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Internet Layer    

The Internet Layer exists for routing and providing a single network interface to the upper layers. IP provides the single network interface for the upper layers.

IP (Internet Protocol)

The Internet Protocol (IP) is a network-layer (Layer 3) protocol that contains addressing information and some control information that enables packets to be routed.  IP has two primary responsibilities: providing connectionless, best-effort delivery of datagrams through an internetwork; and providing fragmentation and reassembly of datagrams to support data links with different maximum-transmission unit (MTU) sizes.

All machines on a TCP/IP network have a unique logical address, an IP address.  The Internet Layer (IP) has a complete picture of the entire network and is responsible for path determination and packet switching.  IP is the transport for TCP, UDP, and ICMP and provides an unreliable service.  It lets the upper layer protocols that use it worry about reliability.  IP will perform as a connectionless service because it handles each datagram as an independent entity.  IP performs packet switching and path determination by maintaining tables that indicate where to send a packet based on its IP address.  IP gets the destination address from the packet.  IP receives segments from the Host-to-Host layer and fragments them into packets.  IP will then reassemble the packets into segments on the receiving end to send to the Host-to-Host layer.  Each packet has the source and destination IP address.  Each router will make path determinations based on the destination IP address.

ICMP (Internet Control Message Protocol)

The Internet Control Message Protocol (ICMP) is a network-layer Internet protocol that provides message packets to report errors and other information regarding IP packet processing back to the source.  ICMP utilizes IP to carry the ICMP data within it through a network.

ICMPs generate several kinds of useful messages, including Destination Unreachable, Echo Request and Reply, Redirect, Time Exceeded, and Router Advertisement and Router Solicitation. If an ICMP message cannot be delivered, no second one is generated. This is to avoid an endless flood of ICMP messages.

When an ICMP destination-unreachable message is sent by a router, it means that the router is unable to send the package to its final destination.  The router then discards the original packet.  Destination-unreachable messages include four basic types: network unreachable, host unreachable, protocol unreachable, and port unreachable.

• Network-unreachable messages usually mean that a failure has occurred in the routing or addressing of a packet.
• Host-unreachable messages usually indicates delivery failure, such as a wrong subnet mask.
• Protocol-unreachable messages generally mean that the destination does not support the upper-layer protocol specified in the packet.
• Port-unreachable messages imply that the TCP socket or port is not available.

An ICMP echo-request message, which is generated by the ping command, is sent by any host to test node reachability across an internetwork.  The ICMP echo-reply message indicates that the node can be successfully reached.  PING - Packet Internet Gropher, uses echo message to test physical connectivity.

An ICMP Redirect message is sent by the router to the source host to stimulate more efficient routing. The router still forwards the original packet to the destination. ICMP redirects allow host routing tables to remain small because it is necessary to know the address of only one router, even if that router does not provide the best path. Even after receiving an ICMP Redirect message, some devices might continue using the less-efficient route.

An ICMP Time-exceeded message is sent by the router if an IP packet's Time-to-Live field (expressed in hops or seconds) reaches zero.  The Time-to-Live field prevents packets from continuously circulating the internetwork if the internetwork contains a routing loop.  Routers discard packets that have reached their maximum hop count and tell the source machine that the packet is expired.  Traceroute - uses ICMP timeouts to find the path a packet takes through the internetwork.

Used to find the MAC address from the known IP address. ARP sends a broadcast asking for the machine with the specified IP address to respond with its MAC address.  If two devices want to communicate, the first device can send a broadcast ARP message requesting the physical address for a specified IP address.  The receiving device responds with its IP address and the first device maintains the entry in its ARP cache.  If a device doesn't exist on the same subnet, the sending device addresses the the default gateway's physical address and sends the packet to the default gateway.

This protocol is used to find an IP address when the MAC address is known.  A machine sends a broadcast with its MAC address and requests its IP address.  An example of a device that uses RARP is a diskless workstation.  Since it can't store its logical network address, it sends its MAC address to a RARP server to requests its IP address.  A RARP server responds to the RARP request with the device's IP address.