Information Technology Reference
In-Depth Information
Figure 6. Operation summary: send
D eterm in e d estin atio n A p p licatio n
Name
notional
Pass to TCP
=
TCP
Establish Connection ( SY N )
/ End Connection (FIN )
notional
B reak m essag e in to seg m en ts ad d
sequence #
T ear in to 1 0
character pieces
A ppend destination Port#
=
A ppend source Port#
=
Append CRC to each
notional CRC
Pass to IP
=
IP
B reak seg m en t in to d atag ram s
n o tio n al
A dd destination IP address to each
=
A dd source IP address to each
=
Lookup First H op IP
=
D eterm ine O utput Connection
=
A d d T im e to L iv e
In itial = 1 0
Each hop -1
Pass to D L L
=
N A L
B reak d atag ram in to fram es
n o tio n al
A dd destination M A C address
=
A dd source M A C address
=
Send to Physical
=
Physical Send to receiving M A C
envelope & w ire
At the receiving end, the process is reversed. There are some differences. For example,
error checking is performed instead of CRC calculation. IP checks for destination and forwards
the packet if necessary. TCP notionally acknowledges each packet.
Example
Figure 8 shows a partially completed header stream for three different messages. This
example follows one segment each of the three messages.
The first from george.edu (10.4.2.1) to thomas.gov (10.4.2.3) takes only one jump,
because the two devices are on the same network segment. The second, from george.edu to
john.org (10.4.5.6) must be routed. The node george does not know where 10.4.5.6 is, so it
sends the packet to its gateway, the Router at MAC4 (10.4.0.0). The packet is addressed to
IP 10.4.5.6 and MAC4, i.e., to john in care of the router. The router knows where 10.4.5.6 is
located, so it can send the packet directly to it.
The third message is from george to martha.edu (10.7.8.9). Again, george does not know
where 10.7.8.9 is, so it directs the packet to its gateway (i.e., to IP 10.7.8.9 and MAC4). This
time, the router does not know either, so it consults its routing table and finds that the router
at 10.7.0.0 is the first hop to 10.7.8.9. The packet is forwarded to 10.7.0.0 (Mac7), which knows
where 10.7.8.9 is, and it sends the packet to its destination.
