At a NANOG Peering BOF, Vijay Gill described various stages in the lifecycle of a peering network, characterized by a change in motivations.
The 5 Stages of Peering Policy Lifecycle
During the initial stages of peering, the network is most likely concerned primarily with reducing their IP transit costs by gaining access to other networks "for free". At this point they are usually interested in peering with every other network willing to peer, regardless of the size of the network. This is generally referred to as an "Open Peering Policy."
As the peering network increase its size, scope, and amount of peering, their motivations begin to include the desire to improve performance and increase their control over their routing. At this stage, the peering network is interested in continuing to expand their peering, and begins to focus their energies on meeting the requirements of more selective networks. Less time and attention is paid to turning up new peering with networks that are considered "less important", though they may still be willing to accept new requests from smaller networks.
As the peering network continues to grow, they begin to think strategically about how to continuing scaling their peering. At this stage they often become "selective", establishing a set of peering requirements based on traffic levels, locations, backbone capacity, ratios, or any combination of the above, in order to limit their peering activities to only networks they consider to be important. The peering network begins to realize that they can increase their traffic levels to larger networks they desire to peer with by not directly peering with customers of those networks, and may begin to depeer some of their smaller or more difficult peers.
By the next stage, the peering network has already established peering with a large portion of the Internet, including the majority of the other networks selective networks. The only networks left to peer with are the "restrictive" networks who are not interested in establishing any new peerings. The peering network continues to increase its selective peering requirements in order to focus only on peering with the remaining few networks, and is very disinterested in establishing new peerings with smaller networks or potential new competetitors. By now the peering network is usually quite large, and begins to leverage external business relationships such as the purchase of transport or fiber in order to secure peering with restrictive peers.
Finally, the peering networks reaches the plateu of the "tier 1" network. They are no longer interested in establishing any new peerings with new networks, and enter the category of "restrictive". By now the network is usually categorized as "huge", and exchanges an enormous amount of traffic with only a handful of other large networks.
During this lifecycle, from a small network just beginning its peering, to a huge network that sends 100% of its traffic to peering, the motivations for public vs private peering changes vastly.
In stage 1, the peering network usually has no choice but to peer publicly. The choice of where and how to peer is largely dictated by the other party, and the peering networks usually chooses locations based solely on the number of other peering partners available there.
In stage 2, the peering network is usually enjoying the capacity and burstability benefits of a relatively large peering port. They usually prefer to keep peers on these large public ports, and any private peers they establish will be based on the requirements of the other party, and will be of a capacity smaller than their public peers.
In stage 3, the peering network is moving some of their largest peers to private peering, but continues to use public peering to aggregate most of the smaller peers. While they are often not looking to establish new peering with any new network who connects to a public peer, the public peer is still an excellent location to conduct trials.
In stage 4, the peering network is attempting to divorce itself from its public peering. The majority of its traffic by volume is now being sent via large private peers, and public peers are not being upgraded. As peers increase their traffic levels, they are moved onto private peering. Towards the end of stage 4, the peering network may seek to disconnect public peering ports it considers less important.
In stage 5, the peering network sees almost no value in public peering. Some may retain presences solely for political reasons, such as avoiding the confrontation of de-peering another network and instead simply limiting the growth of new peering. Other networks may disconnect themselves from public peering completely at this point.