TCP/IP

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CCNA Guide to Cisco
Networking Fundamentals

Fourth Edition

Chapter 3

TCP/IP

CCNA Guide to Cisco Networking Fundamentals, Fourth Edition

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Objectives


Discuss the origins of TCP/IP


Identify and discuss the different layer functions of
TCP/IP


Describe the functions performed by protocols in the
TCP/IP protocol suite, including ICMP, UDP, TCP,
ARP, and RARP

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Objectives (continued)


Use Ping and Trace and describe their functions


Explain how packets are transmitted


Describe the Cisco three
-
layer hierarchical model

CCNA Guide to Cisco Networking Fundamentals, Fourth Edition

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Origins of TCP/IP


Transmission Control Protocol/Internet Protocol
(TCP/IP)


Resulted from a coordinated effort by the U.S.
Department of Defense (DOD)


Advanced Research Projects Agency (ARPA)


Charged with creating a wide area network (WAN)


Results were TCP/IP and ARPANET


DOD funded two projects


The adaptation of TCP/IP to work with UNIX


The inclusion of the TCP/IP protocol with Berkeley
UNIX (BSD UNIX)

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Overview of the TCP/IP Protocol Suite


The TCP/IP model explains how the protocol suite
works to provide communications


Four layers: Application, Transport, Internetwork, and
Network Interface


Requests for Comments (RFCs)


Define, describe, and standardize the implementation
and configuration of the TCP/IP protocol suite

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CCNA Guide to Cisco Networking Fundamentals, Fourth Edition

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Application Layer


Protocols at the TCP/IP Application layer include:


File Transfer Protocol (FTP)


Trivial File Transfer Protocol (TFTP)


Network File System (NFS)


Simple Mail Transfer Protocol (SMTP)


Terminal emulation protocol (telnet)


Remote login application (rlogin)


Simple Network Management Protocol (SNMP)


Domain Name System (DNS)


Hypertext Transfer Protocol (HTTP)

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Transport Layer


Performs end
-
to
-
end packet delivery, reliability, and
flow control


Protocols:


TCP provides reliable, connection
-
oriented
communications between two hosts


Requires more network overhead


UDP provides connectionless datagram services
between two hosts


Faster but less reliable


Reliability is left to the Application layer

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Transport Layer (continued)


Ports


TCP and UDP use port numbers for communications
between hosts


Port numbers are divided into three ranges:


Well Known Ports are those from 1 through 1,023


Registered Ports are those from 1,024 through 49,151


Dynamic/Private Ports are those from 49,152 through
65,535

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Transport Layer (continued)


TCP three
-
way handshake


Establishes a reliable connection between two points


TCP transmits three packets before the actual data
transfer occurs


Before two computers can communicate over TCP,
they must synchronize their
initial sequence
numbers (ISN)


A
reset packet (RST)

indicates that a TCP
connection is to be terminated without further
interaction


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Transport Layer (continued)


TCP
sliding windows


Control the flow and efficiency of communication


Also known as windowing


A method of controlling packet flow between hosts


Allows multiple packets to be sent and affirmed with a
single acknowledgment packet


The size of the TCP window determines the number
of acknowledgments sent for a given data transfer


Networks that perform large data transfers should use
large window sizes

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Transport Layer (continued)


TCP sliding windows (continued)


Other flow control methods include


Buffering


Congestion avoidance

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Internetwork Layer


Four main protocols function at this layer


Internet Protocol (IP)


Internet Control Message Protocol (ICMP)


Address Resolution Protocol (ARP)


Reverse Address Resolution Protocol (RARP)


ARP


A
routed protocol


Maps IP addresses to MAC addresses


ARP tables

contain the MAC and IP addresses of
other devices on the network

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Internetwork Layer (continued)


ARP (continued)


When a computer transmits a frame to a destination
on the local network


It checks the ARP cache for an IP to MAC address
mapping for the destination node


ARP request


If a source computer cannot locate an IP to MAC
address mapping in its ARP table


It must obtain the correct mapping

CCNA Guide to Cisco Networking Fundamentals, Fourth Edition

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Internetwork Layer (continued)

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Internetwork Layer (continued)


ARP request (continued)


A source computer broadcasts an ARP request to all
hosts on the local segment


Host with the matching IP address responds this
request


ARP request frame


See Figure 3
-
7


ARP cache life


Source checks its local ARP cache prior to sending
packets on the local network

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Internetwork Layer (continued)

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Internetwork Layer (continued)


ARP cache life (continued)


Important that the mappings are correct


Network devices place a timer on ARP entries


ARP tables reduce network traffic


Reverse Address Resolution Protocol (RARP)


Similar to ARP


Used primarily by diskless workstations


Which have MAC addresses burned into their network
cards but no IP addresses


Client’s IP configuration is stored on a RARP server


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Internetwork Layer (continued)


RARP request frame


See Figure 3
-
8


RARP client


Once a RARP client receives a RARP reply, it
configures its IP networking components


By copying its IP address configuration information into
its local RAM


ARP and RARP compared


ARP is concerned with obtaining the MAC address of
other clients


RARP obtains the IP address of the local host

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Internetwork Layer (continued)


ARP and RARP compared (continued)


The local host maintains the ARP table


A RARP server maintains the RARP table


The local host uses an ARP reply to update its ARP
table and to send frames to the destination


The RARP reply is used to configure the IP protocol
on the local host


Routers and ARP


ARP requests use broadcasts


Routers filter broadcast traffic


Source must forward the frame to the router

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Internetwork Layer (continued)


ARP tables


Routers maintain ARP tables to assist in transmitting
frames from one network to another


A router uses ARP just as other hosts use ARP


Routers have multiple network interfaces and
therefore also include the port numbers of their NICs
in the ARP table


The Ping utility


Packet Internet Groper (Ping)

utility verifies
connectivity between two points


Uses ICMP echo request/reply messages

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Internetwork Layer (continued)

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Internetwork Layer (continued)

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Internetwork Layer (continued)


The Trace utility


Uses ICMP echo request/reply messages


Can verify Internetwork layer (OSI
-
Network layer)
connectivity


Shows the exact path a packet takes from the
source to the destination


Accomplished through the use of the
time
-
to
-
live
(TTL)

counter


Several different malicious network attacks have
also been created using ICMP messages


Example:
ICMP flood


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Internetwork Layer (continued)

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Network Interface Layer


Plays the same role as the Data Link and Physical
layers of the OSI model


The MAC address, network card drivers, and specific
interfaces for the network card function at this level


No specific IP functions exist at this layer


Because the layer’s focus is on communication with
the network card and other networking hardware

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Understanding Frame Transmission


Each host on a segment evaluates the frame


To determine whether the listed destination MAC
address matches its own or is a broadcast to all hosts


The host makes a copy of the frame and sends the
original along the network path


On the destination host, frames are sent up the
TCP/IP stack


Removing each layer header information


For a packet to be routed on a TCP/IP internetwork


An IP address and MAC address are required for both
the source and destination hosts

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Routers on the Network


A router requires:


An IP address for every network segment to which it is
connected


A separate network interface or port for each network
segment


Computers send frames to destinations that are not
on their segment to the router (default gateway)


The router must determine which subnet should
receive the frame


The router references its
routing table

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Routers on the Network (continued)

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Network to Network


Routers maintain routing tables that they use to route
packets from one network to another


When a network uses TCP/IP, each port on a router
requires an IP address


Allows the router to correctly forward the packet to the
appropriate network segment


On a TCP/IP network, the logical addresses on a
certain segment must be matched


If you move a computer from one segment to another,
the IP address will have to be changed

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Network to Network (continued)

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Dynamic or Static Tables


Routing tables match network addresses with the
addresses of the routers that handle those networks


The tables can be built statically or dynamically


Dynamic updates are provided through
routing
protocols


A
router capable of dynamic routing can choose from
among the various routes on a network


The router communicates with other dynamic routers


To determine the most efficient route from one point to
another on the network

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Dynamic or Static Tables (continued)


Methods to determine the best path across a
network


The distance
-
vector algorithm


The
link
-
state

algorithm

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Transmitting Packets to Remote
Segments


When TCP/IP hosts transmit packets to remote
segments


They contact their default gateway (usually a router)


The router checks its routing tables against the
destination IP address


To locate the appropriate network interface through
which to forward the packet


Router re
-
addresses the frame or sends the packet
to the next router in the path (indirect routing)

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Routing Packets

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Routing Packets (continued)

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Routing Packets (continued)

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Routing Packets (continued)

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The Cisco Three
-
Layer Hierarchical
Model


Cisco Three
-
Layer Hierarchical model


Does not describe how communications take place


Focuses on how best to design a network


Especially a relatively large network or one that is
expected to grow


Each layer of the model is involved in specific
functions


Is typically defined by a particular type of device


The three layers of the model from bottom up are
Access, Distribution, and Core

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CCNA Guide to Cisco Networking Fundamentals, Fourth Edition

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Access Layer


The layer closest to the users, where they attach to
the network


Could be a router if the network is very small


But typically a hub or layer 2 switch


Sometimes called the desktop layer because it deals
with connecting workstations to the network


Frames are delivered to the users at this layer

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Distribution Layer


Separates the Access layer from the Core layer


Implements network policies, and provides many
networking services


Such as Network Address Translation (NAT), firewall
protection, and quality of service (QoS)


IP addressing hierarchy is managed at this layer


IP addressing

is the process of assigning unique IP
addresses to devices on the network


Typically involves routers and includes all of the
router functions


Provides almost all of the connectivity tasks

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Core Layer


Responsible for switching large amounts of data
quickly and efficiently


To prevent slowing down the switching process:


This layer should not be burdened with security or
traffic control measures or any unnecessary additional
equipment


The primary device at this layer is a high
-
end layer 3
switch


Essentially the backbone of the network

Summary


TCP/IP is not limited to transmission control and
Internet protocols


TCP/IP was started by the Defense Advanced
Research Projects Agency (DARPA)


TCP/IP maps to a four
-
layer network model:
Application, Transport, Internetwork, and Network
Interface


The Application layer in the TCP/IP model covers
the Application, Presentation, and Session layers
of the OSI reference model

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Summary (continued)


The TCP and UDP protocols reside at the
Transport layer of the TCP/IP networking model


Both TCP and UDP use port numbers from 1 to
65,535 to establish their communications between
two points


The Internet Protocol (IP) resides at the
Internetwork layer and provides the logical address
that can be passed through a router


You can use the Ping utility with IP and ICMP to
diagnose and troubleshoot network connections

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Summary (continued)


Address Resolution Protocol (ARP) and Reverse
ARP (RARP) reside in the Internetwork layer


The MAC address is the final leg of communication
between hosts


Routing tables can be created manually and
dynamically


Cisco developed the Three
-
Layer Hierarchical
model to help network administrators design more
efficient networks