BLIP / Overview

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BLIP Overview

BLIP is a message-oriented network protocol that lets the two peers on either end of a TCP socket send request and response messages to each other. It's a generic protocol, in that the requests and responses can contain any kind of data you like.

BLIP was inspired by BEEP (in fact BLIP stands for "BEEP-LIke Protocol") but is deliberately simpler and somewhat more limited. That results in a smaller and cleaner implementation, especially since it takes advantage of Cocoa's and CFNetwork's existing support for network streams, SSL and Bonjour. (BLIP is currently a bit under 2,000 lines of code, and the rest of the MYNetwork classes it builds on add up to another 1,500. That's at least an order of magnitude smaller than existing native-code BEEP libraries.)

BLIP is currently implemented in two forms: Objective-C for Mac OS X applications; and pure Python.

Features

  • Each message is very much like a MIME body, as in email or HTTP: it consists of a blob of data of arbitrary length, plus a set of key/value pairs called "properties". The properties are mostly ignored by BLIP itself, but clients can use them for metadata about the body, and for delivery information (i.e. something like BEEP's "profiles".)
  • Either peer can send a request at any time; there's no distinction between "client" and "server" roles.
  • Multiple messages can be transmitted simultaneously in the same direction over the same connection, so a very long message does not block any other messages from being delivered. This means that message ordering is a bit looser than in BEEP or HTTP 1.1: the receiver will see the beginnings of messages in the same order in which the sender posted them, but they might not end in that same order. (For example, a long message will take longer to be delivered, so it may finish after messages that were begun after it.)
  • The sender can indicate whether or not a message needs to be replied to; the response is tagged with the identity of the original message, to make it easy for the sender to recognize. This makes it straighforward to implement RPC-style (or REST-style) interactions. (Responses cannot be replied to again, however.)
  • A message can be flagged as "urgent". Urgent messages are pushed ahead in the outgoing queue and get a higher fraction of the available bandwidth.
  • A message can be flagged as "compressed". This runs its body through the gzip algorithm, ideally making it faster to transmit. (Common markup-based data formats like XML and JSON compress extremely well, at ratios up to 10::1.) The message is decompressed on the receiving end, invisibly to client code.
  • The implementation supports SSL connections (with optional client-side certificates), and Bonjour service advertising.

Much more detail is provided in the protocol documentation.

There is also API documentation.

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