Information Network Systems The application layer. Stephan Sigg

Transcription

1 Information Network Systems The application layer Stephan Sigg Tokyo, November 15, 2012

2 Introduction Introduction to the internet The link layer The network layer The network layer The transport layer The application layer Network management Security and cryptography Wireless and Mobile Networks (1) Wireless and Mobile Networks (2) Multimedia networking (1) Multimedia networking (2) Security in computer networks Content centric networking Stephan Sigg Information Network Systems 2

3 The application layer HTTP FTP Introduction IP stack Application Transport Network Link Physical Stephan Sigg Information Network Systems 3 DNS P2P

4 Outline The application layer Hyper Text Transfer Protocol File Transfer Protocol Electronic mail Domain Name Service (DNS) Peer-to-peer networks Stephan Sigg Information Network Systems 4

5 The application layer We distinguish between two main architectures for the application layer Client-server architecture Peer-to-peer architecture Stephan Sigg Information Network Systems 5

6 The application layer Client-server architecture Server host services requests from client hosts Clients do not directly communicate with each other but via a server Server typically has a fixed well-known IP address and can thus always be contacted E.g. Web, FTP, Telnet, Stephan Sigg Information Network Systems 6

7 The application layer P2P architecture Minimal or no reliance on dedicated servers Direct Communication between pairs of connected hosts (peers) E.g. BitTorrent, Xunlei, Skype, Kankan, PPstream P2P networks are self-scalable since each peer adds not only adds workload but further capacity Stephan Sigg Information Network Systems 7

8 The application layer Sockets Two applications communicate via sockets Application Program Interface (API) between application and network The application has little control (and trouble) with the transport-layer as this is controlled by the socket Stephan Sigg Information Network Systems 8

9 Outline The application layer Hyper Text Transfer Protocol File Transfer Protocol Electronic mail Domain Name Service (DNS) Peer-to-peer networks Stephan Sigg Information Network Systems 9

10 Hyper Text Transfer Protocol HTTP is implemented in a client program and a server program These programs talk to each other by exchanging HTTP messages Many webpages consist of a base HTML file and several objects referenced by their Uniform resource locator (URL) Stephan Sigg Information Network Systems 10

11 Hyper Text Transfer Protocol URL Protocol Hostname in the WWW /somedepartment/ Path name to the object location /picture.gif Filename of the object Stephan Sigg Information Network Systems 11

12 Hyper Text Transfer Protocol Persistent and non persistent connections HTTP uses TCP as its transport protocol Default port: 80 We can distinguish HTTP connections as being either Persistent Non-persistent Stephan Sigg Information Network Systems 12

13 Hyper Text Transfer Protocol Non-Persistent connections A new TCP connection is established each time new data is requested Most contemporary browsers open 5 to 10 parallel TCP connections handling separate request-response transactions Overhead through TCP Connection establishing Stephan Sigg Information Network Systems 13

14 Hyper Text Transfer Protocol An example connection 1. HTTP client initiates connection with server on port 80. A socket is created at both client and server 2. HTTP client sends an HTTP request message to the server including /somedepartment/index.html 3. HTTP server retrieves the object /somedepartment/index.html from its storage and sends it in an HTTP response message 4. HTTP server tells TCP to close connection 5. HTTP client receives the response message and extracts URLs of further elements (e.g. images) 6. For each further element, the steps 1-4 are repeated Stephan Sigg Information Network Systems 14

15 Hyper Text Transfer Protocol Persistent connections Once established connection is maintained for each requested object Default mode of HTTP HTTP server closes a connection when it is not used for a certain time Reduced delay since the connection has to be established only once Stephan Sigg Information Network Systems 15

16 Hyper Text Transfer Protocol HTTP request message Message transmitted in ASCII text Methods: GET (Obtain object at specified address). GET /somedepartment/index.html HTTP/1.1 Host: someschool.edu Connection: close User-agent: Mozilla/5.0 Accept-language: jp. Stephan Sigg Information Network Systems 16

17 Hyper Text Transfer Protocol HTTP request message Message transmitted in ASCII text Methods: POST (Request of a website with specific input to form-fields) POST /wiki/spezial:search HTTP/1.1. Host: de.wikipedia.org Content-Type: application/x-www-form-urlencoded Content-Length: 24 search=katzen&go=artikel. Stephan Sigg Information Network Systems 17

18 Hyper Text Transfer Protocol HTTP request message Message transmitted in ASCII text Methods: HEAD (Reply with empty message (e.g. debugging)). HEAD /bit/archives.zip.torrent HTTP/1.1 Host: Stephan Sigg Information Network Systems 18

19 Hyper Text Transfer Protocol HTTP request message Message transmitted in ASCII text Methods: PUT (Store object at specific path). PUT /repositories/directory HTTP/1.1 Host: localhost Content-Type: application/xml;charset=utf-8 [XML ENCODED DATA]. Stephan Sigg Information Network Systems 19

20 Hyper Text Transfer Protocol HTTP request message Message transmitted in ASCII text Methods: DELETE (Delete object at specified path on the server). DELETE /repositories/directory HTTP/1.1 Host: localhost. Stephan Sigg Information Network Systems 20

21 Hyper Text Transfer Protocol HTTP response message HTTP/ OK Connection: close. DATE: Fri, 10 Aug :44:04 GMT Server: Apache (CentOS) Last-modified: Fri, 10 Aug :44:04 GMT Content-Length: 6823 Content-Type: text/html data...data...data.... Stephan Sigg Information Network Systems 21

22 Hyper Text Transfer Protocol HTTP response message Status messages 200 OK: Request succeeded and information is returned in response 301 Moved Permanently: Object has permanently moved, new URL specified in Location: header of response message 400 Bad Request: Request not understood by server 404 Not Found: Requested document not on this server 505 HTTP Version not Supported Requested HTTP protocol version not supported Stephan Sigg Information Network Systems 22

23 Hyper Text Transfer Protocol Cookies HTTP is a stateless protocol since it does not track the states of clients. To identify users, cookies are frequently utilised Can be used to create a user session layer on top of stateless HTTP Components: 1. Cookie header line in HTTP response message 2. Cookie header line in HTTP request message 3. Cookie file kept on the users end system managed by the browser 4. Back-end database at the web site Stephan Sigg Information Network Systems 23

24 Hyper Text Transfer Protocol Cookies 1. At first contact from a user, Web server creates unique identification number and new corresponding entry in back-end database 2. Web server responds including in the HTTP response a set-cookie: header containing the identification number 3. Receiving browser appends a line to its special cookie file including hostname of server and identification number of cookie. 4. With each follow-up contact with this web server, the browser will send the header line Cookie: with the identification number Stephan Sigg Information Network Systems 24

25 Hyper Text Transfer Protocol Stephan Sigg Information Network Systems 25

26 Hyper Text Transfer Protocol Proxy server (Web cache) A proxy server is a network entity that satisfies HTTP requests on behalf of an origin web server Has own disk storage to keep copies of recently requested objects Browser can be configured to send all HTTP requests first to proxy. Potentially reduces traffic and response time Stephan Sigg Information Network Systems 26

27 Hyper Text Transfer Protocol 1. Browser establishes TCP connection to proxy server and sends HTTP request 2. Proxy server checks whether copy of object is within the local cache. 3. If not, it opens TCP connection to origin server to obtain the object 4. At receiving the object, the proxy server stores it into the local cache and sends an HTTP response message to the client browser Stephan Sigg Information Network Systems 27

28 Hyper Text Transfer Protocol The conditional GET How should the proxy server learn of a stale object in its cache? HTTP defines a conditional GET to verify the validity of an object. Request message uses the GET method and includes If-Modified-Since: header GET /fruit/kiwi.gif HTTP/1.1 Host: If-modified-since: Wed, 7 Sep :23:24 Web server then either answers with the modified page or with an empty HTTP response containing the Not Modified header Stephan Sigg Information Network Systems 28

29 Outline The application layer Hyper Text Transfer Protocol File Transfer Protocol Electronic mail Domain Name Service (DNS) Peer-to-peer networks Stephan Sigg Information Network Systems 29

30 File Transfer Protocol (FTP) File Transfer over a TCP connection Stephan Sigg Information Network Systems 30

31 File Transfer Protocol (FTP) File Transfer over a TCP connection FTP uses two parallel TCP Connections to transfer a file Control connection Used for sending control information between the two hosts (user identification, password, commands to change remote directory, put or get files) Data connection Transmit file Stephan Sigg Information Network Systems 31

32 File Transfer Protocol (FTP) File Transfer over a TCP connection 1. When a user starts FTP session, control TCP connection is established first on server port number 21. User ID and password are sent over the TCP connection. 2. Server initiates TCP data connection to client side. FTP sends exactly one file over the data connection and then closes the connection while the control connection remains open Server maintains state about the user during the session to associate a control connection with each specific user Stephan Sigg Information Network Systems 32

33 Outline The application layer Hyper Text Transfer Protocol File Transfer Protocol Electronic mail Domain Name Service (DNS) Peer-to-peer networks Stephan Sigg Information Network Systems 33

34 Electronic Mail in the Internet Electronic mail One of the most utilised applications in the Internet Asynchronous communication Communication between mail servers using SMTP protocol (not between hosts directly) Stephan Sigg Information Network Systems 34

35 Electronic Mail in the Internet Simple Mail Transfer Protocol SMTP transfers messages from the senders mail server to the recipients mail server Body and header must be 7-bit ASCII TCP connection between SMTP servers Stephan Sigg Information Network Systems 35

36 Electronic Mail in the Internet Talk to an SMTP server C: telnet servername 25 S: 220 *********************** C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 Recipient ok C: DATA S: 354 Enter mail, end with. on a line by itself C: Hello Stephan C: Just writing a short C:. S: 250 Message queued for delivery C: QUIT S: Bye... Stephan Sigg Information Network Systems 36

37 Electronic Mail in the Internet Talk to an SMTP server C: telnet servername 25 S: 220 *********************** C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 Recipient ok C: DATA S: 354 Enter mail, end with. on a line by itself C: Hello Stephan C: Just writing a short C:. S: 250 Message queued for delivery C: QUIT S: Bye... Stephan Sigg Information Network Systems 37

38 Electronic Mail in the Internet Talk to an SMTP server C: telnet servername 25 S: 220 *********************** C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 Recipient ok C: DATA S: 354 Enter mail, end with. on a line by itself C: Hello Stephan C: Just writing a short C:. S: 250 Message queued for delivery C: QUIT S: Bye... Stephan Sigg Information Network Systems 38

39 Electronic Mail in the Internet Talk to an SMTP server C: telnet servername 25 S: 220 *********************** C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 Recipient ok C: DATA S: 354 Enter mail, end with. on a line by itself C: Hello Stephan C: Just writing a short C:. S: 250 Message queued for delivery C: QUIT S: Bye... Stephan Sigg Information Network Systems 39

40 Electronic Mail in the Internet Talk to an SMTP server C: telnet servername 25 S: 220 *********************** C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 Recipient ok C: DATA S: 354 Enter mail, end with. on a line by itself C: Hello Stephan C: Just writing a short C:. S: 250 Message queued for delivery C: QUIT S: Bye... Stephan Sigg Information Network Systems 40

41 Electronic Mail in the Internet Talk to an SMTP server C: telnet servername 25 S: 220 *********************** C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 Recipient ok C: DATA S: 354 Enter mail, end with. on a line by itself C: Hello Stephan C: Just writing a short C:. S: 250 Message queued for delivery C: QUIT S: Bye... Stephan Sigg Information Network Systems 41

42 Electronic Mail in the Internet Talk to an SMTP server C: telnet servername 25 S: 220 *********************** C: HELO crepes.fr S: 250 Hello Task crepes.fr, Write an pleased tovia meet telnet you C: MAIL FROM: Connect to your server via telnet to compose S: 250 Sender ok C: RCPT TO: send S: 250 Recipient ok C: DATA S: 354 Enter mail, end with. on a line by itself C: Hello Stephan C: Just writing a short C:. S: 250 Message queued for delivery C: QUIT S: Bye... Stephan Sigg Information Network Systems 41

43 Electronic Mail in the Internet User agents SMTP server usually accessed by user agent (e.g. POP3, IMAP, HTML) Post Office Protocol Version 3 (POP3) Simple mail access protocol TCP connection with the mail server on port 110 Three phases of a POP3 session Auhorisation (Username and password) Transaction (retrieving of messages and/or message deletion) Update (After the client has quit the connection, the mail server deletes the messages marked for deletion) Stephan Sigg Information Network Systems 42

44 Electronic Mail in the Internet User agents SMTP server usually accessed by user agent (e.g. POP3, IMAP, HTML) Internet Mail Access Protocol (IMAP) Provides additional commands to create remote folders and assign messages to them Server will associate each message with a folder Allows to transmit only parts of a message (e.g. Header only) Stephan Sigg Information Network Systems 43

45 Outline The application layer Hyper Text Transfer Protocol File Transfer Protocol Electronic mail Domain Name Service (DNS) Peer-to-peer networks Stephan Sigg Information Network Systems 44

46 Domain Name System (DNS) DNS Internet hosts can be identified either by their hostname or their IP address Hostname IP address DNS is a directory service that maps IP addresses to hostnames Stephan Sigg Information Network Systems 45

47 Domain Name System (DNS) DNS 1. Browser extracts from an URL the hostname and passes this to the client side of the DNS application (running on the host machine) 2. The DNS client sends a query containing the hostname to a DNS server 3. Eventually, the DNS client receives a reply with the IP address for the hostname 4. With this IP address, the browser can now initiate a TCP connection the the HTTP server process on port 80 Stephan Sigg Information Network Systems 46

48 Domain Name System (DNS) Further DNS services Host aliasing A hostname such as might have several aliases such as google.com is the canonical hostname DNS can provide the canonical hostname and its IP address when supplied with an alias hostname Stephan Sigg Information Network Systems 47

49 Domain Name System (DNS) Further DNS services Mail server aliasing The MX record permits a company s mail server and web server to have the same hostnames. Therefore, DNS can provide the canonical host name for an server provided with the alias address Stephan Sigg Information Network Systems 48

50 Domain Name System (DNS) Further DNS services Load distribution DNS also performs load distribution among replicated servers. Busy sites are replicated over multiple servers each running on different end systems and having different IP addresses With each reply for this alias address, the DNS server will rotate the distribution of the traffic among replicated servers Stephan Sigg Information Network Systems 49

51 Domain Name System (DNS) DNS: A distributed, hierarchical database DNS uses a large number of hierarchically organised servers No single DNS server has all of the mappings for all of the hosts in the Internet Stephan Sigg Information Network Systems 50

52 Domain Name System (DNS) DNS root servers Stephan Sigg Information Network Systems 51

53 Domain Name System (DNS) Root DNS servers 13 Root DNS servers in the Internet Each server Root server is actually a network of replicated servers Alltogether, 247 servers make the Root servers of the internet Stephan Sigg Information Network Systems 52

54 Domain Name System (DNS) Top-level domain (TLD) servers Responsible for top-level domains e.g. com, org, net, edu, gov country specific such as gov, jp, fr, de Stephan Sigg Information Network Systems 53

55 Domain Name System (DNS) Authoritative DNS servers Each organisation with publicly accessible hosts (e.g. web servers and mail servers) must provide publicly accessible DNS records that map names of those hosts to IP addresses These are hosted in the organisations authoritative DNS server Alternatively, the organisation can pay a service provider to store the entries in one of its DNS servers Stephan Sigg Information Network Systems 54

56 Domain Name System (DNS) Iterative and recursive interaction with DNS servers Stephan Sigg Information Network Systems 55

57 Domain Name System (DNS) Recursive interaction with DNS servers Stephan Sigg Information Network Systems 56

58 Outline The application layer Hyper Text Transfer Protocol File Transfer Protocol Electronic mail Domain Name Service (DNS) Peer-to-peer networks Stephan Sigg Information Network Systems 57

59 Peer-to-peer networks A P2P network has minium or no reliance on always-on servers Pairs of intermittently connected hosts (peers) communicate directly Examples: P2P File distribution P2P Distributed Hash Table Stephan Sigg Information Network Systems 58

60 Peer-to-peer networks P2P file distribution Distribution of a large file to a great number of hosts In server-based implementation, enormous burden on the server, consuming large amount of server bandwidth In P2P file distribution, distributed peers share the load for distributing the file. Stephan Sigg Information Network Systems 59

61 Peer-to-peer networks P2P file distribution BitTorrent Collection of all peers distributing a particular file is called torrent Peers in a torrent download equal-sized (e.g. 256 KBytes) chunks of the file from one another Stephan Sigg Information Network Systems 60

62 Peer-to-peer networks P2P file distribution BitTorrent Each torrent has an infrastructure node tracker at which new peers join Each peer periodically informs the tracker that it is still in the torrent When peer enters torrent, it is assigned IP addresses of a set of peers Peer then establishes TCP connections with assigned peers (neighbours) Stephan Sigg Information Network Systems 61

63 Peer-to-peer networks P2P file distribution BitTorrent Neighbours are periodically asked for their already downloaded chunks From missing chunks, rarest are downloaded first and those neighbours are provided with own data that forward data at the highest rate Four peers feeding at highest rates are sent chunks they require (recalculation each 10 seconds) Every 30 seconds, one additional neighbour is chosen at random to be provided with chunks Peers capable of uploading at compatible rates tend to find each other Stephan Sigg Information Network Systems 62

64 Peer-to-peer networks Distributed Hash Table (DHT) Store a database of (key,value)-pairs Each peer will hold only a small subset of all (key,value)-pairs Peers could be organised in a circular fashion Each peer stores (key-value)-pairs for which the key is bigger than the ID of its predecessor and smaller or equal to its own ID Stephan Sigg Information Network Systems 63

65 Peer-to-peer networks Distributed Hash Table (DHT) Values derived by iterative search Robust against dying peers when several successor addresses are stored Stephan Sigg Information Network Systems 64

66 Outline The application layer Hyper Text Transfer Protocol File Transfer Protocol Electronic mail Domain Name Service (DNS) Peer-to-peer networks Stephan Sigg Information Network Systems 65

67 The application layer HTTP FTP Conclusion Comoputer Networking A Top-Down Approach J.F. Kurose and K.W. Ross 6th edition, 2012 Pearson study Addison-Wesley ISBN: Stephan Sigg Information Network Systems 66 DNS P2P

68 Conclusion Do you have any questions? Stephan Sigg Stephan Sigg Information Network Systems 67