A Two-bit Differentiated Services Architecture

K. Nichols, V. Jacobson, L. Zhang

presented by Wendy Edwards

Outline

• Motivation

• Better quality internet services

• Differentiated services– Overview– How it works– Who gets what?– Discussion

Motivation“If That’s Your Best, Your Best Won’t Do”

Twisted Sister (Heavy Metal Band)

• Internet is currently best-effort.

• We want better service for some applications.

• Telephony, multimedia have problems with delays and variable bandwidth.

Approaches

• Integrated Services

• Differentiated Services

Integrated Service Components

• Reservations

• Admission Control

• Scheduling

• Traffic shaping

Why IntServ Won’t Fly

• Scalability problems

• Per flow state at each router

• Each router has to classify each packet

• On-demand reservations are complex

Differentiated Services

• In this paper, two different levels of service proposed, Premium and Assured

• Recommends we use both, using one bit to represent each.

DiffServ Vs. IntServ

• Goals are same, but mechanisms are different

• Bandwidth, rather than delay, will be key parameter

• Mostly static allocation

DiffServ Goals

• Keep forwarding path simple• Push complexity to edges of network• No assumptions about types of traffic (don’t

assume multicasting)• Allocation policy works with long-term and short-

term provisioning• Dominant Internet model remains best effort (most

of the traffic will be best effort)

DiffServ ArchitectureSLA Between AS1 and AS2

Autonomous System 2

Autonomous System 1

Destination

Interior Router maintains two

queues

Egress Router may have to reshape premium

traffic relative to SLA between systems

Ingress Router – doesn’t need to classify, but does

need to police

First Hop Router – “edge” of network

Premium Services

• Guaranteed peak rate

• Shaped/hard-limited, so no bursts

• Cannot be oversubscribed, so it may be under-provisioned

• Gets priority over all other traffic

• Extra traffic may be dropped

Premium Traffic Flow From End-host to Organization’s ISP

first hoprouter

internalrouter

borderrouter

host

borderrouter

ISP

Company A

Unmarkedpacket flow

Packets in premiumflows have bit set

Premium packet flowrestricted to R bytes/sec

Assured Services

• Better than best-effort, though it’s not clear how much better

• “Expected capacity” usage profile

• Unlikely to be dropped if it stays within profile

• Described by average and burst rates

• Extra traffic is sent as best-effort

First Hop/Edge Router

• Classification (detect A or P bit traffic)

• Marking (if traffic conforms to profile, mark)

• Forwarding – premium queue is forwarded first

First Hop Router Overview

ConfigurationInfo

Classifier

Marker

Marker

When a Packet Arrives at Leaf Router

ClearA & P

bits

Packetclassifier

Marker 1

Marker N

Forwardingengine

Arrivingpacket Best effort

Flow

1Flow

N

Markers: service class (Premium or assured), rate (peak for premium, expected for assured), permissible burst size (may not apply to premium)

Two Queues

There are two queues, one for premium and one for assured and best-traffic. The routers check the P-bit of a packet and assign it to the appropriate queue

P-bitSet?

No

Yes

Best Effort Queue

Premium Queue

Assured Service

• In best-effort queue, no congestion = no problem.

• When there is congestion,

– Assured packets within profile are served first, and best-effort traffic is dropped using RED algorithm.

– When some assured packets are not within profile, uses two-tiered RED mechanism called “RIO” (RED with In or Out).

Markers

• Leaf routers have traffic profiles - they classify packets based on packet header

• If no profile present, pass as best effort

• If profile is for Assured Traffic:

– mark in-profile packets (token is available) with A, forward others unmarked where they will be treated as best effort

• If profile is for Premium Traffic:

– delay out-of -profile packets in queue until token becomes available. If queue overflows, drop packets

Marker Diagram

Wait fortoken

Set P bitPacketinput

Packetoutput

Test iftoken Set A bit

token

No token

Packetinput

Packetoutput

Drop on overflow

RED Overview

• Random Early Detection – a mechanism by which the router can more accurately manage its queue length.

• Each router monitors its own queue length and notify sources of imminent congestion.

• RED does this implicitly by dropping packet earlier than it would have to (before buffer space is exhausted) to encourage source to slow down.

• Two thresholds – first one drops packets with probability p, and second drops packets completely. Uses weighted running queue length because traffic is bursty.

• Since RED drops packets randomly, the probability that RED decides to drop a particular flow’s packets is roughly proportional to the share of bandwidth flow is getting.

RED Drop Probabilities

Time

Maxqueue length

Minthreshold

Maxthreshold

Instantaneous queue length

Forced drop

No drop

Probabilisticearly drop

100%

Initial drop probability

maxp WeightedAverageQueue Lengthminth maxth

RIO – RED With In or Out

• Similar to RED, but with two separate probability curves

• Has two classes, “In” and “Out” (of profile)

• “Out” class has lower minimum threshold, so packets are dropped from this class first

• As avg queue length increases, “in” packets are dropped

• Since best-effort is included in the “Out” class, assured traffic can starve best-effort

RIO Algorithm

For each packet arrivalif it is an In packet

calculate the average In queue size avg_in;calculate the average queue size avg_total;

If it is an In packet.if min_in < avg_in < max_in

calculate probability Pin

with probability Pin, drop this packet;else if max_in < avg_in

drop this packet.If it is an Out packet

if min_out < avg_total < max_outcalculate probability Pout;with probability Pout drop this packet;

else if max_out < avg_totaldrop this packet.

RIO Drop Probability Curve

MaxP

1.0

Minout Minin MaxinMaxout

P(drop)

AvgLen

Router Output Interface for Two-bit Architecture

P-bit set?

If A-bit setincr A_cnt

High-priority Q

Low-priority Q

If A-bit setdecr A_cnt

RIO queuemanagement

Packets out

yes

no

Border Router Input Interface Profile Meters

Arrivingpacket

Is packetmarked?

Tokenavailable?

Tokenavailable?

Clear A-bit

Drop packet

Forwardingengine

A s

et

P set

token

toke

n

Not marked

no

no

TSW- Time Sliding Window

• Alternative to RED as a mechanism for traffic policing

• Two components– Rate estimator smooths out burstiness of TCP traffic

and is sensitive to instantaneous sending rate

– Tagging algorithm tags packets as out once traffic exceeds certain threshold

TSW Design

• Three state variables– Win_length – measured in units of time– Avg_rate – rate estimate of last packet arrival– T_front – time of last packet arrival

• Avg_rate and T_front are updated each time a packet arrives, but Win_length is preconfigured

• TSW contains “decaying” function that forgets history over time

TSW Algorithm

Initially

Win_length = a constant;

Avg_rate = connection’s target rate RT

T_front = 0;Upon each packet arrival

Bytes in TSW = Avg_rate * Win_length;

New_bytes = Bytes_in_TSW + pkt_size;

Avg_rate = New_bytes / (now – T_front + Win_length);

T_front = now;

Static Allocation

• Pre-determined, long-term static allocations

• End-to-end bandwidth guarantees based on series of bilateral agreements

• Automatic aggregation

• Manual configuration

Bandwidth Brokers

• “Who gets to use the bandwidth when?”

• Agents that allocate and control bandwidth shares

• Receives requests from peers (or domain it controls) and responds.

• Only need to have limited trust relationships with peers (rather flowspecs in all routers in an end-to-end path)

• Responsibilities– Parcel out region’s marked traffic

– Manage messages sent across boundaries to adjacent BBs

Example 1: Bandwidth Broker

Statically configured example with BB messages exchanged

Example 2: Bandwidth Broker

End-to-end example with static allocation

Example 3: Bandwidth Broker

End-to-end static allocation with no remaining allocation