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University of Delaware CPEG 419 1
Plan for next month or so
Networking Networking at the link layer (LAN networking) Tanenbaum p 318-329 Networking at the Network layer Intro Stallings pp 530-540 Routing Tanenbaum pp 350-384 Global Internet Tanenbaum pp 431-473 QoS Tanenbaum pp 397 – 418
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Today
Networking at the link layer
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Building Bigger LANs from Smaller LANs
LANs are interconnected with•Repeaters•Hubs•Switches•Bridges
•LANs connect to much larger networks through routers.
•LANs are subdivided using VLAN
increasing intelligence
These should all be transparent.
University of Delaware CPEG 419 4
Interconnection Schemes
Hubs or repeaters: physical-level interconnection. Devices repeat/amplify signal. No buffering/routing capability.
Bridges: link-layer interconnection. Store-and-forward frames to destination LAN. Need to speak protocols of LANs it interconnect.
Routers: network-layer interconnection. Interconnect different types of networks.
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Repeater
These connect two wires and make them seems like a longer wire.
They capture the signal on the input, amplify and transmit on the output.
They perform no local functions.
With repeaters, 10Mbps Ethernet can cover 2500m.
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Hub
Hubs are like multi-port repeaters.
Frames that simultaneously arrive at a hub collide even through they don’t arrive on the same wire.
Hubs do not amplify.
Hubs perform no logical function.
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Switch
If there are many hosts on a single LAN, the network might saturate.A switch can alleviate this problem.
When a frame arrives at the switch, it is placed in a buffer. The frame destination address of the frame is analyzed and the frame is placed on the port that leads to the correct destination. (Store and forward).
Typically, only one host is attached to one switch port. So collisions never occur. However, the switch has an internal LAN that must support collision avoidance.
Very good for security!
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BridgesBridges connect different LANS at the link layer (routers do a similar thing at the network layer).
Bridges are like switches, but with a bit more intelligence.
Interconnect LANs of the same type, or LANs that speak different MAC protocols. So they may have to convert header. But this is limited.
Bridge1 4
5 8
LAN A
LAN B
Extended LAN
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Bridges
Why bridges:•A bridge breaks a large LAN into smaller, more manageable ones. •Extend the size (e.g., a 10Mbps Ethernet can’t go more than 2500m.)•Connect LANS of different types.•If one breaks, the others still function. If one is hacked into, the damage is limited.•Traffic load can be managed with hierarchical networks.
High speed LAN (between buildings)
lower speedLAN
(in a building)
bridges lower speedLAN
(in a building)
lower speedLAN
(in a building)
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Bridge Protocol Architecture
IEEE 802.1D specification for MAC bridges.
PHYMACLLC
Station
LAN LANBridge Station
MAC
PHYPHYMAC
LLC
PHY
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Bridges 4
No additional encapsulation. Operate at the data link layer.
Only examine DLL header information. Do not look at the network layer header.
But they may have to do header conversion if interconnecting different LANs (e.g., 802.3 to 802.4 frame).
May interconnect more than 2 LANs. LANs may be interconnected by more than 1
bridge.
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How bridges work
Bridges accept every frame on the LAN to which it is attached.
It stores the frame, decides where it should go, and then forwards it. This is called store and forward (compare to a hub or repeaters).
The difficult task is to decide if and where the frame be forwarded.
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Flooding
Flooding: The bridge transmits every frame it sees onto every link, but the one it came in on.
•Mostly always works.•Does not need any user intervention and simple to program.•Not efficient, we lose the capacity increase associated with hierarchical networks.
•All broadcast frame must be flooded.
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Routing with Bridges
Bridge decides to relay frame based on destination MAC address.
If only 2 LANs, decision is simple.If more complex topologies, routing
is needed, i.e., frame may traverse more than 1 bridge.
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Forwarding Tables
The bridge has a table that maps destinations to out-going links.
Bridge1 4
5 8
LAN A
LAN BExtended LAN
•The bridge accepts all packets from LAN A. •The bridge checks if the destination of the frame is on LAN A or B. •If it is on LAN B, the frame is transmitted onto LAN B. •Otherwise, it drops the frame.
Traffic from B to A is handled similarly.
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Routing
Determining where to send frame so that it reaches the destination.
Routing by learning: adaptive or backward learning.
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Routing with Bridges
3 algorithms: Fixed routing. Spanning tree. Source routing.
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Fixed Routing
Fixed route for every source-destination pair of LANs.
Does not automatically respond to changes in load/topology.
Statically configured routing matrix (pre-loaded into bridge).
If alternate routes, pick “shortest” one.Rij: first bridge on the route from i to j.
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Fixed Routing: Example
LAN A
LAN B LAN C
LAN D E F G
1 2 3
4 5 6 7
101
107
102
103104
105 106
Source LAN
101 102 103 107 105 106
A B C D E F G
A
B 101 102 103 104 105 106
102 101 103 107 105 106
101 103 102 104 105 106
107
102
102
104
101
101
102
105
106
103
103
103
107
107
105
105
106
106
Ex: E-> F: 107; 102; 105.
C
D
E
F
G
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Fixed Routing
Each bridge keeps column for each LAN it attaches.
Table “From X” derived from column “x”.
Every entry that has the number of the bridge results in entry.
101 From A From B
Dest Next hop
B BC
E
FG
A AC AD -E -F AG A
D B
Dest Next hop
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Fixed Routing
Simple and minimal processing.Too limited for internets with
dynamically changing topology.
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Dynamic Routing
•Determine routing tables without any user intervention.•Must learn the network (backward learning).•Must adapt to changes in the network (tables expire and are relearned).
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Address Learning 1
Problem: determine where destinations are.
Bridges operate in promiscuous mode, i.e., accept all frames.
Basic idea: look at source address of received frame to learn where that station is (which direction frame came from).
Build routing table so that if frame comes from A on interface N, save [A, N].
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Address Learning 2
When bridges first start, all tables are empty.
So they flood: every frame for unknown destination, is forwarded on all interfaces except the one it came from.
With time, bridges learn where destinations are, and no longer need to flood for known destinations.
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Backward Learning
Bridges look at frame’s (MAC) source address to find which machine is accessible on which LAN.
LAN 1
LAN 2
LAN 3
LAN 4
B1
B2
If B1 sees frame from C on LAN 2, RT entry (C, LAN2).Any frame to C on LAN1 will be forwarded.But, frame to C on LAN2 will not be forwarded.
CA B
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Address Learning 3RT entries have a time-to-live (TTL). RT entries refreshed when frames
from source already in the table arrive.
Periodically, process running on bridge scans RT and purges stale entries, i.e., entries older than TTL.
Forwarding to unknown destinations reverts to flooding.
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Frame Forwarding
Depends on source and destination LANs. If destination LAN (where frame is going to) =
source LAN (where frame is coming from), discard frame.
If destination LAN != source LAN, forward frame.
If destination LAN unknown, flood frame.
Special purpose hardware used to perform RT lookup and update in few microseconds.
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Loops
Alternate routes: loops.Example:
LAN A, bridge 101, LAN B, bridge 104, LAN E, bridge 107, LAN A.
LAN A
LAN B
E
2
4 5
101
103104
1
107
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Loop: Problems
A
B
LAN 1
LAN 2
B1 B2
1. Station A sends frame to B; bridges B1 and B2 don’t know B.2. B1 copies frame onto LAN1; B2 does the same.3. B2 sees B1’s frame to unknown destination and copies it onto LAN 2.4. B1 sees B2’s frame and does the same.5. This can go on forever.
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Loop Resolution
Goal: remove “extra” paths by removing “extra” bridges.
Spanning tree: Given graph G(V,E), there exists a tree
that spans all nodes where there is only one path between any pair of nodes, i.e., NO loops.
LANs are represented by nodes and bridges by edges.
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Spanning Tree Routing
Aka transparent bridges.Bridge routing table is automatically
maintained (set up and updated as topology changes).
3 mechanisms: Address learning. Frame forwarding. Loop resolution.
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Definitions 1
Bridge ID: unique number (e.g., MAC address + integer) assigned to each bridge.
Root: bridge with smallest ID.Cost: associated with each interface;
specifies cost of transmitting frame through that interface.
Root port: interface to minimum-cost path to root.
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Definitions 2
Root path cost: cost of path to root bridge.
Designated bridge: on any LAN, bridge closest to root, i.e., the one with minimum root path cost.
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Spanning Tree Algorithm 1
1. Determine root bridge.2. Determine root port on all bridges.3. Determine designated bridges.
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Spanning Tree Algorithm 2
Initially all bridges assume they are the root and broadcast message with its ID, root path cost.
Eventually, lowest-ID bridge will be known to everyone and will become root.
Root bridge periodically broadcasts it’s the root.
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Spanning Tree Algorithm 3
Directly connected bridges update their cost to root and broadcast message on other LANs they are attached.
This is propagated throughout network.On any (non-directly connected) LAN,
bridge closest to root becomes designated bridge.
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Spanning Tree: Example
B3
LAN 2
LAN 1
LAN 3 LAN 4
LAN 5
B5
B4B1
B2
10
10
10
10
5
5
5
5
1055
B3
LAN 2
LAN 1
LAN 3 LAN 4
LAN 5
B5
B4B1
B2
10
10
10
10
5
5
5
5
1055
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Spanning Tree: ExampleB1
LAN 1 LAN 2
B2
LAN 3 LAN 4
LAN 5
B4
B5B3
. Only designated bridgeson each LAN allowed toforward frames.
. Bridges continue exchanging info to react to topology changes.
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Source Routing 1
Route determined a priori by sender.Route included in the frame header
as sequence of LAN and bridge identifiers.
When bridge receives frame: Forward frame if bridge is on the route. Discard frame otherwise.
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Source Routing 2
Route: sequence of bridges and LANs.
LAN 3
B1
LAN 1
B3
B2 B4
LAN 2
LAN 4X
Z
X->Z: L1,B1,L3,B3,L2.X->Z: L1,B2,L4,B4,L2
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Source Routing 4
No need to maintain routing table. Frame has all needed routing
information.However, stations need to find route
to destination.
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Route Discovery 1
Finding all routes. If destination is unknown, source sends
broadcast route discovery frame. Frame reaches every LAN. When reply comes back, intermediate
bridges record their id. Source gets complete route information.
Problem: frame explosion.
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Route Discovery 2
Alternative: single route request frame forwarded according to spanning tree.
B3X
Z
B1
B4
LAN 1 LAN 3LAN 2
LAN 4
Z XSingle-routebroadcast
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Route Discovery 3
B3X
Z
B1
B4
LAN 1 LAN 3LAN 2
LAN 4B2
L2, B3, L3, B1, L1
L2, B4, L4, B2, L1
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Route Selection
Select minimum-cost route, e.g., minimum-hop route.
If tie, choose the one that arrived first.
Routes are cached with a TTL; when TTL expires, re-discover route.
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Routers
Operate at the network layer, i.e., inspect the network-layer header.
Usually main router functionality implemented in software.
Store-and-forward.Ability to interconnect heterogeneous
networks: address translation, link speed and packet size mismatch.
Recommended