Section 1'1 The Internet

1
Introduction

• Section 1.1 The Internet
• Section 1.2 IP routers
• Section 1.3 Switch fabrics
• Section 1.4 Circuit switching, packet
  switching, and quasi-circuit switching
• Section 1.5 Optical packet switches

2
The Internet

• Layered structure every layer of protocol has
  its specific objective and its specific message
  format.
• Hyper Text Transfer Protocol (HTTP)
• Transmission Control Protocol (TCP)
• Internet Protocol (IP)
• Ethernet

3
An illustrating example
4
Internet protocol (IP)

• The main objective of IP is to route IP packets
  to the right place.
• Theoretically, every computer that connects to
  the Internet is assigned a unique IP address.
• In every IP packet, both the IP addresses of the
  local host and the server are added so that IP
  routers can use them to route packets.
• In addition to the IP addresses, the information
  of its upper layer protocol is also included in
  every IP packet (in this case, it is TCP). By so
  doing, an IP packet can be forwarded to the
  correct upper layer protocol at the right sever.

5
Some mysteries

• How does the local host find out the IP address
  of the server via its web address?
• How does an IP router route packets by the IP
  address?
• Even when an IP router finds out the IP address
  of the next hop router, how does an IP router
  find out the physical address of the next hop
  router, e.g., the Ethernet address?

6
Domain Name Server (DNS)

• The network of DNS is a hierarchical network that
  is built upon the Internet.
• When the local host is not aware of the IP
  address of a web address, it will send a request
  to its local DNS server.
• If its DNS server cannot resolve the problem, the
  DNS server will send a request to its upper layer
  DNS server and resolve the problem in an
  iterative and recursive manner.

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Address Resolution Protocol (ARP)

• When an IP router does not have the physical
  address of the next hop router, it broadcasts a
  request for the physical address of the next hop
  router through its local area network, e.g., the
  Ethernet.
• The request contains both the physical address
  and the IP address of the router that sends out
  the request, and the IP address of the next hop
  router.
• By examining the IP address, the next hop router
  realizes that the request is addressed to it and
  it needs to send a reply.

8
IP routers

• The main objective of an IP router is to route IP
  packets according to the IP address.
• An IP router usually contains several input ports
  and output ports.
• To route a packet from an input port to an output
  port, an IP router must contain a table that maps
  every IP address to the output port that links to
  the next hop router.

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Routing

• Routing table a table that maps every IP address
  to the output port that links to the next hop
  router.
• Routing protocol the protocol that creates a
  routing table.
• Forwarding table a table converted from a
  routing table for the ease of mapping an IP
  address to the output port of its next hop
  router.

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Forwarding

• The operation that forwards an IP packet from an
  input port to an output port.
• Two steps
• Table lookup it finds the appropriate output
  port.
• Message copying it copies packets from input
  ports to output ports.

11
An IP router
12
Routing software

• The routing software implements the routing
  protocol that creates the routing table and the
  associated forwarding table.

13
Line cards

• A line card is associated with an input port and
  an output port.
• Every line card contains a forwarding table for
  finding out the appropriate output port of an
  arriving packet.
• The IP packet is converted (and sometimes
  segmented) into the packet format used in the
  switch fabric.
• The packet is then sent through the switch fabric
  to the appropriate output port.
• When a packet is received from the switch fabric,
  the packet is buffered (and sometimes
  reassembled) and then sent out from the output
  port in the line card.

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Switch fabric

• The switch fabric performs message copying.
• It copies packets from input ports to output
  ports.
• Sometimes, multiple switch fabrics are installed
  in an IP router for the sake of reliability and
  speedup.

15
Bottleneck of an IP router

• The bottleneck of an IP router is mostly in the
  switch fabric.
• Suppose that an IP router has N line cards and
  each line card is running at rate R bits/sec
  (line rate).
• The rate (or speed) of an IP router is generally
  referred to the aggregated rate of all the line
  cards, i.e., N?R bits/sec.

16
Throughput of an IP router

• Not all the packets can be routed to the output
  ports successfully when each input port is
  running at the full rate R.
• Throughput of an IP router is defined to be the
  percentage of packets that are successfully
  delivered to output ports for a certain traffic
  coming to inputs ports with rate R.
• Not all commercially available IP routers yield
  100 throughput for all kinds of traffic.

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Switches and routers

• Switches and routers are basically the same.
• Traditionally, switches are used for circuit
  switching networks, such as telephone networks,
  that provide connection oriented services.
• Routers are used for packet switching networks
  that provide connectionless services.

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Layer x switches

• Recently, switches are used more often than
  routers.
• A layer x switch implies a switch that uses layer
  x protocol for packet routing.
• A layer 2 switch is an Ethernet switch.
• A layer 4 switch is a switch that uses TCP for
  routing.
• A layer 7 switch is a switch that uses web
  contents for routing.

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Classification of switch fabrics

• Chapter 1
• Output-buffered switches shared memory switches,
  or shared bus switches.
• Input-buffered switches
• Chapter 2 Load balanced Birkhoff-von Neumann
  switches

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Shared Memory Switch

• Packets from all input ports are read into a
  common shared memory.
• A central controller then writes all those
  packets to the destined output ports according to
  an address lookup table.

21
Scalability problem of a shared memory switch

• If there are N input ports and N output ports for
  the switch fabric, then within a time slot N
  packets have to be written into the common shared
  memory and read out from the common shared
  memory.
• If packets arriving at every input port is at the
  rate of R bits/sec, then the common shared memory
  needs to be operated at the rate of 2NR bits/sec.

22
Parallel transmissions
23
Coordinating parallel transmission

• An input port (resp. output port) is allowed to
  transmit (resp. receive) a packet in a time slot.
• Matching one may view input ports as men and
  output ports as women.
• Overheads
• Communication overhead information needs to be
  exchanged among input/output ports for finding a
  matching.
• Computation overhead it takes time to compute a
  matching.

24
Solving the coordination problem

• Use a pre-determined schedule of parallel
  transmissions.
• A pre-determined schedule may not fit the traffic
  demand.
• The idea is then to alter the traffic demand so
  that it fits the pre-determined schedule.
• This requires adding another switch fabric to
  form a two-stage switch fabric.

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Load-balanced Birkhoff-von Neumann switch
26
Load-balanced Birkhoff-von Neumann switch

• The first stage performs load balancing that
  distributes packets evenly to the intermediate
  ports.
• The traffic demand coming to the intermediate
  ports is uniform in spite of the traffic demand
  may be non-uniform.
• As the traffic demand coming to the intermediate
  ports is uniform, packets can be evenly
  distributed to output ports via the switch fabric
  at the second stage.

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Circuit switching

• Telephone networks are based on circuit
  switching.
• Resources, including bandwidth and buffers, are
  reserved along a path for the duration of
  communication.
• quality of service (QoS) is easily guaranteed.
• As resources are reserved for dedicated use,
  resources are not used efficiently.

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Packet switching

• The Internet is a packet-switched network.
• There is no resource reservation.
• Resources could be used more efficiently.
• It is much more difficult to provide QoS.

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Packet switching

• Providing QoS in the Internet in general requires
  implementing several mechanisms.
• traffic policing leaky bucket
• packet schedulingPGPS
• admission control
• IETF RSVP/Intserv architecture
• Network calculus

30
Quasi-circuit switching

• Finding compromises between circuit switching and
  packet switching
• Time in a quasi-circuit switched network is
  partitioned into frames.
• Flows entering a quasi-circuit switched network
  are rate controlled so that the number of bits of
  each flow in every frame is always bounded.
• Via appropriate admission control of flows
  entering a quasi-circuit switched network, the
  links in the network never exceed their
  capacities.
• A quasi-circuit switched network is a
  congestion-free network like a circuit-switched
  network.

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Quasi-circuit switching

• In circuit switching, traffic is completely
  isolated.
• Traffic is completely mixed in packet switching.
• Quasi-circuit switching uses frames to isolate
  traffic.
• It can be viewed as circuit switching at the time
  scale of frames.
• Provide quality of services at the frame level.
• Allow packets to be multiplexed within a frame.
• Achieve statistical multiplexing gain within a
  frame.

32
Optical packet switches

• The access speed of electronic memory is much
  slower than the transmission speed of fiber
  optics.
• Optical signals have to be converted into
  electric signals in electronic IP routers.
• O-E-O conversion is very costly.
• Can IP packets be directly switched in the domain
  of light?

33
Label switching

• In the core network, traffic is aggregated into
  flows.
• Label a much shorter address than an IP address
  could be used.
• It requires a much smaller forwarding table than
  the IP forwarding table.

34
Optical burst switching

• Abstracting the label from an optical packet for
  table lookup might still be too difficult to do.
• Labels are sent through a separate electronic
  network before optical packets are transmitted.
• Table lookup is still done in the electronic
  domain.
• It still has time to reserve the resource for the
  incoming optical packet.

35
Optical buffers

• Unlike electronic devices, it is very difficult
  to build memory using pure optical components.
• Store'' light (and the information contained in
  the light) by circuiting the light in a fiber
  delay line.
• Release'' the light when the information is
  retrieved.
• Switched Delay line (SDL) element use optical
  switches and fiber delay lines to build an
  optical memory.

36
SDL elements

• One has to release the light at the right time to
  the right place.
• The decision is made based on the state'' of
  SDL elements.
• Large memory results in a huge number of states.
• Finding the right control becomes a very
  complicated combinatoric problem.

37
SDL elements

• Certain types of memory can be recursively
  constructed by the SDL elements.
• Time slot interchanges
• N-to-1 FIFO multiplexers
• FIFO queues
• Linear compressors
• Non-overtaking delay lines
• Flexible delay lines