6.033 | Spring 2018 | Undergraduate
Computer System Engineering
Week 6: Networking Part II

Lecture 10 Outline

  1. Introduction 
    • Today: Routing, some addressing.
    • Enormous growth of Internet => routing protocols redesigned to scale, and also to enforce policy.
    • Problem: DV and LS don’t scale to the Internet.
      • DV = Low overhead, but convergence time is proportional to longest path. Good for small networks.
      • LS = Fast convergence, but high overhead because of flooding. Good for MIT-sized networks, but not the Internet.
  2. (Three Ways We Deal With) Scale 
    • Path-vector routing
      • Like DV, but include the full path in the routing advertisements.  Overhead increases (advs are larger), but convergence time decreases (avoid counting to infinity).
      • Overhead is still lower than LS’s.
    • Routing hierarchy
      • Internet is divided into Autonomous Systems (ASes). ASes are universities, ISPs, government branches, etc. Each AS has a unique ID (its AS number). There are tens of thousands of them (but not billions).
      • Use one routing protocol to route across ASes, and a different protocol to route within ASes.
        • Implies that there are devices on the edge of each AS that can “translate” between or “speak” both protocols.
      • BGP is the path-vector protocol used across ASes.
    • Topological addressing
      • Despite being between ASes, BGP still routes to IP addresses (e.g., to, not to AS3).
      • Addresses are given to ASes in contiguous blocks, so that they can be specified succinctly via a particular notation (“CIDR” notation).
      • Keeps advertisements small(er than they would be otherwise).
  3. Policy Routing 
    • ASes also want to implement policy; they want “policy routing”.
    • Policy routing: Switches make routing decisions based on some set of policies set by a human.  Routing protocol must disseminate enough information to enable those policies.
    • What policies are typical in BGP? ASes don’t want to send traffic on a path unless they have financial incentive to do so.
    • Mechanism of enforcement: Selective advertisements. AS1 won’t tell AS2 about a path unless it will make money by letting AS2 use the path.
      • => Each AS will have a different view of the network, and that view will (almost certainly) *not* contain every physical link.
  4. Typical BGP Relationships (which will eventually lead us to typical BGP policies) 
    • Customer/provider
      • Customers pay for access (transit), which the provider provides.
    • Peers
      • Peers provide mutual access to a subset of each other’s routing tables, namely, the subset that contains their transit customers.
      • Why peer? Can save money and improve performance. Sometimes, it may be the only way to connect your customers to some part of the Internet.
      • Why *not* peer? You’d rather have customers.
  5. BGP Relationships => BGP Export Policies 
    • First decision: Which routes do I advertise to which neighbors? These are an AS’s “export policies.”
      • High-level: “Tell everyone about yourself (your internal IPs) and your customers; tell your customer about everyone.”
      • More specifically:
        • Providers export customer’s routes to everyone.
        • A customer exports its provider’s routes to *its* customers.
          • These two should make sense: Since the customer is paying for Internet, the provider should give them a route to as many destinations as possible. Similarly, the provider should allow *other* parts of the network to reach its customers.
        • AS exports *only* customer routes to peers.
          • Why not full table? AS doesn’t want to provide transit for its peers; they’re not paying it for transit.
    • Z will tell X about C; C is a customer of Z, and X and Z are peers.
    • X will tell Z, Y, and T about C1, C2, and C3.
    • Y will tell X about D.
    • X will *not* tell Y about C; it makes no money to provide transit from Y to C.
    • X doesn’t tell Y about T; it would lose money to provide transit from Y to T.
    • In example, Y appears disconnected from part of the network. BGP doesn’t prevent this. In practice, it never happens.
      • Almost every AS is a customer of someone else (i.e., Y would buy transit from someone).
      • Typically: Small ASes buy Internet from Tier-3 ISPs, which buy Internet from Tier-2 ISPs, which buy Internet from Tier-1 ISPs. Tier-1’s are huge; there are only a handful (10-15).
    • Additionally, all Tier-1 ISPs peer with one another. So each Tier-1 ISP can provide global connectivity.
      • This is an example where we need peering in order to reach part of the Internet.
  6. BGP Relationships => BGP Import Policies 
    • If an AS hears about a route to X from multiple neighbors, how does it decide? These are its “import policies.”
    • First: Make money. Prefer routes via customers—which you make money on—to routes via peers —which you don’t make, but don’t lose money on—to routes via providers—which you lose money on.
    • In the case of a tie (which happens often): There are a whole host of other attributes that BGP provides. A common one is AS-hop-count.
    • Each AS sets its own policies.
  7. BGP in Light of Distributed Routing 
    • HELLO protocol: BGP sends KEEPALIVE messages to neighbors.
    • Advertisements: Sent to neighbors. Look different depending on which neighbor.
      • BGP runs on top of TCP, a reliable-transport protocol. Doesn’t have to do periodic advertisements to handle failure. Instead, push advs when routes change.
    • Integration: Via policies described above.
    • Failures: Routes can be explicitly withdrawn in BGP when they fail. Routing loops avoided because BGP is path-vector.
  8. Problems With BGP 
    • Does it scale? Well, it works on the Internet. But…
      • BGP routing tables are getting big (exceeding the amount of memory dedicated to the table in some switches).
      • We see route instability due to misconfigurations or conflicting AS policies. “Route-flap damping” (ignore advs about frequently-changing routes) helps with this, but increases convergence time.
      • ASes can multi-home: Buy Internet from more than one ISP, usually for back-up or load-balancing. More multi-homed networks => bigger routing tables. The load-balancing itself is also tricky.
      • iBGP. An AS actually has multiple BGP routers on its edge, and a protocol called iBGP keeps them all in sync. iBGP requires an AS’s BGP routers to be connected in a complete graph, and so it doesn’t scale particularly well.
      • Basically: Internet has grown enough that scalability of BGP is becoming a concern.
    • Is it secure?
      • Goodness no. ASes can advertise about a prefix that they don’t actually own.
      • Similar problem (and solution) as in DNS. We’ll talk more about it after spring break.
    • Is it secure?
      • The protocol itself: Arguably yes.
      • BGP in practice: No. Again, mo’ money, mo’ problems. Also, human operator error due to the complexity of setting the policies.
Course Info
As Taught In
Spring 2018
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