Differentiated Services
Tarik Cicic
University of Oslo
December 2001
Overview
•
•
•
•
•
•
Motivation for Differentiated Services
DiffServ model, concept of Class of
Service
Components: ingress/egress nodes etc.
Functionality
Per Hop Behavior (PHB)
A critical view of DiffServ
2
Motivation for DiffServ
• ATM has never become widespread
technology
• IntServ with RSVP does not scale due to
signaling and per-flow state overhead
• a solution providing different network
services for different traffic classes is a
more modest but also more realistic aim
3
1
Simple example
• We need to transport real time data (e.g.
video) and traditional best-effort services in
the same network
• define two service classes
• treat them differently: best-effort can wait,
forward real time data ASAP
4
DiffServ Model
• Define a small number of service classes
• at border routers, mark packets as belonging
to one of the defined classes (using policy
checking, “complex lookups”)
• inside the domain, core routers forward the
packets based on the class they belong to
(“simple lookups” only)
5
Architecture Foundations
• Edge routers:
Multi-byte
classifier
Policer
Packet
Marker
Queue manager/scheduler
• Core routers:
DS-byte
classifier
Queue manager/scheduler
• Edge routers are ingress or egress, depending on the
flow direction
6
2
Code Point
• Packet marker in ingress routers assigns a
Differentiated Services Code Point (DSCP)
to each packet
• DSCP is used to classify the traffic in core
routers
• IPv4 DSCP: 8-bit TOS field:
0
1
2
3
4
5
DSCP
6
7
CU
7
DiffServ Network Example
• 1-4 border (edge) routers: examine incoming
packets, mark service class bits in each packet
• 5-7 internal (core) routers: forward packets
according to the class
4
• signalized by
1
5
7
• the DSCP bits
6
3
2
8
DiffServ forwarding
S1
ER1
DiffServ
Cloud
C5
C6
S2
ER4
R2
C7
ER3
R1
ER2
9
3
DiffServ forwarding
€$ | 0000 | S1 | R1
S1
αβ | 0000 | S2 | R2
S2
ER4
DiffServ
Cloud
C5
ER1
R2
C7
C6
R1
ER3
ER2
10
Edge router forwarding
S1 || R1
R1
€$ | 0000 | S1
€$ | 0000
1111 | S1 | R1
Scheduler
Packet marker
Policer
Multibyte classifier
Policies:
S1/R1 Best
S1/*
Good
S5/*
Deny
€$ | 1111 | S1 | R1
ER1
11
DiffServ forwarding
€$ | 1111 | S1 | R1
€$ | 1111 | S1 | R1
S1
ER1
DiffServ
Cloud
C5
ER4
R2
C7
αβ | 1001 | S2 | R2
C6
αβ | 1001 | S2 | R2
S2
ER3
R1
ER2
12
4
Core router forwarding
1111 | S1 | R1
€$ | 1111
€$ | 1111 | S1 | R1
Scheduler
DS-byte classifier
C7
13
DiffServ forwarding
€$ | 1111 | S1 | R1
S1
ER1
DiffServ
Cloud
C5
ER4
C7
C6
αβ | 1001 | S2 | R2
S2
R2
αβ | 1001 | S2 | R2
€$ | 1111 | S1 | R1
ER3
R1
ER2
14
Queuing Schemes (1/4)
• FIFO queuing
– from a Differentiated Services viewpoint FIFO is not
applicable since it does not support quality
differentiation
• Priority queuing
– priority queuing overcomes the shortcomings of FIFOs
by having multiple queues relative various delay classes
– it is straightforward to estimate the latency of the higher
class, but it is difficult to declare much about the delay
of the lower priority classes, because that issue depends
crucially on the traffic load and variations of the higher
class
– difficult to provide delay guarantees
15
5
Queuing Schemes (2/4)
•
•
•
•
Weights for Controlling Delays
Although the incoming packet
stream in an interior node is
largely uncontrollable, the
outgoing of each service class
(queue) can be rigorously
controlled
For example each PHB class i
gets a proportion w(i) of the
whole link capacity when every
queue is non-empty
The challenge is to set perfect
weights - another drawback is
increased problems related to
packet losses
R12
B1
R11
W1
C
R21
R22
B2
W2
Discarded packets
16
Queuing Schemes (3/4)
• Weighted Fair Queuing (WFQ) (per-flow)
• The idea behind WFQ is to compute the time a packet
would need complete a service if a General Process
Sharing (GPS) had been used to to service the packets and
then service the packet according to these finishing times
• GPS is a theoretical scheduling discipline that shares the
bandwidth exactly in proportion to the weight of the
connections
• Three facts make WFQ not appropriate for DiffServ:
– It is difficult to utilize WFQ without per-flow queuing
– WFQ systems need to know the weight of each flow, and if the
requirement changes, a signaling is needed to transmit relevant
information
– WFQ requires quite a hard computational effort
17
Queuing Schemes (4/4)
Target for quality differentiation of CBQ
•
•
•
•
•
•
variation
Class-Based-Queuing (CBQ) Delay
pr node (ms)
“Per-class WFQ”
1
The idea is that a group of users
should not utilize the whole capacity
even though the application they are
using needs high quality
The importance of a packet depends
on the aggregate load level of the
10
class
It is not possible to state that the
packet-loss ratio in Class 1 is always
lower than in Class 2 (without
Class 3
additional traffic controlling
mechanisms)
100
1
Feasible to realize: higher price =>
better quality and more bandwidth
18
Class 1
Class 2
10-4
10-8
Packet loss
ratio
6
SLA/TCA Agreements
•
At “usage” and “network” level:
1. Service-level-agreement (SLA): A contract
that specifies the forwarding service either
between customer and a service provider or
between two service providers
2. Traffic-conditioning agreement (TCA):
Defines the rules used to realize the service,
such as metering, marking, and discarding
(PHB details)
19
Per-Hop Behavior
• DiffServ is more a framework than a detailed
standard
• Per-Hop Behavior (PHB) represents a set of rules
to distinguish and maintain different classes of
service
• a DiffServ domain should have a single treatment
of traffic in each node
• not even PHB standards include implementation
details!
20
PHB class
• A PHB class is a collection of PHBs intended to
be applicable for transmitting packets for one
application
• Technically the service provider is allowed to
remark packets within a class, but not from one
class to another
• An example:
– For handling best-effort traffic one could define a PHB
class consisting of two different PHBs:
• “PHB high importance” with low packet loss ratio
• “PHB low importance” available bitrate
21
7
Expedited Forwarding PHB
• ”Low loss, low latency, low jitter, assured
bandwidth end-to-end service through DS
domain” (RFC2598)
• Departure traffic rate at any transit node must be
less or equal to arrival traffic rate
• Tight traffic control in edge routers!
• ”Premium service” – well suited for multimedia
• Single DSCP (code point):101110 in IPv4
22
Assured Forwarding PHB
• Several (4) independent forwarding classes, with
several (3) drop precedence each
• each of the (four) classes has separate buffer space
and allocated bandwidth
• drop precedence example: MPEG
• otherwise just few constraints: AF presents an
open platform for DiffServ implementation
• DSCP: xxxyy0 (xxx = class, yy = precedence)
23
Typical DS Node Organization
FIFO
EF Handler
Priority
dropping
Priority
Queuing
AF1 Handler
Classifier
AF2 Handler
Scheduler
AF3 Handler
AF4 Handler
Best Effort
FIFO
Weighted
Fair
Queuing
24
8
Analyze of the DiffServ model
• DiffServ does not provide QoS guarantees
• flows belonging to a same CoS will still compete
about the resources
• different CoS can still compete for the resources
(depending on the PHB)
• the network provider can only claim that
“Statistically seen, if the customer sends no more
data than agreed, 99.8% of the packets will be
forwarded.”
25
Admission and policy control
• Both IntServ and DiffServ need to perform the
admission and policy control
• Management issues:
–
–
–
–
–
number of objects (network size, scalability)
distributed or centralized
inter-domain issues, end-to-end
edge-to-edge (intra-domain)
control loop vs. provisioning (adding more resources)
26
RSVP Admission/Policy Control
• RSVP needs help to determine which reservation
requests can be satisfied:
– are the resources sufficient? (admission control)
– does the caller have administrative permission?
(policy control)
• admission control:
– is there sufficient buffer space and processing power to
meet given QoS requirements?
– QoS vector comparison problem
27
9
Management Technologies
• Centralized: Simple Network Management
Protocol (SNMP)
• Policy-based:
– SNMP extensions
– Common Open Policy Service (COPS)
• Active technologies (research area; distributed
systems and network intelligence)
28
Centralized vs. Distributed
Standard
Management by Delegation
Policy Decision Point
ve
n
et w
ork
in g
Network
nodes
Ac
ti
29
COPS
• Policy control: Who should be
allowed to make a reservation?
• payment, criminal record etc.
• COPS (Common Open Policy
Service) protocol
Network node
COPS
PEP
PDP
LDP
IntServ network
30
PEP: Policy Enforcement Point
PDP: Policy Decision Point
LDP: Local Decision Point
10
IntServ vs. DiffServ
Decision
making
Setup
IntServ
DiffServ
Per flow
(individual)
Per class (group)
Per flow
signaling
Administrative
Communicatio Large
n overhead
State in routers Large
Moderate / little
Moderate / no
Yes
No (Best effort
QoS
within the class)
guarantees
No
Replication of E.g. classification
31
functionality
11