Difference between revisions of "QoS"

Revision as of 15:47, 8 February 2007

General

Quality of Service (QoS) refers to control mechanisms in data networks that try to ensure a certain level of performance to a data flow in accordance with requests from the application program. Data equipment tries to ensure the requested level of performance among others by performing

• Call Admission Control,
• Classification of Packets,
• Class based Packet processing
  • Queuing and Scheduling,
  • intelligent dropping (random early detect, RED)
  • Traffic Shaping.

Call Admission Control

Mechanisms like Queuing and Scheduling help to discriminate high prority or "real-time" traffic from other data traffic. However, if the sum of all high priority flows exceed the capabilites of network links or elements, those techniques will fail. In this case, an additional high priority flow will degrade the quality of all existing flows.

Call Admissin Control makes sure that existing flows will keep their quality by rejecting any additional high priority flow. In the terms of telephony, the call is rejected and will either receive a busy signal or is rerouted.

Centralized Static Call Admission Control

Siemens HiPath 8000 has implemented a centralized static Call Admission Control which works in hub-and-spoke or star topologies. Topologies are abstracted to zones with unlimited bandwidth ressources which are interconnected by bottlenecks. If phone A_1 resides in zone_A only, if the bandwidth needed does not exceedand phone B_1 resides in zone_B and those zones are interconnected with a bottleneck link of 150 kbps, the call of 100 kbps is admitted, if no other call is using the bottleneck [link. Now, a second phone A_2 in zone_A wants to start an other 100 kbps call to phone B_2 in zone_B. Since the needed bandwidth would exceed the left bandwidth ressources of 50 kbps, the call will be rejected, preserving the call quality of the first call.

In the case of the HiPath 8000 solution, telephones are classified to be members of certain zones based on IP-addresses or telephone numbers.

Cisco CallManager makes use of the very same centralized static Call Admission Control and also is limited to hub-and-spoke or star topologies.

Network based dynamic Call Admission Control

A standard way of performing Call Admission Control is Ressource ReserVation Protocol (RSVP), i.e. the "IntServ Model". With RSVP, each end system that seeks to send a data flow will send an RSVP PATH message to its communication partner. Network elements along the path that are RSVP aware will add route records to the RSVP PATH messages; others will route the packets like normal IP packets. The communication parter will respond to the RSVP message with a RESV packet and will send it back along the recorded path. RSVP aware network elements now will perform the reservation, if enough bandwith is available or will indicate the failure in the RSVP RESV packet.

RSVP has gained a bad reputation of not being scalable. However, Cisco argues, that the bad reputation is because RSVP had been used for Call Admission Control as well as Queuing and Scheduling. When using RSVP for Call Admission Control only it will be scalable and they recommend to use the "DiffServ model" based on packet marking for Queuing and Scheduling.

Classification of Packets

Network elements like routers or switches classify the ingress traffic based on

• ingress interface
• Information found in the frames (Layer 2) like
  • 802.1p/802.1Q priority (often called COS, class of service; mostly used)
  • L2 MAC Addresses
  • VLAN ID
• Information found in the packets (Layer 3) like
  • IP Precedence/DiffServ Codepoints (DSCP) (mostly used)
  • IP Addresses
  • TCP/UDP port numbers
• or higher-layer information (e.g. Cisco NBAR; usually performed at network edges only, and mapped to DiffServ Codepoints there)

The most commonly classification scheme is to use IEEE 802.1p/Q priority bits for layer 2 switching and the Type of Service/DiffServ Codepoints within the IP header for layer 3 queuing. < add a picture of a frame here, which shows the priority bits within the L2 802.1Q VLAN tag as well as the DSCP-fields in the IP header >

Voice Streams

For voice (real time protocol, RTP) streams, Siemens phones use the DiffServ Codepoint EF, "Expedited Forwarding" (i.e. decimal value 46, binary 101110) on layer 3. Since most network vendors map the first 3 bits to the IEEE 802.1p/Q priority (COS) bits, it is recommended to use COS 5 (binary 101) on layer 2.

Signaling

For signaling the DiffServ Codepoint AF31 (decimal value 26, binary 011010) is used. Since most network vendors map the first 3 bits to the IEEE 802.1p/Q priority (COS) bits, it is recommended to use COS 3 (binary 011) on layer 2.

Class based Packet Processing

If an egress interface is congested, the network element needs to perform an action on packets. It needs to decide, which packets are sent with a higher priority than others, which packets will be stored and queued for later transmissions and which packte will be dropped.

Queuing and Scheduling

In today's most network elements, two ore more packet queues exist per egress interface. Based on the traffic class of a packet, network elements will send the packet to a queue, if the egress interface is congested. The interface's scheduler chooses a packet from a queue and sends it to the egress interface. Several scheduling mechanisms exist:

• priority queuing: as long as the highest priority queue is no empty, packets from this queue are sent only and all other queues need to wait. This can lead to starvation of lower priority packet flows.
• (weighted) round-robin: all queues are served one after the other e.g. queues 1 to 4 are served in the order 1-2-3-4-1-2-3-4 etc. If queue 1 has double weight than the other ones, it will be served like 1-1-2-3-4-1-1-2-3-4- etc. In complex scheduling mechanisms, the packet length will also be taken into account.
• (class-based) fair queuing: packets are classified based on the flow parameters, e.g. source- and destination-IP-addresses and TCP/UDP ports. Idea is to give each flow a fair access to the egress interface. In class-based fair queuing, packets are mapped to traffic classes. Between classes, e.g. weighted round-robin will be performed. Within a traffic class, fair queuing will take place.
• combinations of the above: e.g. Cisco Low Latency Queuing (LLQ) combines priority queuing for real-time applications with class-base weighted fair queuing.

Intelligent Packet Dropping

Random Early Detect (RED) is used ...

Traffic Shaping

External Links

• QoS on Public Wiki