CMPT 371

1. CMPT 371 Data Communications and Networking SMTP

2. SMTP • Simple Mail Transfer Protocol • An application supporting email communication across the Internet • We can use SMTP without connecting through the WWW • Primarily a push protocol (sends to server) • In contrast HTTP is a pull protocol (requests from server)

3. SMTP • Part of the Internet email system which includes • User agents (your mail browser) • Email servers • SMTP is a protocol to communicate between user agents and Email servers, and between different Email servers • For SMTP see RFC 1982 and RFC 2821

4. Mail user agents • A user agent will provide the tools to • Compose mail • Read mail • Send mail (to a recipient through their mail server) • Receive mail (download mail from the server) • Archive received mail • Forward mail • Examples of agents • Thunderbird • Eudora • Outlook

5. Infrastructure of the Email system • Every Email server includes both a SMTP client and an SMTP server • Users sending Email are SMTP clients • Email servers sending mail are SMTP clients • Email servers receiving mail are SMTP servers • Every Email server communicates directly (TCP) with other Email servers it wishes to send Email to. • Emails are sent directly from the senders Email server to the receivers Email server. No other Email servers take part in the transfer of the email.

6. Sending Email: User 1’s Email server User 1’s Email agent User 1 User 2 User 2’s Email server

7. Mail servers: normal operation • When user 1 sends an Email to user 2. The Email will • be sent to user 1’s mail server • be queued on user 1’s mail server • User 1’s mail server will then use TCP to connect to User 2’s Email server • User 1’s mail server will then use SMTP to • send the Email to the user 2’s mail server • When an Email arrives at user 2’s mail server it is placed in user 2’s mail box • User 2 can later retrieve the message

8. Mail servers: problems? • When user 1’s mail server uses SMTP to try to send an Email to user 2’s mail server • User 1’s server will try to create a TCP connection to the user 2’s server • Why might it be impossible to reach user 2’s server? • The requested server is down (not operating) • The requested server does not exist • The requested server is too busy and can not accept the senders TCP connection

9. If user 2’s Email server is not reached • What happens if user 2’s mail server is not available • User 1’s Email server will leave the message in the queue and try to resend it again later • After some number of retries user1 will received a mail from his mail server indicating that the mail he sent could not be delivered • The email user1 sent will be dropped from user 1’s mail server's queue by the mail server

10. SMTP: legacy protocol Recipient’s Mail Server User (sender) agent User (recipient) agent Sender’s Mail Server SMTP messages Through direct TCP connection • SMTP is an relatively old protocol • It requires all messages be sent in ASCII (text) format • If the message is not in text format then it must be somehow translated into text format (even if you are sending images) • Unlike SMTP the newer protocol HTTP does not require this translation

11. MIME extensions For inclusion of non text information in mails Provides methods for translation of non text information into text for transmission using SMTP Information on encodings used will be placed in the header by the sending agent so that the receiving agent can decode the information and reconstruct the non textual information Other information like the RECEIVED line that indicates when the message arrived and from where are added by the receiving mail server before being forwarded to the receiving agent For details see RFC 2045

12. You own SMTP server? • Where should the SMTP server be? • If we place the server on our PC or MAC • Machines (receiver and sender) must always be on • Machines must always be connected to internet • Machines must run server software itself • Usually better to have email servers on a network server that is maintained, always on, and always accessible

13. Sending Email: user1 to user2 User 2 User 1’s Email server SMTP push SMPT push User 1 POP3 IMAP pull User 2’s Email server

14. Agents communicate with servers • Mail access protocols are used by agents communicating with mail servers to request download of mail (pull operation not push so we don’t use SMTP) • Such protocols include • POP3 (post office protocol v3) • IMAP (internet mail access protocol) • HTTP (hotmail and webmail)

15. POP3 (post office protocol) Details in RFC 1939 A simple mail server access protocol (for downloading received mail) The user agent connects to the mail server through a TCP connection Once the connection is made POP3 will check the authorization (username, password etc.) of the user The user agent then performs transactions needed to download the user’s Email

16. POP 3: login session S: +OK POP3 server ready C: user janice S: +OK C: pass tigermoth S: +OK user successfully logged on

17. POP 3: download session User selects “download only” or “download and delete”. Red commands only used when “download and delete” mode is selected C: list S: 1 765 S: 2 225 S: . C: retr 1 S: <what is in the body and header of message 1> S: . C: dele 1 …… C: retr 2 S: <what is in the body and header of message 2> S: . C: dele 2 C: quit S: +OK POP3 server signing off

18. POP3 • The user agent then performs transactions • retrieves the users messages from the mail server across the TCP connection (may delete messages as they are retrieved or not. Two modes available) • records agent requested deletions etc. and replies to such requests • The agent breaks the connection and then the server completes outstanding deletion/modification requests from the agent • Once mail is downloaded the user agent allows classification into folders locally. (Not on the mail server)

19. Pop 3 from multiple clients If a user accesses her email from two different locations, her office and her home then POP has some limitations If POP uses download and delete mode any messages accessed at home have been deleted and cannot be accessed from the office If POP3 uses download only, then the user needs to remember to manually delete mail that she has downloaded in both places

20. IMAP Details in RFC 2060 A more complex and feature rich mail server access protocol Allows organization of mail into folders on the mail server, so that those folders can be accessed from anywhere, (not just the machine they were downloaded to in the case of POP3)

21. IMAP • This requires more commands in the communication protocol to allow the user to instruct the server to move files and save them in particular folders, to create and move those folders. • More flexibility. • Accessing from low bandwidth location you can request only message headers be downloaded, than only download the desired messages that are immediately needed.

22. CMPT 371 Data Communications and Networking DNS

23. Host names • In addition to identifying a host by the IP address of a connected interface we also identify the interface by an hostname • Hostnames are easier for a human to use and remember that the IP address • In the early Internet names were recorded at a central registry at the Network Information Center (NIC) . • New hosts/names were submitted to the central registry and added to the hosts file • The hosts file was available for distribution to all other sites. • This was a flat naming structure

24. Hierarchical name space • The central naming system worked well until the Internet grew larger than it could handle (soon after TCP/IP was adopted) • The central servers could no longer deal with the volume of traffic • The manual updating of names was slow, and maintaining network wide consistency was difficult • Enforcing the use of unique names became more difficult (then impossible)

25. DNS • The primary use of DNS is to answer queries requesting the IP address that corresponds to a given host name. • Reverse queries, finding the name of a host with a particular IP address are also possible • DNS can also support multiple, different names for the same host (computer, mail server, …). • The primary name of a host is the canonical hostname • A host can be called by names other than it’s canonical hostname, these alternate names are called aliases • DNS can also support multiple IPs for the same canonical hostname (used to distribute load)

26. Addresses an names • IP address is a hierarchical system. • An IP address consists of four integers between 0 and 256 separated by .’s (example 192.168.3.1) . • As we scan each group starting at the left and proceeding towards the right we obtain more and more specific information

27. Addresses an names • DNS also uses a hierarchical classification system for domain names. • Domain names may represent a network a sub network or even a host • Consists of . separated names. As we read from left to right each name represents a smaller and more localized network or even a single host

28. Hierarchical name space • The hierarchical DNS name system was designed to replace this original flat namespace in which each machine had a unique name • Administration was decentralized using a distributed database • Local administrators were given responsibility for building and maintaining a database relating IP address and name for their designated groups of local networks

29. DNS Name Tree . Labels may have up to 63 characters Labels (names) may refer to domains (hosts +nets) hosts or networks edu gov us com ca uk arpa fr In-addr ny sun hp nasa ca sfu bc fraser jpl fraser cs

30. DNS Name Tree . All children of a given parent must have unique names Children of different parents can have the same name edu gov us com ca uk arpa fr In-addr sun ny hp nasa ca sfu bc NO! hp1 hp1 fred fraser jpl fraser cs

31. Constructing names: name tree • Start at the leaves of the tree • The domain for the chosen leaf will be the first part of the name. • Add a period to the first part of the name • Check the domain name of the root of the current position in the tree. • If it is not the root of the tree • The domain name of the root of the current position in the tree is added after the period • If it is the root of the tree the name is complete • Repeat steps 3 and 4 until the name is complete

32. jpl.nasa.gov. . edu gov us com ca uk arpa fr In-addr ny sun hp nasa ca sfu bc fraser jpl fraser cs

33. Fully Qualified Domain Name • DNS uses fully qualified domain names • FQDNs are complete domain names including all parts of the domain name from the domain of interest up to the root • Ends in a . to indicate root. For example fraser.sfu.ca. • The terminating . Indicates that the name is absolute ( relative to root, not to any other position in the DNS tree)

34. Domain Names: not fully qualified Domain names that are not fully qualified (do not end at root, like fraser.sfu) may be interpreted by some software as relative to some particular location (other than root) in the DNS tree. Your host must be configured to tell these software applications how to complete a domain name that is not fully qualified (how to convert it to a fully qualified domain name). You must tell the software what locations these names may be relative to.

35. Authority for the DNS namespace • The central internet authority was ICANN (Internet corporation for assigned numbers and names) and is now IANA (Internet assigned numbers authority). • Responsibility for the root level . domain rests with IANA • TLDs, top level directories for the internet namespace • include generic TLDs (gTLDs) like .com or .org for classification of domain names by type of use • include country code TLDs (ccTLDs) like .ca or .us for geographical classification of domain names • Responsibility for administering the TLDs has been delegated to other contractors by IANA

36. DNS Name Tree: Domains Country code Top level domains ccTLDS Root domain Generic Top level domains gTLDS “” edu gov us com ca uk arpa fr In-addr ny hp nasa ca sfu bc jpl fraser cs

37. Authority for the DNS namespace • Any organization to which responsibility for a DNS domain is delegated • must provide at least two independent DNS servers to service that domain • These DNS servers must be geographically separated • These servers must be configured to provide continuous service • may delegate authority for parts of the DNS domain for which they are responsible to other organizations.

38. Authority for the DNS namespace • Each DNS server must know the name/address of the servers it has delegated responsibility to. • The delegator of authority need not inform all organizations it delegates to of changes made by other such organizations. This is an unreasonable load in a rapidly growing/changing internet.

39. Authority for the DNS namespace • Each DNS server must know the name/address of the servers it has delegated responsibility to. • The delegated authority has a responsibility to inform the delegator if address or name of the DNS name server changes. This is necessary to guarantee that address queries can be passed down the tree.

40. Examples of delegation TLD .ca delegates authority to sfu to manage the domain sfu.ca TLD .ca delegates authority to BC to manage top level domain .bc Domain .bc delegates authority to the BC government to manage domain gov.bc.ca

41. DNS Name Tree: sub-trees . us domain edu gov us com ca arpa fr mycomp domain mycomp ny nasa ca sfu bc mynet myhost fraser jpl fraser cs

42. How many DNS servers? • Extrapolating this model we discussed before would have a DNS server for each domain • What is the smallest domain? • 1 host, host name = domain name • Clearly this makes too many servers • 1 local network = 2 DNS servers • Still too many (lots of small networks) • At some reasonable point we need to stop delegating authority

43. Domain Name System • A DNS domain is a sub tree • The name of the domain is the domain name of the node at the root of the sub tree • The domain includes all domains and hosts below the root of the sub tree • .us domain, includes .ca domain and .ny domain • .mycomp domain include .mynet domain and host .myhost • The administrative responsibility for the domain and its subdomains may be arranged in different ways