CS475 – Networks
Lecture 7
Chapter 3 – Internetworking
Assignments
• Reading for Lecture 8: Section 3.2
• Homework 3, Programming Project 2 posted, due Thursday, September 22
Ch. 3 Internetworking
Direct link networks from Chapter 1 must be connected together inputs and outputs. Its job is to forward (switch) a packet
to form a global network. This internetworking is accomplished arriving on an input to the right output.
by switches and routers.
A router interconnects links of different types, dealing with
A switch interconnects links of the same type. It has multiple
__________________.
3.1 Switching and Forwarding
A switch can use one of three methods to determine which
oriented approach, (3) source routing.
output port a packet should be forwarded to: (1) _____________ All three methods assume that each node has a unique identifier
or connectless approach, (2) virtual-circuit or connection-
(an address).
3.1.1 Datagram Forwarding
Dest. Port
A
3
B
0
C
3
D
3
E
2
F
1
G
0
H
0
Forwarding table
for switch 2
Fig 3.2 Example network for datagram forwarding
In datagram forwarding each packet contains the complete
• A switch or link failure can be tolerated if an alternate route
destination address. Each switch contains a ________________
exists.
(routing) table.
For a simple network like the one shown here a network
• A host can send a packet at any time (in contrast to virtual
administrator could construct the forwarding tables manually.
circuit networks).
For more complex networks with dynamically changing
• A host has no way of knowing if the network can deliver the
topologies and multiple paths between nodes, ______________
algorithms are used to construct the table. We will study routing
packet (or if the dest. is up).
algorithms next class.
• Packets are forwarded independently.
The Internet Protocol (IP) is based on datagram forwarding.
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3.1.2 Virtual Circuit Switching
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)
Fig 3.3 An example of a virtual circuit network
Communication between nodes A and B on a VC network
requires that a connection be setup.
In the setup (______________________) 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 The VC tables establish a virtual circuit between hosts A and B.
each switch can construct an entry in the VC table.
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.
• There is at least a ______________ delay for setup before two After the nodes are done communicating the VC should be torn
down to free resources (VC table entries and VC buffers).
nodes can communication.
• While the connection request has to have the full address of B, It is relatively easy to implement different Quality of Service
data frames need only a VCI. (The VCI is changed in the
(________________) guarantees on different VCs.
frame by switch.)
Frame Relay and ATM networks are popular examples of VC
• In the event of a switch or link failure a new connection must
explore ATM in more detail next week (Section 3.3).
be setup.
• Forwarding of the connection request requires a routing
algorithm.
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networks. They are used as backbone networks. We will
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Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode (ATM) was embraced by the
The most significant feature of the ATM cell is its ___________.
telephone industry in the 1980s and early 1990s.
•
Easier to build hardware, write algorithms
ATM is a connection-oriented (virtual circuits), packet-switched
•
Easier to do tasks in parallel, improving scalability
technology. It uses fixed-length 53 byte (5 byte header, 48 byte The payload of 48 bytes was a compromise. Voice data is better
payload) packets called cells.
in smaller units; digital data better in larger units.
Although ATM was originally designed to compete with
Ethernet, it is now used primarily only in _________ networks.
3.1.3 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
_________________.
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
Fig 3.7 Source routing example
(the switch reads the rightmost number)
list.
packet passes through the switch.)
Source routing requires the sending host know enough about the
network topology to build the header (analogous to building
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
forwarding or VC tables).
Note that packet headers must, in general, be of _____________
length with no upper bound on the size of the header.
3.1.4 Bridges and LAN Switches
Switches that forward packets between shared-media LANs (e.g. ports are in different collision domains.
Ethernets) are known as LAN switches or bridges.
Bridges can be used to create an extended LAN. For example a
A bridge (unlike a hub or repeater) _______________ a packet
bridge can be used to connect two Ethernets that are at their
received on one port and retransmits it on all other ports. The
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
_________________ frames need to be forwarded to all ports
forwarding table to only send packets addressed to host A out
(except the port they are received on).
Port 1.
Forwarding tables are constructed automatically. Initially the
Table entries have timeout values to handle moving hosts from
bridge forwards packets to all ports while it inspects the source
one network to another. Frames addressed to hosts not listed in
address in each frame. It builds the table by associating source
the table are forwarded to all other ports.
addresses with the ____________ on which the frame arrives.
Spanning Tree Algorithm
The network at right contains ____________________. Simple
forwarding tables might allow duplicate frames to arrive (along
different paths) or broadcast frames to endlessly cycle.
If we imagine the network as a graph (with bridges as vertices)
then a spanning tree is a _________________ which includes all
vertices but includes no loops. An example graph and a
corresponding spanning tree are shown in Fig. 3.13 (below
right). (The spanning tree shown is one of many possible.)
Fig. 3.10 Extended LAN with loops
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 ___________________ so that if a bridge fails,
Fig. 3.11 (a) cyclic graph (b) spanning tree
a new spanning tree is constructed.
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The algorithm works as follows to allow each bridge to select
•
ports over which it will forward packets:
•
bridge for forwarding frames toward the root. The
Each bridge has a unique ID (B1, B2). The bridge with
designated bridge is the one __________________ to
the smallest ID is the ________________________.
the root (if there is a tie, the lowest ID wins).
The root forwards packets out all ports.
•
On each LAN one bridge is selected as the designated
•
Each bridge is connected to multiple LANs and it
Each bridge computes the shortest path (# of hops) to
participates in election of a designated bridge for each
the root and notes which port is on this path. This port
LAN it is connected to. The bridge forwards frames
is used as the preferred path to the root.
over those ports for which it is the designated bridge.
B5 is the designated bridge for LAN A because it is closer to the Each configuration message contains the following information:
root (B1) than B3. It is also the designated bridge for LAN B
1) The ID for the bridge sending the message
because it has a lower ID than B7.
2) The ID of the bridge thought to be root
Ports not in the spanning tree neither accept or transmit frames.
3) The distance (hops) to the root
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.
In real networks the spanning tree is created by bridges passing
____________________________ messages to each other and
then deciding whether or not they are a root or designated bridge
(refer to the text for details).
Fig. 3.12 Spanning tree with removed ports
Broadcast and Multicast
Bridges forward broadcast frames on each active (selected) port spanning tree are receiving the multicast the segment can be
other than the one on which it is received.
_____________________. Each host in a multicast group must
Multicasts can be handled in the same way, but they can be
periodically send a frame to the multicast address so that the
handled more efficiently. If no hosts on a segment of 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
Packets can be sent (using only MAC addressing) between hosts
spanning tree algorithm scales linearly and broadcast messages
on the the same VLAN. Broadcast messages are restricted to the
are sent across the extended LAN.
same VLAN.
One approach to building larger networks is to use routers to
Frames on VLAN 100 are never forwarded to VLAN 200 (and
connect (extended) LANs. We will discuss routing in Section
vice versa). The packets must be routed between VLANs.
3.3
Newer LAN switches provide support for _________________
LANs (VLAN). VLANs allow a bridged network to be
partitioned into smaller separate LANs.
Ports on LAN switches can be assigned to particular VLANs.
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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