Improving QOS in IP Networks

Thus far: “making the best of best effort”Future: next generation Internet with QoS guarantees

RSVP: signaling for resource reservations Differentiated Services: differential guarantees Integrated Services: firm guarantees

simple model for sharing and congestion studies:

Principles for QOS Guarantees

Example: 1MbpsI P phone, FTP share 1.5 Mbps link. bursts of FTP can congest router, cause audio loss want to give priority to audio over FTP

packet marking needed for router to distinguish between different classes; and new router policy to treat packets accordingly

Principle 1

Principles for QOS Guarantees (more) what if applications misbehave (audio sends higher

than declared rate) policing: force source adherence to bandwidth allocations

marking and policing at network edge: similar to ATM UNI (User Network Interface)

provide protection (isolation) for one class from othersPrinciple 2

Principles for QOS Guarantees (more)

Allocating fixed (non-sharable) bandwidth to flow: inefficient use of bandwidth if flows doesn’t use its allocation

While providing isolation, it is desirable to use resources as efficiently as possible

Principle 3

Principles for QOS Guarantees (more)

Basic fact of life: can not support traffic demands beyond link capacity

Call Admission: flow declares its needs, network may block call (e.g., busy signal) if it cannot meet needs

Principle 4

Summary of QoS Principles

Let’s next look at mechanisms for achieving this ….

Scheduling And Policing Mechanisms

scheduling: choose next packet to send on link; allocate link capacity and output queue buffers to each connection (or connections aggregated into classes)

FIFO (first in first out) scheduling: send in order of arrival to queue discard policy: if packet arrives to full queue: who to discard?

• Tail drop: drop arriving packet• priority: drop/remove on priority basis• random: drop/remove randomly

Need for a Scheduling Discipline

Why do we need a non-trivial scheduling discipline?

Per-connection delay, bandwidth, and loss are determined by the scheduling discipline The NE can allocate different mean delays to

different connections by its choice of service order it can allocate different bandwidths to connections

by serving at least a certain number of packets from a particular connection in a given time interval

Finally, it can allocate different loss rates to connections by giving them more or fewer buffers

FIFO Scheduling

Disadvantage with strict FIFO scheduling is that the scheduler cannot differentiate among connections -- it cannot explicitly allocate some connections lower mean delays than others

A more sophisticated scheduling discipline can achieve this objective (but at a cost)

The conservation law “the sum of the mean queueing delays received by

the set of multiplexed connections, weighted by their fair share of the link’s load, is independent of the scheduling discipline”

Requirements A scheduling discipline must satisfy four

requirements: Ease of implementation -- pick a packet every few

microsecs; a scheduler that takes O(1) and not O(N) time Fairness and Protection (for best-effort connections) --

FIFO does not offer any protection because a misbehaving connection can increase the mean delay of all other connections. Round-robin scheduling?

Performance bounds -- deterministic or statistical; common performance parameters: bandwidth, delay (worst-case, average), delay-jitter, loss

Ease and efficiency of admission control -- to decide given the current set of connections and the descriptor for a new connection, whether it is possible to meet the new connection’s performance bounds without jeopardizing the performance of existing connections

Schedulable Region

Designing a scheduling discipline

Four principal degrees of freedom: the number of priority levels whether each level is work-conserving or non-work-

conserving the degree of aggregation of connections within a

level service order within a level

Each feature comes at some cost for a small LAN switch -- a single priority FCFS

scheduler or at most 2-priority scheduler may be sufficient

for a heavily loaded wide-area public switch with possibly noncooperative users, a more sophisticated scheduling discipline may be required.

Work conserving and non-work conserving disciplines

A work-conserving scheduler is idle only when there is no packet awaiting service

A non-work-conserving scheduler may be idle even if it has packets to serve makes the traffic arriving at downstream switches

more predictable reduces buffer size necessary at output queues and

the delay jitter experienced by a connection allows the switch to send a packet only when the

packet is eligible for example, if the (k+1)th packet on connection A

becomes eligible for service only i seconds after the service of the kth packet, the downstream swicth receives packets on A no faster than one every i secs.

Eligibility times

By choosing eligibility times carefully, the output from a switch can be mode more predictable (so that bursts won’t build up in the n/w)

Two approaches: rate-jitter and delay-jitter rate-jitter: peak rate guarantee for a connection

E(1) = A(1); E(k+1) = max(E(k) + Xmin, A(k+1)) where Xmin is the time taken to serve a fixed-sized packet at peak rate)

delay-jitter: at every switch, the input arrival pattern is fully reconstructed E(0,k) = A (0,k); E(i+1, k) = E(i,k) + D + L where D is

the delay bound at the previous switch and L is the largest possible delay on the link between switch i and i+1

Pros and Cons

Reduces delay jitter: Con -- we can remove jitter at endpoints with an elasticity buffer; Pro--reduces buffers(expensive) at the switches

Increases mean delay, problem?: pro--for playback applications, which delay packets until the delay-jitter bound, increasing mean delay does not affect the perceived performance

Wasted bandwidth, problem?: pro--It can serve best-effort packets when there are no eligible packets to serve

Needs accurate source descriptors -- no rebuttal from the non-work conserving camp

Priority Scheduling

transmit highest priority queued packet multiple classes, with different priorities

class may depend on marking or other header info, e.g. IP source/dest, port numbers, etc..

Priority Scheduling

The scheduler serves a packet from priority level k only if there are no packets awaiting service in levels k+1, k+2, …, n

at least 3 levels of priority in an integrated services network?

Starvation? Appropriate admission control and policing to restrict service rates from all but the lowest priority level

Simple implementation

Round Robin Scheduling multiple classes cyclically scan class queues, serving one from each class (if available) provides protection against misbehaving sources (also guarantees a minimum bandwidth to every connection)

Max-Min Fair Share

Fair Resource allocation to best-effort connections?

Fair share allocates a user with a “small” demand what it wants, and evenly distributes unused resources to the “big” users.

Maximize the minimum share of a source whose demand is not fully satisfied. Resources are allocated in order of increasing demand no source gets a resource share larger than its demand sources with unsatisfied demand s get an equal share

of resource

A Generalized Processor Sharing (GPS) server will implement max-min fair share

Weighted Fair Queueing

generalized Round Robin (offers differential service to each connection/class)

each class gets weighted amount of service in each cycle

Policing Mechanisms

Goal: limit traffic to not exceed declared parameters

Three common-used criteria: (Long term) Average Rate: how many pkts can be sent per unit time

(in the long run) crucial question: what is the interval length: 100 packets per sec or 6000

packets per min have same average!

Peak Rate: e.g., 6000 pkts per min. (ppm) avg.; 1500 ppm peak rate (Max.) Burst Size: max. number of pkts sent consecutively (with no

intervening idle)

Traffic Regulators

Leaky bucket controllers Token bucket controllers

Policing Mechanisms

Token Bucket: limit input to specified Burst Size and Average Rate.

bucket can hold b tokens tokens generated at rate r token/sec unless

bucket full over interval of length t: number of packets

admitted less than or equal to (r t + b).

Policing Mechanisms (more)

token bucket, WFQ combine to provide guaranteed upper bound on delay, i.e., QoS guarantee!

WFQ

token rate, r

bucket size, b

per-flowrate, R

D = b/Rmax

arrivingtraffic

IETF Integrated Services

architecture for providing QOS guarantees in IP networks for individual application sessions

resource reservation: routers maintain state info (a la VC) of allocated resources, QoS req’s

admit/deny new call setup requests:

Question: can newly arriving flow be admitted with performance guarantees while not violating QoS guarantees made to already admitted flows?

Intserv: QoS guarantee scenario

Resource reservation call setup, signaling (RSVP) traffic, QoS declaration per-element admission control

QoS-sensitive scheduling (e.g.,

WFQ)

request/reply

RSVP

Multipoint Multicast connections Soft state Receiver initiated reservations identifies a session using a combination of

dest. Address, transport-layer protocol type, dest. Port number

each RSVP operation applies only to packets of a particular session

not a routing protocol; merely used to reserve resources along the existing route set up by whichever underlying routing protocol

RSVP Messages

Path message originating from the traffic sender to install reverse routing state in each router along

the path to provide recceivers with information about the

characteristics of the sender traffic and end-to-end path so that they can make appropriate reservation requests

Resv message originating from the receivers to carry reservation requests to the routers along the

distribution tree between receivers and senders

PathTear, ResvTear, ResvErr, PathErr

PATH Message

Phop: the address of the last RSVP-capable node to forward this path message.

Sender template: filter specification identifying the sender

Sender Tspec: defines sender traffic characteristics

Optional Adspec: info about the end-to-end path used by the receivers to compute the level of resources that must be reserved

RESV Message

Rspec: reservation specification comprising the value R of bandwidth to be reserved in each router, and a slack term about end-to-end delay

reservation style, FF, WF, SE Filterspec to identify the senders Flowspec comprising the Rspec and traffic

specification (set equal to Sender Tspec) optional ResvConf

Call Admission

Arriving session must : declare its QOS requirement

R-spec: defines the QOS being requested characterize traffic it will send into network

T-spec: defines traffic characteristics signaling protocol: needed to carry R-spec and T-

spec to routers (where reservation is required) RSVP

Intserv QoS: Service models [rfc2211, rfc 2212]

Guaranteed service: worst case traffic arrival: leaky-

bucket-policed source simple (mathematically

provable) bound on delay [Parekh 1992, Cruz 1988]

Controlled load service: "a quality of service closely

approximating the QoS that same flow would receive from an unloaded network element."

WFQ

token rate, r

bucket size, b

per-flowrate, R

D = b/Rmax

arrivingtraffic

Chapter 6 outline

6.1 Multimedia Networking Applications

6.2 Streaming stored audio and video RTSP

6.3 Real-time, Interactivie Multimedia: Internet Phone Case Study

6.4 Protocols for Real-Time Interactive Applications RTP,RTCP SIP

6.5 Beyond Best Effort 6.6 Scheduling and

Policing Mechanisms 6.7 Integrated

Services 6.8 RSVP 6.9 Differentiated

Services

IETF Differentiated Services

Concerns with Intserv: Scalability: signaling, maintaining per-flow router state difficult with large

number of flows Flexible Service Models: Intserv has only two classes. Also want “qualitative”

service classes “behaves like a wire” relative service distinction: Platinum, Gold, Silver

Diffserv approach: simple functions in network core, relatively complex functions at edge routers

(or hosts) Do’t define define service classes, provide functional components to build

service classes