Bridges
What if you need to joins LANs together to make one big LAN (e.g. campus
LAN) because you have a greater distance to cover, or you have too many
hosts for one LAN segment? The IEEE LANs may be bridged (somewhat) easily
because they have a common origin.
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each segment may have different physical & MAC
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everything above MAC (LLC and higher) passes through bridge
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don't do data flow, so congestion is possible
What are the problems of joining IEEE 802 LANs together?
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frame format is different
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requires that bridges reconstruct frames and checksums
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introduces possibilities for new errors, takes time
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data rates are different
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length of data frames (Ethernet - 1518 bytes, TB - 8191 bytes, TR - 5000
bytes)
How do bridges cope with problems?
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802 doesn't provide for splitting large frames into chunks
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buffers can ease the speed differential problems
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bridges must discard frames that are too big
The life of a bridge is spent answering the following question:
Should I forward this frame, or should I filter it?
How is forwarding/filtering decision made? Two strategies
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transparent bridging (TB)
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source routing (SR)
Transparent Bridge (TB)
TB widely used in 802.3 Ethernet LANs
Goal - complete transparency
this required: no changes to LANs, software, hosts, network administrators,
etc
intelligence is in the bridge (transparent to the hosts on the LANs)
Three responsibilities of TB bridge
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learning workstation addresses
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monitors the traffic on each LAN port
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maintains a table of hosts on each LAN
Frame forwarding
<picture of 3 LANs and 2 bridges>
some traffic forwarded, some filtered
algorithm to decide on forward/filter is
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if dest and src LANs are same, filter
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if dest and src are different, forward
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if dest is unkown, use forward
The last case is necessary because initially the table is empty and flooding
algorithm must be used.
If a host is quiet for some time, its entry is purged from the bridge -
this lets bridge and LAN adapt automatically, but causes flooding of traffic
for quiet hosts.
Independence of topology
May have redundant bridges for fault tolerance, but must assure no
topological loops, and must do this automatically, since otherwise not
transparent to network admin.
A spanning tree algorithm is used to create a logical tree from physical
topology (only one path of bridges and LANs between any two LANs)
<picture of two bridges, two paths, two LANs>
<must logically remove one of the bridges>
Bridges communicate with each other via special bridge protocol data
units (BDPU) called Hello BDPUs - exchanged every few minutes. Each is
known by a unique serial number.
One bridge becomes the root (lowest serial number, for instance), every
bridge determines the shortest path to the root, thus every LAN has a unique
path to the root, and hence to every other LAN.
Source Routing (SR)
Developed by IBM for token ring networks.
The TB method is simple to use, just plug the bridge in and it works.
However, TB doesn't take full advantage of the bandwidth of the network
since some bridges are removed from the system and only a single path is
available between any two hosts.
Source Routing improves on the use of redundant resources but requires
the hosts on the network to supply routing information (i.e. not transparent
to the hosts).
Each host sets a bit in the address field marking on or off LAN, and
also must supply a path (LAN-bridge sequence) to destination in header
field of the frames. Bridges are less intelligent than TB, since they just
follow routes constructed by the hosts. Each bridge monitors frames and
looks for its LAN number, if it sees that, then it looks for its bridge
number, if it sees that it forwards it to the next LAN on the path. Three
methods of doing this:
1. software - each bridge copies every frame, checks bit 47 of dest
addr, then checking further if needed
2. hybrid - hardware filters frames with bit 47 set, passes to software
3. hardware - all checking is done in hardware, bridge only handles
frames that are forwarded
Each LAN has a unique number (12 bits) each bridge has a unique number
(4 bits) within any given LAN context (work for the net admin).
How does every host know the path to every other host?
By sending a discovery frame out before the data frame
Every bridge transmits discovery frames
On a response to a discovery frame, each bridge that handles the
packets records itself in the path
This finds all routes to a destination, but can cause an explosion
in discovery frames, and hence congestion
The source can pick the best route to a destination
Thereafter, the source caches the best route to a host, avoiding the
discovery frame
Examples of Bridges
Retix 2200 series
dual-port, Ethernet-Ethernet
max frame filtering rate of 29,000 frames per second
max forwarding rate of 13,650 frames per second
(close to Ethernet max)
frames from 64 to 1518 bytes in size
implements the IEEE 802.1 Spanning Tree algorithm
options for connecting WAN lines
Ungermann-Bass
FDDI to Ethernet/TR
allows for high speed FDDI backbone joining LANs
proprietary algorithm to map between SR and TB
Bridges versus Routers
IP networks are usually wired with routers instead of bridges. Each LAN
corresponds to a subnet.
Example uses, tradeoffs, problems