Cafnet

Overview

CafNet (carry and forward network) is a delay-tolerant stack that enables mobile data muling and allows data to be sent across an intermittently connected network. The CafNet protocols allow cars to serve as data mules, delivering data between nodes that are otherwise not connected to one another. For example, these protocols could be used to deliver data from sensor networks deployed in the field to Internet servers without requiring anything other than short-range radio connectivity on the sensors (or at the sensor gateway node).

Setup

Requirements

Cafnet depends on:

Download

Download the latest release of CafNet from http://cartel.csail.mit.edu/cafnet/cafnet-0.1.tgz.

Installation

Cafnet is a standard setuptools package. To install it with the default options, simply run setup.py install.

Usage

Cafnet is a Twisted daemon plugin. Start it by running twistd cafnet <args>. See cafnet-server.bash and cafnet-client.bash for examples. To see all options and their default values, look at the file cafnet/tap.py.

RPC Interface

All Cafnet addresses are strings.

The RPC interface to Cafnet consists of the following functions.

bind(addr: str, url: str): int

The client must call this after its own RPC server starts listening and before calling any other Cafnet procedures. bind registers the given Cafnet address addr with the given RPC callback URL url so that Cafnet knows where to direct its callbacks. Always returns the integer 0.

schedule(src_addr: str, dst_addr: str, priority: int, ack_type: str, size: int): int

Schedules an ADU for delivery and returns an integer handle to the ADU.

The following user callback RPC interface is expected by Cafnet.

get_msg(id: int): binary

Must return the actual ADU bytes associated with id, as bound by the schedule call.

msg_received(src_addr: str, dst_addr: str, priority: int, ack_type: str, id: int, msg: binary): int

Called when an incoming ADU is received. Must return 0.

ack_received(id: int): int

id is the integer ID of the message that was returned. Must return 0.

RPC Protocol

Each RPC invocation consists of the name of the procedure and the arguments, followed by the return value from the callee to the caller. The name is a string, and arguments and return values can only be integers or (byte) strings. All integers are 4 bytes and big-endian. All strings consist of an integer length followed by that many bytes.

A single TCP connection is maintained for the duration of the RPC; this may be lazily established (i.e., the first time a call needs to be made, establish the connection). The order of return results must correspond to the order of their respective calls.

The protocol was designed to be fast and simple. Once upon a time, Cafnet used XMLRPC, but that proved to be too slow on the CPU-constrained Soekris boxes, particularly since binary blobs had to be sent around and thus base64-encoded, and since a separate TCP connection was made per call. FastRPC was also considered, but it bears overhead in its use of HTTP and thus per-invocation TCP connections, as well as the extra serialization of type information.

Connectivity Updates

To notify Cafnet that network connectivity has been lost, send the Cafnet daemon the signal RTMAX-9. To notify it when connectivity has been re-established, send signal RTMAX-10. The cell-down.bash and cell-up.bash scripts do this.

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