Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!think!harvard!seismo!mcvax!cernvax!kik From: kik@cernvax.UUCP (kik) Newsgroups: net.lan Subject: Re: Repeaters, Bridges, Routers, and Gateways Message-ID: <280@cernvax.UUCP> Date: Wed, 26-Feb-86 07:40:36 EST Article-I.D.: cernvax.280 Posted: Wed Feb 26 07:40:36 1986 Date-Received: Sat, 1-Mar-86 01:24:20 EST References: <1174@decwrl.DEC.COM> <442@ubvax.UUCP> Reply-To: kik@cernvax.UUCP (Crispin PINEY) Organization: CERN, Geneva/Switzerland Lines: 129 Summary: Definitions of terms and services I have a personal interest in this discussion on the terminology of LAN interconnection, since I have just written a paper on a project in which I was involved, interconnecting transparently Ethernets by means of a high-speed, general-purpose backbone network. I have included the (more or less) relevant parts of the paper below: ------------------------------ There are several approaches by which communications can be extended over several networks. They fit into four main categories: gateways, relays, bridges, and repeaters. A gateway translates between protocols. Gateways can operate at different levels in the ISO model. A gateway that operates below the Application layer is also known as a "protocol converter". Gateways are, naturally, specific to the protocols for which they are designed. A relay forwards a packet towards its final destination. This is an operation carried out in the Network layer and is a characteristic mechanism of store-and-forward networks. It allows mixing different communications technologies, but imposes an overall Network level protocol. A bridge is assumed to provide a means of propagating a basic information frame from the source to the destination network, in such a way that the information content is unchanged. This operation is constrained by the OSI model to being performed in the lower sublayer of the Link layer, whence the complete term "Medium Access Control (or MAC)-level Bridge". This approach to intercommunication allows any mix of higher-level protocols to operate within the resulting bridged area network in the same way as within a single local area network. In addition, a bridge can carry out "filtering" on the destination address. This entails forwarding to each segment only the frames whose destination corresponds to an address on that segment. A repeater amplifies or regenerates the physical signals on a link or bus-type network. It is used to overcome distance limitations due to signal degradation. It should, however, be noted that the term buffered repeater is generally used to describe a simple MAC-level bridge. MAC-level Bridge Service Definition =================================== This service definition is based on work being carried out within IEEE project 802 and expected to be incorporated into ISO^8880 and the long-awaited ISO^8802/1. The term station is used to represent a (generally intelligent) device connected to a LAN. The service characteristics are examined with their implication on the required characteristics of the bridge:- topology independence: bridges are invisible to the upper part of level^2 (LLC), and all higher layers. They are not explicitly addressed, except for management functions; MAC transparency: the bridge should not need any modification to the existing MAC specifications (ISO^8802/3,4,5). The assignment of LAN addresses should not be affected by the presence of the bridge; minimal increase in errors: bridges should not significantly increase the incidence of errors between communicating stations. It should be noted, however, that the general bridge service definition does not rule out loss, duplication or misordering of frames; sufficient data length: the bridges should be able to forward at least the minimum useful data unit. The minimum acceptable size is set by IEEE to 263^bytes; limited transit delay: the resultant bridged area network should not exhibit too large an end-to-end transit delay. This upper limit is set by IEEE at 5^seconds. Potential disadvantages ======================= The main problems that can be encountered in a bridged area network fit roughly into the following categories: increased delay: the bridges impose a store-and-forward delay; congestion: congestion in the bridge can lead to several symptoms such as lost frames, irregular traffic, etc.; increased security risks: remote access allows unauthorised use of connected segments; degradation of some service characteristics: the overall service relies on the individual services available along the traversed path. This can lead, for example, to reductions in the maximum frame length compared with that available on each of the end segments; loss of local features: one typical such example is for the ISO^8802/5 token ring, where the MAC-level protocol provides the transmitting station with the information as to whether the receiving station on the ring is present and whether the data was copied ("R" and" "C" bits). These features can obviously not be given end-to-end significance across a bridged area network; management complexity: analysis of performance and diagnosis of end-to-end problems become even more complicated than in a single network. Implications of bridges ======================= In order to be able to provide the services needed by the bridge itself, the LAN interface must have a certain minimal functionality. For reception, it must be capable of receiving all frames travelling on the LAN ("promiscuous" mode), in order to allow the software to select, from the total traffic, the frames to be forwarded. For emission, it must be able to insert any acceptable value for both the source and destination addresses, in order to allow the original frame to be recreated identically on the destination segment. It is also desirable for the emitter to be able to specify the value of the checksum to be included with the frame. ------------------------ I hope that this contributes to the discussion. An effort has been made to match the terminology to that emerging from the standards organisations, insofar as it is relevant to the actual, practical application.