PfR Technology Overview
Moving to Performance Routing
Better Use of WAN Links
Many customers have multiple links to the WAN, being privately held or managed by a Service Provider. In many cases, one link is considered as primary while the other one is considered as a backup which leads to a waste of resources.
Today's challenges in the WAN:
- Traffic increase and badly load-balanced across central site uplinks
- Expensive backup links under utilized
- Load-sharing through complex BGP tricks or PBR
- Manual check of Netflow stats, IP SLA probes and Prefix utilization
What would be the solution:
- Full utilization of expensive network resources
- Efficient distribution of traffic based upon load
- Traffic optimized based upon circuit cost profiles
- Minimization of underutilized expensive WAN paths
Better Application performance and availability
Many customers would like to be aware of fluctuating service levels, Soft Errors or Brownouts. Routing protocols are not aware of such events. This could lead to a bad user experience, to application performance degradation. But still, the routing protocol is up.
- blackout: it is 100% loss of data (control plane is still UP).
- brownout: it is subjective - It is a high loss and very high delay condition.
- Application performance degraded
- Bad user experience
- But no routing problems reported
- Unaware of Soft Errors
- Intermittent reachability (SP flapping)
- Black holes
- New application requirements (low latency)
- But again … no routing problems reported
- Where is the problem?
- WAN Congestion Problem?
- Avoidance of network brownouts and soft errors
- Intelligent Routing based on performance
- Moving to adaptative routing based upon application delay, loss, jitter, bandwidth
One of the key requirement is to be able to route not only based on the prefix but also based on the type of application. Routing should take the best path for an application rather than a prefix.
- Critical application routed on primary link for best SLA
- Use backup links for best effort applications
- But still .. Use backup in case of performance degradation
- Complex configuration based on EEM scripts and Policy Based Routing (PBR).
- Static routing
- Responsiveness to critical application performance requirements
- Time/delay sensitive: voice, vertical apps (trading floor)
- Loss sensitive: video, circuit emulation
- Data center traffic: SAN extension, Internet ISP load balancing
- Transactional traffic: e-commerce transactions, automated B2B, ERP
Traditional routing uses static metrics to provide “reachability” information, but has no link utilization information, no data plane knowledge and is unaware of soft errors.
Cisco Performance Routing (PfR) enhances routing in order to select the best path based on user defined policy. The PfR policy can: minimize cost efficiently distribute traffic load and/or select the optimum performing path for applications
Cisco PfR enables intelligent traffic management that can dynamically route around soft errors in the Enterprise WAN or Internet and makes adaptive routing adjustments based on advanced criteria such as response time, packet loss, jitter, mean opinion score (MOS), availability, traffic load, and cost policies.
Cisco PfR augments traditional routing (BGP, EIGRP, OSPF, PBR) with realtime performance metrics in IOS instrumentation (Interfaces, Netflow, IP SLA, …).
PfR Technical Overview
In any PfR implementation, a master controller (MC) and at least one border router (BR) must be configured. The MC commands and controls the BRs and maintains a central repository for the data collected by the BRs.
The MC function can be collocated (configured) on the same router as the BR, or it can be a dedicated, standalone IOS platform..
BRs are in the user traffic switching path. BRs collect data from their Netflow cache and from the IP SLA probes results, provide a degree of aggregation of this information, and influence the packet switching path to manage user traffic.
The MC is the policy decision maker. Typically, at a large site (data center or campus), the MC is a standalone chassis while at smaller branch locations the MC is typically collocated (configured) on the same platform as the BR. As a general rule, the large locations manage more network prefixes and/or applications than a branch deployment and thus consumes more CPU and memory resources for the MC function. Therefore, it makes a good design practice to dedicate a chassis for the MC at the large sites. The branch typically manages fewer active network prefixes and/or applications and due to the costs associated with dedicating a chassis at each branch, the network manager can collocate the MC and BR on the same platform. CPU and memory utilization should be monitored on platforms operating as MCs and if utilization is high, the network manager should consider an MC platform with a higher capacity CPU and memory.
The MC communicates with the BRs over an authenticated TCP socket, but has no requirement for populating its own IP routing table with anything more than a route to reach the BRs.
Because PfR is a path selection technology, there must be at least two external interfaces under the control of PfR and at least one internal interface. There must be at least one BR configured. If there is only one BR configured, then both external interfaces are attached to the single BR. If more than one BR is configured, then the two or more external interfaces are configured across these BRs. External links, or exit points, are therefore owned by the BR; they may be logical (tunnel interfaces) or physical links (Serial, Ethernet, ...).
For PfR to be able to manage prefixes or application, traffic should cross the BRs from internal interfaces to external interfaces and vice-versa.
Performance Routing Policy Engine
Performance based routing happens in five stages:
- Learn Applications: MC tells BR to learn “interesting” applications, called Traffic Classes.
- This could be destination prefix with or without port, dscp, source prefix or even application using NBAR.
- This profiling process can be entirely automatic based on the top talkers (using Netflow) or configured manually.
- Measure Application performance (Collects traffic class statistics for learned applications):
- Monitor Modes: Passive, Active, Both, Fast, Special (Cat6K)
- Netflow for UDP (bandwidth) and TCP flows (availability, delay, bandwidth, loss)
- IP SLA for TCP and UDP flows (Availability, delay, loss, jitter, MOS).
- Apply Policy
- Use measured application data to determine whether managed traffic-class is out of policy (OOP) and if an alternate path can meet the policy requirements.
- Enforce (re-route traffic)
- Prefix Control: Inject BGP or Static routes
- Application Control: Dynamic Route-map/PBR for traffic classes defined by ACLs, NBAR, unsupported routing protocols (OSPF, ISIS) or, BRs running a mix of routing protocols.
- Verify that the new route match the policy
Reachability must be verified
For PfR to consider an exit interface as a candidate for traffic, reachability to the target network prefix must be verified. When PfR is configured as passive mode (mode monitor passive, Netflow based only), TCP flows must be present across the exit interface to learn the validity of reachability across the exit interface.
Note that a parent route needs to be present to direct traffic for a target network out the external interfaces, in order to allow the Netflow subsystem to identify the validity of reachability through the TCP flows. Given this, if there is no TCP traffic out an exit interface, no passive measurements are available to Netflow/PfR. Or, if there are long-lived TCP flows, flows lasting longer than the PfR monitor period, no TCP SYNc and TCP SYNc/ACK are seen during the monitor period. So in this case, traffic may be active, but because the TCP SYNc and TCP SYNc/ACK is not seen during the monitor period, no delay and reachability can be deduced from this long persistent flow.
For PfR to function optimally in passive monitor mode, more TCP flows equate to more data points for the master controller to analyze and manage. As the number of TCP flows increase, the database becomes more granular, meaning more delay, loss, and reachability information is available for a given network prefix.
The PfR measure phase is the second step in the PfR performance engine. The Traffic Class list is now full of traffic class entries and PfR must measure the performance metrics of these traffic class entries.
Measuring is defined in PfR as the act of measurement performed periodically over a set interval of time where the measurements are compared against a user defined threshold.
PfR measures the performance of traffic classes using active and passive monitoring techniques and it also measures, by default, the utilization of links.
The border routers collect passive monitoring and active monitoring statistics and then transmit this information to the master controller. The PfR measure phase is complete when each traffic class entry in the MTC list has associated performance metric measurements.
- “TCP, UDP, Jitter need IP SLA responder targets defined”
- Mode monitor Both: It is the default (recommended)
- Mode moinitor Fast: recommended for fast convergence
Mode monitor passive
Passive monitoring is the act of PfR gathering information on user packets assembled into flows by Netflow. Passive monitoring is typically only recommended in Internet edge deployments because active probing is ineffective because of security policies that block probing. PfR, when enabled, automatically enables Netflow on the managed interfaces on the Border Routers. By aggregating this information on the Border Routers and periodically reporting the collected data to the Master Controller, the network prefixes and applications in use can automatically be learned.
The border routers report traffic flows identified by Netflow to the master controller. The average delay of the flows, packet loss, and reachability along with the outbound throughput in terms of bits per second is determined for the destination IP prefixes observed in the Netflow data.
Measurements of the TCP traffic flows are characterized by:
- Delay—Time between TCP SYN and TCP SYN/ACK in a TCP three-way handshake.
- Loss—TCP sequence numbers are tracked, loss can estimated when lower sequence numbers than the highest sequence number observed are seen.
- Reachability—Repeated TCP SYNs without an accompanying TCP SYN/ACK identify reachability failures.
- Throughput—Throughput is calculated from NetFlow and measured in bits per second (bps). Measurements of non-TCP traffic flows is characterized by throughput only.
Mode monitor active
Active monitoring is the act of generating Cisco IOS IP Service Level Agreements (SLAs) probes to generate test traffic for the purpose of obtaining information regarding the characteristics of the WAN links. PfR can either implicitly generates active probes when passive monitoring has identified destination hosts, or the network manager can explicitly configured probes in the PfR configuration.
In this mode, IP SLAs probes are generated by the border routers through the current exits and transmitted at the configured probe frequency value. Probes are only generated through alternate paths (exits) in the event the current path is out-of-policy.
The ICMP ECHO requests that are generated by default constitute additional background traffic on the network. When used on the Internet, activating probing may not be desirable in that ICMP packets may be blocked or administratively prohibited and may be considered a threatening or abusive posture to the target hosts.
By default, an active probe is of the type of ICMP echo. If VoIP is to be characterized, the network manager may choose to explicitly configure an active probe. Following is an example from an pfr-map using a traffic-class that matches VoIP streams with probes sent every 2 seconds.
set active-probe jitter 22.214.171.124 target-port 33033 codec g729a set probe frequency 2
It is possible, and practical, to use active monitoring for specific traffic-class while using a global configuration option defaulting to passive monitoring of all other traffic through the TCP flows.
Mode monitor both
Mode monitor both is the default value and combines the capabilities of passive and active monitoring. Up to five IP addresses are actively probed for each destination prefix network learned through passive monitoring. By default, an IP SLA ICMP ECHO probe is automatically generated for the learned IP addresses.
In this mode, IP SLAs probes are generated by the border routers through the current exits and transmitted at the configured (or default) probe frequency value. Unlike mode monitor fast, which is described in a later section, active probing does not probe all exit points continuously. It probes only the current exit point provided the status is INPOLICY and probes are generated after the prefix timer value is exhausted. Probes are only generated through alternate paths (exits) in the event the current path is out-of-policy.
By monitoring both actively and passively, additional data points regarding a network prefix can be obtained through two separate and distinct tools; Netflow for passive measurements and IP SLA for the active measurements. However, the inclusion of active probing also has disadvantages. Mode monitor both is best suited for use within the private internal network of the enterprise.
Important note: With mode monitor both, passive measurements are used to trigger the Out of Policy (OOP) state but the active probing results from the exit interfaces are used with the passive throughput measurement for the traffic class, to select the exit interface.
Mode monitor Fast
This feature was introduced in Cisco IOS Release 12.4(15)T as a key component to the Fast Reroute feature.
This mode generates active probes through all exists continuously at the configured probe frequency. This differs from either active or both modes in that these modes only generate probes through alternate paths (exits) in the event the current path is out-of-policy.
With Fast Reroute, the characteristics of the alternative paths are always known, allowing immediate use as required. If unreachable is determined to be out-of-policy for the current exit, the alternate exit that is in policy is selected as the new current exit.
The unreachable threshold is calibrated in number of failed probes per million probe attempts. If the unreachable value is set to 1, a single probe fails on the current exit, an attempt is made to locate a alternate exit. However, if the alternate exits also have a single failed probe, they are not selected because they too are out-of-policy.
The Fast Reroute feature, therefore, allows rerouting actions to be taken, at an interval approaching the configured probe frequency value. Probe frequency can now be set as low as 2 seconds if fast mode is configured. This allows re-routing at slightly more than the configured probe frequency value. The Fast Reroute feature can reroute OOP traffic in as little as 3 seconds.
The obvious drawback to this feature is the potential for adding additional network traffic overhead associated with the probes themselves and additional CPU resources to the PfR border routers, the source of the active probes.
Probes are always generated unless the prefix is deleted or in the default state,.
The active probe results are used for out-of-policy and to control routing. Passive data collected is for information only, the throughput transmit and receive Kbps values (show oer mast border detail) are used for load balancing.
Apply Policy Phase
The apply policy phase compares the measured performance metrics against well-known or configured thresholds to determine if the traffic is meeting specified levels of service, or if some action is required. If the performance metric does not conform to the threshold, a decision is made by PfR to move the traffic class or exit into another state.
Policies can be applied globally, per Traffic Class or even per link.
PfR Features that Enable Load Balancing
Usage of this policy sets an upper threshold on the amount of traffic a specific link can carry. For example, if the upper threshold for a link is 90 % of total bandwidth, and it is currently running at 95 % of bandwidth, the link is Out-of-Policy (OOP). Cisco PfR will attempt to bring the link back into policy by repeatedly moving prefixes from the over-used link onto other exit links.
Usage of this policy keeps all WAN links within a certain utilization range, relative to each other in order to ensure fair load-sharing across all concerned links. The range functionality allows the network administrator to instruct Cisco PfR to keep the usage on a set of exit links with in a certain percentage range of each other. If the difference between the links becomes too great, Cisco PfR will attempt to bring the link back in to policy by distributing data traffic among the available exit links.
Traffic Class Performance
Usage of this policy enables the customer to define multiple paths that a set of traffic (ie voice traffic) could use as long as all the paths maintain the performance SLA ’s that are needed forth at set of traffic. Hence, a policy that determines voice traffic to have a delay threshold of less than 250 msecs can utilize multiple paths in the network if available, as long as all the paths deliver the traffic within its performance bounds.
PfR Policy Thresholds
PfR policies are defined by comparing measurements for each Traffic Class against default or user-defined thresholds. PfR allows to define different policies for each Traffic Class. One TC can have a policy based on delay and loss (typically for critical applications) whereas another TC would have a policy based on load-sharing across exit points.
- Passive Mode:
- As a general rule, Cisco PfR maintains a short-term counter (statistics from the last 5 minutes) and a long-term counter (statistics from the last 60 minutes).
- Active Mode:
- As a general rule, Cisco PfR maintains a short-term counter (statistics from the last 5 probe results) and a long-term counter (statistics from the last 60 probes results).
Cisco PfR Thresholds can be defined in a relative or absolute mode.
- An absolute threshold is compared against the short term delay value which is 5 min for passive mode, or last 5 probes for active mode.
- Short-term counter exceeds the threshold of N msec.
- Used for delay, loss, mos, unreachable, jitter
- A relative threshold is compared against the short and the long-term value
- Short-term counter exceeds the Long-term counter by X percent.
- Used for delay, loss, unreachable
- Example for delay: if the short term delay = 120ms and the long term delay = 100 ms, then the percent diff is 20. This value is compared to the threshold value configured in the oer-map (or against the default value)
The following tables gives the base units used for each parameter than can be configured in a Traffic Class policy.
Let's take the loss and reachability policy as an example:
- The loss policy -is expressed in Packets per million
- In passive mode: 100 lost out of 10000 TCP packets equal to 10,000 ppm
- In Active mode: 1 loss out 100 probe packets equal to 10000 ppm
- Reachability is expressed in Flows per million
- Passive mode: 100 unreachable out of 10000 TCP flows means 10,000 fpm
- Active mode: 1 probe fail out 5 attempted means 200,000 fpm
Cisco PfR allows multiple policies to be defined for each Traffic Class. To resolve the potential conflict of which policy to run, PfR uses a resolve function which allows to assign a priority for each policy.
By default, PfR assigns the highest priority to delay policies, followed by utilization policies, but unreachable policy always applies and has a default priority of 0 (cannot be changed).
Here is a list of the available policies:
MC(config-pfr-mc)#resolve ? cost Specify OER cost policy resolver settings delay Specify OER delay policy resolver settings jitter Specify OER jitter policy resolver settings loss Specify OER loss policy resolver settings mos Specify OER MOS policy resolver settings range Specify OER range policy resolver settings utilization Specify OER utilization policy resolver settings MC(config-pfr-mc)#
Each resolve is defined with a priority and inherits priorities from the global configuration mode You can change from the global inside pfr-map and Unreachable resolver has always the highest priority as seen before.
PfR Selects a Policy Conforming Exit by following each of the following steps:
- Gather traffic class measurements for all exits
- Gather link utilization for all external interfaces
- Exits with no measurements ignored
- Measurements applied using priority with variance
- Exits within variance are candidates
After All Priorities Examined:
- If a single candidate - Use single candidate
- If multiple candidates includes current exit - Choose current exit
- Else, randomly choose a candidate
An example of resolvers defined in PfR global configuration mode:
pfr master resolve delay priority 4 variance 20 resolve loss priority 6 variance 20 resolve util priority 8 variance 20
PfR Traffic Class States
The table below summarizes the possible TC states:
|DEFAULT||A network prefix may be shown in the default state if it is manually configured or learned, but has not been determined to be in or out-of-policy. Prefixes may revert back to default state if, for some reason, OER can no longer control the prefix. This may happen if all the exits are out-of-policy. The default state means that the parent IP routes control the exit for this destination prefix. This would be the same behavior as if OER were not configured or shutdown.|
|INPOLICY|| The prefix is inpolicy, which means that it meets the policy associated with this prefix or application. The prefix can be inpolicy and being controlled by OER, or inpolicy* and not controlled by OER. The presence of the asterisk (*) on the state attribute indicates the network is known to OER, but is under the control of the parent route. When no asterisk (*) is present, the prefix is being controlled by OER. The state of inpolicy is considered to be a desirable state.
After a few cycles, the Traffic Classes will typically be in the INPOLICY State.
|OOPOLICY|| The prefix or application has been identified as failing to meet its respective policy. If traffic is identified as being out-of-policy, OER moves the traffic to an alternative exit to bring the traffic inpolicy or unmanages the traffic, allowing it to revert back to the default exits as determined by the parent routes in the IP routing table. If the traffic reverts back to the default state, OER will again cycle this traffic, like all other traffic on the network, in an attempt to optimize based on the configured or default OER policy. The state of out-of-policy is considered undesirable.
The prefix or application has been identified as failing to meet its respective policy. The policy parameters (ie. Delay, Loss, etc) are defined in the oer-maps for each Traffic Class.
|HOLDDOWN|| The holddown state is enabled when a traffic class is initially controlled by OER. This holddown concept is applied to prevent churning or erratic behavior of OER managed routes from being injected and withdrawn from the IP routing table (and subsequently being redistributed by some IGP) or BGP tables. Once a prefix has been changed, it enters holddown for the specified (holddown) period, before it can be deemed in or out-of-policy. A network prefix can leave holddown state before the timer expires, if the current exit point experiences an unreachable out-of-policy condition. All other out-of-policy conditions are ignored during holddown state.
When PfR makes a route change to a specific TC, this TC will move to the HOLDDOWN state to avoid erratic behavior. The timer associated is the holddown timer defined in the pfr master configuration.
There are two ways to specify what is flagged as an Out Of Policy (OOP) condition in PfR:
- Relative implies the use of short/long term statistics. You specify a percentage and that is used to gauge if a TC is OOP.
- Example: the short-term delay is 120ms and the long term delay is 90ms, then the percent difference is 33 which is compared to the value that you would configure.
- You now have a threshold that is compared only to the short-term delay.
- The short and long term delay are defined as 5min (with fast mode it is 5 probe cycles) and 60min (fast mode definition 60 probe cycles) averages or till the last time there was an exit change.
! ! Policies for a Business Traffic Class ! pfr-map MYMAP 20 match pfr learn list BRANCH_BUSINESS set delay threshold 150 set mode route control set mode monitor both set resolve delay priority 1 variance 20 set resolve utilization priority 3 variance 20 no set resolve range ! ! ! Policies for a Best Effort Class ! ! pfr-map MYMAP 30 match pfr learn list BRANCH_BE set mode route control set mode monitor both set resolve utilization priority 1 variance 20 no set resolve delay no set resolve range !
Load Interval and Bandwidth
To provide the most granular and accurate information to the master controller, configure the load-interval on internal and external interfaces on the border routers to the minimum value of 30 seconds. Additionally, the bandwidth statement on the interface should also be appropriately configured.
When Cisco PfR is configured in learn mode/passive, TCP flows must be observed by the border routers to manage prefixes. This means to test PfR in a lab environment, some tool to generate actual TCP/UDP traffic and another to introduce delay, loss, etc,. is necessary to observe meaningful results.