Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 1

Bob Braden

USC Information Sciences Institute

30th AnniversarySept 9, 2002

Internet Quality of Service -- Fantasy and Reality

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 2

What is QoS?

• “Qwoss” -- No, it’s not an exotic vegetable...• Fundamental network capability:

Allowing an end user (or collection of users, e.g., a campus or corporation) to control the

attributes of communication service.

• It sounds like almost a no-brainer, but it is actually exceedingly difficult and a bit fuzzy.

• It cuts across many basic technical and economic aspects of the Internet.

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 3

Early History

• Internet designers called it Type of Service (TOS)• 1981: Jon Postel defined Internet Protocol (IP)

[RFC 791]

• He took a SWAG to define a TOS byte in the IP header:– 3 bits of priority (Mandated by DoD -- “precedence”)

– 3 bits of TOS attributes:• Low delay?

• High throughput?

• High reliability?

• This sounded plausible, but ...

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 4

But...

• What do these 3 TOS attributes MEAN?

• How can routers implement them?

• How/when should they be set?

• Is this the “right” set of attributes?

• Need quantifiers?

• How can you prevent a tragedy of the commons?• Administrative control (e.g., military/corporation), or

• Economic control -- charging $$$

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 5

Why do We Need QoS?

• To match network service to application requirements?– Resolve conflicts when network is overloaded, e.g.,

• Interactive services want minimum delay

• File transfer & email want maximum throughput

• Web users want both (!?)

• Or to make some users more equal than others?

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 6

Making Users Unequal

• Basic Internet design: all users are equal!• DoD said: “No way!”

– Required service priority, linked to military hierarchy.

• ISPs want to sell premium service to corporations and government agencies that can afford it.

• Free-market approach -- “QoS Knob”.• Download too slow? Move the slider up, increase

your cost per minute.

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 7

Why is QoS Hard (1)?

1. How do you define the service?– By its effect? Service attributes observable by users in

end systems• ISPs want to write contracts, called “Service-level agreements”

(SLAs), and charge money.

– Or by mechanism? Specific queueing mechanisms in routers

– Users and providers care about effect, not mechanism -- need to pick a service model.

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 8

Service Model...

• User-observable attributes might be:– Reliable delivery of data? (How reliable?)

– Ordered delivery of data?

– Max bandwidth (measured over what interval?)

– Max end-to-end [queueing] delay

– Max jitter (delay variation)

• Which ones matter?

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 9

Effect vs. Mechanism

• Suppose your ISP says: “For an extra $10/month I will give your packets priority over packets from users who don’t pay extra.”

– Priority is a mechanism; what would its service effect be?

– It’s quite hard to connect: effect <=> mechanism; many academic papers have been generated on this topic.

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 10

Service Model...

• It’s quite hard to connect: service <=> mechanism– Internet traffic does not follow any simple statistical

laws; it can be very BURSTY.

– Generally, to build a mechanism matching a useful service model requires traffic shaping/policing mechanisms to place a bound on the burstiness.

• Delay or drop non-conformant packets in each user stream

• Most common: token bucket shaper/policer.

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 11

Why is QoS Hard (2)?

2. You can build mechanisms that operate strictly packet/packet, but to make any QoS guarantee requires per-flow state in routers.– Violates the Internet religion:

“Thou shalt forward IP datagrams using stateless routers.”

• This religion provided simplicity, robustness, generality, and scalability -- not to be given up lightly.

– There are also resource and scalability issues.

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 12

Why is QoS Hard (3)?

3. QoS requires accounting/feedback (e.g., charging) to avoid a tragedy of the commons.– Many technical, business, legal, social problems...

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 13

A Case Study of QoS

• 1991: Internet research community believed that multimedia teleconferencing would become the 1000 pound gorilla (“killer app”) on the Internet.

(The Web had not happened yet!).

– ISI research had played a significant role...• Packet speech experiments on ARPAnet [Danny Cohen @ISI].

• VTC research @ ISI [Steve Casner] & @ BBN.

• VTC technology -- the “MBONE tools” -- developed on DARPA Research Testbed Network (DARTnet) built & operated @ ISI.

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 14

Realtime...

• Packet voice, and to a lesser extent packet video, require “realtime” service -- bounded E2E delay.

• The Internet research community set to work on the technical problems ...

• Developed: Internet Integrated Service.• Mostly DARPA-funded.• 1994: “Integrated Services in the Internet Architecture”

[RFC 1633: Braden@ISI, Clark@MIT, Shenker@PARC]

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 15

Internet Integrated Service (IIS)

• How define service?– Two service models:

• Guaranteed [tight bound on E2E Q delay]

• Controlled Load [loosely defined “good service”]

• How do users request IIS?– RSVP: signaling protocol to request and set up QoS

state in routers.• Initial design: Zhang & Shenker @PARC

• Prototyping and standardization: Estrin, Braden, Berson, Lindell, Herzog @ ISI.

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 16

What Happened to IIS?

• The Web happened, and it became the Internet gorilla, not VTC.

• Some IETF opinion-leaders excommunicated IIS as heretical to the Internet religion.

• Microsoft bought into IIS

– Windows 2000 implements it.

• The RSVP signaling protocol component has been widely adopted/adapted to other Internet signaling applications.

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 17

What Happened?

• ISPs ignored IIS.

– No business case or other incentive.

– E2E IIS requires more collaboration than ISPs can muster

– Another problem, which I have no time to discuss: Multicast!

• The payment problem is not much closer to solution.

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 18

What Happened?

• ISPs have pushed another QoS approach: Differentiated Service.

– Redefine 6 bits of TOS byte to select 64 service classes.

– QoS within ISP clouds rather than E2E.

– Classify packets in boundary routers between ISPs, set these bits for use by the interior routers.

– They can charge large customers for preferred service.

Sept 9, 2002 ISI 30th -- Bob Braden -- QoS 19

Current Status of Internet QoS

• We know much more than we did 20 years ago about QoS service models and mechanisms.

• There is still no Internet-wide deployment of QoS.• However, some forms of QoS are implemented by

router vendors and are deployed in private intranets.

• The IP telephony gorilla is crashing in the underbrush ...