Comp 343/443 	Fall 2008, 25EP-602, 4:15-6:45

Week 4, Sept 17

[mac address validation & arp/dhcp]

Finish Sliding Windows
Ethernet
CRC error-check code
Datagram forwarding

       CRC: 
       		quickie example of "mod2-polynomial" long division: addition = subtraction = XOR
       		
       		Suppose we are given two messages, msg1 and msg2. There are
               	straightforward mathematical means for tweaking last bytes of msg2
        	so crc(msg1) == crc(msg2)
 	        	
        secure hashes (md5, etc)
         	Nobody knows at all how to produce two messages with same hash
         	
        2-D parity (corrects 1-bit errors)
        	fundamental role of error-correcting codes
        	(= "forward error correction")
        	

=======================================================

2.6: Ethernet. See also my notes (link on web page).

    Ethernet: Read section 2.6 on Ethernet
	logical: 
		broadcast bus
		NI interrupts CPU if any of the following apply:
			* packet destaddr matches NI's physical addr
			* packet destaddr is b'cast addr
			* packet destaddr is multicast and NI has "subscribed" to that m'cast addr
			* NI is in promiscuous mode
		Why we need a TYPE field
			
	That covers RECEIVING; what about SENDING?
	
        Ethernet (addresses, collisions, performance)
        	traditional broadcast-bus; role of hub
        		eavesdropping story: 
        		In 1994 I changed the admin password on several remote unix machines, 
        		using telnet. I told no one. Within two hours, someone else logged 
        		into one of the remote machines, using the new password, from inria.fr
        		(then rife with hackers, as I suppose was Loyola)
        		Two months later was the Kevin Mitnick "Christmas Day Attack",
        		launched _from_ apollo.it.luc.edu. 
        			
         	physical addresses, bcast address
        	operation of sending
        	csma/cd
        	collisions
			how CD (collision detect) works
			Signal propagation on the line: 1 bit = 23 m for 10 megabit
			min packet size / max diameter requirement[!]
			SLOT TIME: 
				min packet size
				collision detection time
				max diameter of network
			
			compare max packet size
			
			repeaters: simple amplifiers; max segment length
				need collision signal to be strong enough to be detected!
				change geometry too
				repeaters == hubs!
			Two issues relating to cable length: 
				faintness of signal
				window of opportunity for an undetected collision
			
			
			
			scaling to 100Mbps; min packet revisited
			collisions and hubs
			collisions and switches
			
			
        Exponential backoff algorithm
        
        	Transmit immediately when line is free. This leads to a collision 
        	if someone else was waiting also.
        	
        	After N collisions (including N=1):
        	choose a random k, 0<=k<2^N (choose an N-bit random k)
        	wait k slots
        	try again to transmit. Options: idle/seize_channel, idle/collide, busy
        	
    		hidden bias against hosts that have been waiting longest:
    		"unfairness"
    		
        Timeline of typical exponential backoff
			
	Ethernet myths re capacity
	
	
	Ethernet BRIDGING
	WHy switching avoids collisions, mostly
	
	Half-duplex: data flows in one direction at a time
	Full-duplex: packets can be sent in opposite directions simultaneously; collision-free!
		usually implemented via two half-duplex lines, each with a dedicated direction.
	
	Bridge Learning: first look
		
        IP (with addressing, classes, routing basics)
        	"virtual" addresses, assigned hierarchically
        	addr = net+host
        	local delivery
        	routing
        	*inter*networking
        	

    2.7: FDDI. Omit, except for brief discussion of token idea.

    Basic introduction to Ethernet and Token Ring physical networks
        Ethernet: (below)
        Token Ring:
                Tokens
                Fairness, round-robin allocation
                uniform performance under heavy loads


	Basics of Datagram Routing
	
		A--S1-----S2--D
		   |       |
		   |       |
		B--S3-----S4----S5---F
		   |       |
		   C       E
		   
        Basics of Adaptive (Learning) Bridges (Ethernet Switches)
        	* table size issues
        	* table updates
        	* learning algorithm
        	* b'cast as fallback
        	* bridges v hubs
        	* problem of cycles; spanning-tree algorithm

    P&D 3.2:
        Bridges and Adaptive Bridges; cycles; scalability
        Bridges join separate physical ethernets.
        Packets are propagated, but collisions are *not*.
        Limit to total size: total traffic
                Limits to size: b'cast, table sizes (10^4 v. 10^6)
                Cannot use loop topology
                Delay (we don't want packets arriving late)
        bridges & security: other parties cannot listen in.
        


=====================================================================

CRC error code [We'll do an example of this in class, but no more]

Look at adding up the digits of 123456789. Their sum is 45.
45 is divisible by 9, so 123456789 is also. This is the famous "rule of 9's",
that is based on the fact that 9 = 10-1 and 123456789 was a base-10 number.


Look at adding up the BYTES of a message M.
Bytes are essentially the base-256 "digits" of M, expressed as a huge number.
Internet checksum: fill in checksum field so sum of bytes (actually DWORDS, 
16-bit words) is zero, with overflow ignored.

That means that the sum is divisible by 255. 
By analogy with the Rule of 9's, that means that M as a mega-number
was evenly divisible by 255.

Checksum field is (-remainder), essentially

So, check SUM can be seen as a form of long division.

Good things about division: 
* divisors other than 255 are pretty much immune to reordering!
* divisors other than 255 are pretty darn good at catching errors

Problems:
* takes too long

Fix: do CRC division: fast (relatively), & work is simplified by doing 
long division of "mod 2 polynomials".

* no carries
* 1+1 = 0, 0-1 = 1