Question

As a part of a personal project, I am making an application level protocol (encapsulated in UDP) which is reliable.

For the implementation of reliability, I have to keep track of which packets i send, and which packets are received at the other receiver end. This is done with the help of a sliding window, and it also maintains the flow-control.

1. Is there a way to implement reliability apart from standard sliding window/flow control technique.
2. If No, will someone share his experience/design rationale/code and discuss it over this post.
3. If Yes, have you implemented it, or if you know any implementation of the concept.

Answer 1

After reading all these answers, learning from other implementations and reading some papers, I am writing/sharing what is most pertinent. First let me talk about Flow control, and later i will talk about reliability.

There are two kinds of flow control–

• Rate based — Packets Transmission is done in a timely manner.
• Window based — The Standard Window based, can static or dynamic (sliding window).

Rate Based flow controls are difficult to implement, as they are based on RTT(round trip time — its not as simple as ping’s RTT) calculation. If you decide on providing a proprietary congestion control system, and you are providing it from present release, then you can go for rate-based flow control. Explicit congestion control is also there which is router dependent,so its out of picture.

Window based flow control, a window is used to keep track of all the sent packet, until the sender is sure that receiver has received them. Static window is simple to implement but throughput will be miserable. Dynamic window (also know as sliding window) is a better implementation, but a little complex to implement, and depends on various kind of Acknowledgement mechanisms.

Now Reliability…

Reliability is making sure the receiver has received your packet/information. Which means that receiver has to tell the sender, yes I got it. This notification mechanism is called Acknowledgement.

Now one ofcourse needs throughput for the data transfered also. So you should be able to send as many packets as you can, rather MAX[sender’s sending limit, receiver’s receiving limit], provided you have available bandwidth at both the ends, and throughout the path.

If you combine all the knowledge, although reliability and flow control are different concepts, but implementation wise the underlying mechanism is best implemented, when you use a sliding windows.

so finally in short, I and anyone else if designing a new app protocol and needs it to reliable, sliding window shall be the way to achieve it. If you are planning to implement congestion control then u might as well use a hybrid(window+rate based) approach (uDT) for example.