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. • The OSI Application, Presentation, and Session layers are combined into a single Application layer. • The OSI layers most concerned with data transmission have been combined into a single Host-To-Network layer.
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.
INTERNETWORKING : 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.
Internetworking Devices: • Repeaters: amplify or regenerate weak signals. Used with long-distance cables. • Bridges: store-and-forward forward devices. • Multiprotocol routers: found in the network layer. Takes packets from one line and forward them to another. Similar to bridges, except that these routers can handle the multiple protocols found on different networks. • Transport gateway: make a connection between to networks at the transport layer. • Application gateway: connect application layers together.
The common thread for all internet networks is the network layer protocol, Internet Protocol (IP). This protocol was designed from the start with the internet in mind. • The purpose of IP is to transport datagrams from source to destination, regardless of any other networks that lie in between. • The process begins with the transport layer taking a data stream and breaking it up into datagrams up to 64 kb in size. Each datagram is transmitted through the internet (network layer) to the destination. Then the datagrams are reassembled and given to the transport layer.
The network layer’s purpose is to get packets all the way from the source to their destination. • The data link layer is concerned with moving data from one end of a physical path to another. • The network layer has to worry about hopping through multiple routers along the path. • This implies that the network layer must have some prior knowledge of the topology of the communication subnet (other routers) so that it can provide an appropriate path through it. • It must also choose paths to prevent overloading communication lines.
Services provided to the Transport Layer : • The network layer provides services to the transport layer at the network layer/transport layer interface. This interface is often also the interface between a carrier and a customer. In other words, this interface is often the boundary of the subnet. • The carrier often has control of all the protocols and interfaces up to and including the network layer.
The network layer must perform 3 goals: • The services should be independent of the subnet technology. • The transport layer should be shielded from the number, type, and topology of the subnets present. • The network addresses made available to the transport layer should use a uniform numbering plan, even across LANs and WANs.
The network layer can provide connectionless service or connection-oriented service. • Packets of data in a connectionless service are often called datagrams. • Physical circuits connect telephones. For subnets, we do not have an actual hard-wired pathway. Therefore, subnets are considered to be connected together through a “virtual circuit”. • Virtual circuits in subnets are usually concerned with connection-oriented service.
In a virtual circuit, a pathway is developed between sender and receiver when a connection is established. This pathway is used for all packets between the two machines. When communication is complete, the virtual circuit is terminated. • For connectionless service, no routes are developed in advance. A datagram subnet packet is routed without any regard as to how the previous packet was routed. Sequential packets may take different routes to their destination. • Datagram subnets are less efficient, but extremely robust and adaptable.
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.
Connectionless Networking: 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.
Virtual Circuit • If packets travel through a virtual circuit along a known route, then each router along the path must remember where to forward packets for every currently open virtual circuit. • When a network connection is initiated, a virtual circuit number not already in use on that machine is chosen as a connection identifier. This identifier is only valid locally. It is not a global identifier.
Connectionless Service • For connectionless service (datagrams), routers have a table containing information about which outgoing line to use to reach each possible destination router. • Each datagram must contain the full destination address. Routers compares the destination to a table in order to send the packet to the next router.
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.
Source & Destination Address fields: Every host and router on the internet has an IP address that contains both its network number and its host number. No two machines can have the same IP address. • All IP addresses are 32 bits long. Machines connected to multiple networks have different IP addresses on each network. • Options field:
IP Address Formats Unique addresses are assigned by the Network Information Center (NIC). These addresses are usually written in “dotted decimal notation”.
Subnets • All hosts on a network must have the same network number. As networks grow, they can exceed the number of addresses available for that particular network. • The solution is to allow networks to be split into smaller subsets or subnets, but act like a single network to the outside world. • This is done by splitting the host number.
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. Europe North America Central & South America Asia & Pacific
IPv6 : This is a newer version of IP which, hopefully, will never run out of addresses.