6.829 Fall 2004 PS2 FAQ

Q: If we are submitting our solutions via email, is there some time deadline during the day we should aim for, or is 11:59PM EDT perfectly acceptable?
A: Please make sure you've emailed or handed in (32-G806) your PS2 solutions by midnight eastern time, Friday October 8th.
Q: A question about reading the MIT bgp router table dump. If there is a prefix is specified simply as "214.3.0.0" with no bitmask, what is the correct bitmask for it? Is it /32 or /16? How is it determined for an arbitrary entry without a bitmask?
A: When no mask is given, there is an implicit classfull mask based on the address. In the example you give above, it would be a /24 (since the 3 MSB of 214 are 110).
Q: Is it safe to assume for 1.3 that the routers are _not_ configured to wait to send new advertisements after hearing a withdrawal?
A: Assume that the tables and routing messages at time step n+1 are the product of routing messages and prior state at time n. i.e. all of the routing messages are sent in time step n before evaluating n+1.
Q: From question 2 I know that Level3 is AS 3356, however, I am unable to determine which ASs belong to Internet2. Is this something that I should have figured out in other problems? How do I go about figuring this out?
A: From the MIT BGP table, there should be a small set of ASes which the router peers with, i.e. over all routes there is a small set of next hop ASes. You should be able to look these up using whois to registries such as ARIN.
Q: I'm having trouble working with the router table file. I'm downloading the 23MB fine, but when I try to "gzip -d oix-full-snapshot-2004-09-23-0200.dat.gz" it doesn't return and continues to "unzip" the file until it takes up more than 500 MB, exceeding my Athena quota.
A: Two suggestions: either use a computer with adequate disk space, or keep the file compressed and pipe the decompressed stream to your parser, e.g. "gzip -dc file.gz | ./prog.pl"
Q: For problem 3.2 when it says "for all pairs" do you want the relationships between (on part a for example) 7911 and 31846 as well as for 7911 and 3356, 3356 and 31846? Or just adjacent pairs?
A: We're asking for the relationship between adjacent pairs.
Q: I'm unclear about sibling relationships and how to find them.
A: Consider the following *fictitious* example. You are asked to infer the relations of an ASPATH 7911 209 19092 3908 10947. By constructing the AS graph, one can determine the degree of each AS. Suppose you find that 3908 has the highest degree, so based on the above ASPATH, one would conclude that:
```
7911->209->19092->3908<-10947
```
However, there might be another route in the routeview dump (and not listed as a problem explicitly) with ASPATH 3908 19092 323 2342 123 such that AS 2342 has the highest degree and one similarly concludes:
```
3908->19092->323->2342<-123
```
Combining these 2 pieces of information yields:
```
7911->209->19092<->3908<-10947 
```
Where 19092 and 3908 are siblings.

Last Updated: 09/07/2004 22:10