Networking

1. Networking • A few questions on the course • A few questions about the course

2. 27 of 70 The Chancellor of the University is • Alistair Darling • Lord Patten • Glynis Breakwell • Prince Philip • Lord Tugendhat • Lord Hinton of Bankside

3. That was the “warmup”: E • Darling is Chancellor of the Exchequer • Lord Patten is Chancellor of Oxford • Glynis Breakwell is the Vice-Chancellor • Prince Philip is Chancellor of Cambridge • Lord Tugendhat is the Chancellor • Lord Hinton was the first Chancellor: the mullets (stars) on the unicorn and wyvern supporting the coat of arms are a heraldic pun on his own arms

4. Questions on the course (mostly factual, but there is more to the course than facts)

5. How long is an Ethernet address? • Variable length • 16 bytes • 4 bytes • 6 bytes

6. The answer is D (6 bytes) If you didn’t get that, you’re still pretty confused: After last time, and the previous one, all I can suggest is that you really re-read sections 2.2 and 3.2 of the book.

7. Which of the following will DHCP not tell you • Your IP address • A router’s IP address • A nameserver’s IP address • A nameserver’s name • The subnet mask

8. D — the nameserver’s name • It’s actually not useful • And anyway you can find it out: how?

9. The reset (RST) bit is set in the last packet of a 4-way close • True • False

10. False • RST is not a normal part of TCP operation: it is there to indicate some kind of error/ abnormal circumstance

11. After the 3-way open, MSS is • Negotiated • The minimum of what the two said • The average of what the two said • Possibly different in the two directions

12. D — possibly different There is no negotiation, and it is perfectly possible for MSS to be different in both directions (e.g. one end is on a dial-up link)

13. Consider the following TCP Open scenario: • P→Q: SYN (with MSS) • Q→P: SYN (with MSS, and ACK) • P→lost: ACK What happens next?

14. P’s ACKof Q’s SYN is lost, and • The connection hangs • P eventually times out • P starts sending data • P resends its SYN • Q resends its SYN • P sends a RST

15. C (or possibly E) • Hanging would be a failure of TCP • P thinks the connection’s open, so why time out? • Therefore can send data (which will acknowledge the SYN as well) • Why should P send SYN: it’s been acked • Q hasn’t has SYN acked, so will resend if C doesn’t happen (e.g. SMTP, where the server first sends a 220) • Why should it send RST?

16. Which of these is not in the TCP header • Sequence Number • Options • Receiver Window • Congestion Window • Urgent Pointer

17. D — Congestion Window • Sequence number is always there • Options, if presentare in the header • Receiver Window is always there • Congestion Window is purely maintained by the sender, and is not passed to the other end (its effects may be felt, but that’s another matter) • Urgent pointer is always there

18. A receiver drops packets to indicate that its window is full • True • False

19. False It may drop, and not acknowledge, the data (as in the persist timer examples), but it does always respond to the packet, even if the acknowledgement field is not incremented.

20. Network congestion can be indicated by • Time out • Window =0 • 2 duplicate ACKs • 4 ACKs of the same number • A and D • A,B and C • B and C

21. E (A and D) is right • Timeout is a good indicator of congestion (but the other end might have crashed …) • Not network congestion • Not 2 Duplicate ACKs • 4, with the same number, means 3 are duplicates, so this is congestion

22. The persist timer deals with which problem? • Hosts that go down • Hosts that have no space for more data • Lost SYN packets • Lost window adverts • Lost FIN packets

23. D: lost window adverts Hosts that go down: TCP times out sending Hosts with no space — there’s no cure for this! Lost SYN packets, retransmission deals with this The response to the persist timer will give us a new window size (which may still be same, if there is still no space) Lost FIN packets, retransmission deals with this

24. “slow start” is somewhat of a misnomer • True • False

25. Possibly It does indicate that we start slow (so far, so good), but we get exponentially faster, so most people probably wouldn’t call it slow

26. Sending `window scale’ means • I will send scaled windows • I will understand scaled windows • I will send scaled windows if you will • You can send scaled windows if you want • B and C • B and D

27. E is the best answer • You can’t scale unless the other end says it understands scaling • If you can send them, you must be able to understand them • Certainly correct • Some TCPs send window scale automatically even if the other end didn’t (of course, they can’t use it)

28. In FTP, the control channel • Starts in ASCII or binary depending on the options • Starts in ASCII, but can be switched to binary • Is always in ASCII • Uses XDR

29. C – always ASCII • The other end doesn’t know what the options are, so this can’t be right (a general point about protocols!) • The IMAGE command changes the data channel only • Right • XDR is connected with NFS, not FTP

30. One of these statements is true • SMTP constructs the header from the envelope • The envelope determines who gets the mail • The header determines who gets the mail • The header MUST agree with the envelope on this • The header should agree with the envelope on this

31. B: envelope→sender • If anything, the sending agent (not SMTP) constructs the envelope from the header • Correct • False • Not an RFC requirement: The header might well not agree • It is impossible for C’s machine to check whether C occurs on a mailing list administered by B, or even whether it is a mailing list

32. MIME is • A feature of e-mail only • A feature of the Web only • What Marcel Marceau did • A general-purpose encoding, first invented for mail

33. D — general purpose • It did start out in the mail community, but there is nothing really mail-specific about it • (Marcel Marceau did Mime: different capitalisation!)

34. NFS can be told to handle both binary and ASCII files • True • False

35. False • NFS files are fundamentally binary, and the application has to deal with any differences (e.g. extra CR/LF in ASCII from Windows)

36. Now that we have NFS and the Web, FTP is obsolete • True • False

37. False • It’s less common to type the ftp command, but we see (or at least use) ftp:// URLs. • FTP is fundamentally different from NFS — FTP creates copies, whereas NFS allows multiple computers to access the same file.

38. UDP is used in many circumstances when TCP could be used instead • True • False

39. True • NFS often uses UDP, where TCP would be as good, possibly better • DNS uses UDP (which is simpler), and TCP could be used, though probably less efficient.

40. Therefore the internet could be run without UDP • True • False

41. False • RIP and BOOTP/DHCP both rely on broadcast (or multicast for some RIP v2 installations) and therefore can’t use TCP • Many uses of multicasting (internet radio etc.) would be impossible, which would cause bandwidth/load problems as well

42. It is impossible to build reliability on top of UDP • True • False

43. False • TFTP manages to do it. • TCP builds it on top of IP, and UDP is (by definition) no worse than IP, so clearly it’s possible (which doesn’t mean it’s desirable)

44. Web hosting firms need HTTP 1.0 (or later) because • It allows binary • It allows MIME • It allows the complete URL in GET • It can send the length of the data file

45. C — complete URL • All HTTP allow binary (in the data) • MIME was in 0.9, and anyway isn’t very relevant • Without the complete URL, we wouldn’t know what page we were called on • Size is useful, but not necessary

46. Which of these is not part of the NFS complete suite • XDR • Portmapper • TLS • Remote procedure call

47. C — TLS • eXternal Data Representation is vital: times, lengths etc. • Portmapper to find RPC programs • TLS — Transport Layer Security. Not an intrinsic part. • Remote Procedure Call — the underlying paradigm

48. Recall the ISO 7-layer model • Physical • Link • Network • Transport • Session • Presentation • Application

49. Which layer is MIME? • Physical • Link • Network • Transport • Session • Presentation • Application

50. 6 — Presentation • Describes how the data is meant ot be interpreted at the other end.