Lecture 7
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Reminder: Homework 2, Programming Project
1 due today.
Homework 3, Programming Project 2 out, due
Thursday next week.
Questions?
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Outline
Chapter 3 - Internetworking
3.1 Switching and Bridging
3.2 Basic Internetworking (IP)
3.3 Routing
3.4 Implementation and Performance
3.5 Summary
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Introduction
Direct link networks from Chapter 2 must be
connected together to form a global network. This
internetworking is accomplished by switches and
routers.
A switch interconnects links of the same type. It
has multiple inputs and outputs. Its job is to
forward (switch) a packet arriving on an input to
the right output.
A router interconnects links of different types,
dealing with heterogeneity.
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Switching and Bridging
A switch can use one of three methods to
determine which output port a packet should be
forwarded to: (1) datagram or connectless
approach, (2) virtual-circuit or connection-oriented
approach, (3) source routing.
All three methods assume that each node has a
unique identifier (an address).
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Datagrams
Dest.
Port
A
3
B
0
C
3
D
3
E
2
F
1
G
0
H
0
Forwarding table
for Switch 2
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Fig 3.2 Example network for datagram forwarding
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Datagrams
Each datagram packet contains the complete
destination address. Each switch contains a
forwarding (routing) table.
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A host can send a packet at any time (in contrast to
virtual circuit networks).
A host has no way of knowing if the network can
deliver the packet (or if the destination is up).
Packets are forwarded independently.
A switch or link failure can be tolerated if an alternate
route exists.
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Datagrams
For a simple network like the one shown here a
network administrator could construct the
forwarding tables manually.
For more complex networks with dynamically
changing topologies and multiple paths between
nodes, routing algorithms are used to construct
the table. We will study routing algorithms next
week (Section 3.3).
The Internet Protocol (IP) is based on datagram
forwarding.
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Virtual Circuit Switching
Fig 3.3 An example of a virtual circuit network
SWITCH
In Int.
In VCI
Out Int. Out VCI
1
2
5
1
11
2
3
11
2
7
3
0
7
1
4
Selected virtual circuit entries from switch tables
(for a VC between hosts A and B)
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Virtual Circuit Switching
Communication between nodes A and B on a VC
network requires that a connection be setup.
In the setup (signalling) process host A sends a
setup message to the network. The message
contains the address of host B. As the message
passes through each switch, the switch assigns a
VC identifier (VCI) for that connection on the
upstream link.
The response from B contains the VCI for the
downsteam link so each switch can construct an
entry in the VC table.
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Virtual Circuit Switching
The VC tables
establish a virtual
circuit between
hosts A and B.
Notice that each
link in a VC will
typically have a
different VCI.
Multiple VCIs are normally assigned to a link.
Each VCI would correspond to a different VC.
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Virtual Circuit Switching
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There is at least a RTT delay for setup before
two nodes can communication.
While the connection request has to have the
full address of B, data frames need only a VCI.
(The VCI is changed in the frame by switch.)
In the event of a switch or link failure a new
connection must be setup.
Forwarding of the connection request requires
a routing algorithm.
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Virtual Circuit Switching
After the nodes are done communicating the VC
should be torn down to free resources (VC table
entries and VC buffers).
It is relatively easy to implement different Quality
of Service (QoS) guarantees on different VCs.
Frame Relay and ATM networks were popular
examples of VC networks. They were used as
backbone networks.
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Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode (ATM) was
embraced by the telephone industry in the 1980s
and early 1990s.
ATM is a connection-oriented (virtual circuits),
packet-switched technology. It uses fixed-length
53 byte (5 byte header, 48 byte payload) packets
called cells.
Although ATM was originally designed to compete
with Ethernet, it is now used primarily only in DSL
networks.
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Asynchronous Transfer Mode (ATM)
The most significant feature of the ATM cell is its
fixed length.
Easier to build hardware, write algorithms
 Easier to do tasks in parallel, improving
scalability
The payload of 48 bytes was a compromise.
Voice data is better in smaller units; digital data
better in larger units.
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Source Routing
Fig 3.7 Source routing example
(the switch reads the rightmost number)
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Source Routing
In source routing the source node provides all the
information about the network that is necessary
for the packet to reach the destination. The
information is embedded in the packet header.
In the example on the previous slide the required
switch output port number is included in the
header. Since the number of switches is usually
unknown, the list of ports numbers is rotated so
that the port of the next switch is always at the
head of the list.
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Source Routing
As alternatives to rotating the port list, the port
number could be stripped as in (b) or the header
could contain a pointer to the next port number as
in (c). (The pointer would be updated as the
packet passes through the switch.)
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Source Routing
Source routing requires the sending host know
enough about the network topology to build the
header (analogous to building forwarding or VC
tables).
Note that packet headers must, in general, be of
variable length with no upper bound on the size of
the header.
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Bridges and LAN Switches
Switches that forward packets between sharedmedia LANs (e.g. Ethernets) are known as LAN
switches or bridges.
A bridge (unlike a hub or repeater) buffers a
packet received on one port and retransmits it on
all other ports. The ports are in different collision
domains.
Bridges can be used to create an extended LAN.
For example a bridge can be used to connect two
Ethernets that are at their max. size and form an
extended Ethernet.
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Learning Bridges
Host Port
A
1
B
1
C
1
X
2
Y
2
Z
2
Forwarding Table
A bridge need not forward all frames. The bridge
could use a forwarding table to only send packets
addressed to host A out Port 1.
Broadcast frames need to be forwarded to all
ports (except the port they are received on).
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Learning Bridges
Forwarding tables are constructed automatically.
Initially the bridge forwards packets to all ports
while it inspects the source address in each
frame. It builds the table by associating source
addresses with the port on which the frame
arrives.
Table entries have timeout values to handle
moving hosts from one network to another.
Frames addressed to hosts not listed in the table
are forwarded to all other ports.
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Spanning Tree Algorithm
The network at
right contains
loops. Simple
forwarding tables
might allow
duplicate frames
to arrive (along
different paths) or
broadcast frames
to endlessly cycle.
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Fig. 3.10 Extended LAN with loops
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Spanning Tree Algorithm
If we imagine the
network as a graph
(with bridges as
vertices) then a
spanning tree is a
Fig. 3.11 (a) cyclic graph (b) spanning tree
subgraph which
includes all vertices but includes no loops. An
example graph and a corresponding spanning
tree are shown above. (The spanning tree shown
is one of many possible.)
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Spanning Tree Algorithm
The spanning tree algorithm is a protocol used by
bridges to agree on a spanning tree for an
extended network.
The algorithm allows each bridge to select ports
over which it will and will not forward frames.
The algorithm is dynamic so that if a bridge fails, a
new spanning tree is constructed.
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Spanning Tree Algorithm
The algorithm works as follows to allow each
bridge to select ports over which it will forward
packets:
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Each bridge has a unique ID (B1, B2). The
bridge with the smallest ID is the root bridge.
The root forwards packets out all ports.
Each bridge computes the shortest path (# of
hops) to the root and notes which port is on this
path. This port is used as the preferred path to
the root.
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Spanning Tree Algorithm
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On each LAN one bridge is selected as the
designated bridge for forwarding frames toward
the root. The designated bridge is the one
closest to the root (if there is a tie, the lowest ID
wins).
Each bridge is connected to multiple LANs and
it participates in election of a designated bridge
for each LAN it is connected to. The bridge
forwards frames over those ports for which it is
the designated bridge.
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Spanning Tree Algorithm
B5 is the designated
bridge for LAN A
because it is closer to
the root (B1) than B3.
It is also the
designated bridge for
LAN B because it has
a lower ID than B7.
Fig. 3.14 Spanning tree with removed ports
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Spanning Tree Algorithm
Ports not in the spanning tree neither accept or
transmit frames. Frames from LAN B addressed
(or broadcast frames) to nodes in LAN K would
follow the B1-B5-B7 path.
Note that bridges B3 and B6 are currently unused.
If bridge B5 were to go down the algorithm is
rerun, and bridges B3 and B7 would become
designated bridges for LANs A and B,
respectively. The network would function
normally.
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Spanning Tree Algorithm
In real networks the spanning tree is created by
bridges passing configuration messages to each
other and then deciding whether or not they are a
root or designated bridge (refer to the text for
details).
Each configuration message contains the
following information:
1) The ID for the bridge sending the message
2) The ID of the bridge thought to be root
3) The distance (hops) to the root
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Broadcast and Multicast
Bridges forward broadcast frames on each active
(selected) port other than the one on which it is
received.
Multicasts can be handled in the same way, but
they can be handled more efficiently. If no hosts
on a segment of the spanning tree are receiving
the multicast the segment can be pruned. Each
host in a multicast group must periodically send a
frame to the multicast address so that the bridge
does not prune the segment containing the host.
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Limitations of Bridges
Bridges should be used only to connect 10 or so
LANs. The spanning tree algorithm scales
linearly and broadcast messages are sent across
the extended LAN.
One approach to building larger networks is to
use routers to connect (extended) LANs. We will
discuss routing in Section 3.3.
Newer LAN switches provide support for virtual
LANs (VLAN). VLANs allow a bridged network to
be partitioned into smaller separate LANs.
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Limitations of Bridges
Ports on LAN switches
can be assigned to
particular VLANs.
Packets can be sent
(using only MAC
addressing) between
hosts on the the same VLAN. Broadcast
messages are restricted to the same VLAN.
Frames on VLAN 100 are never forwarded to
VLAN 200 (and vice versa). The packets must be
routed between VLANs.
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In-class Exercises
Problem 3.3 on page 285
Problem 3.13 on page 288
Turn in at the end of class.
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