topic 12 : application layer Flashcards
application layer
- makes up the upper 3 layers of the OSI model (application, presentation, session)
- provides the interface between the applications used to communicate, and the underlying network over which messages are transmitted
- protocols : HTTP, FTP, TFTP, IMAP, DNS
presentation layer functions
1) formatting or presenting data at the source device into a compatible format for receipt by the destination device
2) compressing data in a way that can be decompressed by the destination device
3) encrypting data for transmission and decrypting data upon receipt
session layer functions
1) creates and maintains dialogs between source and destination applications
2) handles the exchange of information to initiate dialogs, keep them active and to restart sessions that are disrupted or idle for a long period of time
client-server model
- client and server processes are considered to be in the application layer
- device requesting the information = client
- device responding to request = server
peer-to-peer network
- 2 or more computers are connected via a network and can share resources (eg printers and files) without having a dedicated server
- every connected end device can function as a server and a client
- One computer might assume the role of 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.
p2p applications
- allows a device to act as both a client and server within the same communication
- some p2p use a hybrid system : each peer accesses an index server to get the location of a resource stored on another peer
COMMON p2p applications
- BitTorrent
- Direct Connect
-eDonkey - Freenet
HTTP and HTTPS
- request/response protocol that specifies the message types used for that communication
3 common message types (HTTP/HTTPS)
1) GET : client request (web browser) for data. client sends GET message to the web server to request HTML pages
2) POST : uploads data files to the web server, such as form data
3) PUT : uploads resources or content to the web server (eg image)
email protocols
- SMTP (simple mail transfer protocol) : used to SEND mail
- POP3 (post office protocol) and IMAP : used for clients to RECEIVE mail
DNS (domain name service)
- DNS protocol : an automated service that matches resource names with the required numeric network address
includes : format for queries, responses and data
DNS message format
1) When a client makes a query, the server DNS process first looks at its own records to resolve
the name.
2) unable to resolve the name by using its stored records, it contacts other servers to resolve the name.
3) 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
DNS hierarchy
- uses it to create a database to provide name resolution
- relies on this hierarchy of decentralized servers to store and maintain these resource records
- each DNS server maintains a specific database file and is only responsible for managing name-to-IP mappings for that small portion of the DNS structure
nslookup command
- allows users to manually query the DNS servers configured on the device to resolve a given host name
- used to troubleshoot name resolution issues and verify the current status of name servers
DHCP
- (for ipv4 service) automates the assignment of ipv4 addresses, subnet masks, gateways and other networking parameters
- dynamic addressing
- when a host connects to the network, DHCP server is contacted and address is requested = DHCP server chooses an address, from configured range of addresses (pool) and assigns (leases) it to the host
- used for general purpose hosts (end user devices)
- static addressing used for network devices (gateway routers, switches etc)
FTP
- allow for data transfers between a client and a server
FTP process
Step 1 - The client establishes the first connection to the server for control traffic using TCP port 21. The traffic consists of client commands and server replies.
Step 2 - The client establishes the second connection to the server for the actual data transfer using TCP port 20. This connection is created
every time there is data to be transferred.
Step 3 - The data transfer can happen in either direction. The client can download (pull) data from
the server, or the client can upload (push) data to the server
SMB (server message block)
- client/server, request-response file sharing protocol
unlike file sharing supported by FTP :
- clients establish long-term connection to servers
- the user of the client can access the resources on the server as though the resource is local to the client host
3 functions of SMB messages
1) start, authenticate and terminate sessions
2) control file and printer access
3) allow an application to send or receive messages to or from another device
