Intro to Networks TCP / IP REFERENCE MODEL The TCP/IP model was the basis for the earliest computer network, the ARPANET. Now the standard model for the successor to ARPANET, the Internet. It is a less well-defined model than the OSI model. A very flexible architecture.
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Intro to Networks
TCP / IP REFERENCE MODEL
The OSI model is unnecessarily complex. The TCP/IP model is poorly defined. Alternatively, we can develop a hybrid model that is easier to understand.
The Network Layer
Many different types of networks exists, with many different protocols in every layer. When 2 or more networks are linked together, they form an internetwork.
THE NETWORK LAYER & THE INTERNET
The internet can be considered as a collection of subnetworks. There is no “set in stone” architecture for the internet. But several major “backbones” exist.
In this virtual circuit, the subnet sees that the connection between point 1 and point 2 is remote and builds a pathway between these points.
An alternative method of networking is the datagram method. Here, the only service the network layer provides to the transport layer is to inject datagrams into the subnet and hope they get to their destination.
IP Packet Format
An IP packet consists of a header and text. The header has a fixed 20-byte part and an optional variable-length part.
Version field: keeps track of the protocol version used to create the packet.
IHL field: describes the length of the header field, in 32-bit words. Minimum value of 5. Maximum of 15.
Type of Service field: allows host to tell the subnet the types of delivery, accuracy, and reliability it wants.
Total Length field: includes everything in the packet. Both header and data. Maximum of 65,535 bytes.
Identification field: needed to allow a destination to determine which datagram a newly arrived fragment belongs to. All fragments of a datagram contain the same identification value.
DF: “Don’t Fragment”. This orders routers not to fragment the message, since the destination is incapable of reassembling it.
MF: “More Fragments”. Indicates that more fragments are being transmitted after this. This bit is zero for the final fragment.
Fragment Offset : tells where in the current datagram this fragment belongs. Allows the receiver to reassemble the entire datagram from its parts.
Time to Live field : a counter used to limit packet lifetimes. Allows for a maximum of 255 seconds. It is decremented by a router after each hop. At zero, the packet is discarded and a warning is sent back to the sender. Prevents packets from wandering around the internet forever.
Protocol field : tells the network layer what to do with the completely reassembled message. Which transport process does this go to ? TCP or UDP are possibilities.
Header Checksum field : verifies the header only, not the frame data. This can detect errors that sometimes occur due to router problems. The header checksum is recomputed after each hop.
IP Address Formats
Unique addresses are assigned by the Network Information Center (NIC). These addresses are usually written in “dotted decimal notation”.
Classless InterDomain Routing (CIDR) :
Due to the popularity of the internet, the IP is rapidly running out of unique addresses. CIDR allocates the remaining unused class C network addresses. Four areas of the world have each been allocated about 32 million addresses.
Central & South America
Asia & Pacific
IPv6 : This is a newer version of IP which, hopefully, will never run out of addresses.