welcome to the intro to networks version 7 for the cisco networking
academy this is module 15 application layer in module 15 we're
going to look at application presentation and session peer-to-peer
web and email protocols ip addressing services and file sharing services
let's take a look first at the application presentation
and session the application layer the upper three layers of the osi model
application presentation and session define
functions of the tcpip application layer so if we look over
here the tcpip model these as they compare to the osi
model the session presentation and application in the tcp model they've all
been collapsed into one layer the application layer provides the
interface between the applications used to
communicate and the underlying network over which the messages
are transmitted some of the most widely known
application layer protocols include http ftp tftp imap
and dns the presentation and session layer
they have three primary functions formatting
or presenting data at the source device into a compatible format
for receipt by the destination device compressing data in a way that can be
decompressed by the destination service and encrypting data for transmission and
decrypting data upon receipt the session layer
functions it creates and maintains dialogue
between the source and destination applications
and it handles the exchange of information to initiate dialogues
keeps them active and to restart sessions that are disrupted
or if they're idle for a long period of time the tcp
application protocols specify the format and control information necessary for
many common internet communication functions
the application layer protocols are used by both the source
and destination services or devices excuse me during a communications
session and for the communications to be successful the application
layer protocols that are implemented on the source and destination host must be
compatible so for example here dns domain name
system uses tcp and udp client 53 translates domain names such as cisco
and ip addresses dhcp our dynamic host configuration protocol
is udp client 68 and server 67 and then web is http you have tcp 80 or
8080 and then you have the 443 which is the
encrypted http protocol peer-to-peer
the client server model client and server processes are considered to be in
the application layer in the client server model the device
requesting the information is called a client
and the device responding to the request is called a server
it's not in our traditional way that we think of servers and clients
so we have like a web server it's it think about it a little bit differently
that whatever's requesting is the client and
whatever is delivering the information is the server in this case
so application layer protocols describe the format of the request
and responses between the client and servers
in a peer-to-peer network two or more computers are connected via a network
and can share resources such as printers and
files without having a dedicated server every connected end device known as
appear can function as both a server and a
client one computer might assume the role of a
server for one transaction while simultaneously serving as a client for
another the roles of client and server are set
on a per request basis so for example this computer here which
is pier 2 can have a printer attached to it
and this computer over here can send a print request to it
and this is acting as the client this is acting as the print server
and it sends the information and it sends it then peer 2 sends it to the
printer a peer-to-peer application allows a device to act as both the
client and a server within the same communication
some p2p applications use a hybrid system where each peer
accesses an index server to get the location of a resource stored on another
pier common p2p applications bittorrent
direct connect e donkey freenet some of the file sharing networks
they can run software that they don't have to have a server
so this computer here can say where is my song.mp3
and then it can go out on the internet or on a network and say i've got a piece
of it i've got a piece of it i've got a piece of it
and then it can construct it off of those different pieces
web and email protocols hypertext transfer protocol and hypertext markup
language when a web address or uniform resource
locator url is typed into a web address the web
browser establishes a connection to the web service
the web service is running on the server that is using the http protocol
to better understand how the web browser and services is interact examined
below how a web page is open a browser so the browser
interprets the three parts of the url it looks to see http that's the protocol
most browsers these days you don't have to type in http though it will convert
that for you then you have the www.cisco.com which is
the server name and then you have the specific file name
in this case index.html in most cases we don't have to put in
the index.html the server just says if you don't have an index.html there we're
going to serve up the default page which is this case index.html
so you would put in http www.cisco.com index.html the server then selects that
page of information and sends it to you step two the browser then checks with
the name server to convert cisco into a numeric ip address
which it uses to connect to the server the client initiates an http request to
a server by sending a get request to the server and ask for
the index.html file in step 3 the response to the server the
server sends the html code for this web page to the browser so it takes the
index.html file in code and it sends it and reconstructs it on
the browser on the client side and then in step four that a browser
deciphers the html code and formats the page for the browser window based on the
code that it received http and https http is a request
response protocol that specifies the message types used for that
communication the three common messages are get
post and put get is a client request for data
a client web browser sends a get message to the web server
post this upslope is this uploads data files to the web server such as form
data and put this uploads resources or
content to the web server such as an image
email protocols email is a store and forward method of sending storing and
receiving electronic messages across the network
email messages are stored in databases on mail servers
and email clients communicate with the mail servers to send and receive that
mail so email protocols are used in operation
for either smtp or pop or imap smtp is simple mail
transport protocol post office protocol or pop and imap are
used for clients to receive mail when a client sends email the client
smtp processes process connects with a server
smtp process on a well-known port 25 after the connection is made the client
attempts to send the email to the server across the connection
and when the server receives the message it either places the message in a local
account if the recipient is local or it forwards
the message to another mail server for delivery the destination email
server may not be online or may be busy if so the smtp spools messages to be
sent at a later date and smtp messages formats require a
message header recipient email address and a sender
email address and a message body so three parts to it pop is used by an
application to retrieve mail from a mail server
when mail is downloaded from the server to the client using pop message
they are then deleted on the server so that's the big distinction here pop
deletes the messages after it reads them so the server starts the pop service by
passively listening to tcp port 110 when the server responds or when a
client wants to make use of the service it sends a request to establish a tcp
connection the connection is established the pop
server sends a greeting the client the pop server then exchange commands and
responses and the email gets downloaded and it gets
deleted off the server pop does not store messages
so it's not recommended for small businesses that need a centralized
backup solution imap is another protocol that describes
a method to retrieve email unlike pop when a user connects to an
imap server copies of the message are downloaded to
the client application but the original messages are kept on the server until
manually deleted so when a user decides to delete
messages the server synchronizes that action and deletes the message from the
server so it'll leave it on the server until
you delete it on your client and when you delete it on your client
it will send a message back up to the server to say hey it's okay to go ahead
and delete that now or move it to the trash can ip
addressing services domain name service domain names were
created to convert numeric ip addresses into simple
recognizable names like cisco.com fully qualified domain names are fqdns
such as http colon slash www.cisco.com are much easier to for people to
remember than 198.132
think about it the same way you remember phone numbers if we could just say hey
call up joe smith in new york and you in your phone now we can do that
of course now with smartphones but think about that which is the regular phone
and you don't even need to know the telephone number the
telephone number gets registered into the system
and you just tell it where you want it you know who you want to call and it
would just convert it for you that's what dns does for you
the dns protocol defines an automated service that matches resource names with
the required numeric addre network address it includes the
format for queries responses and data so what happens is when you create a
fully qualified domain name it gets registered on the domain name
service and then those get propagated through the internet so then
all the dns servers have the same copies of information so
sometimes it can take 48 to 72 hours for a new domain with a new ip address to
get uploaded and moved across the internet
but once it is there and then the dns services get updated
you can just put in the domain name and it gets converted to the
ip address the dns message format the dns server stores different types of
resource records that are used to resolve names
some of those types of records are a for an in-device ipv4 address
ns an authoritative name server and aaa an in-device ipv6 address
promotes pronounced quad a quad a or
mx a mail exchange record when a client makes a query the dns
server processes first looks at its own records to resolve the name
if it's unable to resolve the name by using its stored records it contacts the
other services to resolve the name and after a match is found and returned
to the original requesting server the server temporarily stores the
numbered address in the event that the same name is requested again
and some larger companies will maintain dns servers internally
so that those dns requests are handled internally and
cuts down on some of the network traffic through their isp dns uses the same
message format between servers consisting of a question answer
authority and additional information so you got a question
question in the name for service the answer authority who's got the authority
for that and then the resource records holding additional information
the hierarchy dns uses a hierarchical system to create a database to provide
name resolution each dns server maintains a specific
database file and is only responsible for managing
name to ip mappings for that small portion of the entire dns structure
when a dns server receives a request for the name translation that is not within
its dns zone the dns server forwards the request to
another dns server within the proper zone for translation
so an example of that would be a dot com a dot org
a dot u s a dot a u dot c o a second level domain would be a cisco.com
and then you know cisco.com or ftp.cisco or mail.cisco.com
the nslookup command allows you to see what
default server and ip address so an nslookup is a computer operating system
utility that allows a user to manually query dns servers configured on the
device to resolve a given hostname the utility can also be used to
troubleshoot name resolution issues and to verify current status of the named
servers when the nsl ns lookup command is issued
the default dns server configured for your host is displayed
and the name of a host or domain can be entered into the ns lookup prompt
so here you can see we did an nslookup the default server was dns
sj.cisco.com and it will return the ip address for us
or we can just put in cisco.com and then it will give us the server and the
address name the origin and the v6
dynamic host configuration protocol or dhcp
for v4 service it's automated the assignment of v4 addresses subnet mask
and gateways and other v4 networking diameter parameters
dhcp is considered dynamic addressing compared to static
ip addressing static addressing is manually entered ip addresses on each
client where dynamic it will you set a pool on
your server and it will pass those out when a host connects to the network the
dhcp server is contacted and an address is requested
the dhcp server chooses an address from a configured range or pool
and then it gives out and i gives out that i p address and a lease time
many networks use both dhcp and static addressing so dhcp is used for generic
purpose hosts such as in devices static addressing is used for
network devices such as a gateway router a switch a server or a printer dhcp
operations when v when v4 dhcp configured device
boots up or connects to the network the client broadcast a dhcp discover
message to identify any available dhcp servers
if a server is found it replies with the information
the client sends a dhcp request then that says hey i need an ip address
the server then returns a dhcp acknowledgement
acknowledging the client that the lease has been finalized and if the offer is
no longer valid then the selected server responds with the dhcp negative
acknowledgement and the process must begin with the dhcp
discover again also just a quick note here uh dhcp
after a certain amount of time on its lease
it will the dhcp client will send a request to say hey you know can i still
have this ip address and if the server is available then it
will send a request and it will it will renew the lease
file sharing services file transport protocol or ftp was developed to allow
for data transfers between a client and a server an ftp
client is an application which runs on a computer that's being used to push and
pull data from an ftp server step one is the client establishes a
connection on tcp port 21 the client establishes a second
connection to the server for the actual data transfer using tcp
port 20 and then the data transfer happens and it's a push pull
server message block or smb is a client server request
response file sharing protocol there's three functions of smb messages
it starts authenticates and terminates sessions control file and printer access
and allows an application to send or receive messages to
or from another device and unlike the file sharing supported by ftp
clients establish a long-term connection to the servers and after the connection
is established the user the client can access the resources on the server as
though the resource is local to the client host
okay so what did we cover in this session
we looked at http messages get post put we looked at peer-to-peer networking we
looked at the client server model we looked at dns dhcp ftp client
and the functions of smb i hope this information was helpful for you
be safe and be excellent to each other
you