Gateways are devices that control the flow of traffic into or out of a network. Although definitions differ, for this context a gateway can be thought of as a device that passes packets between subnets (real or virtual), and performs operations above OSI layer 3 (session, flow control, protocol conversion, and application specific).

Gateways can also be the source of vulnerabilities. Gateways are important to wireless networks and mobile wireless devices for several reasons:

• Wireless networks do not afford the same physical levels of security as wired networks. Due to resource constraints, mobile wireless devices are themselves often less secure than wired devices. Wireless security gateways can protect a wired network from untrusted wireless hosts.

Unlike firewalls, for which hosts are either “inside the firewall” or “outside the firewall,” the distinction between inside and outside is somewhat blurred for mobile wireless devices. A company’s trusted workers may need “inside” kinds of connectivity while using wireless devices.

Conversely, visitors may need “outside” kinds of connectivity while connecting to the company’s wired network through an access point inside the corporate firewall. Wireless security gateways address these issues by performing two-way authentication and limiting access privileges on a per-device basis.

• Mobile wireless devices often have limited resources that cannot support the same protocols as wired devices. They may therefore use resource-sharing protocols which must be translated in a protocol gateway to enable interaction with standard Internet protocol services.

For example, a WAP gateway translates protocols in the WAP suite, including WML (HTML), WML Script (CGI), WBMP BMP), WBXML (XML), WSP (HTTP), WTP (TCP/IP), WTLS (SSL), and WDP (UDP).

These kinds of translation pose security issues both because the wireless protocols are often less secure than the corresponding wired protocols and because, in translation, encrypted data takes an unencrypted form inside the gateway.

• Wireless devices often exist on subnets that do not support the full Internet addressing scheme. For example devices may use IP addresses [ipv] reserved for local access only, or otherwise not support all of the capabilities needed for WAN access.

Gateways can provide a bridge between these local subnets and a broader WAN, (i.e., Internet). Common small office home office (SOHO) wireless switches provide NAT to allow local devices to all access the Internet using a single IP address.

Similarly, a Personal Mobile Gateway with WAN connectivity like GSM or GPRS (General Packet Radio Service) can allow Bluetooth, 802.11, or 802.15 devices on a PAN (Personal Area Network) to have full Internet connectivity. The fact that devices behind a NAT gateway do not have unique IP addresses has implications for some security strategies (i.e., IPSEC-AH).

• Mobile wireless devices may be involved in various sorts of commerce, such as M-commerce and downloading multimedia streams with digital rights. Depending on how you look at it, wher e conflicting privacy and ownership interests come into play, “trusted gateways” can bridge the no man’s land, or encapsulate the overlap as a trusted third party.

This space is an area of active research and is, as yet, not as well defined as the other gateway functions. Issues here are closely tied to digital rights management. See for example the Shibboleth project.

The Internet was built on “transparency” and the “end-to-end principle.” Roughly stated, transparency “refers to the original Internet concept of a single universal logical addressing scheme, and the mechanisms by which packets may flow from source to destination essentially unaltered.”

The end-to-end principle holds that functions of data transmission other than transport, such as data integrity and security, are best left to the transmission endpoints, themselves. This allows applications to be ignorant of the transport mechanisms, and transport systems to be ignorant of the data being transported. Gateways, by their nature, violate one or both of these principles.