ch03 IP Addressing.pdf - The Cisco Learning Network

8 Chapter 3: IP Addressing
Internet Layer
is connectionless, there is no need for sequence and acknowledgment numbers. And
second, since there is no flow control, there is no need for a window size field. As you
can see, UDP is a lot simpler, and more efficient, than TCP. Any control functions that
need to be implemented for the connection are not done at the transport layer--instead,
these are handled at the application layer.
Layer-3 of the TCP/IP protocol stack is called the Internet layer. The corresponding layer
in the OSI Reference Model is the network layer. The Internet Protocol (IP) is just one
of the protocols that reside at this layer. It is very
common in the industry to hear people refer to
IP provides a
connectionless, unreliable connection to
other devices. If reliability and flow control
are required, TCP (transport layer) can
provide this.
TCP/IP as just “IP”; however, this is a misnomer,
since IP is just one of many protocols within
TCP/IP. Other IP protocols include ARP, RARP,
ICMP, OSPF, and others. The next few sections
explain the components of an IP packet and some
of the protocols that function at the Internet layer.
IP Datagram
Where the transport layer uses segments to transfer information between machines,
the Internet layer uses datagrams. Datagram is just another word for packet. Table 3-4
shows the components of the IP datagram. Without
any options, the IP header is 20 bytes in length.
The main function of the IP datagram is to
IP is uses a TTL field to
limit the number of hops a packet can
travel. Here are some common protocols
and their protocol numbers: ICMP (1),
IGRP (9), IPv6 (41), TCP (6), and UDP (17).
carry protocol information for either Internet layer
protocols or encapsulated transport layer protocols.
To designate what protocol the IP datagram is
carrying in the data field, the IP datagram carries
the protocol’s number in the Protocol field of the
datagram.
ICMP
The Internet Control Message Protocol (ICMP) is used to send error and control
information between TCP/IP devices. ICMP, defined in RFC 792, includes many
different messages that devices can generate or respond to. Here is a list of these
messages: Address Reply, Address Request, Destination Unreachable, Echo, Echo
Reply, Information Reply, Information Request, Parameter Problem, Redirect,
Subnet Mask Request, Time Exceeded, Timestamp, and Timestamp Reply.