At the most basic level, there are only three BGP policies: pushing traffic through a specific exit point; pulling traffic through a specific entry point; preventing a remote AS (more than one AS hop away) from transiting your AS to reach a specific destination. In this series I’m going to discuss different reasons for these kinds of policies, and different ways to implement them in interdomain BGP.
There are many reasons an operator might want to select which neighboring AS through which to send traffic towards a given reachable destination (for instance, 100::/64). Each of these examples assumes the AS in question has learned multiple paths towards 100::/64, one from each peer, and must choose one of the two available paths to forward along.
In the following network—
From AS65004’s perspective…
Transit providers primarily choose the most optimal exit from their AS to reduce the amount of peering settlement they are paying by using and maintaining settlement-free peering where possible and reducing the amount of time and distance traffic is carried through their network (through hot potato routing, discussed in more detail below).
If, for instance, AS65004 has a paid peering relationship with AS65002, and a contract with AS65003 which is settlement-free so long as the traffic between AS65004 and AS65003 is roughly symmetric. AS65004 has two roughly equal-cost paths (both have the same AS Path length) towards 100::/64. In this situation, AS 65004 is going to direct traffic towards AS65003 to maintain symmetrical traffic flows and direct any remaining traffic towards AS65002.
This kind of balancing is normally done through a controller or network management system that monitors the balance of traffic with AS65003, adjusting the preference of sets of routes to attain the correct balance with AS65003 while reducing the costs of using the link to AS65002 to the minimum possible.
From AS65005’s perspective…
AS65005 can either send traffic originating in AS65001, received from AS65002, and destined to AS65006, to either AS65004—a peer—or AS65006—a customer. The internal path between the entry point for this traffic is longer if the traffic is carried to AS65006, and shorter if the traffic is carried to AS65004. These longer and shorter paths give rise to the concepts of hot and cold potato routing.
If AS65006 is paying AS65005 for transit, AS65005 would normally carry traffic across the longer path to its border with AS65006. This is cold potato routing. AS65005’s reason for choosing this option is to maximize revenue from the customer. First, as the link between AS65005 and AS65006 becomes busier, AS65006 is likely to upgrade the link, generating additional revenue for AS65005. Even if the traffic level is not increasing, steady traffic flow encourages the customer to maintain the link, which protects revenue. Second, AS65005 can control the quality-of-service AS65006 receives by keeping the traffic within its network for as long as possible, improving the customer’s perception of the service they are receiving.
Cold potato routing is normally implemented by setting the preference on routes learned from customers, so these routes are preferred over all routes learned from peers.
If AS6006 is not paying AS65005 for transit, it is to AS65005’s advantage to carry the traffic as short a distance as possible. In this case, although AS65005 is directly connected to AS65006, and the destination is in AS65006, AS65005 will choose to direct the traffic towards its border with AS65004 (because there is a valid route learned for this reachable destination from AS65004).
This is hot potato routing—like the kids’ game, you want to hold on to the traffic for as short an amount of time as possible. Hot potato routing is normally implemented by setting the preference on routes to the same and relying on the IGP metric component of the BGP bestpath decision process to find the closest exit point.
Next week I’ll continue this series on BGP interdomain policies… feel free to leave a comment if you think I’ve explained something incorrectly, etc.