Internet Mail Protocols

Internet Mail Protocols

There are currently three main Internet Email Protocols, SMTP, POP3, and IMAP4. Also there is ODMR which is a variation of SMTP. (There are several other mail protocols, eg UUCP which are currently less widely used, and aren’t currently supported by VPOP3). IMAP4 is only supported by VPOP3 Enterprise, not by VPOP3 Standard.

SMTP Protocol

SMTP (Simple Mail Transfer Protocol) is really intended for permanent connections to the Internet. The SMTP ‘client’ connects to an SMTP ‘server’ to send a message. There is no way to request a specific message using SMTP, but there are extensions to request a server to start sending any messages it has.

When messages are sent using SMTP it is sent in two parts:

1) An Envelope – this contains the email address it was sent from (typically for error reports) and a list of people to receive the message. This is not normally seen by users.

2) The message Data – this contains the message that you typically see.

The Envelope may contain a copy of the information in the Data’s From: and To: header fields, but it may contain other information which is not contained in the message at all (for instance, for mailing list messages or if BCC addressing is used).

If you have an SMTP account with your Internet Provider, then you need to run an SMTP ‘server’ (e.g. VPOP3) on your PC, and then somehow tell your Internet Provider to start sending messages to it after you’ve connected. Some Internet Providers can automatically detect you dialing into them, and immediately start sending mail to your SMTP server. Other Internet Providers need you to issue an SMTP command such as ETRN to their server to trigger mail delivery.

A few Internet Providers require a non-standard action, such as sending a ‘Finger’ command  to their server to trigger mail delivery. You can use an external program, set as the VPOP3 Post-Connection Extension to issue this command.

Once SMTP mail delivery has started there is really no way for the server to reject messages based on any other criteria apart from the data contained in the Envelope (i.e. From or To addresses). Also, once the message has been sent to an SMTP server, the client typically discards its copy.

This means that if VPOP3 is running as your SMTP server to receive mail from your Internet Provider, some useful features such as being able to limit message download size, the Download Rules, leaving messages on the ISP server etc, cannot be used. VPOP3’s SMTP Rules can perform many actions you may require, but they are not as flexible as Download Rules, because VPOP3 cannot see the message header without receiving the entire message.

When sending messages to another site, there are really two ways of doing this, both of which are typically accepted:

The mail server can send the message directly to the destination site’s mail server

The mail server can send the message to a relay-server which then sends the message to the destination site.

We recommend the use of the second method. Some of the reasons for this are:

It is easier to configure, and fits in with most users’ understanding of how mail works.

It is a lot quicker over a dial-up connection. If the first method is used, then any message to more than one recipient typically has to be sent multiple times (once for each recipient). There is also a lot of querying of DNS servers which can be time consuming.

Many Internet Providers (eg AOL) will reject mail that comes directly from a computer on a dial-up connection, as an anti-spam protection method.

 

ODMR Protocol

The ODMR (On Demand Mail Relay) protocol is a variation on SMTP which has been designed to allow SMTP mail delivery to a dynamic IP address. With ODMR, the ODMR client (VPOP3) connects to an ODMR server (at the ISP), logs on with the ISP account details, and from then on acts identically to an SMTP server (so the ODMR client becomes the SMTP server, and the ODMR server becomes the SMTP client).

This allows the advantages of SMTP without requiring a static IP address.

POP3 Protocol

POP3 (Post Office Protocol Version 3) was created for dial-up Internet accounts because of the limitations with the SMTP protocol. When collecting mail from an ISP using POP3, the ‘client’ is the PC at the user’s end, and it is in total control of what messages it receives and which ones it doesn’t.

The POP3 client can also typically view message headers without downloading the entire message, see the message size before downloading it, delete messages without downloading them, or leave messages on the server after downloading them. Because of these POP3 facilities, VPOP3 can do a lot more to help you.

One of the normal problems which occurs if multiple email addresses are directed to a single POP3 mailbox is that the SMTP Envelope (see above) is lost when the message is placed in the mailbox. This means that the explicit message routing information is lost, and all that VPOP3 has to go on is the data in the message headers (eg To, Cc etc). This can cause problems if you receive messages from mailing lists or which include Bccs. Some ISPs get around these problems by extending the POP3 protocol (eg Demon Internet Services do this) or adding special message header fields which VPOP3 can use if you tell it about them.

 

IMAP4 Protocol

IMAP4 (Internet Mail Access Protocol V4) is an email protocol which is sometimes used instead of the POP3 protocol. With IMAP4 email is stored on the mail server and can be accessed from any IMAP4 email client on the network. With POP3 email is downloaded to the mail client where it is accessed.

When using IMAP4 many of the functions of the email client are performed by the mail server instead. This includes things such as searching for messages, moving messages between folders etc.

In most cases the user will not know any difference between using IMAP4 or using POP3, so use whichever is more appropriate.

IMAP4 has some advantages in some situations:

1. Because all email is stored on the mail server it is easier to backup all email in one batch.
2. Users can access their email from anywhere on the network, so if your users do not have a fixed computer to use, IMAP4 can be the solution
3. Users can share mailboxes. The IMAP4 protocol allows several people to log onto a mailbox at once to read messages. This can be useful for ‘noticeboard’ type applications. Access Control Lists allow you to restrict which users can do which tasks in a mailbox.

There are some disadvantages to using IMAP4:

1. Because all mail has to be transferred over the network as it is read it can be slower than reading mail from the local PC where it has been downloaded to using POP3
2. The load on the network is usually much more. Mail is transferred every time it is read, so if a user reads a message at 10 different times, the message is transferred over the network 10 times rather than just once.

 

1. The load on the mail server is much more than with the POP3 protocol. Searching for messages etc will increase the load even more
2. The mail server needs much more mail storage space. When using POP3 the mail is stored on a user’s own PC so the server is not usually affected by large amounts of mail.
3. IMAP4 was not originally designed for remote users. Some email clients allow offline access to IMAP4 mailboxes but because this is not what the protocol was designed for, it can sometimes be unintuitive. The POP3 protocol was designed for remote users, so it is often more efficient and more intuitive.
4. The IMAP4 protocol is not supported by the Standard VPOP3 software, only by VPOP3 Enterprise.

 

 

Internet Routing Protocol

Internet Routing

How does Internet routing work? IP addresses and packet switching provide the technical infrastructure which routing protocols use to transmit packets across the Internet. The Internet Protocol transfers packets between networks and provides the software bridge that knits the whole thing together.

Robert Kahn and Vinton Cerf invented the basic architecture of Internet routing along with their development of the TCP/IP networking protocol

TYPES OF INTERNET ROUTING PROTOCOLS;

Interior Gateway Protocols (IGP)

Exterior Gateway Protocol (EGP)

INTERIOR GATEWAY PROTOCOLS

Interior Gateway protocols (IGP) are used to route Internet communications within a local area network, such as within an office building. The two main types of IGP protocols are described in the following sections, along with an example proprietary protocol for comparison purposes.

Routing Information Protocol (RIP)

Open Shortest Path First (OSPF)

Interior Gateway Routing Protocol (IGRP).

 

ROUTING INFORMATION PROTOCOL

The Routing Information Protocol (RIP) provides the standard IGP protocol for local area networks, and provides great network stability, guaranteeing that if one network connection goes down the network can quickly adapt to send packets through another connection. All RIP routing protocols are based on a distance vector algorithm called the Bellman-Ford algorithm, after Bellman’s development of the equation used as the basis of dynamic programming, and Ford’s early work in the area.

What makes RIP work is a routing database that stores information on the fastest route from computer to computer, an update process that enables each router to tell other routers which route is the fastest from its point of view, and an update algorithm that enables each router to update its database with the fastest route communicated from neighboring routers.

OPEN SHORTEST PATH FIRST

Open Shortest Path First (OSPF) is a particularly efficient IGP routing protocol that is faster than RIP, but also more complex.

The OSPF routing algorithm was created to provide an alternative to RIP, based on Shortest Path First algorithms instead of the Bellman-Ford algorithm. It uses a tree that describes the network topology to define the shortest path from each router to each destination address. Since OSPF keeps track of entire paths, it has more overhead than RIP, but provides more options. The main difference between OSPF and RIP is that RIP only keeps track of the closest router for each destination address, while OSPF keeps track of a complete topological database of all connections in the local network..

Interior Gateway Routing Protocol (IGRP)

Enhanced IGRP uses the same distance vector algorithm and distance information as IGRP. However, the convergence properties and the operating efficiency of enhanced IGRP have improved significantly.

The convergence technology is based on research conducted at SRI International and employs an algorithm referred to as the Diffusing Update Algorithm (DUAL). This algorithm guarantees loop-free operation at every instant throughout a route computation and allows all routers involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in re-computations.

The convergence time with DUAL rivals that of any other existing routing protocol. The initial implementation of IGRP operated in Internet Protocol (IP) networks. Enhanced IGRP extends IGRP so that it is independent of the network-layer protocol. In addition to IP, it now also operates in AppleTalk and Novell IPX networks.

Exterior Gateway Protocols (EGP)

While IGP protocols are used within local networks, Exterior Gateway Protocols (EGP) are used for routing between networks, generally on the Internet backbone itself, linking the different networks together. The following sections provide more information on the two common EGP protocols:

Border Gateway Protocol (BGP)

Exterior Gateway Protocol (EGP).

Border Gateway Protocol (BGP)

The most common Exterior Gateway Protocol in use on the Internet is the Border Gateway Protocol (BGP), ensuring that packets get to their destination network regardless of current network conditions.

Like RIP, the BGP algorithm provides great network stability, guaranteeing that if one Internet network line goes down, BGP routers can quickly adapt to send packets through another connection.

When a BGP router first comes up on the Internet, either for the first time or after being turned off, it establishes connections with the other BGP routers with which it directly communicates. The first thing it does is download the entire routing table of each neighboring router. After that it only exchanges much shorter update messages with other routers.

BGP routers send and receive update messages to indicate a change in the preferred path to reach a computer with a given IP address. If the router decides to update its own routing tables because this new path is better, then it will subsequently propagate this information to all of the other neighboring BGP routers to which it is connected, and they will in turn decide whether to update their own tables and propagate the information further.

Exterior Gateway Protocol (EGP)

This protocol is used throughout the 1980’s and into the mid-1990 was also somewhat confusingly named EGP. However, the EGP protocol had several problems, most notably an inability to scale up to support the growth in the size of the Internet Gateway Protocol (EGP).