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

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

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

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

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

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

Example : Simple Multicast Audio Conference

Example : Audio & Video

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

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

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

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

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

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

RTP Data Transfer Protocol Header

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

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?

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

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

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

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

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

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

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

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.

Results

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

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

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