QoS Protocols & Architectures

by

Harizakis Costas

Presentation Flow

QoS defined QoS protocols :

– RSVP, DiffServ, MPLS, SBM

QoS architectures QoS and multicast environments Protocol comparison … … conclusions !

IP-based Networks - Internet Today

Internet today– Provides “best effort” data delivery– Complexity stays in the end-hosts– Network core remains simple– As demands exceeds capacity, service degrades

gracefully (increased jitter etc.)

Delivery delays cause problems to real-time applications

QoS Defined

The goal :Provide some level of predictability and control beyond the current IP “best-effort” service

Fundamental principleLeave complexity at the “edges” and keep network “core” simple

QoS Metrics

Performance attributes– Service availability– Delay– Delay variation (jitter)– Throughput– Packet loss rate

Vary according to Service Level Agreement (SLA)

Service Level Agreements (SLA)

QUALITY OF SERVICE PARAMETERS

Service Level Application Priority Mapping

1 Non-critical data Similar to Internet today No minimum information rate

guaranteed

Best-effort delivery Unmanaged performance

2 Mission-critical data VPN outsourcing, e-

commerce Similar to ATM VBR

Low loss rate Controlled delay and delay

variation

3 Real time applications Video streaming, voice,

videoconferencing

Low loss rate Low delay and delay variation

QoS Protocol Classification

QoS can be achieved by :– Resource reservation (integrated services)– Prioritization (differentiated services)

QoS can be applied :– Per flow (individual, uni-directional streams)– Per aggregate (two or more flows having something

in common)

QoS Protocols

ReSerVation Protocol (RSVP)

Differentiated Services (DiffServ)

Multi Protocol Labeling Switching (MPLS)

Subnet Bandwidth Management (SBM)

RSVP - Resource Reservation

Attributes– The most complex of all QoS technologies– Closest thing to circuit emulation on IP networks– The biggest departure from “best-effort” IP service

Provides the highest level of QoS in terms of : – Service guarantees– Granularity of resource allocation– Detail of feedback to QoS-enabled applications

RSVP - Integrated Services

Enables integrated services (IntServ)

IntServ types– Guaranteed : as close as possible to a dedicated

virtual circuit– Controlled Load : equivalent to best-effort service

under unloaded conditions

RSVP - Implementation

Host A Host B

PATH RESV

Traffic

Specification

QoS LevelandFilter Specification

RSVP - Implementation

Sender– PATH message containing

traffic specification (bitrate, peak rate etc.)

Receiver– RECV message containing

the reservation specification (guaranteed or controlled) the filter specification (type of packets that the reservation

is made for)

RSVP - Queuing

IntServ uses a token-bucket model to characterize I/O queuing

Token bucket attributes– Token rate– Token bucket depth– Peak rate– Minimum policed size– Maximum packet size

RSVP - Conclusions

Reservations are “soft” – Periodic refresh is necessary

It is a network (control) protocol – Works in parallel to TCP and UDP

APIs are required to specify flow requirements Reservations are receiver-based Has to maintain a state for each flow Multicast reservations

– Merged at replication points, difficult to understood algorithms have to be used though

DiffServ- Prioritization

Description– Applied on flow aggregates– Services requirements are classified – Classification is performed at network ingress points– A predefined per-hop behavior (PHB) is applied to

every service class– Traffic is smoothed according to PHB applied

DiffServ- Traffic Classes

Two traffic classes are available : – Expeditied Forwarding (EF) - 1 codepoint

Minimizes delay and jitter Provides the highest QoS Traffic that exceeds the traffic profile is discarded

– Assured Forwarding (AF) - 12 codepoints 4 classes, 3 drop-precedences within each class Traffic that exceeds the traffic profile is not delivered with

such high probability

DiffServ- Implementation

Classifier

MeterMarker

Conditioner

Maps DSCPs toPHBs

MaintainsDSCP

mappings andassociations

with localpolicies

Accumulatesstatistics

Applies thedefined PHB(scheduling)

DiffServ- Implementation

Type of ServicePrecedence

RFC 1122Must

BeZero

IP Type of Service (TOS)

0 32 4 5 6 71

MBZ

RFC 1349

DiffServ codepoints (DSCPs) redefine the Type-of-Service (ToS) IPv4 field

Precedence bits are preserved Type-of-Service bits are NOT

DSCP

Class Selector Unused

0 32 4 5 6 71

Differenciated ServicesCodepoint (DSCP)

CU

DiffServ- Conclusions

Traffic classes are equivalent to IP precedence service descriptors

– DiffServ unaware routers pass-through DiffServ traffic

Easy to be implemented / integrated even into the network core.

Proper classification can lead to efficient resource allocation and though improved QoS

MPLS - Label Switching

Used to establish fixed bandwidth routes (similar to ATM virtual circuits)

Resides only on routers and is protocol independent Traffic is marked at ingress and unmarked at egress

boundaries Markings are used to determine next router hop (not

priority)

The aim is to simplify the routing process …

MPLS - Implementation

Label Value Exp . S TTL

8320 1

20-bits : Label value used for lookup 8-bits : Time-To-Live

1-bit : Bottom of Label Stack

3-bits : Reserved

The 1st hop router, using the header information (destination address etc.) attaches a label and forwards the packet

Every MPLS-enabled router uses the label as an index to a table defining the next hop and label

MPLS - Conclusions

Labels can be “stacked” – This allows MPLS “routes within routes”

Label Distribution Protocol (LDP)– Distributes labels across MPLS-enabled routers– Ensures they agree on the meaning of labels– Usually transparent to network managers

Implication : – Define a policy management that distributes labels

SBM - Subnet Bandwidth Management

A top-to-bottom QoS approach Applies to the Data Link Layer (OSI layer 2) Makes LAN topologies (e.g. Ethernet) QoS-

enabled Fundamental requirement

– All traffic must pass through at least one SBM-enabled switch

SBM - Implementation

SBM Modules– Bandwidth Allocator (BA)

Hosted on switches Performs admission control

– Requestor Module (RM) Resides in every end-station Maps Layer 2 priority levels and the higher-layer QoS

protocol parameters

SBM - Conclusions

Much like the RSVP protocol Makes the traditional Ethernet, QoS aware Introduces an additional indirection in the

routing mechanism 8-level priority value

QoS Architectures

Application

Physical

Data Link

Network

Transport

Session

Presentation

Top-

to-B

otto

m Q

oS

Host A

Application

Physical

Data Link

Network

Transport

Session

Presentation

Host B

RSVP

DiffServ

SBM

QoS-enabledApplication

QoS API

SBM

RSVP RSVPDiffServ and MPLS

End-to-End QoS

Protocol Comparison

QoS Net App Description

most x Provisioned resources end-to-end (leased lines)

x x RSVP Guaranteed (provides feedback to application)

x x RSVP Controlled Load (provides feedback to application)

x MPLS (Multi-Protocol Label Switching)

x x DiffServ applied at network ingress appropriate to RSVP service level for that flow

x x DiffServ or SBM applied on per-flow basis by source application

x DiffServ applied at network core ingress

x Fair queuing applied by network elements (e.g. WFQ, RED)

least Best effort service

Multicast Environments

RSVP – Heterogeneous receivership makes reservation merging a

difficult task

DiffServ– Its relative simplicity makes it a better fit for multicast support

MPLS– Work is underway, no standards have emerged yet

SBM – Explicit support for multicast

Conclusions

Complexity at the edges – simple network core– Limit RSVP’s use on the backbone– Instead use the DiffServ

DiffServ is a perfect complement for RSVP

ToDo :– Performance attributes for each class still missing– Interworking solution for mapping IP CoS to ATM QoS

References

http://www.nortelnetworks.com/solutions/collateral/qos_wp.pdf

http://www.qosforum.com/white-papers/qosprot_v3.pdf

http://www.qosforum.com/white-papers/Need_for_QoS-v4.pdf