Chapter 5

1
Chapter 5 Protocol Architectures

2
Protocol ArchitectureThe IEEE Protocol Reference
Model

• Introduction
• At the lower layers of the Local Area Networking
  world there are two protocols models that
  predominate the IEEE 802 model and the OSI
  reference model
• Comparison of the OSI reference model to the IEEE
  802 reference model

3
Protocol ArchitectureIntro to the IEEE Protocol
Reference Model

• Physical Layer functions
• Encoding/decoding of signals
• Synchronization
• Bit transmission/reception
• Data Link Layer functions
• Provide one or more addressable service access
  points (SAPs)
• On transmission create a frame containing data,
  addressing, and control/error-detection fields
• On reception disassemble frame and check validity
  of addressing and error detection
• Control access to the physical transmission medium

4
Protocol ArchitectureIntro to the IEEE Protocol
Reference Model

• In the IEEE 802 reference model the data link
  layer functions are split into two sublayers
  the Medium Access Control sublayer and the
  Logical Link Control sublayer
• This separation allows generic higher level
  functions to be grouped into a common Logical
  Link Control that can be used with different
  physical layer specific MAC layers
• The Logical Link Control sublayer provides the
  addressing and is responsible for managing access
  to a shared medium a function not specified for
  the data link layer in the OSI model
• The MAC handles transmission and reception of
  frames and controlling physical access to the
  medium

5
Protocol ArchitectureLogical Link Control

• Principles of Logical Link Control
• Concerned with the transmission of a link-level
  PDU between two stations with no intermediate
  switching nodes
• Where the LLC sublayer fits into the protocol
  stack Figure 5.2
• Different from most other link control protocols
  for two primary reasons
• It must support a shared, multi-access medium
• It doesnt have to be concerned with many medium
  specific functions handled by the MAC sublayer

6
Protocol ArchitectureLogical Link Control
Principles

• Four fundamental services are provided to upper
  layers
• Connectionless Service
• Connection-oriented Service
• Multiplexing of logical connections
• Multicasting broadcasting
• Addressing in the frames is important for all LLC
  services!

7
Protocol ArchitectureLogical Link Control
Addressing

• As mentioned earlier, the process of transferring
  data between networked stations can be broken
  into two steps
• Getting the data to the destination station
• Once at the station, getting the data to the
  destination process within the station
• Such a separation in the process leads to a
  two-level addressing scheme
• The MAC address uniquely identifies a physical
  interface from a LAN to the station multiple
  interfaces are OK for a station but they have
  unique addresses
• The LLC address uniquely identifies a LLC user or
  process executing on the station

8
Protocol ArchitectureLogical Link Control
Addressing

• In addition, group addresses are important
• Broadcast addresses refers to all interfaces
  within a certain context
• Multicast addresses refer to a subset of
  interfaces within some context
• In the context of a group address the interface
  considers the data as addressed to itself
• Possible combinations of individual and group MAC
  LLC addressing Figure 5.5
• In many cases well-known addresses are used for
  specific functions (network control)

9
Protocol ArchitectureLogical Link Control
Addressing

• An example LLC addressing scenario

10
Protocol ArchitectureMedium Access Control

• MAC Techniques
• The key parameters of any MAC technique are where
  how
• Where is control centralized or distributed?
• In a centralized scheme a station must receive
  permission from a controller before transmitting
  while in a decentralized scheme the stations
  collectively follow a procedure to determine who
  is allowed to transmit
• Centralized schemes allow greater control, allows
  the use of simple station logic, and avoids
  ambiguous states that may occur with distributed
  coordination
• Centralized schemes can have a single point of
  failure and also become a performance bottleneck

11
Protocol ArchitectureMedium Access Control
Techniques

• How is the access control synchronous or
  asynchronous?
• Synchronous schemes can be very inefficient in
  data transmission
• Bandwidth allocated even if it is not used
• Three basic asynchronous schemes
• Round robin
• Reservation
• Contention

12
Protocol ArchitectureAsynchronous MAC Techniques

• Round robin each station in turn is given the
  opportunity to transmit
• Can be used in a centralized (polling) or
  decentralized system
• Most efficient when many stations transmit
  streaming traffic
• Reservation each station reserves in advance an
  opportunity to transmit
• Most efficient with streaming traffic
• Can be used in a centralized or decentralized
  control environment
• Contention a station must win the medium to
  transmit
• Best for bursty traffic
• Simple to implement and very efficient with light
  loads system throughput can collapse under heavy
  load
• A strictly distributed scheme most often found in
  LAN products

13
Protocol ArchitectureMedium Access Control Frame
Format

• The exact frame format varies between specific
  MAC protocols, Figure 5.6 shows a very general
  format
• Control field contains any fields needed to
  control the operation of the MAC protocol or
  implement some of it functionality (e.g.
  acknowledgements, priority levels)
• Destination MAC address
• Source MAC address
• Payload usually the LLC Protocol Data Unit
  encapsulated within the MAC frame
• CRC (Cyclic Redundancy Check)

14
Protocol ArchitectureMAC Frame Format

• CRC (Cyclic Redundancy Check) a special code
  mathematically generated from the rest of the
  frame which can detect and sometimes correct
  errors
• The sender generates the CRC and inserts it in
  the field
• Upon reception the receiver also generates the
  CRC and compares it the CRC in the received frame
• If they are the same the frame was received
  without error, otherwise there is at least one
  bit error in the frame
• With the IEEE 802 LAN protocol architecture,
  error detection is the responsibility of the MAC
  sublayer error tracking and recovery is the
  responsibility of the LLC sublayer

15
Protocol ArchitectureMAC Frame Format

• Examination of Figure 5.7 encapsulation in the
  TCP/IP and IEEE 802 protocol stack to put the MAC
  and LLC layers in context

16
Protocol ArchitectureBridges and Routers

• Introduction
• It is very common for organizations to require
  interconnection of LANs
• Two systems used for this purpose bridges
  routers
• Bridges
• Bridges are typically designed to connect
  together LANs that use the same MAC protocols
  (this keeps them simple)
• Advantages to using bridges over a single large
  LAN
• Reliability minimize single points of failure
• Performance increases aggregate throughput
• Security partitions sensitive data
• Geography may be physically impossible to put
  everyone on the same LAN

17
Protocol ArchitectureBridges

• Design characteristics of bridges
• Bridges make no modifications to the control data
  and to end stations they believe they are on one
  large LAN
• Frames must have addressing and routing
  intelligence to know which interface to transmit
  a frame on
• Bridges may connect more than two LANs

18
Protocol ArchitectureBridges

• Protocol Architecture diagram for a bridge
  Figure 5.9a

19
Protocol ArchitectureRouters

• Routers are general-purpose devices used to
  connect dissimilar LANs by operating as a network
  layer relay
• Routers must deal with the following issues
• Different Addressing formats
• Different maximum (and minimum) frame sizes
• Different physical interfaces
• Different levels of reliability
• Other differences in functionality, including
  priority and quality of service mechanisms
• To use routers all systems on the connected
  networks that wish to communicate must share a
  common network layer
• In the TCP/IP protocol stack IP is the common
  glue
• IP is a best effort service it imposes no
  reliability requirements on the lower protocol
  layers

20
Protocol ArchitectureRouters

• Protocol Architecture diagram for a router Fig.
  5.9b
• It is possible and indeed very common to find
  mixed networks of bridges (switches) and routers

21
Protocol ArchitectureOther Networking Devices

• Other devices besides routers and bridges can be
  used for network interconnection
• Another possible but less common method for
  connecting together networks is to use a gateway
• Gateways usually operate at the application layer
  allowing completely different network protocol
  stacks to interoperate
• Gateways are used in security applications to
  provide a high degree of isolation
• Hubs are essentially multiport repeaters with
  very little intelligence all that is necessary
  is the proper propagation of the collision
  detection signals

22
Protocol ArchitectureOther Networking Devices

• As mentioned earlier, a LAN switch is similar to
  a bridge but has additional capabilities
• LAN switches usually perform frame forwarding in
  hardware instead of software for maximum
  performance
• LAN switches can forward multiple frames
  simultaneously bridges can only forward one at a
  time
• LAN switches have two modes of forwarding they
  can perform both cut-through and
  store-and-forward forwarding
• LAN switches are rapidly replacing bridges in
  most networks

23
Protocol ArchitectureAppendix The IEEE 802
Standards

• Introduction
• A widely known and followed set of LAN standards
  has allowed networking to proliferate by
  fostering lower equipment prices promoting
  interoperability
• The IEEE 802 committee was established to develop
  a set of LAN protocols these protocols have
  since been adopted by ANSI and ISO to foster
  truly international standardization
• The committee is focused on Data Link Physical
  layer standards for LANs

24
Protocol ArchitectureAppendix The IEEE 802
Standards

• The IEEE 802 has broken down responsibility for
  standards development into functional areas as
  follows
• 802.1 Higher Layer LAN Working Group
• 802.2 Logical Link Control Working Group
  (inactive)
• 802.3 Ethernet Working Group
• 802.4 Token Bus Working Group (currently
  inactive)
• 802.5 Token Ring Working Group
• 802.6 MAN Working Group (DQDB technology -
  inactive)
• 802.7 Broadband Technical Advisory Group
  (inactive)
• 802.8 Fiber Optic Technical Advisory Group
• 802.9 Isochronous LAN Working Group
• 802.10 Security Working Group
• 802.11 Wireless LAN Working Group
• 802.12 Demand Priority Working Group
  (100VG-AnyLAN)
• 802.14 Cable Modem Working Group
• 802.15 Wireless Personal Area Network Working
  Group
• 802.16 Broadband Wireless Access Study Group

25
Protocol ArchitectureAppendix The Cyclic
Redundancy Check (CRC)

• An error detection mechanism is important for
  data link layer protocols to ensure that reliable
  communication is possible by notifying higher
  layers when they must retransmit data
• Commonly accomplished by adding a frame check
  sequence to the data frame this is used as an
  integrity check by the receiver
• One of the most useful frame check sequences is
  the Cyclic Redundancy Check (CRC)

26
Protocol ArchitectureAppendix The Cyclic
Redundancy Check (CRC)

• How the CRC Works
• Based on a common binary arithmetic principles
• Sender Receiver agree on a common divisor
  polynomial C(x)
• What is transmitted is the n-bit message plus a
  set of k check bits the total nk bits are
  exactly divisible by C(x)
• Heres the process the sender uses
• Multiply M(x) by xk (add k zeros on to M(x)) to
  create M(x)
• Divide M(x) by C(x) and find the remainder
• Subtract the remainder R(x) from M(x) forming
  S(x)
• S(x) is actually the n-bit message followed by
  the k-bit CRC
• The receiver of the message verifies message
  integrity by
• Dividing S(x) by C(x)
• If the division equals zero there is no
  detectable error in the transmission
• A non-zero result indicates errors

27
Protocol ArchitectureAppendix The Cyclic
Redundancy Check (CRC)

• How the CRC Works
• Careful choice of the divisor polynomial
  determines what errors can be caught
• Undetected errors are bit errors that caused the
  received message to be evenly divided by the
  divisor polynomial C(x)
• By choosing a good prime number of sufficient
  length almost all errors can be detected with
  very little overhead Table 5.1
• All of the IEEE 802 MAC frames use a 32 bit CRC
  (as known as CRC-32)
• C(x) 100000100110000010001110110110111 for
  CRC-32

28
LAN Addresses and ARP

• IP address drives the packet to destination
  network
• LAN (or MAC or Physical) address drives the
  packet to the destination nodes LAN interface
  card (adapter card) on the local LAN
• 48 bit MAC address (for most LANs) burned in
  the adapter ROM
• Why not use IP addresses?
• Portability
• Update ROM on reboot
• How about no MAC add.?
• Host receives every packet
• Overhead to host processor

29

• Address allocation
• MAC address allocation administered by IEEE
• A manufacturer buys a portion of the address
  space (to assure uniqueness) in chunks of 224
  addresses
• First 24 bits are fixed
• Manufacturer can use last 24 bits to produce NICs
  with unique add.
• Analogy
• (a) MAC address like student ID number
• (b) IP address like postal address
• MAC flat address ? portability
• IP hierarchical address ? NOT portable (need
  mobile IP)
• Broadcast LAN address 1111.1111
• Each IP node (Host, Router) on the LAN has ARP
  module and Table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address TTLgt
• lt .. gt
• TTL (Time To Live) timer, typically 20 min

30
(No Transcript)
31

• ARP Routing Packet Within a LAN
• Host A wants to send packet to destination IP
  addr XYZ on same LAN
• Source Host first checks own ARP Table for IP
  addr XYZ
• If XYZ not in the ARP Table, ARP module
  broadcasts an ARP packet
• lt XYZ, MAC (?) gt
• ALL nodes on the LAN accept and inspect the ARP
  packet
• Node XYZ responds with unicast ARP pkt carrying
  own MAC addr
• lt XYZ, MAC (XYZ) gt
• MAC address cached in ARP Table

32

• ARP Routing Packet to Another LAN
• Say, route packet from source IP address
  lt111.111.111.111gt to destination address
  lt222.222.222.222gt
• 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

33
Hubs

• Hubs are physical layer devices
• Operate on bits rather than frames
• Broadcast each incoming bit to all other
  interfaces
• Hubs are same as repeaters with additional
  network management functionality
• Multi-tier hub design
• Individual LANs connect to a 10BeseT hub via
  point-to-point connections
• All hubs connected to a backbone hub via
  point-to-point connections
• Entire topology is called a LAN
• Individual LANs ? LAN segments
• All LAN segments belong to the same collision
  domain

34
Backbone Hub

• Benefits of LAN segments connected via backbone
  hub
• Inter-domain communication among hosts
• Extends maximum distance using hubs as repeaters
• Multi-tier approach provides graceful degradation
• Backbone hub can disconnect a malfunctioning
  segment
• Limitations
• Independent collision domains are transformed to
  one large collision domain
• Overall bandwidth is limited by the bandwidth of
  each LAN
• Example 3 LANs each with 10 Mbps ? total
  bandwidth lt 10 Mbps
• If different departments use different LAN
  technologies they cannot be connected through a
  backbone hub
• Restrictions on number of hosts, maximum distance
  between any two hosts, and maximum allowable
  number of tiers

35
Bridge

• Bridge is a layer 2 device
• Examines frames rather than simply broadcasting
  bits
• Uses layer 2 destination addresses for forwarding
• Separate collision domains for each connected LAN
• Example
• Three LANs connected through a bridge

36
Bridge

• Bridges overcome some of the limitations of a
  hub
• Permit inter-LAN communication while preserving
  isolated collision domains
• Bridges can interconnect different LAN
  technologies
• For example, 10 and 100 Mbps Ethernets
• No limit on the size of the LANs connected
  through bridges
• Functions of a bridge
• FilteringWhether a frame should be forwarded to
  another interface or dropped
• Forwarding
• Ability to determine a target interface
• Implemented through a bridge table
• A bridge table entry contains LAN address of a
  node, corresponding bridge interface, and time
  at which the entry was placed in the table

37
Bridge

• Self learning
• A bridge table is populated automatically ? no
  protocol needed
• Table is initially empty
• Bridge forwards copies of a frame whose
  destination address does not have an entry in the
  table to all output interfaces to transmit to all
  LAN segments using CSMA/CD
• For each arriving frame, bridge stores
• Source address field
• Interface from where it arrived from
• Current time
• Aging time after which entry for a source are
  removed if no frames are received with that
  source address
• Bridges are plug-and-play devices
• No involvement from system administrator to
  configure it
• Transparent bridges

38
Bridge

• Problem with hierarchical LANs connected through
  bridges
• Failure of a bridge near the top of hierarchy
  causes all LAN segments below it to be
  unreachable
• Not a fault-tolerant design
• Solution
• Allow multiple, redundant paths between LAN
  segments
• Greatly improves fault tolerance
• Problem frames can cycle and multiply within
  connected LANs
• Cycling and multiplying problem is solved through
  a spanning tree protocol

39
Bridge Spanning Tree Protocol
Bridges determine a spanning tree Communicate
over LANs Result is a subset of original
topology that has no loops Using spanning tree,
bridges virtually disconnect appropriate
interfaces Avoids cycles and multiple copies of
frames Spanning tree algorithm is re-run when one
of the links in spanning tree fails
40
Bridges vs. Routers

• Similarities
• Both are store-and-forward devices
• Both can isolate the collision domains
• Are interchangeable devices
• Differences
• Bridges operate at layer 2 while routers operate
  at layer 3
• Bridges are plug-and-play devices whereas routers
  generally require configuration
• Different efficiency and computational
  requirements

41
Ethernet Switches

• Layer 2 (frame) forwarding, filtering using LAN
  addresses
• Switching
• Assume each host connected through a pair of
  links
• A-to-B and A-to-B simultaneously,
• No collisions
• No CSMA/CD needed
• Often individual hosts, star-connected into
  switch
• Ethernet, but no collisions!

42
An Institutional Network Using Hubs, Ethernet
Switches, and a Router

• Switch and bridge has similar trade-offs
• Both will result in broadcast and flooding storms
• Router is needed at the edge to prevent these
  storms from spilling outside

43
Cut-Through Switching

• An alternative to store-and-forward switching
• For any packet switch router, bridge, or
  Ethernet switch
• Efficiently utilizes empty output buffers
• Cut-through switching
• Frame forwarded from input to output port without
  awaiting for assembly of entire frame
• Start forwarding packet before all of it has
  arrived at input
• Initial part must contain the destination
  information to start forwarding
• Slight reduction in latency
• Delay in gathering entire packet L/R where L is
  packet length and R is transmission rate
• This delay can be eliminated/reduced only when
  output buffer becomes empty before all of the
  packet has arrived at input

44
Cut-Through Switching (Contd)

• Cut-through over store-and-forward
• Limited advantage
• Results in .12 msec (for 100 Mbps Ethernet) to
  1.2 msec (for 10 Mbps Ethernet) reduction for max
  Ethernet packet sizes only when output buffers
  are empty