Comp 443 Week 12: Nov 14

Firewall problems and ulam2:
try ulam3: 10.38.2.67
A few fixes to ERROR ctors

Queuing Disciplines; FIFO+taildrop, Random Drop, fair queuing

TCP

RPC
	Nature of request-reply semantics
	SunRPC
	BLAST, CHAN, SELECT
	

6.3: TCP Congestion avoidance
        self-clocking: sliding windows itself keeps # of packets constant
        
        RTTnoload = travel time with no queuing
        (RTT-RTTnoload) = time spent in queues
        sws*(RTT-RTTnoload) = packets in queues, usually at one router
        	(the "bottleneck" router, right before slowest link)
        	
        CongestionWindow: limits amount of data in transit
        	window = #packets in transit  +  # packets in queues
        additive increase, multiplicative decrease
        timeouts as a sign of congestion

Pick up with slow start

        Reaching equilibrium: slow start
        
	last time: double cwnd each RTT
	This is kind of an oversimplification
        slow start:
                for each ACK received, increment cwnd by 1
                assuming packets travel together, 
                this means cwnd doubles each RTT
                Eventually the network gets "full", and drops a packet.
                Say this is after N RTTs, so cwnd=2^N.
                Then during the previous RTT, cwnd=2^(N-1) worked fine,
                so go back to that previous value: set cwnd = cwnd/2.
        need for further polling for changes in capacity
        Slow increase *after* equilibrium
        slow start threshhold, ssthresh.
        cong. avoidance *only*: on cong., cwnd = curr_window/2
                drop back by 1/2; rationale
        congestion avoidance phase + slow start phase
        

        review of TCP so far
        	slow start + congestion avoidance
        	Need for SS after each loss (just hinted at end of class Week 12)
        	How to *combine* SS and CA
        	self-clocking
        	slow-start on initial startup *and* after timeout
        	
        Note that everything is expressed here in terms of manipulating cwnd.
        
        Summary:
        
        phase	cwnd change, loss	cwnd change, no loss, 
             	                 	per window	per Ack
             	          			
        slow-start	cwnd/2		cwnd *=2	cwnd+=1
        congAvoid	cwnd/2		cwnd+=1		cwnd+=(1.0/cwnd)
        
        real situation: sender realizes lost packet is lost only after protracted continuation
        

        fast retransmit: = TCP Tahoe
        

        Single sender situation
                example A-----R----slow---B, with R having queue size of 4
                bottleneck_queue >= bandwidth*no_load_delay

	Review of Tahoe
	
        fast recovery / TCP Reno
        
        TCP and one connection

        fairness
                same rtt