SMTP

Simple Mail Transfer Protocol (SMTP) is defined in RFC 821. It is the most commonly used protocol for sending email. Even though email is not truly peer-to-peer, SMTP can be used as a protocol over which peer-to-peer applications can be built.

Email offers an asynchronous communication path between any two entities. For instance, User A can log in to her server and send an email to User B without knowing if User B is online at that time. Similarly, User B can at any time retrieve all messages sent to her from her email server, and if needed, send a message to User A.

Note

Even though SMTP provided a strong and reliable framework for sending email, there was a strong need for the capability to add new functionality. This led to Extended SMTP (ESMTP), defined by RFC 1425. ESMTP defines how extensions can be integrated into the basic SMTP protocol, instead of redefining a completely new protocol. It enhances SMTP by providing a framework in which all future extensions can be built in a consistent way.

This fundamentally asynchronous path offered by email can be used as an effective channel for P2P application communication.

However, this might not be feasible in a real-world scenario. It might be possible, so use SMTP in applications where communications are infrequent and response time is not critical.