Approaches to congestion control in network layer PPT.

Approaches to congestion control in the network layer.

Informally, When too many sources sending too much data too fast for a network to handle.

Manifestations: lost of packets (buffer overflow at routers) and long delays (queuing in router buffers).

Two broad approaches to congestion control:

1. End -end congestion control
2. Network assisted congestion control

End-end congestion control:)

• no explicit feedback from the network
• congestion inferred from end-system observed packets loss, delay
• the approach was taken by TCP

Network-assisted congestion control:)

• The router provides feedback to end systems
• A single bit indicating congestion (SNA,DECbit,TCP/IP ECN, ATM)
• explicit rate sender should send at

Goals of congestion control:)

Throughput: Maximize good output and the total number of bits end-end.

Fairness: Give different sessions equal share and maximize the minimum rate session.

Single Link: Capacity(R), session (m) and Each session: R/m

End to end feedback:)

Abstraction: Alarm flag and observable at the end stations.

Simple feedback model:)

Every RTT receives feedback: High congestion, Decrease rate, and Low congestion Increase rate. Also, the Variable rate controls the sending rate.

TCP Congestion Control:)

• Closed-loop, end-to-end, window-based congestion control
• Designed by Van Jacobson in late 1980s, based on the AIMD alg . of Dah-Ming Chu and Raj Jain
• Works well so far: the bandwidth of the Internet has increased by more than 200,000 times

Slow Start: getting to equilibrium gradually but quickly and double cwnd every RTT until network congested and get a rough estimate of the optimal of cwnd.

TCP & AIMD: congestion:)

Dynamic window size [Van Jacobson]

Initialization: Slow start

Steady-state: AIMD

Congestion = timeout : TCP Tahoe

Congestion = timeout || 3 duplicate ACK: TCP Reno & TCP new Reno

Congestion = higher latency : TCP Vegas

Many versions:)

TCP/Tahoe: this is a less optimized version.  It is used for Congestion avoidance.

//slow start is over 

//Congwin > threshold 

Until (timeout or 3dupACKs) { /* loss event */

every ACK: Congwin += 1/Congwin

}

threshold = Congwin/2

Congwin = 1

perform slow start.

TCP/Reno: many OSs today implement Reno type congestion control.Maintains equilibrium and reacts around an equilibrium.

Implements the AIMD algorithm:

• Increases the window by 1 per round-trip time.
• Cuts window size: to half when detecting congestion by 3DUP, to 1 if a timeout and if already timeout doubles timeout.

Initially:
cwnd = 1;
ssthresh = infinite (e.g., 64K);
For each newly ACKed segment:
if (cwnd less than ssthresh)
/* slow start*/
cwnd = cwnd + 1;
else
/* congestion avoidance; cwnd increases (approx.)
by 1 per RTT */
cwnd += 1/cwnd;
Triple-duplicate ACKs:
/* multiplicative decrease */
cwnd = ssthresh = cwnd/2;
Timeout:
ssthresh = cwnd/2;
cwnd = 1;
(if already timed out, double timeout value; this is called exponential backoff)

TCP/Vegas: not currently used

TCP uses different policies to handle the congestion in the network. We describe these policies in this

section.