Data Communications Networking Devices - 4th Ed.pdf

216 __________________________________________________________ WIDE AREA NETWORKSTable 2.4FieldIP header fieldsDescriptionVersionHeader lengthType of serviceThe version of the IP protocol used to createthe datagramHeader length in 32-bit wordsSpeci®es how the datagram should be handled:0 1 2 3 4 5 6 7Precedence D T R UnusedTotal lengthIdenti®cationFlagsFragment offsetTime to liveProtocolHeader checksumSource IP addressDestination IPaddressIP optionsPrecedence indicates importance of the datagramD when set requests low delayT when set requests high throughputR when set requests high reliabilitySpeci®es the total length to include headerand dataUsed with source address to identifyfragments belonging to speci®c datagramsMiddle bit when set disables possiblefragmentationLow-order bit speci®es whether the fragmentcontains data from the middle of the originaldatagram or the endSpeci®es the offset in the original datagramof data being carried in a fragmentSpeci®es the time in seconds a datagram isallowed to remain in the InternetSpeci®es the higher level protocol used tocreate the message carried in the data ®eldProtects the integrity of the headerThe 32-bit IP address of the datagram'ssenderThe 32-bit IP address of the datagram'sintended recipientPrimarily used for network testing ordebugging:0 1 2 3 4 5 6 7Copy Option class Option numberCopy bit set tells gateways that theoption should be copied into all fragments; when set to 0 theoption is copied into the ®rst fragmentThe meaning of the class ®eld is as follows:0 Datagram or network control1Reserved for future use2 Debugging3 Reserved for future useThe option number de®nes a speci®c optionwithin a class