IP Bandwidth Sharing

1. IP Bandwidth Sharing Paul Ferguson Consulting Engineer Internet Architecture Office of the CTO ferguson@cisco.com 1

3. What exactly is “bandwidth sharing”?

4. Bandwidth sharing: • It can be called the “thing” that TCP/IP does to allow bits of information originating from (and destined to) various sources to utilize the same pathways. • IP has been this doing this since Day 1.

5. So what’s the problem?

6. Well, there actually might not be a problem: • Is there congestion? • If “Yes”, then there’s definitely a problem. This is where “bandwidth management” comes into the picture. • If “No”, then there might be a perception or expectation problem (more on that later).

7. Bandwidth Management: • Ensure that the network provides an adequate “appropriate environment” for applications, some of which may have “special” requirements. • Ensure that the network doesn’t melt down.

8. Problem/Objective: • Avoid congestion, or • Provide an “appropriate environment” for applications which have “special” requirements -- “traffic protection”. • Provide an environment which provides the least amount of end-to-end delay.

9. In all cases…. • Ongoing capacity monitoring and planning is required. • If you do not know how much traffic is in your network, then this is a problem (e.g. peak/avg. rates, traffic source/dest.)

10. A Simplified Review of Options

11. Avoiding congestion… • Over-build. Throw bandwidth at it. • Limit aggregate incoming traffic to bandwidth of smallest link. • Neither of these are necessarily realistic.

12. Dealing with congestion… • Allow queues to tail-drop packets. • Or do something else. Several options are available here. More on this later….

13. Do nothing... • Tail-dropping packets can have an adverse impact on all traffic traversing the router, resulting in poor performance for a larger percentage of traffic. • No control for which packets get dropped during congestion events.

14. So something….. • …which provides traffic differentiation in the face of congestion, and/or • ….partition bandwidth to allow protection for a subset of traffic.

15. A brief note on “The most common denominator” • The End-to-End KISS theory of working within the Most Common Denominator -- IP. • “An IP packet may traverse any number of link-layer mediums (e.g. Ethernet, FDDI), so any differentiation done at the link-layer is lost in the end-to-end problem.”

16. Architectural Overview

17. Congestion: We all know what happens when you do nothing….. • It just gets worse. • And people complain. • And sometimes, heads roll when the performance is intolerable.

18. Stateful IETF Integrated Services (Intserv)/RSVP Stateless IETF Differentiated Services (Diffserv) IP Differentiation: Two options

19. Diffserv: Classifier Shaper Policer Scheduler Dropper Intserv: Classifier Shaper Policer Scheduler Dropper Resource Reservation The Building Blocks:

20. Building blocks (2): • Classifier: Classifies packets individually, or as belonging to a flow. • Shaper: Compares incoming traffic to a profile and drops/remarks packets which exceed a threshold. • Policer: Compares incoming traffic to a profile and drops/remarks packets which exceed a threshold.

21. Building blocks (3): • Scheduler: A (non-FIFO) queuing strategy. • Dropper: A (non-taildrop) packet discarding scheme. • Resource Reservation: RSVP

22. Major differences: Intserv & Diffserv • State, or no state. • RSVP has some minor scaling concerns, when individual flows using RSVP grow beyond a few hundred (or perhaps a few thousand). This concern may be somewhat alleviated in the near future with an RSVP reservation aggregation scheme.

23. Diffserv EF PHB: Major points • Strict use of shaping to conform incoming EF traffic to available capacity. • aggregate EF ingress <= % of link capacity set aside for this “service” in core • Packets marked as EF get priority transmission. • Fairly good data protection

24. Diffserv AF PHB: Major points • Packets are simply marked with relative priority. • The service provider can interpret handling at-will. • Provides soft or “squishy” differentiation.

25. What acronym have I, thus far, avoided? • QoS, or Quality of Service. I suggest that “QoS” and “bandwidth management” are intrinsically one and the same in the world of IP. • Further….

26. What about “hard guarantees” on end-to-end delay and jitter? • Well, RSVP gives you a bound on end-to-end maximal queuing times which basically bound delay for flows. But it really doesn’t provide for jitter control. It does, however, “protect” flows and guarantee bandwidth. • Diffserv’s EF PHB, I believe, parallels the Intserv controlled-load service.

27. What about “hard guarantees” on end-to-end delay and jitter? (2) • Remember: TDM and Packet technologies are fundamentally and intrinsically different. Jitter is an issue within the packet world that is generally uncontrollable at an absolute level. (Think: RTP)

28. What about “hard guarantees” on end-to-end delay and jitter? (3) Comparison note: • TDM -- Remember what TDM stands for? There really is no delay or jitter in a TDM world -- everything is timing and timing rates. Basically, this has become an unrealistic (my opinion) standard for VoIP -- hard delay & jitter “guarantees”.

29. What about “hard guarantees” on end-to-end delay and jitter? (4) • RSVP: Fairly hard guarantee on end-to-end “maximal queuing delay”. No guarantees on jitter, although probabilistically good.

30. What about “hard guarantees” on end-to-end delay and jitter? (5) • Diffserv EF & AF PHB: No guarantees on delay or jitter. Semi-soft and squishy “guarantees” on delay, respectively. Jitter still elusive insofar as guarantees are concerned, but with EF jitter is less of a concern. AF jitter probability is directly related to priority ordering.

31. Jitter: • There are no real control mechanisms within the network to control end-to-end jitter. Sure, a consistent queuing scheme might help to make it predictable, but it can never guarantee it.

32. Jitter message (2): • Probably the most effective method of dealing with jitter is to adapt at the end-system (e.g. RTP-based monitoring).

33. Topological significance • Tools (components) used at only the nodes they are needed. • Classify/Mark/Shape packets close to the “edge”, not in the network core.

34. Where’s the architecture? • That’s it. • Before you can effectively design the architecture, you must define it. • Once that is done, you can look at applications and topologies, and decide which method is appropriate.

35. Perception problems • What happens when there is no congestion, and you want to differentiate traffic? What happens, and what would you do? Huge problem. • There is also the problem of UDP (and other non-responsive traffic).

36. Summary • Remember: There are two worlds. One is the global Internet, and the other is private organizational networks. • PATH and RESV state are not always evil, depending on what you really want. • Jitter control is an extremely hard problem to solve in the network.

37. Summary (2): • Jitter is sometimes more easily dealt with by the host, who can readily adapt when using a real-time protocol (e.g. RTP). • Voice is very sensitive to delay & jitter. • Jitter is sometimes very difficult (if not altogether impossible) to remove from packet networks.

38. Summary (3): • It’s generally not practical (or possible) to over-build the network. • Low or No Delay and Jitter are very important for some applications, not for others. • It’s generally very difficult to maintain a balance of economies of scale and sustain network performance.

39. Fin. Thank you. ferguson@cisco.com