ATM Traffic

1
Chapter 13

• ATM Traffic Congestion Control

2
Introduction

• ATM congestion problem overview
• ITU-T and ATM Forum framework for control of
  delay-sensitive traffic
• ATM traffic control mechanisms
• ATM congestion control
• Congestion control schemes for bursty traffic
  (ABR and GFR)

3
Why Typical Tools Are Inadequate for ATM

• Majority of ATM traffic sources are not amenable
  to flow control
• Long-haul ATM ttrans ltlt tprop slow feedback
• Broad range of application types, flow control
  may indiscriminately penalize some
• bandwidth requirements
• traffic patterns
• service requirements
• Very high-speed switching increases volatility
  re control mechanisms

4
Latency/Speed Effects

• Issue rapid insertion rate of ATM cells vs.
  relatively long round-trip delays
• small size (53 bytes) of ATM cell
• small switching delays
• high bandwidth links in ATM networks
• Simplified Example
• 150 Mbps data rate
• tinsert ttrans 2.8 x
  10-6 seconds
• U.S. coast-to-coast roundtrip dprop 48 msec
• Then the number of cells inserted (N) during RTT
  
• 1.7 x 104 cells
  7.2 million bits

53 x 8 bits 150 x 106 bps
48 x 10-3 seconds 2.8 x 10-6 seconds
5
Cell Delay Variation

• General requirement delay should be short
• ATM designed to minimize delay
• For some applications, rate of delivery of cells
  to destination must be constant (ATMs CBR
  service level)
• Contributors to cell delay variation
• network queuing and processing
• variation at UNI (user network interface) due to
  cell processing

6
General Procedure Time Reassembly of CBR Cells
? 1/R inverse of insertion rate
V(0) est. tolerable delay variation
cell arrives late discarded
Slope R cells/sec 1/ ? cell
insertion rate
D(i) end-to-end delay for the ith cell
V(i) V(i-1) ti (ti-1 ?)
7
Origins of Cell Delay Variation at UNI
Interleaving prior to delivery to physical layer
Further delays possible at the physical layer
8
ATM Service Categories

• Constant Bit Rate (CBR)
• fixed data rate required at guaranteed capacity
• Real-Time Variable Bit-Rate (rt-VBR)
• tightly constrained delay and delay variation
• sustained rate guaranteed fast burst rate
• Non-Real-Time Variable Bit-Rate (nrt-VBR)
• no delay variation bound
• Unspecified Bit Rate (UBR)
• best-effort service
• Available Bit Rate (ABR)
• guaranteed minimum capacity, with bursts
• Guaranteed Frame Rate (GFR)
• like UBR/ABR, expressed in terms of frame rate

9
ATM Attributes

• How we describe an ATM traffic flow
• Traffic descriptors
• QoS parameters
• Congestion (for ABR)
• Other (for UBR)

10
Traffic Parameters

• Connection Traffic Descriptor
• Source Traffic Descriptor PCR, SCR, MBS, MCR,
  MFS
• Cell Delay Variation Tolerance (?) upper bound
  on amount of cell delay that is introduced by the
  network interface and the UNI (due to
  interleaving, physical layer overhead,
  multiplexing, etc.)
• Conformance Definition unambiguous specification
  of conforming cells of a connection at the UNI
  (see GCRA, later)

11
Traffic Parameters

• Source Traffic Descriptor
• Peak Cell Rate (PCR) upper bound on traffic
  submitted by source (PCR 1/T, where T minimum
  cell spacing
• Sustainable Cell Rate (SCR) upper bound on
  average rate of traffic submitted by source (over
  a larger T)
• Maximum Burst Size (MBS) maximum number of cells
  sent continuously at PCR
• Minimum Cell Rate (MCR) used with ABR and GFR
  minimum cell rate requested, access to unused
  capacity up to PCR (elastic capacity PCR-MCR?)
• Maximum Frame Size (MFS) maximum size of a frame
  in cells available for GFR service

12
QoS Parameters (as defined by the ATM Forum)

• Peak-to-peak cell delay variation (CDV)
  acceptable delay variation at destination
• Maximum Cell Transfer Delay (maxCTD) maximum
  time between transmission of first bit of a cell
  at the source UNI to receipt of its last bit at
  the destination UNI
• Cell Loss Ratio ratio of lost cells to total
  transmitted cells on a connection

13
Cell Transfer Delay Probability Density
(real-time services)
Variable component of delay, due to buffering
and cell scheduling.
14
Congestion Control and Other Attributes

• Congestion Control
• defined only for ABR service category
• uses network feedback controls
• ABR flow control mechanism (more later)
• Other Attributes (introduced July 2000)
• Behavior class selector (BCS)
• for IP differentiated services (DiffServ)
• provides for different levels of service among
  UBR connections
• implementation dependent, no guidance in specs
• Minimum desired cell rate (MDCR)
• UBR application minimum capacity objective

15
Service Category-Attribute Relationship
16
Traffic Congestion Control Function
Classification
affect more than one connection, effective over
long timeframe
determine if/how network can accommodate
connection at a given QoS
network responds within the round-trip lifetime
of a cell
react immediately to a cell as it is transmitted
17
Chapter 13 Section 2

• ATM Traffic Control (continued)

18
Resource Management Using Virtual Paths

• Multiple VCCs with different QoS requirements in
  same VPC
• Cases to consider
• User-to-User application VPC between pair of
  UNIs, QoS is users responsibility
• User-to-network application VPC between UNI and
  network node, QoS is networks responsibility
• Network-to-network application, QoS is networks
  responsibility

19
Example VCC/VPC Configuration

• How to allocate capacity to each VPC?
• Aggregate peak demand
• Statistical multiplexing

20
Connection Admission Control (CAC)

• Network accepts the connection only if it can
  commit resources that satisfy a given connection
  request
• Service category (CBR, VBR, )
• Connection traffic descriptor (PCR, CDVT,
  conformance definition,.)
• QoS (peak-to-peak CDV, CLR,)
• Cell loss priority (CLP bit 0 or 01)
• If accepted, a traffic contract is awarded to
  the user

21
Procedures Used to Set Traffic Contract Parameters
22
Usage Parameter Control

• Purpose after connection is established, protect
  the network resources from overload/abuse by
  violating connections
• Monitors connection for conformance to the
  traffic contract
• detect violation of assigned parameters
• take appropriate action

23
Usage Parameter Control Function Location
24
Important Announcements

• Project 1 due Thursday, 10/16/03.
• Project 2 will be posted by Thursday, 10/23/03.
• Mid term exam is this Tuesday, 10/21/03, 330pm
  450pm.
• Exam will be OPEN BOOK (no notes)
• mathematical calculator, pen/pencil and Stallings
  textbook only.
• MID-TERM EXAM WILL BE IN ROOM LS119

25
UPC Traffic Management

• Peak Cell Rate Algorithm
• Regulates the peak cell rate and the associated
  CDVT of a connection
• Sustainable Cell Rate Algorithm
• Regulates the sustainable cell rate and
  associated burst tolerance of a connection
• Traffic Shaping
• Smoothes out traffic to reduce clumping
• Reduce delays, ensure fair resource allocation

26
Generic Cell Rate Algorithm (GCRA) Virtual
Scheduling
GCRA (I, L) I Increment L Limit ta(k) Time
of arrival of a cell TAT Theoretical arrival
time At time of arrival ta(1) of the first cell
of connection, TAT ta(1)
Late arrival OK
Early arrival TEST
Early arrival beyond limit NOT OK

• Algorithm takes two arguments, I and L
• With PCR of R, I T 1/R
• CDVT limit, ? L
• Then peak cell rate algorithm is expressed as
• GCRA(T, ?)

I.e., this cell arrived too late.
Early arrival within limit OK
Reset TAT
27
Leaky Bucket Algorithm
28
GCRA Continuous-State Leaky Bucket

• Equivalent to virtual scheduling
• Max Bucket capacity is T?
• Counter X is incremented by T for each compliant
  cell
• Bucket decrements (drains at 1 unit/unit time

GCRA (I, L) I Increment L Limit ta(k) Time
of arrival of a cell X Value of leaky bucket
counter X? Auxiliary variable LCT Last
compliance time At the time of arrival ta(1) of
the first cell, X 0 and LCT ta(1)
29
Depiction of GCRA
30
Cell Arrival at UNI (T4.5?)
Ideal (? 0.5?)
Possible (? 1.5?)
Possible (? 3.5?) Note ? T - ? N ?1
(? / T-?)?
Possible (? 7?) Note ? gt T - ? N ?1 (?
/ T-?)?
31
Sustainable Cell Rate Algorithm

• Uses GCRA (Ts, ?s), where
• Ts 1/Rs is the interarrival time at the
  sustainable cell rate, Rs, and
• ?s is the burst tolerance, or the time scale
  during which cell rate fluctuations are allowed
• ?s is derived from the burstiness of the traffic
  stream
• Burst Tolerance ?s (MBS-1) -

32
Sustainable Cell Rate Algorithm

• Note that, if the traffic stream is constrained
  by both GCRA (T, ?) and GCRA (Ts, ?s) , then
  Maximum Burst Size (MBS) is
• ?s is derived from the burstiness of the traffic
  stream
• Burst Tolerance ?s (MBS-1) -

?s Ts-T
MBS 1
33
UPC Function Possible Actions based on CLP bit
Forward cell or discard it? (see p. 377)
P? Compliance test for parameter P
34
Token Bucket for Traffic Shaping
Tokens are generated and fill the bucket at the
constant rate, ?.
To pass, a token is removed from the bucket. If
bucket is empty, cell is queued to wait for next
token.
Departures rate is smoothed to ?.
35
ABR Traffic Management

• CBR, rt-VBR, nrt-VBR traffic contract with
  open-loop control
• UBR best effort sharing of unused capacity
• ABR share unused (available) capacity using
  closed-loop control of source
• Allowed Cell Rate (ACR) current max. cell
  transmission rate
• Minimum Cell Rate (MCR) network guaranteed
  minimum cell rate
• Peak Cell Rate (PCR) max. value for ACR
• Initial Cell Rate (ICR) initial value of ACR

36
ABR Traffic Management

• ACR is dynamically adjusted based on feedback to
  the source in the form of Resource Management
  (RM) cells
• RM cells contain three fields
• Congestion Indication (CI) bit
• No Increase (NI) bit
• Explicit Cell Rate (ER) field

37
ABR Source Reaction Rules
RIF Fixed rate increase factor (default
1/16) RDF Fixed rate decrease factor (default
1/16)
38
Variations in Allowed Cell Rate
39
Flow of Data and RM Cells ABR Connection
Nrm parameter usually set to 32
FRM cell flow ?
? BRM Cell flow
40
ABR RM Cell Format
41
Initial Values of Cell Fields
42
ABR Parameters
43
ABR Capacity Allocation

• Two Functions of ATM Switches
• Congestion Control throttle back on rates based
  on buffer dynamics
• Fairness throttle back as required to ensure
  fair allocation of available capacity between
  connections
• Two categories of switch algorithms
• Binary EFCI, CI and NI bits
• Explicit rate use of the ER field

44
Binary Feedback Schemes

• Single FIFO queue at each output port buffer
• switch issues EFCI, CI, NI based on threshold(s)
  in each queue
• Multiple queues per port - separate queue for
  each VC, or group of VCs
• uses threshold levels as above
• Use selective feedback to dynamically allocate
  fair share of capacity
• switch will mark cells that exceed their fair
  share of buffer capacity

45
Explicit Rate Feedback Schemes

• Basic scheme at switch is
• compute fair share of capacity for each VC
• determine the current load or degree of
  congestion
• compute an explicit rate (ER) for each VC and
  send to the source in an RM cell
• Several example of this scheme
• Enhanced proportional rate control algorithm
  (EPRCA)
• Explicit rate indication for congestion avoidance
  (ERICA)
• Congestion Avoidance using proportional control
  (CAPC)

46
EPRCA

• Switch calculates mean current load at each
  connection, or the MACR
• MACR(I) (1-?) x MACR(I-1) ? x CCR(I)
• Note typical value for ? is 1/16
• When queue length at an output port exceeds the
  established threshold, update ER field in RMs for
  all VCs on that port as
• ER ? minER, DPF x MCR
• where DPF is the down pressure factor parameter,
  typically set to 7/8.
• Effect lowers ERs of VCs that are consuming
  more than fair share of switch capacity

47
ERICA

• Makes adjustments to ER based on switch load
  factor
• Load Factor (LF) Input rate /Target rate
• where input rate is averaged over a fixed
  interval, target rate is typically 85-90 of link
  bandwidth
• When LF gt 1, congestion is threatened, and ERs
  are reduced by VC on a fair share basis
• Fairshare target rate/number of VCs
• Current VCshare CCR/LF
• newER minoldER, maxFairshare, VCshare

48
GFR Traffic Management

• Simple, like UBR
• no policing or shaping of traffic at end-system
• no guaranteed frame delivery
• depends on higher level protocols (like TCP) for
  reliable data transfer mechanisms
• Like ABR, provides capacity reservation and
  traffic contract for QoS
• guaranteed minimum rate without loss
• Specify PCR, MCR, MBS, MFS, CDVT
• Requires that network recognize frames as well as
  cells
• in congestion, network discards whole frames, not
  just individual cells

49
GFR Mechanism
Manage network resources to ensure fairness and
avoid congestion
Tagging and policing per traffic contract
50
Frame-Based GCRA (F-GCRA)