Week 10
Oct 31, 2007

HUMP scenarios:
	spray
	canceled request
	TFTP/WUMP scenarios:
		
	1.	duplicate REQ
		
		If the first REQ is sent twice (perhaps due to a client timeout),
		two child processes start on the server. The one that the client receives
		DATA[1] from first is the one the client will "latch on" to; the other
		should be sent an ERROR packet.
		
	2.	lost final ACK
		This is addressed with the DALLY state
				
		
	3.	old late duplicates
		This is addressed by having EITHER side (preferably both) choose a new
		port number for each "connection" (ie transfer).
		
	4.	Sequence number wrap: not allowed (but do we check?)
		
	5.	HUMP/CHUMP: two scenarios where we get something other than requested:
		1. Lost REQ: 	REQ "foo" (received but response delayed)
				<abort>
				REQ "bar" (lost, but now delayed DATA1-foo arrives)
				
	6.	2. Malicious flood of DATA[1].bad from port, so
		
					   	BAD guy opens port 6666
					   <---- DATA[1].bad from 666
					   <---- DATA[1].bad from 666
					   <---- DATA[1].bad from 666
					   <---- DATA[1].bad from 666
					   
				REQ "good" --> server
						server creates good

					   <---- DATA[1].bad from 666	LATCH!
						
						
					<----  server sends DATA[1].good
					
					   <---- DATA[1].bad from 666
					   <---- DATA[1].bad from 666
					   <---- DATA[1].bad from 666
						  	
	

CIDR and staying out of jail (allowing change of provider)

                
                Providers P0(A,B,C), P1(D,E), P2(F,G) each with customers shown
                routing tables assuming each customer gets an address from 
                its provider's block
                how longest-match allows customers to move without renumbering
                hidden cost of such moves
                
                 Providers P0(B,C), P1(A,D,E), P2(F,G) each with customers shown
                 (A has moved from P0 to P1)
                 but now we have addrs unrelated to provider, and so A
                 needs to be entered in every table!

		                               /------\
		Consider P0---P1---P2 versus P0___P1___P2

                router pseudo-hierarchy v. addr-alloc true hierarchy
                Don't have to agree, but there is a cost for disagreement
                
                What if B adds a link to P1, in addition to link to A?
                
                How CIDR allows provider-based and geography-based routing
                               
        provider-based addresses 
           Problems:
                route asymmetries
                inefficient routes (send to closest link to dest provider?)
                
                	                     A
                	                     |
                	P1:  r1--------r2----+---R3
                	     |         |         |
                	     |         |         |
                	     |         |         |
                	P2:  s1--+-----s2--------s3
                	         |
                	         B
                	     
                BGP "MED" value (not defined) allows server providers to carry
                the server's *outbound* traffic!
                
                renumbering: threat or menace? [DHCP, NAT]
                        Locators v. EID
                        changing IP addrs midstream
        geographical addresses
           Problems:
                inefficient paths between close sites.
                large sites
                

    Real issue with geographical routing: who carries the traffic?
    Provider-based: *business* model jibes with routing model!!

    New routing picture: destinations are networks, still, but some are organizations
    and some are major providers, with intermediate nets in between.
    Sometimes we might CHOOSE whether to view a large net as one unit,
    or to view it as separate medium-sized subunits (for the sake of visualization,
    assume the subunits have some geographical nature, or other attribute
    so that we can treat them as separate destinations.
    Tradeoff:
    	consolidation => more compact routing table
    	individual subentries => more optimal route selection


    2-step routing: when does it NOT find optimal routes?


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     6.2.5: TCP timeout & retransmission
        original adaptive retransmission: TimeOut = 2*EstRTT,
        EstRTT = \alpha*EstRTT + (1-\alpha)*SampleRTT
        
        RTT measurement ambiguity
        Karn/Partridge algorithm:
                Double Timeout
                Stop recording SampleRTT
                Use doubled Timeout as EstRTT when things resume

        Jacobson/Karels algorithm
                EstRTT = \alpha*EstRTT + (1-\alpha)*SampleRTT
                EstDev = \alpha*EstDev + (1-\alpha)*SampleDev
                TimeOut = EstRTT + 4*EstDev
                
        TCP timers: 
                TimeOut 
                2*MSL Timewait
                persist: sender polls receiver when windowsize = 0
                keepalive

Not covered:               
	Path MTU Discovery
		uses ICMP DONT_FRAG bit


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