Quality of service
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The document discusses the concept of Quality of Service (QoS) in computer networks, comparing best-effort services with guaranteed services, and highlighting basic mechanisms like the leaky bucket and token bucket methods for traffic shaping and resource reservation. It also outlines various QoS issues including flow specification, admission control, and the architecture of Integrated Services and Differentiated Services. Additionally, it covers RSVP protocol for setting up resource reservations and the challenges faced by IntServ in terms of complexity and scalability.
![63
Assured Service
[Clark & Wroclawski ‘97]
• Defined in terms of user profile, how much assured
traffic is a user allowed to inject into the network
• Network: provides a lower loss rate than best-effort
– In case of congestion best-effort packets are dropped first
• User: sends no more assured traffic than its profile
– If it sends more, the excess traffic is converted to best-
effort](https://image.slidesharecdn.com/qualityofservice-200114150012/85/Quality-of-service-63-320.jpg)
![65
Premium Service
[Jacobson ’97]
• Provides the abstraction of a virtual pipe
between an ingress and an egress router
• Network: guarantees that premium packets
(QoS) are not dropped and they experience
low delay
• User: does not send more than the size of
the pipe
– If it sends more, excess traffic is delayed, and
dropped when buffer overflows](https://image.slidesharecdn.com/qualityofservice-200114150012/85/Quality-of-service-65-320.jpg)
Quality of service
- 1. 1 Computer Networks Quality Of Service
- 2. 2 Outline • What is Quality of Service • Basic mechanisms – Leaky and token buckets • Integrated Services (IntServ) • Differentiated Services (DiffServ) • Economics and Social factors facing QoS • QoS Vs. Over Provisioning
- 3. 3 Best Effort vs. QoS • Best Effort: • You get a link to the Internet with at most B bits/sec. • If you don’t like it, switch to another provider. • Quality of Service (QoS) – We provide you some kind of guarantees for: • Bandwidth • Latency • Jitter – I.e., network is engineered to provide some Quality beyond “Not to exceed B bits/s”
- 4. 4 QoS’s Quest The Holy Grail of computer networking is to design a network that has the flexibility and low cost of the Internet, yet offers the end-to- end quality-of-service guarantees of the telephone network. --S. Keshav
- 5. 5 Two Styles of QoS • Worse-case – Provide bandwidth/delay/jitter guarantee to every packet – E.g., “hard real time” • Average-case – Provide bandwidth/delay/jitter guarantee over many packets – Statistical in nature – E.g. “Soft real time”
- 6. 6 Resource Reservation: Example Router RouterSrc Dest 4 Mbps available 6 Mbps available 10 Mbps available Case 1: Source attempts to connect to destination, and attempts to reserve 4 Mbps for the connection Result: Connection accepted. There is enough bandwidth available. Available link bandwidths updated. Case 2: Source attempts to connect to destination, and attempts to reserve 5 Mbps for the connection Result: Failure. There is not enough bandwidth available on one of the links.
- 7. 7 Resource Reservation (cont’d) • Once a connection is accepted, the host must use only the amount of resources reserved. It may not use more than that. • What if the host is malicious and attempts to use more network resources than it reserved?
- 8. 8 Leaky Bucket • Used in conjunction with resource reservation to police the host’s reservation • At the host-network interface, allow packets into the network at a constant rate • Packets may be generated in a bursty manner, but after they pass through the leaky bucket, they enter the network evenly spaced
- 9. 9 Leaky Bucket: Analogy Leaky Bucket Network Packets from host
- 10. 10 Leaky Bucket (cont’d) • The leaky bucket is a “traffic shaper”: It changes the characteristics of packet stream • Traffic shaping makes more manageable and more predictable • Usually the network tells the leaky bucket the rate at which it may send packets when the connection begins • Polices the average rate
- 11. 11 Leaky Bucket: Doesn’t allow bursty transmissions • In some cases, we may want to allow short bursts of packets to enter the network without smoothing them out • For this purpose we use a token bucket, which is a modified leaky bucket
- 12. 12 Token Bucket • The bucket holds tokens instead of packets • Tokens are generated and placed into the token bucket at a constant rate • When a packet arrives at the token bucket, it is transmitted if there is a token available. Otherwise it is buffered until a token becomes available. • The token bucket has a fixed size, so when it becomes full, subsequently generated tokens are discarded
- 13. 13 Token Bucket Network Packets from host Token Generator (Generates a token once every T seconds)
- 14. 14 Token Bucket vs. Leaky Bucket Case 1: Short burst arrivals 6543210 Arrival time at bucket Departure time from a leaky bucket Leaky bucket rate = 1 packet / 2 time units Leaky bucket size = 4 packets 6543210 6543210 Departure time from a token bucket Token bucket rate = 1 tokens / 2 time units Token bucket size = 2 tokens
- 15. 15 Token Bucket vs. Leaky Bucket Case 2: Large burst arrivals 6543210 Arrival time at bucket Departure time from a leaky bucket Leaky bucket rate = 1 packet / 2 time units Leaky bucket size = 2 packets 6543210 6543210 Departure time from a token bucket Token bucket rate = 1 token / 2 time units Token bucket size = 2 tokens
- 16. 16 Flow Specification: Token Bucket • Characterized by two parameters (r, b) – r – average rate – b – token depth • Assume flow arrival rate <= R bps (e.g., R link capacity) • A bit is transmitted only when there is an available token • Arrival curve – maximum amount of bits transmitted by time t r bps b bits <= R bps regulator time bits b slope R slope r Arrival curve
- 17. 17 Quality of service issues • Flow specification – Flow spec: traffic characteristics, QoS requirements (delay, jitter,bandwidth) • Routing – Routing traffic to best meet demand • Resource reservation – End-host signaling to network QoS resource requirements • Admission control – Limiting number of reservations • Packet scheduling – Packet by packet scheduling (fairness, delay) • RSVP addresses reservation
- 18. 18 Integrated Services Example: Data Path Sender Receiver • Per-flow classification
- 19. 19 Integrated Services Example: Data Path Sender Receiver • Per-flow buffer management
- 20. 20 Integrated Services Example Sender Receiver • Per-flow scheduling
- 21. 21 How Things Fit Together Admission Control Data In Data Out ControlPlaneDataPlane Scheduler Routing Routing Messages RSVP messages Classifier RSVP Route Lookup Forwarding Table Per Flow QoS Table
- 22. 22 Service Classes • Multiple service classes • Service: contract between network and communication client – End-to-end service – Other service scopes possible • Three common services – Best-effort (“elastic” applications) – Hard real-time (“real-time” applications) – Soft real-time (“tolerant” applications)
- 23. 23 Worse-case : Guaranteed Services • Service contract – Network to client: guarantee a deterministic upper bound on delay for each packet in a session – Client to network: the session does not send more than it specifies • Algorithm support – Admission control based on worst-case analysis – Per flow classification/scheduling at routers
- 24. 24 Average-case: Controlled Load Service • Service contract: – Network to client: Average delay, jitter, bandwidth, e.g., makes network appear as an unloaded, best effort network with bandwidth and delay – Client to network: the session does not send more than it specifies • Algorithm Support – Admission control based on measurement of aggregates – Scheduling for aggregate possible
- 25. 25 Role of RSVP in the Architecture • Signaling protocol for establishing per flow state • Carry resource requests from hosts to routers • Collect needed information from routers to hosts • At each hop – Consult admission control and policy module – Set up admission state or informs the requester of failure
- 26. 26 RSVP Design Features • IP Multicast centric design • Receiver initiated reservation • Different reservation styles • Soft state inside network • Decouple routing from reservation
- 27. 27 RSVP Reservation Model • Performs signaling to set up reservation state for a session • A session is a simplex data flow sent to a unicast or a multicast address, characterized by – <IP dest, protocol number, port number> • Multiple senders and receivers can be in session
- 29. 29 The Big Picture (2) Network Sender Receiver PATH Msg RESV Msg
- 30. 30 RSVP terminology • Flow descriptor (Flow spec + Filter Spec) • Flow spec (Rate, max burst) – Sender can Explicitly specify flow spec or not specify • Filter Spec (Sender address, TCP/UDP, Port#) – Aids in combining similar flows – Filter can be shared (SE-style) or can use wild cards (all senders on a given port or a given sender on all ports, etc) – The style may be shared or distinct in a sense that all reservations may be handled as one single reservation or there may be a single reservation for each upstream sender respectively.
- 31. 31 RSVP Basic Operations • Sender: sends PATH message via the data delivery path – Set up the path state each router including the address of previous hop • Receiver sends RESV message on the reverse path – Specifies the reservation style, QoS desired – Set up the reservation state at each router • Things to notice – Receiver initiated reservation – Decouple routing from reservation – Two types of state: path and reservation
- 32. 32 RSVP messages • PATH message – sets up state along path followed by packets • RESV message – request for reservation back along setup path path • PATH_TEAR, RESV_TEAR, RESV_CONFIRM, RESV_ERROR, PATH_ERROR
- 33. 33 RSVP Operation Sender Receiver1 Receiver2 Receiver3 Merged reservations Merged reservations
- 34. 34 RSVP PATH MESSAGE • From sender to receiver (unicast or multicast) • Intercepted at each RSVP aware hop • Includes – Sender TSpec : Traffic characteristics of the sender • Token bucket rate, depth, max flow rate, max packet size • forms one side of the ``contract'' between the data flow and the service. – F-flag: specify whether filtered reservation is allowed • Routers store: – Path state, i.e., PHOP address to previous hop (RSVP aware node) – If F-flag is set, store sender and its flowspec – Otherwise, just add new link to multicast tree
- 35. 35 RSVP RESV MESSAGE • From receiver to sender(s) to reserve resources • Sent hop-by-hop using PHOP information • Reservation style and flow description – Reservation style (FF,SE, WF) – Fixed-filter, Shared-explicit, wildcard-filter – Senders to which the reservation applies – Rspec, QoS specific requirements – RSpec is highly specific to the service required, and may include information like bandwidth allocation, maximum delay, or packet loss probabilities etc. • RESV messages processing at each hop – Merging of RESV messages – Forwards resv messages using PHOP
- 36. 36 Route Pinning • Problem: asymmetric routes – You may reserve resources on RS3S5S4S1S, but data travels on SS1S2S3R ! • Solution: use PATH to remember direct path from S to R, i.e., perform route pinning S1 S2 S3 S R S5S4PATH RESV IP routing
- 37. 37 How Is the Token Bucket Used? • Can be enforced by – End-hosts (e.g., cable modems) – Routers (e.g., ingress routers in a Diffserv domain) • Can be used to characterize the traffic sent by an end-host
- 38. 38 Source Traffic Characterization • Arrival curve – maximum amount of bits transmitted during an interval of time Δt • Use token bucket to bound the arrival curve Δt bits Arrival curve time bps
- 39. 39 QoS Guarantees: Per-hop Reservation • End-host: specify – the arrival rate characterized by token-bucket with parameters (b,r,R) – the maximum maximum admissible delay D • Router: allocate bandwidth ra and buffer space Ba such that – no packet is dropped – no packet experiences a delay larger than D bits b*R/(R-r) slope r Arrival curve D Ba slope ra
- 40. 40 End-to-End Reservation • When R gets PATH message it knows – Traffic characteristics (tspec): (r,b,R) – Number of hops • R sends back this information + worst-case delay in RESV • Each router along path provide a per-hop delay guarantee and forward RESV with updated info – In simplest case routers split the delay S1 S2 S3 S R(b,r,R) (b,r,R,3) num hops (b,r,R,2,D-d1) (b,r,R,1,D-d1-d2)(b,r,R,0,0) (b,r,R,3,D) worst-case delay PATH RESV
- 41. 41 Reservation Style • Motivation: achieve more efficient resource utilization in multicast (M x N) • Observation: in a video conferencing when there are M senders, only a few can be active simultaneously – Multiple senders can share the same reservation • Various reservation styles specify different rules for sharing among senders
- 42. 42 Reservation Styles and Filter Spec • Reservation style – use filter to specify which sender can use the reservation • Three styles – Wildcard filter: does not specify any sender; all packets associated to a destination shares same resources • Group in which there are a small number of simultaneously active senders – Fixed filter: no sharing among senders, sender explicitly identified for the reservation • Sources cannot be modified over time – Dynamic filter: resource shared by senders that are (explicitly) specified • Sources can be modified over time
- 43. 43 Wildcard Filter Example • Receivers: H1, H2; senders: H3, H4, H5 • Each sender sends B • H1 reserves B; listen from one server at a time S1 S2 S3 H2 H1 H5 H4 H3 (B,*) (B,*) (B,*) (B,*) (B,*) (B,*) senderreceiver
- 44. 44 Wildcard Filter Example • H2 reserves B S1 S2 S3 H2 H1 H5 H4 H3 (B,*) (B,*) (B,*) senderreceiver (B,*) (B,*) (B,*) (B,*)
- 45. 45 Wildcard Filter • Advantages – Minimal state at routers • Routers need to maintain only routing state augmented by reserved bandwidth on outgoing links • Disadvantages – May result in inefficient resource utilization
- 46. 46 Wildcard Filter: Inefficient Resource Utilization Example • H1 reserves 3B; wants to listen from all senders simultaneously • Problem: reserve 3B on (S3:S2) although 2B sufficient! S1 S2 S3 H2 H1 H5 H4 H3 (3B,*) (3B,*) (3B,*) senderreceiver
- 47. 47 Fixed Filter Example • Receivers: H2, H3, H4, H5; Senders: H1, H4, H5 • Routers maintain state for each receiver in the routing table S1 S2 S3 H2 H1 H3 senderreceiver H5 H4 sender+receiver NextHop Sources H1 S2(H5, H4) H2 H1(H1), S2(H5, H4)
- 48. 48 Fixed Filter Example • H2 wants to receive B only from H4 S1 S2 S3 H2 H1 H3 senderreceiver H5 H4 sender+receiver (B,H4) (B,H4) (B,H4) (B,H4)
- 49. 49 Dynamic Filter Example • H5 wants to receive 2B from any source S1 S2 S3 H2 H1 H3 senderreceiver H5 H4 sender+receiver (B,H4) (B,H4) (B,H4)(2B,*) (B,H4) (B,*) (B,*)
- 50. 50 Soft State • Per session state has a timer associated with it – path state, reservation state • State lost when timer expires • Sender/Receiver periodically refreshes the state • Claimed advantages – no need to clean up dangling state after failure – can tolerate lost signaling packets • signaling message need not be reliably transmitted – easy to adapt to route changes • State can be explicitly deleted by a Teardown message
- 51. 51 Tear-down Example • H4 leaves the group – H4 no longer sends PATH message – State corresponding to H4 removed S1 S2 S3 H2 H1 H3 senderreceiver H5 H4 sender+receiver (B,H4) (B,H4) (B,H4)(2B,*) (B,H4) (B,*) (B,*)
- 52. 52 Tear-down Example • H4 leaves the group – H4 no longer sends PATH message – State corresponding to H4 removed S1 S2 S3 H2 H1 H3 senderreceiver H5 sender+receiver (2B,*) (B,*) (B,*)
- 53. 53 RSVP Soft-state • RSVP control messages need to be sent periodically – State will disappear if not refreshed – Periodic state refresh every t sec (30 sec) – If no refresh within n*t (n=3) , delete state • RSVP messages sent as router-alert message – Intermediate routers intercept packets and update state accordingly
- 54. 54 Soft State (cont) • Per session state has a timer associated with it – Path state, reservation state • State lost when timer expires • Sender/Receiver periodically refreshes the state, resends PATH/RESV messages, resets timer • Claimed advantages – No need to clean up dangling state after failure – Can tolerate lost signaling packets • Signaling message need not be reliably transmitted – Easy to adapt to route changes • State can be explicitly deleted by a Teardown message
- 55. 55 RSVP and Routing • RSVP designed to work with variety of routing protocols • Minimal routing service – RSVP asks routing how to route a PATH message • Route pinning – addresses QoS changes due to “avoidable” route changes while session in progress • QoS routing – RSVP route selection based on QoS parameters – granularity of reservation and routing may differ • Explicit routing – Use RSVP to set up routes for reserved traffic
- 56. 56 Recap of RSVP • PATH message – sender template and traffic spec – advertisement – mark route for RESV message – follow data path • RESV message – reservation request, including flow and filter spec – reservation style and merging rules – follow reverse data path • Other messages – PathTear, ResvTear, PathErr, ResvErr
- 57. 57 Why did IntServ fail? • Economic factors – Deployment cost vs Benefit • Is reservation, the right approach? – Multicast centric view • Is per-flow state maintenance an issue? • More about QoS in general …
- 58. 58 What is the Problem? • Goal: provide support for wide variety of applications: – Interactive TV, IP telephony, on-line gamming (distributed simulations), VPNs, etc • Problem: – Best-effort cannot do it? – Intserv can support all these applications, but • Too complex • Not scalable
- 59. 59 Differentiated Services (Diffserv) • Build around the concept of domain • Domain – a contiguous region of network under the same administrative ownership • Differentiate between edge and core routers • Edge routers – Perform per aggregate shaping or policing – Mark packets with a small number of bits; each bit encoding represents a class (subclass) • Core routers – Process packets based on packet marking • Far more scalable than Intserv, but provides weaker services
- 60. 60 Diffserv Architecture • Ingress routers – Police/shape traffic – Set Differentiated Service Code Point (DSCP) in Diffserv (DS) field • Core routers – Implement Per Hop Behavior (PHB) for each DSCP – Process packets based on DSCP Ingress Egress Ingress Egress DS-1 DS-2 Edge router Core router
- 61. 61 Differentiated Service (DS) Field Version HLen TOS Length Identification Fragment offsetFlags Source address Destination address TTL Protocol Header checksum 0 4 8 16 19 31 Data IP header • DS filed reuse the first 6 bits from the former Type of Service (TOS) byte • The other two bits are proposed to be used by ECN DS Filed 0 5 6 7
- 62. 62 Differentiated Services • Two types of service – Assured service – Premium service • Plus, best-effort service
- 63. 63 Assured Service [Clark & Wroclawski ‘97] • Defined in terms of user profile, how much assured traffic is a user allowed to inject into the network • Network: provides a lower loss rate than best-effort – In case of congestion best-effort packets are dropped first • User: sends no more assured traffic than its profile – If it sends more, the excess traffic is converted to best- effort
- 64. 64 Assured Service • Large spatial granularity service • Theoretically, user profile is defined irrespective of destination – All other services we learnt are end-to-end, i.e., we know destination(s) apriori • This makes service very useful, but hard to provision (why ?) Ingress Traffic profile
- 65. 65 Premium Service [Jacobson ’97] • Provides the abstraction of a virtual pipe between an ingress and an egress router • Network: guarantees that premium packets (QoS) are not dropped and they experience low delay • User: does not send more than the size of the pipe – If it sends more, excess traffic is delayed, and dropped when buffer overflows
- 66. 66 Edge Router Classifier Traffic conditioner Traffic conditioner Scheduler Class 1 Class 2 Best-effort Marked traffic Ingress Per aggregate Classification (e.g., user) Data traffic
- 67. 67 Assumptions • Assume two bits – P-bit denotes premium traffic – A-bit denotes assured traffic • Traffic conditioner (TC) implement – Metering – Marking – Shaping
- 68. 68 TC Performing Metering/Marking • Used to implement Assured Service • In-profile traffic is marked: – A-bit is set in every packet • Out-of-profile (excess) traffic is unmarked – A-bit is cleared (if it was previously set) in every packet; this traffic treated as best-effort r bps b bits Metering in-profile traffic out-of-profile traffic assured traffic User profile (token bucket) Set A-bit Clear A-bit
- 69. 69 TC Performing Metering/Marking/Shaping • Used to implement Premium Service • In-profile traffic marked: – Set P-bit in each packet • Out-of-profile traffic is delayed, and when buffer overflows it is dropped r bps b bits Metering/ Shaper/ Set P-bit in-profile traffic out-of-profile traffic (delayed and dropped) premium traffic User profile (token bucket)
- 70. 70 Scheduler • Employed by both edge and core routers • For premium service – use strict priority, or weighted fair queuing (WFQ) • For assured service – use RIO (RED with In and Out) – Always drop OUT packets first • For OUT measure entire queue • For IN measure only in-profile queue OUT IN Average queue length 1 Dropping probability
- 71. 71 Scheduler Example • Premium traffic sent at high priority • Assured and best-effort traffic pass through RIO and then sent at low priority P-bit set? A-bit set? RIO yes no yes no high priority low priority
- 72. 72 Control Path • Each domain is assigned a Bandwidth Broker (BB) – Usually, used to perform ingress-egress bandwidth allocation • BB is responsible to perform admission control in the entire domain • BB not easy to implement – Require complete knowledge about domain – Single point of failure, may be performance bottleneck – Designing BB still a research problem
- 73. 73 Example • Achieve end-to-end bandwidth guarantee BB BB BB 1 2 3 57 9 sender receiver 8 profile 6 profile 4 profile
- 74. 74 Comparison: Best-Effort, Diffserv, Intserv Best-Effort Diffserv Intserv Service Connectivity No isolation No guarantees Per aggregate isolation Per aggregate guarantee Per flow isolation Per flow guarantee Service scope End-to-end Domain End-to-end Complexity No setup Long term setup Per flow setup Scalability Highly scalable (nodes maintain only routing state) Scalable (edge routers maintains per aggregate state; core routers per class state) Not scalable (each router maintains per flow state)
- 75. 75 Summary • Diffserv more scalable than Intserv – Edge routers maintain per aggregate state – Core routers maintain state only for a few traffic classes • But, provides weaker services than Intserv, e.g., – Per aggregate bandwidth guarantees (premium service) vs. per flow bandwidth and delay guarantees • BB is not an entirely solved problem – Single point of failure – Handle only long term reservations (hours, days)
- 76. 76 Non-technical Factors Impacting QoS • Existing Networks – What is available today to solve our needs? Why switch? • Business Models – How QoS make doing business harders • Deployment Issues – How QoS makes running the network harder.
- 77. 77 Existing Networks • Motivating applications? – Tele/Video conferencing, video distribution, VPN, games. • IP+QoS must be better AND cheaper than: – PSTN with N-way calling – Cable TV with digital recorders (Tivo) – Telecom leased lines (ISDN, ATM, SONET) – Peer to Peer networks
- 78. 78 Business Issues • Service provider offers Qos(premium service) • Must be something customer can: – Understand • Counterexample: Complex statistical reasoning – Verify • 3rd party? • How do you know it works? Simulate a DoS attack? – Reclaim loss if service is not delivered • If you buy a lock and it doesn’t work, do you try to get your $ back? What if no one tried to break in?
- 79. 79 Deployment Issues • Today’s IP operators use simple models to reason about what is a “good network” • Things you worry about: • IP packets • BGP routing • Simple Service Level Agreements (SLA)
- 80. 80 Deployment Issues • QoS introduces extra effort for operators: – shaping, policing, reservation signaling, per-reservation billing and settlement. • QoS deployment changes: – Interface between an ISP and its neighbors – adds whole new complexities for customer and support personnel, – creates the need for accurate service auditing, • Increases the risk of litigation • Tradeoff: – Use QoS vs. make sure utilization is low most of the time? Which is easier?
- 81. 81 Non-technical Issues summary • Working on QoS for IP for 20 years? – Why little/no progress? • QoS benefits must significant to overcome all non-technical obstacles. – Value to users must exceed all costs – A typical technology adoption problem? -> Technically better isn’t always good enough – QWERTY 10x backward compatibility rule? – QoS not cheaper, so 1000x?
- 82. 82 Over Provisioning Vs. QoS • QoS: – Insure resources per flow • Over Provisioning – Domain of traffic engineering – Insure sufficient resources aggregate demand – Formalism for this approach?
- 83. 83 Over Provisioning Formalism • Start with Traffic matrix – Cell is the traffic from source I to dest. J • Insure network delivers adequate performance for all “expected” matrixes