Lecture Notes for Telecommunications

6 December 2004 - The Real Time Protocols

Streaming Services

• Continuous, arbirarily long data streams.

  • Perhaps even infinite.

• Continuous signals are often created via analog-digital conversion.

  • From camera and microphone to packets to speaker and monitor.

Encodings

• Byte-stream encodings.

• Frame-style encodings.

Real-Time Delivery

• Real-time data must be delivered with the same timing behavior it had when it was created.

  • Streaming data is usually real-time data.

• Two important timing behaviors are order and time.

  • Order indicates relative position between data components.

  • Time indicates absolute position for data components.

Best Effort Delivery

• Best effort networks are not real-time in either order or time.

  • Order problems are caused by out-of-order delivery.

  • Time problems are caused by delay and jitter.

  • Out-of-order delivery can be thought of as extreme delay

• Jitter is the variance on delay.

  • Delay is not bad; it can be compensated for.

  • Jitter is bad; it is hard or expensive to compensate for.

Best-Effort Compensations

• Applications must correct best-effort delivery for real-time traffic.

• Ordering problems are dealt with by sequence numbers.

• Delay problems are dealt with by timestamps.

  • Sequence numbers and timestamps are separate solutions.

• Jitter problems are dealt with by buffers.

Best-Effort Loss

• The other characteristic of best-effort delivery is packet loss.

• Retransmission, the usual cure for packet loss, is not real-time.

  • Retransmission can result in arbitrarily long buffers

Handling Packet Loss

• Real-time packet loss is either ignored or regenerated.

• Ignoring data is usually not possible for human receivers.

• Regeneration can be done through interpolation or forward error correction.

RTP

• The real-time protocol (RTP) is a stream-independent protocol for transmitting real-time data via a best-effort network.

  • It's one of a suite of real-time protocols.

  • It's specified by the IETF (rfc 1889).

The RTP Header

RTP Sequence Numbers

• RTP adds a strictly increasing, 16-bit sequence number to each RTP header.

• The RTP receiver uses the sequence number to order payloads.

  • In a playback buffer, for example.

• The initial sequence value is randomly generated.

  • This reduces problems old packets from another stream.

  • It also provides a very small amount of stream security.

RTP Timestamps

• The RTP timestamp is a 32-bit value generated by the application.

• RTP requires that

  • The initial timestamp value must be randomly generated.

  • The timestamp value must be updated continuously.

• RTP does not specify

  • Timestamp units.

  • Timestamp interpretation.

• Stream data generated at the same time may have the same timestamp.

  • This allows inter-stream synchronization.

RTP Stream Translation

• A stream translator accepts an RTP stream and produces another one.

  • Presumably based on the input stream.

• Stream translators can be used to re-encode data.

Stream Multiplexing

• A stream multiplexor accepts several input RTP streams and produces a single RTP output stream.

  • Stream multiplexors are also known as bridges.

Handling Jitter

• Incoming data must be buffered to ensure smooth playback.

  • Jitter may delay the next packet past its expected arrival time.

• The jitter (or playback) buffer holds enough data to ensure smooth playback.

Adaptive QoS

• Applications must adapt to changing conditions on best-effort networks.

  • If they can, QoS becomes an end-to-end property for those applications.

• Media bit rates should be proportional to available bandwidth.

• RTP does not provide the necessary information to allow adapation.

The RTP Control Protocol

• Receivers use the RTP Control Protocol (RTCP) to inform the senders about network conditions.

Synchronization

• Audio and video require separate streams.

  • Rates and encodings are way different.

• Separate but related streams have to be synchronized to ensure sensible playback.

• RTP information isn't enough to establish and maintain synchronization.

• The sender uses RTCP packets for synchronization.

RTCP Synchronization

• Senders periodically transmit RTPC sender reports.

  • A sender has a canonical name to identify all its streams.

• The sender report contains an absolute time associated with RTP timestamps.

Multicast

• RTP is a datagram protocol.

  • Recovery via retransmission is unhelpful.

  • RTP needs to do flow control and sequencing its own way.

  • Datagrams do multicast well.

• Multicast communications allow for richer communication topologies.

  • Cheap one-to-many, or general N-to-M.

RTP Organization

• RTP software organizes into four layers: application, synchronization, rtp-rtcp, and transport (udp).

This page last modified on 14 November 2004.