LECTURE 4. INTER End-hosts (ISP customers) Tier3 ISP (“ Local”) Tier-2 ISP Provider Tier-2 ISP Customer -Provider (“ Regional or CDefault-free”; Has global reachability info) nother)Tier-I ISP Tier 2 ISP Figure 4-3: A more accurate picture of the wide-area Internet routing infrastructure, with various types of ISPs defined by their respective reach. Tier-1 ISPs have"default-free"routing tables (i.e. they don' t have any default routes), and typically have global reachability information. There are a handful of these today (about five or so) mutual access to a subset of each others'routing tables. The subset of interest here is their own transit customers(and the ISPs own internal addresses). Like transit, peering is a business deal, but it may not involve financial settlement. While paid peering is common in some parts of the world, in many cases they are reciprocal agreements. As long as the traffic ratio between the concerned ASs is not highly asymmetric(e.g, 4: 1 is a commonly believed and quoted ratio), there's usually no financial settlement. Peering deals are almost always under non-disclosure and are confidential 4.2.1 Peering v. Transit a key point to note about peering relationships is that they are often between business competitors. The common reason for peering is the observation by each party that a non- trivial fraction of the packets emanating from each one is destined for the others direct transit customers. Of course, the best thing for each of the ISPs to try to do would be to wean away the others customers, but that may be hard to do. The next best thing, which would be in their mutual interest, would be to avoid paying transit costs to their respective providers, but instead set up a transit-free link between each other to forward packets for their direct customers. In addition, this approach has the advantage that this more direct path would lead to better end-to-end performance(in terms of latency, packet loss rate, and throughput)for their customers. It's also worth noticing that a Tier-1 ISP usually will find it essential to be involved in peering relationships with other ISPs(especially other Tier-1 ISPs)to obtain global routing information in a default-free manner. Balancing these potential benefits are some forces against peering. Transit relationships generate revenue; peering relationships usually don't. Peering relationships typically need4 LECTURE 4. INTERDOMAIN INTERNET ROUTING Tier-1 ISP (“Default-free”; Has global reachability info) Tier-3 ISP (“Local”) Tier-2 ISP (“Regional or country-wide) Tier-2 ISP End-hosts (ISP customers) (Another) Tier-1 ISP Customer Provider Customer Provider Tier-2 ISP Figure 4-3: A more accurate picture of the wide-area Internet routing infrastructure, with various types of ISPs defined by their respective reach. Tier-1 ISPs have “default-free” routing tables (i.e., they don’t have any default routes), and typically have global reachability information. There are a handful of these today (about five or so). mutual access to a subset of each others’ routing tables. The subset of interest here is their own transit customers (and the ISPs own internal addresses). Like transit, peering is a business deal, but it may not involve financial settlement. While paid peering is common in some parts of the world, in many cases they are reciprocal agreements. As long as the traffic ratio between the concerned ASs is not highly asymmetric (e.g., 4:1 is a commonly believed and quoted ratio), there’s usually no financial settlement. Peering deals are almost always under non-disclosure and are confidential. ! 4.2.1 Peering v. Transit A key point to note about peering relationships is that they are often between business competitors. The common reason for peering is the observation by each party that a non￾trivial fraction of the packets emanating from each one is destined for the other’s direct transit customers. Of course, the best thing for each of the ISPs to try to do would be to wean away the other’s customers, but that may be hard to do. The next best thing, which would be in their mutual interest, would be to avoid paying transit costs to their respective providers, but instead set up a transit-free link between each other to forward packets for their direct customers. In addition, this approach has the advantage that this more direct path would lead to better end-to-end performance (in terms of latency, packet loss rate, and throughput) for their customers. It’s also worth noticing that a Tier-1 ISP usually will find it essential to be involved in peering relationships with other ISPs (especially other Tier-1 ISPs) to obtain global routing information in a default-free manner. Balancing these potential benefits are some forces against peering. Transit relationships generate revenue; peering relationships usually don’t. Peering relationships typically need