1. RTP: A Transport Protocol for Real-Time Applications
John Paul Kalapurakal
Ronak Kamdar
RFC 3550

2. Background and Motivation
Want to listen to a song?
Download
Listen online
Technical foundation for VoIP

3. Background and Motivation
Quality of Service
End to end delivery service
Application Specific

4. Introduction Transmitting real time data RTP & RTCP Protocol
What it is?
Application level framing
Integrated layer processing
Deliberately incomplete

5. The Problem Statement
Combination of transport layer and application layer protocols to better applications involving real time data

6. Key Concepts Independent of underlying layer
Supports multicast & unicast
Does NOT
Resource reservation
Guarantee QoS
Ensure timely delivery
Prevent out of order packets

7. Example : Simple Multicast Audio Conference

8. Example : Audio & Video

9. Mixers Application: Scale images of different people
on different streams and compose it into one
single stream to simulate a group scene

10. Translator Application: Connection of a group of hosts
speaking only IP/UDP to a group of hosts speaking
only ST-II

11. Mixer vs. Translator The advantage of mixer over translator
Better for low-link bandwidth
The advantage of translator over mixer
For mixer, no control over sources

12. Layered Encoding with layer transmission
Rate Adaptability leads to smallest pipe in the network mesh (Bottleneck)

13. Definitions Port: Different destinations within one host
Transport Address: Network + Port
RTP Session: Different pairs of destination transport addresses

14. More definitions Synchronization Source: Source of a
stream of RTP packets
Contributing Source: Source of packets
that contribute to combined stream.

15. RTP Data Transfer Protocol
Header

16. Multiplexing RTP sessions
Efficient protocol processing requires no. of multiplexing points be minimized
Multiplexing is provided by destination transport addresses

17. Multiplexing RTP sessions
Example
Audio + Video
Should not be carried in a single stream
Why?
Audio + Audio
Multiplexing multiple related sources of the same medium in one RTP session is the norm for multicast
How?

18. RTP Header Extension Profile specific modifications Two options
Can define additional fixed fields
Additional payload information can be carried in the payload section

19. RTCP – RTP control protocol
Feedback on quality of data distribution
Persistent transport level identifier called
the canonical name or CNAME
Scalability
Convey minimal session control
information

20. RTCP Packet Format SR: Sender report RR: Receiver report
BYE: Indicated end of participation
SDES: Source description with CNAME

21. RTCP Packet Format Compound RTCP packet
Multiple RTCP packets concatenated into one
Order not important for processing
Must contain RR, SR and CNAME
Individual RTP participant must send only one compound RTCP packet per report interval
If it exceeds the MTU then use fragmentation

22. RTCP Transmission Interval
Rate at which RTCP packets are sent out
RTP is scalable in terms of users
It scales linearly with no. of participants
Adaptively changes with network traffic

23. Security Secure Real-Time Transport Protocol Encryption
RTCP – random number
RTP – sequence number and time stamp
Weak
DES (Data Encryption Protocol)
Stronger encryption needed
Additional Payloads

24. Congestion Control RTP must provide congestion control
Different from commonly used transport protocols
Defined based on RTP profile

25. RTP over Network and Transport Protocols
RTP relies on the underlying protocols
Ports must be:
Even for – RTP
Odd for – RTCP
Suffers from securities liabilities of underlying protocols.

26. Results

27. VoIP and Skype Protocols to transport voice signals over IP network
Uses secure RTP and RTCP
Challenge: Firewalls, Latency and jitter
Skype uses a proprietary protocol to route calls through other Skype peers on the network, allowing it to traverse symmetric NATs and firewalls

28. Critique Does not guarantee QoS Increased security Too detailed
Strange location in the protocol stack

29. Summary and Conclusions
Incomplete is good!
Achieves the goal of better facilitating real time data transfer
Questions?