detailed_td_algo.txt 5.84 KB
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Initiate Topology Discovery by sending Query to the server on 
   the same machine. The Query packet format is:
	1) Unique ID: Basically its the value returned by "gettimeofday()"
   			which serves two purpose:
			a) Query initiated by different clients running simulatenously
			b) Identifying that its the query packet which was already 
				seen earlier and processed.
	2) TTL: In a sense its the estimate of the diameter of the network.
	3) Factor: "TTL * Factor" is the time till any node waits for its 
			neighbors to respond.
	4) SourceNodeID: The greatest of the MAC Addresses of the machine
			from where the query was initiated.
	5) ImmediateSenderID: Its the ID of the immediate sender of the query.

After sending this query the client waits for the server to respond.

In an "infinite loop" the server does the following:
1. Listen to any query packet on the SERV_PORT
2. We can determine if the latest query packet was already processed earlier 
   or not  based on a list of query that we maintain. We keep adding every new
   query that we encounter in this "Query List".
   (a)If the "query packet" was already processed (as by above) and it is now sent
   by this server then we discard the packet. We can determine who has sent this
   packet by the information returned by "recvfrom" function for UDP receive.

   (b)If the "query packet" was already processed earlier and it is now sent 
   by some other node (neighbors) then the server replies them with its interface
   list. (No neighbor information was supplied in the reply). The fact that the
   packet has come back again tells the possibility of a cycle in the network so
   just telling this node's information is enough to make the cycle complete.

3. If the "query packet is a NEW packet then the server needs to gather its
   neighbors' information and will have to send this back to the node which has
   sent the query.
   To gather this neighbors' information the server broadcasts this query to all
   its interfaces after decrementing the TTL using the broadcast addresses.
   Then the server goes in "select mode" to listen for all replies. It opens sockets
   corresponding to each interface and binds to temperory ports. 
   Based on the replies from all the different interfaces the server makes a list
   of replies (ReplyList). 
   		 The server cann't wait for replies for ever! So, it has a time limit
   after that it stops listening to any reply and sends the current ReplyList back
   to the node which as sent the Query. The "time limit" is determined by "ttl *
   factor" in the query packet.  From this it can be understood that TTL is in
   sense corresponds to the diameter of the network. Factor helps in getting a
   reasonable time out value.

4. After the ReplyList was formed send this to the node which had sent the query.

5. Jump to step 1.
Problems removed from Kristine's Code to follow the above algorithm:
1. TTL was not decremented before forwarding the Query
2. Duplicate Query was not checked in the code correctly
	- TTL was also compared to be equal
3. Before doing select "fd_set" was not saved so it was
   modified after select
4. ResponseList was not returned by the forw_requests funstion
5. It was forwarding the Query on one interface and was blocking
   till it receives the response. Because of this
   		(i)  Timeout was occuring by listening on just one 
		(ii) It resulted in deadlock in case of cycles.
6. Caculation of the size of reply list was wrong
		(i) There were memory problems because size of Query 
			structure was not decremented from the received 
			response before forming the ResponseList
7. Other small small problems like server was not listening on 
	the specific ports where neighbors were sending the response

But apart from this, the algo has an inherent problem:
The server listens to query only after it has received all the replies from the 
neighbors. Because of this the algorithm results in a lock when two nodes are each
waiting for the other to reply and they both time out simulatenously resulting in
missing links. This situation occurs in a cycle of 5 nodes as was discovered while

New Server Algo:

This algorithm is slight modification of the above server algorithm. The main
difference is in the fact that we listen to the query as well as the replies 
at the same point in the algorithm. We do the select on Query socket as well as
the Response socket and process each of them depending on which socket the packet
was received. 
	After receiving a new query the server sets a timeout for the select. After
the timeout occurs server stops waiting for response and then it sends its own
response back to the node which sent the query.

The algorithm is very similar to the above except the fact that we are doing
select on the query socket too.

Set timeout= 0. This blocks the select to listen for the queries.
Run the following steps infinitely:
1. select on socket corresponding to SERV_PORT for listening to any query as well
as any socket open for listening to responses from the neighbors.

2. If there is any query packet on the query socket then process it as done in 
step 2 of the above algorithm. In case of a new query set "timeout" to
"ttl*factor" and goto select statement again.

3. If there are any response packets then make a list of it. Goto select again.

4. If there is "timeout" in select then send the response back and set "timeout"
to 0 again so that it can wait infinitely for any query.

So, as clear from the algorithm the server can respond to any query also while
waiting for replies. This was the drawback in the above algorithm which went in
lock in this scenario.

PS: I have avoided minute details in the algorithm for high level understanding of
the algorithm. Feel free to raise any problem in the above algorithm.