Integrated Services & Differentiated Services
Protocols and the TCP/IP Suite Integrated Services &Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
Introduction Modern Internet applications demand services not provided by a best-effort service model Two complementary, yet fundamentally different, traffic management frameworks have evolved: Integrated Services (IS, ISA, IntServ):reserve resources per session and limit total demand to the capacity that can be handled by the network Differentiated Services (DS, DiffServ): classify traffic into a number of traffic groups and handle traffic based on its group Traffic control mechanisms:queuing discipline, packet discard policy Services are specified within a given domain Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
Internet Traffic Elastic Traffic traffic that can adapt, over a wide range, to delay and throughput changes typically TCP/UDP QoS perceived based on application Inelastic Traffic traffic does not adapt well requires guarantees on:throughput, delay, jitter, packet loss e.g. traffic generated by real-time applications elastic traffic must still be supported Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
IntServ Approach Two key features form core of architecture Resource reservation routers must maintain state of available resource reserved for each session Call/session setup each router on the sessions path must verify availability of required resources for a session and admit sessions only if requirements can be met Call Admission process (more later) Traffic characterization (Tspec) Desired QoS characterization (Rspec) Reservation signaling (RSVP, RFC 2210) Per-element call admission per Tspec and Rspec Chapter 17:Integrated and Differentiated Services Chapter 2 IntServ Implementation
Protocols and the TCP/IP Suite IntServ Implementation Associate each packet with a flow a distinguishable stream of related IP packets that result from a single user activity and demand the same QoS (per RFC 1633) unidirectional, can have multiple recipients typically identified by: source & destination IP addresses, port numbers and protocol type Provide for enhanced router functions to manage flows: Admission control based on requested QoS and availability of required network resources Routing protocol based on QoS (like OSPF/MOSPF) Queuing/scheduling disciplines based on QoS Packet discard policy based on QoS Chapter 17:Integrated and Differentiated Services Chapter 2 IntServ Architecture (ISA) - requirements at each router
Protocols and the TCP/IP Suite IntServ Architecture (ISA) - requirements at each router RSVP Background Functions Primary Forwarding Functions Chapter 17:Integrated and Differentiated Services Chapter 2 ISA: 3 Categories of Service
Protocols and the TCP/IP Suite ISA: 3 Categories of Service Guaranteed Service assured capacity (data rate) specified upper bound on queuing delay through the network no queuing loss (i.e., no buffer overflow) Controlled Load roughly equivalent to best-effort under no-load conditions (dprop + dtrans) no specified upper bound on queuing delay, but will approximate minimum expected transit delay almost no queuing loss Best Effort Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
Leaky Bucket Scheme Used to: Characterize traffic in a flow. Describe the load imposed by a flow. Traffic policing. Note that, during any time period T, the amount of data sent cannot exceed RT+B, and Maximum queuing delay by a packet is B/R. Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
Queuing Disciplines Single FIFO queues have numerous drawbacks relative to QoS demands no special treatment based on priority larger packets get better service connections can get an unfair share of resources IntServ allows for multiple queues one per flow separate discipline per flow fair queuing policy Chapter 17:Integrated and Differentiated Services Chapter 2 Queuing Disciplines (Scheduling)
Protocols and the TCP/IP Suite Queuing Disciplines (Scheduling) FIFO (First-Come-First-Served) Round Robin (Fair Queuing) Drawbacks? Drawbacks? Flows with busy (greedy) sources crowd out others Flows with shorter packets are penalized Flows with shorter packets are penalized Chapter 17:Integrated and Differentiated Services Chapter 2 Processor Sharing Approach
Protocols and the TCP/IP Suite Processor Sharing Approach Processor Sharing (PS) ideal, but not a practical policy transmit only one bit per round per queue with N queues, each queue receives exactly 1/N of the available capacity consider each queue independently to calculate virtual start and finish times for each transmission EXAMPLE QUEUE QUEUE QUEUE Packet 1 Packet Packet 1 Packet Packet 1 Real arrival time, i Transmission time, Pi Virtual start time, Si Virtual finish time, Fi Chapter 17:Integrated and Differentiated Services Chapter 2 Bit-Round Fair Queuing
Protocols and the TCP/IP Suite Bit-Round Fair Queuing Bit-Round Fair Queuing (BRFQ) emulates PS round-robin approach for packets and multiple synchronous queues uses packet length and flow identification (queue) to schedule packets calculate Si and Fias though PS were running when a packet finishes transmission, send next packet based on smallest value of Fi over all queues algorithm is fair on the basis of amount of data transmitted instead of number of packets Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
PS vs. BRFQ Example Drawback? No precedence or priority weighting of flows. Chapter 17:Integrated and Differentiated Services Chapter 2 Queuing Discipline Examples
Protocols and the TCP/IP Suite Queuing Discipline Examples Load equals capacity Chapter 17:Integrated and Differentiated Services Chapter 2 Queuing Discipline Examples
Protocols and the TCP/IP Suite Queuing Discipline Examples Load exceeds capacity Drawbacks? Chapter 17:Integrated and Differentiated Services Chapter 2 Queuing Discipline Priority Queuing
Protocols and the TCP/IP Suite Queuing Discipline Priority Queuing Data Communications and Networking, Forouzan, 2004 Chapter 17:Integrated and Differentiated Services Chapter 2 Queuing Discipline Weighted Fair Queuing
Protocols and the TCP/IP Suite Queuing Discipline Weighted Fair Queuing Data Communications and Networking, Forouzan, 2004 Chapter 17:Integrated and Differentiated Services Chapter 2 Weighted Fair Queue (WFQ)
Protocols and the TCP/IP Suite Weighted Fair Queue (WFQ) Guaranteed Rate (weight) = .5 Guaranteed Rate = .05 Fi = Si + Pi , = weight Maximum delay for flow i Di Bi(Ki-1)Li Ki Lmax Ri Ri m=1Cm Di = max. delay for flow i Bi = token bucket size for flow i Ri= token rate for flow i Ki = number of nodes in flow i path Li= max. packet size for flow i Lmax = max. packet length for all flows through all nodes on flow i path Cm = outgoing link capacity at node m Chapter 17:Integrated and Differentiated Services Chapter 2 Scheduling vs. Queue Management (see RFC 2309)
Protocols and the TCP/IP Suite Scheduling vs. Queue Management (see RFC 2309) Closely related, but different performance issues Scheduling:managing allocation of bandwidth between flows by determining which packet to send next (queuing discipline) Queue Management: managing the length of packet queues by proactively dropping packets when necessary (packet discard policy) Chapter 17:Integrated and Differentiated Services Chapter 2 Random Early Detection (RED)
Protocols and the TCP/IP Suite Random Early Detection (RED) Queuing discipline with proactive packet discard anticipate congestion and take early avoidance action improved performance for elastic traffic by not penalizing bursty traffic avoids global synchronization phenomenon at congestion onset control average queue length (buffer size) within deterministic bounds therefore, control average queuing delay Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
RED Buffer Management Discard probability is calculated for each packet arrival at the output queue based on: the current weighted average queue size, and the number of packets sent since the previous packet discard Chapter 17:Integrated and Differentiated Services Chapter 2 Generalized RED Algorithm
Protocols and the TCP/IP Suite Generalized RED Algorithm calculate the average queue size, avg if avg < THmin queue the packet else if THmin avg < THmax calculate probability Pa with probability Pa discard the packet else with probability 1 Pa else if avg THmax See Figure 17.8, page 489 Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
RED Algorithm avg lags considerably behind changes in actual queue size (weight, wq, is small typ ) avg (1 wq)avg + wqq prevents reaction to short bursts count, number of packets passed without discard, increases incrementally while Thmin < avg < Thmax probability of discard, Pa, increases as count increases helps ensure fairness across multiple flows Chapter 17:Integrated and Differentiated Services Chapter 2 RED Probability Function (Increasing F)
Protocols and the TCP/IP Suite RED Probability Function (Increasing F) Pa = 1/(F x Pmax) - count 1 F = THmax - THmin avg - THmin Chapter 17:Integrated and Differentiated Services Chapter 2 RED Probability Function (Constant F)
Protocols and the TCP/IP Suite RED Probability Function (Constant F) avg - THmin Fconst = THmax - THmin Pb = FxPmax 1 Pa = 1/Pb - count Chapter 17:Integrated and Differentiated Services Chapter 2 RED Performance (vs. Drop Tail Queuing Policy)
Protocols and the TCP/IP Suite RED Performance (vs. Drop Tail Queuing Policy) Pmax = 0.02 Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite Differentiated Services
Chapter 17 continued Differentiated Services Chapter 2 Differentiated Services (DS)
Protocols and the TCP/IP Suite Differentiated Services (DS) ISA and RSVP deployment drawbacks relatively complex may not scale well for large traffic volumes DiffServ solution (RFC2475, 3260) designed as a simple, easily-implemented, low-overhead tool offers a range of services in differentiated service categories scalable and flexible service classification Key characteristics uses existing IPv4 TOS field or IPv6 Traffic Class field (for DS field) SLA established in advance no application changes required built-in aggregation mechanism based on traffic category routers queue and forward based on information carried in the DS Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
DS Domains Contiguous portion of the Internet over which a consistent set of DS policies are agreed and administered Typically under control of a single management entity Services in a domain defined by a Service Level Agreement (SLA) a contract between service provider and user/another domain which specifies QoS parameters detailed service parameters: throughput, drop probability, latency ingress/egress constraints service-based traffic profiles disposition of excess (in violation of SLA) traffic DS field carries a traffic class as specified by the SLA Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
DiffServ Terminology Traffic conditioning functions Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
DS Terminology Service Level Agreement (per RFC 3260): A Service Level Specification (SLS) is a set of parameters and their values which together define the service offered to a traffic stream by a DS domain. A Traffic Conditioning Specification (TCS) is a set of parameters and their values which together specify a set of classifier rules and a traffic profile. A TCS is an integral element of an SLS. Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
DS and IPv4 TOS Fields IP ECN Field, per RFC 3168 & RFC 3260 Replaces 6-bit DS code point, in three pools Pool 1:xxxxx0- standards-based use (e.g , xxx000) Pool 2:xxxx11 experimental/local use Pool 3:xxxx01 experimental/local use, future standards Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
DS Domains/Regions Border node: Per Hop Behavior (PHB) plus traffic conditioning mechanisms Interior node: typically only PHB mechanisms Chapter 17:Integrated and Differentiated Services Chapter 2 DS Traffic Classifier/Conditioner
Protocols and the TCP/IP Suite DS Traffic Classifier/Conditioner Conformance test per SLA (e.g peak rate, burstiness, ) Regulate traffic flow to achieve a specified traffic rate (e.g. with a token bucket) Separate traffic into classes based on fields as specified in the TCS (source IP, dest. IP, source port #, dest. port #, ) Mark with a DS codepoint, or re-mark as necessary (at domain ingress node, or at boundary between domains) Police traffic and drop packets if rate exceeds that specified in the SLA (per metering function) Chapter 17:Integrated and Differentiated Services Chapter 2 Protocols and the TCP/IP Suite
Per-Hop Behavior RFC 2475 definition: a description of the externally observable forwarding behavior of a DiffServ node applied to a particular DiffServ behavior aggregate. Two standard PHBs defined: Expedited Forwarding (RFC 2598) Assured Forwarding (RFC 2597) Expedited Forwarding Premium service with low delay, low-loss, low jitter, and assured bandwidth Domain boundary nodes control traffic aggregate to limit its characteristics (i.e. controlled rate and burstiness) Interior nodes ensure that the aggregates maximum arrival rate is less than its minimum departure rate (i.e. limit the queuing effect) Chapter 17:Integrated and Differentiated Services Chapter 2 Per-Hop Behavior (cont.)
Protocols and the TCP/IP Suite Per-Hop Behavior (cont.) Assured Forwarding designed to offer a service level that is superior to best-effort service based on explicit allocation concept choice of classes offered, each with different traffic profile monitor traffic at boundary nodes, and mark as in or out based on conformance to profile interior nodes handle packets based only on in or out mark in congestion, drop outs before ins implementation defines four AF classes and replaces in/out mark with a drop precedence codepoint simple and easy to implement in nodes Chapter 17:Integrated and Differentiated Services Chapter 2 Differentiated Services Assured Forwarding PHB
Protocols and the TCP/IP Suite Differentiated Services Assured Forwarding PHB ECN Designated AF1x, AF2x, AF3x, AF4x RFC 2597 Chapter 17:Integrated and Differentiated Services Chapter 2 Differentiated Services Assured Forwarding PHB
Protocols and the TCP/IP Suite Differentiated Services Assured Forwarding PHB AF AF AF AF4 Low Medium High 001010 010010 011010 100010 001100 010100 011100 100100 001110 010110 011110 100110 Chapter 17:Integrated and Differentiated Services Chapter 2 Real-Time Traffic Flow
Protocols and the TCP/IP Suite Real-Time Traffic Flow Chapter 17:Integrated and Differentiated Services Chapter 2 Real-Time Packet Transmission
Protocols and the TCP/IP Suite Real-Time Packet Transmission fixed-size packets, generated at fixed intervals data too important for lossy compression Examples: ATC or RT simulations on/off source alternates between fixed size packets with fixed intervals and inactivity Examples: voice telephony or audio conferencing variable packet size at uniform intervals Examples: compressed video as in video conferencing Chapter 17:Integrated and Differentiated Services Chapter 2