Switching and Forwarding

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Switching and Forwarding

• 3.1 Switching and Forwarding
• 3.2 Bridges and LAN Switches
• 3.3 Cell Switching (ATM)
• 3.4 Implementation and Performance

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• Two limitations on the directly connected
  networks
• limit on how many hosts can be attached, examples
• only two hosts can be attached to a
  point-to-point link
• the Ethernet specification allows no more than
  1,024 hosts

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• limit on how large of a geographic area a single
  network can serve, examples
• an Ethernet can span only 2,500 m
• wireless networks are limited by the ranges of
  their radios
• point-to-point links can be quite long

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• Goal
• build networks that can be global in scale
• Problem
• how to enable communication between hosts that
  are not directly connected
• Solution
• computer networks use packet switches to enable
  packets to travel from one host to another, even
  when no direct connection exists between those
  hosts

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• Packet switch
• a device with several inputs and outputs leading
  to and from the hosts that the switch
  interconnects
• Core job of a switch
• take packets that arrive on an input and forward
  (or switch) them to the right output so that they
  will reach their appropriate destination

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• A key problem that a switch must deal with is the
  finite bandwidth of its outputs
• if packets destined for a certain output arrive
  at a switch and their arrival rate exceeds the
  capacity of that output, then we have a problem
  of contention
• the switch queues (buffers) packets until the
  contention subsides, but if it lasts too long,
  the switch will run out of buffer space and be
  forced to discard packets
• when packets are discarded too frequently, the
  switch is said to be congested

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3.1 Switching and Forwarding

• Switch
• a multi-input, multi-output device, which
  transfers packets from an input to one or more
  outputs
• star topology
• switched networks are more scalable (i.e.,
  growing to large numbers of nodes) than
  shared-media networks because of the ability to
  support many hosts at full speed

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A switch provides a star topology
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Scalable Networks

• The figure shows the protocol graph that would
  run on a switch that is connected to two T3 links
  and one STS-1 SONET link

Example protocol graph running on a switch
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• A switch forwards packets from input port to
  output port
• Port selected based on address in packet header
• Advantages
• cover large geographic area (tolerate latency)
• support large numbers of hosts (scalable
  bandwidth)

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Example switch with three input and output ports
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• How does the switch decide on which output port
  to place each packets?
• general answer
• it looks at the header of the packet for an
  identifier that it uses to make the decision
• three common approaches
• datagram (or connectionless) approach
• virtual circuit (or connection-oriented approach)
• source routing

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3.1.1 Datagram Switching

• Sometimes called connectionless model
• Analogy postal system
• No connection setup phase
• no round trip delay waiting for connection setup
• a host can send data as soon as it is ready

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• Each packet is forwarded independently of
  previous packets that might have been sent to the
  same destination
• two successive packets from host A to host B may
  follow completely different paths (perhaps
  because of a change in the forwarding table at
  some switch in the network)

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• A switch or link failure might not have any
  serious effect on communication if it is possible
  to find an alternate route around the failure and
  to update the forwarding table accordingly
• Since every packet must carry the full address of
  the destination, the overhead per packet is
  higher than for the connection-oriented model

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• Source host has no way of knowing if the network
  is capable of delivering a packet or if the
  destination host is even up and running
• Each switch maintains a forwarding (routing) table

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• Example
• the hosts have addresses A, B, C, and so on
• a switch consults a forwarding table (routing
  table) to decide how to forward a packet

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Datagram forwarding an example network
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• The table shows the forwarding information that
  switch 2 needs to forward datagrams

Destination Port
A 3
B 0
C 3
D 3
E 2
F 1
G 0
H 0
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3.1.2 Virtual Circuit Switching

• Sometimes called connection-oriented model
• Analogy phone call
• Explicit connection setup (and tear-down) phase
• it requires that a virtual connection from the
  source host to the destination host is set up
  before any data is sent
• Typically wait full RTT (Round Trip Time) for
  connection setup before sending first data packet

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• If a switch or a link in a connection fails
• the connection is broken and a new one needs to
  be established
• Subsequence packets follow same circuit
• Each switch maintains a Virtual Circuit (VC) table

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• Entry in the VC table on a single switch contains
  
• a virtual circuit identifier (VCI)
• uniquely identifies the connection at this switch
• which will be carried inside the header of the
  packets that belong to this connection

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• an incoming interface
• on which packets for this VC arrive at the switch
• an outgoing interface
• in which packets for this VC leave the switch
• a potentially different VCI that will be used for
  outgoing packets

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• Two classes of approaches to establish connection
  state
• Permanent Virtual Circuit (PVC)
• Switched Virtual Circuit (SVC)

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• Permanent Virtual Circuit (PVC)
• administrator configures the state, in which case
  the virtual circuit is permanent
• administrator can also delete the state, so a
  permanent virtual circuit (PVC) might be thought
  of as a long-lived, or administratively
  configured VC

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• Switched Virtual Circuit (SVC)
• a host may set up and delete a VC by sending
  messages without the involvement of a network
  administrator
• this is referred to as signaling, and the
  resulting virtual circuits are said to be
  switched
• an SVC should more accurately be called a
  signaled VC, since it uses signaling (not
  switching) to distinguish an SVC from a PVC

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• Example
• assume that a network administrator wants to
  manually create a new virtual connection from
  host A to host B
• two-stage process
• connection setup
• data transfer

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(11)
(7)
(5)
(4)
An example of a virtual circuit network
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• The administrator picks a VCI value that is
  currently unused on each link for the connection
• suppose
• VCI 5, the link from host A to switch 1
• VCI 11, the link from switch 1 to switch 2
• VCI 7, the link from switch 2 to switch 3
• VCI 4, the link from switch 3 to host B

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Incoming Interface Incoming VCI Outgoing Interface Outgoing VCI
2 5 1 11
VC table entry at switch 1
Incoming Interface Incoming VCI Outgoing Interface Outgoing VCI
3 11 2 7
VC table entry at switch 2
Incoming Interface Incoming VCI Outgoing Interface Outgoing VCI
0 7 1 4
VC table entry at switch 3
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A packet is sent into a virtual circuit network
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A packet makes its way through a virtual circuit
network
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• Hop-by-hop flow control
• each node is ensured of having the buffers it
  needs to queue the packets that arrive on that
  circuit
• example, an X.25 network-a packet-switched
  network that uses the connection-oriented model

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• X.25 network employs the following three-part
  strategy
• buffers are allocated to each virtual circuit
  when the circuit is initialized
• the sliding window protocol is run between each
  pair of nodes along the virtual circuit, and this
  protocol is augmented with flow control to keep
  the sending node from overrunning the buffers
  allocated at the receiving node

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1. the circuit is rejected by a given node if not
   enough buffers are available at that node when
   the connection request message is processed

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• Examples of virtual circuit technologies
• Asynchronous Transfer Mode (ATM)
• Frame Relay, e.g., Virtual Private Network (VPN)
• Frame Relay operates only at the physical and
  data link layers

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3.1.3 Source Routing

• Neither virtual circuits nor conventional
  datagrams
• All the information about network topology that
  is required to switch a packet across the network
  is provided by the source host

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• Various ways to implement source routing
• method1
• put an ordered list of switch ports in the header
  and to rotate the list so that the next switch in
  the path is always at the front of the list
• for each packet that arrives on an input, the
  switch would read the port number in the header
  and transmit the packet on that output

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Source routing in a switched network (where the
switch reads the rightmost number)
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• method2
• example, rather than rotate the header, each
  switch just strip the first element as it uses it
• method3
• have the header carry a pointer to the current
  next port entry, so that each switch just
  updates the pointer rather than rotating the
  header

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Three ways to handle headers for source routing
(a) rotation, (b) stripping, and (c) pointer.
The labels are read right to left
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3.2 Bridges and LAN Switches

• LANs have physical limitations (e.g., 2500m)
• Bridge (LAN switch)
• connect two or more LANs
• Extended LAN
• a collection of LANs connected by one or more
  bridges
• accept and forward strategy (accept all frames
  transmitted on either of the Ethernets, so it
  could forward them to the other)

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3.2.1 Learning Bridges

• Do not forward when unnecessary
• whenever a frame from host A that is addressed to
  host B arrives on port 1, there is no need for
  the bridge to forward the frame out over port 2

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Illustration of a learning bridge
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Host Port
A 1
B 1
C 1
X 2
Y 2
Z 2

• How does a bridge come to learn on which port the
  various hosts reside?
• each bridge inspects the source address in all
  the frames it receives
• when host A sends a frame to a host on either
  side of the bridge, the bridge receives this
  frame and records the fact that a frame from host
  A was just received on port 1
• in this way, the bridge can build a table just
  like the following table

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Host Port
A 1
B 1
C 1
X 2
Y 2
Z 2
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3.2.2 Spanning Tree Algorithm

• Problem extended LAN has a loop in it
• frames potentially loop through the extended LAN
  forever
• example
• bridges B1, B4, and B6 form a loop

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Extended LAN with loops
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• Solution bridges run a distributed spanning tree
  algorithm
• spanning tree is a subgraph of a graph that
  covers (spans) all the vertices, but contains no
  cycles

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Example of (a) a cyclic graph (b) a
corresponding spanning tree
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• Spanning tree algorithm (developed by Radia
  Perlman)
• each bridge has a unique identifier (e.g., B1,
  B2, B3)
• the algorithm first elects the bridge with the
  smallest ID as the root of the spanning tree
• the root bridge always forwards frames out over
  all of its ports

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• each bridge computes the shortest path to the
  root and notes which of its ports is on this path
• this port is selected as the bridges preferred
  path to the root

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• finally, all the bridges connected to a given LAN
  elect a single designated bridge that will be
  responsible for forwarding frames toward the root
  bridge
• each LANs designated bridge is the one that is
  closest to the root, and if two or more bridges
  are equally close to the root, then the bridges
  identifiers with the smallest ID wins

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Spanning tree with some ports not selected
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• Bridges have to exchange configuration messages
  with each other and then decide whether or not
  they are the root or a designated bridge based on
  these messages
• configuration messages contain
• the ID for the bridge that is sending the message
• the ID for what the sending bridge believes to be
  the root bridge
• the distance, measured in hops, from the sending
  bridge to the root bridge

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• each bridge records current best configuration
  message for each port
• initially, each bridge believes it is the root
• when learn not root, stop generating config
  messages
• in steady state, only root generates
  configuration messages
• when learn not designated bridge, stop forwarding
  config messages
• in steady state, only designated bridges forward
  config messages

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• root continues to periodically send config
  messages
• if any bridge does not receive config message
  after a period of time, it starts generating
  config messages claiming to be the root
• upon receiving a config message over a particular
  port
• the bridge checks to see if that new message is
  better than the current best configuration
  message recorded for that

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• the new configuration message is considered
  better than the currently recorded information
  if
• it identifies a root with a smaller ID or
• it identifies a root with an equal ID but with a
  shorter distance or
• the root ID and distance are equal, but the
  sending bridge has a smaller ID

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• Sequence of events
• assume all the bridges boot at about the same
  time and all the bridges would start off by
  claiming to be the root
• (Y, d, X) denotes a configuration message from
  node X in which it claims to be distance d from
  root node Y

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• Sequence of events on the activity at node B3
• B3 receives (B2, 0, B2)
• since 2 lt 3, B3 accepts B2 as root (B2, 1, B3)
• B3 adds one to the distance advertised by B2 (0)
  and thus sends (B2, 1, B3) toward B5 (B2, 1,
  B3), (B2, 2, B5)
• meanwhile, B2 accepts B1 as root because it has
  the lower ID, and it sends (B1, 1, B2) toward
  B3(B1, 1, B2), (B1, 2, B3)

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1. B5 accepts B1 as root and sends (B1, 1, B5)
   toward B3 (B1, 1, B5), (B1, 2, B3)
2. B3 accepts B1 as root, and it notes that both B2
   and B5 are closer to the root than it is (B1,
   2, B3), (B1, 1, B2), (B1, 1, B5)
3. B3 stops forwarding messages on both its
   interfaces (this leaves B3 with both ports not
   selected)(B1, 1, B2), (B1, 1, B5)

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Spanning tree with some ports not selected
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3.2.3 Broadcast and Multicast

• Since most LANs support both broadcast and
  multicast, then bridges must also support these
  two features
• Broadcast
• each bridge forwards a frame with a destination
  broadcast address out on each active (selected)
  port other than the one on which the frame was
  received
• Multicast
• implemented in exactly the same way, with each
  host deciding itself whether or not to accept she
  message

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3.2.4 Limitations of Bridges

• Do not scale
• Do not accommodate heterogeneity

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Do not Scale

• It is not realistic to connect more than a few
  (tens of) LANs by means of bridges
• the spanning tree algorithm scales linearly,
  i.e., there is no provision for imposing a
  hierarchy on the extended LAN
• bridges forward all broadcast frames and
  broadcast does not scale

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• Virtual LAN (VLAN)
• used to increase the scalability of extended LANs
• allows a single extended LAN to be partitioned
  into several seemingly separate LANs
• each virtual LAN is assigned an identifier
  (sometimes called a color), and packets can only
  travel from one segment to another if both
  segments have the same identifier
• this limits the number of segments in an extended
  LAN that will receive any given broadcast packet

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• Example
• four hosts (W, X, Y, Z) on four different LAN
  segments
• in the absence of VLANs, any broadcast packet
  from any host will reach all the other hosts
• suppose that we define the segments connected to
  hosts W and X as being in one LAN, VLAN 100
• also define the segments that connect to hosts Y
  and Z as being in VLAN 200
• to do his, we need to configure a VLAN ID on each
  port of bridges B1 and B2
• the link between B1 and B2 is considered to be in
  both VLANs

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Two virtual LANs share a common backbone
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• When a packet sent by host X arrives at bridge B2
• the bridge observes that it came in a port that
  was configured as being in VLAN 100
• it inserts a VLAN header between the Ethernet
  header and its payload
• the bridge applies normal rules for forwarding to
  the packet, with the extra restriction that the
  packet may not be sent out an interface that is
  not part of VLAN 100
• thus, even a broadcast packet cant be sent out
  the interface to host Z, which is in VLAN 200

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• An attractive feature of VLANs
• it is possible to change the logical topology
  without moving any wires or changing any
  addresses
• example
• if we want to make the segment that connects to
  host Z be part of VLAN 100, and thus enable X, W
  and Z be on the same virtual LAN, we would just
  need to change one piece of configuration on
  bridge B2

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Do not Accommodate Heterogeneity

• Bridges are fairly limited in the kinds of
  networks they can interconnect
• Bridges make use of the networks frame header and
  so can support only networks that have exactly
  the same format for addresses
• Bridges can be used to connect Ethernets to
  Ethernets, 802.5 (Token Ring) to 802.5, and
  Ethernets to 802.5 rings, since both networks
  support the same 48-bit address format
• Bridges do not readily generalize to other kinds
  of networks, such as ATM