Improve Server Max Bit Rate Violation Messaging To Client

[manedurphy]

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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies: master

• Issues fixed (if any): Allow server-side to enforce rate-limits to avoid possible DoS and protect iperf Daemon #937

• Brief description of code changes (suitable for use as a commit message):

As per the ongoing discussion in iperf/#1684, as part of an effort to enhance the user experience of iperf3 clients, the enhancement proposed is to validate the client's input parameter for --bitrate when it is set and reject the measurement when it violates the server's configured --server-bitrate-limit value. When the client does not provide a limit and the test violates the configured bit rate, then the server should send an informative message to the client to indicate why the test was abruptly stopped.

Client's Requested Bit Rate Exceeds Server's Max

server

./src/iperf3 -s --server-bitrate-limit 5Mbit/1sec
-----------------------------------------------------------
Server listening on 5201 (test #1)
-----------------------------------------------------------
iperf3: error - client's requested bit rate exceeds the server's bitrate limit
-----------------------------------------------------------
Server listening on 5201 (test #2)
-----------------------------------------------------------

client

./src/iperf3 -c 127.0.0.1 --time 5 --parallel 4 --bitrate 10Mbit/1sec
Connecting to host 127.0.0.1, port 5201
iperf3: error - client's requested bit rate exceeds the server's bitrate limit

Client's Bit Rate Exceeds Server's Max Circumstantially

server

./src/iperf3 -s --server-bitrate-limit 5Mbit/1sec
-----------------------------------------------------------
Server listening on 5201 (test #1)
-----------------------------------------------------------
Accepted connection from 127.0.0.1, port 33512
[  5] local 127.0.0.1 port 5201 connected to 127.0.0.1 port 33518
[  8] local 127.0.0.1 port 5201 connected to 127.0.0.1 port 33526
[ 10] local 127.0.0.1 port 5201 connected to 127.0.0.1 port 33540
[ 12] local 127.0.0.1 port 5201 connected to 127.0.0.1 port 33544
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.01   sec  1.15 GBytes  9.86 Gbits/sec
[  8]   0.00-1.01   sec  1.33 GBytes  11.4 Gbits/sec
[ 10]   0.00-1.01   sec  1.32 GBytes  11.2 Gbits/sec
[ 12]   0.00-1.01   sec  1.23 GBytes  10.5 Gbits/sec
[SUM]   0.00-1.01   sec  5.03 GBytes  43.0 Gbits/sec
iperf3: error - total required bandwidth is larger than server limit
-----------------------------------------------------------
Server listening on 5201 (test #2)
-----------------------------------------------------------

client

./src/iperf3 -c 127.0.0.1 --time 5 --parallel 4
Connecting to host 127.0.0.1, port 5201
[  5] local 127.0.0.1 port 33518 connected to 127.0.0.1 port 5201
[  7] local 127.0.0.1 port 33526 connected to 127.0.0.1 port 5201
[  9] local 127.0.0.1 port 33540 connected to 127.0.0.1 port 5201
[ 11] local 127.0.0.1 port 33544 connected to 127.0.0.1 port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  1.15 GBytes  9.86 Gbits/sec    0   1.12 MBytes
[  7]   0.00-1.01   sec  1.33 GBytes  11.3 Gbits/sec    0   1023 KBytes
[  9]   0.00-1.01   sec  1.32 GBytes  11.2 Gbits/sec    0   1.69 MBytes
[ 11]   0.00-1.01   sec  1.24 GBytes  10.6 Gbits/sec    0   1.56 MBytes
[SUM]   0.00-1.00   sec  5.05 GBytes  43.2 Gbits/sec    0
iperf3: error - total required bandwidth is larger than server limit