200 length
• SSRC of sender
• NTP timestamp, MSW
• NTP timestamp, LSW
• RTP timestamp
• Senders packet count
• Senders octet count
• SSRC_1 (SSRC of first source), block
  1
• Senders packet count
• 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
• .

8

• RR receiver report
• Statistics from active receivers
• V2 p rc PRRR201 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 users 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.