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Transcript
Quality of Service Frameworks Hamed Khanmirza Principles of Network University of Tehran What is QoS? • The capability to control traffic handling mechanisms in the network such that the network meets the service needs of certain applications and users subject to network policies. • Applications – Elastic (delay-tolerant) • Tolerate delays and losses • Can adapt to congestion – Non-elastic (Real-Time) • Needs some kind of guarantee from network • QoS Parameters – – – – Bandwidth Latency Jitter Loss Utility Curve Shapes U Elastic BW U Delay-adaptive BW U Hard real-time BW Integrated Services Service characteristic • Enhancing IP Service Model – Add QoS service classes – Explicit resource management at IP level – Per flow state maintained at routers which is • used for admission control and scheduling • set up by signaling protocol, users explicitly request their needs. • This is done with RSVP protocol Integrated Services Example • Achieve per-flow bandwidth and delay guarantees – Example: guarantee 1MBps and < 100 ms delay to a flow Sender Path RSVP Message Receiver Integrated Services Example • Allocate resources - perform per-flow admission control Sender RESV RSVP Message Receiver Integrated Services Example • Install per-flow state Receiver Sender Integrated Services Example • Install per flow state Sender RESV RSVP Message Receiver Integrated Services Example: Data Path • Per-flow classification Receiver Sender Integrated Services Example: Data Path • Per-flow buffer management Receiver Sender Integrated Services Example • Per-flow scheduling Receiver Sender Service Types • Multiple service classes • Service can be viewed as a contract between network and communication client – end-to-end service – other service scopes possible • Three defined services – Best-Effort for (best-effort or elastic) – Guaranteed Service for hard real-time (“Real-Time applications”) – Controlled Load for soft real-time (“tolerant” applications) Differentiated Services What is the Problem? • Goal: providing 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 – Queuing & scheduling – Classification speed – Hardware Restriction • DiffServ aims at providing QoS with simple mechanisms so that it scales and can be deployed. – push the complexity to the “edges” of the network. – Provide weaker guarantee DiffServ Architecture • Ingress routers (Edge Routers) – Perform per aggregate shaping or policing (Behavior Aggregate) – Mark packets with Code Points, each CP represent a Class of Service (DSCP DiffServ Code Point) • Core routers – Implement Per Hop Behavior (PHB) for each DSCP – Process packets based on DSCP DS-2 DS-1 Ingress Ingress Egress Edge router Core router Egress Differentiated Service (DS) Field 0 5 6 7 DS Filed 0 4 Version HLen 8 16 TOS Identification TTL 19 31 Length Flags Fragment offset Protocol Header checksum Source address Destination address IP header Data • 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 Per Hop Behavior (PHB) • Define behavior of individual routers rather than end-to-end services • Two PHBs – Assured Forwarding (AF, A type) – Expedited Forwarding (EF, P type) – Plus, best-effort service! EF PHB (Premium) • Provides the abstraction of a “virtual pipe” between an ingress and an egress router • Network: – No loss – low delay & jitter • User: – Send traffic based on SLA – Excess traffic is delayed, and dropped when buffer overflows • Signaling, admission control may get more elaborate in future (DiffServ/RSVP) Assured Forwarding PHB • Possible service: – strong assurance for traffic within profile – Out-of-profile traffic will be marked as lower class (i.e. BE) • Network: – 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 • IETF defines AF as (RFC 2477) – 4 classes – Each with 3 drop precedence – Order of packets must be preserved Provisioning & Configuration • To provide network QoS, some configuration and provisioning is required • Provisioning: – Static and long-term management tasks • Enhancing network equipment • Interface definition • Link speed and BW • Configuration: – Dynamic and short-term tasks • Direct manipulation of traffic handling mechanisms Service Level Agreement & Policy • Agreements/service provided within a domain – Service Level Agreement (SLA) with ISP • Policy – A high level description of the quality and efficiency objectives to be met by the network – Policy is set by SLA Example of an SLA Traffic submitted by customer c1 and marked with DSCP = EF and destination address in subnet 2.x.x.x and conforming to profile p1 Will be delivered to egress point B with latency not exceeding 100ms and a drop-probability less than 0.1% Traffic submitted by customer c1 and marked with DSCP = EF and destination address in subnet 2.x.x.x and not conforming to profile p1 Will be discarded Example of an SLA Traffic submitted by customer c1 and marked with DSCP = EF and destination address in subnet 3.x.x.x and conforming to profile p2 Will be delivered to egress point C with latency not exceeding 100ms and a drop-probability less than 0.1% Traffic submitted by customer c1 and marked with DSCP = EF and destination address in subnet 3.x.x.x and not conforming to profile p2 Will be discarded Example of an SLA Traffic submitted and marked with and destination and destination Will be discarded by customer c1 DSCP = EF address not in subnet 2.x.x.x address not in subnet 3.x.x.x Traffic submitted by customer c1 and not marked with DSCP = EF Will be delivered with best-effort service P1: Conforming traffic must not exceed 64kbps over any 5msec interval P2: Conforming traffic must not exceed 128kbps over any 2.5msec interval Pushed vs. Signaled Pushed Configuration Sniffer Server monitoring/analysis Sniffer Server monitoring/analysis Sniffer Server monitoring/analysis Sniffer Server monitoring/analysis Sniffer Server monitoring/analysis Sniffer Server monitoring/analysis Sniffer Server monitoring/analysis Signaled Configuration Sniffer Server monitoring/analysis Components of Policy System 3) PDP compares RQ against policies in data store IBM Compatible Policy Data Store 1) Policy is authored in directory and is made available to PDP 4) Admit/Reject decision returned by PDP Functional Layers Policy Server (PDP) No physical 2) PDP pushes config to devices using SNMP, COPS, CLI, … Policy may be updated time to time 2) Policy info exchanged with PDP (using COPS) 1) Signaled RQ Sniffer Server monitoring/analysis Distributed Data Store - Directory Directory Data Data Directory Server Policy Policy PDP Server Server Server Router Router Router Router Router PEP Router Interior Provisioning 45Mbps physical Interface 4.5Mbps allocated to strict priority queue for EF traffic SLS 1.5Mbps EF Traffic to R3 Router Router R1 Router Router R3 R5 R2 Router SLS 3Mbps EF Traffic to R3 Router Assured Service • Large spatial granularity service • Theoretically, user profile is defined irrespective of destination • This makes service very useful, but hard to provision – Over provision? Traffic profile Ingress Multicast Problems in DiffServ • Multicast – Problem • Dynamic trees – Solutions • Different DSCP • Some determined tree structure • Remarking and shaping at boundaries DiffServ Implementations • Two important proposals – – – – RIO Mechanism (1 service) The Scalable Share Differentiation architecture (SSD) Two-Bit architecture RFC (2475) Two-Bit Architecture • Proposes three different levels of service: – Premium Service. – Assured Service. – Best Effort Service. • Two-bit architecture: – Packets get differentiated by two bits in their header. – Premium bit (P-bit) – Assured Service bit (A-bit) Leaf Router Input Functionality Marker 1 Marker N Arriving packet Clear A&P bits MF Packet classifier Best effort Forwarding engine classify packets based on packet header Markers in Leaf Routers Drop on overflow Packet input Wait for token Set P bit Packet output No token Packet input Test if token token Set A bit RIO is applied here Packet output Red with In or Out (RIO) • Similar to RED • With two separate probability curves – In (of profile) – Out (of profile) • “Out” class has – lower Minthresh, so packets are dropped from this class first – Based on queue length of all packets • “In” Class – As avg queue length increases, “in” packets are also dropped – Based on queue length of only “in” packets Dropping probability 1 OUT IN Average queue length Output Forwarding • 2 queues: – High Priority: EF packets – Lower priority queue implements RED “In or Out” scheme (RIO) • Usually scheduling scheme is “Strict Priority” P bit set? EF High-priority Q Send Packet AF If A bit set incr a_cnt Low-priority Q RIO queue management If A bit set decr in_cnt Intra Domain Behavior • 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 Example • Achieve end-to-end bandwidth guarantee 3 2 BB 1 9 8 profile sender 7 BB 6 profile 5 BB 4 profile receiver RFC 2475: Overall Architecture Meter Classifier Marker Classifiers: 1. Multifield Classifier (MF) 2. Behavior Aggregate Classifier (BA) Shaper/ Dropper Traffic Conditioning • Schedulers – Work-conserving – Non-work-conserving • Traffic conditioning uses Non-work-conserving ones • Implementations – Leaky Bucket – Token Bucket – Hybrid approaches • Leaky-Token Bucket • Dual Token Bucket Leaky Bucket • Smoothes traffic and generates constant rate b bits r b/s Token Bucket Filter • Described by 2 parameters: – Token rate r: rate of tokens placed in the bucket – Bucket depth b: capacity of the bucket • Operation: – – – – Tokens are placed in bucket at rate r If bucket fills, tokens are discarded Sending a packet of size P uses P tokens If bucket has P tokens, packet sent at max rate, else must wait for tokens to accumulate Token Bucket Operation Tokens Tokens Tokens Overflow Packet Enough tokens packet goes through, tokens removed Packet Not enough tokens wait for tokens to accumulate Token Bucket • On the long run, rate is limited to r • On the short run, a burst of size b can be sent • Token Bucket 3 possible uses – Shaping • Delay pkts from entering net (shaping) – Policing • Drop pkts that arrive without tokens – Metering (Marking) • Let all pkts pass through, mark ones without tokens Comparison Best-Effort Service • Connectivity • No isolation • No guarantees Diffserv • Per aggregation isolation • Per aggregation guarantee Intserv • Per flow isolation • Per flow guarantee • End-to-end • Domain • End-to-end Complexity • No set-up • Long term setup • Per flow setup Scalability • Highly scalable • (nodes maintain only routing state) • Scalable (edge • Not scalable routers maintains (each router per aggregate maintains per state; core routers flow state) per class state) Service Scope