A Twobit Differentiated Services Architecture

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A Two-bit Differentiated Services Architecture

• K. Nichols, V. Jacobson, L. Zhang
• presented by Wendy Edwards

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Outline

• Motivation
• Better quality internet services
• Differentiated services
• Overview
• How it works
• Who gets what?
• Discussion

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MotivationIf Thats Your Best, Your Best Wont
DoTwisted 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.

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Approaches

• Integrated Services
• Differentiated Services

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Integrated Service Components

• Reservations
• Admission Control
• Scheduling
• Traffic shaping

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Why IntServ Wont Fly

• Scalability problems
• Per flow state at each router
• Each router has to classify each packet
• On-demand reservations are complex

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Differentiated Services

• In this paper, two different levels of service
  proposed, Premium and Assured
• Recommends we use both, using one bit to
  represent each.

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DiffServ Vs. IntServ

• Goals are same, but mechanisms are different
• Bandwidth, rather than delay, will be key
  parameter
• Mostly static allocation

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DiffServ Goals

• Keep forwarding path simple
• Push complexity to edges of network
• No assumptions about types of traffic (dont
  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)

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DiffServ ArchitectureSLA Between AS1 and AS2
Autonomous System 1
Egress Router may have to reshape premium traffic
relative to SLA between systems
First Hop Router edge of network
Interior Router maintains two queues
Destination
Ingress Router doesnt need to classify, but
does need to police
Autonomous System 2
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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

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Premium Traffic Flow From End-host to
Organizations ISP
Company A
Packets in premium flows have bit set
Premium packet flow restricted to R bytes/sec
internal router
ISP
host
border router
first hop router
border router
Unmarked packet flow
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Assured Services

• Better than best-effort, though its 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

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First Hop/Edge Router

• Classification (detect A or P bit traffic)
• Marking (if traffic conforms to profile, mark)
• Forwarding premium queue is forwarded first

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First Hop Router Overview
Configuration Info
Marker
Classifier
Marker
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When a Packet Arrives at Leaf Router
Marker 1
Flow 1
Marker N
Flow N
Forwarding engine
Clear A P bits
Packet classifier
Arriving packet
Best effort
Markers service class (Premium or assured), rate
(peak for premium, expected for assured),
permissible burst size (may not apply to premium)
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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

Best Effort Queue
No
Yes
Premium Queue
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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).

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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

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Marker Diagram
Drop on overflow
Packet input
Packet output
Wait for token
Set P bit
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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 flows
  packets is roughly proportional to the share of
  bandwidth flow is getting.

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RED Drop Probabilities
Instantaneous queue length
Maxqueue length
Time
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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

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RIO Algorithm

• For each packet arrival
• if 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 lt avg_in lt max_in
• calculate probability Pin
• with probability Pin, drop this packet
• else if max_in lt avg_in
• drop this packet.
• If it is an Out packet
• if min_out lt avg_total lt max_out
• calculate probability Pout
• with probability Pout drop this packet
• else if max_out lt avg_total
• drop this packet.

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RIO Drop Probability Curve
P(drop)
1.0
MaxP
AvgLen
Minout
Minin
Maxin
Maxout
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Router Output Interface for Two-bit Architecture
yes
P-bit set?
High-priority Q
Packets out
no
If A-bit set incr A_cnt
Low-priority Q
If A-bit set decr A_cnt
RIO queue management
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Border Router Input Interface Profile Meters
no
Token available?
Clear A-bit
A set
token
Forwarding engine
Arriving packet
Not marked
Is packet marked?
token
P set
no
Token available?
Drop packet
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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

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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

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TSW Algorithm

• Initially
• Win_length a constant
• Avg_rate connections 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

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Static Allocation

• Pre-determined, long-term static allocations
• End-to-end bandwidth guarantees based on series
  of bilateral agreements
• Automatic aggregation
• Manual configuration

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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 regions marked traffic
• Manage messages sent across boundaries to
  adjacent BBs

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Example 1 Bandwidth Broker
Statically configured example with BB messages
exchanged
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Example 2 Bandwidth Broker
End-to-end example with static allocation
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Example 3 Bandwidth Broker
End-to-end static allocation with no remaining
allocation