Outline: Contention-based
Access
18-345: Introduction to
Telecommunication Networks
Lectures 7: Datalink layer
Switching
•
•
•
•
Peter Steenkiste
Aloha
Ethernet MAC
Collisions
Ethernet Frames
Spring 2015
www.cs.cmu.edu/~prs/nets-ece
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Ethernet
Ethernet MAC Features
• First practical local area network, built at Xerox
PARC in 70’s
• Dominant LAN technology:
• Carrier Sense: listen before you talk
– Avoid collision with active transmission
• Collision Detection during transmission
– Cheap
– Kept up with speed race: 10, 100, 1000, … Mbps
– Listen while transmitting
– If you notice interference  assume collision
– Abort transmission immediately – saves time
• Why didn’t ALOHA have this?
Metcalfe’s Ethernet
sketch
– Signal strength is reduced by distance for radio
• May not hear remote transmitter – hidden terminal
– Very difficult for radios to listen and transmit
– More on this later in the course
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Ethernet CSMA/CD:
Making it work
Ethernet MAC – CSMA/CD
Carrier Sense Multiple Access/Collision Detection
Jam Signal: make sure all other transmitters
are aware of collision; 48 bits;
Exponential Backoff:
• If deterministic delay after collision, collision
will occur again in lockstep
• Why not random delay with fixed mean?
Packet?
No
Sense
Carrier
Detect
Collision
Send
– Few senders  needless waiting
– Too many senders  too many collisions
Yes
Discard
Packet
attempts < 16
Jam channel
b=CalcBackoff();
wait(b);
attempts++;
• Goal: adapt retransmission attempts to
estimated current load
attempts == 16
– heavy load: random wait will be longer
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Outline: Contention-based
Access
Ethernet Backoff Calculation
• Delay is set as K slots – control K
• Exponentially increasing random delay
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– Infer senders from # of collisions
– More senders  increase wait time
• First collision: choose K from {0,1}; delay is K
x 512 bit transmission times
• After second collision: choose K from
{0,1,2,3}…
• After ten or more collisions, choose K from
{0,1,2,3,4,…,1023}
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Aloha
Ethernet MAC
Collisions
Ethernet Frames
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2
Collisions
A
B
Minimum Packet Size
C
Time
• Packets must be long
enough to guarantee
all nodes observe
collision
• Depends on packet
size and length of wire
– Propagation delay
• Min packet length > 2x
max prop delay
Space
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Delay & Collision Detection
Scaling Ethernet
• Speed in cable ~= 60% * c ~= 1.8 x 10^8 m/s
• 10Mb Ethernet, 2.5km cable
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–
–
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• What about scaling? 10Mbps, 100Mbps,
1Gbps, ...
~= 12.5us delay
+Introduced repeaters (max 5 segments)
Worst case – 51.2us round trip time!
Corresponds to 512 bits
– Use a combination of reducing network diameter
and increasing minimum minimum packet size
• Reality check: 40 Gbps is 4000 times 10 Mbps
– 10 Mbps: 2.5 km and 64 bytes -> silly
– Solution: switched Ethernet – next lecture
• Also used as slot time = 51.2us for backoff
– After this time, sender is guaranteed sole access to
link
– Specifically, will have heard any signal sent in the
previous slot
• What about a maximum packet size?
– Needed to prevent node from hogging the network
– 1500 bytes in Ethernet
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Ethernet Frame Structure
Outline: Contention-based
Access
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•
•
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• Sending adapter encapsulates IP
datagram (or other network layer protocol
packet) in Ethernet frame
Aloha
Ethernet MAC
Collisions
Ethernet Frames
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Ethernet Frame Structure (cont.)
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Addressing Alternatives
• Broadcast  all nodes receive all packets
• Preamble: 8 bytes
– Addressing determines which packets are kept and
which are packets are thrown away
– Packets can be sent to:
– 101010…1011
– Used to synchronize receiver, sender clock rates
• CRC: 4 bytes
– Checked at receiver, if error is detected, the frame is
simply dropped
• Type: 2 bytes
– Demultiplexing: indicates the higher layer protocol,
mostly IP today but historically more protocols (such
as Novell IPX and AppleTalk)
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• Unicast – one destination
• Multicast – group of nodes (e.g. “everyone playing Quake”)
• Broadcast – everybody on wire
• Dynamic addresses (e.g. Appletalk)
– Pick an address at random
– Broadcast “is anyone using address XX?”
– If yes, repeat
• Static address (e.g. Ethernet)
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Ethernet Address Assignment
Why Did Ethernet Win?
• Failure modes
• Each adapter is given a globally unique 6byte address at manufacturing time
– Token rings – network unusable (or expensive)
• Good performance in common case
– Address space is allocated to manufacturers
– Deals well with bursty traffic
– Usually used at low load
• 24 bits identify manufacturer
• E.g., 0:0:15:*  3com adapter
• Volume  lower cost  higher volume ….
• Adaptable
– Frame is received by all adapters on a LAN and dropped if address
does not match
– To higher bandwidths (vs. FDDI)
– To switching (vs. ATM)
• Special addresses
– Broadcast – FF:FF:FF:FF:FF:FF is “everybody”
– Range of addresses allocated to multicast
• Easy incremental deployment (backwards compatible)
• Cheap cabling, etc
• Adapter maintains list of multicast groups node is
interested in
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And .. It is Easy to Manage
Summary
• CSMA/CD  carrier sense multiple access with
collision detection
• You plug in the host and it basically works
– No configuration at the datalink layer
– Today: may need to deal with security
– Why do we need exponential backoff?
– Why does collision happen?
– Why do we need a minimum packet size?
• Protocol is fully distributed
• Broadcast-based.
• How does this scale with speed?
– In part explains the easy management
– Some of the LAN protocols (e.g. ARP) rely on broadcast
• Ethernet
• Networking would be harder without ARP
– Not having natural broadcast capabilities adds complexity to a
LAN (e.g., ATM)
– What is the purpose of different header fields?
– What do Ethernet addresses look like?
• What are some alternatives to Ethernet design?
• Network managers love it!
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Outline
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•
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Datalink Classification
Error Detection and Correction
Framing
Error and flow control
Medium access control
Contention based access
Bridging and switching
Datalink
Switch-based
Multiple Access
Virtual
Circuits
Packet
Switching
ATM,
MPLS, ..
Bridged
LANs
In ~2 weeks
Scheduled
Access
Random
Access
Token ring,
Ethernet,
FDDI, 802.11 802.11, Aloha
Today
Last lecture
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Outline
Scale
yak yak…
• Bridging and switching:
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Scaling the network
Spanning tree protocol
Why Ethernet?
Something different
• What breaks when we keep adding people
to the same wire?
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Building Larger LANs: Bridges
Scale
• Extend reach of a single shared medium
• Connect two or more “segments” by copying data frames between
them
yak yak…
– Only copy data when needed  key difference from repeaters/hubs
– Reduce collision domain compared with single LAN
– Separate segments can send at once  much greater bandwidth
• What breaks when we keep adding people
to the same wire?
• Only solution: split up the people onto
multiple wires
• Challenge: learning which packets to copy across links
– But how can they talk to each other?
LAN 1
LAN 2
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Frame Forwarding
Transparent Bridges
Bridge
• Design goals:
2
– Self-configuring without hardware or software
changes
– Bridge do not impact the operation of the
individual LANs
MAC
Address
• Three parts to making bridges transparent:
A21032C9A591
1) Forwarding frames
2) Learning addresses/host locations
3) Spanning tree algorithm
99A323C90842
8711C98900AA
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695519001190
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• A machine with MAC Address lies in the
direction of number port of the bridge
Port
Age
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2
2
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2
3
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01
15
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• For every packet, the bridge “looks up” the
entry for the packets destination MAC
address and forwards the packet on that
port.
– Other packets are broadcast – why?
• Timer is used to flush old entries
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Learning Bridges
Spanning Tree Bridges
• Manually filling in bridge tables?
• More complex topologies can provide
redundancy.
– Time consuming, error-prone
• Keep track of source address of packets arriving on every
link, showing what segment hosts are on
– Fill in the forwarding table based on this information
host
host
host
host
host
host
– But can also create loops.
• What is the problem with loops?
• Solution: spanning tree
host
Bridge
host
host
host
host
host
host
Bridge
host
host
host
host
host
host
host
host
Bridge
host
host
host
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Spanning Tree Algorithm
Steps
Spanning Tree Protocol
Overview
Embed a tree that provides a single unique path
to each destination:
1) Elect a single bridge as a root bridge
2) Each bridge calculates the distance of the
shortest path to the root bridge
3) Each LAN identifies a designated bridge,
the bridge closest to the root. It will forward
packets to the root.
4) Each bridge determines a root port, which
will be used to send packets to the root
5) Identify the ports that form the spanning
tree
• Root of the spanning tree is
the bridge with the lowest
identifier.
2 B3
B5 1
– All ports are part of tree
• Each bridge finds shortest path
to the root.
1 B7
1 B2
– Remembers port that is on the
shortest path
– Used to forward packets
B1
• Select for each LAN the
designated bridge that has the
shortest path to the root.
– Identifier as tie-breaker
– Responsible for that LAN
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B6 1
1 B4
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Spanning Tree Algorithm
(part 2)
Spanning Tree Algorithm
• Each node sends configuration
message to all neighbors.
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Identifier of the sender
Id of the presumed root
Distance to the presumed root
E.g. B5 sends (B5, B5, 0)
• When B receive a message, it
decide whether the solution is
better than their local solution.
• Each bridge B can now select
which of its ports make up the
spanning tree:
B3
– B’s root port
– All ports for which B is the
designated bridge on the LAN
B5
B7
B2
B3
B5
B7
B2
• Bridges can not configure their
ports.
– A root with a lower identifier?
– Same root but lower distance?
– Same root, distance but sender
has lower identifier?
– Forwarding state or blocked
state, depending on whether the
port is part of the spanning tree
B1
• After convergence, each bridge
knows the root, distance to root,
root port, and designated bridge for
each LAN.
B6
• Root periodically sends
configuration messages and
bridges forward them over LANs
they are responsible for.
B4
B1
B6
B4
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Spanning Tree Algorithm
Example
•
•
• Bridges make it possible to increase LAN
capacity.
B3
Sends (B2, B2, 0)
Receives (B1, B1, 0) from B1
Sends (B2, B1, 1) “up”
Continues the forwarding forever
Node B1:
– Will send notifications forever
•
Ethernet Switches
Node B2:
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– Packets are no longer broadcasted - they are only
forwarded on selected links
– Adds a switching flavor to the broadcast LAN
B5
B7
B2
• Ethernet switch is a special case of a bridge:
each bridge port is connected to single host.
Node B7:
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Sends (B7, B7, 0)
Receives (B1, B1, 0) from B1
Sends (B7, B1, 1) “up” and “right”
Receives (B5, B5, 0) - ignored
Receives (B5, B1, 1) - better
Continues forwarding the B1
messages forever to the “right”
– Can make the link full duplex (really simple protocol!)
– Simplifies the protocol and hardware used (only two stations
on the link) – no longer full CSMA/CD
– Can have different port speeds on the same switch
B1
B6
• Unlike in a hub, packets can be stored
• An alternative is to use cut through switching
B4
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Ethernet Evolution
Ethernet or 802.3
B/R
A Local Area Network
•
MAC addressing, non-routable
•
BUS or Logical Bus topology
•
Collision Domain, CSMA/CD
•
Bridges and Repeaters for distance/capacity extension
•
1-10Mbps: coax, twisted pair (10BaseT)
Conc..
S7
CSMA - Carrier Sense
Multiple Access
•
Switched solution
•
Little use for collision domains
HUB
•
80% of traffic leaves the LAN
Switch
•
Servers, routers 10 x station speed
•
10/100/1000 Mbps, 10gig coming: Copper, Fiber
•
95% of new LANs are Ethernet
Server
WAN
Capacity?
Reliability?
S9
S1
S5
Current Implementations
Router
Ethernet
WAN
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WAN
LAN
Typical Campus Topology
Early Implementations
S1
S8
S2
S6
S11
S3
S10
S4
S12
CD - Collision Detection
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“Traditional” Topology
Next Lecture
• Hierarchical single tree
• Redundancy for reliability
• Spanning tree (or variant) for loop-freeness
• Discussion of the two papers
– “The design philosophy of the DARPA
Internet Protocols”, Dave Clark, SIGCOMM
88
– “End-to-end arguments in system design”,
Saltzer, Reed, and Clark, ACM
Transactions on Computer Systems,
November 1984
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