lecture 8 - eclass.uoa.gr...lecture 8 traffic engineering - qos atm label switching dimitrios...

Lecture 8

Traffic Engineering - QoS

ATM

Label switching

Dimitrios Klonidis – 14/04/2019

2 Lecture 08 – Traffic Engineering – QoS – ATM – MPLS Intro

Outline

▪ Traffic engineering principles‒ Classification, Scheduling and Polishing

‒ QoS

▪ ATM overview

▪ Label switching ‒ MPLS intro


Traffic engineering principles

QoS, Scheduling, Policing, Classification


Traffic Engineering

▪ Traffic Engineering: A set of processes that aim to put traffic where the network capacity is, or in other words provide a path for carrying data traffic

▪ Target: ‒ Optimize performance …

‒ …for certain traffic demand …

‒ …without affecting existing flows.


Traffic Engineering

▪ Relationship among Capacity, Utilization and QoS (resource reservation).

‒ One can claim that for an underutilized network QoS is not needed.

‒ And if demand exceeds network’s capacity QoS cannot resolve the problem.

➔ Think why?

▪ Currently ISPs do not want to operate beyond 40-50% utilization levels. What if they like to push it to 80-90%?


Quality of Service (QoS)

▪ Quality of Service: ‒ Definition: Provision (by the network) of traffic service guarantees,

such as bandwidth, buffers, delay bounds, jitter, acceptable packet loss

‒ Goal: Ensure fairness among users/traffic streams (fair resource allocation). Use policing, scheduling, priority queueing mechanisms

▪ Applications may have different QoS requirements: real-time applications vs. non-real-time e.g,, IP telephony vs. FTP)

▪ Classify incoming packets, assign them times scheduled for transmission ▪ Discard packets from ill-behaved sources


Applying QoS

▪ Flow and Congestion control are types of QoS‒ Their target is to increase throughput, avoid congestion

• … but not enough themselves to provide true QoS (per service)

‒ Can be done by forming flows (groups) of traffic and treat them separately

• … but applying QoS to each flow can be slow and expensive for routers

▪MPLS has been presented as a solution for traffic engineering in the internet (protocol independent)

▪ SLAs: Service Level Agreements define specific (network) performance metrics (throughput, latency), including service availability and outage notification. Offered by ISPs to their customers [like a customer contract]


IP

Header

TCP

Header

MPLS

LABELTCP Payload

L2

Header

MPLS – The key idea

▪ It is a waste of resources to have to examine every IP packet header (e.g., routing table look-up)

▪ Solution:

➔ create a short, fixed-length identifier: “label” (20 bits). Use the label to identify packets as a unified flow and forward/switch them rapidly.

▪ Characteristics:‒ Uses Layer 2 (e.g., Ethernet, ATM) encapsulation.‒ MPLS applicable to any network layer protocol. It hides the details

(and header) of network layer protocol.‒ Enables integration of IP networks with connection-oriented

networks.


QoS, CoS and Policies

▪ Quality of Service (QoS) classifies network traffic and then ensures that some of it receives special handling.

‒ May track each individual dataflow (sender:receiver) separately.

‒ May include attempts to provide better error rates, lower network transit time (latency), and decreased latency variation (jitter).

▪ Differentiated Class of Service (CoS) is a simpler alternative to QoS.

‒ Doesn't try to distinguish among individual dataflows; instead, uses simpler methods to classify packets into one of a few categories.

‒ All packets within a particular category are then handled in the same way, with the same quality parameters.

▪ Policy-Based Networking provides end-to-end control.‒ The rules for access and for management of network resources are

stored as policies and are managed by a policy server.


•Example: (1Mbps IP phone + data) to share 1.5 Mbps link

−bursts of FTP can congest router, cause audio loss

−want to give priority to audio over data

−Traffic mix (data, IP phone)

Packet marking & classification needed for router to distinguish between different classes; and new router policy to treat packets accordingly

Principle 1

Classification, Scheduling and Polishing


•What if applications misbehave (audio sends higher than declared rate)

−policing: force source adherence to bandwidth allocations

•Marking/classification and policing at network edge:

−similar to ATM UNI (User Network Interface)− Note: If only classification is used providing guaranteed bandwidth to IP phone then if IP phone

exceeds 1Mbps as initially declared, then this bandwidth will be “stolen” from data.

Provide protection (isolation) for one class from others

Principle 2



Allocating fixed (non-sharable) bandwidth to flow: →

but … inefficient use of bandwidth if flows

doesn’t use its allocation

While providing isolation, it is desirable to use resources as efficiently as possible

Principle 3



•Basic fact of life: can not support traffic demands beyond link capacity

•Example: two (competing) IP phone sessions with aggregate

bandwidth request exceeding available capacity

Call Admission: flow declares its needs, network may block call (e.g., busy signal) if it cannot meet needs

Principle 4



Scheduling

▪ Scheduling: ‒ way packets are buffered and scheduled for transmission

(choose next packet to send)

‒ Motivation: • support different classes of service that have heterogeneous

requirements (e.g., bw, delay, loss)• Support for flows with different priorities

‒ Results in better buffer management (memory allocation among competing flows)

‒ Requires flow identification (classification)


Polishing

▪ Policing: ‒ regulate or limit traffic so that it does not exceed declared

parameters.

‒ Can include drastic measures such as packet discard


Resource allocation

▪ Allocation of resources‒ Identify (monitor) the physical and computation resources of a

network and assign the optimum amount to traffic demands according to their service requirements

‒ Requirements:• allow fair share (multiplexing) of resources• should not lead to starvation situations• should not affect existing users, flows or sessions • provide performance bounds (for guaranteed services)• ease of implementation

‒ Not a straightforward process• how much bandwidth is allocated per flow • when packets should be transmitted (choosing eligibility times)• which packets are discarded from the router

‒ Relies on a set of optimization rules and targets • … typically a heuristic approach


Policing Criteria

▪ Three common-used criteria: ‒ (Long term) Average Rate:

• how many pkts can be sent per unit time (in the long run)

• what is the interval length: • E.g. 100 packets per sec or 6000 packets per min have same average!

‒ Peak Rate: • E.g. avg. rate of 6000 pkts per min., but Peak rate of 500 pkts per sec

‒ Maximum burst Size: max. number of packets sent consecutively (with no intervening idle)

▪ ! Recall that Internet traffic is bursty (especially at the network edge)


•FIFO (first in first out) scheduling: send in order of arrival to queue

−real-world example? (bank branch, post office)

−discard policy: if packet arrives to full queue: who to

discard?

tail drop: drop arriving packet

priority: drop/remove on priority basis

random: drop/remove randomly

FiFo scheduling


(Strict) Priority scheduling: transmit highest priority queued packet

•Multiple classes, with different priorities

−class may depend on marking or other header info, e.g. IP source/dest,

port numbers, etc..

−real world example? (airline counter: business and economy class

passenger lines)

Priority scheduling


Round Robin scheduling:

•Multiple classes

•Cyclically scan class queues, serving one from each class (if

available)

•Fair mechanism (→ applied in Waited Fair Queuing)

Round Robin scheduling


QoS implementation models



▪ Best Effort‒ The IP approach → All packets are treated equally

‒ No BW, delay or jitter predictions

‒ Some guaranties through the use of TCP

▪ Integrated Services (IntServ) + RSVP‒ Reservations are made per simplex flow (demand)

‒ Applications request reservations for network resources• granted or denied based on resource availability

‒ Senders specify the resource requirements via a PATH message that is routed to the receiver

‒ Receivers reserve the resources with a RESV message that follows the reverse path



▪ Differentiated Services (DiffServ)‒ The DiffServ Model offers a service that is better than Best-

Effort and more scalable than IntServ

‒ Traffic is first classified • Five forwarding classes are considered

• Process takes part at the edge of a DiffServ network

‒ Forwarding classes are encoded in IP packets • Use of the Differentiated Services Codepoint (DSCP) field of each

packet’s IP header

‒ DiffServ routers apply pre-provisioned Per-Hop Behaviors (PHBs) to packets according to the encoded forwarding class


ATM

A quick technology overview


ATM in a nutshell

▪ ATM : Asynchronous Transfer Mode‒ Layer 2, Virtual Circuit Switching technology‒ End-to-end network technology‒ Use fixed small packets (cells) of 48B payload and 5B header

▪ Key features:‒ Truly supports integrated services and QoS (it was designed that

way)• Able to meet bandwidth demands and stringent delay constraints• Able to provide guaranteed bandwidth and resource allocation.• Support for multiple classes of service• Support for circuit emulation (voice channels)

‒ Ultra high-performance (switching and sophisticated traffic control)

‒ Integration of multiple traffic streams (protocols for x–over–ATM ‒ Efficient packet switching and muxing


Plane Management

Layer Management

Physical Layer

ATM Layer

ATM Adaptation Layer (AAL)

Higher-Layer

Protocols

Control plane

Higher-Layer

Protocols

User plane

The ATM protocol model


The ATM protocol model

End System End SystemSwitch

Voice

Data

Video

AA

AA

LL

P

H

Y

P

H

Y

A

T

M

P

H

Y

A

T

M

P

H

Y

A

T

M

AA

AA

LL

Voice

Data

Video

cells cells

Voice

Data

Video

End System End SystemSwitch

Voice

Data

Video

AA

AA

LL

P

H

Y

P

H

Y

A

T

M

P

H

Y

A

T

M

P

H

Y

A

T

M

AA

AA

LL

Voice

Data

Video

cells cells

Voice

Data

Video

•Adaptation Layer (AAL): Adapts upper layers (e.g., IP) into ATM. Performs data segmentation/reassembly into 48-byte payload. Present only in end-systems

•ATM Layer: Performs cell switching. Adds/ removes 5 byte header to payload

•Physical Layer: Converts to appropriate electrical or optical format


VC

switch

VP

switch

VC

switch

ATM

hostATM

host

VPC

VPI= 0

VCI=25

VPI= 4

VCI=12

VPI= 8

VCI= 2

VPI= 0

VCI=16

VCC

Virtual Paths (VPs) - Virtual Channels (VCs)

•Virtual Channel : a virtual circuit between source and destination (unidirectional)

•Virtual Channel Link (VCL) : specific instance of a VC between two VC connecting points

•Virtual Channel Identifier (VCI): virtual channel identifier, a 16-bit number that uniquely identifies a VCL

•Virtual Channel Connection (VCC) : concatenation of VCLs (unidirectional flow)


•Bundle (thousands of) VCs in (point-to-point) VPs so that all VCs within a VP can benefit from the same management functions

UNIUNI

NNINNI

Virtual Paths (VPs) - Virtual Channels (VCs)


Networking (layered) view


ATM header format

▪ GFC : Generic Flow Control: generally unused

‒ (0 = uncontrolled)

▪ VPI/VCI : used on a per link basis (NNI supports more VPs)

‒ actual number of VCI bits is negotiated between user and network

‒ some VCIs are pre-assigned (e.g., for signalling) -

‒ all cells belonging to same VC have same VCI (max number of VCs within path is 65,636)

▪ PTI : Payload Type Identifier: Indicates type of cell

‒ 1st bit if 0 user cell, if 1 management cell, ‒ 2nd bit indicates whether congestion has been

experienced, 3rd bit ),, Service Data Unit, if set (SDU=1)

‒ 3rd bit denotes the last cell in a multi-cell frame

▪ CLP : Cell Loss Priority‒ used for congestion control (0 = higher priority, 1 =

lower priority - discard first )

▪ HEC : Header Error Control (Check)‒ detects errors in the header only and is able to

correct all single-bit errors (FEC)


IP – ATM comparison


•6 types of messages for establishing and releasing a connection

setupsetup

setup

connect ack

connect

call proceeding

call proceeding

connectconnect

connect ackconnect ack

source switch-1 switch-2 destination

setu

p

setupsetup

setup

connect ack

connect

call proceeding

call proceeding

connectconnect


setupsetup

setup

connect ack

connect

call proceeding

call proceeding

connectconnect


setupsetup

connect ack

connect

call proceeding

call proceeding

connectconnect


source switch-1 switch-2 destination

setu

p

releaserelease

release complete

release complete

release

release complete

rele

ase release

releaserelease complete

release complete

release

release complete

releaserelease

release complete

release complete

release

release complete

rele

ase

Connection set-up


The VP is managed by the network according to each resources and route calculation

Request

Acknowledgement

ATM Connection establishment


•SVC : Switched VC → user-established VC via signaling (dynamically)

ATM

host-B

ATM

host-A

connect to B

Yes/No

UNI signaling NNI signalingconnect to B

Yes/No

set up

connect/reject

Switched VCs


•PVC : Permanent VC → established by the network management (updates VPI/VCI tables)

- provisioned for indefinite use

NMS

ATM

host-B

ATM

host-A

VPI/VCI

VPI/VCI VPI/VCI

VPI/VCI

Permanent VCs


•SPVC : allows PVC- capable end-systems to be supported across an SVC-capable ATM network

PVC PVCSVC

SPVC

UNI UNINNI

ATM network

Soft private VCs


•According to the applications we distinguish into various service (traffic) classes (ATM Forum):

- Constant Bit Rate (CBR)

- Variable Bit Rate (VBR)

- Available Bit Rate (ABR)

- Unspecified Bit Rate (UBR)

•Real-time service classes: CBR, rt-VBR

•Non-real-time service classes: nrt-VBR, UBR and ABR

•Guaranteed services: CBR, rt-VBR, nrt-VBR

•Best Effort: UBR, ABR

ATM service categories


ATM Adaptation Layer (AAL)


ATM service categories

Concepts of CBR/VBR/ABR/UBR


VBR vs. ABR

▪ Both nrt - VBR service and ABR …‒ support variable rate data transmissions and does not preserve

any timing relationships between source and destination.

▪ However…‒ ABR service does not provide any guaranteed bandwidth to the

user. ‒ Rather, the network provides a “best effort” service, in which

feedback (flow control mechanisms) is used to increase the bandwidth available to the user (i.e. the Allowed Cell Rate (ACR)), if the network is not congested and to reduce the bandwidth when there is congestion.

▪ Through such flow control mechanisms, the network can control the amount of traffic that it allows into the network, and minimize cell loss within the network due to congestion.


ATM meets QoS requirements for a diverse range of traffic data

QoS in ATM

▪ QoS monitoring of‒ Cell loss rate‒ Delay‒ Jitter

▪ Traffic based on traffic contract that specifies: ‒ Peak cell rate‒ Average cell rate‒ Burst size

▪ Admission control‒ ATM network determines the set of connections that can be admitted

without QoS violation

▪ Flow control‒ Feedback from ATM network about the congestion levels can be used to

control the traffic from a user


ATM advantages

▪ ATM is scaleable, ‒ i.e., bandwidth can be adapted to meet user requirements

▪ Guaranteed transmission quality‒ Able to match quality demanded by the user

▪ Seamless integration of voice, video, data. ‒ Great for real time services (small cell size = small packetization delay)

▪ Ultra-fast switching‒ Due to synchronous operation

▪ Well standardized,

▪ Enhanced management ‒ (results in cost savings for operators)

▪ Direct compatibility with SONET/SDH backplane‒ (in favor to operators control and management)


Current status

▪ ATM found it main use in metro and less core networks‒ Acted as service differentiation technology for 3-ple play ‒ Direct interface with SONET/SDH in core

▪ It was not fully adopted by the entire community

▪ ATM to the desktop (25 Mbps) never took-off.‒ (Fast) Ethernet prevailed as a much cheaper solution

▪ ATM gradually replaced in metro by MPLS‒ MPLS is a good alternative with lower cost and compatible to access

technology

▪ ATM was somehow successful but replaced by something better.

▪ The ATM characteristics may be repeated in future under a different shape and format!

‒ E.g. Optical Burst Switching OR Optical Packet Switching


Label switching

Combining the benefits from IP and ATM


Moving towards label switching

▪ Software-based routers became network bottlenecks as networks kept growing

‒ need for calculating connections, large number of header addresses, large routing tables

▪ Label switching concept was introduced as an alternative to IP able to facilitate and speed-up switching

▪ Main concept: ‒ use a special tag or label assigned to a packet that identifies and

switches the packet quickly

▪ Goal: ‒ separate route calculation from actual forwarding (done by hardware)

▪ Examples (in order of appearance): ‒ Tag Switching (CISCO), ‒ IP Switching (Ipsilon Networks)‒ MPLS (label swapping),‒ Flow-based switching,


•In order to reduce the amount of information processed (using

software) in a router:

➔ Combine a router with a switch

Router = slow and expensive (for large scale high capacity links!)

Switch = fast and cheap

•Objective: speed data through the network as to reduce latency and

processing (routing)

•Once a flow is detected set up a VC

•Cut-through operation: detour around usual routing paths by taking

fast switching paths (e.g., through ATM switches)

•Use normal hop-by-hop IP forwarding for short-lived packets

Layer 3 intelligence in switching


IP Switch

Controller

ATM

Switch Speed up flows of information destine to the same output.

Process only small amounts of data

Claim:

• - added software is 10% the size of removed software!

• - latency drop (processing) by e factor up to 100!

Layer 3 intelligence in switching


Issues with ATM and IP

▪ ATM‒ ATM has a scalability problem as a backbone technology.‒ ATM is connection-oriented:

• Between any pair of switches / routers, a separate PVC (Permanent Virtual Connection) must be set up.

• If optimal routing is required a full mesh of PVCs needs to be established • Very high complexity (proportional to n2). • In large networks this becomes unfeasible.

‒ ATM does not have powerful routing protocols:• ATM does not provide such powerful routing protocols as IP (OSPF, BGP).

▪ IP‒ QoS is difficult to meet with traditional routers:

• Traditional routers have become the bottleneck in the backbone.‒ Routing is costly:

• Routing is costly ($) since it needs a lot of performance and memory. (@ 2014 ca. 300‘000 BGP routes in Internet backbone; requires 60-120Mb memory to hold these routes)


Towards a labeled approach

▪ Combine the advantages of layer 3 routing + layer 2 packet forwarding

Forwarding:

Label Swapping

Control:

IP Router Software

Control:

IP Router Software

Forwarding:

Longest-match Lookup

Control:

ATM Forum Software

Forwarding:

Label Swapping

IP Router MPLS ATM Switch

Layer 3

Layer 2

▪ Routing is done with IP routing protocols. ‒ No change to the existing Internet backbone routing infrastructure is required

▪ QoS (Quality of Service) is achieved through layer 2 switching.

▪ The routing tables size can be reduced through layer 2 switching.


IP

Header

TCP

Header

MPLS

HeaderTCP Payload

L2

Header

Multi-Protocol Label Switching

▪MultiProtocol Label Switching (RFC3031): ‒ Proposed by IETF standard for carrying multiple network layer

protocols‒ In practice it deals only with IP

▪ Idea: ‒ Use of a short (20 bits) identifier, LABEL, inserted ahead of IP

header at specialized nodes and handled by all the other label switching nodes in the network

‒ The labeled packets are identified as a unified flow and switched rapidly

‒ The IP layer is tunneled with the much faster MPLS‒ MPLS gets information from IP in order to determine the path to

establish follow• No need to go with shortest path


MPLS flow aggregation

▪ Packets with common characteristics form a flow:‒ same source-destination pair,‒ type of service,‒ protocol id, … etc.

▪ Flows are assigned a label

➔So MPLS performs “flow aggregation”

▪ MPLS is possible to offer QoS guarantees for certain traffic

▪ MPLS is used for traffic engineering: ‒ establish performance characteristics for different classes of traffic‒ set up paths that are followed by specific traffic classes (not necessary

shortest paths)

▪ MPLS header typically carries a TTL field, which is reduced at every LSR hop

▪ Easier implementation of source routing ‒ While in IP networks source routing can be only used to force a packet over a

specific route, in MPLS can be easily done by assigning a label


LER

LSR

LER

LSR LSP

MPLS – How it works

▪ Label Edge Router (LER) assigns label to the packet.

‒ LERs handle the entrance/exit of packets into MPLS network

‒ Ingress LER performs the label assignment and packet forwarding classification

‒ Egress LER performs label removal and label distribution (upstream)

▪ Label Switch Router (LSR) switch labelled packets over the label switched paths (LSPs)

▪ LSRs switch packets based only on their labels. MPLS forwarding table distinct from IP forwarding tables


MPLS – How it works ctd.

▪ Since labels are of local significance, LSRs may have to perform label swapping (exchange of labels)

▪ The egress LER has to update the TTL value in IP header, in order to reflect the number of hops that the packet traversed

▪ LSP route discovery can be performed by same protocols as in IP (BGP, OSPF)

▪ Signaling protocol needed to set up forwarding ‒ forwarding is possible along paths that IP alone would not allow

(e.g., source-specific routing) !!‒ use MPLS for traffic engineering


•Packets maintain IP addresses

•3-bit Experimental field: not standard use, could be used as class-of-service

(COS) field

•1-bit S: stack: indicates that the bottom of a stack of labels has been reached

•TTL: IP TTL is copied at the egress and then updated at the egress. MPLS TTL

is decremented at each LSR

PPP or Ethernet

headerIP header remainder of link-layer frameMPLS Header

label Exp S TTL

20 3 1 5

Header position and format


R1R2

D

R3R4R5

0

1

00

A

R6

in out out

label label dest interface

6 - A 0

in out out


10 6 A 1

12 9 D 0

in out out


10 A 0

12 D 0

1

in out out


8 6 A 0

0

8 A 1

MPLS – Forwarding example

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