Tài liệu Campus Quality of Service pdf

Marking is the QoS feature component that “colors” a packet so that it can be identified and distinguished among other packets in QoS treatment: – Differentiated Services Code Point DSCP

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Campus Quality of Service

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the campus

QoS problems

The objective of this module is to define the quality of service (QoS) tools which must be implemented to provide a voice-enabled transport service for IP telephony

A QoS-enabled transport infrastructure is the prime prerequisite for end-to-end IP

telephony, which is covered in the following chapters

The key design consideration here is to make sure the proper hardware and Cisco IOS

releases are in place, along with sufficient bandwidth, to support the added requirements that voice will place on a data network

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Campus QoS

Campus QoS Issues

– Prioritization (Marking and Queuing)

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Campus QoS

Distribution Layer 3

Core Layer 3

Access Layer 2

Distribution Layer 3

Access Layer 2

Server Farm

•Hierarchical design

•Minimize Layer 2

•Switched 10/100 to desktop

This graphic illustrates a typical network of today The 10/100 Ethernet to the desktop has

to be switched Do not support IP Telephony in any way, shape, or form over shared media; switched to the desktop is required, ideally at 100 Mb Gigabit Ethernet trunks should beoverprovisioned, and undersubscribed in a campus network

QoS in the campus today is like an air bag It’s there It’s ve ry comforting that it’s there, but who wants to use it every day? If using it every day, a large call bill develops All of these QoS mechanisms are important and have to be there because they add value to the network

Another important factor is the establishment of a trusted boundary within a network The

PC, the attached device, has typically been viewed as something to entrust, just based on the port But now that voice is running - and in the future, video and data - it is imperative that this trust boundary be defined

Finally, some characteristics must be identified, some way to identify voice, versus video, versus data on the ingress to the network itself

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Need for Campus QoS

Aggregation Speed Mismatch

Packets from Different Applications

Packets that Made It Through;

Rest Are Dropped

Why is QoS needed in the campus? One reason is speed mismatches, going from Gigabit Ethernet to Fast Ethernet Many-to-one means having a lot of things coming down to a common link and oversubscribing it If there is a Catalyst® 6000 with 384 Fast Ethernet ports on it, it is possible to oversubscribe the uplink So potentially, all of these things are reasons to be able to identify voice, and perhaps treat it differently from data

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Steps for Campus QoS

2 Steps for Campus QoS Implementation

• Marking

Marking the packet with a specific priority

Establish a trust boundary.

• Queuing

Assigning packets to one of multiple queues (based on classification) for expedited treatment through the network.

The two steps required for campus QoS are marking and queuing It is necessary to

distinguish among voice, video, and data traffic The traffic must be marked and a trust

boundary established If the traffic is marked at the edge, hopefully, the further into the network, the less the worry about trusting a given packet

Next, use that criteria, however the traffic has been classified, to give it preferential

treatment, perhaps in the wide-area networks If you can classify at the edge, as you go through the wide-area network you can use that classification mechanism as the entrance criteria, for example, to a priority queue for the WAN

The second step is queuing, or assigning packets to one of multiple queues, based on the classification technique employed, within the campus for expedited treatment through the network

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Marking What is it?

Marking is the QoS feature component that “colors” a packet so that it can be identified and distinguished among other packets in QoS treatment:

– Differentiated Services Code Point (DSCP)

– IP Precedence

– QoS-Group

– 802.1p

Packets entering the network may have been marked previously If this marking

is from a trusted source, then classification may be based on the previous mark

If the marking is not from a trusted source, then classification may be used to

determine what the new marking should be

Marking can occur at Layer 2 or Layer 3, however many QoS features are based

on the IP Precedence bit or Differentiated Services Code Point (DSCP) settings

There are methods of marking that will map Layer 2 Class of Service (CoS) bits

to Layer 3 IP Precedence or DSCP settings

A QoS-group is internal to a router It allows us to virtually mark packets as they

come into a router, then use that virtual marking for outbound policy The

biggest advantage to virtual marking is that it does not alter the traffic passing

through the router

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Solutions for Campus QoS Issues

• Marking

• Congestion avoidance (WRED)

• Scheduling & queuing

–Priority queuing

–WRR

• Policing

What are the available classification schemes?

There is CoS, a Layer 2 scheme, 802.1P, and 802.1Q

At Layer 3 are the preferred schemes However, it’s not always possible to classify at Layer

3 The benefit of classifying at Layer 3 is that classification potentially follows a packet from the source to the destination, irrespective of how many hops it traverses This is true

as long as you don't re-classify

Congestion avoidance schemes, such as WRED, don’t really do a great deal for voice

directly It uses the UDP, so dropping a packet is not going to help much But it will

improve handling TCP traffic, allowing you to throttle it back which helps voice indirectly Another issue is queuing and scheduling The current preferred queuing mechanism is

LLQ, which is really PQ-CBWFQ is a rigid traffic prioritization scheme: if packet A has a higher priority than packet B, packet A always goes through the interface before packet B When you define an interface's QoS property as priority queuing, four queues are

automatically created on the interface: high, medium, normal, and low Packets are placed

in these queues based on priority queuing policies you define on the interface Unclassified packets are placed in the normal queue This allows you to exhaust the voice queue before attempting to deliver other classes of traffic

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Weighted round-robin (WRR) scheduling is used on Catalyst 8500 family switch routers (Layer 3 switches) on egress ports to manage the queuing and sending of packets WRR places a packet in one of four queues based on IP precedence, from which it derives a delay priority With WRR, each queue is given a weight This weight is used when congestion occurs on the port to give weighted priority to high-priority traffic without starving low priority traffic The weights provide the queues with an implied bandwidth for the traffic on the queue The higher the weight, the greater the implied bandwidth The queues are not assigned specific bandwidth, however, and when the port is not congested, all queues are treated equally

Finally, policing is another QoS tool QoS Policy Manager (QPM) lets you define QoS

policies at a more abstract level than can be defined using device commands For example, with QPM you can define policies for groups of devices rather than one device at a time You can also create policies that apply to applications or groups of hosts more easily than can be defined using device commands

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Assuming that this Layer 2 device either didn’t have the capability to do this classification

or you chose not to do it at that point, you could classify at the distribution layer The

design recommendation has long been that the distribution layer be a Layer 3 device If using a Catalyst® 6000 with a Policy Feature Card (PFC), or the MFSC, you can map the CoS to a DSCP If using VLANs, you can map the VLAN through an ACL on the VLAN

Or you could even map the IP address to a DSCP This illustrates the importance of

identifying phones uniquely by using a different IP address range The design

recommendation is to use RFC1918 addresses for the phones for network 10

For example, at home you have an IP telephone, but your DSL box at home gives you a network 10 address You’d like to be able to give 10.1 to phones, and 10.2 to PCs Again, you want an easy way at Layer 3 to be able to distinguish one kind of traffic from the other From this point into the network onwards, you don’t have to worry about trust You've

established your trust boundary; in this case, the trust boundary is the telephone itself

You’ve extended the trust to that device; but you’ve achieved classification at Layer 2 and Layer 3

From this point on, you just use your DSCP or IP precedence as the entrance criteria to any fancy queuing you have to achieve You have now achieved end-to-end QoS

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Version Length

Three bits used for CoS (User Priority)

Three bits used for CoS (Class of Service)

Len

Standard IPV4: Three MSB called IP Precedence (DiffServ (DSCP) may use six D.S bits plus two for flow control)

Layer 2 802.1Q/p

Layer 2 ISL

Layer 3 IPV4

ID Offset TTL Proto FCS IP-SA IP-DA Data

FCS

4 Bytes Encapsulated Frame 1…24.5 KBytes

ISL Header

26 Bytes

FCS DATA

PT TAG

4 Bytes SA

DA SFD PREAM.

Classify at Layer 3 or Layer 2

ToS

1 Byte

QoS Tags : Layer 2 = CoS & Layer 3 = ToS

Traffic Classification Types

The goal of protecting voice traffic from being run over by data traffic is accomplished by classifying voice traffic as high priority and then allowing it to travel in the network before low-priority traffic Classification can be done at Layer 2 or at La yer 3 as follows:

• At Layer 2 using the 3 bits in the 802.1p field (referred to as class of service or CoS), which is part of the 802.1Q tag

• At Layer 3 using the 3 bits of DSCP field in the type of service (ToS) byte of the IP header

Classification is the first step towards achieving QoS Ideally, this step should be done as close to the source as possible

Trust Boundaries

The concept of trust is an important and integral one to deploying QoS Once the end

devices have set CoS or ToS values, the switch has the option of trusting them If the switch trusts the values, it does not need to do any reclassification; if it does not trust the values,

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In summary, try to maintain the trust boundary in the wiring closet If necessary, move it down to the distribution layer on a case-by-case basis, but avoid moving it down to the core

of the network This advice conforms with the general guidelines to keep the trust boundary

as close to the source as possible

This discussion assumes a three-tier network model, which has proven to be a scalable

architecture If the network is small, and the logical functions of the distribution layer and core layer happen to be in the same device, then the trust boundary can reside in the core layer if it has to move from the wiring closet

Traffic Classification at Layer 3

Using the 802.1p bits within the 802.1Q tag provides the desired QoS results at Layer 2 When traffic has to cross a Layer 3 boundary, however, it becomes imperative to implement these mechanisms using Layer 3 parameters, such as the three IP precedence bits

(commonly referred to as ToS) or the new DSCP parameter, which uses the six most

significant bits within the ToS byte of the IP header Traffic crosses a Layer 3 boundary when packets are routed between subnets by Layer 3 switches or routers Traffic also

crosses a Layer 3 boundary when packets need to go out of the campus network onto the WAN through edge routers When this happens, Layer 2 classification does not help Layer

3 classification is needed for achieving the desired level of QoS All of the QoS techniques employed by the routers (including the very important WAN QoS) rely on Layer 3

3 before it can cross a Layer 3 boundary

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Handset

8500

Server

GSR

IP is normally thought of as being a “best effort” only protocol But IP has

always had a mechanism for supporting differentiated services The IP ToS field

and the IP Precedence bits provide this capability Because the majority of

applications today are IP-based, why not leverage IP for end-to-end QoS policy

signaling?

IP Precedence takes advantage of in-band signaling The ToS field can be used to bind business policies into network behavior

IP Precedence utilizes the three precedence bits in the IP header ToS field to

specify class of service for each packet You can partition traffic in up to six

classes of service using IP Precedence (two others are reserved for internal

network use) The queuing technologies throughout the network can then use this signal to provide the appropriate expedited handling

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• There is a DS field in header of every IPv4 and IPv6 packet

DSCP is the field identifying what treatment the packet should receive

• DSCP : Differentiated Service Code Point, 6 bits

• CU: Currently Unused, 2 bits

The Internet Engineering Task Force (IETF) defines the six most significant bits

of the 1-byte ToS field as the Differentiated Services Code Point (DSCP) The

priority represented by a particular DSCP value is configurable DSCP values

range from 0 to 63

The slide shows the breakout of the DSCP field Six bits are used for the

Differentiated Service Code Point, and 2 bits are currently unused

Layer 3 IP packets can carry either an IP Precedence value or a DSCP value

MQC supports the use of either value in set and match commands The

recommended settings of the DSCP field are backwards-compatible with IP

precedence (see the following material)

RFC2474, Definition of the Differentiated Services Field (DS Field) in the IPv4

and IPv6 Headers, Dec 98

For additional information, refer to:

http://www.ietf.org/rfc/rfc2474.txt

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What Are 802.1P and Inter-Switch Link?

QoS for a Layer 2 Ethernet switched world!

On trunk ports only

• 802.1P QoS is:

–An IEEE specification

–Focuses on support for QoS over LANs and 802.1Q trunks

–Supports 8 classes of service

• Inter-switch link (ISL) QoS is:

–Cisco specification

–Focuses on support for QoS over ISL trunks

CoS values range from zero for low-priority to seven for high-priority They can

only be applied on trunks (because an encapsulation is only ava ilable on trunks

with space for the bits)

Inter-Switch Link (ISL) frame headers have a 1-byte User field that carries the

CoS value in the three least significant bits

IEEE 802.1P and 802.1Q frame headers have a 2-byte Tag Control Information

field that carries the CoS value in the three most significant bits, which are called the User Priority bits

Other frame types cannot carry CoS values

In general, Layer 2 switches can examine, use, or alter MAC layer markings, not

IP precedence or DSCP settings, since those are Layer 3 Layer 2 markings are

applied on egress trunk ports

Tiêu đề	Campus Quality of Service
Trường học	Cisco Systems, Inc.
Chuyên ngành	Quality of Service
Thể loại	Module
Năm xuất bản	2000
Thành phố	San Jose

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