Transport layer -- May 20041 Transport layer Computer Networks.
Data link layer -- June 20041 Data link layer Computer Networks.
28
Data link layer -- June 2004 1 Data link layer Computer Networks
-
Upload
annie-lynne -
Category
Documents
-
view
260 -
download
6
Transcript of Data link layer -- June 20041 Data link layer Computer Networks.
Data link layer -- June 2004 1
Data link layer
Computer Networks
Data link layer -- June 2004 2
Overview Design issues
Point-to-point links
Local area Networks
Data Link layer Switching
Multiple LANs?
From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs
Data link layer -- June 2004 3
Multiple LANs? Why different LANs in a single organisation?
o Autonomy of owners
o Distance too great for a single LAN
o Load too high for a single LAN
o Reliability: a single defective node will cripple onle a single LAN
o Security: most LANs offer a promiscuous mode (receive all packets)
Solution at data link layer:
Bridge switch
Data link layer -- June 2004 4
From 802.x to 802.y
How do bridges work?
Data link layer -- June 2004 5
Bridges: from 802.x to 802.y Common problems: different
o data rateso maximum frame lengtho securityo quality of service
o frame formats
type Data Rate Mbps
Max length bytes
Support for encrypt QoS
802.3 10-1000 1518
802.11g -11 2312 x x
Data link layer -- June 2004 6
Overview Design issues
Point-to-point links
Local area Networks
Data Link layer Switching
Multiple LANs?
From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs
Data link layer -- June 2004 7
Transparent bridges Goal: interconnect LANs with a bridge and everything should work
perfectly, instantlyo No software changes
o No hardware changes
o No setting of address switches
o No downloading of routing tables
How? Promiscuous mode receive packet…
o Source LAN = destination LAN discard frame
o Destination LAN unknown flooding (on LAN level)
o Destination LAN known forward frame
Learn about LANs? Backward learning: construct routing table
o Initially empty
o Receive frame from node A via LAN L A reachable via L
o Dynamic topologies: attach timestamp to entries
Data link layer -- June 2004 8
Transparent bridges
A
B
C
BridgeL1 L2
from to data
frame
D
L3
packet action Table
host
entries
LAN
initially empty empty
A B flooding to L2 & L3 A L1
Data link layer -- June 2004 9
Transparent bridges
A
B
C
BridgeL1 L2
D
L3
packet action Table
host
entries
LAN
A L1
C A forward to L1 A
C
L1
L2
Data link layer -- June 2004 10
Transparent bridges
A
B
C
BridgeL1 L2
D
L3
packet action Table
host
entries
LAN
A
C
L1
L2
D B flooding to L1 & L2 A
C
D
L1
L2
L3
Data link layer -- June 2004 11
Spanning Tree bridges Bridges in parallel
o To increase reliability loops in topology
o To increase forwarding capacity
Problemo Frame forwarding/flooding forever
Solution:Spanning tree Bridges
Data link layer -- June 2004 12
Transparent bridges Spanning tree bridges:
o Solution transparent to hosts
o Bridges construct spanning tree
one path from every LAN to every other LAN
o All forwarding follows spanning tree
o Spanning tree construction
• Select root of tree: bridge with lowest serial number (unique by
construction; broadcast is used to inform all bridges)
• Distributed algorithm to add (shortest) paths
Data link layer -- June 2004 13
Transparent bridges Spanning tree bridges
Data link layer -- June 2004 14
Remote bridges Interconnect distant LAN
o Bridge on each LAN
o Point-to-point line between bridges
Routing?o No difference
o Point-to-point line = LAN without hosts
Protocolo Select point-to-point protocol
o Inside frame• Complete MAC frame
• Only LLC frame (without MAC header, trailer)
Data link layer -- June 2004 15
Overview Design issues
Point-to-point links
Local area Networks
Data Link layer Switching
Multiple LANs?
From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs
Data link layer -- June 2004 16
Interconnections <> layers Which device in which layer? Frames, packets and headers
Data link layer -- June 2004 17
Interconnections <> layers Repeater
o Analog devices to interconnect cable segments
o Signal on one segment is amplifies and put on other segment
o Not aware of frames, addresses
Hubo Frames arriving on one line are sent
out on all the other lines
o A single collision domain
o Not aware of frames, addresses
Physical layer
Data link layer -- June 2004 18
Interconnections <> layers
Bridgeo Interconnects LANs
o Each line has its own collision domain
Switcho Interconnects hosts
cut-through switcheso Forwarding starts when
header is received
Difference?
Data link layer
Data link layer -- June 2004 19
Overview Design issues
Point-to-point links
Local area Networks
Data Link layer Switching
Multiple LANs?
From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs
Data link layer -- June 2004 20
Virtual LANs Example of a wiring infrastructure
Logical or physical configuration of LANs?
Data link layer -- June 2004 21
Virtual LANs LANs should to reflect organizational structure
o Security
o Load
o Broadcasting
Solutions?o LAN = hub + rewiring
o Rewiring in software VLAN• Implemented by specially-designed VLAN aware
switches/bridges
Data link layer -- June 2004 22
Virtual LANs Example VLANs: White & Gray
How to forward a (broadcast) packet sent out by A?
Data link layer -- June 2004 23
Virtual LANs How to forward a (broadcast) packet sent out by A?
Forwarded by bridge/switch on all Gray ports
What is the color of an incoming frame?
Data link layer -- June 2004 24
Virtual LANs Coloring incoming frames? VLAN color assigned to
o Port
• Does not work for bridges
o MAC address
o Use layer 3
• Could be useful
• Violates independence of layers
o Color frames IEEE 802.1Q
• Changes in Ethernet header to support VLANs
Data link layer -- June 2004 25
IEEE 802.1Q Bridges/switches fill in the VLAN
color Used on lines interconnecting
bridges/switches
Future ethernet cards will be 802.1Q compliant?
Max length increased from
1518 to 1522 bytes
Data link layer -- June 2004 26
IEEE 802.1Q VLAN fields:
o VLAN protocol ID (>max length)
o Pri: hard real-time <> soft real-time <> time-insensitive
o CFI: Canonical Format indicator: 802.5 frame
Data link layer -- June 2004 27
IEEE 802.1Q Configuration of bridges/switches
o Autoconfiguration• If only 802.1Q frames arrive
• Learning
o Manual• For mixed installations
Data link layer -- June 2004 28
Data link layer
Computer Networks
