Week 1 Introduction and Data Link Layer

1
Week 1Introduction and Data Link Layer

2
Layers

• OSI reference model
• Each layer communicates with its peer layer
  through the use of a protocol
• The communication between n and n-1 is known as
  an interface

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Transmission
4
Reception
5
Layers

• Physical Layer
• The physical later is concerned with transmitting
  raw bits over a communication channel.
• The design issues have to do with making sure
  that when one side sends a 1 bit, it is received
  by the other side as a 1 bit, not as a 0 bit.
• Typical questions here ar e how many volts should
  be used to represent a 1 and how many for a 0,
  how many microseconds a bit lasts, whether
  transmission may proceed simultaneously in both
  directions, how the initial connection is
  established and how it is torn down when both
  sides are finished, and how many pins the network
  connector has and what each pin is used for.
• The design issues here deal largely with
  mechanical, electrical, and procedural
  interfaces, and the physical transmission medium,
  which lies below the physical layer. Physical
  layer design can properly be considered to be
  within the domain of the electrical engineer.
• Examples RS232C, X.25, Ethernet

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Layers

• Data Link Layer
• Sometimes called the link layer transmits chunks
  of information across a link.
• It deals with problems as checksumming to detect
  data corruption coordinating the use of shared
  media as in LAN (Local Area Network) and
  addressing (when multiple systems are reachable
  as in a LAN)
• It is common for different links to implement
  different data link layers and for a node to
  support several data link layer protocols, one
  for each of the types of links to which the node
  is attached.
• Example HDLC, SDLC, X.25, Ethernet, ATM.

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Layers

• Network Layer
• The network layer enables any pair of systems to
  communicate with each other.
• A fully connected network is one in which every
  pair of nodes has a direct link between its
  nodes, but this kind of topology does not scale
  beyond a few nodes
• Network layer must find a path through a series
  of connected nodes and nodes along the path
  should forward packets in the appropriate
  direction.
• The network layer deals with problems such as
  route calculation, packet assembly and reassembly
  (when different links on the path have different
  maximum packet sizes), and congestion control.
• Examples IP, IPX, ATM.

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Layers

• Transport Layer
• This layer provides a reliable communications
  stream between a pair of systems
• It deals with errors that can be introduced by
  the network layer, such as lost packets,
  duplicated packets, packet reordering, and
  fragmentation and reassembly
• It is also nice if the transport layer reacts to
  congestion in the network
• Example TCP

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Layers

• Session Layer
• The session layer assumes that a reliable virtual
  point-to-point connection has been made and
  contains specs for the dialog between the two end
  systems such as dialog discipline, data grouping,
  and recovery of an interrupted session. Specs are
  also included for initiating and concluding a
  session. Many network specs contain little or no
  session specs and leave these decisions to the
  applications.
• Presentation Layer
• Provides transformation of data to standardize
  the application interface. Also provides some
  network services such as encryption, compression,
  and text re-formatting.
• Application Layer
• This layer plays the same role as the
  'application interface' in operating systems.
  Provides network services to users (applications)
  of the network in a distributed processing
  environment examples transaction server, file
  transfer protocol, network management, electronic
  mail, and terminal access to remote applications.

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PDUs and SDUs
Application
Application
APDU
PSDU
Presentation
Presentation
PPDU
SSDU
Session
Session
TSDU
SPDU
Transport
Transport
NSDU
TPDU
Network
Network
NPDU
LSDU
Data Link
Data Link
LPDU
PhSDU
Physical
Physical
PhPDU
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Service Models

• Layer n-1 can provide either a connectionless
  service or connection-oriented service
• Communication consists of three phases in a
  CO-service
• Connection setup
• Data transfer
• Connnection release
• Associated with each of these phases are two
  functions
• Layer n initiates the function
• Layer n-1 informs layer n that some layer n
  process in some other node is requesting a
  connection

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Service Models

• Services can vary in their degree of reliability
• Datagram service (also known as best-effort)
  accepts data but makes no guarantees as to
  delivery in that data may be lost, duplicated,
  delivered out of order, or mangled.
• A reliable service guarantees the data will be
  delivered in the order transmitted, without
  corrupting, duplication or loss.

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Examples

• In the TCP/IP protocol suite, network layer is
  connectionless, TCP offers reliable
  connection-oriented service, UDPs datagram
  service
• ATM offers a connection-oriented, unreliable
  service that can be viewed as a network layer.
  For IP over ATM, ATM is viewed by IP as a a data
  link layer
• Its good to know about layering but it should
  not be taken that seriously however it is a good
  learning and communication tool.

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Internet protocol stack

• application supporting network applications
• ftp, smtp, http
• transport host-host data transfer
• tcp, udp
• network routing of datagrams from source to
  destination
• ip, routing protocols
• link data transfer between neighboring network
  elements
• ppp, ethernet
• physical bits on the wire

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TCP/IP Stack
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Layering logical communication

• Each layer
• Distributed entities implement layer functions
  at each node
• entities perform actions, exchange messages with
  peers

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Layering logical communication

• E.g. transport
• take data from app
• add addressing, reliability check info to form
  datagram
• send datagram to peer
• wait for peer to ack receipt

transport
transport
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Protocol layering and data

• Each layer takes data from above
• adds header information to create new data unit
• passes new data unit to layer below

source
destination
message
segment
datagram
frame
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Internet structure network of networks

• roughly hierarchical
• national/international backbone providers (NBPs)
• e.g. BBN/GTE, Sprint, ATT, IBM, UUNet
• interconnect (peer) with each other privately, or
  at public Network Access Point (NAPs)
• regional ISPs
• connect into NBPs
• local ISP, company
• connect into regional ISPs

regional ISP
NBP B
NBP A
regional ISP
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Tiered Networks

• A Tier 1 Network is an IP network which connects
  to the entire Internet solely via Settlement Free
  Interconnection, commonly known as peering.
• Tier 1 - A network that peers with every other
  network to reach the Internet.
• Tier 2 - A network that peers with some networks,
  but still purchases IP transit to reach at least
  some portion of the Internet.
• Tier 3 - A network that solely purchases transit
  from other networks to reach the Internet.

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Routing

• In commercial network routing between autonomous
  systems, hot-potato routing is the practice of
  passing traffic off to another AS as quickly as
  possible, thus using their network for wide-area
  transit.
• Cold-potato routing is the opposite, where the
  originating AS holds onto the packet until it is
  as near to the destination as possible.

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Global Backbone Provider
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Important Properties of a Network

• Scope - A network architecture should solve as
  general a problem as possible
• Scalability - Would work well with very large
  networks and be also efficient with small
  networks
• Robustness The network should continue to
  operate even if nodes or links fail
• Safety barriers A fault does not spread beyond a
  safety barrier, for example a router confines a
  broadcast storm to a single LAN
• Self-stabilization After a failure, the network
  will return to normal operation without human
  intervention within a reasonable time, e.g.,
  routing protocols
• Fault detection
• Autoconfigurability
• Tweakability
• Migration

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How

• A new network "philosophy and architecture," is
  replacing the vision of an Intelligent Network.
  The vision is one in which the public
  communications network would be engineered for
  "always-on" use, not intermittence and scarcity.
  It would be engineered for intelligence at the
  end-user's device, not in the network.
• And the network would be engineered simply to
  "Deliver the Bits" not for fancy network routing
  Fundamentally, it would be a Stupid Network.
• In the Stupid Network, the data would tell the
  network where it needs to go. (In contrast, in a
  circuit network, the network tells the data where
  to go.) In a Stupid Network, the data on it would
  be the boss.

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Scope of this Course

• We will study how a packet finds its way from a
  source to a destination
• Role of Layer 2
• Ethernet, PPP, 802.11
• Role of Layer 3
• IP Addressing
• Routing
• OSPF, BGP
• Internet architecture
• We will also study emerging trends in IP networks
• IP QoS
• MPLS (Multiprotocol Label Switching)
• Traffic Engineering
• Multimedia networking

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The Data Link Layer

• Our goals
• understand principles behind data link layer
  services
• error detection, correction
• sharing a broadcast channel multiple access
• link layer addressing
• reliable data transfer, flow control
• instantiation and implementation of various link
  layer technologies

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Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM and MPLS

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Link Layer Introduction

• Some terminology
• hosts and routers are nodes
• communication channels that connect adjacent
  nodes along communication path are links
• wired links
• wireless links
• LANs
• layer-2 packet is a frame, encapsulates datagram

data-link layer has responsibility of
transferring datagram from one node to adjacent
node over a link
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Link layer context

• transportation analogy
• trip from Princeton to Lausanne
• limo Princeton to JFK
• plane JFK to Geneva
• train Geneva to Lausanne
• tourist datagram
• transport segment communication link
• transportation mode link layer protocol
• travel agent routing algorithm

• Datagram transferred by different link protocols
  over different links
• e.g., Ethernet on first link, frame relay on
  intermediate links, 802.11 on last link
• Each link protocol provides different services
• e.g., may or may not provide reliable data
  transfer over link

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Link Layer Services

• Framing, link access
• encapsulate datagram into frame, adding header,
  trailer
• channel access if shared medium
• MAC addresses used in frame headers to identify
  source, dest
• different from IP address!
• Reliable delivery between adjacent nodes
• seldom used on low bit error link (fiber, some
  twisted pair)
• wireless links high error rates
• Q why both link-level and end-end reliability?

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Link Layer Services (more)

• Flow Control
• pacing between adjacent sending and receiving
  nodes
• Error Detection
• errors caused by signal attenuation, noise.
• receiver detects presence of errors
• signals sender for retransmission or drops frame
• Error Correction
• receiver identifies and corrects bit error(s)
  without resorting to retransmission
• Half-duplex and full-duplex
• with half duplex, nodes at both ends of link can
  transmit, but not at same time

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Adaptors Communicating
datagram
rcving node
link layer protocol
sending node
adapter
adapter

• receiving side
• looks for errors, rdt, flow control, etc
• extracts datagram, passes to rcving node
• adapter is semi-autonomous
• link physical layers

• link layer implemented in adaptor (aka NIC)
• Ethernet card, PCMCI card, 802.11 card
• sending side
• encapsulates datagram in a frame
• adds error checking bits, rdt, flow control, etc.

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Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

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Error Detection

• EDC Error Detection and Correction bits
  (redundancy)
• D Data protected by error checking, may
  include header fields
• Error detection not 100 reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and
  correction

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Parity Checking
Two Dimensional Bit Parity Detect and correct
single bit errors
Single Bit Parity Detect single bit errors
0
0
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Checksumming Cyclic Redundancy Check

• view data bits, D, as a binary number
• choose r1 bit pattern (generator), G
• goal choose r CRC bits, R, such that
• ltD,Rgt exactly divisible by G (modulo 2)
• receiver knows G, divides ltD,Rgt by G. If
  non-zero remainder error detected!
• can detect all burst errors less than r1 bits
• widely used in practice (ATM, HDCL)

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CRC Example

• Want
• D.2r XOR R nG
• equivalently
• D.2r nG XOR R
• equivalently
• if we divide D.2r by G, want remainder R

D.2r G
R remainder
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Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

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Multiple Access Links and Protocols

• Two types of links
• point-to-point
• PPP for dial-up access
• point-to-point link between Ethernet switch and
  host
• broadcast (shared wire or medium)
• traditional Ethernet
• upstream HFC
• 802.11 wireless LAN

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Multiple Access protocols

• single shared broadcast channel
• two or more simultaneous transmissions by nodes
  interference
• collision if node receives two or more signals at
  the same time
• multiple access protocol
• distributed algorithm that determines how nodes
  share channel, i.e., determine when node can
  transmit
• communication about channel sharing must use
  channel itself!
• no out-of-band channel for coordination

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Ideal Mulitple Access Protocol

• Broadcast channel of rate R bps
• 1. When one node wants to transmit, it can send
  at rate R.
• 2. When M nodes want to transmit, each can send
  at average rate R/M
• 3. Fully decentralized
• no special node to coordinate transmissions
• no synchronization of clocks, slots
• 4. Simple

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MAC Protocols a taxonomy

• Three broad classes
• Channel Partitioning
• divide channel into smaller pieces (time slots,
  frequency, code)
• allocate piece to node for exclusive use
• Random Access
• channel not divided, allow collisions
• recover from collisions
• Taking turns
• Nodes take turns, but nodes with more to send can
  take longer turns

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Channel Partitioning MAC protocols TDMA

• TDMA time division multiple access
• access to channel in "rounds"
• each station gets fixed length slot (length pkt
  trans time) in each round
• unused slots go idle
• example 6-station LAN, 1,3,4 have pkt, slots
  2,5,6 idle
• TDM (Time Division Multiplexing) channel divided
  into N time slots, one per user inefficient with
  low duty cycle users and at light load.
• FDM (Frequency Division Multiplexing) frequency
  subdivided.

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Channel Partitioning MAC protocols FDMA

• FDMA frequency division multiple access
• channel spectrum divided into frequency bands
• each station assigned fixed frequency band
• unused transmission time in frequency bands go
  idle
• example 6-station LAN, 1,3,4 have pkt, frequency
  bands 2,5,6 idle
• TDM (Time Division Multiplexing) channel divided
  into N time slots, one per user inefficient with
  low duty cycle users and at light load.
• FDM (Frequency Division Multiplexing) frequency
  subdivided.

time
frequency bands
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Random Access Protocols

• When node has packet to send
• transmit at full channel data rate R.
• no a priori coordination among nodes
• two or more transmitting nodes ? collision,
• random access MAC protocol specifies
• how to detect collisions
• how to recover from collisions (e.g., via delayed
  retransmissions)
• Examples of random access MAC protocols
• slotted ALOHA
• ALOHA
• CSMA, CSMA/CD, CSMA/CA

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Slotted ALOHA

• Assumptions
• all frames same size
• time is divided into equal size slots, time to
  transmit 1 frame
• nodes start to transmit frames only at beginning
  of slots
• nodes are synchronized
• if 2 or more nodes transmit in slot, all nodes
  detect collision

• Operation
• when node obtains fresh frame, it transmits in
  next slot
• no collision, node can send new frame in next
  slot
• if collision, node retransmits frame in each
  subsequent slot with prob. p until success

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Slotted ALOHA

• Pros
• single active node can continuously transmit at
  full rate of channel
• highly decentralized only slots in nodes need to
  be in sync
• simple

• Cons
• collisions, wasting slots
• idle slots
• nodes may be able to detect collision in less
  than time to transmit packet
• clock synchronization

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Slotted Aloha efficiency

• For max efficiency with N nodes, find p that
  maximizes Np(1-p)N-1
• For many nodes, take limit of Np(1-p)N-1 as N
  goes to infinity, gives 1/e .37

Efficiency is the long-run fraction of
successful slots when there are many nodes, each
with many frames to send

• Suppose N nodes with many frames to send, each
  transmits in slot with probability p
• prob that node 1 has success in a slot
  p(1-p)N-1
• prob that any node has a success Np(1-p)N-1

At best channel used for useful transmissions
37 of time!
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CSMA (Carrier Sense Multiple Access)

• CSMA listen before transmit
• If channel sensed idle transmit entire frame
• If channel sensed busy, defer transmission
• Human analogy dont interrupt others!

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CSMA collisions
spatial layout of nodes
collisions can still occur propagation delay
means two nodes may not hear each others
transmission
collision entire packet transmission time wasted
note role of distance propagation delay in
determining collision probability
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CSMA/CD (Collision Detection)

• CSMA/CD carrier sensing, deferral as in CSMA
• collisions detected within short time
• colliding transmissions aborted, reducing channel
  wastage
• collision detection
• easy in wired LANs measure signal strengths,
  compare transmitted, received signals
• difficult in wireless LANs receiver shut off
  while transmitting
• human analogy the polite conversationalist

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CSMA/CD collision detection
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Taking Turns MAC protocols

• channel partitioning MAC protocols
• share channel efficiently and fairly at high load
• inefficient at low load delay in channel access,
  1/N bandwidth allocated even if only 1 active
  node!
• Random access MAC protocols
• efficient at low load single node can fully
  utilize channel
• high load collision overhead
• taking turns protocols
• look for best of both worlds!

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Taking Turns MAC protocols

• Token passing
• control token passed from one node to next
  sequentially.
• token message
• concerns
• token overhead
• latency
• single point of failure (token)

• Polling
• master node invites slave nodes to transmit in
  turn
• concerns
• polling overhead
• latency
• single point of failure (master)

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Summary of MAC protocols

• What do you do with a shared media?
• Channel Partitioning, by time, frequency or code
• Time Division, Frequency Division
• Random partitioning (dynamic),
• ALOHA, S-ALOHA, CSMA, CSMA/CD
• carrier sensing easy in some technologies
  (wire), hard in others (wireless)
• CSMA/CD used in Ethernet
• CSMA/CA used in 802.11
• Taking Turns
• polling from a central site, token passing

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LAN technologies

• Data link layer so far
• services, error detection/correction, multiple
  access
• Next LAN technologies
• addressing
• Ethernet
• hubs, switches
• PPP

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Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

58
MAC Addresses and ARP

• 32-bit IP address
• network-layer address
• used to get datagram to destination IP subnet
• MAC (or LAN or physical or Ethernet) address
• used to get datagram from one interface to
  another physically-connected interface (same
  network)
• 48 bit MAC address (for most LANs) burned in the
  adapter ROM

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LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address FF-FF-FF-FF-FF-FF
adapter
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LAN Address (more)

• MAC address allocation administered by IEEE
• manufacturer buys portion of MAC address space
  (to assure uniqueness)
• Analogy
• (a) MAC address like Social Security
  Number
• (b) IP address like postal address
• MAC flat address ? portability
• can move LAN card from one LAN to another
• IP hierarchical address NOT portable
• depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address TTLgt
• TTL (Time To Live) time after which address
  mapping will be forgotten (typically 20 min)

237.196.7.78
1A-2F-BB-76-09-AD
237.196.7.23
237.196.7.14
LAN
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
237.196.7.88
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ARP protocol Same LAN (network)

• A wants to send datagram to B, and Bs MAC
  address not in As ARP table.
• A broadcasts ARP query packet, containing B's IP
  address
• Dest MAC address FF-FF-FF-FF-FF-FF
• all machines on LAN receive ARP query
• B receives ARP packet, replies to A with its
  (B's) MAC address
• frame sent to As MAC address (unicast)

• A caches (saves) IP-to-MAC address pair in its
  ARP table until information becomes old (times
  out)
• soft state information that times out (goes
  away) unless refreshed
• ARP is plug-and-play
• nodes create their ARP tables without
  intervention from net administrator

63
Routing to another LAN

• walkthrough send datagram from A to B via R
• assume A knows B IP
  address
• Two ARP tables in router R, one for each IP
  network (LAN)
• In routing table at source Host, find router
  111.111.111.110
• In ARP table at source, find MAC address
  E6-E9-00-17-BB-4B, etc

A
R
B
64

• A creates datagram with source A, destination B
• A uses ARP to get Rs MAC address for
  111.111.111.110
• A creates link-layer frame with R's MAC address
  as dest, frame contains A-to-B IP datagram
• As adapter sends frame
• Rs adapter receives frame
• R removes IP datagram from Ethernet frame, sees
  its destined to B
• R uses ARP to get Bs MAC address
• R creates frame containing A-to-B IP datagram
  sends to B

A
R
B
65
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

66
Ethernet

• dominant wired LAN technology
• cheap 20 for 100Mbs!
• first widely used LAN technology
• Simpler, cheaper than token LANs and ATM
• Kept up with speed race 10 Mbps 10 Gbps

Metcalfes Ethernet sketch
67
Star topology

• Bus topology popular through mid 90s
• Now star topology prevails
• Connection choices hub or switch (more later)

hub or switch
68
Ethernet Frame Structure

• Sending adapter encapsulates IP datagram (or
  other network layer protocol packet) in Ethernet
  frame
• Preamble
• 7 bytes with pattern 10101010 followed by one
  byte with pattern 10101011
• used to synchronize receiver, sender clock rates

69
Ethernet Frame Structure (more)

• Addresses 6 bytes
• if adapter receives frame with matching
  destination address, or with broadcast address
  (eg ARP packet), it passes data in frame to
  net-layer protocol
• otherwise, adapter discards frame
• Type indicates the higher layer protocol (mostly
  IP but others may be supported such as Novell IPX
  and AppleTalk)
• CRC checked at receiver, if error is detected,
  the frame is simply dropped

70
Unreliable, connectionless service

• Connectionless No handshaking between sending
  and receiving adapter.
• Unreliable receiving adapter doesnt send acks
  or nacks to sending adapter
• stream of datagrams passed to network layer can
  have gaps
• gaps will be filled if app is using TCP
• otherwise, app will see the gaps

71
Ethernet uses CSMA/CD

• No slots
• adapter doesnt transmit if it senses that some
  other adapter is transmitting, that is, carrier
  sense
• transmitting adapter aborts when it senses that
  another adapter is transmitting, that is,
  collision detection

• Before attempting a retransmission, adapter waits
  a random time, that is, random access

72
Ethernet CSMA/CD algorithm

• 1. Adaptor receives datagram from net layer
  creates frame
• 2. If adapter senses channel idle, it starts to
  transmit frame. If it senses channel busy, waits
  until channel idle and then transmits
• 3. If adapter transmits entire frame without
  detecting another transmission, the adapter is
  done with frame !

• 4. If adapter detects another transmission while
  transmitting, aborts and sends jam signal
• 5. After aborting, adapter enters exponential
  backoff after the mth collision, adapter chooses
  a K at random from 0,1,2,,2m-1. Adapter waits
  K?512 bit times and returns to Step 2

73
Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Bit time .1 microsec for 10 Mbps Ethernet for
  K1023, wait time is about 50 msec

• Exponential Backoff
• Goal adapt retransmission attempts to estimated
  current load
• heavy load random wait will be longer
• first collision choose K from 0,1 delay is K?
  512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten collisions, choose K from
  0,1,2,3,4,,1023

74
CSMA/CD efficiency

• Tprop max prop between 2 nodes in LAN
• ttrans time to transmit max-size frame
• Efficiency goes to 1 as tprop goes to 0
• Goes to 1 as ttrans goes to infinity
• Much better than ALOHA, but still decentralized,
  simple, and cheap

75
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Interconnections Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

76
Hubs

• Hubs are essentially physical-layer repeaters
• bits coming from one link go out all other links
• at the same rate
• no frame buffering
• no CSMA/CD at hub adapters detect collisions
• provides net management functionality

77
Interconnecting with hubs

• Backbone hub interconnects LAN segments
• Extends max distance between nodes
• But individual segment collision domains become
  one large collision domain
• Cant interconnect 10BaseT 100BaseT

hub
hub
hub
hub
78
Switch

• Link layer device
• stores and forwards Ethernet frames
• examines frame header and selectively forwards
  frame based on MAC dest address
• when frame is to be forwarded on segment, uses
  CSMA/CD to access segment
• transparent
• hosts are unaware of presence of switches
• plug-and-play, self-learning
• switches do not need to be configured

79
Forwarding
1
3
2

• How do determine onto which LAN segment to
  forward frame?
• Looks like a routing problem...

80
Self learning

• A switch has a switch table
• entry in switch table
• (MAC Address, Interface, Time Stamp)
• stale entries in table dropped (TTL can be 60
  min)
• switch learns which hosts can be reached through
  which interfaces
• when frame received, switch learns location of
  sender incoming LAN segment
• records sender/location pair in switch table

81
Filtering/Forwarding

• When switch receives a frame
• index switch table using MAC dest address
• if entry found for destinationthen
• if dest on segment from which frame arrived
  then drop the frame
• else forward the frame on interface
  indicated
• else flood

forward on all but the interface on which the
frame arrived
82
Switch example

• Suppose C sends frame to D

address
interface
switch
1
A B E G
1 1 2 3
3
2
hub
hub
hub
A
I
F
D
G
B
C
H
E

• Switch receives frame from from C
• notes in bridge table that C is on interface 1
• because D is not in table, switch forwards frame
  into interfaces 2 and 3
• frame received by D

83
Switch example

• Suppose D replies back with frame to C.

address
interface
switch
A B E G C
1 1 2 3 1
hub
hub
hub
A
I
F
D
G
B
C
H
E

• Switch receives frame from from D
• notes in bridge table that D is on interface 2
• because C is in table, switch forwards frame only
  to interface 1
• frame received by C

84
Switch traffic isolation

• switch installation breaks subnet into LAN
  segments
• switch filters packets
• same-LAN-segment frames not usually forwarded
  onto other LAN segments
• segments become separate collision domains

collision domain
collision domain
collision domain
85
Switches dedicated access

• Switch with many interfaces
• Hosts have direct connection to switch
• No collisions full duplex
• Switching A-to-A and B-to-B simultaneously, no
  collisions

A
C
B
switch
C
B
A
86
More on Switches

• cut-through switching frame forwarded from input
  to output port without first collecting entire
  frame
• slight reduction in latency
• combinations of shared/dedicated, 10/100/1000
  Mbps interfaces

87
Institutional network
mail server
to external network
web server
router
switch
IP subnet
hub
hub
hub
88
How does the IP router different from an Ethernet
switch?
IP Router
Host C
PCs with Ethernet Network Interface Cards (NICs)

• An IP Router is a packet switch whose line
  cards demutliplex out IP datagrams and forward
  packets based on destination IP address and
  routing table entries

89
IP Router vs Ethernet Switch
90
Difference between Ethernet switch and IP router

• Data plane - as packets arrive
• Ethernet switch
• Exact match of destination MAC address of
  incoming packet with destination column entry in
  routing table
• If there is no match, flood packet to all ports
  in the forwarding state
• IP router
• Longest-prefix match - notion of subnet mask
• Default entry match
• If no default entry, drop packet

91
Difference between Ethernet switch and IP router

• Data plane - as packets arrive
• Ethernet switch
• Does not change MAC header
• IP router
• Fields in the IP header are changed, such as TTL

92
Difference between Ethernet switch and IP router

• Addressing
• Ethernet switch
• Flat 6-byte addressing
• Routing tables will be very large because of flat
  addressing
• IP router
• Hierarchical 4-byte (IPv4) and 16-byte (IPv6)
• Advantage address summarization used to decrease
  the number of entries in the routing table

93
Difference between Ethernet switch and IP router

• Routing protocol
• Ethernet switch
• Address learning
• Spanning tree algorithm - "default" ports
• IP router
• OSPF link-state protocol
• RIP, BGP distance-vector protocols

94
Difference between Ethernet switch and IP router

• Ethernet switches
• Characteristics like flooding packets and flat
  addressing makes these packet switches
• Suitable for Local Area Networks (LANs)
• Hence, used within enterprises
• IP routers
• Characteristics like default entry and
  hierarchical addressing (with subnet masks) makes
  these packet switches
• Suitable for Wide Area Networks (WANs)

95
An important difference between Ethernet switch
and IP router

• Ethernet switches
• Plug-and-play
• MAC addresses are hardwired into interfaces (NICs
  and switches' links)
• IP routers
• Needs some administration
• Configure IP addresses of interfaces
• Default router setting

96
"Routing protocol" in Ethernet switches(IEEE
802.1D)

• Address learning
• Spanning tree algorithm
• Two points to note
• The word "bridge" is used here since these
  protocols are run on generic bridges (that
  interconnect any two types of IEEE 802 LANs)
• Current-day interest Ethernet switches run this
  protocol
• A network with a hub is shown as a single line.
  Assume that multiple hosts are connected to each
  hub

M. Veeraraghavan (originals by J. Liebeherr)
97
Operation of transparent bridges

• Three aspects of bridge (switch) operation
• (1) Forwarding of Frames
• (2) Learning of Addresses
• (3) Spanning Tree Algorithm
• Bridges that run spanning-tree algorithm and have
  address learning are essentially connectionless
  packet switches because they perform packet
  forwarding from one link to another based on
  destination addresses carried in the headers of
  incoming packets
• use the term bridge and switch
  interchangeably
• use the term frame and packet
  interchangeably
• The term transparent refers to the fact that
  the hosts are completely unaware of the presence
  of bridges in the network
• Introduction of a bridge does not require hosts
  to be configured.

98
Routing table (called filtering database in
Ethernet switches)

• Each bridge maintains a filtering database
  (routing table) with entries
• lt MAC address, portgt
• MAC address identifies host network interface
  card (NIC)
• port output port number of bridge

99
Frame Forwarding

• Assume an Ethernet frame arrives on port x.

Search if MAC address of destination is listed
for ports A, B, or C in the filtering database.
Notfound ?
Found?
Forward frame on corresponding port if different
from the port on which the frame arrived and the
port state allows it
Flood the frame, i.e., send the frame on all
ports except port x if portstates allow it.
100
Forwarding conditions

• Forward the frame if and only if
• The receiving port is in a forwarding state
• The transmitting port is in a forwarding state
• Either the filtering database indicates the port
  number for the destination MAC address or no such
  entry is present (in which case all ports are
  eligible transmission ports)
• Do not transmit on port on which frame was
  received
• The maximum service data unit size supported by
  the LAN to which the transmitting port is
  connected is not exceeded (e.g., 1500 bytes for
  Ethernet)

101
Address Learning

• In principle, the filtering database could be set
  statically (static routing)
• In the 802.1 bridge, the process is made
  automatic with a simple heuristic
• The source address field of a frame that arrives
  on a port is used by the bridge to update its
  filtering database, which indicates the port
  through which each host is reachable.

Hub
Bridge 2
102
Address Learning

• Algorithm
• For each frame received, the bridge stores the
  source address field in the received frame header
  into the filtering database together with the
  port on which the frame was received.
• All entries are deleted after some time (default
  is 300 seconds).

103
Example

• Consider the following packets ltSrcA, DestFgt,
  ltSrcC, DestAgt, ltSrcE, DestCgt
• What have the bridges learned?

104
Forwarding frames and learning
Learning process writes Filtering database Frame
forwarding reads Filtering database
105
Danger of Loops

• Consider the two LANs that are connected by two
  bridges.
• Assume host n is transmitting a frame F with
  unknown destination.
• What is happening?
• Bridges A and B flood the frame to LAN 2.
• Bridge B sees F on LAN 2 (with unknown
  destination), and copies the frame back to LAN 1
• Bridge A does the same.
• The copying continues
• Wheres the problem? Whats the solution ?

106
Spanning Trees

• IEEE 802.1 has an algorithm that builds and
  maintains a spanning tree in a dynamic
  environment.
• Bridges exchange messages to configure the bridge
  (Configuration Bridge Protocol Data Unit,
  Configuration BPDUs) to build the tree.

107
Concept - Bridge ID

• Each bridge has a unique identifier (8 bytes)
• Bridge ID ltpriority level MAC addressgt
• Priority level 2 bytes Note that a bridge has
  several MAC addresses (one for each port), but
  only one ID using the MAC address of the lowest
  numbered bridge port (port 1)
• Each port within a bridge has a unique identifier
  (port ID).

001235
5124681f34
Bridge
2
3
1
Priority 0x1241
fe64961213
Example above Bridge ID 1241fe64961213
108
Concept - Root bridge of a network

• Root Bridge The bridge with the lowest
  identifier is the root of the spanning tree.

1
LAN A
Bridge 2 with ID 6455421561987
1
LAN B
2
1
Root bridge is bridge 3 since it has the smallest
ID
Bridge 1 with ID 4121121561987
109
Concept - For each bridge

• Root Port Each bridge has a root port which
  identifies the next hop from a bridge to the
  root.
• Root Path Cost For each bridge, the cost of the
  min-cost path to the root
• Example on previous slide What is the root port
  and root path cost of bridge 1
• The root port is port 2 since it leads to the
  root bridge (bridge 3)
• The root path cost is 1 since bridge 1 is one hop
  away from the root bridge (I.e., bridge 3).
• Note We assume that cost of a path is the
  number of hops.

110
Concept - For each LAN

• Designated Bridge, Designated Port Single bridge
  on a LAN that provides the minimal cost path to
  the root for this LAN, and the port on this
  minimal cost path
• if two bridges have the same cost, select the
  one with highest priority (lower bridge ID)
• if the min-cost bridge has two or more ports
  on the LAN, select the port with the lowest
  identifier
• Example for LAN A, the designated bridge is
  bridge 3 since it is the root bridge itself port
  1 is the designated port for LAN B, the
  designated bridge is bridge 1 since this is
  closer to the root bridge than bridge 2. The
  designated port is port 1.

111
Concept - Designated bridge/port

• Even though each LAN is the entity that has a
  designated bridge/designated port, it is each
  bridge that determines whether or not it is the
  designated bridge for the LAN on each of its
  ports.
• Example Bridge 1 in the example determines
  whether it is the designated bridge for LAN A (to
  which its port 2 is connected) and for LAN B (to
  which its port 1 is connected).
• Answer in this case is that bridge 1 is the
  designated bridge for LAN B, but it is not the
  designated bridge for LAN A

112
Steps of Spanning Tree Algorithm

• 1. Determine the root bridge of the whole network
• 2. For all other bridges determine root ports
• 3. For all bridges, determine which of the bridge
  ports are designated ports for their
  corresponding LANs
• The spanning tree consists of all the root ports
  and the designated ports.
• These ports are all set to the forwarding
  state, while all other ports are in a blocked
  state.

113
What we just did

• We just determined the spanning tree for a
  network of LANs and bridges in a centralized
  manner.
• We knew the bridge IDs of all the bridges and
  the port IDs of all the ports in all the bridges.
• We determined the root bridge (the bridge with
  the smallest ID.)
• For each bridge, we determined the shortest path
  to the root by counting hops and thus identified
  the root port.
• For each bridge, we determined which of its ports
  are designated ports for each of its LANs
• However, the network of bridges determines the
  spanning tree in a distributed manner - each
  with limited knowledge.
• This is done using messages called BPDUs.

114
How do the bridges determine the spanning tree?

• With the help of the BPDUs, bridges can
• Elect a single bridge as the root bridge.
• Each bridge can determine
• a root port, the port that gives the best path
  to the root.
• And the corresponding root path cost
• Each bridge determines whether it is a designated
  bridge, for the LANs connected to each of its
  ports. The designated bridge will forward packets
  towards the root bridge.
• Select ports to be included in the spanning tree.
• Root ports and designated ports

115
Short form notation for BPDUs

• Each bridge sends out BPDUs that contain the
  following information

116
Ordering of Messages

• We can order BPDU messages with the following
  ordering relation lt"
• If (R1 lt R2)
• M1 lt M2
• elseif ((R1 R2) and (C1 lt C2))
• M1 lt M2
• elseif ((R1 R2) and (C1 C2) and (B1 lt B2))
• M1 lt M2

lt
M1
M2
ID R1
C1
ID B1
ID R2
C2
ID B2
117
Determine the Root Bridge

• Initially, all bridges assume they are the root
  bridge.
• Each bridge B sends BPDUs of this form on its
  LANs
• Each bridge looks at the BPDUs received on all
  its ports and its own transmitted BPDUs.
• Root bridge is the smallest received root ID that
  has been received so far (Whenever a smaller ID
  arrives, the root is updated)

B
0
B
118
Calculate the Root Path CostDetermine the Root
Port

• At this time A bridge B has a belief of who the
  root is, say R.
• Bridge B determines the Root Path Cost (Cost) as
  follows
• If B R Cost 0.
• If B ? R Cost Smallest Cost in any of BPDUs
  that were received from R 1
• Bs root port is the port from which B received
  the lowest cost path to R.
• Knowing R and Cost, B can generate its BPDU (but
  will not necessarily send it out)

R
Cost
B
119
Determine if the bridge is the designated bridge
for any of the LANs connected to its ports

• At this time B has generated its BPDU
• B will send this BPDU on one of its ports, say
  port x, only if its BPDU is lower (via relation
  lt) than any BPDU that B received from port x.
• In this case, B also assumes that it is the
  designated bridge for the LAN to which the port
  connects.

R
Cost
B
120
Selecting the Ports for the Spanning Tree

• At this time Bridge B has calculated the root
  bridge for the network, its root port, root path
  cost, and whether it is the designated bridge for
  each of its LANs.
• Now B can decide which ports are in the spanning
  tree
• Bs root port is part of the spanning tree
• All ports for which B is the designated bridge
  are part of the spanning tree.
• Bs ports that are in the spanning tree will
  forward packets (forwarding state)
• Bs ports that are not in the spanning tree will
  block packets (blocking state)

121
Adapting to Changes

• Bridges continually exchange BPDUs according to
  the rules we just discussed.
• This allows the bridges to adapt to changes to
  the topology.
• Whenever a BPDU arrives on a port, say port x, B
  bridge determines
• Can B become the designated bridge for the LAN
  that port x is attached to?
• Can port x become the root port?

122
Example 1

• Assume a Bridge with ID 18 has received the
  following as the lowest messages on its 4 ports

Root is 12 85 1 86 Port 2 12.86.18 For
Ports 1,3, 4

• What is the root bridge?
• What is the Root Path Cost?
• What is the root port?
• What is 18s configuration BPDU?
• For which LAN (port), if any, is B the
  designated bridge?

123
Example 2

• Assume a Bridge with ID 92 is receiving the
  following as the lowest messages on its five
  ports

• What is the root bridge?
• What is the Root Path Cost?
• What is the root port ?
• What is 92s configuration BPDU?
• For which LAN (port), if any, is Bridge 92 the
  designated bridge?

124
Network Example (Practice)

• The attached network shows 5 LANs that are
  interconnected by 5 bridges.
• The IDs of the bridges are 1,2,3,4,5 and the
  port IDs are as indicated in the figure.
• The bridges run the spanning tree algorithm.
• Assume that the root cost path is the number of
  hops.
• Assume an initial state.
• Show which messages are exchanged until the tree
  is built.

125
Network Example (Practice Final Answer)

• R Root ports
• D Designated ports
• Show all the BPDUs

126
Failures

• Root bridge periodically transmits configuration
  messages with message age 0
• Bridges receiving these messages transmit them on
  the their designated ports
• If the root or any bridge on the spanning tree
  fails then the configuration messages will time
  out
• At that point, the bridge will discard the
  configuration message and recalculate the root,
  root path cost, and root port.

127
Example
The new root port is 3
The new root port is 5
128
Example
The bridge 92 will assume itself to be the root
and will transmit 92.0.92 on all five ports until
it receives fresh configuration messages from any
of its roots regarding a better root.