December 1998
AN215: ADVANCED NETWORKING

QUESTION 5

Total Marks: 20 Marks

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SUGGESTED SOLUTIONS
Solutions and allocated marks are indicated in green.
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(a) TCP is a protocol used at the transport layer. What functions are provided by the transport layer? [5]
Award marks for these or other relevant answers:
cost optimisation (1 mark); error control (1 mark); flow control (1 mark); sequencing (1 mark); multiplexing (1 mark).

 
(b) The TCP/IP protocol suite is based around a four-layer model, in contrast to the seven-layer OSI model. Name each layer in the TCP/IP suite, and for each layer, provide a suitable example of a protocol or network used in that layer. [6]
Host-to-network, e.g. ethernet (1 mark).
Internet (network) layer, e.g. Internet protocol, IP (1 mark).
Transport layer, e.g. TCP or UDP (1 mark).
Application layer, e.g. Telnet, SMTP, or FTP (1 mark).
Allow credit for other correct examples. In addition, award (2 marks) for listing the layers correctly, or (1 mark) if there is one mistake.

 
(c) The internet protocol suite includes a file transfer protocol. As well as file transfer there are other functions that can be performed with this protocol. Describe four such functions. [4]
Authentication (for restricted remote login; file access only) (1 mark).
List remote directories (1 mark).
File format conversion, e.g. ASCII to EBCDIC or DOS to UNIX (1 mark).
Allows the user to access multiple machines in a single session; proxy transfer (1 mark).
Allow credit for other relevant answers

 
(d) In considering electronic mail, it is often useful to distinguish between the mail transport agent (which handles the transfer of mail messages from one host to another), and the mail user agent (which is concerned with the presentation of messages to the users, and accepting new messages when users type them).

The simple mail transfer protocol (SMTP) covers one of these tasks. Which is it? Describe the features of SMTP.

 [5]
Award up to (5 marks) for points from the following, or other correct observations about SMTP.
SMTP is a mail transport agent (1 mark). Its operation is typically hidden from the user (1 mark). It arranges host-to-host communication (1 mark), and can transfer multiple messages in a single connection (1 mark). It is also able to reverse the direction, whilst remaining connected, and transfer messages from the second host back to the first (1 mark). In addition, it has some functions that deal with destination verification and handling (1 mark).