CPS 356: Introduction to Computer
Networks
Lecture 7: Switching technologies
Ch 2.8.2, 3.1, 3.4
Xiaowei Yang
xwy@cs.duke.edu
Review: link layer functions
• Encoding
– NRZ, NRZI, Manchester, 4B/5B
• Framing: bit, byte, time-synchronization
• Error detection
– Parity bits, Internet checksum, CRC
• Reliable transmission
– Stop-and-wait
– Sliding window
– Concurrent logical channels
• Media Access Control
– Ethernet, Token Ring, Wifi (today)
Ethernet: Physical properties
Sensing the line; if idle, sends signals
CSMA/CD: Carrier Sense
Multiple access with Collision
Detection
10Base5
Collision Domain
• Any host hears any other host
– A single segment
– Multiple segments connected by repeaters
– Multiple segments connected by a hub
Transmitter Algorithm
Begin: Wait until the line is idle and has data to
send, the adaptor sends it, and listens to
collision
– If no, go back to Begin
– else exponentially backoff
•
•
randomly selects a k between [0,2n-1], waits for k x
51.2 μs to try Begin again
Gives up after n reaches 16
Collision detection
• An adaptor senses the signals on the line and
compares it with its own
– If same, no collision; otherwise, collision
– Sends 32-bit jamming sequence after collision
• In the worst case, a sender needs to send 512 bits
(46+14+4 = 64B) to detection collision
– Why?
• One way delay is d
• A needs to send for 2d duration to detect collision
• 2d = 512 μs. On a 10Mps Ethernet, corresponds to 512 bits
• The IEEE 802.3 Baseband 5-4-3
– Five physical segments between any two nodes
– Four repeaters between the nodes.
– Three of these physical segments can have connected node
• Each segment < 500m  Total < 2500m
• Propagation delay for this maximum-extent
Ethernet network is 25.6us
• 2*d = 512us (tolerating errors)
• Minimum Ethernet packet size is 512 bits
(64B)
– Header 14B, payload 46B, CRC 4B
Today
• Types of switching
– Datagram
– Virtual circuit
– Source routing
• Switching hardware design
Packet switching
• Problem: single link networks have limited scale
• Ethernet < 1024 hosts, 2500 meters
• Wireless limited by radio ranges
• Point-to-point links connect only two nodes
• Packet switches enable packets to travel from one
host to another without being directly connected
• A packet switch is a device with several inputs and
outputs leading to and from the nodes that the switch
interconnects
A star topology
• A switch has a limited number
of inputs and outputs ports
• Switches can be connected to
each other to build larger
networks
• Adding a new host may not
reduce the performance for
other hosts
– Not true for shared media
networks
– Why?
Switching
• A switch connects links
• Each link can be of different
types
• A switch runs an appropriate
link layer protocol
• Switching (or forwarding):
receive incoming packets and
send to different outgoing
links
Switching technologies
• Problem: how does a switch decide on which
output port to place each packet?
• Solution: look at the packet header and makes a
decision
– Connectionless: datagram
– Connection oriented: virtual circuit
– Source routing
Challenges
• Contention
– Input rate exceeds output rate
• Multiple input ports may send to the same output port
– Switches queue packets until contention disappears
• Congestion
– When a switch runs out of buffer, it discards packets.
– Too frequent packet loss is said to be congested
Datagram
• Every packet contains the destination address
– A global unique identifier
– Ethernet has 48-bit addresses
• A switch maintains a forwarding table that
maps a packet to an output port
Switch 2’s forwarding table
A
3
B
0
C
3
D
E
F
G
H
Q: how does a switch compute the table?
Features of datagram switching
• Connectionless: hosts can send anytime. No need to wait for
connection to set up
• Unknown network state: not sure whether a packet can reach
the destination
• Independent forwarding: packets may take different paths
• Robust to failures: a failure of a switch may not disrupt
communications
– Switches can re-compute forwarding tables
Virtual circuit switching
• Connection oriented
– Set up a virtual circuit
– Data transfer
• Connection setup phase
– Set up connection state
– A virtual circuit identifier, an incoming interface,
an outgoing interface, and an outgoing virtual
circuit identifier
Virtual circuit table (switch1)
11
5
Incoming
interface
Incoming
VCI
Outgoing
interface
Outgoing
VCI
2
5
1
11
Virtual circuit switching
• Algorithm:
– If a packet arrives on the matching incoming port with the
matching incoming VCI, it will be sent to the
corresponding outgoing port with the corresponding VCI
• VCIs are link-local
How to setup connection state
• Administrator configured
– Permanent virtual circuit (PVC)
– Admin manually sets up VC tables
– Does not suit large networks
• Signaling
– A host sends messages to dynamically setup or tear
down a VC
VC setup
• A host A sends a setup message to first hop switch, including
the final destination address
– Similar to a datagram packet
• The switch picks an unused VCI to identify the incoming
connection, and fills part of the VC table
– Why not let the host pick it?
• Every switch repeats the process until the packet reaches the
destination B
• The destination B sends an ack to inform its upstream switch
the VCI for the connection
IF
2
VCI
5
OF
1
VCI
Setup B
VCI
11
IF
3
OF
2
VCI
Setup B
IF
0
VCI
7
OF
1
Setup B
Setup B
IF
0
VCI
7
OF
1
VCI
VCI
4
VCI
IF
2
VCI
5
OF
1
VCI
VCI
11
IF
3
OF
2
VCI
ACK,
ACK, 4
IF
0
VCI
7
OF
1
VCI
4
VCI
4
Characteristics of VC switching
• Connection setup wait
• Data packets contain a small VCI, rather than the full
destination addresses
• One switch failure tears down the entire connection
• Connection sets up require routing algorithms
– Setup packet is forwarded using a datagram algorithm
VC allows resource reservation
• Buffers can be allocated during the setup phase
to avoid congestion
• An example (X.25)
– Buffers allocated during connection setup
– Sliding window is run between pairs of nodes
(hop-by-hop flow control)
– Circuit is rejected if no more buffer
Quality of service (QoS)
• Connectionless network is difficult to allocate
resources
– Switches send packets independently
– How to associate one packet with other packets?
• Virtual circuit can be used to provide different QoS
– Allocate a fraction of link bandwidth to each circuit
Switching technologies
• Connectionless: datagram
• Connection oriented: virtual circuit
• Source routing
Source routing
• Source host provides all the information for packets
to travel across the network
– Packets carry output port numbers
– Packets carry switch addresses
– Variable header length
Handling source routing headers
• Rotation
• Stripping
– No return path!
• Pointer
Loose or strict source routing
• Strict
– Must visit every node on the path
• Loose
– Waypoints rather than the complete route
Overview
• Types of switching
– Datagram
– Virtual circuit
– Source routing
• Switching hardware design
– How to build switches?
Software switch
• Packets cross the bus twice
– Half of the memory bus speed
• 133Mhz, 64-bit wide I/O bus  4Gpbs
• Short packets reduce throughput
– 1Mpps, 64 bytes packet
– Throughput = 512 Mbps
– Shared by 10 ports: 51.2Mbps
Hardware switches
• Ports communicate with the outside world
– Eg, maintains VC tables
• Switching fabric is simple and fast
Performance bottlenecks
• Input port
– Line speed: 2.48 Gbps
• 2.48x109/(64x8) = 4.83 Mpps
• Buffering
– Head of line blocking
– May limit throughput to only 59%
– Use output buffers or sophisticated buffer
management algorithms to improve performance
Fabrics
• Shared bus
– The workstation switch
• Shared memory
– Input ports read packets to shared memory
– Output ports read them out to links
Fabrics
• Cross bar
– Each output ports need to accept from all input
ports
Fabrics
• Self routing
– a self-routing header
added by the input
port
– Most scalable
– Often built from 2x2
switching units
An example of self-routing
• 3-bit numbers are self-routing headers
• Multiple 2x2 switching elements
– 0: upper output; 1: lower output
Summary
• Types of switching
– Datagram
• Forwarding table
• Simple, difficult to have QoS
– Virtual circuit
• Datagram for connection setup
• Smaller VC identifiers
– Source routing
• Packets carry forwarding information
• Variable header length
• Switching hardware design
– Shared bus, shared memory, cross-bar, self-routing
• Next lecture
– How is forwarding table computed?
– ATM network