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4 transport-sharing

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Fourth lesson of the Computer Networking class. Covers reliable transport principles and the introduction for sharing resources (MAC and congestion control)

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4 transport-sharing

  1. 1. Week 4 Reliable transport Sharing resources
  2. 2. Agenda • Reliable transport • Multiplexing • Connection establishment • Data transfer • Connection release • Sharing resources
  3. 3. Multiplexing applications • How can we multiplex data from several applications running on the same host ?
  4. 4. Multiplexing Request Client Server Source port : 1234 Destination port: 5678 Source port : 5678 Destination port: 1234 Response
  5. 5. Agenda • Reliable transport • Multiplexing • Connection establishment • Data transfer • Connection release • Sharing resources
  6. 6. Connection establishment • How to reliably open a connection ? Connect.req Connect.ind CR Connect.conf Connect.resp CA Connection established Connection established
  7. 7. Segment loss Connect.req() Connect.ind() Connect.conf() CA Connection established Connection established CR Retransmission CR timer expires Connect.resp()
  8. 8. Segments delayed Connect.ind() CR Connect.conf() CA CR First connection established Old previous CR How to detect duplicates ? Connect.req() CA D Connect.resp First connection established First connection stopped First connection stopped
  9. 9. Delayed segments • How to deal with delayed segments ? • Network level guarantee • No packet will survive more than MSL seconds inside the network • Transport entities use on a local clock to detect duplicated connection establishment requests
  10. 10. Three way handshake Host A Host B CR (seq=x) CA (seq=y, ack=x) CA (seq=x, ack=y) Sequence number x read from local transport clock Local state : Connection to B : - Wait for ack for CR (x) - Start retransmission timer Sequence number y read from local transport clock CA sent to ack CR Local state : Connection to A : - Wait for ack for CA(y) Received CA acknowledges CR Send CA to ack received CA Local state : Connection to B : - established - current_seq = x The sequence numbers used for the data segments will start from x The sequence numbers used for the data segments will start from y D(x) D(y) Local state : Connection to A : - established Connection established - current_seq=y Connection established
  11. 11. Three way handshake (2) Host A CR (seq=z) Host B CA (seq=y, ack=z) REJECT (ack=y) Connection cancelled No connection is established Sequence number y read from local transport clock Acknowledges CR segment Local state : Connection to A : - Wait for ack for CA(y) Local state : No connection to B Send REJECT to cancel connection establishment
  12. 12. Three way handshake (3) Host A Host B CR (seq=z) Current state does not contain a CR with seq=x CA (seq=y, ack=x) REJECT (ack=y) CR (seq=z) Retransmission timer expires CA (seq=w, ack=z) CA (seq=z, ack=w) Connection established Sequence number z read from local transport clock Local state : Connection to B : - Wait for ack for CR (z) - Start retransmission timer Current state does not contain a segment with seq=y REJECT ignored Sequence number w read from local transport clock CA sent to ack CR Local state : Connection to A : - Wait for ack for CA(w) Received CA acknowledges CR Send CA to ack received CA Local state : Connection to B : - established - current_seq = z
  13. 13. Three way handshake (4) Host A Host B Invalid CA received from A Send REJECT CR (seq=z) CA (seq=w, ack=z) REJECT (ack=w) CA (seq=z, ack=y) REJECT (ack=z) Sequence number w read from local transport clock Acknowledges CR segment Local state : Connection to A : - Wait for ack for CA(w) Current state does not contain a CR with seq=z No connection is established
  14. 14. Agenda • Reliable transport • Multiplexing • Connection establishment • Data transfer • Connection release • Sharing resources
  15. 15. Reliable data transfer • What are the differences with the reliable protocols of the datalink layer ? • Segments can be reordered • Buffers can change dynamically • Bytestream service
  16. 16. Retransmission policies • Which retransmission policy in reliable transport protocols ? • Alternating Bit • Go-back-n • Selective repeat
  17. 17. Buffer management • A transport entity serves a variable number of applications with a limited buffer • The buffer/window allocated to a given connection may need to change dynamically as connections start and stop
  18. 18. Buffer management A B Data.req(a) Data.ind(a) D(0,a) C(OK,0, w=1) C(OK,0,w=3) Data.req(c) D(2,c) 2 new buffers become available Data.req(b) Data.ind(b) D(1,b) 0 1 2 3 0 1 2 3 0 1 2 3 C(OK,1,w=3) Rwin=1 Swin=3, rwin=1 0 1 2 3 Swin=3, rwin=1 0 1 2 3 Swin=3, rwin=3 0 1 2 3 Data.req(d) D(3,d) 0 1 2 3
  19. 19. Buffer management A B Data.req(b) Rwin=1 Receiver cannot handle segment immediately Data.ind(a) Data.req(a) D(0,a) C(OK,0, w=0) 0 1 2 3 2 new buffers are available Swin=3, rwin=1 0 1 2 3 0 1 2 3 C(OK,0,w=3) Lost segment Swin=3, rwin=0 0 1 2 3 Window blocked No transmission possible Waits for control segment Waits for data segment How to recover from deadlock ? Persitence timer on receiver, resend control segment after timer expiration
  20. 20. Delayed segments A B D(1,b) Timer expiration Retransmission D(3,d) D(1,b) C(OK,0) C(OK,0) C(OK,3) D(0,e) Data.ind(e) C(OK,0) C(OK,1) Data.ind(b) !!!!!!!!!!!! D(0,a) Data.req(a) Data.ind(a) Data.ind(b) Data.ind(e)
  21. 21. Delayed segments • How to deal with them ? • Packets cannot live more than MSL seconds inside the network • Only one segment carrying sequence number x can be transmitted during MSL seconds • upper bound on maximum throughput
  22. 22. Bidirectional transfer • How to efficiently carry data in both directions ?
  23. 23. Piggybacking A B Data.req(a) Data.ind(a) Data.req(b) D(0,0,a) D(1,0,b) Error Discarded Data.req(c) D(2,0,c) D(5,0,w) acks D(0,0,a) Retransmission Segment -> buffer Data.ind(b) D(1,5,b) Data.ind(c) Data.req(d) D(3,6,d) C(OK,2) Data.ind(d) C(OK,3) Data.req(x) D(5,0,w) Data.req(w) Data.ind(w) D(6,0,x) Data.ind(x)
  24. 24. Bytestream • How to provide a bytestream service ?
  25. 25. Byte stream service (2) A B Data.req(abcdef) Data.req(ijkl) Data.req(mnop) Data.ind(ab) D(0,ab) C(OK,1) C(OK,1) D(2,cd) Lost segment D(4,ef) Placed in buffer Data.ind(cdef) D(2,cd) Expiration timer Retransmission D(6,ijklmnop) C(OK,5) Data.ind(ijklmnop) C(OK,13)
  26. 26. Agenda • Reliable transport • Multiplexing • Connection establishment • Data transfer • Connection release • Sharing resources
  27. 27. Connection release • Graceful release • Data transfer is finished and connection must be terminated • Abrupt release • Something went wrong and the connection must be closed immediately • Data can be lost !
  28. 28. Graceful release D(‘a’,1233) DISCONNECT.req (A-B) DISCONNECT.ind(A-B) ACK,1234 DISCONNECT.conf(A-B) ACK,4567 DISCONNECT.req(B-A) DISCONNECT.conf(A-B) DISCONNECT.ind(B-A) DR(B-A,4567) Outgoing connection (A->B) closed Incoming connection (A->B) closed Incoming connection (B->A) closed Outgoing connection (B->A) closed DR(A-B,1234) DATA.ind(‘a’)
  29. 29. Abrupt release CR (seq=z) CA (seq=w, ack=z) CA (seq=z, ack=w) Data.req() D Data.ind() Disc.req() Data.req() D DR Disc.req() Connection closed Connection closed This segment will not be delivered !
  30. 30. Agenda • Reliable transport • Sharing resources • Which resources need to be shared • Medium Access Control • Congestion Control
  31. 31. Network resources • What are the resources that are shared by multiple users inside a network ?
  32. 32. Sharing bandwidth • Several nodes on a single link
  33. 33. Agenda • Reliable transport • Sharing resources • Which resources need to be shared • Medium Access Control • Congestion Control
  34. 34. How to share access to a link ? • Deterministic solutions • Probabilistic solutions
  35. 35. Time Division Multiplexing
  36. 36. ALOHA
  37. 37. The collision problem A B collision
  38. 38. Medium Access Control : ALOHA N=1; while ( N<= max) do send frame; wait for ack on return channel or timeout: if ack on return channel exit while; else /* timeout */ /* retransmission is needed */ N=N+1; end do /* too many attempts */
  39. 39. CSMA • Key idea • Listen to the link before transmitting and only transmit when nobody else transmits
  40. 40. CSMA/CD • Key idea • Listen to link before transmitting • Detect collisions • If a collision occurs, stop transmitting • Caveat • Is it possible to detect all collisions ?
  41. 41. CSMA/CA • Key idea • In wireless networks, we need to avoid collisions by deferring transmissions • Possibility of “reserving” transmission slots
  42. 42. Agenda • Reliable transport • Sharing resources • Which resources need to be shared • Medium Access Control • Congestion Control
  43. 43. Adapting to different bandwidth
  44. 44. Self-clocking
  45. 45. The congestion problem
  46. 46. Fairness • What is the final objective of congestion control ? • On a single link • Fair share • In a large network • Max-min fairness
  47. 47. Max-min fairness • a max-min allocation of bandwidth is an allocation of bandwidth which maximises the allocation of bandwidth to the sources receiving the smallest allocation • a max-min fair allocation is such that in order to increase the bandwidth allocated to one source, it is necessary to decrease the bandwidth allocated to another source which already receives a lower allocation
  48. 48. Congestion control
  49. 49. Congestion control • Additive Increase / Multiplicative Decrease # Additive Increase Multiplicative Decrease if congestion : rate=rate*betaC # MD, betaC<1 else rate=rate+alphaN # AI
  50. 50. How to detect congestion ? • Host-based solutions • Router-based solutions

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