Section 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.
