Is it possible for each node connected to the same router to implement different congestion avoidance technique? Also, is it possible to completely disable congestion avoidance in a node connected to a router. Thanks.
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The answer depends on your definition of "possible".
Internet hosts should adhere to internet standards. According to these documents TCP should implement congestion control algorithm that is "reno-friendly", that is can coexist with Reno (see RFC5681). So, if you need a TCP implementation, that adheres to these standards, then the answer is no.
Enforcing this, is however another issue, which as far as I know does not really have a solution. So, can you still implement whatever congestion control or no congestion control at all, and still connect to Internet, the yes.
Is it actually done? Yes As of now, Linux hosts use TCP Cubic, and Windows uses another congestion control mechanism, whose name I don't remember. They are both Reno friendly and coexist with each other, but they are different and they are different from Reno. Recentrly, Google deployed BBR, which may or may not be Reno friendly. Moreover, realtime multimedia streams (e.g., voice or video conferences) also should use some kind of congestion control, so they contribute to the variety as well.
Will router care? Not Really. Router does not care if the attached hosts implement congestion control or not. A simple router will do exactly the same thing. It will get incomming packets, then either send them, if outgoing interface is free, queue them, if the interface is currently busy transmitting other packets and it has space in a queue for this interface, or drop packets, if the queue is full. More complicated routers can utilize things like active queue management schemes, or quality of service with rate control. This will affect how router handles packets, but it won't affect the functionality of the router. It has to take misbehaving hosts into account.
What will be affected? Applications. Application performance for several flows sharing the same bottleneck will be affected, if the hosts implement different congestion control mechanisms or no congestion control at all. How, actually depends on bottleneck bandwidth, capabilities of the routers, and traffic patterns of the applications. It is not possible to say how. However, there is definitelly a possibility that network will not be able to transmit useful traffic (this is known as congestion collapse). One other important thing that will most likely be affected is fairness, which more or less quantifies how equally several flows sharing the same bottleneck will share available bandwidth. A flow that does not implement congestion control can highjack all available bandwidth. The same applies with flows that use more aggressive congestion control than TCP Reno and flows that don't. So, it is not nice, not to implement congestion control. Of course the router can actually do something about it, but it requires pretty expensive per-flow sheduling (you can search for fair-queueing or flow-queueing), and routers usually do not do this.
References:
Effie
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