I have a HAProxy + NodeJS + Rails Setup, I use the NodeJS Server for file upload purposes.

The problem I'm facing is that if I'm uploading through haproxy to nodejs and a "TCP (Fast) Retransmission" occurs because of a lost packet the TX rate on the client drops to zero for about 5-10 secs and gets flooded with TCP Retransmissions.

This does not occur if I upload to NodeJS directly (TCP Retransmission happens too but it doesn't get stuck with dozens of retransmission attempts).

My test setup is a simple HTML4 FORM (method POST) with a single file input field. The NodeJS Server only reads the incoming data and does nothing else.

I've tested this on multiple machines, networks, browsers, always the same issue.

Here's a TCP Traffic Dump from the client while uploading a file:

.....
TCP 1506    [TCP segment of a reassembled PDU]
>> everything is uploading fine until:
TCP 1506    [TCP Fast Retransmission] [TCP segment of a reassembled PDU]
TCP 66      [TCP Dup ACK 7392#1] 63265 > http [ACK] Seq=4844161 Ack=1 Win=524280 Len=0 TSval=657047088 TSecr=79373730
TCP 1506    [TCP Retransmission] [TCP segment of a reassembled PDU]
>> the last message is repeated about 50 times for >>5-10 secs<< (TX drops to 0 on client, RX drops to 0 on server)
TCP 1506    [TCP segment of a reassembled PDU]
>> upload continues until the next TCP Fast Retransmission and the same thing happens again

The haproxy.conf (haproxy v1.4.18 stable) is the following:

global
    log 127.0.0.1   local1 debug
    maxconn     4096 # Total Max Connections. This is dependent on ulimit
    nbproc      2

defaults
    log global
    mode        http
    option      httplog
    option      tcplog

frontend http-in
    bind *:80

    timeout client 6000

    acl is_websocket   path_beg /node/

    use_backend node_backend  if is_websocket

    default_backend app_backend

# Rails Server (via nginx+passenger)
backend app_backend
    option httpclose
    option forwardfor

    timeout server 30000
    timeout connect 4000
    server app1 127.0.0.1:3000

# node.js
backend node_backend
    reqrep ^([^\ ]*)\ /node/(.*)     \1\ /\2
    option httpclose
    option forwardfor
    timeout queue 5000
    timeout server 6000
    timeout connect 5000
    server node1 127.0.0.1:3200 weight 1 maxconn 4096

Thanks for reading! :)

Simon