CCNA Guide to Cisco
Networking Fundamentals
Fourth Edition
Chapter 3
TCP/IP
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
2
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
3
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)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
4
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
5
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
6
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)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
7
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
8
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
9
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
10
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
11
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
12
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
13
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
14
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
15
Transport Layer (continued)
• TCP sliding windows (continued)
– Other flow control methods include
• Buffering
• Congestion avoidance
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
16
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
17
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
18
Internetwork Layer (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
19
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
20
Internetwork Layer (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
21
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
22
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
23
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
24
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
25
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
26
Internetwork Layer (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
27
Internetwork Layer (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
28
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
29
Internetwork Layer (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
30
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
31
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
32
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
33
Routers on the Network (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
34
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
35
Network to Network (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
36
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
37
Dynamic or Static Tables (continued)
• Methods to determine the best path across a
network
– The distance-vector algorithm
– The link-state algorithm
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
38
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)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
39
Routing Packets
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
40
Routing Packets (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
41
Routing Packets (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
42
Routing Packets (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
43
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
44
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
45
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
46
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
47
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
48
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
49
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
50
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
51