RFC 5681, "TCP Congestion Control", September 2009

Note: This RFC has been updated by RFC 9438

Errata ID: 4068
Status: Held for Document Update
Type: Editorial
Publication Format(s) : TEXT
Reported By: Michael Taylor
Date Reported: 2014-08-04
Held for Document Update by: Martin Stiemerling
Date Held: 2014-11-10

Throughout the document, when it says:

Notes:

The problem is the use of the phrase "new data" in an imprecise manner to sometimes mean "previously unacknowledged data" and other times mean "never before sent data".

For example, in Section 3.1

During slow start, a TCP increments cwnd by at most SMSS bytes for
each ACK received that cumulatively acknowledges new data.

This should read

During slow start, a TCP increments cwnd by at most SMSS bytes for
each ACK received that cumulatively acknowledges previously
unacknowledged data.

I believe that throughout Section 3.1 "new data" refers to "previously unacknowledged data".

However, in Section 3.2 we have

After the fast retransmit algorithm sends what appears to be the
missing segment, the "fast recovery" algorithm governs the
transmission of new data until a non-duplicate ACK arrives.

I believe that here "new data" refers to "previously unsent data".

This is clearer in the following paragraph from Section 3.2

1. On the first and second duplicate ACKs received at a sender, a
TCP SHOULD send a segment of previously unsent data per [RFC3042]
provided that the receiver's advertised window allows, the total
FlightSize would remain less than or equal to cwnd plus 2*SMSS,
and that new data is available for transmission.

Here we can see the use of "previously unsent data" followed by "new data"
which refers to the aforementioned "previously unsent data".

While the meaning of "new data" might be clear to those with extensive experience
in TCP it is imprecise and therefore may be quite confusing to those who are learning about the protocol.

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