Network Protocol

The SANE interface has been designed to facilitate network access to image acquisition devices. In particular, most SANE implementations are expected to support a network backend (net client) and a corresponding network daemon (net server) that allows accessing image acquisition devices through a network connection. Network access is useful in several situations:

• To provide controlled access to resources that are inaccessible to a regular user. For example, a user may want to access a device on a host where she has no account on. With the network protocol, it is possible to allow certain users to access scanners without giving them full access to the system.

  Controlling access through the network daemon can be useful even in the local case: for example, certain backends may require root privileges to access a device. Rather than installing each frontend as setuid-root, a system administrator could instead install the SANE network daemon as setuid-root. This enables regular users to access the privileged device through the SANE daemon (which, presumably, supports a more fine-grained access control mechanism than the simple setuid approach). This has the added benefit that the system administrator only needs to trust the SANE daemon, not each and every frontend that may need access to the privileged device.

• Network access provides a sense of ubiquity of the available image acquisition devices. For example, in a local area network environment, this allows a user to log onto any machine and have convenient access to any resource available to any machine on the network (subject to permission constraints).

• For devices that do not require physical access when used (e.g., video cameras), network access allows a user to control and use these devices without being in physical proximity. Indeed, if such devices are connected to the Internet, access from any place in the world is possible.

The network protocol described in this chapter has been design with the following goals in mind:

1. Image transmission should be efficient (have low encoding overhead).

2. Accessing option descriptors on the client side must be efficient (since this is a very common operation).

3. Other operations, such as setting or inquiring the value of an option are less performance critical since they typically require explicit user action.

4. The network protocol should be simple and easy to implement on any host architecture and any programming language.

The SANE protocol can be run across any transport protocol that provides reliable data delivery. While SANE does not specify a specific transport protocol, it is expected that TCP/IP will be among the most commonly used protocols.