Transcript Socket Programming - Tel Aviv University

Introduction to Computer Networks
Spanning Tree
1
Forming a Spanning Tree
 Bridges transmit special messages (called
configuration message) to each other.
 A bridge will be elected as the root bridge.
 Every bridge calculates the distance of the
shortest path from itself to the root bridge.
 For each LAN, select a designated bridge among
the bridges residing on the LAN.
 For each bridge, choose a port (root port) that
lead to the root bridge.
 Ports to be included in a spanning tree are the
root ports and the ports on which self has been
elected as designated bridge.
2
Forming a Spanning Tree
 A configuration message is transmitted by a
bridge onto a port.
 Received by all the other bridges on the LAN attached
to the port.
 It is not forwarded outside the LAN.
 Contents:
 Root ID: ID of the bridge assumed to be the root.
 Bridge ID: ID of the bridge transmitting this
configuration message.
 Cost: Cost of the shortest path from the transmitting
bridge to the root bridge.
 Port ID: ID of the port from which the configuration
message is transmitted.
3
Forming a Spanning Tree - Rules
 Comparing 2 configuration messages: C1 & C2
 C1 is better than C2 if the root ID in C1 is lower
than that in C2.
 If the root IDs are equal
 C1 is better than C2 if the cost in C1 is lower
than that in C2.
 If the root ID and cost are equal
 C1 is better than C2 if its transmitting ID is
lower than that in C2.
 If the root ID, cost, and transmitting IDs are equal
 C1 is better than C2 if its port ID is lower than
that in C2.
4
Forming a Spanning Tree - Example
Port 1
Port 5
B91
Port 2
Port 4
Port 3
81
0
81
41
1
41
19
12
315 3
125 3
41
41
12
13
90
1
111 2
 Best known root – 41 (Root ID)
 Cost – 12+1 = 13 (Lowest cost)
 Root Port - 4 (Transmitting ID)
 Designated Bridge on Ports 1 (root bridge) & 2 (cost)
 Blocked Ports 3 & 5 (already connected to root)
5
Spanning Tree - Example
Port 1
B5
Port 2
Port 6
Port 4
Port 5
Port 3
1
2
3
4
5
6
B1,
B1,
B1,
B1,
B1,
B1,
11,
B7,Bridge
2
Root
B1
12,
B2, 1
Cost
12
12,
B5,Port
5
Root
1
11,
B17, 5 on Ports 3
Designated
12,
B5, 3 Ports
Blocked
2,4,5,6
12, B4, 3
6
Network Analysis
D
1D
2D
2
3D
R
D
2
2
B3
R
2
R
1
B32
B48
3
D
1D
B15
B11
R
1
3
D1
R
2
1
B76
3
D
R
D
B12
1
B14
2
4
D
2
D
1
D
3
7
Cache & Topology Changes
 Bridges learn and cache the location of hosts.
 A host may move / disappear
 Important for a bridge to “forget” host locations
 Unless frequently reassured that information is correct.
 Done by timing out entries not been recently verified.
 Timeout
 Too long - traffic may not be delivered to the host at
the new place.
 Too short - wastes a lot of network bandwidth
 Solution
 A long value (e.g., 15 seconds) - used in the usual
case, to reduce wasted network bandwidth.
 A shorter value (e.g., forward delay) - used following
a reconfiguration of the spanning tree algorithm
8
Cache & Topology Changes
 A bridge that detects a topology change will send a
message to its parent.
 This message will in turn be forwarded to the
root bridge.
 The root bridge set the topology change flag bit in
its configuration messages
 Sent (every hello time) downstream the spanning
tree.
 For a period that is forward delay + max age.
 The bridges that receive this type of messages use
the shorter timeout value for their caches
 Until the flag is no longer set.
9
Network Analysis – Topology Change
D
1D
2D
2
3D
R
D
2
2
B3
R
2
R
1
B32
B48
3
D
1D
B15
B11
R
1
3
D1
D
R2
1
B76
3
D
R
D
1
R
B14
D2
4
D
2
B12
D
1
D
3
10