Table A
a Internet address
b Resolution Protocol
с Look-up table
d Gateway
e User Datagram Protocol
f Transmission Control Protocol
i Standard used for software that routes data through a gateway
ii Standard used by software that moves information to the correct application on the receiving system of a network
iii Standard used by software that manages communication exchanges between computers on the Internet
iv A 32-bit number identifying a node on an IP network
v Stored information used to route data through a gateway
vi A device for connecting dissimilar networks
2 Mark the following statements as True or False:
a Internet addresses are an integral part of the IP protocol.
b Internet addresses can be written as a series
of numbers. с UDP software provides the final routing for data within the receiving system.
d UDP recovers packets that aren't successfully delivered.
e TCP only works with packet-switched networks.
f TCP only works when it is combined with IP.
Text 8
I. Find the answers to these questions in the following text.
1. Name three different email protocols mentioned in the text.
2. Which email protocol is used to transfer messages between server computers?
3. Why is SMTP unsuitable for delivering messages to desktop PCs?
4. Name two host-based mail systems mentioned in the text.
5. Where are email messages stored in an SMTP system?
6. What happens when you use your Web mail account to access a P0P3 mailbox?
7. Give an advantage and a disadvantage of having an option to leave P0P3 messages on the server.
8. What are the advantages of using the IMAP4 protocol?
Email Protocols
Although the format of a mail message, as transmitted from one machine to another, is rigidly defined, different mail protocols transfer and store messages in slightly different ways. The mail system you're probably used to employs a combination of SMTP and P0P3 to send and receive mail respectively. Others may use IMAP4 to retrieve mail, especially where bandwidth is limited or expensive.
Simple Mail Transfer Protocol
SMTP is used to transfer messages between one mail server and another. It's also used by email programs on PCs to send mail to the server. SMTP is very straightforward, providing only facilities to deliver messages to one or more recipients in batch mode. Once a message has been delivered, it can't be recalled or cancelled. It's also deleted from the sending server once it's been delivered. SMTP uses 'push' operation, meaning that the connection is initiated by the sending server rather than the receiver. This makes it unsuitable for delivering messages to desktop PCs, which aren't guaranteed to be switched on at all times. In host-based mail systems, such as Unix and Web mail, SMTP is the only protocol the server uses. Received messages are stored locally and retrieved from the local file system by the mail program. In the case of Web mail, the message is then translated into HTML and transmitted to your browser. SMTP is the only protocol for transferring messages between servers. How they're then stored varies from system to system.
Post Office Protocol
POP is a message-retrieval protocol used by many PC mail clients to get messages from a server, typically your ISP's mail server. It only allows you to download all messages in your mailbox at once. It works in 'pull' mode, the receiving PC initiating the connection. PC-based P0P3 mail clients can do this automatically at a preset interval. When you use your Web mail account to access a POP3 mailbox, the mail server opens a connection to the P0P3 server just as a PC-based application would. The messages are then copied into your Web mailbox and read via a browser.
Since POP3 downloads all the messages in your mailbox, there's an option to leave messages on the server, so that they can be picked up from different machines without losing any. This does mean that you'll get every message downloaded every time you connect to the server. If you don't clean out your mailbox regularly, this could mean long downloads. When using a Web mail account to retrieve P0P3 mail, be careful about leaving messages on the server - if too many build up, each download will take a long time and fill up your inbox. Many Web mail systems won't recognize messages you've already downloaded, so you'll get duplicates of ones you haven't deleted.
Internet Mail Access Protocol
IMAP is similar in operation to POP, but allows you more choice over what messages you download. Initially, only message headers are retrieved, giving information about the sender and subject. You can then download just those messages you want to read. You can also delete individual messages from the server, and some IMAP4 servers let you organise your mail into folders. This makes download times shorter and there's no danger of losing messages.
II. Re-read the text to find the answers to these questions.
1 Mark the following statements as True or False:
a Different mail systems transfer emails in different ways.
b IMAP4 requires more bandwidth than the other email protocols.
с SMTP is used for sending emails from a PC to a server.
d SMTP delivers messages one at a time.
e SMTP does not allow a delivered message to be cancelled.
f SMTP is only one of many protocols used to send mail between servers.
g POP protocol allows the user to download one message at a time.
