golang http(或gocraft)速率限制

I have a server implementing a REST service using golang's net.http and gocraft.web package.

And I have a client sending JSON request via libcurl, the client is written in C++.

What I notice is that when there are a lot of client threads for sending concurrent JSON requests, there are times when the client gets "Couldn't connect to server" error, but then after a while it can resume sending again.

Since the observation points to rate-limiting, my question is where is rate-limitting happening? Is this at the client side (libcurl), Go's http package, or gocraft.web package?