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Telecommunications architectures

2001: Home assignment 5 - Model answer

Notice! The role of this model answer is instructional only. The answers might not cover everything in detail that the questions asks. The answers give you the content of the answer and idea what it should be. The things that are important in the grading of the homework have been emphasized. If you think there is a mistake in the model answer, please let the course staff know!

1. ICANN (6 points)

The Internet Corporation for Assigned Names and Numbers. ICANN is a non-profit organization. Its responsobilities include IP address space allocation, protocol parameter assignment, domain name system management and root server system management.

ICANN is runned by a board of nineteen directors. Nine At-Large directors (who represent the world's Internet users, some of which have been chosen accorfing to a vote of Internet users), nine selected by ICANN's three supporting organizations (which are advisory bodies) and a president/CEO.

ICANN's functions were formerly taken care by US government (via appropriate organizations). Now that the Internet is a lot more than just a military project, ICANN has been formed as sort of independent international organization.

ICANN has many advisory committees (including the ones of the three supporting organizations. One can always wonder if this is democratic, efficient, economically reasonable and so on. Well justified answers with opinion on some aspect of ICANN are nice.

2. Domain Name System (6 points)

Domain Name System. Hierarchical, domain-based naming system using distributed databases. No central management. DNS maps domain names to IP-addresses, so user doesn't need to know computer numeric address. Using internet would be very inconvenient without DNS (e.g. browsing web at http://130.233.18.62/index.html or sending mail for roskakori@130.233.18.62).
Distributed database. One big hosts-file would be very hard to maintain and impossible to send for every host every day. Name conflicts would be evident.
SOA = Start of Authority, provides the name of the primary source of information about the name server's zone, them email address of its administrator, serial number, flags and timeouts.
A = Address, 32-bit integer, the IP address of the host.
MX = Mail exhange, indicates that domain is willing to accept mail.
NS = Name server, name of the server for this domain
CNAME = Canonical name, allows aliases to be created.
PTR = Pointer, alias for an IP address
HINFO = Host description, CPU and operating system in ASCII.
TXT = Text, uninterpreted ASCII text, allows domains to identify themselves in arbitary ways.

3. Email (6 points)

Email client establishes tcp connection to port 25 on SMTP server (of course, SMTP server may be running on the same machine as the client. If not client may need to fetch the ip address from DNS-server before, but that's another story). Server announces it's presense (220 plus whatever administrator has decided), client send command HELO or EHLO depending witch protocol version it prefers. Server replies. Client sends commands MAIL FROM, RCPT TO and DATA. Server acknowledges and waits for message data. Now client sends the whole message, including fields like To:, From, Reply-To:, Subject and so on. Message is ended by single . on a line. After that, server acknowledges that message has been send. Client ends the connection by QUIT command.
MIME (Multipurpose Internet Mail Extensions) is used everytime when someone wants to send something else than ASCII plaintext. MIME is necessary for sending file attachments, text with non-english characters etc. Different MIME types have been defined: text (rich or plain), images, audio, video, application, message and multipart.
Base64 encoding is used to send messages that don't comply to RFC822 (e.g. binary messages or messages with non-ASCII characters). Base64 breaks up groups of 24 bits to four 6-bit units. These units are sent as legal ASCII characters (A=0, B=1 and so on, then lowercase, ten digits and + and /). Carriage returns and line feeds are ignored, so they can be inserted to comply 1000 character line lenght limit of RFC822. Messages with only few non-ASCII character quated-printable encoding is used. Message is sent in 7-bit ASCII with all the characters above 127 coded as "=" sign and character value in hexadecimal (e.g. =1A).

4. Manual use of protocols (4 points)

kosh ~ 51 % telnet news.tky.hut.fi 119^M^M
Trying 130.233.16.199...
Connected to vara.tky.hut.fi.
Escape character is '^]'.
  S: 200 news.tky.hut.fi InterNetNews NNRP server INN 2.2.2 13-Dec-1999 ready (posting ok).
  C: post
  S: 340 Ok
  C: from: Jukka Larja 
  C: newsgroups: otax.test
  C: subject: testii
  C:
  C: yykaakkoo
  C: .
  S: 240 Article posted
  C: quit
  S: 205 .
^MConnection closed by foreign host.
kosh ~ 52 % exit^M^M

kosh ~ 51 % telnet news.tky.hut.fi 119^M^M
Trying 130.233.16.199...
Connected to vara.tky.hut.fi. 
Escape character is '^]'.
  S: 200 news.tky.hut.fi InterNetNews NNRP server INN 2.2.2 13-Dec-1999 ready (posting ok).
  C: group otax.test   
  S: 211 123 23413 26773 otax.test
  C: article 26773
  S: 220 26773 <9q49vs$1di$1@vara.tky.hut.fi> article
  S: Path: news.tky.hut.fi!not-for-mail
  S: From: Jukka Larja 
  S: Newsgroups: otax.test
  S: Subject: testii
  S: Date: 11 Oct 2001 14:22:52 GMT
  S: Organization: TKK/TKY
  S: Lines: 1
  S: Distribution: otax
  S: Message-ID: <9q49vs$1di$1@vara.tky.hut.fi>
  S: NNTP-Posting-Host: kosh.hut.fi
  S: X-Trace: vara.tky.hut.fi 1002810172 1458 130.233.228.10 (11 Oct 2001 14:22:52 GMT)
  S: X-Complaints-To: usenet@news.tky.hut.fi
  S: NNTP-Posting-Date: 11 Oct 2001 14:22:52 GMT
  S: Xref: news.tky.hut.fi otax.test:26773
  S: 
  S: yykaakkoo
  S: .
  C: quit
  S: 205 .
^MConnection closed by foreign host.
kosh ~ 52 % exit^M^M

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The page has been updated 4.12.2001

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