03 Dll Bridges By Apcoms
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Faisal Amjad CPS 422
CPS 422 Computer Networks DATA LINK LAYER
Bridges are Devices used to inter-connect multiple LANs
BRIDGES
Faisal Amjad CPS 422
Bridges - The Requirement
Faisal Amjad CPS 422
Different departments of an organization may have different types of LANs suited for their own requirements - a need almost always arises to connect them Due to larger distances between buildings it may be cheaper to interconnect using bridges and a wireless medium It is logical to split a very large LAN (as our college network) into a smaller LAN, restricting traffic within a smaller LAN if it is not required outside Bridges are also amplifiers with error correction capability. Can be used to increase the distance.
Faisal Amjad CPS 422
802.3 to 802.5 Bridge
Faisal Amjad CPS 422
Bridges - The Requirement
(Contd)
Bridges partition the LANs and so increase network reliability. If a station continuously transmits garbage (intentionally or otherwise) complete network will be spared The above feature provides security as well, by partitioning the network and not allowing traffic where it is not required
The problems of Bridging
Faisal Amjad CPS 422
Different frame formats – bridging requires reformatting, checksum recalculation and undetected errors induced by a bridge (due to bad buffer bits in a bridge) Different data rates of different LANs, same data rate specification but different on ground (due to different collision characteristics). Values of timers in higher layers specially affect (upper layer’s) fragmented packet transmission. Any one fragment lost due to a delayed frame will result in retransmission of entire packet (even aborting a transmission) Difference in max frame length
1
Faisal Amjad CPS 422
The Source Routing Bridge
Faisal Amjad CPS 422
The sender of a frame is assumed to know whether or not the destination is on the same LAN or not When sending a frame for another LAN, the sender sets the high order bit of source address to 1 and the frame contains the exact path for the frame to follow Path construction is as fol:
Source Routing Bridges
o Each LAN has a unique 12-bit number o Each bridge has a unique 4-bit number o Two bridges may have same numbers but should not be connected between the same two LANs o The path is a sequence of numbers as LAN, bridge, LAN, bridge,.........)
Faisal Amjad CPS 422
G
Source routing bridge is interested only in frames with high order bit of source address set to 1 After this, the bridge scans for the LAN number on which the frame arrived If the LAN number is followed by its own bridge number, the frame will be forwarded on the LAN whose number follows its own bridge number as:
H
LAN 4 Bridges
Stations
A
B1
B
C
LAN 1
B2
D
E
LAN 2
F
LAN number, Bridge number (own), LAN number (frame received from) (to forward frame)
LAN 3
Faisal Amjad CPS 422
Source Routing Bridge-Functioning (Transmission from station A to station H)
•A sets the higher order bit of source address to 1
G
H
•Sets the path as LAN1, B1, LAN2, B2,LAN4 •B1 receives the frame and inspects high order bit of source address
A
LAN 1
B
LAN 4
Bridges
Stations B1
C
D
B2
LAN 2
•Sees that it has received the frame from LAN 1 and finds its own number B1 •Forwards the frame on the LAN whose number follows its own bridge number (in this case LAN2)
E
Faisal Amjad CPS 422
Source Routing Bridge-Functioning
F
LAN 3
Source Routing - Route Discovery
Faisal Amjad CPS 422
When a station wants to send a frame to a station whose path is not known, it FLOODS a Discovery frame. Every bridge forwards this frame on every LAN to which it is connected. The discovery frame ultimately reaches the destination which sends back a reply frame through flooding When the reply comes back, all bridges record their own identity in it, so that a route is constructed. The selection of the best path is done at the source station by running an algorithm designed to do so. Although All the paths are discovered, this technique results in a frame explosion due to flooding. To limit flooding, routes are stored in a cache for a specified duration Caching induces significant administrative intervention
2
Faisal Amjad CPS 422
Transparent Bridges
Faisal Amjad CPS 422
As the name suggests, its primary objective is to ensure complete transparency Transparency means simply a Plug-and-Play operation of the bridge A transparent bridge operates in Promiscuous mode, in which it accepts every frame that it receives and forwards it by reading its source/destination fields Whenever a frame arrives at a bridge, it must decide whether to discard or to forward it If a frame has its destination on a LAN other than the one from which it arrived, it must be forwarded Otherwise it must be discarded
Transparent Bridges
Faisal Amjad CPS 422
Faisal Amjad CPS 422
Transparent Bridges – Route Discovery
Transparent Bridges – Route Discovery
A technique called Backward Learning is used for route discovery Every bridge has a hash table containing all possible destinations and which output line (LAN) to which it belongs Initially there are no entries in the table The bridge resorts to flooding whenever a route is not known (not in the hash table) Whenever a frame arrives whose destination is not known, the frame is flooded and the sender’s address (e.g. A) is noted in the hash table along with the (output port) LAN from which it arrived Next time if a frame destined for station A arrives, it will be forwarded to its known output line
Every entry in the hash table has an associated time value To accommodate topology changes, whenever a frame is received from a particular station, its time entry in the table is also updated If a specific time entry in the table has not been updated for a few minutes, it is purged by the algorithm For the next time flooding will be used
Faisal Amjad CPS 422
Transparent Bridges – Route Discovery The routing procedure for a frame, can be summarized as :
o If source and destination LANs are same, discard it o If source and destination LANs are different, forward it o If source and destination LANs are unknown, use flooding
Spanning Tree Bridges
Faisal Amjad CPS 422
Flooding induces a problem of frame forwarding in infinite loops A spanning tree algorithm calculates a loop free topology Loops can be removed from the topology by constructing a spanning tree of the graph, which has only one route for a given source – destination pair Links in the graph causing loops are ignored and frames not forwarded through them Algorithm is run after the lapse of specified time interval Spanning Tree Bridges implement this technique
3
Assignment # 3
Faisal Amjad CPS 422
Study the issues related to bridging 802 LANs A comprehensive coverage of problems for bridging 802.x to 802.y LANs is given in fig 4-37 Study the comparison of Transparent bridges and Source Routing bridges
4
CPS 422 Computer Networks DATA LINK LAYER
Bridges are Devices used to inter-connect multiple LANs
BRIDGES
Faisal Amjad CPS 422
Bridges - The Requirement
Faisal Amjad CPS 422
Different departments of an organization may have different types of LANs suited for their own requirements - a need almost always arises to connect them Due to larger distances between buildings it may be cheaper to interconnect using bridges and a wireless medium It is logical to split a very large LAN (as our college network) into a smaller LAN, restricting traffic within a smaller LAN if it is not required outside Bridges are also amplifiers with error correction capability. Can be used to increase the distance.
Faisal Amjad CPS 422
802.3 to 802.5 Bridge
Faisal Amjad CPS 422
Bridges - The Requirement
(Contd)
Bridges partition the LANs and so increase network reliability. If a station continuously transmits garbage (intentionally or otherwise) complete network will be spared The above feature provides security as well, by partitioning the network and not allowing traffic where it is not required
The problems of Bridging
Faisal Amjad CPS 422
Different frame formats – bridging requires reformatting, checksum recalculation and undetected errors induced by a bridge (due to bad buffer bits in a bridge) Different data rates of different LANs, same data rate specification but different on ground (due to different collision characteristics). Values of timers in higher layers specially affect (upper layer’s) fragmented packet transmission. Any one fragment lost due to a delayed frame will result in retransmission of entire packet (even aborting a transmission) Difference in max frame length
1
Faisal Amjad CPS 422
The Source Routing Bridge
Faisal Amjad CPS 422
The sender of a frame is assumed to know whether or not the destination is on the same LAN or not When sending a frame for another LAN, the sender sets the high order bit of source address to 1 and the frame contains the exact path for the frame to follow Path construction is as fol:
Source Routing Bridges
o Each LAN has a unique 12-bit number o Each bridge has a unique 4-bit number o Two bridges may have same numbers but should not be connected between the same two LANs o The path is a sequence of numbers as LAN, bridge, LAN, bridge,.........)
Faisal Amjad CPS 422
G
Source routing bridge is interested only in frames with high order bit of source address set to 1 After this, the bridge scans for the LAN number on which the frame arrived If the LAN number is followed by its own bridge number, the frame will be forwarded on the LAN whose number follows its own bridge number as:
H
LAN 4 Bridges
Stations
A
B1
B
C
LAN 1
B2
D
E
LAN 2
F
LAN number, Bridge number (own), LAN number (frame received from) (to forward frame)
LAN 3
Faisal Amjad CPS 422
Source Routing Bridge-Functioning (Transmission from station A to station H)
•A sets the higher order bit of source address to 1
G
H
•Sets the path as LAN1, B1, LAN2, B2,LAN4 •B1 receives the frame and inspects high order bit of source address
A
LAN 1
B
LAN 4
Bridges
Stations B1
C
D
B2
LAN 2
•Sees that it has received the frame from LAN 1 and finds its own number B1 •Forwards the frame on the LAN whose number follows its own bridge number (in this case LAN2)
E
Faisal Amjad CPS 422
Source Routing Bridge-Functioning
F
LAN 3
Source Routing - Route Discovery
Faisal Amjad CPS 422
When a station wants to send a frame to a station whose path is not known, it FLOODS a Discovery frame. Every bridge forwards this frame on every LAN to which it is connected. The discovery frame ultimately reaches the destination which sends back a reply frame through flooding When the reply comes back, all bridges record their own identity in it, so that a route is constructed. The selection of the best path is done at the source station by running an algorithm designed to do so. Although All the paths are discovered, this technique results in a frame explosion due to flooding. To limit flooding, routes are stored in a cache for a specified duration Caching induces significant administrative intervention
2
Faisal Amjad CPS 422
Transparent Bridges
Faisal Amjad CPS 422
As the name suggests, its primary objective is to ensure complete transparency Transparency means simply a Plug-and-Play operation of the bridge A transparent bridge operates in Promiscuous mode, in which it accepts every frame that it receives and forwards it by reading its source/destination fields Whenever a frame arrives at a bridge, it must decide whether to discard or to forward it If a frame has its destination on a LAN other than the one from which it arrived, it must be forwarded Otherwise it must be discarded
Transparent Bridges
Faisal Amjad CPS 422
Faisal Amjad CPS 422
Transparent Bridges – Route Discovery
Transparent Bridges – Route Discovery
A technique called Backward Learning is used for route discovery Every bridge has a hash table containing all possible destinations and which output line (LAN) to which it belongs Initially there are no entries in the table The bridge resorts to flooding whenever a route is not known (not in the hash table) Whenever a frame arrives whose destination is not known, the frame is flooded and the sender’s address (e.g. A) is noted in the hash table along with the (output port) LAN from which it arrived Next time if a frame destined for station A arrives, it will be forwarded to its known output line
Every entry in the hash table has an associated time value To accommodate topology changes, whenever a frame is received from a particular station, its time entry in the table is also updated If a specific time entry in the table has not been updated for a few minutes, it is purged by the algorithm For the next time flooding will be used
Faisal Amjad CPS 422
Transparent Bridges – Route Discovery The routing procedure for a frame, can be summarized as :
o If source and destination LANs are same, discard it o If source and destination LANs are different, forward it o If source and destination LANs are unknown, use flooding
Spanning Tree Bridges
Faisal Amjad CPS 422
Flooding induces a problem of frame forwarding in infinite loops A spanning tree algorithm calculates a loop free topology Loops can be removed from the topology by constructing a spanning tree of the graph, which has only one route for a given source – destination pair Links in the graph causing loops are ignored and frames not forwarded through them Algorithm is run after the lapse of specified time interval Spanning Tree Bridges implement this technique
3
Assignment # 3
Faisal Amjad CPS 422
Study the issues related to bridging 802 LANs A comprehensive coverage of problems for bridging 802.x to 802.y LANs is given in fig 4-37 Study the comparison of Transparent bridges and Source Routing bridges
4
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