Ciscocertified Mail
Mail is the second most important service on the Internet after Web browsing. Journalists always talk about surfing the Web; most new PC purchases are the result of people wanting to dial up and look at Internet Web sites to find information. Keeping in touch is the second major reason for buying a PC, and e-mail is the popular way of doing so. Also, many companies now use e-mail as the primary method of distributing information to customers and distributing information internally.
So, if e-mail breaks, gets lost, or bounces, the effect is very noticeable. Receiving the bounce message four hours after sending an e-mail is very frustrating and adds to the perception that the Internet is unreliable. In fact, all that might be wrong is that the remote mail host has been disconnected.
Configuring mail servers to fit into an ISP backbone should not be that hard for a network engineer. Yet again, following the rule that two or more of everything is good will ensure that there is a reliable e-mail service. Indeed, some ISPs are almost as diligent with their mail server designs as they are with DNS.
The DNS is helpful in providing redundancy for mail systems using the concept of an MX (or mail exchanger) record. A domain can and should have more than one MX record. A number specifier follows, giving the priority of the mail system—the lower the value is, the higher in priority the mail relay is. The one with the lowest value is the primary MX, the intended destination of the e-mail.
Looking at what some organizations have done to configure their e-mail is helpful. The following example shows PIPEX's arrangement and is a good guide for how it should be done:
$ dig pipex.net mx
;; ANSWER SECTION:
pipex.net.
pipex.net.
8H IN MX 8H IN MX
10 shed.pipex.net.
lfallback1.lnd.ops.eu.uu.net.
pipex.net.
8H IN MX
99 fallback.mail.pipex.net.
;; ADDITIONAL SECTION: shed.pipex.net.
158.43.128.176 57m17s IN A 62.189.34.30
lfallback1.lnd.ops.eu.uu fallback.mail.pipex.net. fallback.mail.pipex.net. fallback.mail.pipex.net.
The primary device has an MX of 10, so mail for [email protected] will be delivered to this system if it is available. If the machine does not respond, mail goes to the fallback system with MX of 90. That system has a different IP address than the primary MX and is actually in a different physical location on the ISP's network. If that system also fails, PIPEX has provided a dedicated fallback system with three separate IP addresses.
ISPs that are providing connection services for customers really should offer MX relay services for their customers that are running their own SMTP gateways. Otherwise, the customer has only its own mail host for all e-mail—if the host goes down, e-mail is queued up on the sending system. If the host does not come back on time, the e-mail will be bounced to the sender. A typical setup for all MX records should look like the following:
Customer.com MX 5 mail.customer.com.
Customer.com MX 10 relay0.isp.net.
Customer.com MX 20 relay1.isp.net.
Mail for the customer is delivered directly to the customer's mail host. If it goes down or the circuit to the customer becomes unavailable, the ISP's relay systems take over in order of priority specified by the MX values. (Note that if an ISP is supplying relaying services for its customers, it must be very careful that its mail software is configured to allow relaying only for its own customers. Open relays are considered extremely bad practice and often result in the perpetrator's network being added to a routing black list.)
Usenet News has been around for as long as the Internet has existed in its various forms. Its main popularity centers on the non real-time distribution of information on virtually all possible topics that humankind could be interested in.
Providing a Usenet News (or simply news) service always has been a challenge for ISPs. In our perception, the news heyday was the mid-1990s, with huge volumes of information and materials being distributed around the Internet. A typical full newsfeed was consuming around 20 GB per day, making it both a challenge to store and a challenge to deliver on the typical circuit capacities at the time. It was quite
Continue reading here: Business Model and IP Address Space
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