The network protocol governs the complex web of Internet-based communications. The protocol prescribes a well-specified set of procedures for these packets to follow as they zip from node to node.
The wire that connects each node — computer, modem or routing device — consists primarily of copper wire or fiber-optic cables. Originally designed by the U. But its commercial usage paled in comparison to competing approaches, like Novell Inc. The real power of the Internet is the fact that anybody can talk to anybody. Deciding which gateway to use is called routing.
IP makes the routing decision for each individual packet. Internet gateways are commonly and perhaps more accurately referred to as IP routers because they use Internet Protocol to route packets between networks.
However, if a host is connected to more than one network called a multi-homed host , it can forward packets between the networks. When a multi-homed host forwards packets, it acts just like any other gateway and is in fact considered to be a gateway. Figure shows the use of gateways to forward packets. The hosts or end systems process packets through all four protocol layers, while the gateways or intermediate systems process the packets only up to the Internet Layer where the routing decisions are made.
Systems can deliver packets only to other devices attached to the same physical network. Packets from A1 destined for host C1 are forwarded through gateways G1 and G2. Host A1 first delivers the packet to gateway G1 , with which it shares network A. Gateway G1 delivers the packet to G2 over network B. Gateway G2 then delivers the packet directly to host C1 because they are both attached to network C.
Host A1 has no knowledge of any gateways beyond gateway G1. It sends packets destined for both networks C and B to that local gateway and then relies on that gateway to properly forward the packets along the path to their destinations. Likewise, host C1 sends its packets to G2 to reach a host on network A , as well as any host on network B. Figure shows another view of routing. This figure emphasizes that the underlying physical networks a datagram travels through may be different and even incompatible.
Host A1 on the token ring network routes the datagram through gateway G1 to reach host C1 on the Ethernet. Gateway G1 forwards the data through the X. The datagram traverses three physically different networks, but eventually arrives intact at C1. As a datagram is routed through different networks, it may be necessary for the IP module in a gateway to divide the datagram into smaller pieces.
A datagram received from one network may be too large to be transmitted in a single packet on a different network. This condition occurs only when a gateway interconnects dissimilar physical networks. Each type of network has a maximum transmission unit MTU , which is the largest packet that it can transfer.
This process is called fragmentation. Think of a train delivering a load of steel. In the same way that a railroad is physically different from a highway, an Ethernet is physically different from an X. The format of each fragment is the same as the format of any normal datagram. Header word 2 contains information that identifies each datagram fragment and provides information about how to re-assemble the fragments back into the original datagram.
The Identification field identifies what datagram the fragment belongs to, and the Fragmentation Offset field tells what piece of the datagram this fragment is.
When IP receives a datagram that is addressed to the local host, it must pass the data portion of the datagram to the correct Transport Layer protocol. This is done by using the protocol number from word 3 of the datagram header. Each Transport Layer protocol has a unique protocol number that identifies it to IP. Protocol numbers are discussed in Chapter 2. You can see from this short overview that IP performs many important functions. This protocol is part of the Internet Layer and uses the IP datagram delivery facility to send its messages.
When datagrams arrive too fast for processing, the destination host or an intermediate gateway sends an ICMP Source Quench Message back to the sender. This tells the source to stop sending datagrams temporarily. If the unreachable destination is a network or host, the message is sent by an intermediate gateway. But if the destination is an unreachable port, the destination host sends the message.
We discuss ports in Chapter 2. A gateway sends the ICMP Redirect Message to tell a host to use another gateway, presumably because the other gateway is a better choice. This message can be used only when the source host is on the same network as both gateways. To better understand this, refer to Figure If a host on the X. On the other hand, if a host on the token ring network sent a datagram to G1 , the host could not be redirected to use G2.
This is because G2 is not attached to the token ring. When a system receives an echo message, it replies and sends the data from the packet back to the source host.
The ping command uses this message. TCP provides reliable data delivery service with end-to-end error detection and correction. UDP provides low-overhead, connectionless datagram delivery service.
Both protocols deliver data between the Application Layer and the Internet Layer. Applications programmers can choose whichever service is more appropriate for their specific applications.
The User Datagram Protocol gives application programs direct access to a datagram delivery service, like the delivery service that IP provides.
This allows applications to exchange messages over the network with a minimum of protocol overhead. UDP is an unreliable, connectionless datagram protocol.
Within your computer, UDP will deliver data correctly. UDP uses bit Source Port and Destination Port numbers in word 1 of the message header to deliver data to the correct applications process. Figure shows the UDP message format. Why do applications programmers choose UDP as a data transport service? There are a number of good reasons.
If the amount of data being transmitted is small, the overhead of creating connections and ensuring reliable delivery may be greater than the work of re-transmitting the entire data set. Applications that fit a query-response model are also excellent candidates for using UDP. The response can be used as a positive acknowledgment to the query.
Imposing another layer of acknowledgment on any of these types of applications is inefficient. Applications that require the transport protocol to provide reliable data delivery use TCP because it verifies that data is delivered across the network accurately and in the proper sequence.
TCP is a reliable , connection-oriented , byte-stream protocol. Simply stated, a system using PAR sends the data again unless it hears from the remote system that the data arrived OK. The unit of data exchanged between cooperating TCP modules is called a segment see Figure Each segment contains a checksum that the recipient uses to verify that the data is undamaged.
If the data segment is received undamaged, the receiver sends a positive acknowledgment back to the sender. If the data segment is damaged, the receiver discards it. After an appropriate timeout period, the sending TCP module re-transmits any segment for which no positive acknowledgment has been received. TCP is connection-oriented. It establishes a logical end-to-end connection between the two communicating hosts.
Control information, called a handshake , is exchanged between the two endpoints to establish a dialogue before data is transmitted. TCP indicates the control function of a segment by setting the appropriate bit in the Flags field in word 4 of the segment header.
The type of handshake used by TCP is called a three-way handshake because three segments are exchanged. Figure shows the simplest form of the three-way handshake. It is the principal communications protocol responsible for the formats and rules for exchanging data and messages between computers on a single network or several internet-connected networks.
Its main purpose is to deliver data packets between the source application or device and the destination using methods and structures that place tags, such as address information, within data packets. TCP and IP are separate protocols that work together to ensure data is delivered to its intended destination within a network. IP obtains and defines the address—the IP address—of the application or device the data must be sent to.
TCP is then responsible for transporting and routing data through the network architecture and ensuring it gets delivered to the destination application or device that IP has defined. In other words, the IP address is akin to a phone number assigned to a smartphone. TCP is the computer networking version of the technology used to make the smartphone ring and enable its user to talk to the person who called them.
It was developed by the United States Department of Defense to enable the accurate and correct transmission of data between devices. It breaks messages into packets to avoid having to resend the entire message in case it encounters a problem during transmission. Packets are automatically reassembled once they reach their destination.
Every packet can take a different route between the source and the destination computer, depending on whether the original route used becomes congested or unavailable. As a connection based protocol, the TCP establishes and maintains a connection between applications or devices until they finish exchanging data. It determines how the original message should be broken into packets, numbers and reassembles the packets, and sends them on to other devices on the network, such as routers, security gateways, and switches, then on to their destination.
TCP also sends and receives packets from the network layer, handles the transmission of any dropped packets, manages flow control, and ensures all packets reach their destination. A good example of how this works in practice is when an email is sent using SMTP from an email server. To start the process, the TCP layer in the server divides the message into packets, numbers them, and forwards them to the IP layer, which then transports each packet to the destination email server.
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