IPv4
Contents
Version
Internet header lenght
Type of service
Type of Service: 8 bits
The Type of Service provides an indication of the abstract parameters of the quality of service desired. These parameters are to be used to guide the selection of the actual service parameters when transmitting a datagram through a particular network. Several networks offer service precedence, which somehow treats high precedence traffic as more important than other traffic (generally by accepting only traffic above a certain precedence at time of high load). The major choice is a three way tradeoff between low-delay, high-reliability, and high-throughput.
Bits 0-2: Precedence.
Bit 3: 0 = Normal Delay, 1 = Low Delay.
Bits 4: 0 = Normal Throughput, 1 = High Throughput.
Bits 5: 0 = Normal Relibility, 1 = High Relibility.
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
The use of the Delay, Throughput, and Reliability indications may increase the cost (in some sense) of the service. In many networks better performance for one of these parameters is coupled with worse performance on another. Except for very unusual cases at most two of these three indications should be set.
Total Length
Дължината на дейтаграмата, измерва се в октети (байтове). Включва интернет хедъра и данните. Максималната дължина на дейтаграмата е 65 535 октета.
Минималната допустима дължина е 576 октета.
Identification
16 bits
An identifying value assigned by the sender to aid in assembling the fragments of a datagram.
This field is used for uniquely identifying the IP datagrams. This value is incremented every-time an IP datagram is sent from source to the destination. This field comes in handy while reassembly of fragmented IP data grams.
Flags
Flags: 3 bits
- 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. If this field contains the value zero, then the datagram must be destroyed. This field is modified in internet header processing. The time is measured in units of seconds, but since every module that processes a datagram must decrease the TTL by at least one even if it process the datagram in less than a second, the TTL must be thought of only as an upper bound on the time a datagram may exist. The intention is to cause undeliverable datagrams to be discarded, and to bound the maximum datagram lifetime.
Protocol
8 bits
This field indicates the next level protocol used in the data
portion of the internet datagram. The values for various protocols are specified in "Assigned Numbers"
Header Checksum
Protocol
8 bits
http://www.iana.org/assignments/protocol-numbers/protocol-numbers.xml
This field indicates the next level protocol used in the data
portion of the internet datagram. The values for various protocols are specified in "Assigned Numbers"
Header Checksum
Options
The options might not end on a 32-bit boundary. The internet header must be filled out with octets of zeros. The first of these would be interpreted as the end-of-options option, and the remainder as internet header padding.
Every internet module must be able to act on every option. The Security Option is required if classified, restricted, or compartmented traffic is to be passed.
