ࡱ> `_`!pHt2:prKC` . >xcdd``dd``baV d,FYzP1n:&! KA?H1: < ǀqC0&dT20 `[YB2sSRs=bF: pi#9\ J? |tHL@l&9vkTb'ܤri&.S\ HE"?0UL g+ wcN%4v)b*- `PĤ\Y\ ba\~fZm`!ӊoY[ۑr: xcdd`` @c112BYL%bpu @c112BYL%bpu+-.p0*24531 ?T-3/2/2001 (Hanoch Levy, CS, TAUO =(TCP Behavior and Performance , 0&Workshop on QoS Hanoch Levy April 2004&'(#IP and TCP IP finds the way to send the packet sent from end to end. IP is UNRELIABLE (pkts lost + delayed) TCP: Transport Control Protocol Above IP Tasks: Guaranteed arrivals Flow control LZZ#Z#MdWhat / Why Flow Control (Control the flow of packets out of source Purposes: Don t flood recipient Don t flood the network Don t send data no-body can take ( BW waste ). .5`5` The General Philosophy Sender sends packets Receiver sends ACKS for packets received. ACKS serve kind of an indication regarding how much can send more. General notion: Receiver gives sender indication of how much more can be sent. "TCP Sliding Window %!Window properties (fixed size)Closes when left side moves to right (packets sent by sender) Opens when right side move to right (acks by received by sender). Shrinks when right moves to left. RFC strongly opposes this  but must be able to cope with this. 7:@c~&" Window size   Not  how open . Rather   how large . Receiver can determine the size of the window. Measure can be by bytes/packets. TCP: bytes. V)& Slow Start  * normal start : when starting, sender can send as much as the widow size right away. Slow Start (TCP): Desire: rate of packets limited by rate of acks Add a congestion window (cwnd)  to the control. Sender can send up to the MIN of congestion window and advertised window. h I01      ,a,'Congestion WindowStarts at 1 (segment = about 500 bytes) Every time an ack received: Increase congestions window by 1 (segment). Cwnd=1, pckt 1, Ack 1 Cwnd=2, pckt 2, 3 Ack 2 ,3 Cwnd = 4, pckt 4, 5, 6, 7 Ack 4 5 6 7  Exponential growth rZZZZ ZZ ZZ-ZN    77 "-)How large should the window be  Desire: fill up the pipe: That is: Capacity (bits) = bandwidth (bit/sec) x round trip time (sec) Algorithm aim: reach that neighborhood (by trial and error). ~%)6(-%)6( /+Time out and RetransmissionDA retransmission timer used when expecting an ack from receiver How large:  roughly as round trip time Other timers: Persist timer Keepalive timer 2MSL timer xx-)--6),.T 0,Estimating RTTMeasure for each packet sent / ack received: Record by the value M. Compute R via a smoothing factor as follows: Retransmission timeout (RTO)= Where is an estimator of delay variance (recommended value =2)LsZZf-Zse1- Variance  xA more sophisticated algorithm is used (rather than taking the fixed value) Dynamically estimate the variance as well.y-y2.$Congestion avoidance: lost packets. %%tWhen sense congestion: reduce window. Algorithm: Init: cwnd1 segment, ssthresh65536 bytes. Send: MIN of cwnd and receiver window At congestion (timeout on packt, or duplicate ack): Ssthresh of current window If duplicate ack: cwnd ssthresh If timeout: cwnd 1(segment) At ack reception: increase cwnd  2-S-4----2#&40   9    8 .#   50 If cwnd < ssthresh: do slow start Increase cwnd exponentialy Else : do congestion avoidance Increase cwnd linearily: at most 1 segment for RTT$--5-3-  5 %P?;2*How congestion is sensed: Duplicate acks. ++$At congestion loss or delayed packet. receiver may receive out of order TCP rule: Sensing out of order: When receiver discovers out of order send DUPLICATE ACK Duplicate ack == signal for sender about congestion.dW-P-5-?  P5'=4More sophistication See Stevens Papers. ->5 Questions  Is TCP behavior too noisy? Will it be less noisy  if session interest is  selfish ? W-W<3 References  W. R. Stevens,  TCP/IP illustrated , Vol I. 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