Section G.1. IP Datagram Header

G.1 IP Datagram Header

This description is taken from pages 11 to 15 of RFC 791, Internet Protocol.

Internet Header Format                                            0                   1                   2                   3         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |Version|  IHL  |Type of Service|          Total Length         |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |         Identification        |Flags|      Fragment Offset    |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |  Time to Live |    Protocol   |         Header Checksum       |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |                       Source Address                          |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |                    Destination Address                        |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |                    Options                    |    Padding    |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      Version:  4 bits        The Version field indicates the format of the internet header.       This document describes version 4.      IHL:  4 bits        Internet Header Length is the length of the internet header in 32      bit words.  The minimum value for a correct header is 5.      Type of Service:  8 bits        The Type of Service indication the quality of service desired.      The meaning of the bits is explained below.          Bits 0-2:  Precedence.        Bit    3:  0 = Normal Delay,      1 = Low Delay.        Bits   4:  0 = Normal Throughput, 1 = High Throughput.        Bits   5:  0 = Normal Reliability 1 = High Reliability.        Bit  6-7:  Reserved for Future Use.             0     1     2     3     4     5     6     7        +-----+-----+-----+-----+-----+-----+-----+-----+        |                 |     |     |     |     |     |        |   PRECEDENCE    |  D  |  T  |  R  |  0  |  0  |        |                 |     |     |     |     |     |        +-----+-----+-----+-----+-----+-----+-----+-----+            Precedence              111 - Network Control            110 - Internetwork Control            101 - CRITIC/ECP            100 - Flash Override            011 - Flash            010 - Immediate            001 - Priority            000 - Routine      Total Length:  16 bits        Total Length is the length of the datagram, measured in octets       (bytes), including internet header and data.      Identification:  16 bits        An identifying value assigned by the sender to aid in assembling      the fragments of a datagram.      Flags:  3 bits        Various Control Flags.  The Flag bits are explained below:          Bit 0: reserved, must be zero        Bit 1: (DF) 0 = May Fragment,  1 = Don't Fragment.        Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments.              0   1   2          +---+---+---+          |   | D | M |          | 0 | F | F |          +---+---+---+      Fragment Offset:  13 bits        This field indicates where in the datagram this fragment belongs.      The fragment offset is measured in units of 8 octets (64 bits).      The first fragment has offset zero.      Time to Live:  8 bits        This field indicates the maximum time the datagram is allowed to      remain in the internet system.       Protocol:  8 bits        This field indicates the Transport Layer protocol that the data      portion of this datagram is passed to.  The values for various      protocols are specified in the "Assigned Numbers" RFC.      Header Checksum:  16 bits        A checksum on the header only.  Since some header fields change      (e.g., time to live), this is recomputed and verified at each      point that the internet header is processed.  The checksum      algorithm is:          The checksum field is the 16 bit one's complement of the one's        complement sum of all 16 bit words in the header.  For purposes        of computing the checksum, the value of the checksum field is        zero.      Source Address:  32 bits        The source IP address.  See Chapter 2 for a      description of IP addresses.      Destination Address:  32 bits        The destination IP address.  See Chapter 2 for a description of IP      addresses.      Options:  variable        The options may or may not appear in datagrams, but they must be      implemented by all IP modules (host and gateways).  No options     were used in any of the datagrams examined  in Chapter 13.