The Application Layer. Chapter 7. DNS – The Domain Name System: RFC 1034, 1035. The DNS Name Space : No rule against registering under two top-level domains but not many organizations except multinationals do it( For Eg. Yahoo.com and Yahoo.co.in
The Application Layer
A portion of the Internet domain name space.
Domain_name TTL Class Type value
TTL: How much stable/reliable is this entry, for heavily used site this could be big and infrequently used site it could be small.
Class: Theoretically, for Internet related information it is IN, for other purposes other codes may be used, but in practice other codes are rarely seen.
The principal DNS resource records types.
A portion of a possible DNS database for cs.vu.nl.
Part of the DNS name space showing the division into zones.
How a resolver looks up a remote name in eight steps.
As resource records trace back through various name servers, they are entered into a cache there.
This information is not authoritative, since the changes made at cs.yale.edu before the use of the entry are not reflected in the cache entry.
Hence the TTL field. Sites which have held their IP addresses for long can have a large value(1 day or 86400secs) for this field and others can have smaller value depending upon how frequently they change.
Some smileys. They will not be on the final exam :-).
Architecture: Header and Body.
An example display of the contents of a mailbox.
RFC 822 header fields related to message transport.
Some fields used in the RFC 822 message header.
MIME – Multipurpose Internet Mail Extensions: RFC 2045-2049
RFC 822 headers added by MIME.
The MIME types and subtypes defined in RFC 2045.
A multipart message containing enriched and audio alternatives.
An SMTP client (running on the sender’s mail server) establishes a TCP connection at port 25 with SMTP server (running on receiver’s mail server).
SMTP server is continuously listening to port 25 as explained earlier.
If the server is ready to accept mail from the client it informs else try later.
Once the connection is established, client announces whom the mail is coming from and whom is it meant for.
If such a recipient exists at the server’s side it gives a go-ahead.
Client sends the message and the server acknowledges. Send more if required else connection is released.
(a) Sending and reading mail when the receiver has a permanent Internet connection and the user agent runs on the same machine as the message transfer agent. (b) Reading e-mail when the receiver has a dial-up connection to an ISP.
Once the receiver downloaded the message, it was erased from the mailbox at the ISP.
The only copy was at the user’s hard disk and if it crashes all the mails are gone.
A comparison of POP3 and IMAP.
(a) A Web page (b) The page reached by clicking on Department of Animal Psychology.
The parts of the Web model.
(a) A browser plug-in. (b) A helper application.
A multithreaded Web server with a front end and processing modules.
A server farm.
(a) Normal request-reply message sequence.
(b) Sequence when TCP handoff is used.