6.033 Computer System Engineering

Transcription

1 MIT OpenCourseWare Computer System Engineering Spring 2009 For information about citing these materials or our Terms of Use, visit:

2 L2: end to end layer Dina Katabi Some slides are from lectures by Nick Mckeown, Ion Stoica, Frans Kaashoek, Hari Balakrishnan, Sam Madden, and Robert Morris

3 client stub End-to-end layer presentation Layer server stub End-to-end layer RPC Data Header Data Header RPC D H Network Layer D H D H D H D H D H Link Layer

4 Network layer provides best effort service Packets may be: Lossed Delayed (jitter) Duplicated Reordered Problem: Inconvenient service for applications Solution: Design protocols for E2E modules Many protocols/modules possible, depending on requirements

5 This lecture: some E2E properties At most once At least once Exactly once? Sliding window Case study: TCP Tomorrow: Network File System (NFS)

6 At Least Once client Data server client Data server an RTT X Timeout and Retransmission Data Sender persistently sends until it receives an ack Challenges: Duplicate s What value for timer

7 Duplicate problem Client Req Req Server timeout Req Req Req Req 2 Req Req 3 Problem: Request 2 is not delivered violates at-least once delivery

8 Solution: nonce timeout Client Server Req N Req N N N N Req N2 Req N2 Req N Req N N2 N N Req N2 Req N2 Label request and ack with unique identifier that is never re-used

9 Engineering a nonce Use sequence numbers Challenges: Wrap around? Failures? Client Server Req Req timeout Req 2 Req 2 Req Req 2 Req 2 Req 2

10 Timer value Fixed is bad. RTT changes depending on congestion Pick a value that s too big, wait too long to retransmit a packet Pick a value too small, generates a duplicate (retransmitted packet). Adapt the estimate of RTT adaptive timeout

11 Adaptive Timeout: Exponential weighted moving averages Samples S, S 2, S 3,.. Algorithm EstimatedRTT = T 0 EstimatedRTT = α S + (- α) EstimatedRTT where 0 α What values should one pick for α and T 0? Adaptive timeout is also hard

12 At Most Once Challenges client Req Req server 2 Ok Ok req req Process request Process request Server shouldn t process req Server should send result preferably

13 Idea: remember sequence number client server Req Req Process request Ok Ok Ok Ok req req Ok Ok 2 Resend Server remembers also last few responses

14 Problem: failures client 2 Req Req Ok Ok Ok Ok req req server 0 0 Ok Ok Ok Ok Performed request twice! How to maintain the last nonce per sender (tombstone)? Write to non-volatile storage? Move the problem? (e.g., different port number) Make probability of mistake small? How about exactly once? (Need transactions)

15 How fast should the sender sends? Host A Data Data 2 Host B Waiting for acks is too slow Throughput is one packet/rtt Say packet is 500 bytes RTT 00ms Throughput = 40Kb/s, Awful! Overlap pkt transmission

16 Send a window of packets Host A Send? OK, 3 pkts Host B Idle Assume the receiver is the bottleneck Maybe because the receiver is a slow machine 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 Sliding Window Host A Send? OK, 3 pkts Host B Idle Senders advances the window whenever it receives an ack sliding window But what is the right value for the window?

18 The Right Window Size Assume server is bottleneck Goal: make idle time on server zero Assume: server rate is B bytes/s Window size = B x RTT Danger: sequence number wrap around What if network is bottleneck? Many senders? Sharing? Next lecture

19 Negative Host A D D2 X D3 D4 Nack-2 D2 Host B D D3 Minimize reliance on timer Add sequence numbers to packets Send a Nack when the receiver finds a hole in the sequence numbers Difficulties Reordering Cannot eliminate acks, because we need to ack the last packet

20 E2E layer in Internet HTTP, RTP, Sun RPC, TCP or UDP Application Transport End-to-End Layer IP Network Ethernet, WiFI,... Link The 4-layer Internet model

21 UDP

22 Transmission Control Protocol (TCP) Connection-oriented Delivers bytes atmost-once Bidirectional s are piggybacked x,? x+, y+ Host A Syn x ack x+, syn y Data x+, ack y+ Host B y, x+

23 TCP header

24 Closing a TCP connection Host A Host B x,y fin x y, x timed wait ack x+ fin y ack y+ timeout closed closed

25 Connect/SYN (step of the 3 way handshake) (Start) Closed Close Listen Close SYN/SYN+ (step 2 of the 3 way handshake) Listen SYN Received RST SEND/SYN SYN/SYN+ (simultaneous open) SYN Sent Close/FIN Close/FIN Data exchange occur Established SYN+/ (step 3 of the 3 way handshake) FIN/ Active close FIN/ FIN WAIT FIN+/ Closing Passive close Close WAIT Close/FIN FIN WAIT 2 FIN/ TIMED WAIT Timeout Last (Go back to start) Closed Unusual event Client/reciever path Sender/server path Figure by MIT OpenCourseWare.