B227 Lecture Overheads - Topic 4: Bridges

Bridges

To connect up multiple LANs

Some of the LANs may be different, so the bridge will have to convert the header of the frames coming from one LAN to another.

Figure 4-35 Tanenbaum textbook p35

Operate on DLL

The bridges do not look at the contents of the packets as given by the network layer of the sending machine. It copies the packets without worrying about what the bits in the payload actually mean..

Reasons for Having Bridges

As oppose to connecting everything up as one LAN

Requirements of different sections of an organisation different
Cover large area, cheaper to have bridges than running copper wires.
Too many machines - load can become very high if put in a single LAN due to the fact that all frames are broadcast to every machine.
Distance too much, round-trip time too high.
Realiability - problem machines and links will only affect one LAN
Security - filters traffic to and from the right places – like the firewalls we will study in higher layers.

Bridges between 802 LANs

Problems to deal with:

Frame formats all different – conversion requires CPU time

Figure 4-36 Tanenbaum textbook p307 – comparison of 802 frame formats

LANs run at different speeds – bridges have to buffer frames from faster LANs to slower LANs

It is possible that bridges will have to start discarding frames it doesn’t have enough memory.

Maximum frame lengths for LANs are different

If a large frame from a LAN needs to send to another LAN which cannot support a frame that large, the bridge CANNOT just break the frame into smaller frames, since 802 specifications doesn’t define how to put frames back together again – so the receiving machines will not be able to reconstruct the original large packet. The bridge will have no choice but to discard, and let the sending station work out (after repeatedly resending the same frame) that the receiver is unreachable.
The only solution is to configure the LANs so that they have maximum frame lengths which are within the limit of other LANs connected to it through bridges. This can be very inefficient.
Read the textbook p308-309 on problems converting from specific 802.x to specific 802.y (eg. how to add priority in 802.3 frames, which doesn’t have it). Figure 4-37 p310 also has a table summarising what bridges do when converting the frames – it recommends panic (!) if the frame is too long.

Transparent Bridges

Bridges which can be hooked up to existing connectors between two 802 LANs and work

Do not need hardware or software reconfiguration on any machine

Do not disrupt the operation of the LANs.

How Transparent Bridges Work

Operates in promiscuous mode

That is, it accepts all frames from all LANs it is connected to. Note that all 802 network interfaces operate in promiscuous mode as well, since they accept all frames in the broadcast network. It’s just that they discard the frames if the destination address is not them.

Keeps a table of where destinations are, so that it knows which LAN to forward frames to.

Uses backward learning to construct table

Looks at the source address for all frames to know which LAN the source machines are in.

Note here that this is the first time we start looking at the service of routing (how to move data through intermediate machines to get from sender to receiver) . Up until now, we have either been discussing sending from one machine to an adjacent machine, or sending out to all machines in a broadcast network. Routing is a key service in the network layer. In 802 specifications, routing is involved in the bridges as well.

After reading a frame:

If frame’s source and destination LAN are the same – discard

If frame’s source and destination LAN are the different – forward to destination LAN

If frame’s destination LAN is unknown (the frame destination address is not in table) – broadcast to every LAN.

Spanning Tree Bridges

When joining multiple LANs using transparent bridges, it is possible we can have loops.

Eg. From figure 4-39 p 312, frame F from LAN1 is received by bridge B1. B1 converts F to F1 forwards to LAN2 and bridge B2 reads it. B2 now convert F1 to F3 and forwards to LAN1 again. B1 then reads F3, converts to F5, and forwards back to LAN2. The cycle can continue forever.

Possible solution: impose a tree structure on the network joined by the bridges.

Each node in a tree is a LAN, each link is a bridge.

In a tree structure, there can be no loops.

All frames will have one and only one way of getting from one LAN to any other.

Source Routing Bridges

In source routing, the path to be used is set by the sending machine.

The path is put into the frame header.

This means that when source routing bridges are added to a LAN, the LAN needs to be reconfigured so that it produces frames with this new field if required.

All bridges inspect this path in the frame to determine where to forward it.

If a source machine doesn’t know the full route, it sends discovery frames to the bridges to determine the route.

Comparison of 802 Bridges

Figure 4-42 p316

Remote Bridges

In the previous bridges, we physically plug the bridges to all the LANs it is connected to.

When the LANs are remote (not in physical proximity), we set up a point-to-point line from the bridge in one LAN to the bridge in another

Eg. using PPP over the telephone line to connect the bridges.

Figure 4-43 p318

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