CSCI 6433 Class 10

1. Dave Roberts CSCI 6433Class 10

2. Agenda • DHCP • SMTP

3. DHCPv4: How does a host get started?

4. What Addresses Are Needed? • Address of default router • Network mask • Addresses of mail server, DNS server • MTU of local network • TTL value to use for IP datagrams • IP address of host

5. What About Manual Configuration? • People make mistakes • There aren’t enough people • Hosts may be dumb and unable to remember settings So—we use IP to get the IP addresses that we need. The “limited broadcast” IP address is used to broadcast even before the client knows its own IP address.

6. Retransmission • Responsibility for reliable communication on the client • Requires that UDP checksum be on • Requires that do not fragment bit be on • Client sets timeout, retransmits when timeout expires • Delay has a random number added • After failure, delay is doubled, up to 60 seconds

7. Message Format

8. Address Allocation Methods • Manual allocation: Each IP address is allocated to a single device. DHCP communicates that address • Automatic allocation: DHCP automatically assigns an IP address permanently to a device, selecting from a pool • Dynamic allocation: DHCP assigns an IP address from a pool for a time chosen by the server or until the client gives it up (most used, by far)

9. DHCP Lease Length • DHCP administrator must pick lease length for IP addresses • Short lease lengths make most effective use of IP addresses • Long lease lengths produce more network stability • Lease lengths might be from one hour to months

10. DHCP Lease Allocation Process

11. Summary • DHCPv4 is run by hosts at startup • It configures the host for network operation What about v6?

12. Startup with IPv6 • Designers wanted startup without DHCP • Designed stateless autoconfiguration • But ISPs and network managers wanted more control, so DHCPv6 was also developed

13. DHCPv6 • Server for each network; host contacts server • Host generates link-local address, uses link-local multicast to reach server • DHCPv6 allows for authentication • RFC for DHCPv6 is twice the size as RFC for DHCPv4

14. Stateless Autoconfiguration • Uses IPv6 Neighbor Discovery Protocol (NDP) • Host can generate an IPv6 address and verify its uniqueness

15. NDP • Router Discovery • Next-hop routes • Neighbor discovery • Neighbor unreachability detection • Address prefix discovery • Configuration parameter discovery • Stateless autoconfiguration • Duplicate address detection • Address resolution • DNS server discovery • Redirect

16. ICMPv6 Message Types for NDP • Router Solicitation • Router Advertisement • Neighbor Solicitation • Neighbor Advertisement • Redirect

17. ICMPv6 Router Solicitation

18. ICMPv6 Router Advertisement

19. ICMPv6 Neighbor Solicitation

20. ICMPv6 Neighbor Advertisement

21. ICMPv6 Redirect

22. Summary • For IPv6, configurations can be managed by DHCP or automatically • Either method can provide all the parameters needed on startup • DHCP allows for more complete control of which hosts can connect to a network

23. Electronic mail

24. Electronic Mail Email allows users to send memos across the Internet. • Notes can be short or quite large • Notes can have multiple attachments • Must work when remote machine is unreachable

25. Actors in the Email System • Post Office • SMTP servers move email between each other • SMTP servers store email for delivery to end users • Users • POP clients pick up email from SMTP servers • POP clients hand outgoing mail to SMTP servers

26. Components

27. Delayed Delivery Sender and receiver do not need to be connected to the server at the same time

28. Questions • What protocol do SMTP servers use to deliver messages? • How does SMTP differ between v4 and v6?

29. SMTP Transaction Sequence

30. Alias Expansion, Mail Forwarding

31. Internetworking and Email • TCP/IP internet makes universal delivery service possible • Mail systems built on TCP/IP are inherently reliable because of end-to-end delivery • Alternatively, mail gateways are used • Allow mail transfer between different systems • When gateway has a message, sender discards it Question: Is email through a gateway better or worse than a direct SMTP transfer?

32. TCP/IP Email Standards • Format and computer interaction are specified separately • Format: header, blank line, body • Body unspecified • Header is key word, colon, value • Some keywords required, others optional • Header is readable

33. Email Addresses local-part @ domain-name • domain-name: name of mail destination • local-part: address of a mailbox at destination Note: when gateways are employed, mail addresses are site-dependent

34. SMTP • Simple Mail Transport Protocol • Focuses on how mail delivery system passes messages from one machine to a server on another machine • Does not specify anything about user interface • Does not specify how mail is stored

35. Assured Delivery • SMTP server forms TCP/IP connection with receiving server • Once receiving server has put message into safe store, then it acknowledges and sender discards message • If SMTP can’t transfer message on the first try, it keeps trying • After several days of failure, SMTP reports failure to deliver.

36. SMTP Highlights • All communications is readable ASCII text • Transcript of interactions is readable • Each message is acknowledged separately • Addresses of the form local-part@domain-name

37. Example Transfer Client establishes connection Sent by server Sent by client S: 220 Beta.GOV SMPT ready C: HELO Alpha.EDU S: 20 Beta.GOV C: MAIL FROM: Smith@Alpha.EDU S: 250 OK C: RCPT TO: Green@Beta.GOV 550 No such user here RCPT TO: Brown@Beta.GOV 250 OK C: DATA S: 354 Start mail input;end with <CR><LF>.<CR><LF> C: ..sends body of mail message C: <CR><LF>.<CR><LF> S: 250 OK C: QUIT S: 221 Beta.GOV Service closing transmission channel

38. Mail Retrieval and Mailbox Manipulation • POP3—Post Office Protocol • POP3 client creates TCP connection to POP3 server on mailbox computer • Mailbox computer runs two servers: • SMTP to place mail into user mailboxes • POP3 server to allow user to extract messages from user mailbox • POP3 retrieves messages, deletes from user mailbox • Two servers must coordinate use of the user mailbox

39. Internet Message Access Protocol • POP stores messages offline; not compatible with the use of multiple computers • IMAP allows message access, manipulation from multiple computers • Platform-independent access to mail Question: How does server resource usage of IMAP compare with POP?

40. MIME Extension for Non-ASCII Data MIME—Multipurpose Internet Mail Extensions • Defined to allow transmission of non-ASCII data through mail • MIME allows arbitrary data to be encoded in ASCII, transmitted as standard email message • MIME message tells recipient type of data, type of encoding used • Data type and subtype is specified • MIME information is in 822 mail header

41. MIME Content Types

42. MIME Example From: bill@acollege.edu To: john@example.com MIME-Version: 1/0 Content-Type: image/gif Content-Transfer Encoding: base64 ….data for the image….

43. Mail Retrieval, Mailbox Protocols • Post Office Protocol—client login with userid/password • Client can then retrieve, delete messages • Server computer must run two servers: POP and SMTP • POP and SMTP must coordinate use of mailbox

44. Summary • Email is a very important Internet service • Separate standards are used for message format and transfer • SMTP—how a mail system on one machine transfers to a server on another • POP3—how a user can retrieve contents of a mailbox • IMAP—user protocol for use from multiple computers • MIME allows arbitrary data to be exchanged using SMTP