Network Management Quality of Service I

Transcription

1 Network Management Quality of Service I Patrick J. Stockreisser p.j.stockreisser@cs.cardiff.ac.uk

2 Lecture Outline Basic Network Management (Recap) Introduction to QoS Packet Switched Networks (Recap) Common QoS approaches Best Effort Service (Integrated Service) (Differentiated Service) Common QoS Principles

3 Network Management Network management means different things to different people. Network management is the execution of a set of functions required for controlling, planning, allocating, deploying, coordinating, and monitoring the resources of a network. Generally it is a service that employs a variety of tools, applications, and devices to assist human network managers in monitoring and maintaining networks.

4 History Early 1980s Great expansion in the area of network deployment. Cost benefits Productivity Gains By mid-1980s, Growing difficulties in management of the networks A result of deploying many different (and sometimes incompatible) network technologies By late-1990s Wireless technologies emerge (standards agreed) A new range of issues arise Existing network management technologies unsuitable

5 Network Management Automation The problems associated with network expansion affect both day-to-day network operation management and strategic network growth planning. Each new network technology requires its own set of experts. In the early 1980s, the staffing requirements alone for managing large, heterogeneous networks created a crisis for many organizations. An urgent need arose for automated network management.

6 Some Network Management Functions Security: Ensuring that the network is protected from unauthorized users. Performance: Eliminating bottlenecks in the network. Reliability: Making sure the network is reliable to users and responding to hardware and software malfunctions. Availability:

7 Quality of Service In computer networking, the traffic engineering term Quality of Service (QoS) refers to control mechanisms that can provide different priority to different users or data flows, or guarantee a certain level of performance to a data flow in accordance with requests from the application program.

8 An Analogy for Quality of Service Consider two competing cargo airlines (A and B) operating out of New York JFK airport with a service to London Heathrow. Both airlines have the same aircraft, both airlines charge the same rate per package shipped Both airlines offer seven flights a week. There is little to differentiate the service offered by these two companies. These competing airlines offer the same throughput capacity in terms of packages shipped per week.

9 Same Service? Same QoS? Imagine this: Airline A offers one flight per day for each day of the week Airline B offers all of its seven flights on a single day of the week. So while both airlines provide the same throughput capacity over the weekly period, they differ greatly in the actual service provided. Depending on your delivery needs these different airline service models will succeed or fail in quite dramatic fashion. Eg: an online real-time mail order business Eg: stock supply for a for a large warehouse Obviously, the business requirements for delivery define which service model works best. Real-time (daily) demand needs a very regular and consistent service whereas irregular (weekly) batch demand does not.

10 Some other QoS definitions QoS is described in terms of a set of user perceived characteristics of the performance of a service. It is expressed in user-understandable language and manifests itself as a number of parameters, all of which have either a subjective or objective values QoS refers to the capability of a network to provide better service to selected network traffic over various technologies QoS is the collective effect of service performance which determine the degree of satisfaction of a user of the service

11 OSI End to End QoS

12 The simplest of networks Network Client Server Packets are sent between nodes.

13 Network Basics Computers running on the Internet communicate to each other using either the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP) When you write programs that communicate over a network, you are programming at the application layer. Typically, you don't need to concern yourself with the TCP and UDP layers.

14 Packet Review (Packet) (frame) (block) (cell) (segment) A network breaks a message into parts of a certain size in bytes. These are the packets Each packet carries the information that will help it get to its destination: the sender's IP address, the intended receiver's IP address, something that tells the network how many packets this message has been broken into and the number of this particular packet. The packets carry the data in the protocols that the Internet uses: Transmission Control Protocol/Internet Protocol (TCP/IP). Each packet contains part of the body of your message. A typical packet contains perhaps 1,000 or 1,500 bytes.

15 Packet Sending Each packet is then sent off to its destination by the best available route A route that might be taken by all the other packets in the message or by none of the other packets in the message. This makes the network more efficient. Load balancing Problem Avoidance

16 Networks

17 Packet Structure Most packets are split into three parts: 1. Header - The header contains instructions about the data carried by the packet. These instructions may include: Length of packet (some networks have fixed-length packets, while others rely on the header to contain this information) Synchronization (a few bits that help the packet match up to the network) Packet number (which packet this is in a sequence of packets) Protocol (on networks that carry multiple types of information, the protocol defines what type of packet is being transmitted: , Web page, streaming video) Destination address (where the packet is going) Originating address (where the packet came from) 2. Payload - Also called the body or data of a packet. This is the actual data that the packet is delivering to the destination. If a packet is fixed-length, then the payload may be padded with blank information to make it the right size. 3. Trailer - The trailer, sometimes called the footer, typically contains a couple of bits that tell the receiving device that it has reached the end of the packet. It may also have some type of error checking. The most common error checking used in packets is Cyclic Redundancy Check (CRC). CRC is pretty neat. Here is how it works in certain computer networks: It takes the sum of all the 1s in the payload and adds them together. The result is stored as a hexadecimal value in the trailer. The receiving device adds up the 1s in the payload and compares the result to the value stored in the trailer. If the values match, the packet is good. But if the values do not match, the receiving device sends a request to the originating device to resend the packet.

18 A Packet Header Example

19 Network Problems Dropped packets The routers might fail to deliver (drop) some packets if they arrive when their buffers are already full. Some, none, or all of the packets might be dropped, depending on the state of the network, and it is impossible to determine what will happen in advance. The receiving application must ask for this information to be retransmitted, possibly causing severe delays in the overall transmission. Delay It might take a long time for a packet to reach its destination, because it gets held up in long queues, or takes a less direct route to avoid congestion. Alternatively, it might follow a fast, direct route. Thus delay is very unpredictable. Jitter Packets from source will reach the destination with different delays. This variation in delay is known as jitter and can seriously affect the quality of streaming audio and/or video. Out-of-order delivery When a collection of related packets is routed through the Internet, different packets may take different routes, each resulting in a different delay. The result is that the packets arrive in a different order to the one with which they were sent. This problem necessitates special additional protocols responsible for rearranging out-of-order packets to an isochronous state once they reach their destination. This is especially important for video and VoIP streams where quality is dramatically impacted by both latency or lack of isochronicity. Error Sometimes packets are misdirected, or combined together, or corrupted, while en route. The receiver has to detect this and, just as if the packet was dropped, ask the sender to repeat itself.

20 Internet Problems The Internet today does not make any promises about QoS an application will receive. An application will receive whatever level of performance (e.g. end-to-end packet delay and loss) that the network is able to provide at that moment. Delay-sensitive multimedia applications cannot request any special treatment. All packets are treated equal at the routers, including delaysensitive audio and video packets. Network congestion (or interfering traffic) can severally limit the performance of an application (especially audio-video streaming, Multimedia, VoIP and other new applications).

21 QoS Question Hence, what new architectural components can be added to the Internet architecture to shield an application from such congestion and thus make high-quality networked multimedia applications a reality? Guarantees for: low latency high bandwidth real-time performance

22 Common QoS Approaches Best Effort Services (no guarantees) The network does not provide any guarantees that data is delivered or that a user is given a guaranteed quality of service level or a certain priority. Integrated Services (resource reservation) The network resources are assigned according to the application QoS request and subject to the bandwidth management policy Differentiated Services (prioritisation) Network traffic is classified and network elements give preferential treatment to classifications identified having more demanding requirements.

23 Best Effort In a best effort network all users obtain best effort service Each service obtains variable bit rate and delay. Removing features such as recovery of lost or corrupted data and pre allocation of resources, the network operates more efficiently, and the network nodes are inexpensive. Application sends data whenever it feels like, as much as it feels like without requiring any permission. Network elements try their best to deliver the packets to the destination without any bounds on delay, latency, jitter, etc. Network elements can give up to deliver without informing either the sender or the receiver. Conventional IP routers only provide best-effort service. The simplicity of routers is a key factor why IP has been much more successful than more complex protocols such as X.25 and ATM.

24 Best-Effort Post Office Analogy The post office service delivers letters using a best effort delivery approach. The delivery of a certain letter is not scheduled in advance no resources are pre allocated by the post office The postman will make his "best effort" to try to deliver a message but may be delayed if: All of a sudden, too many letters arrive at the post office The postal address is incomplete The postman s van breaks down The sender is not informed if a letter has been delivered successfully. However, the sender can pay extra for a delivery confirmation receipt, This requires that the carrier get a signature from the recipient to prove the successful delivery

25 Scenario Example: Consider two hosts H1, H2 sending packets via router R1 to R2, and subsequently to hosts H3 and H4 Let us assume the LAN speeds are significantly higher than 1.5 Mbps, and focus on the output queue of router R1; Where and why could we encounter packet delay?

26 Packet Delay Demand > Capacity? Packet delay and packet loss will occur if the aggregate sending rate of the H1 and H2 exceeds 1.5 Mbps.

27 QoS: Four Principles To tackle the problems which may arise in such scenarios we will look at 4 QoS Principles: Packet Classification Isolation Scheduling and Policing High Resource Utilisation Call Admission and Blocking

28 Another Problem A 1 Mbps audio application (e.g. a CD-quality audio call) shares the 1.5 Mbps link between R1 and R2 with an FTP application that is transferring a file from H2 to H4 In the best-effort Internet, the audio and FTP packets are mixed in the output queue R1 and (typically) transmitted in a first-in-first-out (FIFO) order Burst of packets from FTP source could potentially fill up the queue, causing IP audio packets to be excessively delayed or lost to buffer overflow at R1

29 Packet Marking A solution is to give priority to audio packets, as FTP does not have timing constraints hence the notion of distinguishing the types of packets via Traffic Class field in IPv6. Principle 1: Packet marking allows a router to distinguish among packets belonging to different classes or traffic.

30 Another Scenario Imagine the FTP user has purchased platinum service (i.e. high priced) Internet access from its ISP, while the audio user has purchased a cheap, low-budget Internet service. Should the cheap user s audio packets be given priority over FTP packets in this case?

31 Packet Classification A more reasonable solution is to distinguish packets on the basis of the sender s IP address. More generally, we see that it is necessary for a router to classify packets according to some criteria. A router must be able to distinguish between packets according to a policy decision One way to achieve this is through marking the packets, however, this does not mandate that a certain QoS will be given. Principle 1 (new): Packet classification allows router to distinguish among packets belonging to different classes of traffic.

32 Another Scenario Suppose the outer knows it should give priority to packets from the 1 Mbps audio application Since outgoing link is 1.5 Mbps, FTP packets receive lower priority, they will still, on average, receive 0.5 Mbps of transmission service Suppose audio applications starts transmitting at greater than 1.5 Mbps (link capacity) This may lead to starvation of FTP packets Similarly for multiple audio applications sharing a link A non-compliant flow could degrade performance

33 Flow Isolation There is a need for a degree of isolation among flows, in order to protect one flow from another misbehaving flow Principle 2: It is desirable to provide a degree of isolation among traffic flows, so that one flow is not adversely affected by another misbehaving flow

34 Policing Policing Mechanism: A monitoring (policing) mechanism put in place to ensure that traffic flows meet some predefined criteria If a policed application misbehaves, the policing mechanism will take some action e.g., drop or delay packets that are in violation of the criteria so that the traffic actually entering the network conforms to the criteria Packet classification and marking mechanism (principle 1) and the policing mechanism (principle 2) are co-located at the edge of the network, either in the end system, or at an edge route.

35 Bandwidth Enforcement Traffic isolation can also be achieved by the link level protocol providing fixed bandwidth to each application flow Audio - 1 Mbps FTP Mbps Here, audio and FTP flows see a logical link with capacity 1.0 and 0.5 Mbps, respectively

36 Enforcement Issues When bandwidth is enforced, a given flow cannot use bandwidth not being used by another application (it can only use a maximum of its own limit) For example, if the audio flow goes silent (e.g., if the speaker pauses and generates no audio packets), the FTP flow would still not be able to transmit more than 0.5 Mbps over the R1-to- R2 link this is clearly wasteful Principle 3: While providing isolation among flows, it is desirable to use resources (e.g., link bandwidth and buffers) as efficiently as possible.

37 Another Example Consider two competing applications transmitting at 1 Mbps, with a link capacity (R1-to-R2) of 1.5 Mbps In this case the combined rate for the two applications is 2 Mbps (higher than link capacity) No marking, isolation or classification will help solve this problem Each app gets 0.75 Mbps of link (half of link capacity) Each app gets 25% packet loss This quality is unacceptable So its better not to transmit any packet at all

38 QoS Guarantees The network should provide the minimum quality of service to enable an application to run, or block the application example callblocking on a telephone network (where endto-end quality of service is necessary) Hence, in the previous case either the minimum QoS is guaranteed, or the application is stopped, as it would not be usable.

39 QoS Requirements Implicit with the need to provide a guaranteed QoS to a flow is the need for the flow to declare its QoS requirements This process of having a flow declare its QoS requirement, and then having the network either accept the flow (at the required QoS) or block the flow (because the resources needed to meet the declared QoS requirements can not be provided) is referred to as the call admission process Principle 4: A call admission process is needed in which flows declare their QoS requirements and are then either admitted to the network (at the required QoS) or blocked from the network (if required QoS can not be provided by the network)

40 Scheduling and Policing Mechanisms Packets from various sources are multiplexed together and queue for transmission at the output buffer of a link A Link Scheduling Policy determines how these packets are then selected for transmission The plays an important role in providing QoS guarantees First-In-First-Out: Packets arriving at link output queue are buffered if link is busy transmitting. If not sufficient buffering space, then invoke a Packet Discarding Policy. In FIFO policy, packet departure from buffer is based on time of arrival first packet to arrive is the first to leave Packet Discarding Policy: determines whether packets will be dropped (lost) when queue is full can be based on removing already buffered packets

41 Scheduling Priority and Round Robin Priority: packets arriving at output link are classified into one of two more priority classes priority value is based on information carried in packet header (such as the Traffic Class field in IPv6) A different queue is maintained for each priority class with the highest priority queue given preference when transmitting packets. Round Robin: assumes existence of queues, but a round robin scheduler alternated service between classes. A work-conserving scheduler based on the round-robin strategy will keep the link busy, always checking for low priority (class) packets when the high priority (class) queue is empty Weighted Fair Queuing (WFQ): similar to round robin, except that each class may receive a differentiated amount of service in any interval of time. Each class i is assigned a weight w i

42 Weighted Fair Queuing In WFQ, during any interval of time, if there are class i packets to send, class i will be guaranteed to receive a fraction of service equal to: w i /Σw i where the sum in the denominator is taken over all classes that also have packets queued for transmission Alternatively, we can say that with WFQ queues, a link with transmission rate R, class i will always achieve a throughput of at least R x w i / Σ w i WFQ plays an essential role in guaranteeing of QoS

43 Policing Policing is used to regulate the rate at which packets can be inserted into a network An important part of a QoS architecture Three important policing criteria: Average rate: limit the long-term average rate (packets per time period) at which a flow s packets can be sent into the network. Must determine time interval over which the average value is calculated. For instance, average rate of 100 packets/sec is more constrained than 6000 packets/minute Peak rate: constrains the maximum number of packets that can be sent over relatively short period of time (compared to average rate) Burst size: the number of packets that can be sent instantaneously into the network a limit case of the peak rate

44 Leaky Bucket A Leaky Bucket algorithm can be used to characterise these policing limits It consists of a bucket that can hold up to b tokens which determines the burst size New tokens added to bucket at r tokens/sec if bucket is full, a newly generated token is ignored Max number of packets that can enter the network within any time interval t, is rt+b Can use multiple Leaky buckets in series Leaky Bucket Game Demo

45 Leaky Bucket Analogy Leaky bucket Wireless networks Bucket size Water drops from the tap Water drops leaking from the bottom Water drops prevented from entering the bucket by the tap Water from an extra pipe Water drops overflowing The task: to maintain a high water level in the bucket, while restricting the overflow rate to 1% or below Bandwidth Capacity New Packet to Send Packets Sending Completed Blocked Packets (terminated before the service starts) Hand over packets (switching from a neighbouring wireless port) Dropped Packets (terminated in the middle of a packet flow, service very annoying*) The task: to maintain a high level of channel utilization in the router, while restricting the dropping probability to 1% or below

46 Lecture Review In this lecture we have: Looked at the primary principles behind QoS Looked at the existing best effort approach A brief look at scheduling and policing techniques Round Robin Leaky Bucket