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QoS in the Internet – Proposals and Prospects COMP680E by M. Hamdi 1 Introduction • For Many Years the Internet was primarily used for networking research. File transfer and email were the most popular applications: They do not need any performance guarantee from the underlying network. • The current Internet cannot provide any resource guarantees: the service is best effort • New applications such as VoIP, video conferencing, e-commerce apps are sensitive to network performance (e.g., delay and bandwidth guarantees). COMP680E by M. Hamdi 2 Current State of Internet • The internet service did not change by much: – Uses best-effort service model – No guarantee of timeliness or delivery – No service discrimination – Bandwidth and network congestion problems – Unpredictable network response time COMP680E by M. Hamdi 3 What is QoS • The capability to provide resource assurance and service differentiation so that delay, jitter or loss sensitive applications can perform satisfactorily is referred to as quality of service (QoS). • Can be provided through relative prioritization of resource allocation to different flows/packets in the network. COMP680E by M. Hamdi 4 Resource Allocation • Many problems in the Internet come down to the issue of resource allocation. • Packets get delayed or dropped because network resource cannot meet the traffic demands. • A network consists of shared resources : bandwidth, buffers, serving traffic from competing users. • To support QoS, the network must allocate resources and decide who should get how much resources. COMP680E by M. Hamdi 5 Integrated Services • Based on per flow resource reservation. • Applications must make a reservation before transmitting traffic. • Application characterizes its traffic and resource requirement. • Network uses routing protocol to find a path. • A reservation protocol is used to install the reservation state along that path. COMP680E by M. Hamdi 6 Integrated Services (cont’d) • At each hop admission control checks whether sufficient resources are available to accept the new reservation. • Resource reservation enforced by packet classification and scheduling mechanisms. • Two service models are introduced: guaranteed service and controlled load. • Guaranteed service provides deterministic worst case delay • Controlled load provides less firm guarantee : it’s close to a lightly loaded best-effort network. COMP680E by M. Hamdi 7 Integrated Services (cont’d) • Overhead of setting up reservation is high. • Scalability problem : Backbone will have a large number of flows and keeping flow information is not feasible. COMP680E by M. Hamdi 8 Differentiated Service • Users traffic is divided into a small number of forwarding classes. • For each forwarding class, the amount of traffic that users can inject is limited at the edge of the network. • Edge of a differentiated Services network responsible for mapping packets to their appropriate forwarding classes. • Packet classification is usually based on service level agreement. COMP680E by M. Hamdi 9 Differentiated Service (Cont’d) • Nodes at the edge of the network perform traffic policing to ensure conformance. • Non-conforming traffic may be dropped, delayed or marked with a different forwarding class. • Forwarding class is directly encoded into the packet header. • Interior nodes use this info. to differentiate the treatment of packets. • Does not require resource reservation. COMP680E by M. Hamdi 10 Differentiated Service (Cont’d) • Forwarding classes apply to traffic aggregates rather than individual flows. • No scalability problem. COMP680E by M. Hamdi 11 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 DS-2 DS-1 Ingress Ingress Egress Edge router Egress Core router COMP680E by M. Hamdi 12 Multiprotocol Label Switching (MPLS) • A short fixed length label is encoded into the packet header and is used for packet forwarding. • When a label switch router (LSR) receives an MPLS packet, it uses incoming label to find the next hop and the corresponding outgoing label. • In MPLS, the path a packet traverses is called label switched path (LSP). • Network protocol independent COMP680E by M. Hamdi 13 Multiprotocol Label Switching (MPLS) • Works alongside existing routing technologies and provide a mechanism for explicit control over routing paths. • Used for traffic engineering, guaranteeing QoS and VPN. COMP680E by M. Hamdi 14 Traffic Engineering • The basic problem : Given a network and traffic demands, how can traffic flows in the network be organized so that an optimization objective can be optimized. • Typically optimal operating point is reached when traffic is evenly distributed : leads to min queuing delay and packet loss. • This cannot be achieved through destination based IP routing • Advanced routing techniques constraint-based routing are used. COMP680E by M. Hamdi 15 Traffic Engineering (cont’d) • Routes are computed with multiple constraints and aims for efficient resource utilization. • Appropriate path selection with uniform traffic distribution and the congestion avoidance techniques improve the network performance • MPLS can be used to achieve this goal. COMP680E by M. Hamdi 16 Internet QoS Overview • • • • Integrated services Differentiated Services MPLS Traffic Engineering COMP680E by M. Hamdi 17 State information • No State Vs. Soft State Vs. Hard State IP No State diffserv No State inside the network Flow information at the edges Intserv/ RSVP ATM Circuit Switched Soft State Hard State Dedicated Circuit Packet Switched COMP680E by M. Hamdi 18 Integrated Services • Early 1990 IETF started Inegrated Services working group to standardize a new resource allocation architecture. • Based on a per flow resource reservation. • Goal is to preserve the datagram model of IPbased networks and at the same time support resource reservation for real-time applications. COMP680E by M. Hamdi 19 QoS Router Queue management Policer Per-flow Queue Scheduler Classifier Policer Per-flow Queue Policer Per-flow Queue Scheduler Classifier Policer shaper shaper Per-flow Queue COMP680E by M. Hamdi 20 Basic Approach • A set of mechanisms and protocols is used for making explicit resource reservation. • To receive performance guarantee from the network, resource reservation must be set up before the application can start transmitting packets. • Sender starts the setup of a reservation by sending characteristics and resource requirement of the flow. • The network can accept the new application flow only if sufficient resource is there. • Once reservation is setup successfully, the application can start sending data packets. COMP680E by M. Hamdi 21 Key Components QoS routing agent Admission control Reservation setup agent Resource reservation table Control plane Flow identification Packet scheduler Data plane COMP680E by M. Hamdi 22 Key Component (cont’d) • Control Plane sets up resource reservation. • Data plane forwards data packets based on reservation state. • To setup reservation, app first characterizes its traffic flow and specifies QoS requirements : referred to as flow specification • The reservation setup request is then sent to the network. COMP680E by M. Hamdi 23 Key Component (cont’d) • Router upon getting the request, interacts with QoS routing agent to find the next hop. • It then coordinates with the admission control module to determine if there are sufficient resources to meet the requested resources. • Once reservation set up is successful, the information for the reserved flow is installed into the resource reservation table. • Info. in the resource reservation table is used to configure the flow identification module and the packet scheduling module in the data plane. COMP680E by M. Hamdi 24 Route Selection • IntServ does not specify any route selection of its own. • It relies on existing routing protocols to forward its control packets further. • Obviously a more efficient routing protocol which can find a path that is likely to have sufficient resources is desired. COMP680E by M. Hamdi 25 Reservation Setup • To setup reservation, a reservation set up protocol is needed that goes hop by hop along the path to install the reservation state in the routers. • The reservation protocol must also deal with changes in the network topology. • In IntServ, RSVP has been developed as the resource reservation protocol. COMP680E by M. Hamdi 26 Admission Control • In order to provide guaranteed resources for reserved flows, a network must monitor its resource usage and admit a new flow only if it has sufficient resource. • It has two functions : to determine if a new flow reservation can be set up based on the admission control policies and to monitor and measure the available resources. COMP680E by M. Hamdi 27 Flow Identification • Router must examine every incoming packet and decide whether the packet belongs to one of the reserved flows. • IP flow is identified by src addr, dest addr, proto ID, src port, dst port : five-tuple. • These five fields of the incoming packet is compared against the five-tuple of all the flows in the reservation table for flow identification. COMP680E by M. Hamdi 28 Packet Scheduling • Packet scheduler responsible for resource allocation • Directly affects delay, jitter and packet loss • Primary task is to select a packet to transmit when outgoing link is ready such that the QoS promised to flows is provided COMP680E by M. Hamdi 29 Service Models • Describe interface between the network and its users. • IntServ has standardized two basic service models: – Guaranteed service – Controlled load service COMP680E by M. Hamdi 30 Guaranteed Service • Provides guaranteed bandwidth and strict bounds for delay. • Intended for apps that require highest assurance on bw and delay : mission critical apps, intolerant playback apps. • Can be viewed as a virtual circuit with guaranteed bw. • Provides bounds on maximal queuing delay. COMP680E by M. Hamdi 31 Controlled load service • Strict bw assurance and delay bound comes at a price : resources have to be reserved for the worst case. • For some apps a service model with less strict guarantees and lower cost would better serve their needs. • End-to-end behavior somewhat vague. • A very high percentage of packets will be successfully delivered by the network to the receivers. • The transit delay experienced by a very high percentage of packets will not greatly exceed min delay. COMP680E by M. Hamdi 32 RSVP • A resource reservation protocol defined under IntServ. • Used by hosts to communicate service requirements to the network and by routers in the network to establish reservation state along a path PATH (1) (6) (2) (5) (3) (4) RESV COMP680E by M. Hamdi 33 Protocol Overview (Cont’d) • RESV must follow the exact same reverse path upstream. • They create reservation state in each node along the paths • After receiving RESV msg sender can start sending data packets. COMP680E by M. Hamdi 34 DiffServ • Differentiated Services (DiffServ) is proposed by IETF as a scalable QoS solution for the next generation Internet. • Developed for relatively simple, coarse methods of providing different levels of service for Internet traffic. • Divides traffic into a small number of classes and allocates resources on a per class basis. • Core of a diffserv network distinguishes between small number of forwarding classes rather than individual flows. COMP680E by M. Hamdi 35 DiffServ (cont.) • Complex per-flow classification and scheduling used in intServ (causes scalability) not needed. • 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 COMP680E by M. Hamdi 36 Basic Approach • Traffic is divided into a small number of groups called forwarding classes • Forwarding class that a packet belongs to is encoded into a field in the IP packet header. • Each forwarding class represents a predefined forwarding treatment in terms of drop priority and bandwidth allocation. COMP680E by M. Hamdi 37 Basic Approach (cont.) • Achieves scalability by implementing traffic classification and conditioning functions at network boundary nodes • Classification involves mapping packets to different forwarding classes. • Conditioning : checking whether traffic flows meet the service agreement and dropping/remarking nonconformant packets. • Interior nodes forward packets based solely on the forwarding class. COMP680E by M. Hamdi 38 Per Hop Behavior (PHB) • Forwarding treatments at a node • Each PHB is represented by a 6-bit value called DSCP • All packets with the same code points are referred to as a behavior aggregate (BA) and they receive the same forwarding treatment. • May describe forwarding behavior in either relative or absolute terms – Minimal bw for BA : absolute term – Allocate bw proportionally : relative • Typically implemented by means of buffer management and packet scheduling. COMP680E by M. Hamdi 39 Differentiated Services Field • Uses 6 bits in the IP header to encode forwarding treatment • These 6 bits are those out of the IP TOS field (8 bits long) • DiffServ redefines existing IP TOS field to indicate forwarding behavior • Replacement field, called DS field supersedes existing definition of TOS • First 6 bits used as DSCP to encode the PHB, remaining 2 bits are currently unused (CU). COMP680E by M. Hamdi 40 Differentiated Service (DS) Field 0 5 6 7 DS Field 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 to determine the PHB COMP680E by M. Hamdi 41 Assured Forwarding (AF) • The basic idea came from RIO scheme • In RIO scheme packets are marked as in or out • During congestion, out packets are dropped first : in/out bit indicates drop priorities • AF standard extended the basic in or out marking in RIO into four forwarding classes and within each forwarding class, three drop precedences COMP680E by M. Hamdi 42 Assured Forwarding (AF) (cont.) • Customers can subscribe to the service built with AF forwarding class and their packets will be marked with appropriate AF DSCPs. • Drop priorities within each forwarding class are used to select which packets to drop during congestion • When backlogged packets from an AF forwarding class exceed a specified threshold, packets with highest drop priority is dropped first, then packets with lower drop priority COMP680E by M. Hamdi 43 Mechanism for assured service - summary Host First-hop Marking packets according to the service profile Out- and in- dropper RIO scheme, packets are treated preferentially COMP680E by M. Hamdi 44 Expedited Forwarding (EF) • Proposed to characterize a forwarding treatment similar to that of a simple priority queuing. • Forwarding treatment of traffic aggregate must equal or exceed a configurable rate • Should receive this rate independent of load of other traffic passing through the node • Provides low delay and low loss service COMP680E by M. Hamdi 45 EF implementation • Several queuing mechanisms can be used to implement EF PHB – Priority queuing with token bucket • Priority of EF traffic should be highest in the system • Token bucket is used to limit the total amount of EF traffic so that other traffic will not starve – WFQ can be used such that weight assigned to EF traffic has relative priority than other traffic COMP680E by M. Hamdi 46 DiffServ – Summary DiffServ Domain Classification / Conditioning PHB LLQ/WRED Premium Gold Silver Bronze COMP680E by M. Hamdi 47 Functionality at DiffServ Routers COMP680E by M. Hamdi 48 Proportional QoS Using a proportional QoS model, we not only guarantee that a higher priority class receives better service, but we also quantify the differentiation between different classes: COMP680E by M. Hamdi 49 Proportional Differentiation • Definition If qi is the QoS metric of interest, and si is the differentiation factor for class i, we have: qi si qj sj (i, j 1...N ) For example: Given two classes 1 and 2, and the QoS metric is packet loss rate, s1=1; s2=2, the packet loss rate of class 2 should be twice that of the loss rate of class 1. COMP680E by M. Hamdi 50 Proportional Differentiation • Pros – Controllable Differentiation level between service classes can be controlled by network operator; – Predictable Performance of higher classes is consistently better than the performance of lower Class even in short time scale; COMP680E by M. Hamdi 51 Proportionally differentiated packet delay Waiting Time Priority (WTP) Scheduling Class 0 One packet need to be scheduled Class 1 Class N On-line priority measurement is done COMP680E by M. Hamdi 52 Proportionally differentiated packet delay Waiting Time Priority (WTP) Scheduling Class 0 Class 1 Class N Class 1 has the highest priority COMP680E by M. Hamdi 53 Proportionally differentiated packet delay • Wait Time Priority (WTP) Scheduling – Suppose class i is backlogged at time t, and that wi(t) is the head waiting time of class i at t; – We have normalized head waiting time of class i at t as: ~ (t ) w (t ) / s w i i i – When a packet need to be scheduled, a backlogged class j is selected for ~ (t ) j arg max w i iB ( t ) COMP680E by M. Hamdi 54 Performance Proportional average packet delay: COMP680E by M. Hamdi 55 Proportionally differentiated loss rate • Buffer Management On-line priority measurement is done Class 0 Class 1 One packet arrives Class 2 Total buffer size 20 COMP680E by M. Hamdi 56 Proportionally differentiated loss rate • Buffer Management Class 0 Class 1 Class 0 has the lowest priority Class 2 Total buffer size 20 COMP680E by M. Hamdi 57 Proportionally differentiated loss rate • Proportional Loss Rate (PLR) dropper – Suppose there are two counters for each class i, counter ai records packet arrival history of class i, counter di records packet drop history of class i; – We have normalized packet loss rate of class i as: ~ li di /( ai si ) – When a packet needs to be dropped, a backlogged class j is selected for ~ j arg min li (t ) iB ( t ) COMP680E by M. Hamdi 58 Performance Proportional packet loss rate: COMP680E by M. Hamdi 59 Architecture I Parameters Timer Counter sets Update Resetting counterwhen counter Packet condition is arrival fulfilled CPU Dropper Drop one packet COMP680E by M. Hamdi Waiting time recorder Scheduler Schedule a packet60 MPLS COMP680E by M. Hamdi 61 Why MPLS? • MPLS stands for: “Multi-Protocol Label Switching” • Goals: – Bring the speed of layer 2 switching to layer 3 • May no longer be perceived as the main benefit: Layer 3 switches – Resolve the problems of IP over ATM, in particular: • Complexity of control and management • Scalability issues – Support multiple layer 2 technologies COMP680E by M. Hamdi 62 Basic Idea • MPLS is a hybrid model adopted by IETF to incorporate best properties in both packet routing & circuit switching IP Router Control: MPLS Control: IP Router Software IP Router Software Forwarding: Forwarding: Longest-match Lookup Label Swapping COMP680E by M. Hamdi ATM Switch Control: ATM Forum Software Forwarding: Label Swapping 63 Basic Idea (Cont.) • Packets are switched, not routed, based on labels • Labels are filled in the packet header • Basic operation: – Ingress LER (Label Edge Router) pushes a label in front of the IP header – LSR (Label Switch Router) does label swapping – Egress LER removes the label • The key : establish the forwarding table – Link state routing protocols • Exchange network topology information for path selection • OSPF-TE, IS-IS-TE – Signaling/Label distribution protocols: • Set up LSPs (Label Switched Path) • LDP, RSVP-TE, CR-LDP COMP680E by M. Hamdi 64 MPLS Operation 1a. Routing protocols (e.g. OSPF-TE, IS-IS-TE) exchange reachability to destination networks 1b. Label Distribution Protocol (LDP) establishes label mappings to destination network 4. LER at egress removes label and delivers packet IP IP 2. Ingress LER receives packet and “label”s packets 3. LSR forwards packets using label COMP680E by M. Hamdiswapping 65 Label switching technologies • Main objectives: a forwarding technique – Improve Internet forwarding technology performance – As a result, scale the (WAN) Internet infrastructure • Several pre-standard industry flavors – "Tag switching", "IP switching", "Fast IP", ... • IETF Standardization – First drafts produced mid 97 COMP680E by M. Hamdi 66 Label Switching • Uses concept of edge and core where: – conventional routing done at the edge Conventional Routing at edge Switching in core – switching (i.e.make forwarding decision not based on destination address) done in the core COMP680E by M. Hamdi 67 Labels 7423 H • In core, forwarding decision based on “logical forwarding references” 7423 R 3 7423 R 7 R • “Logical forwarding reference” called Label (1) label COMP680E by M. Hamdi 7423 68 Label Switching Routers • In the core nodes capable of switching on labels called: Specific device 7423 H 7423 – Label Switching Routers (LSR) 3 LSR 7423 • LSR is a specific device LSR 7 – neither regular router – nor regular ATM switch Label Switching Domain COMP680E by M. Hamdi 7423 69 Label Switching rationale • Select output port at "hardware speed" Simple look-up Using an Index – – – – – Label 3 Outgoing port short of fixed length in fixed position at beginning of packet with no internal structure COMP680E by M. Hamdi 0 1 2 3 4 5 6 7 70 Relationships with QoS • Entry may also contain information about what resources all the pkts carrying this label may use Label 3 Outgoing Resources port 0 1 2 3 4 5 6 7 e.g. – outgoing queue – drop reference level COMP680E by M. Hamdi 71 Label local significance • No reason index (labels) to forwarding tables be the same in all nodes • Label to be agreed by two communicating LSRs • Have strictly local significance, as 3 LSR 7 LSR 2 – X.25 virtual channels numbers – ATM VCIs and VPIs COMP680E by M. Hamdi 72 Label Switched Path (LSP) • Label Switched Path (LPS) – concatenation of labels 3 LSR 7 LSR 2 – constitutes, from node to node, the path followed by all pkts carrying those labels COMP680E by M. Hamdi 73 FEC and Label Switched Paths Forwarding Equivalence Classes • Pkts following an LSP form an FEC “a set of pkts forwarded in the same manner” Example of Label and FEC binding 5 LSR LSR 2 LSR FEC: All packets forwarded in the same manner by that router • An FEC maps to a label l Form a Class (FEC) l carry same label 3 on this section COMP680E by M. Hamdi 74 Further FEC partition Example of further partition of an FEC • Further FEC partition possible (but not mandatory) Set of Packets • Taking Same path • Best Effort PHB label x LSR • If packets have different QoS requirements LSR label y Set of Packets • Taking Same path • Expedited PHB COMP680E by M. Hamdi 75 Label swapping • Label swapping in a core Label Switching Network (LSN) is similar to forwarding over ATM or X.25 virtual circuits – replace incoming label with outgoing label for next hop – Implement QoS/priority as appropriate – forward to output port • In addition, LSRs must check if pkt has reached final destination (egress point) COMP680E by M. Hamdi 76 Core MPLS Switching Routers MPLS switches may be Core switching • Routers enhanced with MPLS functionality R R R – If IP, can be: • IPv4 router • IPv6 router MPLS-capable Routers • ATM switches (1) Core switching – supporting MPLS protocols – implementing regular layer 3 routing – called ATM-LSR or LS-ATM ATM R ATM R ATM R MPLS-capable and routingcapable ATM switch COMP680E by M. Hamdi 77 Where to code the label? • Labeling a pkt: either Layer 2 header Layer 3 header Label – overwriting an existing field in a layer 2 header, or – inserting an extra header between layer 2 and layer 3 headers - the MPLS header label set in existing field Layer 2 header COMP680E by M. Hamdi MPLS header Layer 3 header Label label inserted 78 Example of layer 2 label Core switching • When Label Switching Routers are ATM-LSR ... ATM R ATM R ATM R Layer 2 header Layer 3 header Label then, ... VPI • … Label carried in the VPI/VCI field (28 bits) VPI VCI COMP680E by M. Hamdi VCI VCI Payload CLP HEC 8 79 Traffic Engineering & MPLS COMP680E by M. Hamdi 80 Traffic Engineering • Concerned with the performance optimization of operational networks • Main objective is to reduce congestion hot spots and improve resource utilization across the network through carefully managing the traffic distribution inside the network • Cost savings that results in more efficient use of resources (e.g. bw) helps to reduce overall cost of operation for service providers. • IP routing is based on destination and used simple metrics such as hop count • IP routing can lead to poor resource utilization COMP680E by M. Hamdi 81 The Fish Problem D G F Head A C E COMP680E by M. Hamdi Tail B 82 The Fish Problem (cont.) • There are two paths from A and B to G. • But only one of the two paths (shortest path) will be used for traffic • Leads to unbalanced traffic distribution • Problem caused by two properties of IP routing – IP routing is destination based. Thus for each destination network there is typically only one path in the routing table : traffic distribution tends to be unbalanced COMP680E by M. Hamdi 83 The Fish Problem (cont.) – Decision making in current routing is based on local optimization : any node simply selects a path that is best from its own perspective. It does not take into account the overall system objective and have a global view of the network in terms of traffic distribution COMP680E by M. Hamdi 84 Optimization Objectives • The main aim of TE is to improve network performance through optimization of resource utilization in the network. • Common optimization objectives are – Minimizing congestion and packet losses in the network – Improving link utilization – Minimizing total delay experienced by packets – Increasing number of customers with the current assets COMP680E by M. Hamdi 85 Optimization Objectives (cont.) • ISPs would like to avoid hot spots in the network – Mathematically means minimize the maximum link utilization – Means lower total delay and loss – Leaves more space for future traffic growth since available bandwidth is maximized COMP680E by M. Hamdi 86 Constraint-Based Routing • Conventional IP routing is based on an algorithm that optimizes a particular scalar metric • With constraint based routing path is optimal w.r.t. some scalar metric, at the same time it does violate a set of constraints : – Performance constraint • Path with certain minimum available bw – Administrative constraints • Path that excludes certain links in the network COMP680E by M. Hamdi 87 Constraint-Based Routing (cont.) • Plain IP routing cannot support constraint based routing – Constraint-based routing requires path calculation at the source • Because different source may have different constraints for a path to the same destination • Constraints associated with a particular source router are only known to that router • In plain IP routing paths are computed in a distributed fashion by every router; does not take into account constraints of different sources COMP680E by M. Hamdi 88 Constraint-Based Routing (cont.) – When a path is determined by the source, forwarding along such a path cannot be provided using the destination-based IP forwarding – Path computation at the source needs to have information about attributes associated with individual links (e.g. link utilization). • There is no mechanism to distribute this information in the network through plain IP routing • IP routing protocol can be augmented to support these functionality COMP680E by M. Hamdi 89