NetNotes

TCP congestion control and queueing

TCP mechanisms for traffic regulation and congestion-control

Standard TCP implementations have several mechanisms for congestion-avoidance:

• TCP Slow-start - this is the standard method for increasing the amount of data being transported - starting exponentially at one segment, and after a certain point - in a linear way (congestion-avoidance behaviour), and for regulating this amount in cases of congestion after the traffic source;
• TCP congestion-avoidance - works together with TCP Slow-start and defines a means of regulating the volume of data to be sent in TCP - de facto providing something like flow-control at the source;
• TCP Fast retransmit - defines an accelerated retransmission of part of the data, upon receipt of a minimum of 3 duplicate acknowledgements, without waiting for the TCP retransmission timeout to expire;
• TCP Fast recovery - after sending the missing segment of the fast-restransmit mechanism, TCP goes on with linear growing of the amount of information in transport (congestion avoidance), instead of starting a new slow-start from the beginning;

TCP Slow-start

During a TCP session establishment one of the variables being initialized at the end nodes is the cwnd - congestion window. › Continue reading

Tags: congestion avoidance, congestion control, QoS, queueing, slow-start, TCP

Port-channel note

We have the following setup of a port-channel between to Catalyst 3750s - SW1 Gi0/1 and Gi0/2, bundled in a port-channel are connected to Gi0/1 and Gi0/2 on SW2:

SW1:

interface GigabitEthernet1/0/1
description Member of PC1
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,2,3,4
switchport mode trunk
switchport nonegotiate
channel-group 1 mode on

interface GigabitEthernet1/0/2
description Member of PC1
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,2,3,4
switchport mode trunk
switchport nonegotiate
channel-group 1 mode on

interface Port-channel1
description to SW2
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,2,3,4
switchport mode trunk
switchport nonegotiate

SW2:

interface GigabitEthernet1/0/1
description Member of PC1
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,2,3,4
switchport mode trunk
switchport nonegotiate
channel-group 1 mode on

interface GigabitEthernet1/0/2
description Member of PC1
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,2,3,4
switchport mode trunk
switchport nonegotiate
channel-group 1 mode on

interface Port-channel1
description to SW1
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,2,3,4
switchport mode trunk
switchport nonegotiate

In order for the port-channel to form successfully, all port members need to have identical: › Continue reading

Tags: port-channel, Switching