1 Introduction: Network Applications

Transcription

1 1 Introduction: Network Applications Some Network Apps Web Instant messaging Remote login P2P file sharing Multi-user network games Streaming stored video clips Internet telephone Real-time video conference Massive parallel computing Network App Concept

2 Processes Communicating A process sends/receives messages to/from its socket Processes Communicating For a process to receive messages, it must have an identifier A host has a unique 32-bit IP address Identifier includes both the IP address and port numbers (16-bit) associated with the process on the host e.g. or telnet About App Layer Protocols App Layer Protocols define: Types of messages exchanged, eg, request & response messages Syntax of message types: what fields in messages & how fields are delineated Semantics of the fields, ie, meaning of information in fields Rules for when and how processes send & respond to messages Public-domain protocols (e.g. HTTP, SMTP,..., RFCs) Proprietary protocols (e.g. KaZaA) Net App Requirements 2

3 2 Web: HTTP HyperText Transfer Protocol Web page consists of objects Object can be HTML file, JPEG image, Java applet, audio file,... Web page consists of base HTML-file which includes several referenced objects Each object is addressable by a URL e.g. HyperText Transfer Protocol Web s application layer protocol Client/server model client: browser that requests, receives, displays Web objects server: Web server sends objects in response to requests HTTP 1.0 (1996): HTTP 1.1 (1997): Client initiates TCP connection (creates socket) to server, port 80 HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) HTTP is stateless HTTP Request/Response 3

4 HTTP connections Nonpersistent HTTP At most one object is sent over a TCP connection HTTP/1.0 uses nonpersistent HTTP Persistent HTTP Multiple objects can be sent over single TCP connection between client and server HTTP/1.1 uses persistent connections in default mode RTT: Round Trip Time Persistency and Pipelining Nonpersistent requires 2 RTTs per object OS must work and allocate host resources for each TCP connection (browsers often open parallel TCP connections to fetch referenced objects) Persistent server leaves connection open after sending response subsequent HTTP messages between same client/server are sent over connection Persistent without pipelining 4

5 client issues new request only when previous response has been received 1 RTT for each referenced object Persistent with pipelining (default in HTTP/1.1) client sends requests as soon as it encounters a referenced object as little as 1 RTT for all the referenced objects Request Message GET, POST, HEAD, PUT, DELETE Response Message 200, 301, 400, 404, 505,... User-server state: cookies Four components 1. cookie header line in the HTTP response message 2. cookie header line in HTTP request message 3. cookie file kept on user s host and managed by user s browser 5

6 4. back-end database at Web site What cookies can bring authorization shopping carts recommendations user session state (Web ) Cookies and privacy cookies permit sites to learn a lot about you you may supply name and to sites search engines use redirection & cookies to learn yet more advertising companies obtain info across sites User-server state: cookies Web Cache (Proxy) User sets browser: Web accesses via cache Browser sends all HTTP requests to cache object in cache: cache returns object else cache requests object from origin server, then returns object to client Cache acts as both client and server Typically cache is installed by ISP (university, company, residential ISP) 6

7 Web Cache (Proxy) 3 File Transfer: FTP FTP: File Transfer Protocol FTP: File Transfer Protocol Transfer file to/from remote host Client/server model (1985) Active FTP Client contacts server port 21 from client port N (>1023) Server contacts client port N+1 from port 20 Passive FTP Client contacts server port 21 from client port N (>1023) Server starts listening to port M (>1023) and tells client about it Client contact server port M from client port N+1 e.g. 7

8 FTP: File Transfer Protocol FTP uses TCP for all connections, non-persistant, only one file transfer per connection Control connection: out of band FTP server maintains state : current directory, earlier authentication Commands: USER, PASS, LIST, RETR, STOR,... Return codes (similar to HTTP) 331 Username OK, password required 425 Can t open data connection 452 Error writing file 4 Mail: SMTP & POP/IMAP Mail Protocols SMTP: Simple Mail Transfer Protocol TCP to server port 25 Three phases of transfer greeting (handshake) transfer of messages closure Commands: HELO/EHLO, MAIL FROM, RCPT TO, DATA, QUIT Return codes: 220, 250, 354, 221,... Messages must be 7-bit ASCII Persistant connections 8

9 Mail Message Format (1982) MIME: Multimedia Mail Extension html Additional lines in msg header declare MIME content type MIME-version: 1.0 Content-Transfer-Encoding: e.g. base64 Content-Type: e.g. image/jpeg POP: Post Office Protocol (POP3, 1996) Commands: USER, PASS, LIST, RETR, DELE, QUIT Return codes: +OK, -ERR POP3 is stateless across sessions IMAP: Internet Mail Access Protocol (IMAP4rev1, 1996) More powerful than POP allows user to organize messages in folders can retrieve parts of messages IMAP keeps user state across sessions names of folders and mappings between message IDs and folder name 5 Name Resolution: DNS DNS: Domain Name System Distributed database implemented in hierarchy of many name servers Core Internet function, implemented as application-layer protocol Provides Hostname to IP address translation IP address to hostname translation Host aliasing 9

10 Mail server aliasing Load distribution (Authorative information about your network!) (new uses appear, e.g. RBL, SPF) UDP to server port 53 (switch to TCP same port if reply > 512 bytes) Interaction of DNS servers Authorative and Local/Caching servers Recursive and Iterative Queries DNS: Resource Records RRs: (Name, Value, Type, TTL) SOA Start Of Authority NS Name Server A Address, define the canonical name of an IP address CNAME Canonical NAME, define alias to canonical name PTR PoinTer Record, reverse mapping DNS: Message Format 10

11 6 Next Week Next Week Transport Layer - Chapter three remember: talk app protocols over tcp with telnet! 11