Download Chapter 7—packet

Chapter 7—packet-switching
• Chapter 5: data-link layer protocols (some
protocols are nearly same for transport layer)
• Chapter 6: MAC sublayer (for broadcast
networks).
• This chapter: network layer
– Telephony networks (i.e., circuit-switching) for stream
of real time voice
– Packet-switching networks for short messages, burst
information, as well as real-time applications.
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Two views of networks
• External view: what services provided to transport layer
by a network (i.e. network layer)
– Need connection setup or not
– What QoSs are provided
– Services should be independent from underlying networks so that
transport layer can run over any networks as long as the
networks provide the services
• Internal view:
– physical topology,
– datagram message transfer or virtual circuit information transfer,
– addressing and routing, congestion control.
• In this chapter, networks indicate systems to transfer
information from one end to the other. It is reasonable to
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consider networks to be network layer here.
Comparisons of two views by examples
• Broadcast networks and packet-switched networks
• From external view, both networks provides
transfer of information between users, not too
much different
• But from internal view, very different:
– A broadcast network (such as LANs) is small,
addressing is simple, frame transferred in one hop so no
routing is needed
– In a packet-switching network, addressing must
accommodate large-scale networks and routing is
necessary.
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What we will discuss
•
•
•
•
•
•
•
Network services and internal network operations
Physical view of networks,
Datagram and virtual circuit
Routing
Shortest path algorithms
ATM networks
Traffic management and congestion control (if
have time)
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Transfer of a block of info. VS. a sequence of blocks
t1
t0
Network
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Figure 7.1
Services provided by network layer
•
Two basic kinds of services
– One view: Transfer of a block of information VS. a sequence of blocks
– Second view: connectionless VS. connection-oriented.
– IP provides connectionless service VS. ATM provides connection-oriented service
•
Other services such as:
– Best-offer connectionless services
– Low-delay connectionless service
– Connection-oriented reliable stream service
– Connection-oriented transfer of packets with delay and bandwidth guarantees
•
End-to-end argument:
– Functions should be placed as close as possible to the application since the application is in best
position to determine whether a function is being carried out completely and correctly,
– Therefore, network layer should provide minimum functions required to meet application
requirements and performance. Leave more functions to upper layers such as transport layer.
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protocol stack: network provides minimum required services to transport layer
Messages
Messages
Segments
Transport
layer
Transport
layer
Network
service
Network
service
Network
layer
Network
layer
Network
layer
End system Data link
layer
a
Data link
layer
Data link
layer
Data link End system
layer
b
Physical
layer
Physical
layer
Physical
layer
Physical
layer
Network
layer
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Figure 7.2
Discussion of connectionless vs. connection-oriented
Transport layer:
connectionless
Network layer:
connectionless(IP)
connection-oriented
connection-oriented (ATM)
Both connection-oriented and connectionless services in the upper layer can be
implemented over a connectionless network (layer) (such as IP), similarly, can
be implemented over a connection-oriented network (layer) (such as ATM).
In network layer, connectionless service is also called datagram service,
connection-oriented service called virtual-circuit service.
Thinking: differences (or implications) of connection-oriented in transport layer and in
network layer. What the connection between two ends in transport layer means?
Which is preferred?
From end-to-end argument, connectionless network (IP) is preferred because
IP makes intermediate nodes as simple as possible and put the burden on end
systems. Moreover, it makes the network grow easily (more scalable).
Why ATM which is connection-oriented?
Main reason is QoS because connection-oriented network can easily guarantee
bandwidth, delay, etc. Detail motivation behind ATM will be discussed later.
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Most basic functions of a network (layer)
Routing and forwarding.
Priority and scheduling (to provide QoS)
Congestion control (also in transport layer)
Segmentation (to deal with different frame sizes of underlying systems)
Alternatively, network (layer) sends error message to let edge system
to do segmentation.
Addressing
1. to deal with different address formats when
interconnecting networks.
2. Hierarchical address for scalability.
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Packet network topology --Access multiplexer
Access node forwards
packets into backbone
packet networks.
Multiplexer combines multiple bursty flows
into one aggregated flow.
Network access
MUX
.
.
.
Node
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Figure 7.4
Packet network topology --LAN
LANs were introduced for sharing of resources in a organization.
They are basic building components for Wide Area Networks.
(a)
(b)
LAN
LAN 1
Bridge
LAN 2
Now nearly every LAN is connected to packet-switching network (the
Internet) to share information globally and to make the Internet
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extremely large.
Figure 7.5
Campus network
Gateway
Organization
Servers
To internet or
wide area
network
s
s
Backnone network may
be connected by links,
LAN, or ATMs.
Backbone
R
R
R
S
Departmental
Server
R
S
S
R
R
s
s
s
s
s
s
s
s
s
LANsextended LANs subnetworks (by backbone networks) campus network
.
 university network, finally connect to Internet, Servers provide various services
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Figure 7.6
bridges, routers and gateway,… play key-role in networks
Domain: indicate the routers run the same routing protocols.
Interdomain level
Border routers
Autonomous system
or domain
Border routers
Internet service
provider
Autonomous systems: included
domains under a single administrations
LAN level
Intradomain level
•Various networks are connected through ISP.
•Tremendous small office and home (SOHO)
users connect to the Internet through ISP,
using dynamic IPs.
Intradomain and interdomain levels
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Figure 7.7
National service provider A
(a)
National service provider B
NAP
NAP
National service provider C
(b)
NAP
RA
Route
server
RB
LAN
RC
National service providers are connected by NAPs (national access14point)
Figure 7.8
Route server distributes routing information to routers.
Packet-switching topology
• What a big picture of topology for picket-switching
networks!!!
– Hierarchical structure, from LANs  campus, university
and organization networksInternet
– ISPs provide backbone networks to connect tremendous
different networks and enormous home PCs
– Switches (bridges, routers, gateways): key elements
– Various servers provides rich services to users
– Domain, intradomain, interdomain for easy administration
and network management.
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Simplified picture of switched networks
1.
2.
Transmission lines and packet switches
They provide connectivity between any source and destination dynamically
Transmission
link
Packet
switch
Network
3. The resources are allocated when needed, so can be shared among multiple users
4. Information can be transferred in connection-oriented or connectionless manner.
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Figure 7.9
Brief discussion of switches
• Switches are generic term for switching
devices in switched networks
– Circuit-switches in telephony network
– LAN switches in LAN connection, i.e. bridges,
Ethernet switch,…
– Packet switches in pack switching networks,
i.e. routers and gateways.
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Structure of a packet switch/router
Control
Line Card
…
…
N
Line Card
Line Card
1
2
Line Card
3
Line Card
…
3
Line Card
Line Card
…
Line Card
Interconnection
Fabric
1
2
N
Components: input ports, output ports, interconnection fabric, controller
A line card handles several pairs of input/output ports and implements physical layer and data
link layer functions: symbol timing, line coding, framing, physical addressing, error checking,
MAC protocol, data link protocol, and buffering (speed mismatch between lines and fabric).
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Figure 7.10
A PC or workstation can be configured to become a switch
1.Several NICs are inserted into expansion slots,
2. Install routing and other protocols in the computer
NIC Card
3
NIC Card
N
NIC Card
…
…
1
2
NIC Card
CPU
Main Memory
I/O
Bus
Operations:
1. NIC gets frames from networks and de-encapsulate packets
2. Packets are transferred into memory using I/O bus
3. CPU performs required routing and protocol processing
4. Packets are transferred from memory to an appropriate NIC
5. The NIC forms new frame and sends out
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Figure 7.11
Three basic resources and bottlenecks in switches
• Processing, memory and bus bandwidth
– Processing implements protocols, hence processing
capacity places a limit on maximum rate at which
switch can operate
– Memory stores packets, hence the amount of memory
determines the rate at which packets are lost, placing
another limit on switch load, moreover memory
bandwidth also place limit on switch rate
– I/O bus bandwidth places a limit on total rate at which
information can be transferred between ports.
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1
1
2
2
…
…
Input port demultiplexes incoming packet stream;
packets are routed to output port;
output port multiuplexes outgoing packet stream
N
N
Switches/routers play a key role in controlling packet flows, thus
efficiently using network bandwidth and optimizing performance.
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Figure 7.12
Connectionless packet switching—originated from message switching
•message has header with source & destination address
• CRC check bit are used to detect errors
Message
Message
Message
Subscriber
A
Message
Network
nodes
Subscriber
B
•Each switch check error, if yes, ask retransmission, if not find next hop.
•Message enter into a QUEUE to wait for line free to transmit
•Increased utilization of line is at the expense of queuing delay
•Loss of message may occur because of insufficient buffer.
•End-to-end error recovery is needed
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Figure 7.13
Delays in message switching
Source
T
t
Switch 1
t
p
Switch 2
Destination
t
t
Delay
Minimum Delay = 3p + 3T,
( remember it,will use it later)
p: propagation delay
T: message transmission time
Back
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Figure 7.14
Packet switching VS. Message switching
Why packet switching?
1. Long message has more chance to incur error than short packet
Examples: L=1M=106 bits over two hops. Bit error rate: p=10-6
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the probability without error: Pc=(1-p)L=(1-10-6)10 eLp=e-1  1/3.
So on average, 1/(1/3)=3 tries for 1st hop. Similarly, 3 tries
for 2nd hop, Total six tries, 6M.
Suppose divided into 105 bit packets, the no-error probability 0.9.
So each packet needs 1/0.9=1.1 times on average. So entire message
Get transmitted over each hop using 1.1M bits transmission.
So total 2.2M bit transmission over two hops.
2. Long message is not suitable for interactive application because
it causes very long waiting delay on other messages. Moreover
the delay for one message is generally longer than the total delay
of packets the message is divided into.
Example: come soon.
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Operations: 1. Address information is included in header
2. CRC for error recovery
3. switches inspect destination address in header to determine next hop
4. Packets are put in QUEUE to wait for line becoming available
5. Sharing lines among multiple packets, high utilization is at the expense of queue delay
6. Packets travel independently and may along different paths
Packet 1
Packet 1
Packet 2
Packet 2
Packet 2
7. Route may be detoured,thus bypassing failure and congestion
8. Packets may arrive out of order, resequencing may be required
Datagram packet switching
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Figure 7.15
Assume three packets along the same path and transmitted in succession
Source
Switch 1
t
1
2
3
p
t
1
Switch 2
2
3
p+P
t
1
2
3
p+P
t
Destination
3 hops
L hops
3p + 2(T/3) first bit received
Lp + (L-1)P first bit received
3p + 3(T/3) first bit released
Lp + LP first bit released
3p + 5 (T/3) last bit released
Lp + LP + (k-1)P last bit released
where T = k P
Delay in packet switching networks
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Figure 7.16
Delay comparison of message switch vs. packet switch
• Support two switches and 3 packets /message
– p: propagation delay, T: message transmission time, P: packet
transmission time, and T=3P
– Delay in message switching: 3p+3T
see detail
– Delay in packet switching: 3p+T+2T/3
• Support L-1 switches and k packets/message:
– T=kP
– Delay in message switching: Lp+LT =Lp+LkP
– Delay in packet switching (k packets): Lp+LP+(k-1)P
• Results: message switching involves additional delay of (L1)(k-1)P.
• Note, in the above, we neglect queuing and processing at each
switch.
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Routing table in connectionless packet switching
Destination
address
Next hop
0785
3245
1345
2343
1566
3784
2458
7612
Designing routing table is a key issue in packet-switching networks
Which requires: knowledge of network topology and traffic levels.
Moreover, size of table will become very large when network size increase
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Figure 7.16
Example –IP internetworks
• Objective of IP is to provide connectionless
transfer service across heterogeneous
networks
• Read it through textbook.
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Virtual-circuit packet switching
• Establishment of connection between source &
destination prior to the transfer of packets See figure
• Connect request is sent by source and connect
confirm is replied by destination See figure
• Signal exchanges is performed in virtual-circuit setup
See figure
• All packets from a connection follow the same path.
• Admission control could be used to limit load on
networks or specific links
• Routing is easier once connection was setup routing
table
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Virtual-circuit switching
Packet
Packet
Question: why called virtual-circuit?
Because resources, e.g., switches, buffers and lines, are shared
by packets from multiple connections, not dedicated to one
specific connection.
back
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Figure 7.17
t
Connect
request
CC
CR
CC
CR
Connect
confirm
1
2
3
1
2
t
Release
3
t
1
2
3
t
Beginning latency and delay of packets in virtual-circuit packet switching
Back
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Parameters such as buffer, bandwidth, delay requirements
were set in every switch along the path during setup
SW
1
Connect
request
Connect
confirm
SW
2
…
Connect
request
SW
n
Connect
request
Connect
confirm
Signaling message exchanges in virtual-circuit setup
Back
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Figure 7.20
Example of virtual-circuit routing table for an input port
Entry for packets
with identifier 15
Identifier
Output
port
Next
identifier
12
13
44
15
15
23
27
13
16
58
7
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Each connection is identified by VCI (virtual-circuit identifier).
VCI may be different along the path for the same connection: the input VCI in a
switch is different from output VCI for the same connection, called local VCIs.
VCI is in the header of a packet, which is much smaller than IP address, as in IP protocol.
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Table lookup will find the output port # and output VCI based on input VCI, which is faster.
Figure 7.21
Pro and con about virtual-circuit switching
• Pro:
– Header is shorter, resources allocated during
setup, admission and congestion are easier.
– Minimum delay reduces further. See figure
• Con:
– Every router needs to maintain state information
about all connections
– Once there is a failure, all affected connections
must be set up again.
• Example –ATM networks
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Forward a packet as soon as the header is received and table lookup is carries out.
Assumptions: no error check, all lines are available.
Source
t
Switch 1
2
1
3
t
Switch 2
2
1
3
t
1
Destination
2
3
t
Minimum Delay = 3p+T
Back
Cut-through packet switching
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Figure 7.22
Assume three packets along the same path and transmitted in succession
Source
Switch 1
t
1
2
3
p
t
1
Switch 2
2
3
p+P
t
1
2
3
p+P
t
Destination
3 hops
L hops
3p + 2(T/3) first bit received
Lp + (L-1)P first bit received
3p + 3(T/3) first bit released
Lp + LP first bit released
3p + 5 (T/3) last bit released
Lp + LP + (k-1)P last bit released
where T = k P
Delay in packet switching networks
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Figure 7.16
Example –ATM networks
• Connection-oriented networks
• All information must be divided into fixed-length
very small packets called cell (53 bytes). (Why?)
• The setup gives a chance for negotiating parameters
between user requirement and network commitment,
resources are allocated along the path
• The connection is defined by a chain of local
identifiers VCIs.
• Assume low-error rate optical channel, so error
control is done only end-to-end
• Is intended to support a large range of applications
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