Distributed Systems 3. Network Quality of Service (QoS)

Transcription

1 Distributed Systems 3. Network Quality of Service (QoS) Paul Krzyzanowski 1

2 What factors matter for network performance? Bandwidth (bit rate) Average number of bits per second through the network Delay (latency) Average time for data to get from one endpoint to its destination Jitter Variation in end-to-end delay Loss (packet errors and dropped packets) Percentage of packets that don t reach their destination 2

3 Why do we care? We want to provide users with Good interactive performance Glitch-free, delay-free streaming video Good quality IP telephony 3

4 Quality of Service Goals Provide applications with the data flow performance they need: Bandwidth, delay, jitter, loss Examples VoIP: low bandwidth, low latency (<150 ms), low jitter (<30 ms) Video streaming: high bandwidth, longer latency OK Remote login: low bandwidth, relatively low latency, high reliability File transfer: high bandwidth, high latency OK, high jitter OK, high reliability 4

5 Why do we have QoS problems? We don t have unlimited resources! On-host resources Multiple processes may want to send packets Network Available bandwidth Bandwidth mismatch Network routing & switching Processing delays Buffer space in routers & switches Too many queued packets queue overflow & dropped packets congestion Address quality of service with resource allocation & prioritization 5

6 Contributors to congestion Bandwidth mismatch 10 Gbps 50 Mbps Aggregation 1 Gbps 1 Gbps Extra traffic resulting from Faulty hardware/firmware Excess retransmission from data corruption High-traffic apps or misbehaving hosts (e.g., malware generating floods) Congestion leads to packet loss 6

7 Contributors to latency Packetization on host Create IP & MAC packets Serialization Send data onto the network One packet at a time others are queued Propagation delay Processing delay on routers Input queue output queue and packet inspection Queuing delay Wait for interface to be available 7

8 Contributors to jitter Packets arrive at a router or switch at the same time Used to lead to collision on a shared link Buffers store packets Transmitted FIFO, not all at once! Dequeuing creates jitter FIFO output queue Dynamic re-routing Transport layer Caching out of order packets delayed delivery Out-of-order packets need to be buffered May need to be retransmitted Retransmission Transport-layer retransmission End-to-end (huge!) jitter 8

9 Contributors to packet loss Buffer overrun on routers/switches (queue full) Bad transmission lines (e.g., signal degradation & noise) Faulty hardware 9

10 QoS Aware Networks A QoS-aware network will allow an application to specify its needs and will reserve sufficient capacity in the network to meet those goals Admission Control Applications request a particular quality of service from the network If the request is granted, the application has the contract Traffic Control Classifies, queues, and schedules traffic within the network 10

11 Service Models for QoS No QoS (best effort) Default behavior for IP with no QoS No preferential treatment Host is not involved in specifying service quality needs Soft QoS (Differentiated Services) No explicit setup Identify one type of service (data flow) vs. another Certain classes get preferential treatment over others Hard QoS Network makes commitment to deliver the required quality of service Host makes a reservation Traffic flows are reserved 11

12 Approaches to Network Resource Allocation Router-based vs. Host-based Router-based Each router decides which packet to forward & when Host-based Each host observes network conditions and adjusts its behavior Reservation vs. feedback-based Reservation-based Host asks for specific capacity from the network Each router allocates buffer space for that flow to satisfy the request. If a router cannot grant, reservation is denied Feedback-based Hosts sends data without reserving resources Host gets congestion feedback from a router or detects packet loss & adjusts its transmission rate 12

13 Congestion Control Congestion: Caused by more incoming traffic than we can route Three approaches Admission control (input) Prevent congestion that has been detected from getting worse Do not allow packets for a specific flow, new flows, new virtual circuits Choke packets (feedback) Queue management (output) 13

14 Congestion Control: Choke Packets If a router determines congestion, send back a choke packet Packet contains destination address identifying the port that is congested This is a request from the router not to send packets to that destination (voluntary) IP handles congestion by dropping packets Explicit Congestion Notification (ECN) allows end-to-end notification of congestion (RFC 3168) 14

15 Classifier Scheduler Controlling Jitter output queues Incoming packet Transmitted packet Multiple queues per output port Queue based on priority Each packet is assigned to a queue Layer 2: 802.1p priority bits, MPLS EXP field Layer 3: IP ToS bits, protocol type, source/destination address Layer 4: port numbers Any combinations 15

16 Classifier Scheduler Controlling Jitter: scheduling packets output queues Incoming packet Transmitted packet Simplest was a single FIFO queue Round robin from multiple queues Each class of service gets a fair share not what we want Priority If there are always packets in a high priority queue, we get head of line blocking (simply packets holding up other packets) Weighted Fair Queuing (WFQ) Each queue gets a priority and a minimum % of link speed 16

17 Flow Detection Flow: stream of related packets between hosts In IP, set of packets from one address:port to another address:port with same protocol Soft state Keeping state for a flow makes resource allocation easier (e.g., route, average packet size, packet arrival rate) This is called a soft state. There is usually no signaling information that is used to create these states. Flow detection in routers: Network controls flow rate by dropping or delaying packets Define rules for traffic queuing, shaping, and policing drop TCP packets over UDP Discard specific UDP flows to ensure QoS for other flows 17

18 Queue Management With Flows Flow-based weighted fair queuing Per-flow queues Each queue is assigned a weight (volume) Flow 1 Class-based weighted fair queuing User-defined classes Protocols Input interfaces Specific source and/or destination IP addresses Flow 2 Flow 4 Flow 3 Each class is assigned Bandwidth Queue limit (# packets) Weight 18

19 Bandwidth Management Traffic Shaping Goal: regulate average rate of data transmission per flow Queue packets during surges and release later Example: high-bandwidth link to low-bandwidth link Traffic Policing Goal: Monitor network traffic and discard offenders Discard traffic that exceeds allotted bandwidth 19

20 Traffic Shaping: Leaky Bucket Visualization Bucket with a hole Filled up at a varying rate Water leaks at a constant rate Bucket = packet queue buffer If a packet comes in and bucket is full, discard packet Buffer overrun If there is nothing to transmit (bucket is empty) Buffer underrun Convert an uneven flow of packets into an even flow 20

21 Traffic Shaping: Token Bucket Bucket holds tokens that are generated at a certain rate To transmit a packet The bucket must hold and destroy a token(s) The token bucket allows a host to save up permission to send large bursts later Bucket size determines maximum burstiness 21

22 Traffic Shaping: Token Bucket Desired average rate: r bytes/second Add a token every 1/r seconds If # tokens > b (bucket is full), discard the token When packet arrives (size = n bytes): if # tokens is < n Traffic shaping: queue (delay) the packet until there are enough tokens Traffic policing: drop the packet else transmit the packet and remove n tokens (in implementation, the tokens are just one number, not a collection) 22

23 Token bucket vs. Leaky bucket Token bucket: may be bursty Leaky bucket: cannot be bursty 23

24 Quality of Service in IP IP was not designed with QoS controls in mind IP cannot (normally) take advantage of QoS controls offered by an underlying network (LAN) You can t count on a specific underlying network for end-to-end service Some QoS mechanisms created as add-ons 24

25 Quality of Service Problems in IP Bandwidth mismatches and aggregation Lead to congestion Inefficient packet transmission (not really a QoS issue) 59 bytes to send 1 byte in TCP/IP! 20 bytes TCP + 20 bytes IP + 18 bytes ethernet Unreliable delivery Software to the rescue TCP/IP But leads to jitter Unpredictable packet delivery No controls on bit rate, delay, jitter Packets may take different routes, resulting in different performance 25

26 Inefficient Packets Some software generates lots of tiny packets Can lead to head-of-line blocking Nagle s algorithm: Buffer new data if unacknowledged TCP/IP data outstanding Header/packet compression Link-to-link Header compression (RFC 3843) Payload compression (RFC 2393) Trade-off: $ delivery vs. $ compression Routers have to be aware 26

27 Differentiated Services (soft QoS) Treat some traffic as better than other Statistical - no guarantees Identify class of service & priority Router can use this data to make scheduling/dropping decisions Use on Internet (especially across ISPs) limited due to peering agreement complexities 27

28 Differentiated Services (DiffServ) ToS field in IP header (bits 9 16) Differentiated Services Control Point (DSCP): first six bits Differentiated Services (DS) field p p p x x x - - DSCP value identifies class of service Each router uses this value to determine how to prioritize the packet it affects per hop behavior of the router Priority: 0-7 Priority is used if these three bits are 0 RFC 2474, 2475, December

29 Hard QoS: Integrated Services IntServ: Integrated Services (RFC 1633) End-to-end reservation of services Traffic specification (TPSEC) Define token bucket: rate & size Request specification (RPSEC) Specify levels of assurance Best effort (no reservation) Controlled Load: soft QoS data rates may increase or packet loss may occur Guaranteed: hard QoS tight bounds on delay Underlying mechanism Resource Reservation Protocol (RSVP) 29

30 Reservation & Delivery Protocol RSVP: ReSerVation Protocol Each unidirectional data stream is a flow Hosts request specific quality of service per flow Routers reserve resources RFC 2205 Every device through which data flows must support RSVP 30

31 Other mechanisms Multi-Protocol Layer Switching (MPLS) Make packet routing more efficient more like layer 2 Each packet is assigned a routing label (based on destination & packet priority) Routers make queuing and forwarding decisions based on this label MPLS routers can create end-to-end circuits using any protocol QoS parameters Traffic Class ECN (Explicit Congestion Notification) 31

32 Other mechanims 802.1p: control QoS at the MAC layer (layer 2) Eight traffic classes defined in an ethernet frame This is not an IP QoS mechanism Priority Traffic 0 Background 1 Best effort 2 Excellent effort 3 Critical 4 Video <100 ms latency 5 Voice <10 ms latency 6 Internetwork control 7 Network control 32

33 Media Delivery Protocols Real-Time Control Protocol (RTCP) Provides feedback on QoS (jitter, loss, delay) RFC 3550 RTP: Real-Time Transport Protocol Not a routing protocol No service guarantees Provides: Payload identification sequence # time stamp RTP/RTCP do not provide QoS controls 33

34 Virtual Circuit Networks Avoid having to choose a route for every packet Route established when a connection is set up All traffic from source to destination flows on the same route Packets only need to contain a virtual circuit number, not destination address IP networking does not work this way TCP/IP s virtual circuit is modeled entirely in the TCP driver in the OS Routers are unaware of the presence of a virtual circuit 34

35 ATM: Asynchronous Transfer Mode Late 1980 s Goal: Merge voice & data networking low but constant bandwidth high but bursty bandwidth 35

36 ATM Traditional voice networking Circuit switching Too costly Poor use of resource Does not lend to multicasting ATM: wide area networking with QoS controls Based on fixed-size packets over virtual circuits Virtual circuit must be established before any traffic is sent Route is established and all participating switches commit to QoS Fixed-size cells provide for predictive scheduling Large cells will not hold up smaller ones Rapid cell switching 36

37 ATM ATM cell 53-byte cell: 48-byte data, 5-byte header Sender specifies traffic type on circuit setup: CBR Constant bit rate bandwidth Uncompressed video, voice VBR Variable bit rate Avg, peak bandwidth Compressed video, voice ABR Available bit rate Minimum bandwidth web access UBR Unspecified bit rate ftp 37

38 ATM Small cells lots of interrupts 622 Mbps link: 12 million interrupts per second ATM hardware supports an ATM Adaptation Layer (AAL) Converts cells to variable-sized (larger) packets: AAL 1: for CBR AAL 2: for VBR AAL 3/4: ABR data AAL 5: ABR data, simplified AAL 6: MPEG-2 video IP traffic: transmitted as data using AAL5 38

39 The End 39