Data-Link Layer-Framing Takes Placeĭata-link layer protocols, such as PPP, format the IP datagram into a frame. In case the datagram exceeds the allowable byte size for network packets and must be fragmented. Information in the IP header includes the IP addresses of the sending and receiving hosts, datagram length, and datagram sequence order. IP attaches an IP header to the segment or packet's header in addition to the information added by TCP or UDP. So they can be delivered effectively to the receiving host. IP then determines the IP addresses for the datagrams, IP prepares them for delivery by formatting them into units called IP datagrams.
Internet LayerĪs shown in Figure 4-1, both TCP and UDP pass their segments and packets down to the Internet layer, where they are handled by the IP protocol. UDP does not use the three-way handshake. The application layer determines whether the receiving UDP process acknowledges that the packet was received. The sending UDP process attempts to send the packet to its peer UDP process on the receiving host. UDP attaches a header to each packet, whichĬontains the sending and receiving host ports, a field with the length of the packet, and a checksum. Instead, UDP takes the message received from the application layer and formats it into UDP packets. Unlike TCP, it does not check to make sure that data arrived at the receiving host. This exchange of control information is referred to as a three-way handshake. The sending TCP sends another ACK segment, then proceeds to send the data. The receiving TCP returns a segment called an ACK to acknowledge the When the sending TCP wants to establish connections, it sends a segment called a SYN to the peer TCP protocol running on the receiving host. TCP uses segments to determine whether the receiving host is ready to receive the data. The TCP protocols on both hosts use the checksum data to determine whether data has transferred without error. Segment headers contain sender and recipient ports, segment ordering information, and a data field known as a checksum. TCP divides the data received from theĪpplication layer into segments and attaches a header to each segment. Figure 4-1 shows how the TCP protocol receives the stream from the rlogin command. TCP is often called a "connection-oriented" protocol because it ensures the successful delivery of data to the receiving host. The end result depends on whether TCP or UDP has handled the information.
When the data arrives at the transport layer, the protocols at the layer start the process of data encapsulation. Transport Layer-Data Encapsulation Begins This type of packet is referred to as a message. Therefore, the packet containing the command must be formatted in a manner that UDPĮxpects. NIS+ uses the UDP transport layer protocol. Suppose a user wants to mount a file system on a remote host, thus initiating the NIS+ application layer protocol. Not all application layer protocols use TCP, however. Therefore, rlogin sends this data as a TCP stream. TCP expects to receive data in the form of a stream of bytes containing the information The rlogin command uses the TCP transport layer protocol. Suppose the user issues an rlogin command to log in to the remote host, as shown in Figure 4-1. The application protocol associated with the command or message formats the packet so that it can be handled by the appropriate transport layer protocol, TCP or UDP. The packet's history begins when a user on one host sends a message or issues a command that must access a remote host. Application Layer-User Initiates Communication This section summarizes the life cycle of a packet from the time the user issues a command or sends a message to the time it is received by the appropriate application on the receiving host. Figure 4-1 How a Packet Travels Through the TCP/IP Stack Moreover, each layer has a different term for the altered packet, as shown in the following figure. When a protocol on the sending host adds data to the packet header, the process is called data encapsulation. As the packet travels through the TCP/IP protocol stack, the protocols at each layer eitherĪdd or remove fields from the basic header. The packet is the basic unit of information transferred across a network, consisting, at a minimum, of a header with the sending and receiving hosts' addresses, and a body with the data to be transferred. Data Encapsulation and the TCP/IP Protocol Stack
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