Network Delivery Mechanisms - A Practical Guide to Video and Audio Compression - page 338

the stack, sometimes called the physical layer. This layer is also sometimes referred to as
the transport but that embodies some higher-level functionality that helps correct for
errors.
The data become wrapped in increasingly structured containers as the layers are tra-
versed, and each layer adds some small overhead in order to manage its specific respon-
sibilities. Figure 19-3 shows how this works.

19.7.1 TCP Versus UDP

Somewhat higher up in the stack is a layer called the universal datagram protocol (UDP).
This UDP describes an interface that guarantees a timely delivery of a packet from one
node to the next. It does not guarantee 100% reliable delivery of a complete series of pack-
ets in a session since some may be delivered via another route and may not reach the des-
tination. The UDP also does not guarantee that they will all arrive in the right order.
A further layer in the protocol referred to as TCP addresses the shortcomings of
UDP. The TCP acronym is short for Transmission Control Protocol and this layer guaran-
tees that all the required packets will be sent and that they will be delivered to the call-
ing application in the client in the correct sequence. This layer implements sophisticated
buffering algorithms to ensure that sequencing is correct and also supports acknowledg-
ment messages to the sender. If necessary, it can request that a packet be retransmitted to
fill any gaps.
The downside is that TCP will guarantee delivery but not necessarily in a timely
fashion. This is why your client player offers you the opportunity to select TCP or UDP
connections in the preferences panel.

19.7.2 Irregular Packet Delivery (Jitter)

Delayed packets when using the UDP protocol may manifest the problems in a different
way from when you use TCP. A delayed packet in TCP simply slows down the transfer.

Application
Data

TCP or UDP
Header

Application
Data

IP
Header

TCP or UDP
Header

Application
Data

Ethernet
Header

IP
Header

TCP or UDP
Header

Application
Data

Ethernet
Trailer

Figure 19-3 TCP packetizing
structure.

Ethernet frame