Sliding Window Protocols, Connection Management, and TCP Reliability

Transcription

1 1 Sliding Window Protocols, Connection Management, and TCP Reliability

2 2 Outline Review Sliding window protocols Go-back-n Selective repeat Connection management for reliability TCP reliability Overview RTT measurement

3 3 Review: Services Provided by Transport Layer Multiplexing/demultiplexing Reliable data transfer Flow control Congestion control

4 rdt3.0 Sender 4

5 Another Look at rdt3.0 sender receiver data 0 waiting for 0 sending 0 data 0 ACK for 0 waiting for 1 sending 1 data 1 waiting for 0 5

6 6 A Summary of Questions How to improve the performance of rdt3.0? What if there are duplication and reordering? What about duplex reliable data transfer? How to determine the right timeout value?

7 7 Outline Review Sliding window protocols Go-back-n Selective repeat Connection management TCP reliability Overview RTT measurement

8 8 Sliding Window Protocols: Pipelining Pipelining: sender allows multiple, in-flight, yet-to-beacknowledged pkts range of sequence numbers must be increased buffering at sender and/or receiver Two generic forms of pipelined protocols: go-back-n, selective repeat

9 9 Go-Back-n Sender: k-bit seq # in pkt header window of up to w, consecutive unack ed pkts allowed w ACK(n): ACKs all pkts up to, including seq # n - cumulative ACK Note: ACK(n) can mean two things: I have received upto and include n, or I am waiting for n timer for each in-flight pkt or the packet at base only timeout(n): retransmit pkt n and all higher seq # pkts in window

10 GBN Sender: Extended FSM w { block sender if (new packets ACKed) { advance base; if (more packets waiting) send more packets } if (base == nextseqnum) stop_timer else start_timer for the packet at new base 10

11 11 GBN Receiver: Extended FSM Receiver simple: ACK-only: always send ACK for correctly-received pkt with highest in-order seq # need only remember expectedseqnum out-of-order pkt: discard (don t buffer) -> no receiver buffering! ACK pkt with highest in-order seq #

12 12 GBN in Action window size = 4

13 13 Selective Repeat Sender window Window size N: N consecutive unacked seq # s Receiver individually acknowledges all correctly received pkts ACK(n) means received packet with seq# n only Buffer size at receiver: window size Sender only resends pkts for which ACK not received sender timer for each unacked pkt

14 14 Selective Repeat: Sender, Receiver Windows w w

15 15 Selective Repeat sender data from above : unacked packets is less than window size w, send; otherwise block app. timeout(n): resend pkt n, restart timer ACK(n) in [sendbase,sendbase+w-1]: mark pkt n as received update sendbase to the first packet unacked receiver pkt n in [rcvbase, rcvbase+w-1] send ACK(n) if (out-of-order) mark and buffer pkt n else /*in-order*/ deliver any in-order packets otherwise: ignore

16 Selective Repeat: Ambiguity 16 Example: seq # s: 0, 1, 2, 3 window size=3 receiver sees no difference in two scenarios! incorrectly passes duplicate data as new in (a) Q: what relationship between seq # size and window size?

17 17 A Summary of Questions How to improve the performance of rdt3.0? sliding window protocols What about duplex reliable data transfer? What if there are duplication and reordering? How to determine the right timeout value?

18 18 Outline Review Sliding window protocols Go-back-n Selective repeat Connection management TCP reliability Overview RTT measurement

19 19 Connection Management: Setup The previous protocols assume we start with sequence number 0. Is there any problem with this approach? The objectives of connection setup agree on initial sequence numbers agree on other initial parameters

20 SYN: indicates connection setup 20 Three Way Handshake (TWH) [Tomlinson 1975] Host A Host B SYN(seq=x) ACK(seq =x), SYN(seq =y) ACK(seq=y) DATA(seq =x+1)

21 21 Scenarios with Duplicate Request Host A Host B no such request SYN(seq=x) ACK(seq =x), SYN(seq =y) REJECT(seq =y) accept? reject

22 22 Scenarios with Duplicate Request/Reply Host A Host B no such request SYN(seq=x) ACK(seq =x), SYN(seq =y) ACK(seq=z) accept? reject REJECT(seq=y)

23 23 Connection Management: Close Host A Host B DATA DATA FIN no data is delivered after FIN

24 The First Attempt: Agreeing on Connection Close client server init. close I am done. Are you done too? close I am done too. Goodbye! close Is this approach right? 24

25 The two blue armies need to agree on whether or not they will attack the white army. They achieve agreement by sending messengers to the other side. If they both agree, attack. Otherwise, no. Note that a messenger can be captured! Can they achieve agreement? 25 General Case: The Two-Army Problem

26 26 Four Way Teardown Host A Host B close FIN ACK FIN close timed wait closed ACK