TCP Congestion Control & Network Optimization: RTT, Avoidance & Notification, Study notes of Electrical and Electronics Engineering

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ECE 8833 TCP Congestion Domain Names (DNS)^ Jan 13th

Some Reading:

-^ RFCs on web page: IP, UDP, TCP, Congestion, DNS

RTT estimate

• RTTnew

= (a)RTT

+ (1-a)RTTold

pkt

• Enhanced scheme, Van Jacobson, 1988• Uses “enhanced measurement”– Accounts for

average^ plus

variance

• RTTpkt

often measured at 500ms precision

Round Trip Timeout (RTO) • RTO = Avg + (4)Deviation – Initialized to Avg = 0, Deviation = 6 (seconds) • Retransmit after RTO, – And again at 2xRTO – And again at 4xRTO – And again at 8xRTO – And so on (doubling multiplier each time) – Until receiving an ack (or finally terminating)

TCP Windows

• Network temporarily “stores” data while intransit • Simple formula (complex implications)^ • Average bandwidth (B)^ • Average Delay (D)^ • Stored data (S)^ S = D x B^ Delay-Bandwidth Product

Delay-Bandwidth

• Bandwidth = Data/Delay^ – TCP acknowledges data sent^ – If you limit the amount of

unacknowledged

data:^ • Ie, sender quits sending when it reaches a certainnumber bytes that have not been acknowledged^ • Then: bandwidth is reduced accordingly – This is the basis of TCP congestion control

TCP bandwidth algorithm • Start with slow sending rate (small CWND) • Grow CWND for every ack – (note that as CWND exceeds packet size,multiple acks create geometric CWND growth) • When a packet is lost, slow down to ½current rate and grow “slowly” (lineargrowth)

Slow Start

• Slow Start^ – Uses congestion window (CWND)^ • Measured in bytes,^ • Incremented in segment size (MSS)^ – CWND initialized to one MSS^ • Grows by MSS for each packet that is received andacknowledged before RTO^ • Exponential growth^ – Attempting to approximate:^ • CWND = Delay x Bandwidth

Congestion Avoidance • When a “lost packet” is detected: – Set SSTHRESH = ½ min( CWND, RCVR_Win) • New variable, SSTHRESH, is “

Slow Start Threshold

”

• If “lost packet” is due to timeout, set CWND = MSS • Growing CWND on successful packet (ack):^ • Previously “add MSS to CWND for every successful packet –^ If CWND<SSTHRESH, add MSS

( Slow Start )

-^ If CWND>SSTHRESH, add 1/CWND

(Congestion Avoidance)

-^ Note that congestion avoidance is linear

growth, slow start is exponential^ growth

Fast Retransmit & Fast Recovery • More modifications (proposed 1990, VJ), “duplicate acks”^ –^ Indicates a packet was lost but later packets arrived (CWND >>MSS) ,OR packets were reordered along the way^ –^ Receiver must “ack” immediately for incorrect packet (cannot delay) • If three or more “duplicate” acks received:^ –^ “Mild” congestion, one packet was lost, others flowing …^ –^ Don’t overreact:^ • Set SSTHRESH = ½ CWND^ • Retransmit the missing packet^ • Set CWND = SSTHRESH + 3 segments^ – Add one segment to CWND for any more duplicate acks^ • On next non-duplicate ack, set CWND = SSTHRESH from above^ – Continue with normal CWND>SSTHRESH congestion avoidance

Extensions & Tricks

-^ Problems with synchronization:^ – What if queue fills up and drops many packets at thesame time^ • Causes all TCP connections to slow down at the same time^ • Network is suddenly “underutilized”^ • Solution: try to throw away a few packets before queues arecompletely full •^ In special cases:^ – Can slow one application down by throwing away itspackets more often^ – Same as giving one application more queue space ormore priority

Explicit Congestion Notification • RFC 3168 (Sept 2001):

http://www.ietf.org/rfc/rfc3168.txt

• Instead of dropping a packet, set a “congestion” bit inthe packet header^ • 1) Negotiate ECN capable sender/receiver^ – Use bit pair in

IP^ header (NOT TCP header)

• Set bit pair to 10 or 01 to indication ECN capable – Leave at 00 for not ecn capable • 2) Queues set congestion bit pair to 11 when consistently nearfull – This is a notification to the receiver that the sender-> receiverlink was full • 3) Receiver sets new bit from RESERVED field in TCP header(receiver tells sender) called ECN-Echo (ECE) • 4) Sender reacts to ECE bit in ack packet as if a packet waslost (congestion avoidance),

without resending

• 5. Sender replies with TCP RESERVED field CWR bit to letreceiver know that it is responding to congestion