Transcript Differential Services - Riga Technical University

Differentiated Services (DiffServ)
Definition of DiffServ Per Domain
Behaviors and Rules for their
Specification
Papadimitriou Panagiotis
21/01/2004
1
Why DiffServ?

Internet currently provides “Best Effort” service
(inadequate, Quality of Service not provided).

The IETF defined models, IntServ and DiffServ, are
simply two ways of considering the fundamental
problem of providing QoS for a given IP packet.

DiffServ mainly aims for seamless end-to-end QoS
deployment with complete management,
provisioning, and signaling support.
2
DiffServ at a glance

There is a clear need for relatively simple and
coarse methods of providing differentiated classes
of service for Internet traffic to support various types
of applications and specific business requirements.

The differentiated services framework enables
quality-of-service provisioning within a network
domain by applying rules at the edges to create
traffic aggregates and coupling each of these with a
specific forwarding path treatment in the domain
through use of a codepoint in the IP header.
3
DiffServ at a glance (2)
Differentiated Services allows an
approach to IP QoS which:




is modular
is increamentally deployable
is scalable
introduces minimal per-node complexity
4
Definitions

Differentiated Services Domain (DS Domain):
a contiguous portion of the Internet over which a consistent set of
differentiated services policies are administered in a coordinated
fashion

Differential Services Boundary:
the edge of a DS domain, where classifiers and traffic conditioners
are likely to be deployed

Per Domain Behaviour (PDB):
the expected treatment that an identifiable or target group of packets
will receive from “edge to edge” of a DS domain

Per Hop Behaviour (PHB):
defines the service at each hop; may be relative (compared to other
PHBs) or absolute (in bandwidth or delay terms)
5
Definitions (2)

Traffic Aggregate:
collection of packets with a codepoint (DSCP) that maps to the same
PHB, usually in a DS domain or some subset of a DS domain

Behaviour Aggregate (BA):
a collection of packets with the same codepoint (DSCP) crossing a
link in a particular direction

Service Level Specification (SLS):
a set of parameters and their values which together define the
service offered to traffic stream by a DS domain

Autonomous System (AS):
an independantly administered domain of the Internet
6
DiffServ Code Point Field

Packets are first divided into classes by marking the type of
service (ToS) byte in the IP header.

A 6-bit bit-pattern (called the Differentiated Services Code
Point [DSCP]) in the IPv4 ToS Octet or the IPv6 Traffic Class
Octet is used as shown below:
7
DiffServ Architecture

A DS-Domain is made up of
DS Ingress nodes,
DS Interior nodes (in the core), and
DS Egress nodes.

Functionally all DS Ingress and Egress nodes can be
categorized as a Boundary node, since they act as a
demarcation point between the DS-Domain and the non-DSaware network.
8
DiffServ Architecture (2)
AS2
AS1
Y
X
X
AS3
Y
AS4


X
Y
X
AS5
The letters X and Y represent the DS boundary routers
DS boundary routers contain traffic conditioners that ensure
and enforce perfomance (e.g., shapers and policers)
9
DiffServ Traffic Conditioner Block




Classifier: selects a packet in a traffic stream based on the
content of some portion of the packet header
Meter: checks compliance to traffic parameters (e.g., Token
Bucket) and passes results to marker and shaper/dropper to
trigger action for in/out-of-profile packets
Marker: writes/rewrites the DSCP value
Shaper: delay some packets for them to be compliant with the
10
profile
DiffServ Traffic Conditioner Block (2)
A traffic conditioner typically:




classifies the incoming packets into pre-defined
aggregates
meters them to determine compliance to traffic
parameters (and determines if the packet is in
profile, or out of profile),
marks them appropriately by writing/re-writing the
DSCP,
and finally shapes (buffers to achieve a target flow
rate) or drops the packet in case of congestion.
11
Defining PDBs




Each PDB has measurable, quantifiable attributes
that can be used to describe what happens to its
packets as they enter and cross the DS domain.
PDB attributes may be absolute or statistical and
they may be parameterized by network properties.
A PDB is applied to a target group of packets
arriving at the edge of the DS domain.
The target group is distinguished from all arriving
packets by use of packet classifiers.
12
Defining PDBs (2)
The action of the PDB on the target group has 2 parts:


The first part is the the use of traffic conditioning to
create a traffic aggregate. During traffic
conditioning, conformant packets are marked with a
DSCP for the PHB associated with the PDB.
The second part is the treatment experienced by
packets from the same traffic aggregate transiting
the interior of a DS domain, between and inside of
DS domain boundaries.
13
The effects of traffic conditioning on
the target group



This effect is quantified by the relationship of the
emerging traffic aggregate to the entering target
group.
This relationship can depend on the arriving traffic
pattern as well the configuration of the traffic
conditioners.
There may be a “loss rate” on the arriving target
group that results from sending too much traffic or
the traffic with the wrong temporal characteristics.
14
Crossing the DS Domain

DSCPs should not change in the interior of DS
domain as there is no traffic conditioning being
applied.

If it is necessary to reapply the kind of traffic
conditioning that could result in remarking, there
should be a DS domain boundary at that point,
though such an “interior” boundary can have “lighter
weight” rules in its TCA (Traffic Conditioning
Aggrements).
15
Crossing the DS Domain (2)




A PDB operates between N ingress points and M
egress points at the DS domain boundary (N ≥ 1,
M ≥ 1).
Even in the degenerate case where N = M = 1, PDB
attributes are more complex than the definition of
PHB attributes since the concatenation of the
behavior of intermediate nodes affects the former.
A complex case with N > 1, M > 1 involves splits
and merges in the traffic path.
Analytic, simulation and experimental work will be
necessary to understand even the simpler PDBs.
16
Constructing PDBs

A DS domain is configured to meet the network operator's
traffic engineering goals for the domain independently of the
performance goals for a particular flow of a traffic aggregate.

Multiple PDBs may use the same PHB. The specification of a
PDB can contain a list of PHBs and their required
configuration, all of which would result in the same PDB. In
operation, it is expected that a single domain will use a single
PHB to implement a particular PDB, though different domains
may select different PHBs.

Multiple PDBs might use the same PHB in which case the
transit performance of traffic aggregates of these PDBs will be
the same.
17
PDBs using PHB Groups
The use of PHB groups to construct PDBs can be
done in several ways:

A single PHB member of a PHB group might be used to
construct a single PDB.
The traffic conditioning for that PDB and the required
configuration of the particular PHB would be defined in such a
way that there is no dependence or relationship with the
manner in which other PHBs of the group are used.

A single PDB can be constructed using more than one PHB
from the same PHB group.
18
PDBs using PHB Groups (2)

A set of related PDBs might be defined using a PHB group.
This is appropriate when the traffic conditioners that create the
traffic aggregates associated with each PDB have some
relationships and interdependencies such that the traffic
aggregates for these PDBs should be described and
characterized together.
The transit attributes will depend on the PHB associated with
the PDB and will not be the same for all PHBs in the group,
though there may be some parameterized interrelationship
between the attributes of each of these PDBs.
19
Format for Specification of PDBs

1. Applicability Statement
All PDB specs must have an applicability statement that
outlines the intended use of this PDB and the limits to its
use.

2. Technical Specification
In general, rules or guidelines to create the PDB.
The technical specification must list the classification and
traffic conditioning required (if any) and the PHB (or PHBs)
to be used with any additional requirements on their
configuration.
20
Format for Specification of PDBs (2)

3. Attributes
PDB’s attributes tell how it behaves under ideal conditions
if configured in a specified manner
Drop rate, throughput, delay bouns, etc.
Attributes must be explicit, quantifiable and defensible

4. Parameters
Maximum number of hops
Minimum bandwidth
Total number of entry / exit points of the PDB
Minimum buffer size for the PDB at a network node
21
Format for Specification of PDBs (3)

5. Assumptions
PDBs will be usually specified assuming lossless links, no
link failures and relatively stable routing

6. Example uses
A PDB specification must give example uses to motivate
the understanding of ways in which a diffserv network could
make use of the PDB

7. Security Considerations for each PDB
22
On PDB Attributes

Topic for Discussion:
Under what conditions can we join the output of a DS domain
to another under the same traffic conditioning and
expectations?


Although there are many ways in which traffic might be
distributed, creating quantifiable, realizable PDBs that can be
concatenated into multi-domain services limits the realistic
scenarios.
A PDB's attributes with a clear statement of the conditions
under which the attributes hold is critical to the composition of
multi-domain services.
23
On PDB Attributes (2)
There are two ways to characterize PDBs with
respect to time:

Properties over "long" time periods, or average behaviors.
A PDB specification should report these as the rates or
throughput seen over some specified time period.

Properties of “short” time behavior, usually expressed as the
allowable burstiness in a traffic aggregate.
The short time behavior is important in understanding buffering
requirements (and associated loss characteristics) and for
metering and conditioning considerations at DS boundaries
24
Reference Best Effort PDB



We define as a reference a Best Effort PDB, a PDB that has
little in the way of rules or expectations
A Best Effort (BE) PDB is for sending “normal internet traffic”
across a diffserv network
The definition and use of this PDB is to preserve, to a
reasonable extent, the pre-diffserv delivery expectation for
packets in a diffserv network that do not require any special
differentiation
25
A look at IntServ/RSVP




The Intergated Services (IntServ) model relies on the
Resource Reservation Protocol (RSVP) to signal and reserve
the desired QoS for each flow in the network.
Two types of service can be requested via RSVP (assuming
all network devices support RSVP along the path from the source to
the destination):
The first type is a very strict guaranteed service that provides for firm
bounds on end-to-end delay and assured bandwidth for traffic that
conforms to the reserved specifications.
The second type is a controlled load service that provides for a better
than best effort and low delay service under light to moderate
network loads. Thus, it is possible (at least theoretically) to provide
the requisite QoS for every flow in the network, provided it is
signaled using RSVP and the resources are available.
26
Multi-Protocol Label Switching (MPLS)




A standards-based technique used to manage and optimize
traffic flow for large-scale networks.
In an MPLS network, incoming packets are assigned a label
by a label edge router (LER). Label switch routers (LSRs) use
these labels to forward the packets through the network along
a label switch path (LSP). Each LSR removes the existing
label and assigns a new one.
MPLS combines the advantages of bridges (Layer 2 switching,
which is used in ATM and frame relay) and routers (Layer 3
switching, which is used in IP).
MPLS serves to create faster and more scalable networks to
facilitate quality of service, class of service, and the use of
VPNs.
Discuss: Differences between DiffServ and MPLS?
27
DiffServ vs MPLS





Diffserv, as mentioned before, takes the IP TOS (type of
service) field, renames it the DS byte, and uses it to carry
information about IP packet service requirements.
It operates at Layer 3 only and does not deal with lower
layers.
On the other hand, MPLS specifies ways that Layer 3 traffic
can be mapped to connection-oriented Layer 2 transports like
ATM and Frame Relay.
MPLS adds a label containing specific routing information to
each IP packet and allows routers to assign explicit paths to
various classes of traffic.
It also offers traffic engineering and techniques that can boost
IP routing efficiency
28
DiffServ Issues/Problems

Topic for Discussion:
Drawbacks of DiffServ?

Provisioning
Unlike RSVP/IntServ, DiffServ needs to be provisioned.
Setting up the various classes throughout the network requires
knowledge of the applications and traffic statistics for
aggregates of traffic on the network.
This process of application discovery and profiling can be
time-consuming, although there are tools that can make life
easier (e.g. NBAR application discovery, Protocol Analyzers)
29
DiffServ Issues/Problems (2)

Billing and Monitoring
Management is still a big issue. Even though packets/sec,
bytes/sec and many other counters are available via the
class-based Management Information Base (MIB), billing and
monitoring are still difficult issues.

QoS and Routing
One of the biggest drawbacks of DiffServ comes from the fact that
signaling/provisioning happens separate from the routing process.
Thus, there may exist a path (e.g. OSPF, ISIS, EIGRP) in the
network that has the required resources, even when DiffServ fails to
find the resources.
30