1. CIS 235: Networks Fall, 2007 Western State College Computer Networks Fall, 2007 Prof Peterson

2. CIS 235: Networks Fall, 2007 Western State College Friday Mike is returning on Friday for one last presentation. We will work together on a wiki page to preserve Mike’s material. Take notes and ask questions. There will be a short homework in which we all work together to distill and elaborate the information he provides. I’ll assign specific tasks Monday. This will be the last homework assignment!

3. CIS 235: Networks Fall, 2007 Western State College ARP I’m stealing a topic from 5.4.2 so that we’ll understand Mike on Friday. Read this! ARP is how the Link level addressing scheme (which is???) interfaces with the Network level addressing scheme (which is???) What does an ARP table do?

4. CIS 235: Networks Fall, 2007 Western State College ARP Details ARP tables are built on the fly Lots of broadcasts – you can see what others are doing Works on a single subnet Similar to DNS Basic idea is to send out a query (IP address) on broadcast and get back a Mac address

5. CIS 235: Networks Fall, 2007 Western State College DV Routing Algorithm Why should we handle routing with a separate protocol? Who talks on this protocol? What is an “asynchronous” algorithm? What is the Bellman-Ford equation? What is a routing loop? Why would you want to change a link cost?

6. CIS 235: Networks Fall, 2007 Western State College Distance Vector Algorithm (5) Iterative, asynchronous: each local iteration caused by: local link cost change DV update message from neighbor Distributed: each node notifies neighbors only when its DV changes neighbors then notify their neighbors if necessary Network Layer4-6 wait for (change in local link cost or msg from neighbor) recompute estimates if DV to any dest has changed, notify neighbors Each node:

7. CIS 235: Networks Fall, 2007 Western State College Network Layer4-7 x y z x y z 0 2 7 ∞∞∞ ∞∞∞ from cost to from x y z x y z 0 from cost to x y z x y z ∞∞ ∞∞∞ cost to x y z x y z ∞∞∞ 710 cost to ∞ 2 0 1 ∞ ∞ ∞ 2 0 1 7 1 0 time x z 1 2 7 y node x table node y table node z table D x (y) = min{c(x,y) + D y (y), c(x,z) + D z (y)} = min{2+0, 7+1} = 2 D x (z) = min{c(x,y) + D y (z), c(x,z) + D z (z)} = min{2+1, 7+0} = 3 32

8. CIS 235: Networks Fall, 2007 Western State College Network Layer4-8 x y z x y z 0 2 7 ∞∞∞ ∞∞∞ from cost to from x y z x y z 0 2 3 from cost to x y z x y z 0 2 3 from cost to x y z x y z ∞∞ ∞∞∞ cost to x y z x y z 0 2 7 from cost to x y z x y z 0 2 3 from cost to x y z x y z 0 2 3 from cost to x y z x y z 0 2 7 from cost to x y z x y z ∞∞∞ 710 cost to ∞ 2 0 1 ∞ ∞ ∞ 2 0 1 7 1 0 2 0 1 7 1 0 2 0 1 3 1 0 2 0 1 3 1 0 2 0 1 3 1 0 2 0 1 3 1 0 time x z 1 2 7 y node x table node y table node z table D x (y) = min{c(x,y) + D y (y), c(x,z) + D z (y)} = min{2+0, 7+1} = 2 D x (z) = min{c(x,y) + D y (z), c(x,z) + D z (z)} = min{2+1, 7+0} = 3

9. CIS 235: Networks Fall, 2007 Western State College Distance Vector: link cost changes Network Layer 4-9 Link cost changes: r node detects local link cost change r updates routing info, recalculates distance vector r if DV changes, notify neighbors “good news travels fast” x z 1 4 50 y 1 At time t 0, y detects the link-cost change, updates its DV, and informs its neighbors. At time t 1, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV. At time t 2, y receives z’s update and updates its distance table. y’s least costs do not change and hence y does not send any message to z.

10. CIS 235: Networks Fall, 2007 Western State College Distance Vector: link cost changes Network Layer4-10 Link cost changes: r good news travels fast r bad news travels slow - “count to infinity” problem! r 44 iterations before algorithm stabilizes: see text Poisoned reverse: r If Z routes through Y to get to X : m Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) r will this completely solve count to infinity problem? x z 1 4 50 y 60

11. CIS 235: Networks Fall, 2007 Western State College Autonomous Systems How do we deal with the huge number of routers on the Internet? What is a gateway router? What is the difference between inter-AS and intra-AS routing? What is our AS at Western? http://logbud.com/visual_trace http://www.arin.net/whois/

12. CIS 235: Networks Fall, 2007 Western State College Router Protocols RIP OSPF BGP What are these about? What do they do? How do these affect the scalability of networks?