The HTTP 1.1 protocol states that single-user clients should not maintain more than two connections with any server or proxy. This is the reason for browser limits. For more information, see RFC 2616 – Hypertext Transfer Protocol, section 8 – Connections.

Modern browsers are less restrictive than this, allowing a larger number of connections. The RFC does not specify how to prevent the limit being exceeded. Either connections can be blocked from opening or existing connections can be closed.

Some Diffusion™ protocols like HTTP Polling (XHR) use up to two simultaneous connections per Diffusion client. It is important to understand that the maximum number of connections is per browser and not per browser tab. Attempting to run multiple clients within the same browser might cause this limit to be reached.

Reconnection can be used to maintain a larger number of clients than is usually allowed. When TCP connections for HTTP requests are closed, the Diffusion sends another HTTP request which the server accepts. Be aware of cases where Diffusion tries to write a response to closed polling connections before the client can re-establish them. This behavior results in an IO Exception and the Diffusion server closes the client unless reconnection is enabled. When the client tries to re-establish the poll, it is aborted.

Another way to get around browser limits is by providing multiple subdomains. Each subdomain is allowed the maximum number of connections. By using numbered subdomains, a client can pick a random subdomain to connect to. Where the DNS server allows subdomains matching a pattern to be resolved as the same server, tab limits can be mitigated substantially.