Network Delivery Mechanisms - A Practical Guide to Video and Audio Compression - page 339

A delayed packet that arrives too late with UDP will be out of sequence and cannot be
used. The consequence is dropped frames and a rather jittery viewing experience.

19.7.3 Packet Sequence Errors

Packets arriving out of sequence might be handled gracefully if you are downloading or
buffering rather than playing live to the window as packets are received. Players are get-
ting better at handling this sort of problem. Network latency and error handling is also
being designed into the transport protocols so that the players do a better job under
extreme situations.
These artifacts are not evident in broadcasting systems because the flow of bits
through the system is uni-directional and not subject to routing and transfer problems.
The transmission chain is simply a first in-first out (FIFO) buffer. This preserves sequence
order and guarantees delivery. Momentary dropouts will be error-corrected but will result
in some latency.

19.7.4 Lost Packets

Packets that get lost completely and never arrive present the player with a problem.
Handling the dropout in a graceful manner that the user won't notice is challenging. The
forward error-correction support may help by providing sufficient redundancy coding to
allow the missing data to be reconstructed. This imposes some load on the CPU but the
dropouts by nature are infrequent and if they do happen often, the content negotiation
could kick in and force the streamer to deliver at a lower bit rate. This might help to alle-
viate the problem if it is due to bandwidth limits somewhere in the route between the
client and server.
The codec has some additional strategies for dealing with missing parts of the image;
newer codec designs are far more resilient when faced with data loss and may have a
variety of alternative strategies for locating a suitable fragment of data to replace the
missing part.
The H.264 syntax specifies that the decoder should cache a large number of frames
that it can refer to for missing data. The structure is organized so that the P- and B-frame
references can also be computed at the slice level. A single reconstructed frame might use
component slices from several different source frames. Older codecs refer to entire frames.
This fine-grain approach helps H.264 video handle dropouts more effectively. When a
dropout occurs, the reconstructed error correction will be less noticeable.

19.7.5 Dropped Frames

The TCP guarantees delivery of all packets in the right order but with the penalty that
there may be some delays. On the other hand, UDP delivers all the packages it can within
the lowest latency but will occasionally lose a packet.
To avoid problems when this happens, the streaming protocols add error-correction
support to the transport and the parity algorithms help to fill in missing parts of the data