Week 7: Oct 10

UDP socket programming details

TCP

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

TCP - P&D 5.2
	more semantic issues: 
		connected v connectionless
		connection is between two sockets: a socketpair
		one socket on host A can be part of multiple distinct socketpairs
		reliable v unreliable
		stream v packets
		
        what TCP does; header fields
  
  TCP Header Format

                                    
    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Acknowledgment Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data |           |U|A|P|R|S|F|                               |
   | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
   |       |           |G|K|H|T|N|N|                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |         Urgent Pointer        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            TCP Header Format

connection establishment
        
    Logical issue of what is a "connection"
    connection state on top of stateless lower layer
    
          basic strategy of 3-way handshake
          ISN issue
          
          	outline
          	versus one 2-way handshake
          	
          TCP sliding windows		mostly done earlier
             basic outline
             examples, window-size flow control
                fast sender, slow receiver
                fast receiver, slow sender
                slow router
             Keeping the pipe full: bandwidth*delay!  p. 301
	eoc: just got to TIMEWAIT and final ACK
	
	Introduction to TCP state diagram, as far as connection establishment
	Initial Sequence Numbers (ISNs)
	SYN, ACK, FIN, RST packets
	
	
	TIMEWAIT and final ACK again
	old late duplicates problem: TIMEWAIT again!

	what a connection is: machine state at each endpoint
	TCP should handle:
		lost packets
		damaged packets
		reordered packets
		duplicated packets
		widely varying delay

	
	OLD LATE DUPLICATES problem
	   TCP example
	   ISN negotiation, choice
	   MSL 
	   TIMEWAIT
	   TIMEWAIT and final ACK 
	   ISN rationale 1: old late duplicates
	   

	   ISN rationale 2: distinguishing new SYN from dup SYN

	From Dalal & Sunshine's original paper on the TCP 3-way handshake:
	
		2-way handshake: can't confirm both ISNs
			
		4-way handshake:
		    1	--SYN->
		    2	<-ACK--
		    3	<-SYN--
		    4	--ACK->
		FAILS if first SYN is very very old
		The ack at line 2 is ignored. LHS thinks the SYN on line 3
		is a new request, and so it acks it. It would then send
		its own SYN (on what would be line 5), but it would be ignored.
		At this point A and B have different notions of ISN_A.
		
		3-way handshake

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

An example exchange, showing SEQ and ACK fields for each packet.
Each side chooses its ISN: A chooses 500 and B chooses 3000.


A				B
SYN, ISN_A = 500
				SYN+ACK, ISN_B=3000, ACK=501
				
ACK 3001

Data="ABC", SEQ=501,ACK=3001

				ACK SEQ=3001, ACK=504	// 3001 *would* be next byte
				
DATA="EF", SEQ=504, ACK=3001
DATA="G", SEQ=506
				
				ACK=507 DATA="xyzw" SEQ=3001
				
ACK=3005 SEQ=507 DATA="HI"
				ACK=509, SEQ=3005
				
ACK=3005, SEQ=509, FIN

				ACK=510, SEQ=3005	// FIN is like 1 byte
				
				ACK=510, SEQ=3005, DATA="goodbye"
				ACK=510, SEQ=3012, FIN
				
ACK=3013, SEQ=510