Reliability & Flow Control

Transcription

1 Read 7.E Reliability & Flow Control Prof. ina Kat abi Some slides are from lectures by Nick Mckeown, Ion Stoica, Frans Kaashoek, Hari Balakrishnan, and Sam Madden 1

2 Previous Lecture How the link layer delivers data over a link How the network layer performs routing and forwarding Hierarchical Rout ing and Addressing 2

3 Hierarchical Routing Interior router Border router domain- 1 domain- 3 domain- 2 Int ernet: collect ion of dom ains/net works Inside a dom ain: Route over a graph of routers Between dom ains: Route over a graph of dom ains Address: concatenation of om ain Id, Node Id 3

4 Hierarchical Routing Advantage scalable Smaller tables Smaller messages elegat ion Each domain can run its own routing protocol isadvant age Mobilit y is difficult Address depends on geographic locat ion Sup-optim al paths E.g., in the figure, the shortest path between the two machines should traverse the yellow domain. But hierarchical routing goes directly between the green and blue domains, then finds the local destination path traverses m ore rout ers. 4

5 This Lecture Transport Layer Reliable data transm ission Flow Cont rol Mult iplexing < num ber>

6 Review of the Transport Layer Athena.MIT.ed u ina Application Layer Transport Layer Leland.Stanford.ed u Nick O.S. ata Header ata Heade r O.S. H Network Layer H H H H H Link Layer 6

7 Layering HTTP, FTP TCP Applicat ion Transport End-to-End Layer IP Net work Ethernet Link The 4-layer Internet model 7

8 Transport Layer Net work layer provides best-effort service Loss, delay, jit t er, duplicat es, reordering, Not convenient for applicat ions Transport layer builds on the best effort service to provide applications with a convenient environm ent Reliability: at least once at most once Perform ance: flow and congestion control Ordering ata integrity (checksum ) Tim eliness (rem ove jitter) Also transport provides multiplexing between 8 multiple applications

9 This Lecture Transport Layer Reliable data transm ission Flow Cont rol Mult iplexing 9

10 At Least Once Host A ata 1 Host B Host A ata 1 Host B an RTT ACK X ata 2 Timeout and Ret ransm issio n ata 1 Sender persistently sends until it receives an ack How long should the tim eout be? Fixed is bad. RTT changes depending on congestion Pick a values that s too big and it will wait too long to retransmit a packet, Pick a value too small, and it will unnecessarily retransmit packets. Adapt the estimate of RTT adaptive timeout 10

11 RTT Measurem ents (collect ed by Caida) 11

12 Adaptive Tim eout Samples S 1, S 2, S 3,.. Algorit hm Estim atedrtt = T 0 Estim atedrtt = α S + (1- α) Estim atedrtt where 0 α 1 What values should one pick for α and T 0? Adaptive tim eout is also hard 12

13 ifferent Approach: NACK Host A X Nack-2 2 Host B 1 3 Minimize reliance on timer Add sequence numbers to packet s Send a Nack when the receiver finds a hole in the sequence num bers ifficult ies Reordering Cannot elim inate acks, because we need to ack the last packet 13

14 At Most Once Suppress duplicat es Packets must have ids to allow the receiver to distinguish a duplicate from a new packet Receiver should keep track of which packet ids have been delivered t o applicat ions To simplify tracking, senders pick m onotonically increasing packet ids, i.e., sequence num bers Receiver delivers packets to application in order. It keeps track of the largest id delivered so far 14

15 This Lecture Transport Layer Reliable data transm ission Flow Cont rol Mult iplexing 15

16 Host A How fast should the sender sends? ata 1 ACK ata 2 Host B Waiting for acks is too slow Throughput is one packet/rtt Say packet is 500B RTT 100m s Throughput = 40Kb/s, Awful! Overlap pkt t ransm ission 16

17 Send a window of packets Host A Host B Send? OK, 3 pkts Idle Assume the receiver is the bottleneck Maybe because the receiver is a slow m achine Receiver needs to tell the sender when and how much it can send The window advances once all previous packets are acked too slow 17

18 Sliding Window Host A Host B Send? OK, 3 pkts Idle Senders advances the window whenever it receives an ack sliding window But what is the right value for the window? 18

19 The Right Window Size Note that if W/RTT < Bottleneck Capacity If W/RTT > Bottleneck Capacity under utilization Large queues 19

20 This Lecture Transport Layer Reliable data transm ission Flow Cont rol Mult iplexing 20

21 A1 App Multiplexing by Transport Multiple applications run on the sam e m achine but use different ports A2 App Application Layer B1 App B2 App P1 P2 Transport Layer TCP P1 P2 P1 P2 H H Net work Layer P2 P1 H H 21