CCNA Discovery
Chapter 7
Oh, so they have
Internet on
computers now!
ISP’s Provide Services
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Email
Web Hosting
Media Streaming
IP Telephony
File Transfer
ISP’s must guarantee a level of service based on:
• Reliability
• Availability
Characteristics of Reliability
• Redundant Hardware
• Redundant Hardware
• Redundant Hardware
• Fault Tolerance - The measure of equipment
robustness is fault tolerance, the longer the
MTBF, the greater the fault tolerance.
• The longer the MTBF (mean time between
failure), the greater the reliability
Availability
• Five nines (99.999%) Uptime (.001 Downtime)
I'm learnding!
TCP/IP Protocols
TCP/IP vs OSI Models
Similarities
• Use of layers to visualize
the interaction of protocols
and services
• Comparable Transport and
Network Layers
• Used in the networking field
when referring to protocol
interaction
Differences
•OSI model breaks the function of the
TCP/IP Application Layer into separate
distinct layers. The upper three layers of
the OSI model specify the same
functionality as the Application Layer of
the TCP/IP model.
•The TCP/IP protocol suite does not
specify protocols for the physical
network interconnection. The two lower
layers of the OSI model are concerned
with access to the physical network and
the delivery of bits between hosts on a
local network.
TCP/IP vs OSI Models
TCP/IP model is based on actual protocols and standards
developed, whereas the OSI model is a theoretical guide for
how protocols interact.
TCP vs. UDP (Transport Needs)
TCP (Connection-Oriented Protocol)
• TCP is a reliable, guaranteed-delivery protocol.
TCP specifies the methods hosts use to
acknowledge the receipt of packets, and
requires the source host to resend packets that
are not acknowledged. TCP protocols also
govern the exchange of messages between the
source and destination hosts to create a
communication session.
• High Overhead
TCP vs. UDP (Transport Needs)
UDP (Connectionless Protocol)
• Low overhead
• No Flow Control
• No error recovery function
• “Best Effort” and may arrive out of order or lost.
Encapsulation
• 7.2.2.2 view animation
TCP Handshake
Great Homer, how are you
doing?
How are you
doing, Barney?
I’m hungry.
TCP Handshake (3 Way handshake)
1. A --> B SYN My sequence
number is X
2. A <-- B ACK Your sequence
number is X ;
expect X + 1
next
3. A <-- B SYN My sequence
number is Y
4. A --> B ACK Your sequence
number is Y ;
expect Y + 1
next.
**Because step 2 and 3 are combined
into one message, it is called a threeway handshake. The following diagram
might better illustrate this process.
Why use the three-way handshake?
• It synchronizes both ends of a connection by
allowing both sides to agree upon initial
sequence numbers.
TCP Timer
• When a host sends message segments to a
destination host using TCP.
• The timer allows sufficient time for the message
to reach the destination host and for an
acknowledgement to be returned. If the source
host does not receive an acknowledgement
from the destination within the allotted time, the
timer expires and the source assumes the
message is lost and reset.
TCP Sequence Numbers
• TCP specifies how messages are reassembled
at the destination host. Each TCP segment
contains a sequence number. At the destination
host, the TCP process stores received
segments in a TCP buffer. By evaluating the
segment sequence numbers, the TCP process
can confirm there are no gaps in the received
data. When data is received out of order it can
also reorder the segments as necessary.
Me fail English? That’s
unpossible.
UDP
Very Simple Protocol: it is not connectionoriented and does not provide the sophisticated
retransmission, sequencing, and flow control
mechanisms of TCP, UDP has a much lower
overhead.
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Domain Name System (DNS)
Simple Network Management Protocol (SNMP)
Dynamic Host Configuration Protocol (DHCP)
Routing Information Protocol (RIP)
Trivial File Transfer Protocol (TFTP)
Online games
Supporting Multiple Services
• The task of managing multiple simultaneous
communication processes is done by the
Transport Layer.
• To differentiate the segments and datagrams
for each application, both TCP and UDP have
header fields that can uniquely identify these
applications for data communications purposes.
Port Numbers
• In the header of
each segment or
datagram, there is a
source and
destination port.
• When a client
application sends a
request to a server
application, the
destination port
contained in the
header is the port
number that is
assigned to the
application running
on the server.
*IP and MAC Address’ do
not change!
Port Numbers
• For example, if you want to talk to Mr Buggi on
the phone, you first need to know Mr. Buggi’s
phone number, then you should know Mr
Buggi’s name. Here the name of Mr. Buggi is
the port number. As for computer, knowing the
IP address is just one thing. It also need to
know which software need to transfer the data.
Port Numbers to know
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HTTP – 80
SMTP – 25
NETSTAT – 15
FTP – 20 & 21
SSH – 22
Telnet – 23
DNS – 53
BOOTP-67 & 68
TFTP – 69
SNMP – 161
BGP – 179
UPS – 401
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HTTPS – 443
POP3 - 110
Kazaa – 1214
Windows Live
Messaging – 1863
• Civilization – 2056
• Halo: Combat
Evolution – 2302
• Nintendo Wi-Fi
(Wii ?) - 29920
• The TCP processes on the sending and
receiving hosts are slightly different. Clients are
active and request connections, while servers
are passive, and listen for and accept
connections.
• Server processes are usually statically
assigned well-known port numbers from 0 to
1023. Well-known port numbers enable a client
application to assign the correct destination port
when generating a request for services.
Sockets and Socket Pairs
• The combination of the Transport Layer port
number and the host's Network Layer IP
address uniquely identifies a particular
application process running on an individual
host device. This combination is called a
socket. A socket pair, consisting of the source
and destination IP addresses and port
numbers, is also unique and identifies the
specific conversation between the two hosts.
• What would the source and destination sockets
be?
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Socket Example:
• Source: 192.168.1.17:25
• Destination: 192.168.2.39:8080
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Socket Pair
• 192.168.1.17:25, 192.168.2.39:8080
Naming – DNS and HOSTS
• Used HOSTS before DNS
• A HOSTS file is still used by virtually all
computer systems. A local HOSTS file is
created when TCP/IP is loaded on a host
device. As part of the name resolution process
on a computer system, the HOSTS file is
scanned even before the more robust DNS
service is queried. A local HOSTS file can be
used for troubleshooting or to override records
found in a DNS server.
• Malware/Virus’ can attach it
HOSTS Example
DNS (Domain name Service) 53
• Maps name-to-IP addresses for internal hosts
• Forwards name resolution requests to a
caching-only server (maintained by ISP)
DNS
Made up of:
1. Resource Records and Domain Namespace
2. Domain Name Servers
3. Resolvers
• The root DNS server may
not know exactly where
the host H1.cisco.com is
located, but it does have a
record for the .com top
level domain. Likewise, the
servers within the .com
domain may not have a
record for H1.cisco.com
either, but they do have a
record for the cisco.com
domain. The DNS servers
within the cisco.com
domain do have the record
for H1.cisco.com and can
resolve the address.
The name H1.cisco.com is referred
to as a fully qualified domain name
(FQDN) or DNS name, because it
defines the exact location of the
computer within the hierarchical
DNS namespace.
DNS Dynamic Updates
• Dynamic updates enable DNS client computers
to register and dynamically update their
resource records with a DNS server whenever
changes occur.
DNS Name Resolution
• Forward lookup (Primary/Secondary)
• Reverse lookup (Primary/Secondary)
Forward Lookup Zone
• A forward lookup zone is a standard DNS zone
that resolves fully qualified domain names to IP
addresses. This is the zone type that is most
commonly found when surfing the Internet.
When typing a web site address, such as
www.cisco.com, a recursive query is sent to the
local DNS server to resolve that name to an IP
address so as to connect to the remote web
server.
Reverse Lookup Zone
• A reverse lookup zone is a special zone type that
allows you to resolve an IP address to a fully
qualified domain name. Some applications use
reverse lookups to identify computer systems who
are actively communicating with them. There is an
entire reverse lookup DNS hierarchy on the Internet
that will enable any publicly registered IP address to
be resolved. Many private networks choose to
implement their own local reverse lookup zones to
help identify computer systems within their network.
Reverse lookups on IP addresses can be found
using the ping -a <ip address> command.
• When registering a Domain Name you must
also name two DNS servers for redundancy.
Ideally on two different networks.
SERVICES – HTTP & HTTPS
• HTTP
• Many different versions. Most recent version allows
for a single web server to host multiple sites. Also
multiple requests and response messages can be
used with the same connection.
• NOT SECURE – sent in plain text (request and
response)
• HTTPS
• Secure – authentication and encryption with SSL
• Requires additional server time
• When contacting an HTTP server to download
a web page, a uniform resource locator (URL)
is used to locate the server and a specific
resource. The URL identifies:
1. Protocol being used
2. Domain name of the server needing to be
accessed
3. Location of the resource on the server
http://example.com/example1/index.htm
Proxy Server
Proxy server
• Speed - caching allows resources requested by one user to be available to
subsequent users without having to access the actual server where the page
is stored.
• Security - proxy servers can be used to intercept computer viruses and
other malicious content and prevent them from being forwarded onto clients.
• Filtering - proxy servers can view incoming HTTP messages and filter
unsuitable and offensive web content.
FTP
20 and 21
• FTP is a connection-oriented protocol that uses
TCP to communicate between a client FTP
proces
• FTP implementations include the functions of a
protocol interpreter (PI) and a data transfer process
(DTP). PI and DTP define two separate processes that
work together to transfer files. As a result, FTP
requires two connections to exist between the client
and server, one to send control information and
commands, and a second one for the actual file data
transfer.s and an FTP process on a server.
Protocol Interpreter (PI) 21
• The PI function is the main control connection
between the FTP client and the FTP server. It
establishes the TCP connection and passes
control information to the server. Control
information includes things such as commands
to navigate through a file hierarchy, as well as
renaming or moving files.
DTP (Data Transfer Process)
• This function is enabled only when the user
wants to actually transfer files to or from the
FTP server. Unlike the PI connection, which
remains open, the DTP connection closes
automatically when the file transfer is complete.
Active vs Passive FTP
• Passive- a client initiates a request to the server
and opens a port for the expected data. The
server then connects to the client on that port
and the data transfer begins.
• Active- the FTP Server opens a random source
port (greater than 1023). The server forwards
its IP address and this random port to the FTP
client over the control stream. The server then
waits for a connection from the FTP client in
order to begin the data file transfer.
EMAIL: SMTP, POP3 and IMAP
• Store and forward method (Stored in databases
on mail servers)
EMAIL: SMTP, POP3 and IMAP
• Sends: SMTP (25)
• dbuggi@esu6.org
• Receives: POP3 (110)(deleted from serer and
IMAP (not deleted from server)