Information Technology Reference
In-Depth Information
The datagram can contain up to 65 536 bytes (64 kB) of data. If the data to be transmitted
is less than, or equal to, 64 kB, then it is sent as one datagram. If it is more than this then the
sender splits the data into fragments and sends multiple datagrams. When transmitted from
the source each datagram is routed separately through the internet and the received fragments
are finally reassembled at the destination.
The fields in the IP datagram are:
•
Version
. The TCP/IP
version number
helps gateways and nodes interpret the data unit
correctly. Differing versions may have a different format. Most current implementations
will have a version number of four (IPv4).
•
Type of service
. The
type of service
bit field is an 8-bit bit pattern in the form
PPPDTRXX
, where
PPP
defines the priority of the datagram (from 0 to 7). The precedence
levels are:
111 (network control)
110 (Internetwork control) 101 (CRITIC/ECP)
100 (flash override)
011 (flash)
010 (immediate)
001 (priority)
000 (routine)
D
sets a low delay service (0 = normal delay, 1 = low delay).
T
sets high throughput (0 = normal throughput, 1 = high throughput).
R
sets high reliability (0 = normal reliability, 1 = high reliability).
The
XX
bits are currently not used (and set to 00).
•
Header length
(4 bits). The
header length
defines the size of the data unit in multi-
plies of four bytes (32 bits). The minimum length is five bytes and the maximum is
65 536 bytes. Padding bytes fill any unused spaces.
•
Identification
(16 bits). A value which is assigned by the sender to aid the assembly of
the frames of a datagram.
•
D
and
M
bits. A gateway may route a datagram and split it into smaller fragments. The
D
bit informs the gateway that it should not fragment the data and thus it signifies that a re-
ceiving node should receive the data as a single unit or not at all. The
M
bit is the 'more
fragments' bit and is used when data is split into fragments. The
fragment offset
contains the fragment number. The bit settings are:
D
(don't fragment) - 0 = may fragment, 1 = don't fragment.
M
(last fragment)
- 0 = last fragment, 1 = more fragments.
•
Fragment offset
(13 bits). Indicates which datagram this fragment belongs to. The frag-
ment offset is measured in units of eight bytes (64 bits). The first fragment has an offset
of zero.
•
Time-to-live
(8 bits). A datagram could propagate through the internet indefinitely. To
prevent this, the 8-bit
time-to-live
value is set to the maximum transit time in seconds
and is set initially by the source IP. Each gateway then decrements this value by a defined
amount. When it becomes zero the datagram is discarded. It also defines the maximum
