This preview shows page 1. Sign up to view the full content.
Unformatted text preview: Data and Computer Communications Communications
Chapter 13 – Congestion in Data Chapter Networks Networks
Eighth Edition by William Stallings Lecture slides by Lawrie Brown Congestion in Data Networks Congestion
At St. Paul's a great throng crammed the platform. She At saw a sea of faces, each stamped with a kind of purposeful, hungry urgency, a determination to get into this train. As before, when she was on the Northern Line, she thought there must be some rule, some operating law, that would stop more than a limited, controlled number getting in. Authority would appear and stop it. and —King Solomon's Carpet, Barbara Vine (Ruth Rendell) What Is Congestion? What congestion occurs when the no of packets being congestion transmitted through the network approaches the packet handling capacity of the network packet congestion control aims to keep no of packets congestion below a level at which performance falls off dramatically dramatically a data network is a network of queues generally 80% utilization is critical finite queues mean data may be lost Queues at a Node Queues Interaction of Queues Interaction Ideal Network Network Utilization Effects of Congestion No Control Mechanisms for Congestion Control Congestion Backpressure Backpressure iif node becomes congested it can slow down or f halt flow of packets from other nodes halt cf. backpressure in blocked fluid pipe may mean that other nodes have to apply control on may incoming packet rates incoming propagates back to source can restrict to high traffic logical connections used in connection oriented nets that allow hop used by hop congestion control (eg. X.25) by not used in ATM nor frame relay only recently developed for IP Choke Packet Choke a control packet control generated at congested node sent to source node eg. ICMP source quench
• • • from router or destination source cuts back until no more source quench message sent for every discarded packet, or anticipated is a rather crude mechanism Implicit Congestion Signaling Implicit transmission delay increases with congestion hence a packet may be discarded source detects this implicit congestion indication useful on connectionless (datagram) networks eg. IP based
• (TCP includes congestion and flow control - see chapter 17) used in frame relay LAPF Explicit Congestion Signaling Explicit network alerts end systems of increasing network congestion congestion end systems take steps to reduce offered load Backwards congestion avoidance notification in opposite direction congestion to packet required to congestion avoidance notification in same direction as congestion packet required packet Forwards Explicit Signaling Categories Explicit Binary a bit set in a packet indicates congestion indicates how many packets source may send common for end to end flow control supply explicit data rate limit nodes along path may request rate reduction eg. ATM Credit based Rate based Traffic Management Traffic fairness provide equal treatment of various flows different treatment for different connections traffic contract between user and network carry best-effort or discard excess traffic quality of service reservations Congestion Control in Packet Switched Networks Switched send control packet to some or all source send nodes nodes requires additional traffic during congestion may react too quickly adds to overhead either backwards or forwards rely on routing information end to end probe packets add congestion info to packets in transit Frame Relay Congestion Control Congestion minimize discards maintain agreed QoS minimize probability of one end user monopoly simple to implement create minimal additional traffic distribute resources fairly limit spread of congestion operate effectively regardless of traffic flow minimum impact on other systems minimize variance in QoS FR Control Techniques FR difficult for frame-relay joint network & end-system responsibility techniques: discard strategy congestion avoidance explicit signaling congestion recovery implicit signaling mechanism Traffic Rate Management Traffic must discard frames to cope with congestion arbitrarily, no regard for source no reward for restraint so end systems transmit as no fast as possible fast Committed information rate (CIR)
• • • data in excess of this liable to discard not guaranteed in extreme congestion situations aggregate CIR should not exceed physical data rate Committed burst size Excess burst size Operation of CIR Operation Relationship Among Congestion Parameters Parameters Congestion Avoidance using Explicit Signaling Explicit network alerts end systems of growing network congestion using congestion backward explicit congestion notification forward explicit congestion notification frame handler monitors its queues may notify some or all logical connections user response reduce rate ATM Traffic Management ATM high speed, small cell size, limited overhead bits still evolving reasons existing tools are inadequate for ATM majority of traffic not amenable to flow control feedback slow due to reduced transmission time feedback compared with propagation delay compared wide range of application demands different traffic patterns different network services high speed switching and transmission increases high volatility volatility Latency/Speed Effects Latency/Speed consider ATM at 150Mbps takes ~2.8x10-6 seconds to insert single cell time to traverse network depends on time propagation delay and switching delay propagation assume propagation at two-thirds speed of light iif source and destination on opposite sides of f 3 USA, propagation time ~ 48x10--3 seconds USA, seconds given implicit congestion control, by the time given dropped cell notification has reached source, 7.2x106 bits have been transmitted 7.2x10 this is not a good strategy for ATM Cell Delay Variation Cell for ATM voice/video, data is a stream of cells delay across network must be short rate of delivery must be constant there will always be some variation in transit delay cell delivery to application so that constant delay bit rate can be maintained to application bit Timing of CBR Cells Timing Network Contribution to Cell Delay Variation Cell in packet switched networks is due to in queuing delays and routing decision time queuing in Frame relay networks is similar in ATM networks less than frame relay ATM protocol designed to minimize ATM processing overheads at switches processing ATM switches have very high throughput only noticeable delay is from congestion must not accept load that causes congestion Cell Delay Variation At The UNI At application produces data at fixed rate 3 layers of ATM processing causes delay interleaving cells from different connections operation and maintenance cell interleaving iif using synchronous digital hierarchy frames, f these are inserted at physical layer these cannot predict these delays Origins of Cell Delay Variation Origins Traffic and Congestion Control Framework Control ATM layer traffic and congestion control ATM should support QoS classes for all foreseeable network services foreseeable should not rely on AAL protocols that are should network specific, nor higher level application specific protocols application should minimize network and end to end should system complexity system Timings Considered Timings timing intervals considered: cell insertion time round trip propagation time connection duration long term determine whether a given new connection can be determine accommodated accommodated agree performance parameters with subscriber traffic control strategy then must: now review various control techniques Resource Management Using Virtual Paths Virtual separate traffic flow according to service separate characteristics on a virtual path characteristics user to user application user to network application network to network application cell loss ratio cell transfer delay cell delay variation QoS parameters concerned with are: Configuration of VCCs and VPCs VCCs Allocating VCCs within VPC Allocating all VCCs within VPC should experience all similar network performance similar options for allocation: aggregate peak demand
• • set VPC capacity to total of all peak VCC rates will meet peak demands, but often underutilized statistical multiplexing
• set VPC capacity to more than average VCC rates • will see greater variation but better utilization Connection Admission Control Control first line of defense user specifies traffic characteristics for new user connection (VCC or VPC) by selecting a QoS connection network accepts connection only if it can meet network the demand the traffic contract peak cell rate cell delay variation sustainable cell rate burst tolerance Usage Parameter Control Usage UPC function monitors a connection to UPC ensure traffic obeys contract ensure purpose is to protect network resources purpose from overload by one connection from done on VCC and VPC peak cell rate and cell delay variation sustainable cell rate and burst tolerance UPC discards cells outside traffic contract Selective Call Discard Selective when network at point beyond UPC when discards (CLP=1) cells discards aim to discard lower-priority cells when aim congested to protect higher-priority cells congested note. can’t distinguish between cells originally note. labeled lower priority, verses those tagged by UPC function UPC Traffic Shaping Traffic UPC provides a form of traffic policing can be desirable to also shape traffic smoothing out traffic flow smoothing reducing cell clumping token bucket Token Bucket for Token Traffic Shaping Traffic GFR Traffic Management GFR guaranteed frame rate (GFR) as simple as UBR guaranteed from end system viewpoint from places modest requirements on ATM network end system does no policing or shaping of traffic may transmit at line rate of ATM adaptor no guarantee of delivery so higher layer (eg. TCP) must do congestion control ensures application can send at min rate with no loss if no congestion, higher rates maybe used user can reserve capacity for each VC Frame Recognition Frame GFR recognizes frames as well as cells when congested, network discards whole frame when rather than individual cells rather all cells of a frame have same CLP bit setting CLP=1 AAL5 frames lower priority (best effort) CLP=0 frames minimum guaranteed capacity GFR Contract Parameters GFR Peak cell rate (PCR) Minimum cell rate (MCR) Maximum burst size (MBS) Maximum frame size (MFS) Cell delay variation tolerance (CDVT) Components of GFR System Supporting Rate Guarantees Supporting Tagging and Policing Tagging discriminates between frames that discriminates conform to contract and those that don’t conform set CLP=1 on all cells in frame if not gives lower priority maybe done by network or source network may discard CLP=1 cells policing Buffer Management Buffer deals with treatment of buffered cells congestion indicated by high buffer congestion occupancy occupancy will discard tagged cells in preference to will untagged cells untagged including ones already in buffer to make room may do per VC buffering for fairness cell discard based on queue-specific cell thresholds thresholds Scheduling Scheduling preferential treatment to untagged cells separate queues for each VC make per-VC scheduling decisions enables control of outgoing rate of VCs VCs get fair capacity allocation still meet contract GFC Conformance Definition GFC UPC function monitors each active VC to ensure traffic conforms to contract tag or discard nonconforming cells frame conforms if all cells conform a cell conforms if: rate of cells is within contract all cells in frame have same CLP frame satisfies MFS parameter frame
• check if either last cell in frame or cell count < MFS QoS Eligibility Test QoS two stage filtering process a frame is tested for conformance to contract if not, may discard or tag set upper bound & penalize cells above upper bound do expect attempt to deliver tagged cells under GFR contract for VC set lower bound on traffic frames in traffic flow below threshold are eligible determine frames eligible for QoS guarantees GFR VC Frame Categories GFR nonconforming frame cells of this frame will be tagged or discarded cells will receive a best-effort service cells will receive a guarantee of delivery cells conforming but ineligible frames conforming and eligible frames form of cell rate algorithm is used Summary Summary congestion effects congestion control traffic management frame relay congestion control ATM congestion control ...
View Full Document
- Spring '09