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The Internet Layer Protocols
There are two main reasons for the Internet layers existence: routing, and providing a single network interface to the upper layers. None of the upper- or lower-layer protocols have any functions relating to routing. The complex and important task of routing is the job of the Internet layer. The Internet layers second job is to provide a single network interface to the upper-layer protocols. Without this layer, application programmers 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 applicationone for Ethernet, another one for Token Ring, and so on. To prevent this, IP provides one single network interface for the upper-layer protocols. That accomplished, its then the job of IP and the various Network Access protocols to get along and work together.
All network roads dont lead to Romethey 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 model go through IP. The following sections describe the protocols at the Internet layer. These are the protocols that work at the Internet layer:
Internet Protocol (IP)
Internet Control Message Protocol (ICMP)
Address Resolution Protocol (ARP)
Reverse Address Resolution Protocol (RARP)
Internet Protocol (IP)
The Internet Protocol (IP) essentially is the Internet layer. The other protocols found here merely exist to support it. IP contains the big picture and could be said to see all, in that it is aware of all the interconnected networks. It can do this because all the machines on the network have a software, or logical, address called an IP address. IP looks at each packets address. Then, using a routing table, it decides where a packet is to be sent next, choosing the best path. The Network Accesslayer protocols at the bottom of the model dont possess IPs enlightened scope of the entire network; they deal only with physical links (local networks).
Identifying devices on networks requires answering these two questions:
1. Which network is it on?
2. And what is its ID on that network?
The first answer is the software, or logical, address (the correct street). The second answer is the hardware address (the correct mailbox). All hosts on a network have a logical ID called an IP address. This is the software, or logical, address and contains valuable encoded information greatly simplifying the complex task of routing. IP receives segments from the Host-to-Host layer and fragments them into datagrams (packets). IP then reassembles datagrams back into segments on the receiving side. Each datagram is assigned the IP address of the sender and of the recipient. Each router (layer-3 device) that receives a datagram makes routing decisions based upon the packets destination IP address.
Internet Control Message Protocol (ICMP)
The Internet Control Message Protocol (ICMP) works at the Network layer and is used by IP for many different services. ICMP is a management protocol and messaging service provider for IP. Its messages are carried as IP datagrams.
Periodically, router advertisements are announced over the network, reporting IP addresses for the routers network interfaces. Hosts listen for these network infomercials to acquire route information. A router solicitation is a request for immediate advertisements and may be sent by a host when it starts up.
Address Resolution Protocol (ARP)
The Address Resolution Protocol (ARP) finds the hardware address of a host from a known IP address. Heres how it works: When IP has a datagram to send, it must inform a Network Access protocol, such as Ethernet or Token Ring, of the destinations hardware address on the local network. (It has already been informed by upper-layer protocols of the destinations IP address.) If IP doesnt find the destination hosts hardware address in the ARP cache, it uses ARP to find this information. As IPs detective, ARP interrogates the local network by sending out a broadcast asking the machine with the specified IP address to reply with its hardware address. In other words, ARP translates the software (IP) address into a hardware addressfor example, the destination machines Ethernet board addressand from it, deduces its whereabouts. This hardware address is technically referred to as the media access control (MAC) address or physical address. Figure 3.8 shows how an ARP might look to a local network.
Reverse Address Resolution Protocol (RARP)
When an IP machine happens to be a diskless machine, it has no way of initially knowing its IP address, but it does know its MAC address. The Reverse Address Resolution Protocol (RARP) discovers the identity of the IP address for diskless machines by sending out a packet that includes its MAC address and a request for the IP address assigned to that MAC address. A designated machine, called a RARP server, responds with the answer, and the identity crisis is over. RARP uses the information it does know about the machines MAC address to learn its IP address and complete the machines ID portrait.