Differentiated Services

1. Differentiated Services • Avoids the resource reservation process • Operates on a per-hop behavior (PHB) basis • Classifies packets by marking the headers; Routers discriminate packets based on their markings • Packet marking is done on the basis of a service level agreement (SLA) between the host and the ISP • Provides service assurances but no QoS guarantee

2. Differentiated Services in the Internet • Basic concept • DiffServ architecture • Expedited forwarding • Assured forwarding • An Integrated Model • Efficient Techniques for DiffServ

3. Differentiated Services: Basic concepts • Differentiated Services (DiffServ) is proposed by IETF as a scalable QoS solution for the next generation Internet. • Achieves scalability by implementing complex classification and conditioning functions only at network boundary nodes, and by applying per-hop behaviors (PHB) to aggregates of traffic that are marked using the DS field. • PHB permits a reasonable granular means of allocating buffer and bandwidth at each core node among competing streams. • Per flow state maintenance is not required

4. Intermediate marker Intermediate marker BB BB BB DiffServ Architecture Leaf marker Intermediate marker BB Leaf marker --end user --edge router --core router

5. DiffServ Concepts • The host and the domains have service level agreements (SLA) with the ISPs they will need to use. • DS field in the TOS byte is used for marking the packets. • The packets are classified and policed at the ingress of the ISP networks at the edge routers. • Packets are marked as in-profile or out-of-profile based on their respective SLAs. • During congestion, the routers preferentially drops the out-of-profile packets. • Marking and dropping of packets is done on an aggregate basis using a single queue.

6. DiffServ Requirements • Should accommodate a wide variety of services • Should work with existing applications without the need for API changes • Should not depend on hop-by-hop signaling • Should avoid state maintenance in core routers • Should utilize only aggregated classification states within core routers • Should permit interoperability with non-DS compliant network nodes • Should accommodate incremental deployment

7. Service Level Agreement (SLA) • SLA is usually negotiated prior to the packet transmission between a host and the DS domain to which it is attached or between two neighboring domains • The service level includes parameters such as the flow rate, burst size, delay requirements, etc. • SLA can be negotiated statically or dynamically. Dynamic SLA negotiation is on a need basis. • Markers are used to ensure conformance of SLA

8. DiffServ Codepoint (DSCP) • Each service class has its own DiffServ Code Point (DSCP), which is a part of the DS field. • DSCP are encoded using 6 bits of the ToS byte of IPV4 header or the Traffic Class byte of IPv6. • DSCP is used to define the PHB. • The value of the DSCP is derived from the SLA. • Each packet is assigned a DSCP at the edge of the domain. • Core routers check the DSCP to determine the forwarding priority.

9. Per-Hop Behavior (PHB) • PHB describes the forwarding behavior of a DS node applied to a particular DS behavior aggregate • PHB is the means by which a node allocates resources to behavior aggregates • PHB is specified in terms of their resource (e.g. buffer, bandwidth) priority relative to other PHBs, or in terms of their relative observable traffic characteristics (delay, loss, etc.) • PHB is selected at a node by mapping the DS codepoint in a received packet

10. Classes of Services • Currently has two classes of services other than the best-effort service: • EF (expedited forwarding) and AF (assured forwarding). • EF defines the Premium Service and AF defines the Assured Services. • Premium service is similar to leased lines, and is useful for hard real-time applications. • Assured service is adequate for most multimedia applications.

11. Traffic Classification and Conditioning • Traffic Conditioning Agreement (TCA): specifies classifier rules and any corresponding traffic profiles and metering, marking, discarding and/or shaping rules which are to be applied to traffic streams selected by the classifier • Classifiers are used to segregate packets matching some specified rules and steer to the traffic conditioner (TC) for further processing • Two types of classifiers are defined: • Behavior aggregate (BA) • Multi-field (MF)

12. Packet Classifier and Traffic Conditioner METER PACKETS CLASSIFIER MARKER SHAPER DROPPER

13. Packet Marking • Traffic Profile: in-profile and out-of-profile • Traffic Conditioners: meter, marker, shaper, dropper r bits/sec LEAKY BUCKET FORWARDING ENGINE PACKET CLASSIFIER IN IN/OUT REMARKING OUT

14. Flow Control in Routers • Fair Queuing: Provides fair services to the flows • Problem: requires state maintenance • Tail-dropping scheme: When the buffer gets full, drop all the incoming packets • Problems: • Bias against bursty traffic • Global synchronization problem • DECbit Gateways: the gateway uses a congestion-indication bit in packet headers to provide feedback about congestion in the network • Has bias against bursty traffic • The congestion bit is set on the basis of the average queue length for the last period plus the current busy period

15. RED Scheme • Random Early Detection (RED) is used for flow control in routers • It eliminates the global synchronization problem and the impact of bursty effect P(drop) 1.0 Pmax MAX-thr MIN-thr qlen-avg MIN-thr MAX-thr

16. maxout minout RED-In/Out (RIO) Queue • Pout = Pmaxout (avgout+in – minout) / (maxout – minout) • Pin = Pmaxin (avgin – minin) / (maxin – minin) • If avgout+in < minout, no packet dropped, if avgout+in > minin, all Out packets are dropped • If avgin < minout, no packet dropped, if avgin > minin, all Out packets are dropped maxin minin

17. RIO Scheme P_out (drop) P_in (drop) 1.0 1.0 P_max P_max Avg_in Avg_tot MIN-thr MAX-thr MIN-thr MAX-thr

18. Premium Service (PS) • PS provides guaranteed services (low delay, low-jitter) for clients that generate fixed peak bit-rate traffic; each of these client will have and SLA with its ISP • The ISP guarantees the availability of the agreed bandwidth by reserving a portion of the total capacity. The client is not expected to exceed the peak rate; excess traffic is dropped • SLA negotiation can be static or dynamic • Suitable for Internet telephony, video conferencing, creating virtual lease lines for VPNs etc.

19. Assured Services • Bandwidth is allocated on the basis of static bandwidth allocation • Classification and policing are done at the ingress routers of the DS domain • Packets classified as in-profile or out-of-profile are enqueued in the same Assured Queue (AQ) • AQ is managed by the RIO queue management scheme • The conformant traffic perceive predictable service from the network

20. Premium Service (PS) - Implementation • PS is designated by setting up the P-bit in the DS field at the boundary nodes • All packets with the P-bit set enters a premium queue (PQ) and are served before packets in the other queue(s). (PQ and the other queues could use WFQ for service) • The ingress router does admission control to allow conformant flows. Non-conformant flows are dropped • The first-hop router performs traffic shaping and sets the designated P-bit.

21. Example of Premium Service – Static SLA bandwidth broker host core router boundary router

22. Example of Premium Service – Dynamic SLA bandwidth broker host core router boundary router

23. Example of Assured Service bandwidth broker host core router boundary router

24. An Integrated DiffServ Model • Provides both premium services and assured services • Benefits: • Both services are only as good as their admission control schemes • Overallocation of resources for premium services can be utilized by assured services • Assured services allow burst to happen naturally, while premium service employ static allocation of peak rates • Location and functional blocks needed for implementation of the schemes are very similar; integration is quite feasible • Packets are marked as the edge router: A-bit is set for assured traffic, P-bit is set for premium traffic

25. Integrated Scheme: Border Router Functionality Token available? Clear A bit No A-Bit Token Forwarding Engine Is packet marked? Token P-Bit Token Available? Drop packet No

26. A Random Early Demotion and Promotion (REDP) Marking Scheme • REDP is an intermediate marker • Works for Assured Service • Assured Service is proposed to provide an “explicit minimum throughput” assurance for individual flows. • May also be beneficial for Premium Service • In-profile packets: Green • Out-of-profile packets: Red

27. Motivation: • Previously proposed Leaky bucket(LB) marker is simple, but may not work well under deficient or surplus SLAs. • In case of deficient SLA, a “green” packet has to be demoted to “red”. But because of the phase effect of UDP and TCP flows, a simple leaky bucket marker will be UNFAIRindemotion. • In case of surplus SLA, it is desirable to promote those demoted “green” packets. But a simple leaky bucket marker CANNOT do this. • The Random Early Demotion and Promotion (REDP) marker is designed to solve these two problems.

28. Supports at Internet Edge • Leaf marker works for individual flow. In profile packet are marked as “green”, out of profile packets are marked as “red” • Intermediate marker works for flow aggregate. It demotes “green” packets to “red” when the SLA is deficient. Leaf Marker Intermediate Marker ISP Network Domain Local Network Domain Domain Boundary

29. Support in Internet Core • EF packets are enqueued in a higher priority queue • AF packets and best-effort packets are mixed in the other queue. AF packets have lower drop probability. EF AF + Best-Effort Packet classifier Forwarding Engine Internet Core Router

30. Adding a Third Color “Yellow” • “yellow” is used to “memorize” the demoted “green” packets • In case of SLA deficiency, only “yellow” packets get promoted State Transition of a Packet in the REDP Marker Red R > negotiated rate Green Yellow R < negotiated rate R =< negotiated rate R >= negotiated rate R: the aggregate “green” packet rate

31. Random Demotion and Promotion • Balanced region: a packet is forwarded with its original color. • Demotion region: a “green” packet is demoted with the following probability: Pdemo = (TL-Tokennum) Pmax / TL • Promotion region: a “yellow” packet is promoted with the following probability: Ppromo = (Tokennum-TH)Pmax / (b-TH) Token Collector r bits/sec b promotion region TH balanced region TL demotion region 0

32. Why Random? • Periodical UDP traffics have serious Phase Effect. • A leaky bucket marker will cause unfair demotion. • Random demotion decisions could improve fairness.

33. Simulation Topology 10 Mbps 10 Mbps 1.6 Mbps (bottleneck) domain boundary 1.6Mbps AF rate Simulations for UDP: • Simulation(ns simulator): four udp flows (0.6Mbps rate, 0.5Mbps originally marked as “green”) compete for the 1.6Mbps bottleneck link.

34. Results: • REDP marker removes the Phase Effects encountered by the leaky bucket marker Bandwidth of the Four UDP Flows

35. Why Random (cont.) ? • TCP traffic is not as periodical as UDP traffic, but also has Phase Effect. Round Trip Time (RTT) is the period. • By setting the RTT of the four flows as 26ms, 28ms, 26ms, 28ms respectively, we can observe the unfairness.

36. Bandwidth of the Four TCP Flows Simulation Results for TCP: • REDP marker can also remove the Phase Effect of TCP flows, hence providing a fairer bandwidth sharing.

37. Why Promotion? • Demoted packets may not really get dropped soon. • Promotion of these demoted packets could decrease the drop probability of the packet in the remaining path. • Promotion could increase the assurance of the “explicit minimum throughput” of a flow under some situations.

38. flow1: 500 kbps AF 10 Mbps flow2: 0 kbps AF 10 Mbps 2 Mbps 10 Mbps 1.2 Mbps (bottleneck) flow3: 500 kbps AF domain boundary 1 600 kbps AF domain boundary 2 1.2 Mbps AF flow4: 0 kbps AF Simulations: • At boundary 1, 400 kbps among the 1000 kbps “green” packets has to be demoted but not dropped. At boundary 2, if we can promote those demoted packets, they won’t get dropped at the bottleneck link.

39. UDP Flows Simulation Results (UDP) • For UDP flows, 3-color promotion (REDP) could increase the assurance of flow1 and flow3 to 500kbps, while 2-color promotion does not work. • 2-color promotion: randomly select a “red” packet to promote • 3-color promotion: “yellow” is used to memorize those demoted green packets. Only “yellow” are promoted.

40. Simulation Results (TCP) • TCP has the similar results as UDP TCP Flows

41. Three Drop Precedences vs. Two Drop Precedences • Previous 3-color promotion assumes the use of RIO scheme in core routers. • “green” is deemed as “In”, “yellow” and “red” are deemed as “Out”. • If the core router could support 3-drop precedences, then “yellow” could get priority over “red”, hence would increase the assurance.

42. Simulation results: UDP TCP • Same as the promotion simulation, change the 2Mbps link to 1.2Mbps. • 3-drop precedences could further increase the assurance.

43. Relative Service Differentiation • The network traffic is differentiated into N service classes. • Class i is better than class (i-1) for 1<=I<=N in per-hop performance measure • Provides more flexibility than assured services. • Models: • Strict prioritization • Price differentiation • Capacity differentiation

44. Proportional Differentiation Model • Certain class of performance metrics should be proportional to the differentiation parameters the network operator chooses. • Even though the actual quality level of each class will vary with the class levels, the quality ratio between classes should remain fixed and controllable by the network operator independent of class loads.

45. QoS Guarantee with Diffserv • DiffServ provides QoS assurances; several application would need QoS guarantee • Techniques: • Premium service model • Using dynamic packet states (DPS) using a distributed implementation of virtual clock • Using delay classes at the core routers and through feedback mechanism to adjust the flow rate dynamically

46. DiffServ – In Summary • Traffic entering a network is classified and possibly conditioned at the boundaries of the network, and assigned to different behavior aggregates • Each behavior aggregate is identified by a single DS codepoint • Within the core of the network, packets are forwarded according to the PHB associated with the DS codepoint.