Peering
(Redirected from Peering agreement)
Categories: Section stubs | Internet architecture
Peering is the practice of exchanging Internet traffic between the data networks of different Internet service providers. It involves three elements:
- the physical interconnection of the networks,
- technical liaison between the networks to allow exchange of routes, and
- the commercial and contractual peering agreements.
Internet service providers (ISPs) set up peering points, the physical locations where exchanges happen and negotiate peering agreements (which are essentially legal contracts setting out the exact details of how traffic is to be exchanged). Most peering points are located in colocation centres, where the different network operators 'co-locate' their Points of Presence.
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History of peering
In the early days of the Internet, a single backbone network existed in the form of first the ARPANET and later the NSFNET. All other networks connected with one another via the Internet backbone, and routing information was conveyed between the backbone and the other networks via the Exterior Gateway Protocol (EGP).
The modern Internet no longer has a single backbone in the traditional sense. Rather, it has many backbones, consisting of the individual backbones of various commercial ISPs and private networks. These networks are interconnected either at an Internet Exchange Point or through private interconnections between two or more networks. Their operations all rely upon the Border Gateway Protocol (BGP) which allows them to coordinate the operation of the Internet without the need for any central authority.
Types of peering agreements
The act of peering can be done as:
- a private peering connection between the networks,
- a peering via an Internet Exchange Point (ones that are independent of any single provider), which typically runs a large LAN that multiple ISPs connect to, allowing them to make multiple peering connections over a single physical connection
Providers with large traffic volumes, often known as Tier 1 carriers, tend to peer without charge with other large providers, and charge for peering with smaller ISPs. Because they provide each other with settlement-free interconnection, they are sometimes known informally as the "SFI club".
As of 2005, the "SFI club" included Qwest, MCI, Sprint, AOL, Verio/NTT, Level 3, Global Crossing, Savvis, and AT&T.
Providers with smaller volumes tend to converge at Internet Exchange Points, which provide them with a commercially neutral venue for peering.
Peering vs. transit
In a normal peering arrangement, only traffic between the two ISPs' directly attached customers is exchanged over the peering connection, and neither ISP can see beyond the other's network to the wider Internet. This is generally enforced by a combination of packet filtering and BGP policy filters.
Peering is generally a voluntary arrangement performed by mutual consent. It is generally only practiced where the traffic in each direction is roughly balanced, so that neither provider gains a competitive advantage over the other by the existence of the peering connection; rather, both gain economic advantage relative to other ISPs by having the peering interconnect.
When one provider carries packets for another to or from the wider Internet, this is known as an Internet transit arrangement. In the very early Internet, the assumption was made that all networks would provide transit for one another; however, in the commercial Internet, this is commercially unsustainable. Unlike settlement-free peering, transit is almost always a commercial service provided by the transit-providing ISP. However, ISPs are known to temporarily provide transit for one another informally and without compensation as mutual support in emergencies.
Peering and BGP
A great deal of the complexity in the BGP routing protocol exists to aid the enforcement and fine-tuning of peering and transit agreements.
De-peering and its effects
In some situations de-peering can occur, where ISPs disconnect their networks from each other. This is generally when one ISP wishes to charge the other to exchange traffic rather than continue to peer for free. This depeering is usually a commercial decision by a larger ISP which has access to a large amount of content or subscribers. It is seen as an opportunity by the large ISP to charge smaller players for the privilege of connecting. De-peering usually results in degraded performance for customers as traffic is forced to take less optimal paths; in the worst case some parts of the internet can become unreachable to hosts on either side of the de-peered networks. This occurred on October 5th 2005 when two backbone NSP's in the USA, Level 3 Communications and Cogent Communications de-peered following a commercial determination by Level 3 that it was significantly larger and therefore Cogent should pay it to continue peering [1].
See also
- Autonomous system
- BGP
- Default-free zone
- Settlement-free interconnect ("SFI club")
- Interconnect agreement
- Internet traffic engineering
- NANOG