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0. LOGISTICS, REVIEW, OUTLINE
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0.1 Logistics
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* Assignment 2 out
* Late policy: Beyond one week, 50% reduction with every day.
* Haven't heard from 7 of you out of 22. I am assuming you are using up your
  late days. If not, please let me know.
* Might occasionally update assignment for clarity. Will send out email. Use instructions here to synchronize.
https://help.github.com/articles/merging-an-upstream-repository-into-your-fork/
* Assignment 1 quick feedback:
--> Some of you had trouble finding a wired machine, but I didn't know about
it! Please let me know if you want to finish this part of the assignment, and I
can setup an account for you.
--> Will try and use EC2 for this in the future.
--> Please check your assumptions before the assignment, not after you hand in
the assignment.

0.2 Review
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* So far, have discussed the application and transport layers
* It's all been at the end host until now.
* Application: Interface with the user: creating and consuming data
* Transport layer: Reliability, preventing congestion collapse
* The transport layer assignment should clarify much of what we discussed.

0.3 Today
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* Will move on to the routing layer: how packets get from one end to another
* Two broad aspects to it: the control plane and the data plane
* Control plane: deciding where to send packets to, i.e., populating look-up tables
* Data plane: Actually sending these packets, i.e., looking up the look-up table on every packet.
* What are the time scales for the control and data planes?
* This week: control plane
* Next week: Recess and guest lecture
* Week after: Midterm
* Week after: data plane
* Three topics for today:
  --> Link state protocols (mention SDN here as well)
  --> Distance vector protocols
  --> Comparing the two
(assume bi directional links)

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1. LINK STATE PROTOCOLS
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* Conceptually want to determine the shortest path from source to destination
* How did you do this in your algorithms class?
* Can we use this approach?
* Does every router have the required information to compute shortest paths?
* Link state protocols:
  --> Disseminate information about your neighbors to EVERYONE in the network
  --> Once everyone has ALL the information, run Dijkstra's (or any other) shortest path algorithm.
* Example
* This centralized approach is the basis of Software-Defined Networking:
   * Instead of replicating the route computation at each router, why not just do it once and send it out to everyone?
   * Also, what if you want to do more fine-grained routing based on source address, source port, etc.

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2. DISTANCE VECTOR PROTOCOLS
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* Every node maintains a vector of current best distances to each destination.
* Nodes send this to their neighbors alone
  * Every node receiving an advertisement compares its best distance to the destination against the best destination through that neighbor.
  * If it finds a better path through that neighbor, it updates the path.
* Example

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3. COMPARING THE TWO
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* Link state sends information about neighbors alone to everyone
* DV sends information about everyone to neighbors alone
* Link state consumes more link capacity; DV takes longer to converge.
* Both are used: OSPF (link state), wireless mesh networks (distance vector)