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178 lines
5.6 KiB
Markdown
178 lines
5.6 KiB
Markdown
---
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title: "17-app-layer"
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aliases:
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tags:
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- cosc203
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- lecture
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sr-due: 2022-10-20
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sr-interval: 3
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sr-ease: 250
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---
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# Client-server vs peer to peer
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- social network
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- email
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- multi player games
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- video streaming
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- P2P file sharing
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- voice over IP e.g., skype
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- remote ogin
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Principles
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- run of different end systems
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- communicate over network
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- no need to run application specific software on netwrk-core devices
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## client-server
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sercer
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- provide services to clients
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- always on
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- often in data centers, for scaling
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- needs to powerful
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- multiple client requesting info at the same time
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- have specialzed software
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client
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- request services by contacting and communicating with server
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- may be intermittently connected
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- do not communicate directly with each other
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## peer 2 peer
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- no alwasy on server
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- decentralize resource on a network
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- end systems are both client and servers
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- end systems communcatae directly
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- take advantage of distrivuted shared resources (bandwidth, CPU storage)
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- operate in dynamic environment with frequent join and leave
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- e.g., bitTorrent, blockchain
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## communication
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- process: network application running with a host
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- client proces and server process
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- processes in different hosts communicate by exhanging messages
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- process must have identifiers
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- ip address to differentiate processes in different hosts
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- IPv4: 10.96.35.21 • IPv6: fe80::44b:184c:7ebb:6524
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- port number to differentiate processes in same host
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- HTTP server (80), mail server (25)
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# functions of application protocols
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protocols define:
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- types of messages
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- e.e.g, request, response
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- message syntax
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- what fields in messages and how field are delineated
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- semantics
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- meaning on information in fields
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- communcation rules
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- when and how proceses send and response to messages
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## WEB http
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- client server model
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- browser requests, recieves and displays web objects
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- server sends objects in response to requests
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- HTTP
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- communcation betwen browsers and servers
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- https
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- extensioin of http that uses transport layer security for security
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methods
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- get, head, post, put (request to store a webpage), delete, link (connect two existing resources), unlink
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web cache
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- store data to be used later
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- makes loading faster
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- browser sends all request to cache
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- if object is not in cache, the cache requests from the server
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- files are stored as representation (a snapshot)
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- files expire after their time to live is finished
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- we need to check if cache is up to date
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- conditional get: If-modified-since:
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- if up to date then: HTTP/1.0 304 Not Modified
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## Email
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three components
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- user agents
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- software to compose, read, reply etc email
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- handle local mailboxes
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- e.g., outlook, thunderbird
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- mail servers
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- mailbox contains emails for user
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- each box has a unique address
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- localpart@domain
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- message queue of outgoing to send messages
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- protocols
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- 
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- SMTP to send (simple mail transfer protocol)
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- send from user agent to mail server and between mail servers
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- simple ascii protocol
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- POP (post office protocol)
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- fetch email from mail server
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- simple but limited
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- delete and keep mode (delete from server of keep)
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- IMAP (internet mail access protocol)
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- more functions
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- e.g., check email header before downloading, partially download email
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- allow users to organize mails on server
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- MIME (multi prpose internet mail extensions)
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- supplementary protocol to allow non-ASCII dat to be send through emails
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- WAN connect LANS together
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- email format
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- 
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- envelope
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- contains address of sender and reciever
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- used by smtp to transport email
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- message
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- header
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- defines sender, reciever, subject etc
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- body
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- contains actial information
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- email spoofing can make the addresses in header and in envelope different
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## DNS
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- ip adresses are hard to remember
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- need to map betwen host name and ip address
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- hierarchical, domain-based naming scheme implemented using a distributed databse system
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- core internet function, but implemented as an application layer protocol
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- tree structure 
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- domain name space
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- each node has a label : the root label is a null string
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- each node has a domain name: a sequence of labels separated by dots, reading from the node up to the root.
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- 
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- name servers
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- name space is divided into non-ovelapping zones
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- each zone had name servers to hold information
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- top level dns servers are responsible for domains such as .com .org .net and country domains
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- authoritative DNS servers: maintained by organization or service providers
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- 
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DNS caching
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- once (any) name server learns mapping, it caches mapping, and immediately returns a cached mapping in response to a query
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- caching improves response time
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- cache entries timeout (disappear) after some time (TTL)
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- top-level domain names typically cached in local name servers
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- cached entries may be out-of-date
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- if named host changes IP address, may not be known Internet-wide until all TTLs expire!
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- best-effort name-to-address translation!
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DNS name resolution: recursive query
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- e.g., host at cs.otago.ac.nz wants IP address for ai.cs.waikato.ac.nz
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- 
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# Socket programming
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- socket
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- software component that has
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- local/remote socket address: IP + port no.
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- transport layer protocol: TCP or UDP
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- socket api
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- application programming interface usually provided by the operating system
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- 
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