Is "Peering" in networking, always exchanged directly?

Question

Do ISPs always exchange traffic directly through Peering? Or not necessarily true? I heard that peering is an interconnection that charged for no money (free) unlike Transit providers. If that the case, then why most ISPs do not benefit from Peering?

Answer 1

The Wikipedia article on peering has a pretty comprehensive introduction.

do ISPs always exchange traffic directly through Peering?

No, although they may have direct connections with other networks for peering quite often they will use an internet exchange to facilitate peering, allowing an ISP to use a single connection to the IX to (potentially) peer with a large number of other members rather than establishing dedicated links to each of those.

Also an ISP may also still need to buy transit for routes they can't reach (efficiently) by peering for instance.

I heard that peering is an interconnection that charged for no money (free) unlike Transit providers.

There is no such thing as a free lunch... The connection itself will cost money to maintain, to name one.

The public internet exchanges charge a membership fee per 100 Mbit/s or 1/10/100 Gbit/s port.

But the cost associated with peering is a fraction of the price tag for buying transit.

then why most ISPs do not benefit from Peering?

What makes you think they don't?

Answer 2

You can think of networking interchange as either peering or transit. Peering can be further broken down into settlement free peering and paid peering.

Regional networks are populated by what we call tier 1 network providers. These tier 1 networks can reach the entire internet region through settlement free peering. Other networks in that region or without buy transit from these tier 1 providers.

In the US, for example, tier 1 providers include:

1. AT&T
2. Verizon
3. Sprint (Softbank Broadband)
4. Century Link (Qwest)
5. Level 3 (with Global Crossing now)
6. NTT/Verio
7. Cogent (maybe?)

See generally, drpeering.net and peeringdb.com

Answer 3

Peering only makes sense for ISPs if traffic is approximately equal in both directions, meaning that if peers would need to pay for the traffic to each other than balance would fluctuate around zero, so peering simplifies their accounting. Also, make sure you read this https://en.wikipedia.org/wiki/Peering.

Answer 4

Imagine an airlines only responsibility was shifting people from a to b, and the people didn't care how they were shifted, as long as they got there in a timely manner. It would make a lot more sense for British Airways to say to American Airlines, 'hey, you've got a few spare seats there, do you fancy taking some of our passengers too' In return American Airlines knows that British Airways will forward passengers into Europe whenever it has space to do so.

That is exactly the concept of peering, it is one provider giving away their free capacity in exchange for the use of one of their peers free capacity.

As for directness, once the traffic is on company b's network, it will take advantage of whatever peering agreements are available for its forward routing.