Hot-potato and cold-potato routing

This article is about the use of these terms in the context of routing policy. For the packet-switching technique sometimes also called "hot potato routing", see deflection routing.

In commercial network routing between autonomous systems which are interconnected in multiple locations, hot-potato routing is the practice of passing traffic off to another autonomous system as quickly as possible, thus using their network for wide-area transit. Cold-potato routing is the opposite, where the originating autonomous system holds onto the packet until it is as near to the destination as possible.

Hot-potato routing

Hot-potato routing is the normal behavior of most settlement-free peering agreements. Hot-potato routing has the effect that the network receiving the data bears the cost of carrying it between cities. When the traffic ratio (the ratio of traffic flowing in one direction to the traffic flowing in the other direction between peers) is reasonably even, this is considered fair, because the networks will share evenly in carrying traffic exchanged by their customers between cities.

The marginal cost of carrying traffic between cities depends on how the network has purchased those links; some networks own dark fiber, which can be upgraded by merely replacing the equipment on each end of the fiber, and possibly the amplifiers along the path between cities. In other cases, the network has an agreement with a telco that allows for a specific amount of bandwidth, and upgrading involves paying more money to the telco.

(n.) A form of routing in which the nodes of a network have no buffer to store packets in before they are moved on to their final predetermined destination. In normal routing situations, when multiple packets contend for a single outgoing channel, packets that are not buffered are dropped to avoid congestion. But in hot potato routing, each packet that is routed is constantly transferred until it reaches its final destination because the individual communication links can not support more than one packet at a time. The packet is bounced around like a "hot potato," sometimes moving further away from its destination because it has to keep moving through the network. This technique allows multiple packets to reach their destinations without being dropped. This is in contrast to "store and forward" routing where the network allows temporary storage at intermediate locations. Hot potato routing has applications in optical networks where messages made from light can not be stored in any medium.

Cold-potato routing

Cold-potato routing, on the other hand, is more expensive to do, but keeps the traffic under your control for longer, allowing operators of well-provisioned networks to offer a higher quality of service to their customers. It can also be preferred when connecting to content providers; if content providers use hot-potato routing, they may escape from paying for the cost of links between cities.

Cold-potato routing is prone to misconfiguration as well as poor coordination between two networks. In such scenarios, packets can be routed further distances as well as allow another autonomous system to manipulate routing in your network for various purposes. Cold-potato routing requires a level of trust between two networks that either side will not attempt to "cheat" the other.

Some content networks favor the use of cold-potato routing (multi exit discriminator exchange/honoring) in order to deliver content from replicated server farms closer to the end-user.

Route announcement policy

The terms can also be used to describe the route announcement policy of a network: by choosing to announce their network at a large number of points at the periphery of another autonomous system, a provider can pull incoming traffic onto their network as soon as possible, ensuring that the traffic stays on their network all the way to their customer's connection.