System information

The Network Layer
Book Title
7-92
Figure 7-1 The Internet Protocol Suite and the OSI Reference Model
Creation and documentation of the Internet Protocol suite closely resemble an academic research
project. The protocols are specified in documents called Requests for Comments (RFCs). RFCs are
published and then reviewed and analyzed by the Internet community. Protocol refinements are
published in new RFCs. Taken together, the RFCs provide a colorful history of the people,
companies, and trends that have shaped the development of what is today the world's most popular
open-system protocol suite.
The Network Layer
IP is the primary Layer 3 protocol in the Internet protocol suite. In addition to internetwork routing,
IP provides fragmentation and reassembly of datagrams and error reporting. Along with TCP, IP
represents the heart of the Internet Protocol suite. The IP packet format is shown in Figure 7-2.
Figure 7-2 The IP Packet Format
OSI reference model
Application7
Presentation6
Session5
Transport4
Network3
Link2
Physical
Internet Protocol suite
NFS
XDR
RPC
FTP, Telnet,
SMTP, SNMP
TCP, UDP
IPRouting protocols
ARP, RARP
Not specified
1
ICMP
Version
Identification Flags Fragment offset
Time-to-live Protocol Header checksum
Source address
32 bits
Destination address
Options (+ padding)
Data (variable)
Type-of-service Total lengthIHL