Lecture 21. Reminders: Homework 6 due today, Programming Project 4 due on Thursday Questions? Current event: BGP router glitch on Nov.

Transcription

1 Lecture 21 Reminders: Homework 6 due today, Programming Project 4 due on Thursday Questions? Current event: BGP router glitch on Nov. 7 Tuesday, November 8 CS 475 Networks - Lecture 21 1

2 Outline Chapter 6 - Congestion Control and Resource Allocation 6.1 Issues in Resource Allocation 6.2 Queuing Disciplines 6.3 TCP Congestion Control 6.4 Congestion-Avoidance Mechanisms 6.5 Quality of Service 6.6 Summary Tuesday, November 8 CS 475 Networks - Lecture 21 2

3 Issues in Resource Allocation Congestion control refers to the efforts make by network nodes to prevent or respond to overload conditions. (Congestion control is different from flow control, but they share some of the same control mechanisms.) Resource allocation refers to the process by which network elements try to meet the competing demands that applications have for network resources primarily link bandwidth and buffer space in routers. Tuesday, November 8 CS 475 Networks - Lecture 21 3

4 Issues in Resource Allocation Resource allocation is partially implemented in the routers or switches inside the network and partially in the transport protocol. End systems use signaling protocols to convey resource requirements to network nodes. The nodes respond with information about resource availability. Tuesday, November 8 CS 475 Networks - Lecture 21 4

5 Network Model We will discuss resource allocation with respect to a network that is packet switched connectionless and best-effort. Tuesday, November 8 CS 475 Networks - Lecture 21 5

6 Network Model Packet Switched Network Links in a packet-switched network will typically have different bandwidths. Although a given source may have enough capacity on an outgoing link to send a packet. A slow link somewhere in the middle can cause a bottleneck. Tuesday, November 8 CS 475 Networks - Lecture 21 6

7 Network Model Connectionless Flows Connectionless networks do not use setup messages to reserve network resources prior to transmitting data. Although our network is connectionless and packet-switched we will see that the concept of a flow is important when discussing resource allocation. A flow is a sequence of packets traveling between a source and destination along the same path. Tuesday, November 8 CS 475 Networks - Lecture 21 7

8 Network Model Connectionless Flows Multiple flows passing through a network. Tuesday, November 8 CS 475 Networks - Lecture 21 8

9 Network Model Connectionless Flows Routers may maintain soft state about a flow so that it can perform more efficiently. (Routers in connected networks maintain hard state.) A flow can be detected implicitly by inspecting packet source and destination addresses or explicitly via flow setup messages. Tuesday, November 8 CS 475 Networks - Lecture 21 9

10 Network Model Service Model We will initially assume that our network is a best effort network. No guarantees are made on either throughput or delay. Some networks can guarantee certain levels of performance (throughput, delay, etc). Such a network is said to provide quality of service (QoS). This type of network is covered in Section 6.5, which we will be skipping. Tuesday, November 8 CS 475 Networks - Lecture 21 10

11 Taxonomy Router-Centric vs Host-Centric Resource allocation mechanisms can be categorized as being either router-centric or hostcentric. In a router-centric design, the router decides when packets are forwarded and dropped. Routers inform sending hosts of how much data they can send. In a host-centric design, hosts observe the network and adjust their behavior accordingly. Tuesday, November 8 CS 475 Networks - Lecture 21 11

12 Taxonomy Reservation vs. Feedback In a reservation-based resource allocation scheme a host asks the network for capacity at the time a flow is established. If a route can not satisfy the request the flow may be rejected. (A router-centric design is implied.) In a feedback-based approach hosts begin sending data and adjust sending rates based on feedback. Feedback may be explicit (in the form of messages sent back from routers) or implicit. Tuesday, November 8 CS 475 Networks - Lecture 21 12

13 Taxonomy Window-Based vs. Rate-Based In a window-based resource allocation method windows are used to indicate to the sender how much data should be transmitted. Both TCP flow and congestion control methods are windows based. Alternatively, a sender could be asked to transmit at a particular rate (bits per second). Rate-based requests are common in networks that support Quality of Service. Tuesday, November 8 CS 475 Networks - Lecture 21 13

14 Evaluation Criteria Effective Resource Allocation We want to allocate resources so that we maximize throughput while minimizing delay. Many allocation methods seek to optimize the power of the network: Power = Throughput/Delay For a given allocation scheme, power is optimized at a particular load value. Tuesday, November 8 CS 475 Networks - Lecture 21 14

15 Evaluation Criteria Fair Resource Allocation Unless there is a statement to the contrary, we will assume that fair resource allocation implies that all flows using a link should have equal throughput. Given a set of flow throughputs (x 1, x 2,..., x n ) one proposed fairness index is: n x i 2 f x 1, x 2,, x n = i=1 n n i=1 x i 2 Tuesday, November 8 CS 475 Networks - Lecture 21 15

16 Queuing Disciplines A router's queuing discipline governs how packets are buffered while waiting to be transmitted. The queuing algorithm affects throughput (by determining which packets get transmitted) and buffer space (by deciding which packets get dropped). We will examine the first-in-first-out (FIFO) and fair queuing (FQ) algorithms. Tuesday, November 8 CS 475 Networks - Lecture 21 16

17 FIFO FIFO is the simplest of all queuing algorithms. It is currently also the most widely used algorithm on Internet routers. (a) FIFO Queue, (b) tail drop Tuesday, November 8 CS 475 Networks - Lecture 21 17

18 FIFO FIFO is a scheduling discipline. Routers also use a drop policy to determine which packets are dropped when the queue is full. FIFO routers typically use a tail drop drop policy. A simple variation on FIFO queuing is to have multiple priority queues. Packets could contain a field that indicates the packet priority (the IP Type of Service or TOS field could be used for this purpose). Tuesday, November 8 CS 475 Networks - Lecture 21 18

19 Fair Queuing FIFO queues do not separate packets by flow. It is possible for an ill-behaved source to use up most of the available bandwidth. We will see that TCP uses host-centric congestion control. It is possible for an application to use UDP instead and bypass TCP's congestion control mechanism. Tuesday, November 8 CS 475 Networks - Lecture 21 19

20 Fair Queuing With fair queuing (FQ) a queue is maintained for each flow and the flows are serviced in roundrobin fashion. FQ would prevent a UDP flow from using more than its fair share of network bandwidth. Tuesday, November 8 CS 475 Networks - Lecture 21 20

21 Fair Queuing Fair queuing is not quite that simple due to different packet lengths. We want to ensure equal bit rates for all flows rather than equal packet rates. To ensure equal bit rates we compute the time when the router would finish transmitting the packet as: F i = max(f i 1, A i ) + P i where F i is the finish time, A i is the arrival time and P i is the transmit time. Tuesday, November 8 CS 475 Networks - Lecture 21 21

22 Fair Queuing The F i values are treated as time stamps and we transmit packets in order of increasing time stamp. An arriving packet is not permitted to interrupt the transmission of packet even if it has a smaller F i value. Tuesday, November 8 CS 475 Networks - Lecture 21 22

23 Fair Queuing FQ is work-conserving, the link is never idle as long as there is one packet in a queue. If one active flow is sharing a link with several idle flows, the active flow will use the full link capacity. If there are n active flows then each flow will get no more than 1/n th of the link capacity. If a source tries to send data at a faster rate, the queue will fill and packets will be dropped. Tuesday, November 8 CS 475 Networks - Lecture 21 23

24 Fair Queuing A variation is weighted fair queuing (WFQ) in which each flow is given a weight. With three flows with weights of 1, 2, and 3, the flows would get 1/6, 1/3, and 1/2 of the link capacity respectively. WFQ could be used on classes of traffic instead of on flows. The IP TOS field could be used to identify classes. The Differentiated Services architecture (Section 6.5) uses this approach. Tuesday, November 8 CS 475 Networks - Lecture 21 24

25 In-class Exercises To be turned in as part of Homework 7 Problem 6.7 on pages Problem 6.10(a) on page 565 Tuesday, November 8 CS 475 Networks - Lecture 21 25