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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?

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  • How would a small, local ISP in Washington peer with a small, local ISP in Copenhagen? And if they found some way to do it, how could they possibly exchange enough traffic to justify the cost? Sep 8, 2015 at 0:51
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    Please do not ask new questions by editing existing questions. It tends to invalidate existing answers and also once you have marked an answer as accepted it is unlike to attract much more attention.
    – HBruijn
    Sep 12, 2015 at 10:52

4 Answers 4

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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?

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  • i know it's natural for any ISP to pay for their internet connection fees. i only asked if two ISPs were to agree on peering. does peering itself means that the traffic itself is routed directly from the two points?
    – Semphie94
    Sep 7, 2015 at 22:21
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    Yes, if two networks peer with each other they can exchange traffic directly without having a third party network (transit network) in between. Sep 7, 2015 at 23:16
  • @Sander Steffann Thanks for your clarification. if you've had answered my question, i would have already accepted your answer right away! ;)
    – Semphie94
    Sep 7, 2015 at 23:24
  • @Semphie94 They can't peer with each other without a connection to peer over, and someone has to pay the costs of that connection. Sep 8, 2015 at 0:53
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    Yes, there is no implied guarantee or even requirement that traffic routed from A to B follows the same path in reverse from B to A.
    – HBruijn
    Sep 8, 2015 at 9:30
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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

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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.

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    The idea that traffic flows need to be roughly symmetrical for peering to be valuable is quite flawed, large content distributors (the Facebook's YouTube's , Netflix, AWS etc) are just as keen to reduce the cost of outgoing traffic as the big consumers, the ISP's for users like you and me, are to reduce the cost for incoming traffic.
    – HBruijn
    Sep 7, 2015 at 22:18
  • @HBruijn I think he meant settlement-free peering. Sure, it still makes sense to peer with asymmetric traffic with settlements. (Or in the rare case where asymmetric traffic exists without asymmetric cost, but that's very rare. Generally, outbound traffic is much cheaper than inbound traffic.) Sep 8, 2015 at 0:55
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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.

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  • in other words, if two ISPs agreed on peering. they should exchange traffic directly without any third party? and for free too?
    – Semphie94
    Sep 7, 2015 at 22:17
  • If its not for free, its not a peering agreement, its transit. I'm not sure how you could have a peering agreement with a third party in between. They need to be directly connected to each other at the point of peering
    – Michael B
    Sep 7, 2015 at 22:21
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    There are (more in Europe than the U.S. I think)the public internet exchanges that facilitate
    – HBruijn
    Sep 7, 2015 at 22:27
  • Your example of peering isn't really accurate, and although peering is often done for free paid peering exists as well. Sep 7, 2015 at 23:14
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    Peering isn't about letting others use your unused capacity, it's about directly exchanging traffic from or to your own network without going over a transit network. Sep 7, 2015 at 23:33

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