Side_1_360

(destination unreachable, packet too big, time
exceeded, parameter problem) and informational
messages (echo request, echo reply).

4 User Datagram Protocol

• UDP
  Avoiding addressing each of the processes util-
  ising IP explicitly, protocol port numbers are
  introduced as explained in Section 6. Each pro-
  tocol port is identified by an integer. To commu-
  nicate with a remote process, the destination IP
  address and port number of the process have to
  be known. Then, for a connectionless protocol
  this information has to be included in every mes-
  sage. Hence, while IP addresses commonly refer
  to machines/IP layer processes, further identi-
  fiers must be used to identify the transport pro-
  cess.

Two protocols dominate in the transport layer;
Transmission Control Protocol (TCP) and User
Data Protocol (UDP). The former is connection-
oriented while the latter is connectionless.

Simplified, UDP can be seen as placing a short
header in front of the user data before putting it
into an IP packet. Hence, the User Datagram
Protocol, UDP, is seen as one of the simplest
transport protocols utilising IP. The format of
the UDP header is depicted in Figure 7.

As depicted, the UDP header is divided into four
16 bit fields. The Sourceand Destination ports
identify the UDP processes at the two ends (fill-
ing in the Source port is optional). The Length
field gives the number of bytes in the UDP
packet (sum of header and data). The UDP
checksumfield can be used to detect bit errors
introduced in the header and user data. A pseudo
header is added before the checksum is calcu-
lated allowing the receiving process to check
that the correct destination and protocol port
have been used.

Examining the fields in the UDP header, it is
recognised that UDP provides an unreliable con-
nectionless delivery service based on IP. How-
ever, UDP adds the ability to multiplex several
processes on a given host, see Figure 8.

5 Transmission Control

Protocol – TCP

Several applications are asking for more reliable
transfers than UDP can provide. For those, TCP
can be used. TCP provides a reliable transfer ser-
vice, as packet loss does not need to be dealt
with by the application. Two basic features are
part of TCP; acknowledgement of transferred
data, and window for unacknowledged data
units. The former makes sure that the data is cor-
rectly transferred by explicitly giving acknowl-
edgements. Introducing windows on the sender

side allows a number of packets to be sent

before being acknowledged, increasing the utili-

sation of the network. In contrast to UDP, TCP

is connection-oriented. That is, prior to sending

data a TCP connection is established between

the two end-points.

TCP was specified to support a dependable flow

of user data to be carried by IP packets, that is,

over an unreliable network. TCP was initially

defined in [RFC793], and some corrections and

extensions described in [RFC1122] and

[RFC1323], respectively. A TCP entity com-

monly accepts a flow of user data and divides it

into packets smaller than 64 kbytes (commonly

1500 bytes are used), and sends each packet to

the IP entity. As a network has its maximum

transfer unit (MTU), this value is commonly

respected by the TCP entity when deciding upon

the packet size in order to avoid fragmentation

in the IP layer. At the receiver side, the IP packet

is transferred to the TCP entity that reconstructs

the original user data flow.

TCP uses a sliding window mechanism for effi-

cient transmission and flow control. Sending

multiple packets before an acknowledgement

has to arrive at the sender increases the transfer

rate and therefore the link utilisation. As the

window has a limited width, it can also be used

to limit the data volume that can be sent and

therefore the transfer rate. The window size to

use depends on conditions in the network or the

receiver (controlled by reporting on window

width to be used and delaying acknowledge-

ments). Moreover, as the sender is adjusting the

window width based on the packet dropping

(lack of timely acknowledgements), dropping

packets in the network during congestion will

also limit the sender’s transmission rate. Figure 7 UDP header format

Figure 8 UDP demultiplexing

based on port numbers

Destination port

UDP checksum

01631

Source port

Length

Port i Port j Port k

IP

UDP-

demultiplexing