I have been given the task of installing SQUID server with follwing features in it:-

1) It should support two internet connections simultaneously and if one of the connection is down then it should automatically switch to other connection.

2) We should know which system is using how much bandwidth in the network.All details should be maintained that which system is pening which websites and downloading what data.

3) Limitations and restrictions are applied like how much maximum data can be downloaded or attached in a mail.

Please give me ideas how i can configure SQUID with these features and i will be very grateful to you.

You can do 1st with iptables. I do not know if squid can do that without netfilter.

If your squid is working fine and logging the requests, your 2nd issue will be solved without much issue. Squid logs all the data passing through.

You can use delay pools for your download and bandwidth restrictions. But I am bit suspicious about the mail attachments. It should be done by your mail server.