Quality of service: who goes first
When a link is busy, someone has to wait. Quality of service (QoS) decides who. Follow a voice call and a big photo backup through one 10 Mb/s uplink: the router classifies each packet, marks it in the IP header, gives each class its own queue, sends voice first and shares the rest fairly, then uses policing and shaping to keep each class within its rate.
5 quick questions
Pick an answer to see why. Your score saves with your progress.
01Which need matters most for a voice call?
A call sends only about 80 kb/s, but each packet must arrive within about 150 ms and at an even pace. A backup is the opposite: it wants throughput and hardly notices delay.
02What is the DSCP?
The Differentiated Services Code Point uses 6 bits of the IP header (RFC 2474), so it has 64 values. Routers read it to decide how to treat the packet.
03Which DSCP value is the standard mark for voice, Expedited Forwarding (EF)?
EF is 46, binary 101110 (RFC 3246). 0 is the default (DF, best effort) and 1 is Lower Effort (LE, RFC 8622) for traffic that should go last.
04With strict priority for voice on a 10 Mb/s link, why can a voice packet still wait about 1.2 ms?
Priority decides what goes next, but a packet already on the wire finishes first. 1,500 bytes × 8 ÷ 10,000,000 bit/s = 1.2 ms.
05What is the difference between policing and shaping?
A policer has no queue: over the rate means drop now. A shaper buffers the excess, so nothing is lost but packets wait a little longer.
Glossary
- Quality of service (QoS)
- Rules a router uses, when a link is busy, to decide which packets go first, which wait and which may be dropped.
- Jitter
- Variation in delay from one packet to the next. Voice and video need it small, or sound and pictures stutter.
- Throughput
- How many bits per second actually get through. Downloads and backups care about it most.
- Classification and marking
- Sorting packets into classes by their headers (addresses, ports, protocol), then writing the class into the packet so later routers can read it.
- DSCP
- Differentiated Services Code Point: 6 bits in the IPv4 or IPv6 header (RFC 2474) that carry a packet's class, such as EF 46, DF 0 or LE 1.
- Strict priority queue
- A queue that is always served first when it has packets waiting. Used for voice, and rate-limited so it cannot starve the others.
- Weighted fair queueing
- Serving several queues in turn, each getting a share of the link in proportion to its weight; a share nobody uses goes to the others.
- Policing and shaping
- Two ways to hold traffic to a rate: a policer drops or re-marks the excess straight away, a shaper delays it in a queue and sends it out evenly.
Notes and sources
- The weights (3 : 1), the 1 Mb/s voice policer and the 2 Mb/s backup shaper are illustrative settings. The voice packet is G.711 with 20 ms of sound per packet; the 798 ms wait is a full 1 MB buffer on a 10 Mb/s link, from the queues and buffering lesson.
- 192.168.1.20 and 192.168.1.30 are private home addresses (RFC 1918); 198.51.100.1, 192.0.2.40 and 203.0.113.20 are documentation addresses (RFC 5737).
- Each tray draws at most 7 envelopes; real queues can hold hundreds of packets.
- QoS only matters where packets queue: at the slowest link, usually your uplink. Many networks, including most of the internet, ignore or reset DSCP marks they do not trust.
- Downloads queue at the ISP's end of your line, out of your router's reach; home routers approximate QoS for them by shaping incoming traffic a little below the line rate.
- RFC 2474: Definition of the Differentiated Services Field (DS Field)
- RFC 2475: An Architecture for Differentiated Services
- RFC 3246: An Expedited Forwarding PHB
- RFC 8622: A Lower-Effort Per-Hop Behavior (LE PHB)
- RFC 4594: Configuration Guidelines for DiffServ Service Classes
- ITU-T G.114: One-way transmission time
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