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Interexchange Carrier MPLS Network Design Study

Chapter Description

USCom is a fictitious nationwide data and long-distance voice service provider in the U.S. This chapter discusses the current USCom MPLS network design, its evolution, and how USCom characteristics and objectives influenced the corresponding design decisions that were made in order to illustrate how design decisions should stem from the characteristics of your company.

Quality of Service Design

The quality of service (QoS) marketed by a service provider and actually experienced by its customer base, is a key element of customer satisfaction. This is particularly true because most customers have already migrated, or soon will migrate, their mission-critical applications, as well as their voice and video applications, to IP services. In turn, this means that QoS is a key element of service provider competitiveness and success in the marketplace for both Internet and Layer 3 MPLS VPN services.

The levels of performance offered as part of Internet services in some parts of the world (such as the U.S.) have increased tremendously in recent years. This section discusses the Internet SLA that USCom offers in such a context. It also reviews the Layer 3 MPLS VPN SLA that USCom offers. Its objective is to allow customers to successfully carry all their mission-critical applications, as well as converge their data, voice, and video traffic. Finally, this section presents the design, both in the core and on the edge, deployed by USCom to meet these SLAs.

SLA for Internet Service

USCom offers an SLA for its Internet service. This SLA is made up of availability commitments, as well as performance commitments, as summarized in Table 3-6.

Table 3-6 USCom Internet SLA Commitments

SLA Parameter

SLA Commitment

Service availability (single-homed, no backup)

99.4%

Mean Time To Repair (MTTR)

4 hours

POP-to-POP Round-Trip Time (RTT)

70 ms

POP-to-POP Packet-Delivery Ratio (PDR)

99.5%

The availability commitments are provided to each Internet site. They are characterized by service availability of 99.4 percent (for a single-homed site attached via a leased line and without dial/ISDN backup) and an MTTR of 4 hours. Higher-availability commitments are offered with optional enhanced access options such as dial/ISDN backup and dual homing. Service availability is defined as the total number of minutes in a given month during which the Internet site can transmit and receive IP packets to and from the USCom backbone, divided by the total number of minutes in the month. USCom calculates service availability and MTTR based on trouble ticket information reported in the USCom trouble ticketing system for each site and customer.

Because USCom does not manage the Internet CE routers, the performance commitments of its Internet SLA are not end-to-end (not site-to-site). Instead, they apply POP-to-POP. The performance commitments are made up of an RTT of 70 ms and a PDR of 99.5 percent, which apply between any two POPs.

Using active measurements and averaging, USCom computes the POP-to-POP RTT and PDR. Dedicated devices located in every POP are used to generate two sets of sample traffic every 5 minutes to every other POP. The first sample traffic is a series of ten ICMP ping packets, which the sample traffic source uses to measure RTT. The second sample traffic is a series of ten UDP packets. The sample traffic destination uses them to measure the PDR (the ratio of received sample packets divided by the total number of transmitted sample packets). The worst RTT value measured over every hour is retained as the "worst hourly value." These "worst hourly values" are then averaged over the day, and the daily averages are averaged over the month. This yields the monthly average RTT value to be compared against the 70-ms SLA commitment specified in Table 3-6.

SLA for the Layer 3 MPLS VPN Service

Several considerations influenced the SLA definition of the Layer 3 MPLS VPN service. The first was the need to offer QoS levels that allow VPN customers to converge their data, voice, and video applications onto a common infrastructure. The second was the fact that it is relatively easy and cheap for USCom to "throw bandwidth" at the QoS problem in the core (from POP to POP). As discussed in the next section, the most attractive approach for USCom to offer the appropriate QoS to all traffic of all application types, including the most demanding applications such as voice, was to offer indiscriminately the highest required QoS to all the traffic. In turn, this means that the SLA needed to specify only a single set of POP-to-POP performance commitments that were applicable to all traffic.

USCom handles all traffic the same way in the core and does not allocate more resources to some types of traffic over others. Therefore, there is no need for the company to charge differently depending on the mix of traffic types from the customer or to limit the rate at which some traffic types from a given customer site might enter the network. This results in a very simple service for both USCom and the customer in which the customer can transmit as much traffic as he wants, of any traffic type, without USCom's having to know or even care about it. In turn, this means that the service charge for a Layer 3 MPLS VPN site is a flat fee that depends only on the site's port access speed.

The next consideration was the fact that, unlike in the core, "throwing bandwidth" at the QoS problem on the access links (CE-to-PE and PE-to-CE links) is not easy or cheap for USCom. This is primarily because these links are dedicated to a single customer and need to be provisioned over access technology where capacity is usually still scarce or at a premium. This means that congestion is to be expected on some of the CE-to-PE and PE-to-CE links. Therefore, prioritization mechanisms (such as differentiated services) are required on these links to protect the high QoS of the most demanding and important applications.

The final main SLA consideration was the fact that USCom does not manage the CE routers that access the Layer 3 MPLS VPN service. This means that prioritizing traffic onto the CE-to-PE links, and the resulting QoS experienced by various applications on this segment, is entirely under the control of the customer (because such mechanisms have to be performed by the device that transmits onto the link) and conversely is entirely out of USCom's control. In turn, this means that USCom cannot offer any SLA performance commitment for the CE-to-PE segment. This does not mean that QoS cannot be achieved on that segment. Instead, it recognizes that such QoS is under the customer's operational domain and thus is the customer's responsibility.

Similarly, USCom does not restrict in any way the proportion of each traffic type that a CE router can send, nor does it restrict which remote VPN site this traffic is destined for. Therefore, USCom has no way of knowing, or controlling, how much traffic will converge onto a remote PE router for transmission to a given CE router on the corresponding PE-to-CE link. In addition, sizing of the corresponding PE-to-CE link is under the customer's control. Thus, although USCom manages the upstream device on the PE-to-CE link, it cannot offer any SLA performance commitment on the PE-to-CE link either.

To help illustrate this point, imagine that a given customer has a VPN containing five sites—S1, S2, S3, S4, and S5—each of which is connected to its respective PE router via a T1 link. Imagine that S1, S2, S3, and S4 all transmit 500 kbps worth of voice traffic destined for S5. The egress PE router that attaches to S5 receives 2 Mbps of voice traffic destined for S5. It is clear that no matter what scheduling/prioritization mechanism may be used on the link to S5 by this PE router, and leaving aside any other traffic, trying to squeeze 2 Mbps of voice traffic onto a T1 link will result in poor QoS, at least on some voice calls (if not all).

However, the upstream end of the PE-to-CE link (the PE router itself, where prioritization and differentiation have to be implemented to offer appropriate QoS to the various applications on that segment) is not in the customer operational domain. So, unlike the CE-to-PE link case, the customer is not in a position to implement the required mechanisms independent of USCom to ensure that the right QoS is provided to all applications on the PE-to-CE link.

To ensure that the customer can achieve QoS on the PE-to-CE link in cases where that link may become congested, USCom decided to offer a service option (the "PE-to-CE QoS" option) whereby USCom would activate on the PE-to-CE link a fixed set of DiffServ per-hop behaviors (PHBs) that the customers can use as they see fit. Using this service offering, by properly sizing the PE-to-CE link, by controlling the amount of traffic for a given type of application (such as voice) sent from and to a given CE router, and by triggering the appropriate PHB made available by USCom for that type of traffic, the customer can ensure that the required QoS is provided to important applications. To trigger the appropriate PHB for a given packet, the customer simply needs to mark the packet's DS field in the IP header according to the Differentiated Services Codepoint (DSCP) values specified by USCom for each supported PHB. This marking must be performed at the ingress VPN site (on the ingress CE router) so that the DS field is already marked when the egress PE router applies the PHB during transmission of the packet onto the PE-to-CE link (after forwarding takes place and the MPLS header is popped). This is described in detail in the "QoS Design on the Network Edge" section.

The "PE-to-CE QoS" service option is marketed with a flat fee corresponding to the value-add provided to the end user and reflecting the extra processing load on the PE router. This option is kept very simple, without any customizable parameters.

For each application to experience the required QoS end-to-end, the corresponding requirements must be met on every segment of the traffic path—that is, within the ingress VPN site, from the ingress CE router to the ingress PE router, from USCom POP to POP, from egress PE router to egress CE router, and finally within the egress VPN site. The following summarizes how the USCom SLA performance commitments play out across each segment of the end-to-end path:

  • POP-to-POP—USCom offers arbitrarily to all traffic (independent of its application type and/or actual requirements) SLA performance commitments across the backbone (from POP to POP). This is compatible with the most stringent requirements of any application (including voice).
  • CE-to-PE link—USCom provides no SLA performance commitments on this segment. It is the customer's responsibility to ensure that the QoS required by the various applications is offered appropriately. The customer may achieve this by ensuring that the CE-to-PE link is sufficiently overengineered for the total aggregate load or by deploying DiffServ mechanisms on the ingress CE router, including classifying traffic into separate DiffServ classes and applying the corresponding PHBs.
  • PE-to-CE link—USCom provides no SLA performance commitments on this segment. However, as a service option, USCom can take the responsibility of applying DiffServ PHBs. It is then the customer's responsibility to use these PHBs, in combination with traffic control on ingress CE routers and capacity planning for the PE-to-CE link, to ensure that the QoS required by the different applications is provided on that segment.
  • Layer 3 VPN sites—Because this is entirely out of the USCom realm of operation, USCom leaves it to the customer to ensure that the right QoS is offered inside the VPN. The customer may achieve this by overengineering the VPN site network (that is, via switched Gigabit Ethernet technology) and/or deploying DiffServ within the VPN Site.

These SLA points are illustrated in Figure 3-13.

Figure 13

Figure 3-13 USCom VPN SLA Performance Commitments and Customer Responsibility

As with the Internet SLA, all the performance commitments apply POP-to-POP. The RTT and PDR commitments provided in the Internet SLA are appropriate for any multimedia application, so those are also used in the Layer 3 MPLS VPN SLA. However, because the performance commitments must meet the QoS requirements of all applications, including real-time/VoIP, a jitter commitment is added in the VPN SLA to the RTT and PDR commitments.

As with the RTT and PDR, USCom uses active measurement and averaging to compute the jitter. The same series of sample traffic of ten UDP packets sent every 5 minutes that is used to measure the packet delivery ratio is also used to measure the jitter. Note that the sample source and sample destination do not need to synchronize their internal clocks because jitter can be computed by the destination only using its local timestamp on packet arrival and analyzing the variation over the known transmitted interpacket interval. The worst value measured over every hour is retained as the "worst hourly value." These "worst hourly values" are then averaged over the day, and the daily averages are averaged over the month.

Table 3-7 lists the Layer 3 MPLS VPN SLA commitments.

Table 3-7 USCom Layer 3 MPLS VPN SLA Commitments

SLA Parameter

SLA commitment

Service availability (single-homed, no backup)

99.4%

Mean Time To Repair (MTTR)

4 hours

POP-to-POP Round-Trip Time (RTT)

70 ms

POP-to-POP Packet-Delivery Ratio (PDR)

99.5%

POP-to-POP jitter

20 ms

Optional "PE-to-CE QoS"

Optional support of three PHBs on the PE-to-CE link

When unable to meet the commitments listed in Table 3-6 over the one-month measurement period, USCom offers refunds to its VPN customers in the form of service credits. The SLA specifies how the service credits are computed, depending on the observed deviation from the commitment for each SLA parameter.

QoS Design in the Core Network

This section presents the QoS design USCom deployed in the core network to support the Internet SLA and the Layer 3 MPLS VPN SLA performance commitments described in the previous sections. As discussed in the section "USCom's Network Environment," thanks to its DWDM optical infrastructure, and thanks to the use of Gigabit Ethernet switching within its POPs, USCom can enforce an overengineering policy. Therefore, it can maintain a low aggregate utilization everywhere in the core without incurring any excessive additional capital expenditure. USCom elected to take full advantage of this by

  • Relying exclusively on aggregate capacity planning and overengineering to control QoS in the core and not deploying any DiffServ mechanisms or MPLS Traffic Engineering. This results in simpler engineering, configuration, and monitoring of the core.
  • Pushing this overengineering policy approach further so that, in most cases, the aggregate utilization is kept low even during a single link, node, or SRLG failure. In turn, this ensures that QoS is maintained during most failures. (Protection against SRLG failure is discussed later, in the "Network Recovery Design" section.)
  • Factoring in a safety margin when determining USCom's maximum utilization for capacity planning purposes to compensate for the shortcomings of capacity planning. This is discussed in the "Core QoS Engineering" section of Chapter 2, "Technology Primer: Quality of Service, Traffic Engineering, and Network Recovery."

Thus, USCom is adhering to the 1/1/0 model (or 3/1/0 model when the PE-to-CE QoS option is used) presented in the "QoS Models" section of Chapter 2.

The maximum distance between any two POPs in the USCom network is 4000 km. Assuming 25 percent of extra distance to cope with a longer actual physical route and additional distance when transiting via intermediate POPs, the one-way maximum distance is 5000 km and the round-trip maximum distance is 10,000 km. Assuming a 5-ms per 1000 km of light propagation delay through fiber, the maximum round-trip propagation delay in the USCom network is 50 ms.

The SLA RTT commitment of 70 ms leaves 20 ms of round-trip queuing delay. Assuming a maximum of 12 hops in one direction, such a round-trip queuing delay is safely met if the delay at each hop is kept below 0.8 ms. In fact, the round-trip queuing delay is likely to be significantly better than 20 ms because delay commitment is statistical in nature and therefore does not accumulate linearly. However, USCom uses the simpler linear rule because exact accumulation formulas are not strictly known, and estimate functions are quite complex.

Similarly, the SLA jitter commitment of 20 ms can be safely met if the jitter is kept below 0.8 ms at every hop, which is all the more true if the queuing delay itself is bounded at 0.8 ms, as identified to meet the RTT commitment.

USCom determined through mathematical analysis and simulation of aggregate queuing through a single hop and by applying an empirical safety margin that the per-hop queuing delay requirement can be safely met with a maximum aggregate utilization of 70 percent for any of the link speeds used in its core. In other words, USCom characterized the shape of the QoS versus utilization curve (discussed in the section "The Fundamental QoS Versus Utilization Curve" in Chapter 2) for its particular environment and various core link speeds.

This analysis also indicated that the level of loss caused by excessive queue occupancy under such conditions would be well below what is necessary to achieve the SLA's packet delivery ratio (in fact, it would actually be negligible). However, the packet delivery ratio also accounts for other causes of loss, such as those due to failures and routing reconvergence.

Based on this, USCom specified its capacity planning policy whereby additional core capacity is provisioned whenever

  • The measured link utilization exceeds 40 percent in the absence of any failure in the network.

    or

  • The link utilization exceeds 70 percent in the case of a single failure of a link, node, or SRLG.

Clearly, this policy ensures that POP-to-POP performance commitments are met because the link utilization is significantly below the maximum aggregate utilization in the absence of failure and is below or equal to the maximum aggregate utilization in the case of a single failure.

To enforce this policy, USCom monitors link utilization at 10-minute intervals on every link. When the utilization reaches 40 percent, an alarm is triggered. If this level is reached in the absence of failure and is not caused by any exceptional event, additional capacity is provisioned.

Also, USCom uses a network engineering and simulation tool with "what-if" analysis capabilities. On a regular basis, the measured maximum utilization figures for all links are fed into the tool. The tool then determines what the maximum utilization would be on all links should any link, node, or SRLG fail. If this exceeds 70 percent and cannot be reduced by adjusting IS-IS metrics (without redirecting excessive traffic onto another link), additional capacity is provisioned.

It is therefore clear that as long as USCom can enforce its high overengineering policy (based on the capacity planning rule of keeping utilization on all links below 40 percent in the absence of failure and below 70 percent in the presence of failure), the SLA performance commitments can be met without deploying any additional QoS tools in the core network, such as MPLS DiffServ or MPLS Traffic Engineering.

Because the DWDM optical core is currently far from reaching capacity limitations (that is, all lambdas used on a given fiber), the link provisioning lead time is only a few weeks. Because traffic growth on the USCom backbone is relatively steady and free from huge spikes (as shown in Figure 3-14), USCom felt it will indeed be able to enforce its overengineering policy, at least in the next one to two years. Thus, USCom has not yet deployed MPLS DiffServ or MPLS Traffic Engineering. However, if in the longer term enforcing the high overengineering policy becomes difficult, USCom will then consider such technologies.

Figure 14

Figure 3-14 USCom Utilization and Traffic Growth

In summary, although USCom offers tight POP-to-POP SLA commitments for Internet and Layer 3 MPLS VPN traffic, its core QoS design is very simple. It relies entirely on capacity planning, with enforcement of a high overengineering policy applied on an aggregate basis to all traffic. It does not involve any additional QoS mechanism in the core network.

QoS Design on the Network Edge

This section presents the QoS design deployed by USCom on the customer-facing interfaces of the PE routers. Because USCom does not implement any differentiated service in the core network and does not care about the mix of traffic classes received from a CE router, no QoS mechanism is configured on the ingress of the PE routers for both Internet customers and Layer 3 MPLS VPN customers.

On the egress side of the PE router, by default no QoS mechanisms are activated. However, if the Layer 3 MPLS VPN customer requests the PE-to-CE QoS option, a fixed QoS service policy is applied on the egress side of the PE router that activates three DiffServ PHBs.

Because USCom does not perform any QoS mechanism on the ingress side of the PE router or in the core, the Precedence field (or even the full Differentiated Services field) of an IP packet is carried transparently through the USCom network. Its value at the time of transmission by the egress PE router onto the PE-to-CE link (that is, after popping of the complete MPLS header) is unchanged from when the packet was transmitted by the customer ingress CE router. Therefore, USCom can use the Precedence field in the IP header as the classification criteria to apply the PHBs on the PE-to-CE link. To control which packets receive what PHB, the customer just has to mark the Precedence field on the ingress CE router (or upstream of it) in accordance with the Precedence-to-PHB mappings defined by USCom and specified in Table 3-8.

Table 3-8 Precedence-to-PHB Mapping for the PE-to-CE QoS Option

Precedence Values

PHB*

Targeted Traffic

0, 1, 2, 3

BE

Best-effort traffic

4, 6, 7

AF41

High-priority traffic

5

EF

Real-time traffic

*See the section "The IETF DiffServ Model and Mechanisms" in Chapter 2.

For example, the customer could configure its CE router to

  • Set the Precedence field to a value of 5 when sending a VoIP packet to the CE-to-PE link.
  • Set the Precedence field to a value of 4 when sending an Enterprise Resource Planning (ERP) packet to the CE-to-PE link.
  • Set the Precedence field to a value of 0 when sending other packets to the CE-to-PE link.

The result of this configuration is that when USCom transmits packets to the PE-to-CE link, it applies the EF PHB to the voice packets, the AF41 PHB to the ERP packets, and the BE PHB to the rest of the traffic. This effectively allows the customer to ensure that its applications are prioritized as it sees fit on the PE-to-CE link in case of congestion on that link.

Note that the customer would also probably configure its CE router to apply some custom PHBs on the CE-to-PE link to manage potential congestion on the CE-to-PE link. This set of custom PHBs does not have to be the same as the ones applied by USCom for the PE-to-CE QoS option, but it must be consistent with it, and its DS-field-to-PHB mapping must be consistent with the one from USCom. For example, the customer could decide to perform finer differentiation and activate a set of four PHBs with the Precedence-to-PHB mappings shown in Table 3-9.

Table 3-9 Sample Precedence-to-PHB Mapping for Custom CE PHBs

Precedence Values

PHB

Targeted Traffic

0, 1, 2

BE

Best-effort traffic

3

AF31

High-priority noninteractive traffic

4, 6, 7

AF41

High-priority interactive traffic

5

EF

Real-time traffic

End-to-end QoS operation when the PE-to-CE QoS option is not used, and when it is used by a customer, are illustrated in Figures 3-15 and 3-16, respectively.

Figure 15

Figure 3-15 End-to-End QoS Operations Without the PE-to-CE QoS Option

Figure 16

Figure 3-16 End-to-End QoS Operations with the PE-to-CE QoS Option

USCom elected to perform classification for the PE-to-CE QoS based on Precedence rather than the full DS field because it offers the end customer the flexibility to perform traffic marking either on the Precedence field or on the full DS field. For example, if the customer elected to mark VoIP packets with the DS field set to the EF DSCP (101110), these packets would be classified by the egress PE router appropriately because the first 3 bits of the packet's DS field, which constitute the Precedence field, are set to 101, which is Precedence 5.

Because USCom wanted to support a simple fixed set of PHBs for the PE-to-CE QoS option without any customizable parameters, it selected a versatile set of PHBs, as shown in Table 3-10, and a versatile PHB instantiation intended to be suitable for typical customer needs.

Table 3-10 PHB Instantiation for the PE-to-CE QoS Option

PHB

Instantiation

EF

Priority queue with 40% of the link bandwidth allocated.

In the absence of congestion, bandwidth is not limited.

In the presence of congestion, bandwidth is limited to 40% (excess is dropped) to protect the mission-critical applications expected to be handled by the AF41 PHB.

AF41

Class queue with most of the remaining bandwidth allocated (50% of the link bandwidth). This ensures strong prioritization of AF41 over BE.

In case of contention across all classes, this queue is granted 50% of the link bandwidth. However, this queue is not limited to 50%. It can use more if the other queues are not currently using their allocated bandwidth.

Random Early Detection (RED), as discussed in the section "The IETF DiffServ Model and Mechanisms" of Chapter 2, optimizes performance for TCP traffic, which is expected to be common in this class.

BE

Class queue with remaining bandwidth allocated (10% of the link bandwidth).

In case of contention across all classes, this queue is granted 10% of the link bandwidth. However, this queue is not limited to 10%. It can use more if the other queues are not currently using their allocated bandwidth.

Random Early Detection (RED) optimizes performance for TCP traffic, which is expected to be common in this class.

Example 3-9 illustrates how USCom configures PHB instantiation using Cisco IOS Modular QoS CLI (MQC) and applies it as the egress service policy of a PE router for a PE-to-CE link (see [QoS-CONF] and [QoS-REF] for details on how to configure QoS on Cisco devices using MQC). Note that the service policy is applied on the ATM1/0/0.100 and ATM1/0/0.101 interfaces because the PE-to-CE QoS option has been requested for the corresponding attached site. Expressing bandwidth as a percentage of the link bandwidth (rather than in absolute values) in the policy map is extremely convenient. It allows the use of a single policy map on all physical and logical interfaces regardless of their actual link speed.

Example 3-9 Egress Service Policy for the PE-to-CE QoS Option

ip vrf v101:USPO
 description VRF for US Post Office
 rd 32765:239
 route-target export 32765:101
 route-target import 32765:101
!
ip vrf v102:SoccerOnline
 description VRF for SoccerOnline International
 rd 32765:240
 route-target export 32765:102
 route-target export 32765:102
!
ip vrf v103:BigBank
 description VRF for BigBank of Massachusetts
 rd 32765:241
 route-target export 32765:103
 route-target import 32765:103
!
interface ATM1/0/0.100 point-to-point
 description ** BigBank_Site2 with PE-to-CE QoS option
 ip vrf forwarding v103:BigBank
 ip address 23.50.0.17 255.255.255.252
 pvc 10/50
  vbr-nrt 1200 1000 2
  encapsulation aal5snap
  service-policy out policy-PE-CE-QoS
!
interface ATM1/0/0.101 point-to-point
 description ** SoccerOnline_Site1 International with PE-to-CE QoS option
 ip vrf forwarding v102:SoccerOnline
 ip address 23.50.0.9 255.255.255.252
 pvc 10/60
  vbr-nrt 1500 1500 3
  encapsulation aal5snap
  service-policy out policy-PE-CE-QoS
!
interface ATM1/0/0.102 point-to-point
 description ** US Post Office_Site10 without PE-to-CE QoS option
 ip vrf forwarding v101:USPO
 ip address 23.50.0.13 255.255.255.252
 pvc 10/50
  vbr-nrt 1200 1000 2
  encapsulation aal5snap
!
class-map class-PrecHigh
  match precedence 4 6 7
class-map class-PrecVoice
  match precedence 5
!
policy-map policy-PE-CE-QoS
  class class-PrecVoice
   priority percent 40
  class class-PrecHigh
   bandwidth percent 50
   random-detect
  class class-default
   bandwidth percent 10
   random-detect

In summary, the USCom QoS edge design is very simple: By default, no QoS mechanism is activated on the PE routers. When a customer selects the PE-to-CE QoS option, a fixed service policy is applied in the egress direction onto the PE-to-CE link in order to instantiate a traditional set of three PHBs targeted at real-time, mission-critical, and best-effort applications.

6. Traffic Engineering Within the USCom Network | Next Section Previous Section