Graphics Programs Reference
In-Depth Information
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Network Layer
The network layer is like a worldwide postal service providing an addressing
and delivery method used to send things everywhere. The protocol used at
this layer for Internet addressing and delivery is, appropriately, called Internet
Protocol (IP); the majority of the Internet uses IP version 4.
Every system on the Internet has an IP address, consisting of a familiar
four-byte arrangement in the form of
xx.xx.xx.xx
. The IP header for packets
in this layer is 20 bytes in size and consists of various fields and bitflags as
defined in RFC 791.
From RFC 791
[Page 10]
September 1981
Internet Protocol
3. SPECIFICATION
3.1. Internet Header Format
A summary of the contents of the internet header follows:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example Internet Datagram Header
Figure 4.
Note that each tick mark represents one bit position.
This surprisingly descriptive ASCII diagram shows these fields and their
positions in the header. Standard protocols have awesome documentation.
Similar to the Ethernet header, the IP header also has a protocol field to
describe the type of data in the packet and the source and destination
addresses for routing. In addition, the header carries a checksum, to help
detect transmission errors, and fields to deal with packet fragmentation.
The Internet Protocol is mostly used to transmit packets wrapped in
higher layers. However, Internet Control Message Protocol (ICMP) packets
