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TCP/IP and the DoD Model

127

Urgent Pointer: 0

TCP Options:

Option Type: 2 Maximum Segment Size

Length: 4

MSS: 1460

No More HTTP Data

Frame Check Sequence: 0x6E203132

And here again, you can see that the response from the server shows the

source is 80 and the destination is the 1144 sent from the originating host--
all's well.

The Internet Layer Protocols

In the DoD model, there are two main reasons for the Internet layer's exist-
ence: routing, and providing a single network interface to the upper layers.

None of the other upper- or lower-layer protocols have any functions

relating to routing--that complex and important task belongs entirely to the
Internet layer. The Internet layer's second duty is to provide a single network
interface to the upper-layer protocols. Without this layer, application pro-
grammers would need to write "hooks" into every one of their applications
for each different Network Access protocol. This would not only be a pain
in the neck, but it would lead to different versions of each application--one
for Ethernet, another one for Token Ring, and so on. To prevent this, IP pro-
vides one single network interface for the upper-layer protocols. That
accomplished, it's then the job of IP and the various Network Access proto-
cols to get along and work together.

All network roads don't lead to Rome--they lead to IP. And all the other

protocols at this layer, as well as all those at the upper layers, use it. Never
forget that. All paths through the DoD model go through IP. The following
sections describe the protocols at the Internet layer:

Internet Protocol (IP)

Internet Control Message Protocol (ICMP)

Address Resolution Protocol (ARP)

Reverse Address Resolution Protocol (RARP)

Internet Protocol (IP)

Internet Protocol (IP) essentially is the Internet layer. The other protocols
found here merely exist to support it. IP holds the big picture and could be said

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