Transcript best-effort service!

Quality of Service
Frameworks
Hamed Khanmirza
Principles of Network
University of Tehran
What is QoS?
• The capability to control traffic handling mechanisms in
the network such that the network meets the service needs
of certain applications and users subject to network
policies.
• Applications
– Elastic (delay-tolerant)
• Tolerate delays and losses
• Can adapt to congestion
– Non-elastic (Real-Time)
• Needs some kind of guarantee from network
• QoS Parameters
–
–
–
–
Bandwidth
Latency
Jitter
Loss
Utility Curve Shapes
U
Elastic
BW
U
Delay-adaptive
BW
U
Hard real-time
BW
Integrated Services
Service characteristic
• Enhancing IP Service Model
– Add QoS service classes
– Explicit resource management at IP level
– Per flow state maintained at routers which is
• used for admission control and scheduling
• set up by signaling protocol, users explicitly request their
needs.
• This is done with RSVP protocol
Integrated Services Example
• Achieve per-flow bandwidth and delay guarantees
– Example: guarantee 1MBps and < 100 ms delay to a flow
Sender
Path RSVP Message
Receiver
Integrated Services Example
• Allocate resources - perform per-flow admission
control
Sender
RESV RSVP Message
Receiver
Integrated Services Example
• Install per-flow state
Receiver
Sender
Integrated Services Example
• Install per flow state
Sender
RESV RSVP Message
Receiver
Integrated Services Example: Data Path
• Per-flow classification
Receiver
Sender
Integrated Services Example: Data Path
• Per-flow buffer management
Receiver
Sender
Integrated Services Example
• Per-flow scheduling
Receiver
Sender
Service Types
• Multiple service classes
• Service can be viewed as a contract between
network and communication client
– end-to-end service
– other service scopes possible
• Three defined services
– Best-Effort for (best-effort or elastic)
– Guaranteed Service for hard real-time (“Real-Time
applications”)
– Controlled Load for soft real-time (“tolerant”
applications)
Differentiated Services
What is the Problem?
• Goal: providing support for wide variety of applications:
– Interactive TV, IP telephony, on-line gamming (distributed
simulations), VPNs, etc
• Problem:
– Best-effort cannot do it
– Intserv can support all these applications, but
• Too complex
• Not scalable
– Queuing & scheduling
– Classification speed
– Hardware Restriction
• DiffServ aims at providing QoS with simple mechanisms so
that it scales and can be deployed.
– push the complexity to the “edges” of the network.
– Provide weaker guarantee
DiffServ Architecture
• Ingress routers (Edge Routers)
– Perform per aggregate shaping or policing (Behavior Aggregate)
– Mark packets with Code Points, each CP represent a Class of
Service (DSCP DiffServ Code Point)
• Core routers
– Implement Per Hop Behavior (PHB) for each DSCP
– Process packets based on DSCP
DS-2
DS-1
Ingress
Ingress
Egress
Edge router
Core router
Egress
Differentiated Service (DS) Field
0
5 6 7
DS Filed
0
4
Version HLen
8
16
TOS
Identification
TTL
19
31
Length
Flags
Fragment offset
Protocol
Header checksum
Source address
Destination address
IP
header
Data
• DS filed reuse the first 6 bits from the former
Type of Service (TOS) byte
• The other two bits are proposed to be used by
ECN
Per Hop Behavior (PHB)
• Define behavior of individual routers rather than
end-to-end services
• Two PHBs
– Assured Forwarding (AF, A type)
– Expedited Forwarding (EF, P type)
– Plus, best-effort service!
EF PHB (Premium)
• Provides the abstraction of a “virtual pipe”
between an ingress and an egress router
• Network:
– No loss
– low delay & jitter
• User:
– Send traffic based on SLA
– Excess traffic is delayed, and dropped when buffer
overflows
• Signaling, admission control may get more
elaborate in future (DiffServ/RSVP)
Assured Forwarding PHB
• Possible service:
– strong assurance for traffic within profile
– Out-of-profile traffic will be marked as lower class (i.e. BE)
• Network:
– lower loss rate than best-effort
– In case of congestion best-effort packets are dropped first
• User: sends no more assured traffic than its profile
– If it sends more, the excess traffic is converted to best-effort
• IETF defines AF as (RFC 2477)
– 4 classes
– Each with 3 drop precedence
– Order of packets must be preserved
Provisioning & Configuration
• To provide network QoS, some configuration and
provisioning is required
• Provisioning:
– Static and long-term management tasks
• Enhancing network equipment
• Interface definition
• Link speed and BW
• Configuration:
– Dynamic and short-term tasks
• Direct manipulation of traffic handling mechanisms
Service Level Agreement & Policy
• Agreements/service provided within a domain
– Service Level Agreement (SLA) with ISP
• Policy
– A high level description of the quality and efficiency
objectives to be met by the network
– Policy is set by SLA
Example of an SLA
Traffic submitted by customer c1
and marked with DSCP = EF
and destination address in subnet 2.x.x.x
and conforming to profile p1
Will be delivered to egress point B with
latency not exceeding 100ms and a
drop-probability less than 0.1%
Traffic submitted by customer c1
and marked with DSCP = EF
and destination address in subnet 2.x.x.x
and not conforming to profile p1
Will be discarded
Example of an SLA
Traffic submitted by customer c1
and marked with DSCP = EF
and destination address in subnet 3.x.x.x
and conforming to profile p2
Will be delivered to egress point C with
latency not exceeding 100ms and a
drop-probability less than 0.1%
Traffic submitted by customer c1
and marked with DSCP = EF
and destination address in subnet 3.x.x.x
and not conforming to profile p2
Will be discarded
Example of an SLA
Traffic submitted
and marked with
and destination
and destination
Will be discarded
by customer c1
DSCP = EF
address not in subnet 2.x.x.x
address not in subnet 3.x.x.x
Traffic submitted by customer c1
and not marked with DSCP = EF
Will be delivered with best-effort service
P1:
Conforming traffic must not exceed 64kbps over any 5msec
interval
P2:
Conforming traffic must not exceed 128kbps over any
2.5msec interval
Pushed vs. Signaled
Pushed
Configuration
Sniffer Server
monitoring/analysis
Sniffer Server
monitoring/analysis
Sniffer Server
monitoring/analysis
Sniffer Server
monitoring/analysis
Sniffer Server
monitoring/analysis
Sniffer Server
monitoring/analysis
Sniffer Server
monitoring/analysis
Signaled
Configuration
Sniffer Server
monitoring/analysis
Components of Policy System
3) PDP compares
RQ against policies
in data store
IBM Compatible
Policy Data
Store
1) Policy is authored in
directory and is made
available to PDP
4) Admit/Reject
decision returned
by PDP
Functional Layers
Policy Server
(PDP)
No physical
2) PDP pushes config to
devices using SNMP,
COPS, CLI, …
Policy may be updated
time to time
2) Policy info
exchanged with
PDP (using COPS)
1) Signaled RQ
Sniffer Server
monitoring/analysis
Distributed Data Store - Directory
Directory
Data
Data
Directory Server
Policy
Policy
PDP
Server
Server
Server
Router
Router
Router
Router
Router
PEP
Router
Interior Provisioning
45Mbps physical Interface
4.5Mbps allocated to strict
priority queue for EF traffic
SLS
1.5Mbps
EF Traffic to R3
Router
Router
R1
Router
Router
R3
R5
R2
Router
SLS
3Mbps
EF Traffic to R3
Router
Assured Service
• Large spatial granularity service
• Theoretically, user profile is defined irrespective of
destination
• This makes service very useful, but hard to
provision
– Over provision?
Traffic profile
Ingress
Multicast Problems in DiffServ
• Multicast
– Problem
• Dynamic trees
– Solutions
• Different DSCP
• Some determined tree structure
• Remarking and shaping at boundaries
DiffServ Implementations
• Two important proposals
–
–
–
–
RIO Mechanism (1 service)
The Scalable Share Differentiation architecture (SSD)
Two-Bit architecture
RFC (2475)
Two-Bit Architecture
• Proposes three different levels of service:
– Premium Service.
– Assured Service.
– Best Effort Service.
• Two-bit architecture:
– Packets get differentiated by two bits in their header.
– Premium bit (P-bit)
– Assured Service bit (A-bit)
Leaf Router Input Functionality
Marker 1
Marker N
Arriving
packet
Clear
A&P bits
MF
Packet
classifier
Best effort
Forwarding
engine
classify packets based on packet header
Markers in Leaf Routers
Drop on overflow
Packet
input
Wait for
token
Set P bit
Packet
output
No token
Packet
input
Test if
token
token
Set A bit
RIO is applied
here
Packet
output
Red with In or Out (RIO)
• Similar to RED
• With two separate probability curves
– In (of profile)
– Out (of profile)
• “Out” class has
– lower Minthresh, so packets are dropped from this class first
– Based on queue length of all packets
• “In” Class
– As avg queue length increases, “in” packets are also dropped
– Based on queue length of only “in” packets
Dropping
probability
1
OUT
IN
Average queue length
Output Forwarding
• 2 queues:
– High Priority: EF packets
– Lower priority queue implements RED “In or Out”
scheme (RIO)
• Usually scheduling scheme is “Strict Priority”
P bit set?
EF
High-priority Q
Send Packet
AF
If A bit set
incr a_cnt
Low-priority Q
RIO queue
management
If A bit set
decr in_cnt
Intra Domain Behavior
• Each domain is assigned a Bandwidth Broker (BB)
– Usually, used to perform ingress-egress bandwidth
allocation
• BB is responsible to perform admission control in
the entire domain
• BB not easy to implement
– Require complete knowledge about domain
– Single point of failure, may be performance bottleneck
– Designing BB still a research problem
Example
• Achieve end-to-end bandwidth guarantee
3
2
BB
1 9
8 profile
sender
7
BB
6
profile
5
BB
4 profile
receiver
RFC 2475: Overall Architecture
Meter
Classifier
Marker
Classifiers:
1. Multifield Classifier (MF)
2. Behavior Aggregate Classifier (BA)
Shaper/
Dropper
Traffic Conditioning
• Schedulers
– Work-conserving
– Non-work-conserving
• Traffic conditioning uses Non-work-conserving
ones
• Implementations
– Leaky Bucket
– Token Bucket
– Hybrid approaches
• Leaky-Token Bucket
• Dual Token Bucket
Leaky Bucket
• Smoothes traffic and generates constant rate
b bits
r b/s
Token Bucket Filter
• Described by 2 parameters:
– Token rate r: rate of tokens placed in the bucket
– Bucket depth b: capacity of the bucket
• Operation:
–
–
–
–
Tokens are placed in bucket at rate r
If bucket fills, tokens are discarded
Sending a packet of size P uses P tokens
If bucket has P tokens, packet sent at max rate, else
must wait for tokens to accumulate
Token Bucket Operation
Tokens
Tokens
Tokens
Overflow
Packet
Enough tokens 
packet goes through,
tokens removed
Packet
Not enough tokens
 wait for tokens to
accumulate
Token Bucket
• On the long run, rate is limited to r
• On the short run, a burst of size b can be sent
• Token Bucket 3 possible uses
– Shaping
• Delay pkts from entering net (shaping)
– Policing
• Drop pkts that arrive without tokens
– Metering (Marking)
• Let all pkts pass through, mark ones without tokens
Comparison
Best-Effort
Service
• Connectivity
• No isolation
• No guarantees
Diffserv
• Per aggregation
isolation
• Per aggregation
guarantee
Intserv
• Per flow isolation
• Per flow guarantee
• End-to-end
• Domain
• End-to-end
Complexity
• No set-up
• Long term setup
• Per flow setup
Scalability
• Highly scalable
• (nodes maintain
only routing
state)
• Scalable (edge
• Not scalable
routers maintains
(each router
per aggregate
maintains per
state; core routers flow state)
per class state)
Service Scope