Internet Mail: The SMTP Model

Transcription

1 Internet Mail: The SMTP Model User File System Sender- SMTP SMTP Commands Replies Receiver- SMTP File System Simple Mail Transfer Protocol (SMTP) to transfer s Depending on the operating system, sender and receiver SMTP entity may be integrated into one process A receiver SMTP entity either refuses messages for non-local users or forwards them The mail user normally uses a graphical interface to the sender SMTP entity Internet mail only supports the services message transfer and message duplication Communication Networks -11. The Internet 632 Internet Mail: An Exemplary SMTP Dialogue R: 220 MIT-Multics.Arpa Simple Mail Transfer Service Ready S: HELO Isi-VaxA.Arpa R: 250 MIT-Multics.Arpa S: MAIL FROM: Smith@Isi-VaxA.Arpa R: 250 OK S: RCPT TO: Jones@MIT.Multics.Arpa R: 250 OK S: DATA R: 354 Start mail input; end with <CRLF>.<CRLF> S: < Text Line 1 > < etc. >. R: 250 OK S: QUIT R: 221 MIT-Multics.Arpa Service closing transmission channel Communication Networks -11. The Internet 633 Prof. Jochen Seitz 1

2 Internet Mail: Accessing the Mail Folder Using POP3 Mail Server SMTP Received Messages Sent Messages Internet File System Mail Client POP 3 Post Office Protocol POP in version 3 With POP3, the mail client gets the messages from the mail folder in the mail server Communication Networks -11. The Internet 634 Internet Mail: Accessing the Mail Folder Using IMAP Internet Message Access Protocol (IMAP) IMAP provides access to the mail folder on the mail server just like POP IMAP is a superset of POP Unlike POP, IMAP offers services to process and analyze messages on the server: The state of the mail account is maintained on the server Even if a user uses different end devices or applications to access his mail account, he will always have a consistent view on his messages IMAP offers selective and partial transfer of messages IMAP4 additionally provides authentication ( Kerberos ) and access to one folder by several users / user groups Communication Networks -11. The Internet 635 Prof. Jochen Seitz 2

3 The MIME Standard SMTP only provides transfer of ASCII messages Multipurpose Internet Mail Extensions (MIME) extend the message body with format information: Content-Type: defines the type of the message body, e. g. Text, Multipart, Message, Application (Binary), Image, Audio, Video and X- private. Content-Transfer-Encoding: defines the transfer syntax, in which the message body is encoded, e. g. Base 64, Quoted Printable, 7bit, 8bit and binary. Largely compatible to traditional Internet mail Message Header (incl. MIME format information) Message Body Text Picture Audio Sequence Communication Networks -11. The Internet 636 FTP- File Transfer Protocol User Interface User Client Control Entity FTP Commands FTP Replies Server Control Entity File System Client File Transfer Entity Data Server File Transfer Entity File System Client Server Communication Networks -11. The Internet 637 Prof. Jochen Seitz 3

4 What is the World-Wide? an internet-wide distributed hypermedia information retrieval system [Liu et al. 1994] a global seamless environment in which all information (text, images, audio, video, computational services) that is accessible from the Internet can be accessed in a consistent and simple way by using a standard set of naming and access conventions [Master Magazine 1996] the universe of network-accessible information, the embodiment of human knowledge [W3C 1999] Communication Networks -11. The Internet 638 Architecture and Protocols Client/Server architecture Synchronous communication (Request/Response) Resources Unit for communication between client and server Static or dynamic Page Page Page Page Client Request Response Server Resources HTTP Communication Networks -11. The Internet 639 Prof. Jochen Seitz 4

5 Unique Identification of Resources Resources must be identified uniquely By name, By address / location. Each resource in the Internet should be identifiable resources, FTP resources, news resources, mailboxes, directories, services,... Identification scheme must be extensible complete representable in ASCII Communication Networks -11. The Internet 640 Uniform Resource Name (URN) Unified Naming / Addressing: URN [RFC 1737, RFC 2141] <urn> ::= "urn:" <nid> ":" <nss> <nid> = Namespace Identifier <nss> = Namespace Specific String Examples: urn:isbn: The 1968 book The Last Unicorn, identified by its book number urn:isan: e o The 2002 film Spider-Man, identified by its audiovisual number Characteristics: Globally unique Enduring Scalable and extensible Supporting existing applications Independent Die Internet-Protokollwelt - 9. Anwendungen im Internet 641 Prof. Jochen Seitz 5

6 URN Name Resolution RDS 1. NID 2. URN Resolver Address Client URN Resolver Server Required infrastructure for resolving URNs Resolver Discovery Service, RDS: Retrieve the address of the URN resolver responsible for NID URN Resolver: Resolve the Namespace Specific String to a URL or URC Die Internet-Protokollwelt - 9. Anwendungen im Internet 642 Uniform Resource Locator, URL Uniform addresses for all web resources Definition of URL schemes: [RFC 1738, 4248, 4266] http, https, ftp, news, nntp, mailto, telnet, ldap,... scheme-specific-part: ["//"][user[":"password]"@"]host[":"port]["/"url-path] Definitions are managed by the Internet Assigned Numbers Authority (IANA) Relative URLs are possible Die Internet-Protokollwelt - 9. Anwendungen im Internet 643 Prof. Jochen Seitz 6

7 Differentiation: and Internet The Internet is an interconnection of heterogeneous networks: Network protocol IP provides transparency Different transport protocols are available (TCP, UDP, SCTP,...) There is a huge amount of application protocols (Telnet, FTP, NNTP, SMTP, HTTP,...) The World-Wide was originally just one of many Internet services, but other services can easily and seamlessly be integrated into the World-Wide Communication Networks -11. The Internet 644 Flexibility of a Client WWW Server HTTP FTP Server NEWS Server Mail Server.. FTP NNTP SMTP FTP: Listing of available files Transfer of files to and from an FTP server NNTP: Access to news groups using a NEWS-URL Direct link from a news article to a web page SMTP: Embedded links to mail addresses: mailto:name@address Internet WWW Client Communication Networks -11. The Internet 645 Prof. Jochen Seitz 7

8 Transaction 2 Transaction 1 Transaction 3 HTTP Transaction Client Server User Get Resource Analysis of page Request Request Get Resource Get Resource Communication Networks -11. The Internet 646 Process of HTTP/1.0 Protocol=HTTP Connect TCP Socket GET /index.html HTTP/1.0 ACCEPT: */* <CR><LF> Close TCP Socket Client DNS-Lookup TCP Connection Connect TCP Socket <HTML> <HEAD> <title>homepage der Technischen Universitaet Ilmenau</title> </HEAD> <BODY>... Inhalt... Close TCP Socket Server Communication Networks -11. The Internet 647 Prof. Jochen Seitz 8

9 References Requests for Comments (I) K. Sollins and L. Masinter, "Functional Requirements for Uniform Resource Names," Internet Engineering Task Force, Request for Comments 1737, December K. Sollins, "Architectural Principles of Uniform Resource Name Resolution," Internet Engineering Task Force, Request for Comments 2276, January T. Berners-Lee, R. Fielding, and H. Frystyk, "Hypertext Transfer Protocol -- HTTP/1.0," Internet Engineering Task Force, Request for Comments 1945, May T. Berners-Lee, R. Fielding, and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax," Internet Engineering Task Force, Request for Comments 3986, January R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1," Internet Engineering Task Force, Request for Comments 2616, June R. Moats, "URN Syntax," Internet Engineering Task Force, Request for Comments 2141, Mai P. Hoffman, "The telnet URI Scheme," Internet Engineering Task Force, Request for Comments 4248, October P. Hoffman, "The gopher URI Scheme," Internet Engineering Task Force, Request for Comments 4266, November Communication Networks -11. The Internet 648 References Requests for Comments (II) P. HOFFMAN, L. MASINTER, and J. ZAWINSKI, "The mailto URL scheme," Internet Engineering Task Force, Request for Comments 2368, July D. CONNOLLY and L. MASINTER, "The 'text/html' Media Type," Internet Engineering Task Force, Request for Comments 2854, June R. L. SUNBERG, "File Transfer and Error Recovery," Internet Engineering Task Force, Request for Comments 133, April J. KLENSIN, "Simple Mail Transfer Protocol," Internet Engineering Task Force, Request for Comments 5321, October M. CRISPIN, "Internet Message Access Protocol -- Version 4rev1," Internet Engineering Task Force, Request for Comments 3501, March J. MYERS and M. ROSE, "Post Office Protocol - Version 3," Internet Engineering Task Force, Request for Comments 1939, May N. FREED and N. BORENSTEIN, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies," Internet Engineering Task Force, Request for Comments 2045, November Communication Networks -11. The Internet 649 Prof. Jochen Seitz 9

10 References References COMER, D.E. (2005): Internetworking with TCP/IP, Vol. 1: Principles, Protocols, and Architecture. 5th edition, New York: Prentice Hall International. ISBN PERLMAN, R. (1999): Interconnections: Bridges and Routers. Boston; San Francisco; New York: Addison Wesley, 2nd edition. ISBN STALLINGS, W. (2000). Data and Computer Communications. Prentice Hall, Upper Saddle River, New Jersey, 6th edition. ISBN STEVENS, W.R. (1994). TCP/IP Illustrated, Vol. 1 The Protocols. Boston; San Francisco; New York: Addison-Wesley. ISBN Communication Networks -11. The Internet 650 Prof. Jochen Seitz 10