1. CMPE 252A: Computer Networks
Lecture 17
CMPE252A Fall 2004

2. Announcements Homework 5 posted. Tutorial on ns-2: Monday, 11.29.
Due on Dec. 1st.
Tutorial on ns-2: Monday,
CMPE252A Fall 2004

3. Today’s plan
Reliable multicast transport.
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4. Reliable Multicast Transport
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5. Protocols
SRM.
RMTP.
PGM.
pgmcc.
RLM.
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6. Challenges Heterogeneity. Scalability. Implosion: feedback.
Exposure: repair.
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7. Mechanisms
Error recovery.
Congestion control.
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8. Error recovery
Sender-reliable.
Receiver-reliable.
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9. Sender-reliable Receiver ACKs.
Wait for ACKs from all receivers. Re-send on timeout or selective ACK.
: easy resource management.
: wait for ACK.
-: receiver state in sender not scalable.
-: ACK implosion.
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10. Receiver-reliable Receiver NACKs lost packet.
: no state at sender - good for multicast.
-: does not provide 100% reliability.
: NACK implosion.
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11. Implosion
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12. Implosion
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13. Implosion
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14. Implosion
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15. Implosion
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16. Implosion
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17. Implosion
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18. Implosion
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19. Implosion
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20. Implosion
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21. Implosion
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22. Implosion
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23. Implosion
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24. Retransmission Who retransmits? How to retransmit? Problem: exposure
Sender- versus receiver-based.
How to retransmit?
Unicast, multicast, scoped multicast, retransmission group, …
Problem: exposure
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25. Exposure
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26. Exposure
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27. Exposure
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28. Exposure
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29. Exposure
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30. Exposure
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31. Exposure
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32. Exposure
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33. Exposure
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34. Exposure
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35. Exposure
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36. Exposure
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37. Exposure
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38. Exposure
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39. Exposure
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40. Exposure
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41. Router-assisted recovery
“Ideal router”.
A single request is sent upstream.
A single repair is multicast from the nearest repairer to the subtree down the lossy link. R A R A A A A
router
receiver R A
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42. Scalable Reliable Multicast
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43. Scalable Reliable Multicast
SRM.
Originally designed for wb.
Receiver-reliable.
NACK-based.
Every member may multicast NACK or data.
Need much more elaborate explaination of why SRM is designed this way, e.g., get the previous microsoft guy’s slides of SRM, showing
(1) why NACK instead of ACK;
(2) why need NACK suppression and retransmission suppression;
*Maybe* a little bit discussion of why not use tree-like structure: because these replication applications are essentially PEER-TO-PEER, and all members are senders. We believe in this situation, random suppression with a “good” timer will perform better than dynamic/static tree structures.
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44. SRM Request Suppression
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45. SRM Request Suppression
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46. SRM Request Suppression
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47. SRM Request Suppression
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48. SRM Request Suppression
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49. SRM Request Suppression
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50. SRM Request Suppression
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51. SRM Request Suppression
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52. SRM Request Suppression
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53. SRM Request Suppression
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54. SRM Request Suppression
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55. SRM Request Suppression
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56. SRM Request Suppression
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57. Deterministic Suppression
time d d
data 2d d
session msg d d
nack
repair
= sender 3d
= repairer d 4d
= requestor
Delay = C1dS,R
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58. SRM Star Topology
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59. SRM Star Topology
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60. SRM Star Topology
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61. SRM Star Topology
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62. SRM Star Topology
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63. SRM Star Topology
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64. SRM Star Topology
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65. SRM Star Topology
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66. SRM Star Topology
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67. SRM Star Topology
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68. SRM Star Topology
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69. SRM Star Topology
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70. SRM Star Topology
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71. SRM: Stochastic Suppression
time 2d d
data 1 d
repair
session msg d 2
NACK d 3
= sender
Delay = U[0,C2] +C1dS,R
= repairer
= requestor
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72. SRM (summary) NACK/Retransmission suppression.
Delay before sending.
Delay based on RTT estimation.
Deterministic + Stochastic components.
Periodic session messages.
Full reliability (e.g., last packet lost).
Estimation of distance matrix among members.
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73. What’s missing?
Losses at link (A,C) causes retransmission to the whole group.
Solution: only retransmit to those members who lost the packet.
Request from nearest member. A B E F S C D
0.99
sender
receiver
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74. Local Recovery Application-level hierarchy. TTL scoped multicast.
Router supported.
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75. RMTP
Reliable Multicast Transport Protocol by Purdue and AT&T Research Labs.
Designed for file dissemination (single-sender).
Deployed in AT&T’s billing network.
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76. Tree-based protocol Solution to feedback implosion and local recovery.
Receivers are grouped into local regions under designated receivers (DR).
DR representative of local region.
Only DRs send feedback to source.
Receivers send feedback to DR (ACK processor).
DR does local recovery based on feedback from receivers.
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77. Tree-based hierarchy Receivers grouped into local regions.
Receiver unicasts periodic ACKs to its ACK Processor (AP), AP unicasts its own ACK to its parent. DR R1 R2 DR R3 DR R4 R5
Receiver dynamically chooses closest statically configured Designated Receiver (DR) as its AP. R R R R R R DR
DR/AP R
receiver R*
router
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78. Choosing DRs Pre-defined set of receivers that can act as DRs.
DRs periodically send special packet using multicast tree.
TTL decremented at each hop.
Receiver chooses DR whose packet has highest TTL = closest DR.
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79. Error control DR checks retransmission requests periodically.
Multicast or unicast retransmission.
Based on percentage of requests.
Immediate transmission request.
Used for late joining receivers.
Data cache: buffers entire file during session.
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80. Congestion Control Retransmission requests as congestion indicator.
Window-based.
Sender computes number of retransmission requests over pre-configured time interval.
If threshold exceeded, cwnd=1.
Otherwise, cwnd=cwnd+1 up to pre-configured maximum window size.
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81. RMTP: Comments : Heterogeneity : Position of DR critical
Lossy link or slow receiver will only affect a local region.
: Position of DR critical
Static hierarchy cannot adapt local recovery zone to loss points
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82. PGM Cisco’s reliable multicast protocol. Single source protocol.
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83. Protocol overview NACK-based with probabilistic suppression.
Repair only forwarded to “NACKers”.
Can make use of router support.
Feedback aggregation.
Selective repair forwarding.
Source path message (SPM): generated hop-by-hop by PGM routers.
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84. PGM: Request forwarding
NACK + random delay.
Forwarded upstream towards the source.
Only one NACK is forwarded for every packet loss.
SPM: identify next PGM router upstream. S R A R A A2 A A1 R
router A
receiver
NACK
RTX
SPM
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85. PGM Repair
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86. PGM Repair
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87. PGM Repair
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Data loss
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88. PGM Repair
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89. PGM Repair Src (x+1) Next sequence number sent; nodes will notice the
loss.
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90. PGM Repair
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(x+1)
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91. PGM Repair
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(x+1)
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92. PGM Repair Src (x?) noticing the loss, sending a NACK
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93. PGM Repair
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94. PGM Repair Src (x?) NACKS have now created state at necessary routers
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95. PGM Repair
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out goes the repair
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96. PGM Repair
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97. PGM Repair
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98. PGM Repair
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99. PGM Repair
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100. Problem: Repeated Retransmissions
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101. Repeated Retransmissions
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102. Repeated Retransmissions
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data loss
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103. Repeated Retransmissions
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104. Repeated Retransmissions
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Next sequence number
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105. Repeated Retransmissions
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106. Repeated Retransmissions
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107. Repeated Retransmissions
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108. Repeated Retransmissions
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109. Repeated Retransmissions
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110. Repeated Retransmissions
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repair
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111. Repeated Retransmissions
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112. Repeated Retransmissions
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113. Repeated Retransmissions
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114. Repeated Retransmissions
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115. Repeated Retransmissions
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116. Repeated Retransmissions
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117. Repeated Retransmissions
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118. Repeated Retransmissions
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2nd. repair
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119. Repeated Retransmissions
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120. Repeated Retransmissions
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121. Repeated Retransmissions
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122. Repeated Retransmissions
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123. PGM Summary Pros: Cons: Avoids sending repairs everywhere.
Routers need to look at multicast transport state (sequence numbers).
Can result in duplicate repairs (depending on timing).
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124. pgmcc Single-rate multicast congestion control mechanism.
Single rate as opposed to hierarchical rate decomposition.
Single-source multicast.
Developed for PGM.
But does not require router support.
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125. pgmcc features End-to-end. Use ACKs and NACKs. Window-based.
Sender-based RTT measurements.
Representatives (ackers).
Receiver with worst throughput.
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126. Protocol overview Sender monitors receiver reports (NACKs).
Select acker.
Window-based congestion control (a la TCP) between sender and acker.
Acker sends ACKs for every data packet.
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127. Protocol components (1)
Receiver reports:
Sent as part of NACKs.
Used to determine lowest throughput receiver, or acker.
Reports receiver’s measured loss rate.
Used to measure RTT.
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128. Protocol components (2)
Acknowledgments:
Acker generates an ACK for each data packet received.
Contain loss report.
Sequence number of ack’ed packet.
Bitmap with status of most recent 32 packets.
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129. Protocol components (3)
Window-based controller
Window-based congestion control between sender and acker.
Similar to TCP: AIMD.
When sessions starts, slow-start up to small ssthresh.
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130. Acker election and tracking
Acker is receiver with lowest throughput.
Uses TCP relation between throughput and loss, i.e.,
Bias in favor of current acker.
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131. RLM Receiver-driven Layered Multicast. Use multiple multicast groups.
Receiver initiated rate adaptation.
Cope with heterogeneity.
Use multiple multicast groups.
Assemble content by combining data from multiple groups.
Receivers add/remove layers
Applicable to video.
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132. RLM Layered video encoding. Each layer uses its own multicast group.
Basic layer.
Additional layer for enhanced video.
Each layer uses its own multicast group.
On spare capacity, receivers add layer.
On congestion, receivers drop layer.
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133. Layered media streams R1 R1 10Mbps 10Mbps 512Kbps S R R 10Mbps 128Kbps
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134. When to add layer? Join experiments used to probe for bandwidth.
Receivers try to add layers at specific times.
Spontaneous subscription to next layer.
If join-experiment causes congestion, receiver drops layer.
Otherwise, receiver succeeds.
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135. RLM Adaptation Learning strategy. Shared learning.
Receivers learn over time level of subscription that causes congestion.
Join-timer performing exponential backoff.
Shared learning.
For large groups, receivers learn from one another.
Receiver multicasts join message; if experiment fails, other receivers know.
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136. Join experiments Layer 3 timer exponentially increased Congestion4 3 B 2 A 1
time
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