CS544 Computer Networks

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CS544Computer Networks

• Professor Mike Kain
• Class 4 Slides
• Winter 2008-2009

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Tonights Topics

• Review Class 3 major topics
• Protocol analysis paper due at 1159pm
• Submit to KP (tanetworks_at_cs.drexel.edu or through
  BlackBoard)
• Be sure to include work breakdown and references
  as part of submission.
• Protocol design paper due in 2 weeks Feb 11
  (can hand in early)
• Midterm starts Sunday 2/1 (hopefully) -gt Sat 2/7.
  

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Protocol Analysis

• What did the paper show you?
• Possibly many ways to solve a problem (e.g. file
  transfer)
• Lots of tradeoffs during protocol design and
  implementation
• Protocol inherits what the Internet looked like
  at the time and may not adapt as the Internet
  changes
• Trust (node or application)
• How the internet is connected
• Security
• Services / PDU
• Developers had lots of common ideas (looked at
  the same protocols)

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Discussion

• So, what did you get out of the paper?

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Major topics for tonight

• PPP
• Ethernet (IEEE 802.3)
• Wireless (IEEE 802.11)
• LLC (IEEE 802.2)
• Bridges Switches
• VLANs (IEEE 802.1q)
• Review for midterm

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Protocol Design Paper

• Due in two weeks (February 11th)
• Design a stateful protocol to solve a real-world
  problem.
• Define
• Service
• Messages
• DFA
• Security
• Extensibility

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1st submission

• Your groups first draft is due February 11th.
  It should cover all of the requirements
• Well give comments on the design and give ideas
  for submission.
• You will resubmit the design along with the
  implementation at the end of the semester (they
  should be equal).

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Link Layer Major Duties

• Breaking data into "frames (inbound and
  outbound)
• Flow Control across the link
• Acknowledgements
• Error Detection and Retransmission
• Sequencing (lost, duplicate)
• Broadcasting (resolving access to the media)
• Negotiation of service (QoS)
• Shows some kind of link to upper layers.

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Class 3 Major Topics

• Sliding Window Protocol (SWP / ARQ)
• Sequence Numbers (next to receive)
• ACK / NAK
• Timeouts
• Checksum
• Sending Window / Receiving Window
• Pipelining / Piggybacking
• Retransmission Strategy
• Go Back N / Selective Repeat

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Okay, so what can I do with this?

• How the SWP operates is controlled by many
  factors
• Reliability do I retransmit lost frames?
• How fast do I slide my windows?
• Do I ACK every PDU?
• Do I wait for every PDU before sending up to
  higher layer?
• Others? Can show many different QoS through SWP.

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SWP/ARQ example
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Shared Media

• Concepts
• Contention
• Collision
• Persistence
• Carrier Sense
• Collision Detection
• Token

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MAC tree
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Examples of Random

• ALOHA
• Slotted ALOHA
• CSMA
• CSMA/CD
• CSMA/CA

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Examples of Scheduled

• Token
• Reservation
• Polling
• Examples of Static
• FDMA
• TDMA
• CDMA

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Point-to-Point Protocol (PPP)

• RFC 1661, etc.
• Provides framing over serial links
• Shows evolution of protocol
• Frame (RFC 1662)

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PPP DFA (pg 331 Leon-Garcia)
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Problems and opportunities

• Cant have flag occur in data have to add extra
  bytes to make it special
• Makes data packet variable in length depending on
  data
• GFP (General Framing Procedure) goal is to make
  frame constant size so that it can be decoded
  easier (like ATM).
• Seeing much more use because of cellular 3G
  networks reason for GFP due to constrained
  devices / power.

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So, how do these MACs look?

• Ethernet (IEEE 802.3)
• Originally defined for a bus topology
• Defined as a 1-persistent CSMA/CD with Binary
  Exponential Backoff for collision resolution
• BEB maintain a counter of frame collisions, and
  backoff a random number between 0 and 2N-1
  slots for contenting again

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Ethernet (continued)

• So, for 1 collision, back off 0 or 1 slots
  what is a slot?
• Time that it takes signal to get from one end of
  the cable and back.
• For collisions 1-9, (0-gt2n-1), for 10, maxxed at
  (0-gt1023 slot times)
• So, what does this do for persistence?

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Ethernet PDU format
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Ethernet Physical Layers

• 10Base5 (Thick Coax) bus
• 10Base2 (Thin Coax) bus
• 10BaseT (Twisted Pair gt CAT5) star
• 10BaseF point-to-point
• collision domain group of stations to which a
  collision affects.
• How do we go faster?

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Faster Ethernet

• 100BaseT (IEEE 802.3u)
• 1000BaseT (IEEE 802.3z)
• 10Gigabit Ethernet (IEEE 802.3ae) pairs with
  SONET
• How do we go faster?

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Other IEEE protocols

• Token Bus (IEEE 802.4)
• Token Ring (IEEE 802.5)
• Actually a ring made acknowledgements easy!
• FDDI (ANSI) two rings
• A/C bits in trailer

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Wireless (IEEE 802.11)

• Wireless removes wires from the computer, but
  makes it more complex due to
• Mobile can leave and enter range of signal
  easily
• Signals more susceptible to interference and
  dissipation than wired
• Devices are usually running on batteries (power
  limited)
• hidden terminal problem

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Wireless topologies

• Each station connected to an ACCESS POINT
  (AP/WAP)
• APs grouped into DS (Distribution System)
• When each station powers on, it creates an
  association with the closest AP (could be
  cleartext or encrypted)

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802.11 PDU
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Sending in 802.11 (DCF)

• Try to send if medium busy calculate a random
  backoff time.
• When to send?
• SIFS for ACK frames, CTS frames, segmented data
  or AP control frames
• PIFS for priority access
• DIFS for regular data and management
• short, medium, and long

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Sending in 802.11 (DCF) cont.

• Can also use RTS/CTS when they feel there is a
  hidden terminal.
• Stations can also set NAV field to tell others
  how long they are transmitting.

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New technologies

• Broadband (IEEE 802.16)
• WiMax
• HSPA / EVDO
• 3G

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LLC (IEEE 802.2)

• A basic sliding window protocol between SAPs
  (SSAP DSAP) provides a class of service
• COS1 unacked connectionless
• COS2 acked connectionless
• COS3 acked connection-oriented

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Bridges Switches

• Bridge a device at the Data Link layer which
  connects one or two different MAC types.
• Deals with speed, acknowledgements, media
  contention, etc.
• Can learn which stations are on which sides of
  them.
• For example 802.3 vs 802.11

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Switches

• Switches MAC level devices which operate in the
  center of the star to put the MAC frame down the
  right wire.

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VLANs

• Switches allow grouping of Ethernet connections
  into virtual LANs to minimize collision domain
  (if any) mostly speed
• Either by port (in the device) device on only
  one VLAN
• Or Tagged (after SSAP) with VLAN id tag device
  can be on multiple VLANs (IEEE 802.1q)

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Link Layer Authentication

• When the link layer greets, it may ask the other
  side for credentials
• 802.2 (LLC)
• 802.1 (middle protocols, including
  authentication)
• 802.x (MACs)
• CHAP, RADIUS, NAC, 802.1x.
• May include other information such as anti-virus
  signature date, firewall status, etc.
• Shows even more sublayering possible.

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The midterm

• What to expect?
• Master concepts and be able to use examples to
  illustrate concepts
• Major concepts (nothing small).
• 6 questions in essay form
• Nothing that can be googled.
• You have 4 hours from the time that you check out
  the exam in BlackBoard to submit your answers.
• Can submit a separate doc/pdf rather than type
  into Blackboard.

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Review

• Class 1
• Class 2
• Class 3
• Class 4

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Next week

• MIDTERM! Due by Saturday February 7th!
• No class February 4th
• Protocol Design due February 11th.