Hi,
I need to send a "huge" amount of TCP data (60-100kBytes) over a slow wireless
connection (PPP, around 3-4 kbit/s). With my first attempts I sometimes see TCP
retransmissions from my LwIP stack, so I did some measurements on this. I know
that a 1024 bytes packet will take around 2.5 seconds, in addition to the
travel time of the returning ACK I should not do a TCP retransmission before 5
seconds.
I did some measurements over the wire then:
- Opening a TCP connection from my device to a TCP test server
- Sending a 1024 bytes payload TCP packet over to the server (which is
immediately ACKed)
- Pulling the wire to the server
- Sending another 1024 bytes packet
- Tracing all this in Wireshark and see after which time LwIP does the first
retransmission
With this setup I see a first retransmission sent out around 13 seconds after
the initial packet which was not ACKed. That's OK and will definitely work also
on my slow PPP connection.
But then a bit different:
- Opening a TCP connection from my device to a TCP test server
- Sending a 1024 bytes payload TCP packet over to the server (which is
immediately ACKed) <- LEAVING THIS STEP AWAY
- Pulling the wire to the server
- Sending another 1024 bytes packet
- Tracing all this in Wireshark and see after which time LwIP does the first
retransmission
Now LwIP sends the first retransmission right after around 2.5 seconds (which
is way too early for my slow connection).
So, I'm wondering why LwIP uses totally different times to the first TCP
retransmission in both scenarios. This is followed by a major question: How can
I set a defined minimum retransmit time of 5 seconds for my connection?
A second issue/question:
As I have this amount of data stored in my memory, but the outgoing connection
is that slow, I'd like to have knowledge how many packets are "in the transmit
queue" in LwIP here.
When my NETCONN_WRITE sends the data out to LwIP (using NETCONN_COPY), the data
is copied into LwIP's memory. So I can repeat that a few times until LwIP's
buffers are full (remember: The outgoing PPP connection for this is slooooow)
and my NETCONN_WRITE blocks.
To get around this and avoid filling up LwIP's buffers (which would block also
local Ethernet connections like my webserver), a "dumb" way would be to have a
fixed delay of 3 seconds between two consecutive NETCONN_WRITE to this TCP
connection.
More elegant would be if there is any way/variable to "poll" if there are still
packets in the queue / in the transmit process for my TCP connection. Once the
buffers are empty again, I could send the next packet over to LwIP.
Any ideas here?
Thanks a lot,
Marco
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