QoS Architectures in Computer Networks

1
QoS Architectures in Computer Networks

• Prof. A. Sahoo
• KReSIT
• IIT Bombay

2
Introduction

• For Many Years Internet was primarily used for
  networking research. File transfer, email were
  the most popular application They do not need
  any performance guarantee from underlying
  network.
• New applications such as VoIP, video
  conferencing, e-commerce apps are sensitive to
  network performance.

3
Introduction (contd)

• Internet cannot provide any resource guarantees
  the service is best effort
• Internet does not provide service differentiation
  all packets are treated equal.
• But applications such as VoIP require low delay,
  jitter and packet loss whereas file transfer app
  can tolerate fair amount of delay and loss. Thus
  there is a need to differentiate between packets
  of such applications.

4
Current State of Internet

• Uses best-effort service model
• No guarantee of timeliness or delivery
• No service discrimination
• Bandwidth and network congestion problems
• Unpredictable network response time

5
What is QoS

• The capability to provide resource assurance and
  service differentiation so that delay, jitter or
  loss sensitive applications can perform
  satisfactorily is often referred to as quality of
  service (QoS).
• can be provided through relative prioritization
  of resource allocation to different flows/packets
  in the network.

6
Resource Allocation

• Many problems in the Internet come down to issue
  of resource allocation.
• Packets get delayed or dropped because network
  resource cannot meet the traffic demands.
• A network consists of shared resources
  bandwidth, buffer, serving traffic from competing
  users.
• To support QoS network must allocate resources
  and decide who should get how much resources.

7
Resource Allocation (contd)

• Current Internet does not support active resource
  allocation.
• Network treats all packets equally and serves
  them FCFS.
• Hence current Internet offers best effort
  service.
• Adequate for some apps (e.g. file transfer), but
  not suitable for realtime apps.

8
Integrated Services (Intserv)
9
Integrated Services

• Based on per flow resource reservation.
• Apps must make a reservation before transmitting
  traffic.
• App characterize its traffic and resource
  requirement.
• Network uses routing protocol to find a path.
• A reservation protocol is used to install the
  reservation state along that path.

10
Integrated Services (contd)

• At each hop admission control checks whether
  sufficient resources are available to accept the
  new reservation.
• Resource reservation enforced by packet
  classification and scheduling mechanisms.
• Two new service models were introduced
  guaranteed service and controlled load.
• Guaranteed service provides deterministic worst
  case delay
• Controlled load provides less firm guarantee
  its close to a lightly loaded best-effort
  network.

11
Integrated Services (contd)

• Overhead of setting up reservation is high.
• Scalability problem Backbone will have a large
  number of flows and keeping flow information is
  not feasible.

12
Basic Approach

• A set of mechanisms and protocols is used for
  making explicit resource reservation.
• To receive performance guarantee from the network
  resource reservation must be set up before the
  application can start transmitting packets.

13
Basic Approach (Contd)

• Sender starts the setup of a reservation by
  sending characteristics and resource requirement
  of the flow.
• The network can accept the new application flow
  only if sufficient resource is there.
• Once reservation is setup successfully,
  application can start sending data packets.

14
Key Components
QoS routing agent
Admission control
Reservation setup agent
Resource reservation table
Control plane
Flow identification
Packet scheduler
Data plane
15
Key Component (contd)

• Control Plane sets up resource reservation.
• Data plane forwards data packets based on
  reservation state.
• To setup reservation, app first characterizes its
  traffic flow and specifies QoS requirements
  referred to as flow specification
• The reservation setup request is then sent to the
  network.

16
Key Component (contd)

• Router upon getting the request, interacts with
  QoS routing agent to find the next hop.
• It then coordinates with the admission control
  module to determine if there are sufficient
  resources to meet the requested resources.
• Once reservation set up is successful, the
  information for the reserved flow is installed
  into the resource reservation table.
• Info. in the resource reservation table is used
  to configure flow identification module and the
  packet scheduling module in the data plane.

17
Route Selection

• IntServ does not specify any route selection of
  its own.
• It relies on existing routing protocols to
  forward its control packets further.
• Obviously a more efficient routing protocol which
  can find a path that is likely to have sufficient
  resources is desired.

18
Reservation Setup

• To setup reservation a reservation set up
  protocol is needed that goes hop by hop along the
  path to install the reservation state in the
  routers.
• The reservation protocol must also deal with
  changes in the network topology.
• In IntServ, RSVP has been developed as the
  resource reservation protocol.

19
Admission Control

• In order to provide guaranteed resources for
  reserved flows, a network must monitor its
  resource usage and admit a new flow only if it
  has sufficient resource.
• It has two functions to determine if a new flow
  reservation can be set up based on the admission
  control policies and to monitor and measure the
  available resources.

20
Flow Identification

• Router must examine every incoming packet and
  decide whether the packet belongs to one of the
  reserved flows.
• IP flow is identified by src addr, dest addr,
  proto ID, src port, dst port five-tuple.
• These five fields of the incoming packet is
  compared against the five-tuple of all the flows
  in the reservation table for flow identification.

21
Packet Scheduling

• Packet scheduler responsible for resource
  allocation
• Directly affects delay, jitter and packet loss
• Primary task is to select a packet to transmit
  when outgoing link is ready such that the QoS
  promised to flows is provided

22
Service Models

• Describe interface between the network and its
  users.
• IntServ has standardized two basic service
  models
• Guaranteed service
• Controlled load service

23
Flow Specification

• A service contract that specifies the traffic
  that the source will send
• If application violates the contract then it may
  not get the QoS expected.
• This is done by policing the traffic to ensure
  that it conforms to its traffic description.

24
Flow characterization

• Peak rate highest rate at which a source can
  generate traffic.
• Can be calculated from packet size and the
  spacing between two packets.
• Average rate The avg. transmission rate over a
  time interval.
• Typically calculated with a moving time window.
• Burst The max amount of data that can be
  injected at peak rate.

25
Flow specification (contd)

• In IntServ, traffic is described in terms of
  leaky bucket parameters.
• It has two parameters token arrival rate r and
  bucket depth b.
• Token gets into bucket at the rate r and packet
  is sent only if there are enough tokens.
• When a packet is sent, tokens equal to the packet
  size is removed from the bucket.

26
Guaranteed Service

• Provides guaranteed bandwidth and strict bounds
  for delay.
• Intended for apps that require highest assurance
  on bw and delay mission critical apps,
  intolerant playback apps.
• Can be viewed as a virtual circuit with
  guaranteed bw.
• Provides bounds on maximal queuing delay.

27
Controlled load service

• Strict bw assurance and delay bound comes at a
  price resources have to be reserved for the
  worst case.
• For some apps a service model with less strict
  guarantees and lower cost would better serve
  their needs.
• End-to-end behavior somewhat vague.
• A very high percentage of packets will be
  successfully delivered by the network to the
  receivers.
• The transit delay experienced by a very high
  percentage of packets will not greatly exceed min
  delay.

28
RSVP

• A resource reservation protocol defined under
  IntServ.
• Used by hosts to communicate service requirements
  to the network and by routers in the network to
  establish reservation state along a path

29
Basic Features

• Simplex Reservation
• Makes reservation only in one direction.
• Treats sender as logically distinct from a
  receiver
• For two way communication, the two ends must
  establish reservation for both directions.
• Receiver Oriented
• Receivers of a flow initiates and maintains the
  resource reservation.

30
Basic Features (Contd)

• Routing Independent
• Designed to operate with current and future
  unicast and multicast routing protocols
• The path for a flow is done separately by routing
  protocols
• Policy Independent
• RSVP transports and maintains traffic control and
  policy control parameters that are opaque to RSVP
• Control params are passed to relevant control
  modules for processing.

31
Basic Features (Contd)

• Soft State
• RSVP maintains soft states providing graceful
  support for dynamic membership changes and
  automatic adaptation to routing changes.
• Reservation state has a timer associated with the
  state. When timer expires, the state is
  automatically deleted.
• RSVP periodically refreshes the reservation state
  to maintain the state along the paths.

32
Basic Features (Contd)

• Reservation Style
• RSVP provides several reservation models or
  styles to fit a variety of applications
• Can be used to share a reservation among traffic
  streams from multiple senders or to select a
  particular sender.

33
Protocol Overview
34
Protocol Overview (Contd)

• Two primary RSVP msgs PATH and RESV
• PATH msgs are sent from source towards the
  receivers.
• Used to pass characteristics of the path.
• Installs path state in each node along the way
• Includes IP address of previous hop (needed to
  send RESV msg)
• After receiving PATH msg receiver can request a
  reservation by sending RESV msg.

35
Protocol Overview (Contd)

• RESV must follow the exact same reverse path
  upstream.
• They create reservation state in each node along
  the paths
• After receiving RESV msg sender can start sending
  data packets.

36
IntServ References

• R. Braden, D. Clark, S. Shenker, Integrated
  Services in the Internet Architecture an
  Overview, RFC1633
• J. Wroclawski, The Use of RSVP with IETF
  Integrated Services, RFC2210.
• J. Wroclawski , Specification of the
  Controlled-Load Network Element Service, RFC2211
• S. Shenker, C. Patridge, R. Guerin,
  Specification of Guaranteed Quality of Service,
  RFC2212
• R. Braden, L.Zhang et. al., Resource Reservation
  Protocol (RSVP), RFC2205

37
Differentiated Service
38
DiffServ

• Differentiated Services (DiffServ) is proposed by
  IETF as a scalable QoS solution for the next
  generation Internet.
• Developed for relatively simple, coarse methods
  of providing different levels of service for
  Internet traffic.
• Divides traffic into a small number of classes
  and allocates resources on a per class basis.
• Core of a diffserv network distinguishes between
  small number of forwarding classes rather than
  individual flows.

39
DiffServ (cont.)

• Complex per-flow classification and scheduling
  used in intServ (causes scalability) not needed.
• Operates on a per-hop behavior (PHB) basis
• Classifies packets by marking the headers
  Routers discriminate packets based on their
  markings
• Packet marking is done on the basis of a service
  level agreement (SLA) between the host and the
  ISP
• Provides service assurances but no QoS guarantee

40
Basic Approach

• Traffic is divided into a small number of groups
  called forwarding classes
• Forwarding class that a packet belongs to is
  encoded into a field in the IP packet header.
• Each forwarding class represents a predefined
  forwarding treatment in terms of drop priority
  and bandwidth allocation.

41
Basic Approach(cont.)

• Achieves scalability by implementing traffic
  classification and conditioning functions at
  network boundary nodes
• Classification involves mapping packets to
  different forwarding classes.
• Conditioning checking whether traffic flows
  meet the service agreement and dropping/remarking
  non-conformant packets.
• Interior nodes forward packets based solely on
  the forwarding class.

42
Basic Approach(cont.)

• Resource allocation for aggregated traffic rather
  than individual flows
• Performance assurance to individual flows in a
  forwarding class provided through prioritization
  and provisioning rather than per-flow reservation
• Traffic policing on the edge and class-based
  forwarding in the core
• Define forwarding behaviors not services

43
Basic Approach(cont.)

• Guarantee by provisioning rather than reservation
• Allocate resources to forwarding class and
  control the amount of traffic for these classes
• Provides only service assurance no bw or delay
  guarantee
• Based on SLAs, not dynamic signaling
• Focus on a single domain, not end-to-end
• Forwarding classes can be defined for a single
  domain and between domains service providers can
  extend or map their definitions through bilateral
  agreement

44
Per Hop Behavior (PHB)

• Forwarding treatments at a node
• Each PHB is represented by a 6-bit value called
  DSCP
• All packets with the same code points are
  referred to as a behavior aggregate (BA) and they
  receive the same forwarding treatment.
• Basic building block in diffserv for resource
  allocation to different BAs.

45
PHB (cont.)

• May describe forwarding behavior in either
  relative or absolute terms
• Minimal bw for BA absolute term
• Allocate bw proportionally relative
• Typically implemented by means of buffer
  management and packet scheduling.

46
Services

• Describes the overall treatment of a customers
  traffic within a DS domain or end-to-end.
• This is what is visible to the customers PHBs
  are hidden inside the network node.
• Realizing a service involves many components to
  work together mapping of traffic to specific
  PHBs, traffic conditioning at the boundary,
  network provisioning, PHB-based forwarding in the
  core

47
Services (cont.)

• In diffserv, services are defined in the form of
  a Service Level Agreement (SLA) between a
  customer and its service provider
• One important element of SLA in diffserv is the
  traffic conditioning agreement (TCA).
• TCA details the service parameters for traffic
  profiles and policing actions.

48
Services (cont.)

• This may include
• Traffic profiles, such as token bucket parameters
  for each of the classes
• Performance metrics throughput, delay
• Actions for non-conformant packets
• In addition to TCA, an SLA may also contain other
  characteristics and business-related agreements
  such as availability, security, monitoring,
  auditing, billing.

49
Services (cont.)

• SLAs may be static or dynamic
• Services can be defined in either quantitative or
  qualitative terms
• Services may have different scopes
• All traffic from ingress node A and any egress
  nodes
• All traffic between ingress node A and egress
  node B

50
Diffserv Architecture
Leaf marker
Intermediate marker
BB
Leaf marker
--end user
--edge router
--core router
51
Packet Classifier and Traffic Conditioner
52
Classifier

• Divides an incoming packet stream into multiple
  groups based on predefined rules
• Two basic types of classifiers
• Behavior aggregate (BA)
• Multifield (MF)
• BA classifier selects packets based solely on
  DSCP value in the packet header
• BA classifier is used when DSCP has been set
  (marked) before the packet reaches the classifier

53
Classifier (Cont.)

• MF classifier uses a combination of one or more
  fields of the five-tuple (src addr, src port,
  dest addr, dest port, proto ID) in the packet
  header for classification
• Classification policies may specify a set of
  rules and corresponding DSCP values for marking
  the matched packets

54
Traffic Conditioner

• Performs traffic policing function to enforce the
  TCA between customer and service providers
• Four basic elements meter, marker, shaper and
  dropper

55
Meter

• For each forwarding class meter measures the
  traffic flow from a customer against its traffic
  profile
• In-profile packets are allowed to enter the
  network
• Out-profile packets are further conditioned based
  on TCA

56
Marker

• Sets the DS field of a packet to a particular
  DSCP, adding marked packet to forwarding class.
• May act on unmarked packets or remark previously
  marked packets.
• Can occur at different locations
• Can be marked by the application
• Marked by the first-hop routers on LAN
• Such marking is usually associated with an MF
  classification

57
Marker (cont.)

• Marking can be done on non-conforming packets
• Packets may be marked with a special DSCP to
  indicate non-conformance
• These packets would be dropped first in the event
  of network congestion
• Since packets travel through different domains,
  packets that have been marked may be remarked (to
  a different DSCP).

58
Marker (cont.)

• When packet remarked with new DSCP receives worse
  forwarding treatment than from previous DSCP
  PHB demotion
• With better forwarding treatment PHB promotion

59
Shaper

• Shapers delay non-conformance packets in order to
  bring the stream into compliance.
• A stronger form of policing than marking
• Shaping may also be needed at a boundary node to
  a different domain (to make sure that the traffic
  is conformant before entering the next domain)
• Usually has finite buffer, so may also drop
  packets when buffer is full

60
Dropper

• Discards packets in a traffic stream in order to
  bring the stream into compliance with a traffic
  profile.
• Strongest policing entity
• Can be implemented as a special case of a shaper
  by setting the shaper buffer size to zero.

61
Differentiated Services Field

• Uses 6 bits in the IP header to encode forwarding
  treatment
• These 6 bits are those out of the IP TOS field (8
  bits long)
• DiffServ redefines existing IP TOS field to
  indicate forwarding behavior
• Replacement field, called DS field supersedes
  existing definition of TOS
• First 6 bits used as DSCP to encode the PHB,
  remaining 2 bits are currently unused (CU).

62
Differentiated Services Field (cont.)

• xxxxx0 standard action
• xxxx11 experimental and local use
• xxxx01 experimental and local use but may be
  subject to standard action (in case pool 1 is
  exhausted)

63
Assured Forwarding (AF)

• One of the two PHB groups standardized by IETF.
• Four forwarding classes and three drop
  precedences within each forwarding class.
• The three drop precedences within each forwarding
  class are used to select which packet to drop
  during congestion
• Highest drop precedence is dropped first.

64
Assured Forwarding (AF)
65
Expedited Forwarding (EF)

• Proposed to characterize a forwarding treatment
  similar to that of a simple priority queuing.
• Forwarding treatment of traffic aggregate must
  equal or exceed a configurable rate
• Should receive this rate independent of load of
  other traffic passing through the node
• Provides low delay and low loss service
• Code point lt101110gt used for EF PHB

66
References

• An Architecture for Differentiated Services
  RFC 2475
• A Framework for Integrated Services Operation
  over Diffserv Networks RFC 2998
• Random Early Detection Gateways for Congestion
  Avoidance IEEE/ACM Trans. On Networking vol.
  1, No-4, August 1993
• Explicit Allocation of Best-Effort Packet
  Delivery Service IEEE/ACM Trans. On
  Networking, vol. 6, no-4, August 1998.

67
Multi Protocol Label Switching (MPLS)
68
MPLS Basics

• Multi Protocol Label Switching is arranged
  between Layer 2 and Layer 3

69
MPLS Basics (cont.)

• MPLS Characteristics
• Mechanisms to manage traffic flows of various
  granularities (Flow Management)
• Is independent of Layer-2 and Layer-3 protocols
• Maps IP-addresses to fixed length labels
• Interfaces to existing routing protocols (RSVP,
  OSPF)
• Supports ATM, Frame-Relay and Ethernet

70
Label

• Generic label format

71
Label (cont.)

• Label distribution
• MPLS does not specify a single method for label
  distribution
• BGP has been enhanced to piggyback the label
  information within the contents of the protocol
• RSVP has also been extended to support
  piggybacked exchange of labels.

72
Label (cont.)

• IETF has also defined a new protocol known as the
  label distribution protocol (LDP) for explicit
  signaling and management
• Extensions to the base LDP protocol have also
  been defined to support explicit routing based on
  QoS requirements.

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Label Edge Router - LER

• Resides at the edge of an MPLS network and
  assigns and removes the labels from the packets.
• Support multiple ports connected to dissimilar
  networks (such as frame relay, ATM, and
  Ethernet).

74
Label Switching Router - LSR

• Is a high speed router in the core on an MPLS
  network.
• ATM switches can be used as LSRs without changing
  their hardware. Label switching is equivalent to
  VP/VC switching.

75
Positions of LERs LSRs
76
Forward Equivalence Class - FEC

• Is a representation of a group of packets that
  share the same requirements for their transport.
• The assignment of a particular packet to a
  particular FEC is done just once (when the packet
  enters the network).

77
Label-Switched Paths - LSPs

• A path is established before the data
  transmission starts.
• A path is a representation of a FEC.

78
LSP Details

• MPLS provides two options to set up an LSP
• hop-by-hop routing
• Each LSR independently selects the next hop for
  a given FEC.
• explicit routing
• Is similar to source routing. The ingress LSR
  specifies the list of nodes through which the
  packet traverses.
• The LSP setup for an FEC is unidirectional. The
  return traffic must take another LSP!

79
MPLS Operation

• The following steps must be taken for a data
  packet to travel through an MPLS domain.
• label creation and distribution
• table creation at each router
• label-switched path creation
• label insertion/table lookup
• packet forwarding

80
Step 1

• Label creation and label distribution
• Before any traffic begins the routers make the
  decision to bind a label to a specific FEC and
  build their tables.
• In LDP, downstream routers initiate the
  distribution of labels and the label/FEC binding.
  
• In addition, traffic-related characteristics and
  MPLS capabilities are negotiated using LDP.
• A reliable and ordered transport protocol should
  be used for the signaling protocol.

81
Step 2

• Table creation
• On receipt of label bindings each LSR creates
  entries in the label information base (LIB).
• The contents of the table will specify the
  mapping between a label and an FEC.
• mapping between the input port and input label
  table to the output port and output label table.
• The entries are updated whenever renegotiation of
  the label bindings occurs.

82
Example of LIB Table
83
MPLS Operation Example
84
Step 3

• Label switched path creation
• The LSPs are created in the reverse direction to
  the creation of entries in the LIBs.

85
MPLS Operation Example
86
Step 4

• Label insertion/table-lookup
• The first router (LER1) uses the LIB table to
  find the next hop and request a label for the
  specific FEC.
• Subsequent routers just use the label to find the
  next hop.

87
MPLS Operation Example
88
Step 5

• Packet forwarding
• When a packet arrives at LER1, it determines the
  FEC of the packet.
• LER1 inserts the label for that FEC, finds the
  next hop the FEC (which is LSR1) and forward the
  packet to LSR1.
• Each subsequent LSR, i.e., LSR2 and LSR3, will
  examine the label in the received packet, replace
  it with the outgoing label and forward it.
• When the packet reaches LER4, it will remove the
  label because the packet is departing from an
  MPLS domain and deliver it to the destination.
• The actual data path followed by the packet is
  indicated by the broken red lines.

89
MPLS Operation Example
90
Advantages of Label Switching

• Simpler packet forwarding paradigm
• IP lookup involves longest-prefix match, which
  requires extensive preprocessing and multiple
  memory access. With label switching, packets are
  forwarded by doing an exact match against a short
  label by looking up the label switching table
• Makes forwarding independent of routing
  architectures. Once Label Switching Paths (LSP)
  are established packet forwarding is always the
  same. Thus new routing schemes can be developed
  without changes in the forwarding logic

91
Advantages of Label Switching (cont.)

• Better forwarding granularity. For current IP
  based routing granularity is destination-based.
  But sometimes more granularity is desired e.g. an
  ISP may want to know from which interface a
  particular packet came from. Label switching
  allows multiple granularities e.g. packets from a
  particular ingress interface can be put into an
  LSP.
• Can be used for traffic engineering

92
References

• Multiprotocol Label Switching Architecture
  RFC 3031
• MPLS Technology and Applications Bruce Davie,
  Yakov Rekhter, Morgan Kaufmann Publishers
• RSVP-TE Extensions to RSVP for LSP Tunnels
  RFC 3209
• LDP Specification RFC 3036