15-441 Communications and Networking

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15-441 Communications and Networking

• Lecture 7
• Gregory Kesden

The switching portion of these slides evolved
from Prof. Steenkistes slides circa 2000.
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Real-World Link Layer Protocols Original
Ethernet

• Wired physical layer
• 1-persistence CSMA/CD
• Manchester encoding
• Binary exponential backoff upon collision
• IEEE 802.3, the adopted standard, has a slightly
  different frame format than the original Ethernet
  but the distinction is not drawn in
  conversation.
• 10Base5, garden hose wiring (now obsolete) up
  to 500m and 100 stations
• 10Base2, think coax -- up to 185m and 30
  stations
• 10Base-T, twisted pair, up to 100m and 1024
  stations

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Real-World Link Layer Protocols Ethernet, cont.

• Binary exponential backoff?
• Upon a collision, wait and try again, up to 16
  times.
• How long to wait
• For the 0-9th collisions, pick a random number
  between
• 0, 2i-1, and skip that number of slots
  (opportunities to send).
• For collisions 10-15, stick with 0, 210-1,
  which is 1023
• Give up after that.

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Real-World Link Layer Protocols Ethernet, cont.

• What exactly is a slot?
• The time to send a minimum frame.
• The standard sets a maximum length of 2500m
• The reference configuration is a 10Mbps LAN with
  4 repeaters
• Given this, the maximum round-trip propagation
  time is approximately 50 microseconds (10-6
  seconds).
• At the specd 10Mbps, this makes for a 500 bit
  minimum frame size. Add a bit of padding for
  safety and round up to a power-of-two and get a
  minimum frame size of 512 bits.
• So the minimum frame time is 51.2 microseconds.
  This is a slot time.
• Obviously, this increases as the speed of the
  underlying network increases, e.g. 6400 bytes for
  a 1Gbps network of up to 2500m in length.

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IEEE 802.15 Bluetooth
S
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M
M
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S
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Piconet
Piconet
Scatternet
Up to 7 active slaves/piconet, range 10m. Up to
255 parked slaves.
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IEEE 802.15 Bluetooth, cont

• RF physical layer
• 2.4GHz Band
• 79 1MhZ bands
• FSK, 1 bit/Hz
• Frequency hopping at 1600 hops/second
• Fairly allocates channels
• Reduces multipath fading
• Limited security benefit
• Shares same bands as 802.1 and can interfere with
  each other.
• Bluetooth hops aster, so it causes more damage to
  802.11 than vice-versa.

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IEEE 802.15 Bluetooth, cont.

• Baseband layer is the lower piece of the data
  link layer.
• Defines slots
• Defines 625 microsecond timeslots
• even for self, odd for slaves.
• 259 bits of 625 are settling time
• 366 are usable
• 126 data access code and header
• 240 bits for data
• If five slots are combined for a single frame,
  2781 of 3125 bits are available, since settling
  and other overhead are needed only once.
• A link is a logical abstraction
• Synchronized Connection Oriented (SCO) for
  real-time data
• Asynchronous Connection-Less (ACL) for
  packet-switched data

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IEEE 802.15 Bluetooth, cont.

• L2CAP is top half of data link
• Breaks packets into frames
• Multplexing and demultiplexing of packet sources
  (higher level senders and receivers)
• Quality of service negotiation for example,
  balancing needs of small packet and large-packet
  senders.
• Application/Profile layer
• Defines 13 types of applications and higher-level
  stacks for them
• Dial-up, fax, cordless telephony, file transfer,
  synchronization, link management, service
  discovery, c

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Building Bigger LANs
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A Bridge-based Network

• Switches are connected by point-point links.
• Packets are forwarded hop-by-hop by the switches
  towards the destination.
• Forwarding is based on the address
• How do nodes exchange packets over a link?
• How does a switch work?
• How do adjacent switches manage the link?

Point-Point link
Switch
PCs at Work
PC at Home
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Bridges

• Multiple LANS may be connected to form one
  logical LAN
• Since bridges are at the link layer, they do not
  examine network headers, c.
• Reasons
• to increase scale,
• control load,
• allow for long distances,
• ease administration,
• security/protection

LAN
LAN
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Bridge Architecture

• Takes in packets in one interface and has to
  forward them to an output interface based on the
  address.
• A big intersection
• Same idea for bridges, switches, routers address
  look up differs
• Control processor manages the switch and executes
  higher level protocols.
• E.g. which way?, c.
• The switch fabric directs the traffic to the
  right output port.
• The input and output ports deal with transmission
  and reception of packets.

Control Processor
Switch Fabric
Input Port
Output Port
Output Port
Input Port
Output Port
Output Port
Input Port
Input Port
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Bridge Fabric Options

• Crossbar switch.
• Requires lots of hardware but good performance
• Multistage interconnection networks an
  alternative
• Bus-based switches.
• Fabric consists one (or more) fast shared buses
• Each input port has a slot time slot on the bus
• Shared memory switch.
• Switch is one large memory
• Input ports write packets to memory and output
  ports read packets from memory
• Does not scale well need very fast memory
• Hybrid solutions.

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I/O Port Functions

• Input port identifies the outgoing port and
  buffers packets if there is contention for the
  switch fabric.
• Output port queues packets and a scheduler
  determines the order in which packets are sent
  over the outgoing link.
• Many buffering options exist.
• Input buffering, output buffering, internal
  buffering
• Typically a combination is used
• Buffer management can limit throughput, e.g. head
  of line blocking

Switch Fabric
Address Lookup
Scheduler
Address Lookup
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A Simple Bus-based Architecture
Input Ports
Bus
Output Ports
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A Crossbar Architecture
Input Ports
Output Ports
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The Knockout Architecture
input buses
concentrators
buffers
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Transparent Bridges

• Backward learning
• Plug and go
• Listen to traffic on all interfaces
• Store each machine that transmits in hash table
  along with interface.
• Periodically purge old entries, just in case a
  machine moves.
• When a frame hits the wire, look it up in the
  hash table and forward it to the correct LAN.
• If it originated on the correct LAN, do not
  forward it.
• If the destination of the frame is not in the
  hashtable, flood all attached LANs.

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Transparent Bridges, cont.
frame1
frame2
LAN
LAN
frame
frame12
frame22
host
Consider what can happen if a redundant bridge is
used for robustness. Both bridges could copy
the frame, creating a duplicate.Then, this
duplicate could be copied twice, and so
onforever
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Bridges with Spanning Trees

• The solution is for the bridges to communicate
  with each other and build spanning trees that
  represent the network.
• One bridge is selected to be the root of the
  tree, perhaps by serial number bullying or
  other broadcast-based approach.
• Then, a minimum spanning tree is constructed from
  each LAN, through necessary bridges, to the root.
  
• This spanning tree is used to determine how to
  forward a frame. Connections that are not in the
  tree cannot be used even if some bridges are
  left out.
• The algorithm continues to run to stay up-to-date
  and detect bridge failures, host moves, c.

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Switched Ethernet

• Ethernet switches are a type of bridge that have
  a many-to-one point-to-point relationship.
• Many low-bandwidth legs share a high-bandwidth
  common bus.
• Typically one leg, has a very high bandwidth
  consider it to be the uplink. Imagine several
  floors or departments, each of which has its own
  leg, and then one leg going out to an intranet or
  the Internet.
• Each port on the switch forms its own collision
  domain. If multiple stations are connected on the
  same port, as through a concentrator or hub,
  collisions can occur among them.
• Among ports, collisions can be mitigated by
  buffering inputs

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The Network Layer
The solution-specific protocol used by the
application program.
Hides host-specific and/or user-specific nuiances
Extends the host-to-host abstraction provided by
the Transport layer to include more specific,
complex features.
Hides the network(s) from the user and provides
a host-to-host(s) abstraction.
You are here
Network
Moves data from one network to another
Packages data for transmission/reception over a
single network.
Transmits and receives via a particular media
over a single network.
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A Network Packet
Network Layer
Packet
Link Layer
Frame

• Much as the physical layer wasnt concerned with
  the framing performed by the link layer,
• the link layer just views the network layers
  headers, c just as it does the users data
• as payload.
• From the perspective of the link layer, the
  network layer packet is just data.

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The Network Layer

• The physical layer is important, because we need
  some way of transmitting and receiving signals
  over the physical media.
• The data link layer is important, because we need
  some way of organizing the communication to allow
  multiple hosts to send and receive messages given
  the ability of the media to carry them.
• The network lay is important because hosts that
  are members of separate but interconnected
  networks may want to communicate.

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Encapsulation
data
data
Packet (data)
Network
Packet (data)
Network
Network
Frame(packet)
Frame(packet)
current
current
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Network vs. Internetwork

• What distinguishes separate, but interconnected
  networks from a single inter-network?
• One trivial (and non-informative) answer If a
  data link protocol is sufficient for the machines
  to communicate, they are part of a single
  network. If a network-layer protocol is required,
  the machines are on separate networks that,
  compose (at least in part) an inter-network.
• A slightly better answer If the machines are all
  connected to the same communications channel,
  they are part of a single network
• An even better answer If the machines are
  connected to a single communications channel, or
  several communications channels that logically
  form one communications channel (as through
  bridging), they are all part of the same network.
• A yet better answer If the answer to the
  question How do I get there? is interesting,
  the machines are not connected via one network
  they are connected via an inter-network.

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The Bottom Line

• The job of the network layer is to answer the
  famous question, How do I get there from here?

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Bridges Know it All

• Earlier today we discussed connecting
  communication channels together using bridges to
  form a single network.
• The important thing to remember about bridges is
  that they are designed to combine to channels to
  make them operate like one.
• But think about what they must do learn the
  location of every host on the network.

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Bridges Too Much To Remember

• On those occasions when they dont know a hosts
  location, there is a very high price the frame
  must be flooded to every machine on the network.
• If there are too many machines, it is too costly
  to know the location of all of them especially
  since the lookup must be very, very fast.
• If the network is large, traffic is likely to be
  very high this makes flooding frames to all
  interfaces very, very costly.
• It would be impossible for one machine to know,
  and rapidly access, the location of every machine
  on the Internet, never mind manage it
  efficiently.

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Routers and Network Addresses

• Routers are the network-level equivalent of
  bridges. They connect networks to allow
  communication between/among them.
• But, unlike bridges, they do not attempt to know
  every machine. Instead, they take advantage of
  hierarchical addressing and only know how to get
  a packet to the right network.
• Once at the right network, the link-layer can
  deliver the frame.

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Internetworks and Hierarchies

• What is needed is some more extensible scheme for
  finding machines
• One way of dealing with scale is to organize
  things into a hierarchy.
• Each level of the hierarchy can handle only one
  portion of the job.

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Consider a mailing address

• Gregory Kesden
• School of Computer Science
• CMU
• Pittsburgh, PA USA
• The letter carrier Brazil isnt likely to know
  where I live! How does
• a letter get from Brazil to me?
• International mail is directed to a sorting
  station in the U.S. This sorting station knows
  how to get the letter to a sorting station in
  Pennsylvania.
• The Pennsylvania station knows how to get the
  mail to Pittsburgh
• The Pittsburgh station knows how to get it to CMU
• Etc, etc, etc.
• By breaking things down into a hierarchy, we have
  to make more decisions, but each decision is more
  manageable.

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Network Level Addressing

• Data link layer addresses are flat not
  hierarchical. As discussed, these dont scale
  well and, as a result, limit the size of an
  individual network.
• Network layer addresses, much like postal
  addresses are hierarchical.
• The first part of the address identifies the
  network. The second part of the address
  identifies the individual host within the
  network.
• The networkhost pair is globally unique, but the
  host id, without the network id, is not
  necessarily unique.
• Sub-networks (sub nets), or hierarchies with an
  individual network, are also possible.

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Internet Protocol (IP) Addresses

• There are three different classes of IP
  addresses
• Class A Those used for large networks (typically
  very large corporations. Exception MIT has a
  class A address. Some other universities had them
  in the past, but voluntarily gave them up,
  because there are very few MIT didnt give
  theirs up.)
• Class B Those used for medium sized networks
• Class C Those used for small networks

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Address Resolution Protocol (ARP)

• ARP is the protocol used to convert from an IP
  address to a MAC address.
• A host that has an IP address, but needs to know
  the MAC address broadcasts a request to all
  machines on the LAN. This broadcast uses the
  LANs broadcast address. Each host on the LAN
  receives this request. The host associated with
  the request IP address replies.
• The host with the matching IP address unicasts
  its MAC address to the sender. The results of the
  ARP request are cached on the requestor.
• Failure to flush the ARP cache after moving
  interface cards around is a common cause of
  self-solving mysteries for novice sys. admins.
• ARP Storms can result when many systems are
  turned on at the same time.
• ARP includes no security requestors believe
  whatever they are told from whomever they are
  told.
• There is a reverse ARP, RARP, that can convert
  a MAC address to a name, but it is complex and
  has largely been replaced by DHCP.

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IP Addresses
1
7
24
0
Network
Host
Class A
2
14
16
10
Network
Host
Class B
3
21
8
110
Network
Host
Class C
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IP Addresses A Few Thoughts

• There are very few addresses for large networks
  and very many for small networks this makes
  sense given the number of parties needing 224
  host LANs versus those needing 28 host LANs.
• There are 232 (about 4 billion) possible
  addresses, but many are lost due to fragmentation
  most groups get blocks of addresses for a whole
  network, but dont use nearly so many machines.
  This fragmentation has generated a shortage of
  network addresses.

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How Does a Host Get an IP Address?

• Ethernet MAC addresses are built in serial
  numbers provided by the manufacturer (in theory).
  
• But this wont work for IP addresses their
  address must be related to the network on which
  they live.
• A networks administrators is assigned a block of
  addresses (a whole network worth) from IANA
  (Internet Assigned Number Authority).
• The administrator then assigns these addresses to
  individual devices.

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Static Address Assignment

• The easiest way to assign IP addresses is a once
  and for all approach the administrator
  statically assigns an IP address to each device.
  With luck, the user will give it back when the
  device dies or is retired.
• This devices uses this address every time.
• Static assignment isnt particularly efficient
  for some types of devices
• Mobile hosts, home machines, c may only require
  network connectivity during part of the day, but
  will require an IP address during this time.
• If an address is permanently assigned to these
  transient devices, it is wasted much of the time.
• Users often dont return the address of dead or
  retired machines this is also wastage.
• This is bad, since IP addresses are in short
  supply.

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Dynamic Address Assignment and DHCP

• In order to avoid the wastage associated with
  permanent, static IP address assignment, many
  networks use dynamic address assignment. One such
  protocol is DHCP (Dynamic Host Configuration
  Protocol)
• Each time a machine reboots, it sends out a
  discovery message via a broadcast to a special IP
  address (255.255.255.255).
• The DHCP server, which hears this message,
  replies with the assigned IP address. The host
  then assumes this address.
• Since hosts may be transient, the address is
  leased to machine for a period of time. Unless
  the machine renews its lease, it will expire and
  the address can be reassigned to another machine.
  
• Obviously, the DHCP server needs to be assigned a
  block of IP addresses. This is done by the system
  administrator.

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More about DHCP

• Broadcast messages to special addresses, such as
  the one used for DHCP are not communicated by
  routers to other networks if they were, only
  one DHCP server could exist in the world.
• As a result, if the same administration is
  responsible for multiple networks (or
  subnetworks), the available addresses would need
  to be partitioned among the DHCP servers. This
  isnt particularly efficient and is an
  administrative burden.
• To avoid this, DHCP relay servers can be used.
• One DHCP server can be configured to handle
  multiple networks (or subnetworks).
• One relay server can be placed within each
  network.
• The relay server listens for the DHCP discovery
  broadcast and relays it to the DHCP server, which
  then responds directly to the requesting host
  with its IP address.