1.3.1 HTTP Encapsulation Protocols

This document defines three HTTP encapsulation protocols, LongLived, KeepAlive and Polling. Each of these HTTP encapsulation protocols is designed to replace TCP as the transport for SSTP. Multiple encapsulation protocols exist because of different proxy implementations and lack of proxy standards. All of the HTTP encapsulated connections specified in this document are designed to traverse firewalls and HTTP proxies.

These encapsulation protocols are used to navigate firewalls and proxies when both are working together. When allowed by firewall rules, these encapsulation protocols can be used to connect directly to a target server on port 80/TCP. If a direct connection to the target server is blocked by a firewall, these encapsulation protocols can be used to traverse an HTTP proxy. The server listens on the well-known HTTP port 80/TCP. The HTTP proxy typically listens on the well-known HTTP port 80/TCP or the alternate well known port 8080/TCP.

SSTP is the preferred protocol for client and server communication because it avoids the overhead associated with HTTP encapsulation of SSTP. HTTP Encapsulation of SSTP protocols are adaptable to various types of network topologies that block SSTP traffic on 2492/TCP. Because the encapsulation protocols are optimized for different network topologies, each protocol has its own advantages and constraints. This adaptability comes with the cost of additional protocol overhead, in the form of additional headers, message exchanges and connection management. For performance reasons, described in section 1.3.4, HTTP encapsulation   connections are used when direct SSTP connections to a server are blocked and when both Secure Tunnel and SOCKS proxy connections fail.