A Network Policy is a set of rules governing the network. It can include traffic monitoring and controlling mechanisms. We should implement network policies in order to help implement QoS in a controlled manner. Network policies can also help with security implementations.
A Policy Decision Point (PDP) is the point in the network where decisions about policy are made. This is not to say that the PDP makes the policy – it just enforces them! A PDP can be almost any device in a network, including routers, switches, gateways, and servers. A PDP lets us enable multiple-media applications, WAN efficiency, and alignment of resources to business priorities, just name a few.
Policies are stored in a Policy Repository, which is an LDAP database. These policies are accessed by the PDP (sometimes referred to as a Policy Server). The PDP then distributes the policies to network devices, also referred to as Policy Enforcement Points. Typical policy management functions include:
1. Maximization of current bandwidth usage
2. Mapping network resources for increased performance without bandwidth cost
3. Identifying traffic flows for applications and services
4. Supporting advanced traffic prioritization
5. Administering rules or policies for network behavior
This is not a comprehensive list. There are many more functions of network policies, and I won’t begin to discuss them here. Nearly every corporation with a data or voice network has network policies. Often they are invisible to the end user; sometimes they are put in writing so that users can see them. Network policies are an excellent way to manage network traffic and should almost always be used to ensure Quality of Service.
Network management is also critical to network design. This includes the ability to manage individual devices on the network (with or without being physically present at the device), using fault management techniques, and identifying problems on a proactive basis. Of course, these are not the only things we need to think about when discussing network management, but they are very important. The last one is probably the most important – if we wait for a problem to arise before we fix it, we are asking for trouble. Fixing problems proactively is the key to ensuring network stability.
Showing posts with label prioritization. Show all posts
Showing posts with label prioritization. Show all posts
Traffic Flow Design Considerations
Traffic engineering involves the measurement and control of traffic. Main VoIP traffic sources are IP phones and VoIP Trunks. We need to be able to perform traffic engineering on individual nodes and consider the traffic between nodes. Traffic measurement is an analysis of existing traffic flow and the patterns of a network. This will help us to determine how many users and calls a VoIP network can support. Some operational metrics from the system include the number of calls and users, the duration of calls, the number of concurrent calls, call volume profiles, location and call flow, and the number and length of faxes sent and received. In order to assemble this traffic information and make a judgment we need to look at a number of design considerations:
1. System capacity for busy hour
2. Interfaces for critical connections
3. Single points of failure at critical points in the network
4. Is there sufficient power
5. If a device fails, does network design provide redundant paths?
Once again, I will reiterate that we should ONLY use switched media Ethernet networks with VoIP. Shared media, such as hubs, will cause problems. We need to ensure that QoS is being used, such as 802.1p, VLANs, port prioritization, and the rest. If we are fortunate enough to be using Layer 3 switching, we should try to use DiffServ to manage applications with different types of QoS services. And, of course, we should look to increase bandwidth as necessary.
When considering routing across a WAN, we have some additional factors to consider. WANs can require a large amount of memory and processing capacity. When designing WAN traffic flow, we should consider performance, resiliency, stability, and route cost. As always, performance is a large part of WAN design. When considering performance, we should:
1. Be aware of CPU utilization required for router intelligence
2. Move the heavy burden of processing routers to out-of-band processors
3. Use network routers for packet forwarding and handling
4. Deploy Layer 2 QoS Switching between policy switch stacks to reduce policy switch stack and router CPU utilization for LAN-Side traffic
5. Reduce the number of router zones (or areas)
The complexity of a network will dramatically affect its resiliency. Network instability, or Route Flapping, can occur on a more complex network. In addition, we need to consider route cost or metrics that will be applied to a route. In order to guarantee a scalable and reliable enterprise WAN service providers should offer a competitive Service Level Agreement (SLA). The SLA should clearly guarantee the maximum latency and available bandwidth. Increased reliability and performance requirements of converged networks put added pressures for the establishment of strong SLAs with our service providers. If our service provider of choice will not give us an acceptable SLA we should begin the process of looking for another service provider!
1. System capacity for busy hour
2. Interfaces for critical connections
3. Single points of failure at critical points in the network
4. Is there sufficient power
5. If a device fails, does network design provide redundant paths?
Once again, I will reiterate that we should ONLY use switched media Ethernet networks with VoIP. Shared media, such as hubs, will cause problems. We need to ensure that QoS is being used, such as 802.1p, VLANs, port prioritization, and the rest. If we are fortunate enough to be using Layer 3 switching, we should try to use DiffServ to manage applications with different types of QoS services. And, of course, we should look to increase bandwidth as necessary.
When considering routing across a WAN, we have some additional factors to consider. WANs can require a large amount of memory and processing capacity. When designing WAN traffic flow, we should consider performance, resiliency, stability, and route cost. As always, performance is a large part of WAN design. When considering performance, we should:
1. Be aware of CPU utilization required for router intelligence
2. Move the heavy burden of processing routers to out-of-band processors
3. Use network routers for packet forwarding and handling
4. Deploy Layer 2 QoS Switching between policy switch stacks to reduce policy switch stack and router CPU utilization for LAN-Side traffic
5. Reduce the number of router zones (or areas)
The complexity of a network will dramatically affect its resiliency. Network instability, or Route Flapping, can occur on a more complex network. In addition, we need to consider route cost or metrics that will be applied to a route. In order to guarantee a scalable and reliable enterprise WAN service providers should offer a competitive Service Level Agreement (SLA). The SLA should clearly guarantee the maximum latency and available bandwidth. Increased reliability and performance requirements of converged networks put added pressures for the establishment of strong SLAs with our service providers. If our service provider of choice will not give us an acceptable SLA we should begin the process of looking for another service provider!
Traffic Shaping at Level 4 and Above
Best Effort Networks, operating in layers 2, 3, and 4, are prone to delay and packet loss under congestion. These layers are used for IP traffic.
RTP Prioritization provides a strict priority queuing scheme for voice traffic. It identifies voice traffic by RTP port numbers and classifies traffic by specifying an RTP port range. If traffic matches the specified range, it is classified as voice and is placed in low latency queuing Priority Queue. If traffic does not fall within the specified port range it is classified by service policy of low latency queuing scheme.
TCP/IP Classification prioritizes packets by the packet header. There are four fragmentation methods:
1. Frame Relay Fragmentation is limited to Frame Relay connections. It enables data frames to be fragmented into smaller pieces and interleaved with real-time frames. It also enables real-time voice packets and non-real-time data packets to be carried together on lower speed links without causing excessive delay in real-time links.
2. ATM Fragmentation is limited to ATM Connections. It fragments packets into 53-byte cells for the interleaving of real-time and data packets.
3. PPP Fragmentation is limited to PPP links and splits packets into multiple smaller packets and encapsulates them into PPP frames before queuing them for transmission. Higher priority VoIP packets interrupt and transmit ahead of the larger, lower-priority data packets that were already queued and can interleave packets to minimize delay.
4. Protocol (IP) Fragmentation only transmits the layer 4 and higher protocol information in the first packet in a series of fragments.
Policy Management is another method of Layer 4 QoS. You can implement policy management on network components, such as devices, rules, actions, and policies. Policy management can actually be used on any layer of the OSI Model. Policy management controls traffic flow based on traffic conditions, schedules, and actions.
RTP Prioritization provides a strict priority queuing scheme for voice traffic. It identifies voice traffic by RTP port numbers and classifies traffic by specifying an RTP port range. If traffic matches the specified range, it is classified as voice and is placed in low latency queuing Priority Queue. If traffic does not fall within the specified port range it is classified by service policy of low latency queuing scheme.
TCP/IP Classification prioritizes packets by the packet header. There are four fragmentation methods:
1. Frame Relay Fragmentation is limited to Frame Relay connections. It enables data frames to be fragmented into smaller pieces and interleaved with real-time frames. It also enables real-time voice packets and non-real-time data packets to be carried together on lower speed links without causing excessive delay in real-time links.
2. ATM Fragmentation is limited to ATM Connections. It fragments packets into 53-byte cells for the interleaving of real-time and data packets.
3. PPP Fragmentation is limited to PPP links and splits packets into multiple smaller packets and encapsulates them into PPP frames before queuing them for transmission. Higher priority VoIP packets interrupt and transmit ahead of the larger, lower-priority data packets that were already queued and can interleave packets to minimize delay.
4. Protocol (IP) Fragmentation only transmits the layer 4 and higher protocol information in the first packet in a series of fragments.
Policy Management is another method of Layer 4 QoS. You can implement policy management on network components, such as devices, rules, actions, and policies. Policy management can actually be used on any layer of the OSI Model. Policy management controls traffic flow based on traffic conditions, schedules, and actions.
Implementing QoS At Layer 3
At Layer 3, Differentiated Services (DiffServ) provides a scalable mechanism for prioritizing packets and providing QoS on a VoIP network. There are three main levels identified by DiffServ. The highest priority level is Expedited Forwarding (EF). Expedited Forwarding packets get priority over all other packets. Assured Forwarding (AF) is the mid-level prioritization. Anything that is not EF or AF is classified as Default, and receives the lowest priority. The DiffServ identifier is found within the DiffServ Code Point (DSCP) found in the header of the IP packet.
DiffServ identifies Per-Hop Behaviors (PHB) for each packet that is sent through. A packet classified as EF will receive the highest priority, and is ideal for low-latency applications such as VoIP. An EF package will almost never be discarded from a DiffServ-capable device.
Assured Forwarding packets are subdivided into three behavior groups – Low Drop, Medium Drop, and High Drop. Each of these groups is further divided into four classes, Class 1, Class 2, Class 3, and Class 4. This gives us a total of twelve AF classifications. The drop precedence of these classifications is often defined by the queuing methods of QoS defined in the previous post. The most common methods are Fair Queuing or Weighted Fair Queuing. The level of forwarding assurance is determined by the load of the AF class and the drop precedence of the packet (in the case of congestion within a class).
A Default (DE) packet is sent using a best-effort behavior. If no DiffServ argument is included in a packet it is assumed to be part of the default category.
There are multiple points where a package can be marked for DiffServ. It can be marked using the end point within the device or application from which it originates, it can be marked using intelligent switches where the device is connected to a network based on its TCP/UDP Port, IP Address, or VLAN. The packet can be marked at a core or edge router where the packet encounters links. Traffic entering a network is classified, and possibly conditioned, at the boundaries of the network and is assigned to different behavior aggregates identified by a single DSCP. DiffServ edge router and switch interfaces can be configured to trust or distrust previously marked DSCP or 802.1p tagged packets. If the packet is trusted, the DSCP tag will remain the same. If it is distrusted, the DiffServ router will assign a new DSCP tag.
There are three types of nodes on a DiffServ domain: Edge nodes, Boundary Nodes, and Interior Nodes. The Edge node is the first IP Aware device that applies QoS policies. Edge Nodes and Boundary Nodes are configured to trust or distrust the DSCP identification and forward the packets from one node to another.
DiffServ identifies Per-Hop Behaviors (PHB) for each packet that is sent through. A packet classified as EF will receive the highest priority, and is ideal for low-latency applications such as VoIP. An EF package will almost never be discarded from a DiffServ-capable device.
Assured Forwarding packets are subdivided into three behavior groups – Low Drop, Medium Drop, and High Drop. Each of these groups is further divided into four classes, Class 1, Class 2, Class 3, and Class 4. This gives us a total of twelve AF classifications. The drop precedence of these classifications is often defined by the queuing methods of QoS defined in the previous post. The most common methods are Fair Queuing or Weighted Fair Queuing. The level of forwarding assurance is determined by the load of the AF class and the drop precedence of the packet (in the case of congestion within a class).
A Default (DE) packet is sent using a best-effort behavior. If no DiffServ argument is included in a packet it is assumed to be part of the default category.
There are multiple points where a package can be marked for DiffServ. It can be marked using the end point within the device or application from which it originates, it can be marked using intelligent switches where the device is connected to a network based on its TCP/UDP Port, IP Address, or VLAN. The packet can be marked at a core or edge router where the packet encounters links. Traffic entering a network is classified, and possibly conditioned, at the boundaries of the network and is assigned to different behavior aggregates identified by a single DSCP. DiffServ edge router and switch interfaces can be configured to trust or distrust previously marked DSCP or 802.1p tagged packets. If the packet is trusted, the DSCP tag will remain the same. If it is distrusted, the DiffServ router will assign a new DSCP tag.
There are three types of nodes on a DiffServ domain: Edge nodes, Boundary Nodes, and Interior Nodes. The Edge node is the first IP Aware device that applies QoS policies. Edge Nodes and Boundary Nodes are configured to trust or distrust the DSCP identification and forward the packets from one node to another.
QoS At Layer 2
There are methods of implementing quality of service at different layers. I would like to begin by talking about the methods of implementing QoS at Layer 2.
The Ethernet Standard specifies a frame as having a maximum size of 1,518 bytes. The IEEE defined an additional 4-byte tag for Ethernet Headers, increasing the maximum size to 1,522 bytes. This tag contains two valuable pieces of information that pertain to QoS, the 802.1w VLAN ID Field, and the 802.1p Priority Field.
I will begin by discussing the 802.1p Priority field. Three bits in the IEEE Ethernet Protocol Specification can be used to prioritize packets at the user level. The three bits allow for up to eight levels of prioritization (0-7). Any 802.1p switch is capable of reading this field and can then retransmit the packet across the network with the higher priority. The higher the number in the priority field, the higher priority the switch will give to the packet. The IEEE has made some recommendations for the numbering system to be used here, but it is at the discretion of the network administrator. IEEE recommendations are to use 0 for Best Effort, 1 for Background, 2 as a spare, 3 as Excellent Effort, 4 as Controlled Load, 5 as Video, 6 as Voice, and 7 as Network Control.
The 802.1q specification, also known as VLAN Tagging, refers to the process of identifying which VLAN a packet is coming from. A VLAN, or Virtual LAN, makes a physical switch into smaller logical groupings. For example, a 24-port switch may be divided into three smaller 8-port virtual switches to eliminate leakage of data from one section to the next. This is very useful in a VoIP deployment, as data and voice VLANs can be created to minimize the effect of data transmission on the voice network. VLANs work to reduce the number of collisions and lost packets on a network. There are two types of VLAN – Port-based and policy-based. A Port-based VLAN will assign certain ports on a switch to be members of a VLAN. This is the most common type of VLAN. A policy-based VLAN can be based on a variety of information, such as IP Address, MAC Address, or Protocol (i.e. IP vs. IPX). Most new switches support Port-based VLANs. It is possible to improve QoS by putting IP phones on a separate VLAN from data and other devices, thereby avoiding collisions with these devices.
To ensure QoS we can also select queuing methods for the packets that come into our router. There are six methods of queuing for QoS:
1. First-in First-Out is a straight-forward method of queuing. The first packet received by a switch is the first on sent out by the switch
2. Priority Queuing tells the switch to process high priority traffic first
3. Custom Queuing lets the administrator assign traffic to a certain queue to control how much data is sent each time to a particular queue
4. Fair Queuing uses a round-robin approach – each queue receives the same amount of bandwidth
5. Weighted Round Robin lets the administrator allocate different amounts of bandwidth to different priority queues while ensuring that lower priority queues still receive some bandwidth
6. Weighted Fair Queuing identifies logical conversations (high volume traffic between 2 peers) and makes sure that those conversations do not monopolize the link.
The Resource Reservation Protocol (RSVP) is a method used to reserve resources (or bandwidth) on an RSVP-Compliant router. Resources are allocated on a first-come-first-serve basis. A device requests a certain amount of bandwidth, and the router replies that the bandwidth is now reserved. It will not release the bandwidth until the device is done. This protocol does not carry data; it only acts as a control mechanism for RDP packets. It is not very widely used today.
Traffic Shaping is used to help alleviate network congestion. Network congestion can occur when multiple users contend for access to the same resources. Traffic Shaping lets you determine which packets are dropped due to congestion and which packets receive priority. It can take action based on various characteristics of traffic. It also relies on queuing mechanisms.
IP Address prioritization is as simple as it sounds – packets from certain IP addresses are given priority over packets from other IP addresses. This is ideal for devices with static IP Addresses, such as Call Servers, Signaling Servers, VoIP Gateways, etc. A Network Administrator can configure routers to filter and prioritize all packets originating from these IP Addresses and know they are from VoIP devices.
The Ethernet Standard specifies a frame as having a maximum size of 1,518 bytes. The IEEE defined an additional 4-byte tag for Ethernet Headers, increasing the maximum size to 1,522 bytes. This tag contains two valuable pieces of information that pertain to QoS, the 802.1w VLAN ID Field, and the 802.1p Priority Field.
I will begin by discussing the 802.1p Priority field. Three bits in the IEEE Ethernet Protocol Specification can be used to prioritize packets at the user level. The three bits allow for up to eight levels of prioritization (0-7). Any 802.1p switch is capable of reading this field and can then retransmit the packet across the network with the higher priority. The higher the number in the priority field, the higher priority the switch will give to the packet. The IEEE has made some recommendations for the numbering system to be used here, but it is at the discretion of the network administrator. IEEE recommendations are to use 0 for Best Effort, 1 for Background, 2 as a spare, 3 as Excellent Effort, 4 as Controlled Load, 5 as Video, 6 as Voice, and 7 as Network Control.
The 802.1q specification, also known as VLAN Tagging, refers to the process of identifying which VLAN a packet is coming from. A VLAN, or Virtual LAN, makes a physical switch into smaller logical groupings. For example, a 24-port switch may be divided into three smaller 8-port virtual switches to eliminate leakage of data from one section to the next. This is very useful in a VoIP deployment, as data and voice VLANs can be created to minimize the effect of data transmission on the voice network. VLANs work to reduce the number of collisions and lost packets on a network. There are two types of VLAN – Port-based and policy-based. A Port-based VLAN will assign certain ports on a switch to be members of a VLAN. This is the most common type of VLAN. A policy-based VLAN can be based on a variety of information, such as IP Address, MAC Address, or Protocol (i.e. IP vs. IPX). Most new switches support Port-based VLANs. It is possible to improve QoS by putting IP phones on a separate VLAN from data and other devices, thereby avoiding collisions with these devices.
To ensure QoS we can also select queuing methods for the packets that come into our router. There are six methods of queuing for QoS:
1. First-in First-Out is a straight-forward method of queuing. The first packet received by a switch is the first on sent out by the switch
2. Priority Queuing tells the switch to process high priority traffic first
3. Custom Queuing lets the administrator assign traffic to a certain queue to control how much data is sent each time to a particular queue
4. Fair Queuing uses a round-robin approach – each queue receives the same amount of bandwidth
5. Weighted Round Robin lets the administrator allocate different amounts of bandwidth to different priority queues while ensuring that lower priority queues still receive some bandwidth
6. Weighted Fair Queuing identifies logical conversations (high volume traffic between 2 peers) and makes sure that those conversations do not monopolize the link.
The Resource Reservation Protocol (RSVP) is a method used to reserve resources (or bandwidth) on an RSVP-Compliant router. Resources are allocated on a first-come-first-serve basis. A device requests a certain amount of bandwidth, and the router replies that the bandwidth is now reserved. It will not release the bandwidth until the device is done. This protocol does not carry data; it only acts as a control mechanism for RDP packets. It is not very widely used today.
Traffic Shaping is used to help alleviate network congestion. Network congestion can occur when multiple users contend for access to the same resources. Traffic Shaping lets you determine which packets are dropped due to congestion and which packets receive priority. It can take action based on various characteristics of traffic. It also relies on queuing mechanisms.
IP Address prioritization is as simple as it sounds – packets from certain IP addresses are given priority over packets from other IP addresses. This is ideal for devices with static IP Addresses, such as Call Servers, Signaling Servers, VoIP Gateways, etc. A Network Administrator can configure routers to filter and prioritize all packets originating from these IP Addresses and know they are from VoIP devices.
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