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Increase maximum supported number of simultaneously active TCP connections (Photon) #1350

merged 7 commits into from Jan 22, 2018


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commented Jul 5, 2017


Currently, the firmware allows up to 5 TCP connections (sockets) to be active at the same time. With one socket used by the system to communicate with the cloud, applications can establish (or accept) only 4 TCP connections.


Increase maximum number of simultaneously active TCP connections to 10. This requires increasing size of the LwIP's TCP PCBs (Protocol Control Blocks) pool from current 5 to 10 entries. The pool is allocated statically, so after this change static RAM usage in the part2 increases by ~800 bytes.

Ideally, we would also want to increase number of preallocated TX/RX buffers accordingly (currently there are 7 buffers available for each direction), but this doesn't seem to be feasible due to static RAM constraints.

Depending on the application's data flow, limited number of TX/RX buffers may lead to increased number of retransmissions between the peers and, in some cases, to connection errors. This PR includes a set of tests which stress-test the TCP implementation under various scenarios.

Steps to Test

Run wiring/tcp_client and app/tcp_server tests.

Example App

#include "application.h"


namespace {

const Serial1LogHandler logHandler(115200, LOG_LEVEL_WARN, {
    { "app", LOG_LEVEL_ALL }

const char* const SERVER_HOST = ""; // Google
const unsigned SERVER_PORT = 80;

const int MAX_CONNECTIONS = 10;

} // namespace

void setup() {
    Vector<TCPClient> clients;
    for (int i = 0; i < MAX_CONNECTIONS; ++i) {"Opening connection %d of %d...", i + 1, MAX_CONNECTIONS);
        TCPClient c;
        if (c.connect(SERVER_HOST, SERVER_PORT)) {
        } else {

void loop() {



  • User is totes amazing for contributing!
  • Contributor has signed CLA (Info here)
  • Problem and Solution clearly stated
  • Run unit/integration/application tests on device
  • Added documentation
  • Added to after merging (add links to docs and issues)

TODO: Update reference docs


[PR #1350] [Photon/P1] Increase maximum supported number of simultaneously active TCP connections from 5 to 10 (cloud connection consumes 1).

@sergeuz sergeuz added this to the 0.8.0 milestone Jul 5, 2017

@sergeuz sergeuz requested a review from avtolstoy Jul 5, 2017

@sergeuz sergeuz added the enhancement label Jul 5, 2017

@m-mcgowan m-mcgowan modified the milestones: 0.8.0, 0.8.0-rc.2 Nov 24, 2017

@avtolstoy avtolstoy referenced this pull request Jan 15, 2018
4 of 6 tasks complete

@m-mcgowan m-mcgowan merged commit c6e57ab into fix/tcpserver-threading Jan 22, 2018

2 checks passed

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@m-mcgowan m-mcgowan deleted the feature/max_tcp_conn_10 branch Jan 22, 2018

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