Operation Manual

IP
SIP Software Release 6-0-0
J613-M0524-00
19
contains the information necessary to move the packet across the internet. It
must be able to cope with missing and duplicated packets as well as possible
fragmentation (and reassembly) of the original packet.
Packets are sent using a connectionless transport mechanism. A connection
is not maintained between the source and destination addresses; rather, the
destination address is placed in the header and the packet is transmitted on
a best effort basis. It is up to the intermediate systems (routers and
gateways) to deliver the packet to the correct address, using the information
in the header.
Successive packets may take different routes through the network to the
destination. There is a strong analogy with the postal delivery system in that
letters are placed in individually addressed envelopes and put into the
system in the ‘hope’ that they will arrive. Like an internet, the postal system
is very reliable. In an internet, higher layers (such as TCP and Telnet) are
responsible for ensuring that packets are delivered in a reliable and
sequenced way.
In contrast to a connectionless transport mechanism, a connection-oriented
transport mechanism requires a connection to be maintained between the
source and destination for as long as necessary to complete the exchange of
packets between source and destination. X.25 is an example of a connection-
oriented protocol. A good analogy to X.25 would be a telephone call, in
which both parties verify that they are talking to the correct person before
exchanging highly sequenced data (if both talk at once then nothing
intelligible results!), and the connection is maintained until both parties have
finished talking. Its not hard to imagine the chaos if the telephone system
delivered words in the wrong order.