1. TCP: Reno vs Cubic
Matt Kane, Ryan Chu

2. Agenda Brief history of congestion control reno cubic
Experiment description
Results

3. Reno
In the late 80s, the internet suffered major performance issues caused by high volume of packets being sent
after a certain amount of packets offered to the network, the amount of goodput (useful packets) dropped dramatically
One internet backbone dropped from 32kbits/s to 40 bits/s
Van Jacobson is said to have saved the internet from congestion collapse
Introduced congestion control principles in TCP Tahoe/Reno ( )
ie congestion window
Upon receiving 3 duplicate ACKS, Reno halves the congestion window, sets the slow-start threshold to the new cwnd, perform fast retransmit, entering fast recovery phase.

4. Cubic Derived from BIC BIC was more aggressive BIC was more complex
BIC was not as TCP fair/friendly
TCP congestion-control algorithm for high bandwidth networks with high latency
The congestion window is a cubic function of time since the last congestion event

5. Reno vs Cubic

6. Experiment
Upload a large file (multiple GBs) using both reno and cubic and measure the throughput.
Which will have a higher throughput?
Initial thought: cubic

7. Linux congestion control
Find out and modify Linux congestion control:
/proc/sys/net/ipv4/tcp_available_congestion_control
shows all congestion control algorithms that can be used
cubic reno
/proc/sys/net/ipv4/tcp_congestion_control
shows CURRENT congestion control algorithm
to change, write “cubic” or “reno”
cubic is default from version
NOTE: need root permission to change

8. Linux congestion control
Changing TCP congestion control
Method 1: setsockopt()
Method 2: iperf
-Z reno
-Z cubic
#include <netinet/tcp.h>
...
if (setsockopt(msgsock, IPPROTO_TCP, TCP_CONGESTION, optval, optlen) < 0) {
perror("setsockopt");
return 1; }

9. Results Using setsockopt() Trouble sending more than 1.4GB
results were inconsistent

10. Results
Using iperf

11. Discussion Why was there no difference in throughput? Ideas:
Is iperf actually changing the congestion control algorithm?
Network is not fast enough for cubic to make a difference
max out ~100 MB/sec

12. Questions