CSE
561: Computer Communication and Networks
Class
Notes: Oct 16, 2002
Tom: Sure, today after class, or after 4:45pm, not tomorrow.
Monday all afternoon after class.
Janet: email for appointment.
Yes, by all means. "The key is not to prevent people from stealing your ideas; it is to get them to steal your ideas"
evaluation metrics for routing:
- reliability : make sure that everybody find the same route.
- scalability : no centralized point of control.
- cost
- performance (related to revenue)
Baseline approach -- SPF:
- concentrates on reliability
- ignores performance
BGP started in the SPF world:
- In BGP an entire organization == one node; shortest path in terms of the number of ASs crossed
- Initial design ignored cost and performance
So how do we add performance to the Interdomain Routing?
- Use link weights to approximate performance
- For the sake of scalability, link weights are adjusted locally
- System is dynamically unstable
How is it done in Intradomain Routing?
- The link weights are usually set centrally by humans on basis of trafic flows and link capacities;
- The problem is NP hard.
- It does not allow for splitting traffic among different routes unless paths have equal weights (but if the weights are equal, both paths get equal shares of the traffic).
- To address the problem: using circuits/ fractional routing. Make the network more regular (like tree).
Shortest AS routing:
- Might be reliable
- Might not be optimal
From the point of view of a single ISP, it has:
- Transit relationships -- buy a way to reach the rest of the Internet
- Peering -- free exchange traffic
- Customers -- people who buy transit from it
- Who to buy transit from
- Which ASs to use for outgoing traffic
§ This is done in a straightforward way based on the IP prefix; regional carriers may price quite low for transit to their customers but quite a lot for the rest of the Internet.
§ General rule: first try to send to customers, then to peers, then on transit links;
§ You have to worry a bit about performance, too... (some cheap links are not as good)
- Which links to use to send traffic to a given AS
§ Choose the closest (early exit)
- How to influence incoming traffic?
§ Pricing;
§ Prefix filtering (choose which routes to advertise; e.g. do not advertise yourself to your peers as a connection to locations that you yourself have to pay for)
§ AS link padding (e.g. I can advertise different entry points with different links weights). Reliability is a problem.
§ MEDs (Multi Exit Discriminators) can tell the other ASs which particular links to use to enter your net.
§ "Community Attrbutes", e.g. send traffic for ATT customers directly to ATT; the community attributes tell us who its customers are; (the problem is that community attributes can lie...)
During the Internet Bubble "The market value of a BGP engineer was a million dollars"