RTP runs in end-systems.

RTP QoS:

• Protocol does not provide any QoS guarantees
• RTP encapsulation is only seen at the end systems. Routers provide best-effort-service making no special effort

RTP on top of UDP

• RTP packets are encapsulated in UDP segments
• RTP libraries provide transport-layer interface that extends UDP
  • Port numbers, IP addresses
  • Payload type identification
  • Packet sequence numbering
  • time stamping
• Application collects encoded data in chunks, ex: every 20ms 160B in a chunk
• Audio chunk + RTP Header form RTP packet, encapsulated in UDP segment
• RTP header indicates type of audio encoding in each packet
• Header also contains sequence number and time stamps

RTP Header:

1. Payload Type (7b):
   • Indicates type of encoding currently being used. If sender changes encoding during call, sender informs receiver via payload-type field
2. Sequence # (16b):
   • Increment by one for each RTP packet sent
   • Detect packet loss, restore packet sequence
3. Timestemp (32b):
   • Sampling instant of the first byte in the packet

RTCP - Real time control protocol:

• Works in conjunctions with RTP
• Each participant in RTP session periodically sends RTCP control packets to all other participants
• Each RTCP packet contains sender and/or receiver reports
• Feedback used to control performance

Each RTP session: - Typically a single multicast address - All RTP/RTCP packet belonging to the session use multicast address

• RTP and RTCP packets are distinguished via distinct port numbers