ccna guide to cisco networking fundamentals fourth edition chapter 3 tcp/ip

CCNA Guide to Cisco Networking Fundamentals

Fourth Edition

Chapter 3TCP/IP

CCNA Guide to Cisco Networking Fundamentals, Fourth Edition 2

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


Objectives (continued)

• Use Ping and Trace and describe their functions

• Explain how packets are transmitted

• Describe the Cisco three-layer hierarchical model


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)


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


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)


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


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



• 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



• 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



• TCP sliding windows (continued)– Other flow control methods include

• Buffering

• Congestion avoidance


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


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



• 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



• 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



• 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



• 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



• 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



• 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


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


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


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


Routers on the Network (continued)


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


Network to Network (continued)


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


Dynamic or Static Tables (continued)

• Methods to determine the best path across a network– The distance-vector algorithm– The link-state algorithm


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)


Routing Packets


Routing Packets (continued)


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


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


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


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



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


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 chapter 3 tcp/ip

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