Comp [34]43 Week 7: Oct 8, 2008

Midterm next week

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Chapter 5: transport
UDP - P&D 5.1

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Length             |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        unreliable, unconnected (stateless). General uses: NFS, boot, RPC
        checksum & algorithm
            checksum can also be updated; eg in IP header
            0000 checksum never occurs naturally, can be used as "checksum not used"
        pseudo-header: include some IP header fields in UDP checksum
        ports & final destinations
        lost packets due to queue overflow	 (skipped)
                at router
                at host

        socket programming interface (P&D 1.3)  // done in java
                udpclient/udpserver
                simpletalk client, server: TCP and UDP versions
                socket, bind, sockaddr_in, gethostbyname, 

        Note how socket programming interface corresponds to the actual options:
        	* create a socket
        	* ask for a specific port number
        	* send or receive  
        

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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
          
          Sequence number value is the number of the 1st byte of the packet,
          or what the number would be if there <i>were</i> a first byte.
          
          Ack is the number of the next byte we are expecting
          
          SYNs count as 1 byte (so do FINs)
          
          A				B
          send SYN, seq=0		
          				send syn+ack, seq=0, ack=1	;; expecing 1
          send ACK, seq=1,ack=1
          
          send "abc", seq=1, ack=1
          				send ACK, seq=1, ack=4
          send "defg", seq=4, ack=1
          				send seq=1, ack=8
          send "foobar", seq=8, ack=1
          				send seq=1, ack=14, "hello"
          send seq=14,ack=6, "goodbye"
          send seq=21, ack=6, FIN	send seq=6, ack=21	;; crossing
          				send seq=6, ack=22
          				
          				send seq=6, ack=22, FIN
          send seq=22, ack=7
          
          
          ISN issue
          
          	outline
          	versus one 2-way handshake
          	
          	Adding 1000 to ISN_A: this affects SEQ on A's side, ACK on B's side
          	
          TCP sliding windows		mostly the same as "normal" sliding windows, 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

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TCP state diagram

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           TCP state diagram
           
                                                Functional Specification



                                    
                              +---------+ ---------\      active OPEN  
                              |  CLOSED |            \    -----------  
                              +---------+<---------\   \   create TCB  
                                |     ^              \   \  snd SYN    
                   passive OPEN |     |   CLOSE        \   \           
                   ------------ |     | ----------       \   \         
                    create TCB  |     | delete TCB         \   \       
                                V     |                      \   \     
                              +---------+            CLOSE    |    \   
                              |  LISTEN |          ---------- |     |  
                              +---------+          delete TCB |     |  
                   rcv SYN      |     |     SEND              |     |  
                  -----------   |     |    -------            |     V  
 +---------+      snd SYN,ACK  /       \   snd SYN          +---------+
 |         |<-----------------           ------------------>|         |
 |   SYN   |                    rcv SYN                     |   SYN   |
 |   RCVD  |<-----------------------------------------------|   SENT  |
 |         |                    snd ACK                     |         |
 |         |------------------           -------------------|         |
 +---------+   rcv ACK of SYN  \       /  rcv SYN,ACK       +---------+
   |           --------------   |     |   -----------                  
   |                  x         |     |     snd ACK                    
   |                            V     V                                
   |  CLOSE                   +---------+                              
   | -------                  |  ESTAB  |                              
   | snd FIN                  +---------+                              
   |                   CLOSE    |     |    rcv FIN                     
   V                  -------   |     |    -------                     
 +---------+          snd FIN  /       \   snd ACK          +---------+
 |  FIN    |<-----------------           ------------------>|  CLOSE  |
 | WAIT-1  |------------------                              |   WAIT  |
 +---------+          rcv FIN  \                            +---------+
   | rcv ACK of FIN   -------   |                            CLOSE  |  
   | --------------   snd ACK   |                           ------- |  
   V        x                   V                           snd FIN V  
 +---------+                  +---------+                   +---------+
 |FINWAIT-2|                  | CLOSING |                   | LAST-ACK|
 +---------+                  +---------+                   +---------+
   |                rcv ACK of FIN |                 rcv ACK of FIN |  
   |  rcv FIN       -------------- |    Timeout=2MSL -------------- |  
   |  -------              x       V    ------------        x       V  
    \ snd ACK                 +---------+delete TCB         +---------+
     ------------------------>|TIME WAIT|------------------>| CLOSED  |
                              +---------+                   +---------+

                      TCP Connection State Diagram
                               Figure 6.

	half open
	simultaneous open
	

						  	
===============================================================	
	
Anomalous TCP scenarios
        Duplicate SYN           (cf Duplicate RRQ in the TFTP protocol)
                recognized because of same ISN
        Loss of final ACK       (cf TFTP)
                any resent FIN will receive a RST in response
        Old segments arriving for new connection
                solved by TIMEWAIT
        Sequence number wraparound (WRAPTIME < MSL)
                Note WRAPTIME = time to send 4 GB
                WRAPTIME = 100 sec => 40 mbytes/sec => >300 Mbits/sec
        Client reboots (application restart isn`t an issue)
                Could an old connection be accepted as new?
                

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