Policy Routing for Fun and Profit
Sangoma is a manufacturer of PCI-based WAN interface cards. For demonstrations and redundancy, we have two separate high-speed Internet connections: a full T1 carrying Frame Relay, using our PCI S5148 T1/E1 modem, and an ADSL connection carrying PPPoE over ATM, using our PCI S518 ADSL modem. The ADSL line is shared with our fax machine, the only telephone line not connected to our PBX. We use two different ISPs for the services. The ADSL and fax telephone line are provided by Bell Canada's Sympatico service, and the T1 Frame Relay connection is provided by MCI.
The combination of the installed T1 and ADSL Internet links provide reliable connectivity, but the resultant bandwidth is excessive for our requirements. Sangoma's Web site is hosted by Earthlink in Atlanta, so our Internet access requirements are not too different from any other company's, primarily e-mail and Web access, with some FTP traffic mainly as uploads to the FTP server on our Web site. We could manage without a fixed IP address, although we do find it valuable that the T1 link is provisioned with a range of fixed addresses.
All our Internet servers are Linux-based. Although we support Windows, FreeBSD, Solaris and other popular operating systems, Linux is our most important platform, and only Linux has the rich toolset of traffic management routines we need. The layout is shown in Figure 1.
The ADSL line is inexpensive, especially after the rebate we get for using our own ADSL modem instead of the consumer-grade external ADSL modem normally provided as part of the service. T1 in Canada is expensive as compared to the US; the cost of a standard unrestricted T1 Internet link can exceed $1,900 CAN ($1,450 US) per month.
Sangoma resells Internet access to two other tenants in our building to offset costs somewhat. The other parties sharing our connections host Web and VPN services, so they need both fixed addresses and high outbound bandwidth, which is why they are interested in sharing our T1 line. The private and public segments of the T1 line are shown in Figure 2.
The two Sangoma Linux machines could be combined into one quite easily. The combination router would have another NIC to support the public network segments to customers A and B. Sangoma's firewall would operate between Sangoma's private LAN segment and all the other public segments, which include the Frame Relay T1 link, the ADSL link and the public Ethernet link.
The financial contributions from customers A and B are not enough to pay for a full T1 at Canadian prices. The solution for us was to employ a usage-based service for T1. This is a so-called burstable T1 service, which is about half the price of a full bandwidth T1 line. The T1 use is unrestricted up to the full bandwidth of 1,536Mbps full duplex. Billing is based on the 95th percentile of the bandwidth used. Traffic is sampled in five-minute intervals, and the average bandwidth for the five minutes is calculated. At the end of the month, these five-minute bins are arranged in decreasing order of bandwidth. The top 5% are discarded, and the billing rate depends on the next highest bin for the month. The trigger throughput in our case is 128kbps. If our 95th percentile throughput exceeds 128kbps, the monthly line charge is incremented by a minimum of $300.
This complicated billing structure is hard for subscribers to understand. It looks like a good deal to the customer but is difficult to manage and hard to measure. The billing rate is measured at the 5% level, where the rate of change of the usage curve is near a maximum. So, many users find themselves paying high bills that depend on the bandwidths of only one or two five-minute bins out of the more than 8,500 bins measured each month. Unless one's traffic is consistently low, one quickly finds that the surcharges are such that one may as well bite the bullet and take the full T1, even though the average throughput may be well below 128kbps.
The major plus is that the highest 5% of bandwidth usage for each month is “free”. This amounts to about 36 hours per month at any bandwidth without penalty, so almost a full working week out of the month is available at full line speed. Figure 3 shows the ideal traffic pattern for achieving the highest throughput for the lowest cost on our burstable T1 service. Essentially, the aim of the traffic control logic is to restrict the T1 bandwidth to 128kbps after the first free 36 hours of unrestricted bandwidth has been consumed in a given month.
So how do we manage to take the bait without the hook? With a series of scripts and dæmons that use a combination of policy routing, IP accounting and traffic shaping we can manage the network intelligently, so both we and our co-users get maximum performance at minimum cost.
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