Bootcast -- using multicast to distribute files to multiple nodes

bootcast is a file transport protocol that reduces bandwidth when multiple clients request access to the same files at once. This can be useful for instance when netbooting multiple nodes in a single cluster. Using a different file transport protocol like (T)FTP or HTTP will quickly saturate the network when more clients boot at the same time.

Design notes

bootcast was designed as a drop-in replacement for (T)FTP or HTTP based clients like wget or curl. A bootcast client can simply request a file from a bootcast server and the file will be downloaded. This is different from most other multicast implementations where a file is simply pushed to different clients.

Behind the scenes the initial bootcast request will trigger to send the bootcast server to start a broadcast of the requested file. The bootcast server will respond with the connection details for the client in order to join the TCP multicast group. Subsequent requests will not trigger a new read from the file, but will simply join in the existing multicast group.

When a client has received all pages from the bootcast server it will send a request indicating that it will leave the broadcast. When all clients have left the bootcast the bootcast server will stop the cast. Also, when several iterations of bootcasts have occurred without new join requests or leave requests, the bootcast server will stop the broadcast.

Protocol notes

A client can request a bootcast file by peforming a join request. This is a simple HTTP GET request like this GET /join/<path>. The server will respond with a text document containing the following lines:

CONNECT=<multicast_group>:<port>
TOKEN=<a client session identifier>
PAGES=<the number of pages or file blocks>
PAGESIZE=<the size of each block in bytes>

The client can then bind to a listening socket as described in the CONNECT string. Each block of data sent by the server will have the following data fields.

1. a four byte integer containing the size of the current page.
2. a four byte integer containing the page index (starting with 1)
3. a four byte integer which is reserved for future expansion.
4. a four byte integer which is reserved for future expansion.

Each value will be transferred in network byte order.

Once the client has downloaded all pages from the server, it will leave the cast. This is done by sending a HTTP GET request like GET /leave/<path>. The client must identify itself by providing an HTTP X-TOKEN header containing the client session identifier. If the client does not provide this header, its request will be ignored. No response body will be sent back to the client.

Reference implementation

A simple reference implementation of the bootcast protocol can be downloaded from http://github.com/hansthen/bootcast. This will install a sample bootcast client and a sample bootcast server.