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All this conceptual talk about the OSI reference model and TCP/IP s layers is nice and all, but how does it come together in practical terms When data works its way through a network, it must be in a format that both sender and receiver can understand TCP/IP accommodates this communication with its Internet layer This layer maps directly to the OSI model s network layer and is based on a fixed-message format called the IP datagram Think of a datagram as a basket that holds all the parts of your message For instance, when you download a Web page to your browser, what you finally see was delivered piece by piece inside datagrams
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Datagrams and packets are easily confused: A datagram is a unit of data, whereas a packet is the physical message entity (created at levels 3 and up) that actually passes through a network Although the terms are used interchangeably (like engine and motor ), they really are two different things but we don t need to bore you with anything other than a high-level explanation The important thing to note here is that messages are broken into pieces, sent across the network, and then reassembled at the receiving station
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The IP Datagram Format
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Taking a closer look at a IP version 4 datagram, we see a number of fields for both the payload and message-handling data Figure 4-3 illustrates a datagram s layout If you take another look at the datagram format from Figure 4-3, you ll notice that the leftmost fields are a consistent size in each datagram Without some standardization in these fields, the resulting bits would be an undecipherable mess of zeroes and ones To the right of the datagram are the variable-length packets There are two basic components on each datagram: the header and the data portion Think of a datagram as the Space Shuttle The header can be thought of as all the computers in the shuttle s cockpit It handles all the information needed by the various routers and computers that will send the packet on its way, and will be used to maintain order when it s time to bring this packet together with its siblings to form the message The payload is like the Shuttle s cargo bay it s what s being carried, either by the Shuttle or the datagram This is what the various datagram fields do: VER The version of IP being used by the station that originated the message The current version is IP version 4 This field lets different versions coexist in an internetwork HLEN For header length, this tells the receiver how long the header will be so that the central processing unit (CPU) knows where the data field begins
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Frag Offset
IP Options
Protocol
Padding
Length
HLEN
Frags
Figure 4-3 The IP datagram format is variable in length
VER 4
Data
Variable length
4:
TCP/IP and Name Resolution
Service type A code to tell the router how the packet should be handled in terms of level of service (reliability, precedence, delay, and so on) Length The total number of bytes in the entire packet, including all header fields and the data field ID, frags, and frags offset These fields identify for the router how to handle packet fragmentation and reassembly, and how to offset for different frame sizes that might be encountered as the packet travels through different LAN segments using different networking technologies, such as Ethernet, Fiber Distributed Data Interface (FDDI), and so on TTL Short for Time to Live, a number that is decremented by one each time the packet is forwarded When the counter reaches zero, the packet is dropped TTL prevents router loops and lost packets from endlessly wandering internetworks Protocol The transport protocol that should be used to handle the packet This field almost always identifies TCP as the transport protocol to use, but certain other transports can be used to handle IP packets Header checksum A checksum is a numerical value used to help ensure message integrity If the checksums in all the message s packets don t add up to the right value, the station knows that the message was garbled Source IP address The 32-bit address of the host that originated the message (usually a PC or a server) Destination IP address The 32-bit address of the host to which the message is being sent (usually a PC or a server) IP options Used for network testing and other specialized purposes Padding Fills in any unused bit positions so that the CPU can correctly identify the first bit position of the data field Data The payload being sent For example, a packet s data field might contain some of the text making up an e-mail
So why bother with breaking the message into pieces and then transmitting them It s sort of the same reason why the president and vice president don t fly together If a bird gets sucked into Air Force One s engine and it goes down, we still have the vice president to lead the country If a message is corrupted during transmission, only that portion of the message needs to be resent, not the entire thing Another benefit is that no lone host needs to wait an inordinate amount of time before being able to send its own message
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