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sf.97dec16.txt
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1997-12-15
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ECN, TCP, and congestion on the ACK-path:
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>I have one general question, namely whether
>pure acks have the ECN-aware bit set. Becaue they're *not* actually
>adaptive themselves.
For the initial TCP implementations with ECN, pure acks should not have
the ECN-capable bit set. This is because it is not completely obvious
what the TCP sender should do in response to congestion on the
ack-path. Cut its window in half? Or better, tell the receiver to
send fewer acks, and to begin rate-pacing in compensation.
For current non-ECN-capable TCP implementations, the drop of a single
pure ack packet does not generally result in a decrease in the
subsequent arrival rate of ack packets. For current TCP
implementations, if a single ack packet is dropped, the TCP sender will
refrain from whatever window increase it would have made if that ack
packet had arrived, and when a subsequent ack arrives at the sender, the
sender will send a burst of back-to-back packets. Thus, for current
TCP implementations, the main effect of a dropped ack packet is to
prevent an increase in the TCP sender's congestion window, and to
increase the burstiness of the sending process.
One benefit of ECN for TCP is that it opens the door for explicit
congestion control on the ack path. This is likely to be particularly
useful for TCP connections over asymmetric paths, where the "return"
path is relatively low-bandwidth. But the ECN draft leaves the
details for future research.