1. Telecooperation Technische Universität Darmstadt Copyrighted material; for TUD student use only Internet Standards: E-Mail Julian Schröder-Bernhardi, Dirk Bradler Telecooperation Group TU Darmstadt

2. Prof. Dr. M. Mühlhäuser Telekooperation © 2 A bit of history E-Mail predates the Internet –First electronic mail existed on time-sharing computers Sending mail is one of the earliest services on the Internet –Used most bandwidth until early 90ies –E-Mail is one of the few actual standards in the net (STD 11) – most protocols are just Request for Comments –Was a driving force for interconnecting networks through the Internet

3. Prof. Dr. M. Mühlhäuser Telekooperation © 3 Standards for E-Mail Layered model –Layers are independet from each other –Various protocols for each layer Mail-Transport Headers Body Headers Body Network TCP/IP UUCP POP IMAP SMTP STD11 / RFC2822, … RFC2822, RFC2045ff, … RFC2045

4. Prof. Dr. M. Mühlhäuser Telekooperation © 4 Mail Transport Standard Internet Mail Transport protocols (SMTP, POP) are text based –Human readable –Easier to debug –Possibly easier to implement Downside –Uses the smallest common base of “what is text” –Text = 7-Bit US-ASCII, i.e., standard Latin characters, Arabic digits, punctuation marks –Other characters or binary data have to be mapped upon ASCII

5. Prof. Dr. M. Mühlhäuser Telekooperation © 5 SMTP Simple Mail Transfer Protocol – STD 10 –Really very simple 1.Connect to SMTP server 2.Specify sender- and receiver-address 3.Dump header + body of the mail May contain different “From” and “To” header fields than the ones given at (2) No authentication, no authorization in original protocol Ever wondered where all the spam is coming from?

6. Prof. Dr. M. Mühlhäuser Telekooperation © 6 POP Post Office Protocol – RFC 1939 SMTP servers are designed to be always online –No problem for multiuser machines –Not feasible for most PC users (especially before DSL) Solution: POP –Stores mail on a server –POP clients must actively poll new emails –Clients are expected to delete mails on the server Text based, few commands Responses always start with “+OK” or “-ERR” Responses containing more than one line always end with a.

7. Prof. Dr. M. Mühlhäuser Telekooperation © 7 POP sample session Connect to POP server +OK Really Simple POP3 Server ready Send credentials user +OK Name is a valid mailbox pass +OK Login succeeded Get number of mails stat +OK 5 140875 Retrieve Mail retr 1 +OK 1826 octets. Exit quit +OK Goodbye

8. Prof. Dr. M. Mühlhäuser Telekooperation © 8 E-Mail E-Mail is an old protocol with much legacy –Very limited range of characters – 7 bit –CRLF to separate lines instead of automatic line-wrap –Maximum line length: at most 998 characters, 78 characters recommended Headers are just standard text –Separated from body by empty line –Syntax: “:” –Header-Names are case-insensitive, Header-Values may be case-sensitive –Values may span over several lines: line continuation signaled by CRLF followed by one or more space characters E-Mail headers are defined in various documents –Standard headers – RFC 2822: From, To, CC, Subject, Date

9. Prof. Dr. M. Mühlhäuser Telekooperation © 9 MIME Multipurpose Internet Mail Extensions –RFCs 2045-2047 Extends plain text mail by: –Sending non-ASCII characters –Sending binary data –Multipart messages: attachments, formatted mail Important headers –Mime-Version: Must be 1.0, E-Mails without this header are not MIME compliant –Content-Type Define type of data – useful for selecting viewing/editing-program Also used outside mail now (HTTP, operating systems) –Content-Transfer-Encoding: Representation of non-ASCII characters

10. Prof. Dr. M. Mühlhäuser Telekooperation © 10 Binary mail transfer How can I send binary data? Simple idea –Map range [0;255] to [0;127] –Form group of 7 bytes (byte = 8 bits) –Create extra-character out of most significant bit –Send 8 7-bit “bytes” Does not work –Maximum line length: no guarantee that CRLF is part of data –Characters 0-31 are control characters, mail servers are allowed to change them  corrupt data –What if the number of bytes is not divisible by 7?

11. Prof. Dr. M. Mühlhäuser Telekooperation © 11 Base 64 Same idea for binary data –Uses less characters –Guaranteed to work, even if servers change control characters –Content-Transfer-Encoding: Base64 Idea (RFC2045) –Group 3 bytes together –Make 4 groups of 6 bits each out of them –Convert each 6-bit-group into character [A-Za-z0-9+/] –Add CRLF after 78 characters –If data not divisible by 3: stuff with 0-bytes, use “=” as encoding of stuffed 0 to indicate this –  Base64 has ~33% overhead to binary transmission

12. Prof. Dr. M. Mühlhäuser Telekooperation © 12 Quoted printable Second way of sending binary data –Content-Transfer-Encoding: quoted-printable Any byte may be represented by equals-sign + 2-digit hexadecimal encoding of the byte (e.g. “A”  “=41”) Any standard US-ASCII-character may also be used without encoding (with exception of the equals-sign) “Quoted unreadable” –Used if standard text is interspersed with some non-ASCII characters, e.g., German text with umlauts –Compared to Base64, the text is still almost readable without explicit decoding

13. Prof. Dr. M. Mühlhäuser Telekooperation © 13 Multipart messages MIME-mails may contain other MIME-mails recursively –Indicated by special Content-Type –multipart/mixed: attachments –multipart/alternative: Mail has different representations (e.g. plain text and HTML), the client may select any alternative How are these mails separated in the content? –Content-Type defines a boundary = a line that separates the mail parts –Split the mail-body along the boundary – each chunk is a separate mail –Each mail may have its own header, separated from its body by an empty line

14. Prof. Dr. M. Mühlhäuser Telekooperation © 14 Assignment 2 Create a simple POP-mail client –Only for receiving mail, no mail sending needed –Simple POP server for local testing provided at the homepage Mail client should display –Plain text mails –MIME-mails with Base64 and Quoted-Printable encoding in the body –MIME mails with attachments Saving attachments should be possible Each alternative of multipart/alternative mails should be displayed