When the IP protocol encapsulates the transport layer packet How does it know to which upper layer protocol UDP or TCP the packet should be delivered?

TCP/IP is a large family of protocols that is named after its two most important members. Figure 1 shows the TCP/IP protocols used by CICS® TCP/IP, in terms of the layered Open Systems Interconnection (OSI) model, which is widely used to describe data communication systems. For CICS users who might be more accustomed to SNA, the left side of Figure 1 shows the SNA layers, which correspond very closely to the OSI layers.

Figure 1. TCP/IP protocols compared to the OSI model and SNA

The protocols implemented by TCP/IP Services and used by CICS TCP/IP are shown in the right hand column in Figure 1: Transmission Control Protocol (TCP) In terms of the OSI model, TCP is a transport-layer protocol. It provides a reliable virtual-circuit connection between applications; that is, a connection is established before data transmission begins. Data is sent without errors or duplication and is received in the same order as it is sent. No boundaries are imposed on the data; TCP treats the data as a stream of bytes. User Datagram Protocol (UDP) UDP is also a transport-layer protocol and is an alternative to TCP. It provides an unreliable datagram connection between applications. Data is transmitted link by link; there is no end-to-end connection. The service provides no guarantees. Data can be lost or duplicated, and datagrams can arrive out of order. Internet Protocol (IP) In terms of the OSI model, IP is a network-layer protocol. It provides a datagram service between applications, supporting both TCP and UDP.

IP Datagram Encapsulation and Formatting IP Datagram General Format

In the chapter describing the OSI Reference Model, I looked at several ways that protocols at various layers in a networking protocol stack interact with each other. One of the most important concepts in inter-protocol operation is that of encapsulation. Most data originates within the higher layers of the OSI model. The protocols at these layers pass the data down to lower layers for transmission, usually in the form of discrete messages. Upon receipt, each lower-level protocol takes the entire contents of the message received and encapsulates it into its own message format, adding a header and possibly a footer that contain important control information. Encapsulation is explained in general terms in a separate topic.

A good analogy for how encapsulation works is a comparison to sending a letter enclosed in an envelope. You might write a letter and put it in a white envelope with a name and address, but if you gave it to a courier for overnight delivery, they would take that envelope and put it in a larger delivery envelope. (I actually have written a complete description of this sort of analogy, if you are interested.)

Due to the prominence of TCP/IP, the Internet Protocol is one of the most important places where data encapsulation occurs on a modern network. Data is passed to IP typically from one of the two main transport layer protocols: TCP or UDP. This data is already in the form of a TCP or UDP message with TCP or UDP headers. This is then encapsulated into the body of an IP message, usually called an IP datagram or IP packet. Encapsulation and formatting of an IP datagram is also sometimes called packaging—again, the implied comparison to an envelope is obvious. The process is shown in Figure 85.

Figure 85: IP Datagram Encapsulation

This is an adaptation of Figure 15, the very similar drawing for the OSI Reference Model as a whole, showing specifically how data encapsulation is accomplished in TCP/IP. As you can see, an upper layer message is packaged into a TCP or UDP message. This then becomes the payload of an IP datagram, which is shown here simply with one header (things can get a bit more complex than this.) The IP datagram is then passed down to layer 2 where it is in turn encapsulated into some sort of LAN, WAN or WLAN frame, then converted to bits and transmitted at the physical layer.

If the message to be transmitted is too large for the size of the underlying network, it may first be fragmented. This is analogous to splitting up a large delivery into multiple smaller envelopes or boxes. In this case, each IP datagram carries only part of the higher-layer message. The receiving device must reassemble the message from the IP datagrams. So, a datagram doesn't always carry a full higher-layer message; it may hold only part of one.

The IP datagram is somewhat similar in concept to a frame used in Ethernet or another data link layer. The important difference, of course, is that IP datagrams are designed to facilitate transmission across an internetwork, while data link layer frames are used only for direct delivery within a physical network. The fields included in the IP header are used to manage internetwork datagram delivery. This includes key information for delivery such as the address of the destination device, identification of the type of frame, and control bits. The header follows a specific format described in the following topic.

After data is encapsulated into an IP datagram, it is passed down to the data link layer for transmission across the current “hop” of the internetwork. There, it is of course further encapsulated, IP header and all, into a data link layer frame such as an Ethernet frame. An IP datagram may be encapsulated into many such data link layer frames as it is routed across the internetwork; on each hop the IP datagram is removed from the data link layer frame and then repackaged into a new one for the next hop. The IP datagram, however, is not changed (except for some control fields) until it reaches its final destination.

IP Datagram Encapsulation and Formatting IP Datagram General Format

The TCP/IP Guide (http://www.TCPIPGuide.com)

© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.

BC 440 - Quiz 2 .prepared 8.30.19.docx

Macroeconomicss T12&13.docx

Cabling and Systems Worksheet v33-1.docx

kking0038mft6106week4.docx

Muscle Rules KEY (1).docx

A27C9E65-6410-4688-AA86-C4FECEBEDC02.jpeg

ISDS 361B Ch 4 Quiz with Ans.docx