fwb@siemens.UUCP (02/17/86)
The MAP 2.1 document from General Motors defines bridges, routers, and
gateways (and repeaters in the bridge definition). I will quote from the
document. There is no copyright notice on it, so I guess this is OK.
Please try to imagine better graphics in the figures.
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3.3.2 _BRIDGES_
Bridges are transparent devices used to connect segments of a single LAN.
The Data Link protocols of these segments are usually the same. A bridge
can extend a network beyond the design specification of a single segment or
physically isolate the segments from one another. By comparison to an (IEEE
802 baseband) repeater which provides transparent network expansion at the
Physical Layer, the bridge provides network expansion at the Data Link Layer.
...
Bridges may be used to provide connection of two identical network types
such as two IEEE token bus implementations. A bridge may also be used in
coupling dissimilar IEEE 802 networks. The architecture of a bridge is
shown in Figure 3.3-1.
User User
7 7
6 6
5 5
4 4
3 ----- 3
2 2 2 2
1 1 1 1
^ ^ ^ ^
<___|_____| |_____|___>
Segment A Segment B
3.3-1 BRIDGE ARCHITECTURE
3.3.3 _GATEWAYS_
Gateways are devices that connect different network architectures by
performing protocol translation. The primary difference between a gateway
and a bridge is that the gateway utilizes all seven OSI layers where a
bridge is restricted to layers 1 and 2.
...
Examples of gateway applications include connecting a MAP network to an
Allen-Bradley Datahighway, a Modicon Modbus, a DEC Decnet, an IBM SNA, or an
HP DS/1000 network. In terms of OSI architecture, a gateway can be
illustrated as shown in Figure 3.3-2.
User ----- User
7 7 7 7
6 6 6 6
5 5 5 5
4 4 4 4
3 3 3 3
2 2 2 2
1 1 1 1
^ ^ ^ ^
<___|_____| |_____|___>
Segment A Segment B
3.3-2 GATEWAY ARCHITECTURE
3.3.4 _ROUTERS_
Routers are commonly used to connect several networks together at a common
point. In this configuration the Router provides path selection and
alternate routing based on destination network layer addresses and status of
connected networks, an illustration in OSI terms is as shown in Figure
3.3-3. A router has one common network address for all attached networks.
----INTERNET---
3 3 3
2 2 2
1 1 1
^ ^ ^
<_____| | |_____>
NETWORK A | NETWORK C
\ /
NETWORK B
3.3-3 ROUTER ARCHITECTURE
-----------------------------------------------------
Frederic W. Brehm (ihnp4!princeton!siemens!fwb)
Siemens Research and Technology Laboratories
105 College Road East
Princeton, NJ 08540
(609) 734-3336fwb@siemens.UUCP (02/17/86)
Oops, please excuse a mistake in one of the figures. The gateway
architecture links two NETWORKS, not two SEGMENTS. Here is the corrected
figure.
User ----- User
7 7 7 7
6 6 6 6
5 5 5 5
4 4 4 4
3 3 3 3
2 2 2 2
1 1 1 1
^ ^ ^ ^
<___|_____| |_____|___>
NETWORK A NETWORK B
3.3-2 GATEWAY ARCHITECTURE
-----------------------------------------------------
Frederic W. Brehm (ihnp4!princeton!siemens!fwb)
Siemens Research and Technology Laboratoriesmwg@petrus.UUCP (Mark Garrett) (02/24/86)
++ > Gateways are devices that connect different network architectures by > performing protocol translation. The primary difference between a gateway > and a bridge is that the gateway utilizes all seven OSI layers where a > bridge is restricted to layers 1 and 2. > Frederic W. Brehm (ihnp4!princeton!siemens!fwb) > Siemens Research and Technology Laboratories Do gateways really go to layer 7? This would imply that a gateway sometimes needs to know about what specific application is running through the network. I can't imagine a need to know anything above the session layer. Also, someone mentioned that a bridge can do protocol conversion between dissimilar networks. Perhaps if this conversion is simple (changing one address for another, and one control code to another syntax with the same meaning etc) - but if it requires anything like a microprocessor and software, as opposed to a little finite state machine (ie PAL), I would call it a gateway. This isn't any official definition, just what makes sense to me. Comments? Mark Garrett Bellcore
mark@cbosgd.UUCP (Mark Horton) (03/03/86)
In article <30@petrus.UUCP> mwg@petrus.UUCP (Mark Garrett) writes: >Do gateways really go to layer 7? This would imply that a gateway sometimes >needs to know about what specific application is running through the network. >I can't imagine a need to know anything above the session layer. > >Also, someone mentioned that a bridge can do protocol conversion between >dissimilar networks. I'm not the world's foremost authority on gateways, but here's what "gateway" means to me: A "gateway" is a very general term, used to represent a box or other service which connects two networks together, so that users of one network can communicate with users of the other network. It can go at just about any level of the OSI model. If you have two dissimilar networks, but they both support a service which is similar (e.g. transport, or mail) then you can build a gateway as follows. First, get a host which is on both networks. Then, determine the OSI layer at which the two similar services sit. For this example, I'll use "connection oriented transport" as the service, which sits at layer 4. Now, build into your gateway a translation between transport A and transport B. This translation logically sits "above" layer 4, but it's clearly "below" the session layer. So we'll call it layer 4.5. The resulting tower picture looks like this: --------- ---------- 7. Appl | X.400 | |SMTP/822| --------- ---------- 6. Pres | X.409 | | TELNET | --------- ---------- 5. Sess | OSI | | null | --------- --------------------- ---------- | | | gateway | | | | TP0 | --------------------- | TCP | 4. Tran | | | TP0 | | TCP | | | --------- ---------- ---------- ---------- 3. Net | X.25 | | X.25 | | IP | | IP | --------- ---------- ---------- ---------- 2. DL | HDLC | | HDLC | |Ethernet| |Ethernet| --------- ---------- ---------- ---------- 1. Phys | 56KB |----------| 56KB | |Ethernet|------|Ethernet| --------- ---------- ---------- ---------- It should be clear in looking at this picture (with electronic mail standards filled in at the higher levels) that it doesn't matter that the layers lower than the gateway (1-4) are quite different, but it doesn't matter. These two networks operate independently of each other in the first 4 layers. At the layers higher than the gateway, however, they also speak different protocols, and this DOES matter. Just hooking together two transports doesn't help SMTP and X.400 talk to each other. To make that work, you'd have to build a different kind of gateway: one that sits above layer 7. (This is in fact how the various mail gateways between networks work, and was probably what one previous poster had in mind.) Back in the mid-1970's, a machine which was on both the ARPANET and TELENET was viewed as a gateway between them. If you had a login on the machine, you could connect to it over one network, log in, and go out over the other network. This was the simplest, crudest case, but it did work. I've seen similar setups done at the physical level with dialups and port switches, interconnecting different switches with the dialup world. In general, it's possible to install a gateway at any level. A typical modem might be viewed as a gateway at the physical level. An IBM style protocol converter is a gateway at the data link level. IP is a gateway at the network level. There are two approaches to take, once you've built a gateway such as the above. One is to take the position that the higher levels don't matter - X.25 did this for years, using an X.25 network level connection as if it were a remote login application service. A variation on this is to put the gateway above layer 7, so there are no higher levels to matter (although this really just shunts the conversion problem onto layer 8, e.g. the user.) This is how mail has worked for years, using addresses like cbosgd!mark@Berkeley to route mail through the ARPANET to UUCP. A second approach is to start building new layers on top of the bridged service, on both sides. If you can get the same layers on both sides, they can talk, and the gateway becomes smoothly integrated. This is what was done with TCP/IP - once the new IP service was created, a whole set of appliations (plus TCP and TELNET layers) were defined. This is also what the Domain system does for mail, using uniform addresses like mark@cbosgd.ATT.COM on both sides of the gateway. By the way, if the two layers being connected together (e.g. transport in the examble above) are compatible, then as a special case, the gateway is called a "bridge". For example, a pair of modems serves as a bridge between two compatible RS232 cables. Ethernet bridges (as most except UB seem to use the term) fit above the data link layer (or whichever layer you put Ethernet at.)