From the December 95 I/S newsletter: ========== The Way of E-Mail -- Routings and Reasons for Occasional Delays Computing Support Services - Training The Way of E-Mail: Routings and Reasons for Occasional Delays Postmaster, Network Services Susan B. Jones, Publication Services A few times this fall, you may have noticed that you didn't receive e-mail for hours, or perhaps even a day. Network Services had reports from several people that they weren't notified of a meeting until after the meeting was held, due to delays in e-mail delivery. What causes delays like these, and just how dependable is e-mail? The good news is that about 99% of the 400,000 e-mail messages that get processed at MIT each day are delivered in a timely fashion. On the other hand, you shouldn't send out anything of an emergency nature strictly via e-mail. IS can't guarantee instant delivery due to the nature of the technology. How E-Mail Gets Routed All e-mail coming into MIT (and some being sent from MIT) goes to a server called a hub, where it is processed and sent on to its next stop. If the mail is for an MIT e-mail user, it is sent to a post-office server (also known as a POP server) where it is stored until the recipient downloads it. The average delay for mail coming from outside MIT is about eight minutes. Mail sent from one MIT user to another is usually delivered to a post-office server within seconds. However, a message can be queued on a server and not be delivered immediately for several reasons. A server may become overloaded and need to store messages in a queue until it can catch up later when the load goes down. Overloading can occur when someone sends a mass mailing, or "spam" (see box). Another cause is when a host goes out of control by sending a message repeatedly in a rapid-fire manner. This can happen with so-called "vacation" programs, for example. There isn't much that can be done to guard against such barrages. When these situations occur, the servers will queue mail and possibly go as far as refusing to accept new mail in order to protect the system. Sometimes a site cannot be reached. This occurs routinely on the Internet and, when it happens, messages destined for this site are queued for later delivery. If a significant portion of the Internet is unavailable (for example, due to an outage), thousands of mail servers start to queue mail. This happened on December 4, when the amount of queued mail was so extraordinary that the MIT post offices had a difficult time keeping up with messages sent to MIT users. The result was a few thousand messages queued for several hours. The Bottom Line The important part of this to keep in mind is that while MIT has an e-mail system that usually delivers mail very rapidly, there is nothing in the technology used that guarantees this response time. Message queueing and the delays that result from this are a normal response of the system to overloads or other perturbations. IS Network Operations does its best to avoid and minimize the impact of these occurrences, but users need to set their expectations accordingly. ------------------- Don't Spam the Net E-mail is a fast, convenient form of communication. It's easy to send e-mail to multiple individuals and multiple mailing lists. But this ability to send messages to many people also makes it easy to misuse the system. The general rule is: use e-mail to communicate with specific users, not to broadcast announcements to the user community at large. These guidelines are not based on etiquette alone: the mail system doesn't have the capacity to process a very large number of e-mail messages at once. When a user sends an announcement to a huge list of recipients, the mail servers get overloaded, disks fill up, and staff intervention is required. This results in a degradation of service for all users. Electronic chain letters are especially abusive of the mail system and network. They waste valuable computing resources, and may be considered harassing.