Application Level Protocols

Transcription

1 Application Level Protocols Nixu Oy PL 21 (Mäkelänkatu 91) Helsinki, Finland tel fax info@nixu.fi

2 Contents Client-Server Store and Forward Push Pull Poll Connection Connectionless Copyright 2000 Nixu Oy 2/45 Application Level Protocols

3 Application Level Protocols Applications handle different kinds of content eg. , web pages, voice Different types of content require different kinds of protocols Applications level protocols Transfer the application s content Transfer information about the capabilities of the participants Use lower layer protocols to avoid doing unnecessary work Copyright 2000 Nixu Oy 3/45 Application Level Protocols

4 Network relations The network entities use different behavioral models on all protocol layers Client-Server Store and Forward Push Pull Poll Master-Slave Connection Connectionless Copyright 2000 Nixu Oy 4/45 Application Level Protocols

5 Client-Server Examples: A WWW client connects to a WWW server and requests a document Xeyes program requests the X server for information about mouse cursor position Client is the active participant Sessions are initiated by the client Server is passive and waits for contact Client-server model is usually used to distribute data or CPU Copyright 2000 Nixu Oy 5/45 Application Level Protocols

6 Store and Forward Example: SMTP server recieves a message and stores it to disk, after the message is stored, server tries to contact next server and transmit the message forward to it > An SMTP server acts both as a server and as a client Store and forward makes data loss in transit less likely, at a cost Copyright 2000 Nixu Oy 6/45 Application Level Protocols

7 Push Example: A news server contacts a second news server and asks if it has a certain article (identified by message-id). If not, it pushes the article to the second server A message is sent from one server to another using the SMTP protocol Push is used to update information at another server The pushing sender has responsibility of transmitting new information Copyright 2000 Nixu Oy 7/45 Application Level Protocols

8 Pull Examples: A news server contacts a second news server and asks what articles it has (identified by message-id). Then it retrieves those articles it does not already have An client program contacts a POP or IMAP server and asks for new for an user-id Pull is used to retrieve information when it suits the client The pulling client has responsibility of receiving the information it needs, this usually requires periodic checks Copyright 2000 Nixu Oy 8/45 Application Level Protocols

9 Poll Example: An SNMP network management station asks each router in a network for a count of transmitted packets at each interface The polling entity requests information, polled entities do not (usually) send anything by themselves Polled entities require minimal configuration, polling entity can decide when to poll (manage load) Often used in hard real time to ensure that the controlling program has time to loop through all the code Copyright 2000 Nixu Oy 9/45 Application Level Protocols

10 Master-Slave Example: An Ethernet network is used for hard real time by having one station which initiates all communications, other stations can only reply Master dominates slaves completely Copyright 2000 Nixu Oy 10/45 Application Level Protocols

11 Connection Examples: An user connects to a Unix server from a PC using Telnet protocol A WWW client program connects to a WWW server using HTTP/1.1 protocol over a TCP protocol and stays connected until all the elements of a WWW page are received > Two connections at different levels In a connection both ends share a state IP network is not aware of a connection A connection can be broken by network fault Copyright 2000 Nixu Oy 11/45 Application Level Protocols

12 Connectionless data transfer Examples: A DNS resolver sends a DNS server an UDP packet, containing a DNS query A network management station queries routers using SNMP packets in UDP packets, if no reply is received after retries, an alarm is generated In connectionless data transfer the entities transferring information are responsible of knowing the status of communication A DNS server does not care The DNS resolver must retry if the query or reply are lost (UDP is defined as unreliable) or if server is down Avoids the setup cost of a connection Copyright 2000 Nixu Oy 12/45 Application Level Protocols

13 What Protocols Do? Protocols are the language different network entities use to talk to each other Windows Netscape can send to a Sendmail program running on Unix operating system, because they talk same language Internet protocols provide layers of abstraction and rely on other protocols to operate together Copyright 2000 Nixu Oy 13/45 Application Level Protocols

14 Internet Protocol Model SNMP, NFS DNS FTP, HTTP, SMTP, Telnet, SSH etc.. Ping UDP TCP Internet Protocol (IP) ICMP Data Link protocols (Ethernet, PPP, ATM, Frame Relay...) Physical media Copyright 2000 Nixu Oy 14/45 Application Level Protocols

15 SMTP mail transfer Simple Mail Transfer Protocol SMTP server recieves a message and stores it to disk After the message is stored, server tries to contact next server and transmit the message forward to it An SMTP server acts both as a server and as a client Copyright 2000 Nixu Oy 15/45 Application Level Protocols

16 POP and IMAP mail read Post Office Protocol Internet Message Access Protocol An client program contacts a POP or IMAP server and asks for new e- mail for an user-id Copyright 2000 Nixu Oy 16/45 Application Level Protocols

17 How the mail travels Host sends using SMTP Server forwards mail using SMTP Client retrieves mail using POP or IMAP Sender Sender s Reciever s local server local server Reciever Copyright 2000 Nixu Oy 17/45 Application Level Protocols

18 A SMTP Protocol Sample 220 tcm.hut.fi SMTP/smap Ready. helo jalopeno.nixu.fi 250 (jalopeno.nixu.fi) pleased to meet you. mail from: 250 Sender Ok rcpt to: 250 OK data 354 Enter mail, end with "." on a line by itself From: kiravuo@jalopeno.nixu.fi To: kiravuo@hut.fi Subject: Greetings Hi, dude!. 250 Mail accepted quit 221 Closing connection Copyright 2000 Nixu Oy 18/45 Application Level Protocols

19 Telnet Network Terminal Protocol Remote login over the network to a host of any operating system > Usually multi-user operating system, like Unix or VMS Offered by TCP/IP protocol suite Application layer protocol, at the top of TCP/IP protocol architecture A very common service > Typically Telnet client is included with the operating system (Windows, Unix) > The server is also included in most multiuser operating systems (Unix) Client always initiates the connection Server waits for connections from the client Copyright 2000 Nixu Oy 19/45 Application Level Protocols

20 Network Virtual Terminal Defines the way how data and commands are sent accross the network Client software translates keystrokes and command sequences from the user s terminal into NVT format Server software translates incoming data and commands from NVT format into the format remote system requires At startup 7-bit US ASCII representation 95 printable characters 33 control codes Copyright 2000 Nixu Oy 20/45 Application Level Protocols

21 Network Virtual Terminal In addition NVT defines control characters for example ENTER/RETURN -> mapped into CR-LF transmission Remote terminal will be controlled with virtual keys Control functions are encoded using escape sequences Reserved octet IAC (Interpret as command) Copyright 2000 Nixu Oy 21/45 Application Level Protocols

22 Telnet commands Command Decimal encoding Meaning IAC 255 Interpret next octet as command DON T 254 Denial of request to perform specified option DO 253 Approval to allow specified option WON T 252 Refusal to perform specified option WILL 251 Agreement to perform specified option IP 244 Interrupt Process (terminate running program) BRK 243 Break (break key or attention signal) NOP 241 No operation Copyright 2000 Nixu Oy 22/45 Application Level Protocols

23 Telnet commands Request for server interrupt: IAC IP = IAC as data: IAC IAC = Forcing the server to read control function Needed when buffers are filled or Remote process is in endless loop Telnet uses an out of band signal TCP URGENT DATA bit is set Urgent data bypasses flow control and reaches the server immediately Copyright 2000 Nixu Oy 23/45 Application Level Protocols

24 Option negotiation Request WILL X Will you let me use option X? Response DO X or DON T X I agree to let you use option X I don t agree to let you use option X Request DO X Use option X Response WILL X or WON T X I will start using option X I won t start using option X Copyright 2000 Nixu Oy 24/45 Application Level Protocols

25 Option negotiation Request WON T X or DON T X I want to disable option X I want you to disable option X Responce DON T X or WON T X Options that are not understood are simply declined Copyright 2000 Nixu Oy 25/45 Application Level Protocols

26 Telnet connections Client has two connections to two servers Client connects Server Another server to a server A connection may be formed from one server to another one Copyright 2000 Nixu Oy 26/45 Application Level Protocols

27 DNS, Domain Name System The service that provides the name to IP address translation for the Internet Distributed database with local caching distributed systems are scalable distributed systems handle network errors well cache improves performance Distributed administration (adding or deleting data) General usage: not for just one application or protocol Only the name space structure and query protocol fixed Three main components: name space structure, resolvers and name servers Copyright 2000 Nixu Oy 27/45 Application Level Protocols

28 Name Space Structure "." com fi se namesurfer tele nixu dat ns ns2 tapas ns Copyright 2000 Nixu Oy 28/45 Application Level Protocols

29 DNS query DNS is a connectionless protocol and uses simple UDP packets A typical DNS query (eg. what is the IP address of can be completed in only two packets, query and response Usually DNS query is passed from the initial server to other servers In connectionless data transfer the entities transferring information are responsible of knowing the status of communication A DNS server does not care The DNS resolver must retry if the query or reply are lost (UDP is defined as unreliable) or if server is down Avoids the setup cost of a connection Copyright 2000 Nixu Oy 29/45 Application Level Protocols

30 Querying client 1 query answer 9 name- server resolver cache every Internet host 2 3 query answer cache & database name server host other ns s 1: client sends query for the IP-address of 2: resolver checks if it has the data. No. 3: resolver sends query to the nearest name server. Copyright 2000 Nixu Oy 30/45 Application Level Protocols

31 4-5: name server checks if it has the actual data or if the data is in the cache. No. 6: it asks a root name server. 7: Root name server gives a referral to fi name servers. Saves them into the cache. 6: it asks a fi name server and 7: gets back addresses of nixu.fi name servers. Saves them into the cache. 6: it asks a nixu.fi name server and 7: gets an authoritative IPaddress answer because nixu.fi name server has the address in its database (see phase 5). 8-9: the answer is relayed to the client. Copyright 2000 Nixu Oy 31/45 Application Level Protocols

32 The name server queried by resolver (3) received a recursive query ("keep querying until you get an answer"). Other servers received an iterative/nonrecursive query (6) ("just give me the nearest address you ve got"). Copyright 2000 Nixu Oy 32/45 Application Level Protocols

33 HTTP Overview Application-level protocol for distributed, collaborative,hypermedia information systems. Used by Web browsers to communicate with WWW servers. Generic, stateless, object-oriented Since 1990 HTTP/1.0 (later version 1.1) Based on a request/response paradigm. On HTTP/1.0 connection is closed after each request/response change while on HTTP/1.1 connection may be used for more than one request/response change. HTTP/1.0 specified in RFC 1945 HTTP/1.1 in RFC 2068 Copyright 2000 Nixu Oy 33/45 Application Level Protocols

34 HTTP Communication (client) Client (browser) opens a TCP connection to an HTTP server (e.g. Apache) by default to port 80. Client observes the URL: > "http": use HTTP protocol > "//": absolute URL > " WWW server > ":8080": use port 8080 Client sends a request line, some optional request headers and a blank line to server. Copyright 2000 Nixu Oy 34/45 Application Level Protocols

35 HTTP Communication (server) Server sends a response line, some response headers, a blank line and a document and closes the connection (on HTTP/1.1 connection is not closed) Every object on a page is requested separately. > HTML page with 3 pictures: with HTTP/1.0 four separate requests and connections. Server responce may be HTML, graphics, audio, VRML or Java... Copyright 2000 Nixu Oy 35/45 Application Level Protocols

36 HTTP Client request HTTP/1.0 supports 3 commands: GET, HEAD and POST Format of a simple request is following: Request-Method Document-Address HTTP/Version (e.g. "GET /index.html HTTP/1.0") After request line there can be zero to n request headers which consists of Request-Header:Value pairs. Request is ended by a blank line. (In POST command additional data can be sent after blank line.) Copyright 2000 Nixu Oy 36/45 Application Level Protocols

37 HTTP Server Response Format of response line is following: HTTP/Version Status-Code Message (e.g. HTTP/ OK) Unspecified amount of response headers might follow (header:value pairs). At least Content-Type header should be send (identifies the type of data that follows). Blank line after final response header. Response Document (or nothing when HEAD command is used). Copyright 2000 Nixu Oy 37/45 Application Level Protocols

38 Example 1 bash-2.03$ telnet Trying... 3 Connected to jalopeno.nixu.fi. 4 Escape character is ^]. 5 HEAD / HTTP/ HTTP/ OK 8 Date: Mon, 12 Apr :26:06 GMT 9 Server: Apache/ Last-Modified: Fri, 26 Feb :28:20 GMT 11 Connection: close 12 Content-Type: text/html Connection closed. 15 bash-2.03$ Copyright 2000 Nixu Oy 38/45 Application Level Protocols

39 HTTP Methods Command GET HEAD POST PUT DELETE OPTIONS TRACE Explanation Normal method to request documents Method to request document headers Method to send data to server Method to send a document to server and request server to store it at specified URI Client requests server to remove a specified URI Method to query available options for server in general or for specified URI Requests the server to return the attached document unchanged. Used for debugging Copyright 2000 Nixu Oy 39/45 Application Level Protocols

40 HTTP Request Headers Header Accept Connection Content-Length Cookie From Host If-Modified-Since Explanation Usually username:password encoded in base64 If Keep-Alive used connection is not closed after each request on HTTP/1.0 (default behaviour for HTTP/1.1) On POST method length of data area Returns information supplied via a Set-Cookie header (in previous connection) address of person responsible for the request. Used mostly by Web indexing programs (robots) Host and port as listed in the original URL Conditional GET Copyright 2000 Nixu Oy 40/45 Application Level Protocols

41 Header Pragma Cache-Control Referer User-Agent Explanation No-cache server should send a fresh copy of document (not cached) No-cache Identical to Pragma except used on HTTP/1.1 Specifies URL of the page that contained the cross-reference Specifies type of browser Copyright 2000 Nixu Oy 41/45 Application Level Protocols

42 HTTP Response Status Line HTTP/Version Status-Code Reason-Phrase Status-Code categories 1xx: Informational - Not used, reserved for future use 2xx: Success - Action was successfully received, understood, and accepted. 3xx: Redirection - Further action must be taken in order to complete the request 4xx: Client Error - Request contains bad syntax or cannot be fulfilled 5xx: Server Error - Server failed to fulfill an apparently valid request Copyright 2000 Nixu Oy 42/45 Application Level Protocols

43 Predefined status codes (HTTP/1.1) "200" ; OK "201" ; Created "202" ; Accepted "203" ; Non-Authoritative Information "301" ; Moved Permanently "400" ; Bad Request "404" ; Not Found "500" ; Internal Server Error "505" ; HTTP Version not supported Copyright 2000 Nixu Oy 43/45 Application Level Protocols

44 HTTP Response Headers Most of the headers are optional but Content-Type header should be included in all responses. Allow Content-Encoding Content-Length Content-Type Expires Last-Modified Lists the set of request methods permitted for the specified resource Describes the decoding mechanism that must be used to obtain the MIME media type specified in the Content-Type header Number of bytes in the file MIME type and subtype Time and date when document expires Time and date when document was changed last time Copyright 2000 Nixu Oy 44/45 Application Level Protocols

45 Location Server Set-Cookie WWW-Authenticate New location of the requested document Information about the HTTP server name/value pair to be stored by browser. This pair will be transmitted in the Cookie header in future requests to the same URL Gives authorization type and realm that the client has to supply in an Authorization header Copyright 2000 Nixu Oy 45/45 Application Level Protocols