The word gateway [ ˈgeɪtweɪ ] (English for exit and entry , literally gate way ) describes a component (hardware and / or software) in computer science that creates a connection between two systems. The term gateway implies that the forwarded data is processed. This can be data in all layers of the OSI model . Depending on the shift on which the processing takes place, there are more specific terms for the corresponding systems.
The way a gateway works is very different depending on the layer in the OSI model:
- Layer 1 Media converters work here . B. convert electrical signals into optical signals. Also Powerline adapters are among the gateways to layer first
- Layer 2 The translation between different network types takes place on Layer 2, e.g. B. between Ethernet and Token Ring . From this layer an effective change of the transported data is possible. The Ethernet headers are replaced by a token from Token Ring and vice versa.
- Layer 3 In networks that are divided into subnets like IP addresses , the routers work here . A router forwards packets between the connected subnets. The actual routing does not change any data on Layer 3. An exception to this is Network Address Translation, which changes data on Layer 3.
- Layer 4 In the TCP / IP stack , port address translation is a common example of a gateway. Port numbers of TCP or UDP are exchanged in the transported packets. Another example of Layer 4 gateways is the termination of TLS connections. The gateway decrypts the incoming packets and then forwards them to another system. Encryption using TLS can also be used for forwarding.
- Layer 5-7 gateways on these layers implement a conversion between different application protocols, e.g. B. from fax to e-mail or from analog telephony to VoIP . A gateway on these layers that examines and changes the contents of the packets without translating them into another protocol is called an application layer gateway or, more often, a proxy .
In practice, the different gateways are often found in the same systems. Firewalls are the best example of this . Even the simplest firewalls cover layers 1 - 4 with their functions, while more expensive devices have functions for all seven layers.
Differentiation from the standard gateway as a router (default gateway)
In the early days of IP , it was not uncommon for one to connect networks of different types with one another and thus inevitably convert their protocols. Because IP was confronted with protocols such as DECnet, SNA and Novell's IPX / SPX. The term default gateway from the IP network configuration should make it clear to the administrator that he can enter a gateway here. But what is actually used there depends on the respective network architecture.
With the predominance of the IP protocol, the router increasingly took the place of the gateway. There are now hardly any gateways in this segment, as the networks communicate almost exclusively via the IP protocol. It is no longer necessary to implement a protocol.
Instead of converting protocols, the default gateway of an IP configuration nowadays only forwards all network inquiries that do not belong to a subnet to another subnet and thus simply fulfills the functions of a router, which is why the term "default router" would be more appropriate today. Gateways are therefore often equated with routers in common parlance, although a gateway is not necessarily a router.
Routers work on the third layer (network layer) of the OSI reference model , whereas a gateway can be implemented on all layers.
According to RFC 4949 , typical forms of a gateway are: bridge, router and proxy server.
In the home, a combination of DSL router and DSL modem is sometimes referred to as an Internet gateway. In simple terms, these devices combine the function of connecting networks with one another ( routing ) with the ability to use different protocols for this purpose (gateway). When using DSL , IP packets from the home network are usually sent to the provider's network using the PPPoE protocol .
The protocol designation of a standard gateway can be described as multilayered on the implementation level because, in contrast to a simple router, there is the ability to start up independently and temporarily independent of a main system. This applies not only to WAN activities, but also to all processes that are possible on operating systems today.
On the other hand, the Internet gateway can be another name for establishing a VPN connection via a secure tunnel.
A VPN gateway enables secure access to a remote company network that is normally not publicly accessible via a public network such as the Internet. Various services such as e-mail, intranet or shared drives, which are actually only available internally in the LAN, can be used via a tunneled connection.
The gateway as a network service provider should not be confused with the “service gateway” concept, as represented by OSGi , for example - even if there are certainly overlaps.
In telecommunications , a gateway is also referred to as a network transition . Soft switches or media gateways serve as an interface between different network types that take over the digital transcoding of the different media (voice). A gateway can be used, for example, between a circuit-switched network ( ISDN ) and an IP-based packet-oriented network ( NGN ).
In automotive engineering , gateways are often found between the different data buses such as CAN in different speed versions, e.g. B. MOST bus . A typical configuration in an automobile consists of a "fast" CAN bus for engine control and similar real-time control devices and a "slow" CAN bus for control devices with little and seldom data ("comfort bus", for tire pressure monitoring or fuel sensors) . For certain purposes ( diagnosis ), some data must also be made available on the other bus. For this purpose the gateway copies the data from one bus to the other. This gateway can act as a separate control device or it can be part of an existing, larger control device (for example the instrument cluster ), which must serve connections to all bus variants.