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  1. 1. 13PIT101 Multimedia Communication & Networks UNIT - I Dr.A.Kathirvel Professor & Head/IT - VCEW
  2. 2. Unit - I Open Data Network Model – Narrow Waist Model of the Internet - Success and Limitations of the Internet – Suggested Improvements for IP and TCP – Significance of UDP in modern Communication – Network level Solutions – End to End Solutions – Best Effort service model – Scheduling and Dropping policies for Best Effort Service model
  3. 3. Open Data Network Models
  4. 4. LAYERED TASKS We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office. Topics discussed in this section: Sender, Receiver, and Carrier Hierarchy
  5. 5. Figure 1 Tasks involved in sending a letter
  6. 6. THE OSI MODEL Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s. Topics discussed in this section: Layered Architecture Peer-to-Peer Processes Encapsulation
  7. 7. Note ISO is the organization. OSI is the model.
  8. 8. Figure 2 Seven layers of the OSI model
  9. 9. Figure 3 The interaction between layers in the OSI model
  10. 10. Figure 4 An exchange using the OSI model
  11. 11. LAYERS IN THE OSI MODEL In this section we briefly describe the functions of each layer in the OSI model. Topics discussed in this section: Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer
  12. 12. Figure 5 Physical layer
  13. 13. Note The physical layer is responsible for movements of individual bits from one hop (node) to the next.
  14. 14. Figure 6 Data link layer
  15. 15. Note The data link layer is responsible for moving frames from one hop (node) to the next.
  16. 16. Figure 7 Hop-to-hop delivery
  17. 17. Figure 8 Network layer
  18. 18. Note The network layer is responsible for the delivery of individual packets from the source host to the destination host.
  19. 19. Figure 9 Source-to-destination delivery
  20. 20. Figure 10 Transport layer
  21. 21. Note The transport layer is responsible for the delivery of a message from one process to another.
  22. 22. Figure 11 Reliable process-to-process delivery of a message
  23. 23. Figure 12 Session layer
  24. 24. Note The session layer is responsible for dialog control and synchronization.
  25. 25. Figure 13 Presentation layer
  26. 26. Note The presentation layer is responsible for translation, compression, and encryption.
  27. 27. Figure 14 Application layer
  28. 28. Note The application layer is responsible for providing services to the user.
  29. 29. Figure 15 Summary of layers
  30. 30. TCP/IP PROTOCOL SUITE The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application. Topics discussed in this section: Physical and Data Link Layers Network Layer Transport Layer Application Layer
  31. 31. Figure 16 TCP/IP and OSI model
  32. 32. ADDRESSING Four levels of addresses are used in an internet employing the TCP/IP protocols: physical, logical, port, and specific. Topics discussed in this section: Physical Addresses Logical Addresses Port Addresses Specific Addresses
  33. 33. Figure 17 Addresses in TCP/IP
  34. 34. Figure 18 Relationship of layers and addresses in TCP/IP
  35. 35. Example 1 In Figure 19 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.
  36. 36. Figure 19 Physical addresses
  37. 37. Example 2 Most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below: 07:01:02:01:2C:4B A 6-byte (12 hexadecimal digits) physical address.
  38. 38. Example 3 Figure 20 shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection.
  39. 39. Figure 20 IP addresses
  40. 40. Example 4 Figure 21 shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination.
  41. 41. Figure 21 Port addresses
  42. 42. Note The physical addresses will change from hop to hop, but the logical addresses usually remain the same.
  43. 43. Example 5 A port address is a 16-bit address represented by one decimal number as shown. 753 A 16-bit port address represented as one single number.
  44. 44. Narrow waist Model of the Internet
  45. 45. Fundamental Goal • “technique for multiplexed utilization of existing interconnected networks” • Multiplexing (sharing) – Shared use of a single communications channel • Existing networks (interconnection)
  46. 46. Fundamental Goal: Sharing Packet Switching • No connection setup • Forwarding based on destination address in packet • Efficient sharing of resources Tradeoff: Resource management potentially more difficult.
  47. 47. Type of Packet Switching: Datagrams • Information for forwarding traffic is contained in destination address of packet • No state established ahead of time (helps fate sharing) • Basic building block • Minimal assumption about network service Alternatives • Circuit Switching: Signaling protocol sets up entire path out-of-band. (cf. the phone network) • Virtual Circuits: Hybrid approach. Packets carry “tags” to indicate path, forwarding over IP • Source routing: Complete route is contained in each data packet
  48. 48. An Age-Old Debate Circuit Switching • Resource control, accounting, ability to “pin” paths, etc. Packet Switching • Sharing of resources, soft state (good resilience properties), etc. It is held that packet switching was one of the Internet’s greatest design choices. Of course, there are constant attempts to shoehorn the best aspects of circuits into packet switching. Examples: Capabilities, MPLS, ATM, IntServ QoS, etc.
  49. 49. Stopping Unwanted Traffic is Hard February 2000 March 2006
  50. 50. Research: Stopping Unwanted Traffic • Datagram networks: easy for anyone to send traffic to anyone else…even if they don’t want it! cnn.com Possible Defenses • Monitoring + Filtering: Detect DoS attack and install filters to drop traffic. • Capabilities: Only accept traffic that carries a “capability”
  51. 51. The Design Goals of Internet, v1 • Interconnection/Multiplexing (packet switching) • Resilience/Survivability (fate sharing) • Heterogeneity – Different types of services Decreasing – Different types of networks Priority • Distributed management • Cost effectiveness “This set of goals might seem to be nothing more than a checklist of all the desirable network features. • Ease of attachment It is important to understand that these goals are in • Accountability order of importance, and an entirely different network architecture would result if the order were changed.” These goals were prioritized for a military network. Should priorities change as the network evolves?
  52. 52. Fundamental Goal: Interconnection • Need to interconnect many existing networks • Hide underlying technology from applications • Decisions: – Network provides minimal functionality – “Narrow waist” email WWW phone... SMTP HTTP RTP... Applications TCP UDP… IP ethernet PPP… CSMA async sonet... copper fiber radio... Technology Tradeoff: No assumptions, no guarantees.
  53. 53. The Internet Protocol Suite FTP HTTP DNS TCP TFTP UDP TCP UDP IP Applications Waist Data Link Ethernet SONET 802.11 Physical The Hourglass Model The waist facilitates interoperability 53
  54. 54. The “Curse of the Narrow Waist” • IP over anything, anything over IP – Has allowed for much innovation both above and below the IP layer of the stack – An IP stack gets a device on the Internet • Drawback: very difficult to make changes to IP – But…people are trying – NSF GENI project: http://www.geni.net/
  55. 55. Interconnection: “Gateways” • Interconnect heterogeneous networks • No state about ongoing connections – Stateless packet switches • Generally, router == gateway • But, we can think of your home router/NAT as also performing the function of a gateway Home Network Internet
  56. 56. Network Address Translation • For outbound traffic, the gateway: – Creates a table entry for computer's local IP address and port number – Replaces the sending computer's non-routable IP address with the gateway IP address. – replaces the sending computer's source port • For inbound traffic, the gateway: – checks the destination port on the packet – rewrites the destination address and destination port those in the table and forwards traffic to local machine
  57. 57. NAT Traversal • Problem: Machines behind NAT not globally addressable or routable. Can’t initiate inbound connections. • One solution: Simple Traversal of UDP Through NATs – STUN client contacts STUN server – STUN server tells client which IP/Port the NAT mapped it to – STUN client uses that IP/Port for call establishment/incoming messages Home Network 1 Relay node More next time. Home Network 2
  58. 58. Goal #2: Survivability • Network should continue to work, even if some devices fail, are compromised, etc. • Failures on the Abilene (Internet 2) backbone network over the course of 6 months Thanks to Yiyi Huang How well does the current Internet support survivability?
  59. 59. Goal #2: Survivability Two Options • Replication – Keep state at multiple places in the network, recover when nodes crash • Fate-sharing – Acceptable to lose state information for some entity if the entity itself is lost Reasons for Fate Sharing • Can support arbitrarily complex failure scenarios • Engineering is easier Some reversals of this trend: NAT, Routing Control Platform
  60. 60. Goal #3: Heterogeneous Services • TCP/IP designed as a monolithic transport – TCP for flow control, reliable delivery – IP for forwarding • Became clear that not every type of application would need reliable, in-order delivery – Example: Voice and video over networks – Example: DNS – Why don’t these applications require reliable, in-order delivery? – Narrow waist: allowed proliferation of transport protocols
  61. 61. Topic: Voice and Video over Networks • Deadlines: Timeliness more important than 100% reliability. • Propagation of errors: Some losses more devastating than others Loss i A chor Fra e (I-Frame) Propagates to Depe de t Fra es (P and B-Frames)
  62. 62. Goal #3b: Heterogeneous Networks • Build minimal functionality into the network – No need to re-engineer for each type of network • “Best effort” service model. – Lost packets – Out-of-order packets – No quality guarantees – No information about failures, performance, etc. Tradeoff: Network management more difficult
  63. 63. Research: Network Anomaly Detection • Operators want to detect when a traffic flow from ingress to egress generates a “spike”. • Problem: Today’s protocols don’t readily expose this information. • Management/debuggability not initially a high priority!
  64. 64. Goal #4: Distributed Management Many examples: • Addressing (ARIN, RIPE, APNIC, etc.) – Though this was recently threatened. • Naming (DNS) • Routing (BGP) No single entity in charge. Allows for organic growth, scalable management. Tradeoff: No one party has visibility/control.
  65. 65. No Owner, No Responsible Party “Some of the most significant problems with the Internet today relate to lack of sufficient tools for distributed management, especially in the area of routing.” • Hard to figure out who/what’s causing a problem • Worse yet, local actions have global effects…
  66. 66. Local Actions, Global Consequences “…a glitch at a small ISP… triggered a major outage in Internet access across the country. The problem started when MAI Network Services...passed bad router information from one of its customers onto Sprint.” -- news.com, April 25, 1997
  67. 67. Goal #5: Cost Effectiveness • Packet headers introduce high overhead • End-to-end retransmission of lost packets – Potentially wasteful of bandwidth by placing burden on the edges of the network Arguably a good tradeoff. Current trends are to exploit redundancy even more.
  68. 68. Goal #6: Ease of Attachment • IP is “plug and play” Anything with a working IP stack can connect to the Internet (hourglass model) • A huge success! – Lesson: Lower the barrier to innovation/entry and people will get creative (e.g., Cerf and Kahn probably did not think about IP stacks on phones, sensors, etc.) • But…. Tradeoff: Burden on end systems/programmers.
  69. 69. Goal #7: Accountability • Note: Accountability mentioned in early papers on TCP/IP, but not prioritized • Datagram networks make accounting tricky. – The phone network has had an easier time figuring out billing – Payments/billing on the Internet is much less precise Tradeoff: Broken payment models and incentives.
  70. 70. Success and Limitations of the Internet • Success of Internet – e-com, Internet Marketing etc.. • The quality of information resources might not always be reliable and accurate. • Searching of information can be very tedious. • Internet is definetly not 100% secure. • Performance and speed are the main limitations to today's Internet
  71. 71. Transport Protocols • Provide logical communication between application application processes running on transport network different hosts data link physical • Run on end hosts – Sender: breaks application messages into segments, and passes to network layer – Receiver: reassembles segments into messages, passes to application layer • Multiple transport protocol available to applications – Internet: TCP and UDP network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical 71
  72. 72. Internet Transport Protocols • Datagram messaging service (UDP) – No-frills extension of “best-effort” IP • Reliable, in-order delivery (TCP) – Connection set-up – Discarding of corrupted packets – Retransmission of lost packets – Flow control – Congestion control (next lecture) • Other services not available – Delay guarantees – Bandwidth guarantees 72
  73. 73. Multiplexing and Demultiplexing • Host receives IP datagrams – Each datagram has source and destination IP address, – Each datagram carries one transport-layer segment – Each segment has source and destination port number • Host uses IP addresses and port numbers to direct the segment to appropriate socket 32 bits source port # dest port # other header fields application data (message) TCP/UDP segment format 73
  74. 74. Unreliable Message Delivery Service • Lightweight communication between processes – Avoid overhead and delays of ordered, reliable delivery – Send messages to and receive them from a socket • User Datagram Protocol (UDP) – IP plus port numbers to support (de)multiplexing – Optional error checking on the packet contents SRC port DST port checksum length DATA 74
  75. 75. Why Would Anyone Use UDP? • Finer control over what data is sent and when – As soon as an application process writes into the socket – … UDP will package the data and send the packet • No delay for connection establishment – UDP just blasts away without any formal preliminaries – … which avoids introducing any unnecessary delays • No connection state – No allocation of buffers, parameters, sequence #s, etc. – … making it easier to handle many active clients at once • Small packet header overhead – UDP header is only eight-bytes long 75
  76. 76. Popular Applications That Use UDP • Multimedia streaming – Retransmitting lost/corrupted packets is not worthwhile – By the time the packet is retransmitted, it’s too late – E.g., telephone calls, video conferencing, gaming • Simple query protocols like Domain Name System – Overhead of connection establishment is overkill – Easier to have application retransmit if needed “Address for www.cnn.com?” “” 76
  77. 77. Transmission Control Protocol (TCP) • Connection oriented – Explicit set-up and tear-down of TCP session • Stream-of-bytes service – Sends and receives a stream of bytes, not messages • Reliable, in-order delivery – Checksums to detect corrupted data – Acknowledgments & retransmissions for reliable delivery – Sequence numbers to detect losses and reorder data • Flow control – Prevent overflow of the receiver’s buffer space • Congestion control – Adapt to network congestion for the greater good 77
  78. 78. An Analogy: Talking on a Cell Phone • Alice and Bob on their cell phones – Both Alice and Bob are talking • What if Alice couldn’t understand Bob? – Bob asks Alice to repeat what she said • What if Bob hasn’t heard Alice for a while? – Is Alice just being quiet? – Or, have Bob and Alice lost reception? – How long should Bob just keep on talking? – Maybe Alice should periodically say “uh huh” – … or Bob should ask “Can you hear me now?”  78
  79. 79. Some Take-Aways from the Example • Acknowledgments from receiver – Positive: “okay” or “ACK” – Negative: “please repeat that” or “NACK” • Timeout by the sender (“stop and wait”) – Don’t wait indefinitely without receiving some response – … whether a positive or a negative acknowledgment • Retransmission by the sender – After receiving a “NACK” from the receiver – After receiving no feedback from the receiver 79
  80. 80. Challenges of Reliable Data Transfer • Over a perfectly reliable channel – All of the data arrives in order, just as it was sent – Simple: sender sends data, and receiver receives data • Over a channel with bit errors – All of the data arrives in order, but some bits corrupted – Receiver detects errors and says “please repeat that” – Sender retransmits the data that were corrupted • Over a lossy channel with bit errors – Some data are missing, and some bits are corrupted – Receiver detects errors but cannot always detect loss – Sender must wait for acknowledgment (“ACK” or “OK”) – … and retransmit data after some time if no ACK arrives 80
  81. 81. TCP Support for Reliable Delivery • • • 81 Checksum – Used to detect corrupted data at the receiver – …leading the receiver to drop the packet Sequence numbers – Used to detect missing data – ... and for putting the data back in order Retransmission – Sender retransmits lost or corrupted data – Timeout based on estimates of round-trip time – Fast retransmit algorithm for rapid retransmission
  82. 82. TCP Segments 82
  83. 83. TCP “Stream of Bytes” Service Host A Host B 83
  84. 84. …Emulated Using TCP “Segments” Host A Segment sent when: TCP Data 1. 2. 3. TCP Data Host B 84 Segment full (Max Segment Size), Not full, but times out, or “Pushed” by application.
  85. 85. TCP Segment IP Data TCP Data (segment) TCP Hdr IP Hdr • IP packet – No bigger than Maximum Transmission Unit (MTU) – E.g., up to 1500 bytes on an Ethernet • TCP packet – IP packet with a TCP header and data inside – TCP header is typically 20 bytes long • TCP segment – No more than Maximum Segment Size (MSS) bytes – E.g., up to 1460 consecutive bytes from the stream 85
  86. 86. Sequence Numbers Host A ISN (initial sequence number) Sequence number = 1st byte TCP Data TCP HDR TCP Data Host B 86 ACK sequence number = next expected byte TCP HDR
  87. 87. Initial Sequence Number (ISN) • Sequence number for the very first byte – E.g., Why not a de facto ISN of 0? • Practical issue – IP addresses and port #s uniquely identify a connection – Eventually, though, these port #s do get used again – … and there is a chance an old packet is still in flight – … and might be associated with the new connection • So, TCP requires changing the ISN over time – Set from a 32-bit clock that ticks every 4 microseconds – … which only wraps around once every 4.55 hours! • But, this means the hosts need to exchange ISNs 87
  88. 88. TCP Three-Way Handshake 88
  89. 89. Establishing a TCP Connection A B Each host tells its ISN to the other host. • Three-way handshake to establish connection – Host A sends a SYN (open) to the host B – Host B returns a SYN acknowledgment (SYN ACK) – Host A sends an ACK to acknowledge the SYN ACK 89
  90. 90. TCP Header Source port Destination port Sequence number Flags: SYN FIN RST PSH URG ACK Acknowledgment HdrLen 0 Flags Advertised window Checksum Urgent pointer Options (variable) Data 90
  91. 91. Step 1: A’s Initial SYN Packet A’s port B’s port A’s Initial Sequence Number Flags: SYN FIN RST PSH UR G ACK Acknowledgment 20 0 Flags Advertised window Checksum Urgent pointer Options (variable) A tells B it wants to open a connection… 91
  92. 92. Step 2: B’s SYN-ACK Packet B’s port A’s port B’s Initial Sequence Number Flags: SYN FIN RST PSH URG ACK A’s ISN plus 1 20 0 Checksum Flags Advertised window Urgent pointer Options (variable) B tells A it accepts, and is ready to hear the next byte… … upon receiving this packet, A can start sending data 92
  93. 93. Step 3: A’s ACK of the SYN-ACK A’s port B’s port Sequence number Flags: SYN FIN RST PSH URG ACK B’s ISN plus 1 20 0 Flags Advertised window Checksum Urgent pointer Options (variable) A tells B it wants is okay to start sending … upon receiving this packet, B can start sending data 93
  94. 94. What if the SYN Packet Gets Lost? • Suppose the SYN packet gets lost – Packet is lost inside the network, or – Server rejects the packet (e.g., listen queue is full) • Eventually, no SYN-ACK arrives – Sender sets a timer and wait for the SYN-ACK – … and retransmits the SYN-ACK if needed • How should the TCP sender set the timer? – Sender has no idea how far away the receiver is – Hard to guess a reasonable length of time to wait – Some TCPs use a default of 3 or 6 seconds 94
  95. 95. SYN Loss and Web Downloads • User clicks on a hypertext link – Browser creates a socket and does a “connect” – The “connect” triggers the OS to transmit a SYN • If the SYN is lost… – The 3-6 seconds of delay may be very long – The user may get impatient – … and click the hyperlink again, or click “reload” • User triggers an “abort” of the “connect” – Browser creates a new socket and does a “connect” – Essentially, forces a faster send of a new SYN packet! – Sometimes very effective, and the page comes fast 95
  96. 96. TCP Retransmissions 96
  97. 97. Automatic Repeat reQuest (ARQ) • Automatic Repeat Request – Receiver sends acknowledgment (ACK) when it receives packet – Sender waits for ACK and timeouts if it does not arrive within some time period Sender Timeout • Simplest ARQ protocol – Stop and wait – Send a packet, stop and wait until ACK arrives Receiver Time 97
  98. 98. Packet lost 98 Timeout Timeout Timeout Timeout Timeout Timeout Reasons for Retransmission ACK lost DUPLICATE PACKET Early timeout DUPLICATE PACKETS
  99. 99. How Long Should Sender Wait? • Sender sets a timeout to wait for an ACK – Too short: wasted retransmissions – Too long: excessive delays when packet lost • TCP sets timeout as a function of the RTT – Expect ACK to arrive after an RTT – … plus a fudge factor to account for queuing • But, how does the sender know the RTT? – Can estimate the RTT by watching the ACKs – Smooth estimate: keep a running average of the RTT • EstimatedRTT = a * EstimatedRTT + (1 –a ) * SampleRTT – Compute timeout: TimeOut = 2 * EstimatedRTT 99
  100. 100. Example RTT Estimation RTT: gaia.cs.umass.edu to fantasia.eurecom.fr 350 RTT (milliseconds) 300 250 200 150 100 1 8 15 22 29 36 43 50 57 64 71 time (seconnds) SampleRTT 100 Estimated RTT 78 85 92 99 106
  101. 101. A Flaw in This Approach • An ACK doesn’t really acknowledge a transmission – Rather, it acknowledges receipt of the data • Consider a retransmission of a lost packet – If you assume the ACK goes with the 1st transmission – … the SampleRTT comes out way too large • Consider a duplicate packet – If you assume the ACK goes with the 2nd transmission – … the Sample RTT comes out way too small • Simple solution in the Karn/Partridge algorithm – Only collect samples for segments sent one single time 101
  102. 102. Yet Another Limitation… • Doesn’t consider variance in the RTT – If variance is small, the EstimatedRTT is pretty accurate – … but, if variance is large, the estimate isn’t all that good • Better to directly consider the variance – Consider difference: SampleRTT – EstimatedRTT – Boost the estimate based on the difference • Jacobson/Karels algorithm – See Section 5.2 of the Peterson/Davie book for details 102
  103. 103. TCP Sliding Window 103
  104. 104. Motivation for Sliding Window • • 104 Stop-and-wait is inefficient – Only one TCP segment is “in flight” at a time – Especially bad when delay-bandwidth product is high Numerical example – 1.5 Mbps link with a 45 msec round-trip time (RTT) • Delay-bandwidth product is 67.5 Kbits (or 8 KBytes) – But, sender can send at most one packet per RTT • Assuming a segment size of 1 KB (8 Kbits) • … leads to 8 Kbits/segment / 45 msec/segment  182 Kbps • That’s just one-eighth of the 1.5 Mbps link capacity
  105. 105. Sliding Window • Allow a larger a ou t of data i flight – Allow sender to get ahead of the receiver – … though ot too far ahead 105
  106. 106. Receiver Buffering • Window size – Amount that can be sent without acknowledgment – Receiver needs to be able to store this amount of data • Receiver advertises the window to the receiver – Tells the receiver the amount of free space left – … and the sender agrees not to exceed this amount Window Size Data ACK’d 106 Outstanding Un-ack’d data Data OK to send Data not OK to send yet
  107. 107. TCP Header for Receiver Buffering Source port Destination port Sequence number Flags: SYN FIN RST PSH URG ACK Acknowledgment HdrLen 0 Flags Advertised window Checksum Urgent pointer Options (variable) Data 107
  108. 108. Fast Retransmission 108
  109. 109. Timeout is Inefficient • Timeout-based retransmission – Sender transmits a packet and waits until timer expires – … and then retransmits from the lost packet onward 109
  110. 110. Fast Retransmission • Better solution possible under sliding window – Although packet n might have been lost – … packets n+1, n+2, and so on might get through • Idea: have the receiver send ACK packets – ACK says that receiver is still awaiting nth packet • And repeated ACKs suggest later packets have arrived – Sender can view the “duplicate ACKs” as an early hint • … that the nth packet must have been lost • … and perform the retransmission early • Fast retransmission – Sender retransmits data after the triple duplicate ACK 110
  111. 111. Effectiveness of Fast Retransmit • When does Fast Retransmit work best? – Long data transfers • High likelihood of many packets in flight – High window size • High likelihood of many packets in flight – Low burstiness in packet losses • Higher likelihood that later packets arrive successfully • Implications for Web traffic – Most Web transfers are short (e.g., 10 packets) • Short HTML files or small images – So, often there aren’t many packets in flight – … making fast retransmit less likely to “kick in” – Forcing users to like “reload” more often…  111
  112. 112. Tearing Down the Connection 112
  113. 113. Tearing Down the Connection B A time • Closing the connection – Finish (FIN) to close and receive remaining bytes – And other host sends a FIN ACK to acknowledge – Reset (RST) to close and not receive remaining bytes 113
  114. 114. Sending/Receiving the FIN Packet • Sending a FIN: close() – Process is done sending data via the socket – Process invokes “close()” to close the socket – Once TCP has sent all of the outstanding bytes… – … then TCP sends a FIN 114 • Receiving a FIN: EOF – Process is reading data from the socket – Eventually, the attempt to read returns an EOF
  115. 115. Suggested improvement for IP and TCP 2.115
  116. 116. • • • • • • The TCP/IP data path has improved pathlength and scalability, and it provides virtual storage constraint relief. Communications Server does the following: Reduces extended common storage area (ECSA) consumption for TCP/IP workloads Communications Server housed portions of inbound datagrams in ECSA, and in certain circumstances, system outages caused by ECSA usage spikes could occur. Communications Server does not use ECSA to hold inbound IP traffic. Reduces system pathlength for the TCP/IP data path. This results in more efficient TCP/IP communications (potentially lower utilization of the LPAR), and can lead to improved network response time if the z/OS image is currently MIPs-constrained. Improves scalability. The UDP layer is enhanced to enable more efficient processing of incoming datagrams when an application has multiple threads concurrently reading datagrams from the same datagram socket. With this enhancement, the UDP layer now wakes up only a single thread to process an incoming datagram, which reduces overhead by avoiding the unnecessary resumption and suspension of multiple threads for every incoming datagram.
  117. 117. Significance of UDP in modern communication 2.117
  118. 118. • • • • In situations where your really want to get a simple answer to another server quickly, UDP works best. In general, you want the answer to be in one response packet, and you are prepared to implement your own protocol for reliability or resends. DNS is the perfect description of this use case. The costs of connection setups are way to high (yet, DNS does support a TCP mode as well). Another case is when you are delivering data that can be lost because newer data coming in will replace that previous data/state. Weather data, video streaming, a stock quotation service (not used for actual trading), or gaming data come to mind. Another case is when you are managing a tremendous amount of state and you want to avoid using TCP because the OS cannot handle that many sessions. This is a rare case today. In fact, there are now user-land TCP stacks that can be used so that the application writer may have finer grained control over the resources needed for that TCP state. Prior to 2003, UDP was really the only game in town. One other case is for multicast traffic. UDP can be multicasted to multiple hosts whereas TCP cannot do this at all.
  119. 119. Telecommunications • Tele (Far) + Communications • Early telecommunications – smoke signals and drums – visual telegraphy (or semaphore in 1792) • Telegraph and telephone – Telegraph (1839) – Telephone (1876) • Radio and television • Telephony – Voice and Data
  120. 120. Communications and Networks • Data Communications – Transmission of signals • Encoding, interfacing, signal integrity, multiplexing etc. • Networking – Topology & architecture used to interconnect devices • Networks of communication systems
  121. 121. Network Trends (1980-Present) Voice, Image, Data, Video Microcontroller Microcontroller Networking Wireless Integrated Systems!
  122. 122. Communication Systems • • • Process describing transfer of information, data, instructions between one or more systems through some media – Examples • people, computers, cell phones, etc. • Computer communication systems Signals passing through the communication channel can be Digital, or analog – Analog signals: continuous electrical waves – Digital signals: individual electrical pulses (bits) Receivers and transmitters: desktop computers, mainframe computers, etc. Communication channel Communication media R R R X X X T X Amp/Adaptor
  123. 123. Communication Systems
  124. 124. Communications Components • Basic components of a communication system – Communication technologies – Communication devices – Communication channels – Communication software
  125. 125. A Communications Model
  126. 126. Communications Tasks Transmission system utilization Addressing Interfacing Routing Signal generation Recovery Synchronization Message formatting Exchange management Security Error detection and correction Network management Flow control
  127. 127. Data Communications Model
  128. 128. Communication Technology Applications voice mail instant messaging e-mail newsgroups collaboration Twitter telephony groupware chat rooms videoconferencing global positioning system (GPS)
  129. 129. Communication Technologies - Applications • Different technologies allowing us to communicate – Examples: Voice mail, fax, email, instant message, chat rooms, news groups, telephony, GPS, and more • Voice mail: Similar to answering machine but digitized • Fax: Sending hardcopy of text or photographs between computers using fax modem • Email: electronic mail – sending text, files, images between different computer networks - must have email software – More than 1.3 billion people send 244 billion messages monthly! • Chat rooms: Allows communications in real time when connected to the Internet
  130. 130. Communication Technologies – Applications (cont) • Telephony: Talking to other people over the Internet (also called VoIP) – Sends digitized audio signals over the Internet – Requires Internet telephone software • Groupware: Software application allowing a group of people to communicate with each other (exchange data) – Address book, appointment book, schedules, etc. • GPS: consists of receivers connected to satellite systems – Determining the geographical location of the receiver – Used for cars, advertising, hiking, tracking, etc.
  131. 131. Communication Devices • Any type of hardware capable of transmitting data, instructions, and information between devices – Functioning as receiver, transmitter, adaptor, converter – Basic characteristics: How fast, how far, how much data! • Examples: Dial-up modems, ISDN, DSL modems, network interface cards
  132. 132. Communication Devices(Cont) – Dial-up modem: uses standard phone lines • Converts digital information into analog • Consists of a modulator and a demodulator • Can be external, internal, wireless – ISDN and DSL Modem: Allows digital communication between networks and computers • Requires a digital modem • Digital is better than analog – why? – Cable modem: a modem that transmits and receives data over the cable television (CATV) network • Also called broadband modem (carrying multiple signals) • The incoming signal is split • Requires a cable modem – Network interface cards: Adaptor cards residing in the computer to transmit and receiver data over the network (NIC) • Operate with different network technologies (e.g., Ethernet)
  133. 133. Communication Software • Examples of applications (Layer 7) take advantage of the transport (Layer 4) services of TCP and UDP – Hypertext Transfer Protocol (HTTP): A client/server application that uses TCP for transport to retrieve HTML pages. – Domain Name Service (DNS): A name-to-address translation application that uses both TCP and UDP transport. – Telnet: A virtual terminal application that uses TCP for transport. – File Transport Protocol (FTP): A file transfer application that uses TCP for transport. – Trivial File Transfer Protocol (TFTP): A file transfer application that uses UDP for transport. – Network Time Protocol (NTP): An application that synchronizes time with a time source and uses UDP for transport. – Border Gateway Protocol (BGP): An exterior gateway routing protocol that uses TCP for transport. BGP is used to exchange routing information for the Internet and is the protocol used between service providers.
  134. 134. Communication Channels • A channel is a path between two communication devices • Channel capacity: How much data can be passed through the channel (bit/sec) – Also called channel bandwidth – The smaller the pipe the slower data transfer! • Consists of one or more transmission media – Materials carrying the signal – Two types: • Physical: wire cable T1 T1 lines • Wireless: Air destinatio lines n network server T3 lines T1 lines
  135. 135. Physical Transmission Media • A tangible media – Examples: Twisted-pair cable, coaxial cable, Fiber-optics, etc. • Twisted-pair cable: – One or more twisted wires bundled together (why?) – Made of copper • Coax-Cable: – Consists of single copper wire surrounded by three layers of insulating and metal materials – Typically used for cable TV • Fiber-optics: – Strands of glass or plastic used to transmit light – Very high capacity, low noise, small size, less suitable to natural disturbances
  136. 136. Physical Transmission Media twisted-pair cable woven or braided metal plastic outer coating copper wire insulatin g material optical fiber core glass cladding protective coating twisted-pair wire
  137. 137. Wireless Transmission Media • Broadcast Radio – Distribute signals through the air over long distance – Uses an antenna – Typically for stationary locations – Can be short range • Cellular Radio – A form of broadcast radio used for mobile communication – High frequency radio waves to transmit voice or data – Utilizes frequency-reuse
  138. 138. Wireless Transmission Media • Microwaves – Radio waves providing high speed transmission – They are point-to-point (can’t be obstructed) – Used for satellite communication • Infrared (IR) – Wireless transmission media that sends signals using infrared light- waves - Such as?
  139. 139. Physical Transmission Media Wireless channel capacity: 100 Mbps is how many bits per sec? Which is bigger: 10,000 Mbps, 0.01Tbps or 10Gbps?
  140. 140. Networks • • • • • Collection of computers and devices connected together Used to transfer information or files, share resources, etc. What is the largest network? Characterized based on their geographical coverage, speed, capacities Networks are categorized based on the following characteristics: – Network coverage: LAN, MAN, WAN – Network topologies: how the computers are connected together – Network technologies – Network architecture
  141. 141. Network coverage • • • Local Area Networks: – Used for small networks (school, home, office) – Examples and configurations: • Wireless LAN or Switched LAN • ATM LAN, Frame Ethernet LAN • Peer-2-PEER: connecting several computers together (<10) • Client/Server: The serves shares its resources between different clients Metropolitan Area Network – Backbone network connecting all LANs – Can cover a city or the entire country Wide Area Network – Typically between cities and countries – Technology: • Circuit Switch, Packet Switch, Frame Relay, ATM – Examples: • Internet P2P: Networks with the same network software can be connected together (Napster)
  142. 142. LAN v.s WAN LAN - Local Area Network a group of computers connected within a building or a campus (Example of LAN may consist of computers located on a single floor or a building or it might link all the computers in a small company. WAN - A network consisting of computers of LAN's connected across a distance WAN can cover small to large distances, using different topologies such as telephone lines, fiber optic cabling, satellite transmissions and microwave transmissions.
  143. 143. Network Topologies • Configuration or physical arrangement in which devices are connected together • BUS networks: Single central cable connected a number of devices – Easy and cheap – Popular for LANs • RING networks: a number of computers are connected on a closed loop – Covers large distances – Primarily used for LANs and WANs • STAR networks: connecting all devices to a central unit – All computers are connected to a central device called hub – All data must pass through the hub – What is the problem with this? – Susceptible to failure
  144. 144. Network Topologies personal computer personal computer personal computer personal computer personal computer personal computer personal computer personal computer personal computer host compute r printer file server
  145. 145. Network Architecture • • Refers to how the computer or devices are designed in a network Basic types: – Centralized – using mainframes – Peer-2-Peer: • Each computer (peer) has equal responsibilities, capacities, sharing hardware, data, with the other computers on the peer-to-peer network • Good for small businesses and home networks • Simple and inexpensive – Client/Server: • All clients must request service from the server • The server is also called a host • Different servers perform different tasks: File server, network server, etc. client laser printer client serve r client
  146. 146. P2P vs Client-Server Peers make a portion of their resources, such as processing power, disk storage or network bandwidth, directly available to other network participants, without the need for central coordination by servers or stable hosts Peer-to-Peer Examples
  147. 147. (Data) Network Technologies • Vary depending on the type of devices we use for interconnecting computers and devices together • Ethernet: – LAN technology allowing computers to access the network – Susceptible to collision – Can be based on BUS or STAR topologies – Operates at 10Mbps or 100Mbps, (10/100) – Fast Ethernet operates at 100 Mbps / – Gigabit Ethernet (1998 IEEE 802.3z) – 10-Gigabit Ethernet (10GE or 10GbE or 10 GigE) • 10GBASE-R/LR/SR (long range short range, etc.) • Physical layer – Gigabit Ethernet using optical fiber, twisted pair cable, or balanced copper cable Project Topic
  148. 148. (Data) Network Technologies • Token Ring – LAN technology – Only the computer with the token can transmit – No collision – Typically 72-260 devices can be connected together • TCP/IP and UDP – Uses packet transmission • 802.11 – Standard for wireless LAN – Wi-Fi (wireless fidelity) is used to describe that the device is in 802.11 family or standards – Typically used for long range (300-1000 feet) – Variations include: .11 (1-2 Mbps); .11a (up to 54 Mbps); .11b (up to 11 Mbps); .11g (54 Mbps and higher Project Topic
  149. 149. (Data) Network Technologies • 802.11n – Next generation wireless LAN technology – Improving network throughput (600 Mbps compared to 450 Mbps) – thus potentially supporting a user throughput of 110 Mbit/s • WiMAX – Worldwide Interoperability for Microwave Access – Provides wireless transmission of data from point-to-multipoint links to portable and fully mobile internet access (up to 3 Mbit/s) – The intent is to deliver the last mile wireless broadband access as an alternative to cable and DSL – Based on the IEEE 802.16(d/e) standard (also called Broadband Wireless Access) http://www.broadcom.com/collateral/wp/802_11n-WP100-R.pdf Project Topic
  150. 150. Network Technologies • Personal area network (PAN) – A low range computer network – PANs can be used for communication among the personal devices themselves – Wired with computer buses such as USB and FireWire. • Wireless personal area network (WPAN) – Uses network technologies such as IrDA, Bluetooth, UWB, Z-Wave and ZigBee • Internet Mobile Protocols – Supporting multimedia Internet traffic – IGMP & MBONE for multicasting – RTP, RTCP, & RSVP (used to handle multimedia on the Internet) • VoIP RTP: Real-time Transport Protocol Project Topic
  151. 151. Network Technologies • • • • • Zigbee – High level communication protocols using small, low-power digital radios based on the IEEE 802.15.4 – Wireless mesh networking proprietary standard Bluetooth – Uses radio frequency – Typically used for close distances (short range- 33 feet or so) – Transmits at 1Mbps – Used for handheld computers to communicate with the desktop IrDA – Infrared (IR) light waves – Transfers at a rate of 115 Kbps to 4 Mbps – Requires light-of-sight transmission RFID – Radio frequency identification – Uses tags which are places in items – Example: merchandises, toll-tags, courtesy calls, sensors! WAP – Wireless application protocol – Data rate of 9.6-153 kbps depending on the service type – Used for smart phones and PDAs to access the Internet (email, web, etc) Project Topic
  152. 152. Network Examples • • • • IEEE 802.15.4 – Low-rate wireless personal area networks (LR-WPANs) – Bases for e ZigBee, WirelessHART, and MiWi specification – Also used for 6LoWPAN and standard Internet protocols to build a Wireless Embedded Internet (WEI) Intranets – Used for private networks – May implement a firewall • Hardware and software that restricts access to data and information on a network Home networks – Ethernet – Phone line – HomeRF (radio frequency- waves) – Intelligent home network Vehicle-to-Vehicle (car2Car) - http://www.car-to-car.org/ – A wireless LAN based communication system to guarantee European-wide inter-vehicle operability Car2Car Technology: http://www.youtube.com/watch?v=8tFUsN3ZgR4 Project Topic
  153. 153. Network Examples • Interplanetary (Internet) Network http://www.ece.gatech.edu/research/labs/bwn/deepspace/ Project Topic
  154. 154. Network Example: Telephone Networks • • • • • • • • • Called the Public Switched Telephone Network (PSTN) World-wide and voice oriented (handles voice and data) Data/voice can be transferred within the PSTN using different technologies (data transfer rate bps) Dial-up lines: – Analog signals passing through telephone lines – Requires modems (56 kbps transfer rate) Switching Technologies: ISDN lines: Technologies: – Integrated Services Digital Network •Circuit Switching – Digital transmission over the telephone lines •Packet Switching – Can carry (multiplex) several signals on a single line •Message Switching DSL •Burst Switching – Digital subscribe line – ADSL (asymmetric DSL) • receiver operated at 8.4 Mbps, transmit at 640 kbps T-Carrier lines: carries several signals over a single line: T1,T3 Frame Relay ATM: – Asynchronous Transfer Mode – Fast and high capacity transmitting technology – Packet technology Project Topic
  155. 155. Network Example: Optical Networks • Fiber-to-the-x – Broadband network architecture that uses optical fiber to replace copper – Used for last mile telecommunications – Examples: Fiber-to-the-home (FTTH); Fiber-to-the-building (FTTB); Fiber-to-the premises (FTTP) • Fiber Distribution Network (reaching different customers) – Active optical networks (AONs) – Passive optical networks (PONs) Project Topic
  156. 156. Network Example • Smart Grid – Delivering electricity from suppliers to consumers using digital technology to save energy • Storage Area Networks • Computational Grid Networks http://rekuwait.wordpress.com/2009/06/18/smart-electric-grid/ Project Topic
  157. 157. Network Example: Telephone Networks
  158. 158. Network Examples
  159. 159. Network Examples Public Telephone Network T-Carrier ATM Dedicated Lines DSL What about Cable Internet Services? Dail-up ISDN
  160. 160. solutions 2.160
  161. 161. Cluster-based Storage Systems Ethernet: 1-10Gbps Client Commodity Ethernet Switch Round Trip Time (RTT): 100-10us Servers
  162. 162. Cluster-based Storage Systems Synchronized Read 1 R R R R 2 3 Client 1 Switch 2 3 4 4 Client now sends next batch of requests Storage Servers Data Block Server Request Unit (SRU)
  163. 163. Synchronized Read Setup • Test on an Ethernet-based storage cluster • Client performs synchronized reads • Increase # of servers involved in transfer – Data block size is fixed (FS read) • TCP used as the data transfer protocol
  164. 164. TCP Throughput Collapse Cluster Setup Collapse! 1Gbps Ethernet Unmodified TCP S50 Switch 1MB Block Size • TCP Incast • Cause of throughput collapse: coarse-grained TCP timeouts
  165. 165. Solution: µsecond TCP + no minRTO Throughput (Mbps) Our solution Unmodified TCP more servers  High throughput for up to 47 servers Simulation scales to thousands of servers
  166. 166. Overview • Problem: Coarse-grained TCP timeouts (200ms) too expensive for datacenter applications • Solution: microsecond granularity timeouts – Improves datacenter app throughput & latency – Also safe for use in the wide-area (Internet)
  167. 167. Outline • Overview • Why are TCP timeouts expensive? • How do coarse-grained timeouts affect apps? • Solution: Microsecond TCP Retransmissions • Is the solution safe?
  168. 168. TCP: data-driven loss recovery Seq # 1 2 3 Ack 1 4 Ack 1 5 Ack 1 Ack 1 3 duplicate ACKs for 1 (packet 2 is probably lost) Retransmit packet 2 immediately In datacenters data-driven recovery in µsecs after loss. 2 Ack 5 Sender Receiver
  169. 169. TCP: timeout-driven loss recovery Seq # 1 2 3 Timeouts are expensive (msecs to recover after loss) 4 5 Retransmission Timeout (RTO) Retransmit packet 1 Ack 1 Sender Receiver
  170. 170. TCP: Loss recovery comparison Timeout driven recovery is slow (ms) Data-driven recovery is super fast (µs) in datacenters Seq # 1 2 3 4 5 Seq # 1 2 3 4 5 Retransmission Timeout (RTO) 1 Sender Retransmit 2 Sender Ack 1 Receiver Ack 1 Ack 1 Ack 1 Ack 1 Ack 5 Receiver
  171. 171. RTO Estimation and Minimum Bound • Jacobson’s TCP RTO Estimator – RTOEstimated = SRTT + (4 * RTTVAR) • Actual RTO = max(minRTO, RTOEstimated) • Minimum RTO bound (minRTO) = 200ms – TCP timer granularity – Safety (Allman99) – minRTO (200ms) >> Datacenter RTT (100µs) – 1 TCP Timeout lasts 1000 datacenter RTTs!
  172. 172. Outline • Overview • Why are TCP timeouts expensive? • How do coarse-grained timeouts affect apps? • Solution: Microsecond TCP Retransmissions • Is the solution safe?
  173. 173. Single Flow TCP Request-Response R Data Data Data Client Switch Response sent Request sent Server Response resent time Response dropped 200ms
  174. 174. Apps Sensitive to 200ms Timeouts • Single flow request-response – Latency-sensitive applications • Barrier-Synchronized workloads – Parallel Cluster File Systems • Throughput-intensive – Search: multi-server queries • Latency-sensitive
  175. 175. Link Idle Time Due To Timeouts Synchronized Read 1 R R R R 2 4 Client 1 3 Switch 2 3 4 4 Req. sent Rsp. sent 4 dropped 1 – 3 done Link Idle! Server Request Unit (SRU) Response resent time
  176. 176. Client Link Utilization Link Idle! 200ms
  177. 177. 200ms timeouts  Throughput Collapse Cluster Setup Collapse! 1Gbps Ethernet 200ms minRTO S50 Switch 1MB Block Size • [Nagle04] called this Incast • Provided application level solutions • Cause of throughput collapse: TCP timeouts • [FAST08] Search for network level solutions to TCP Incast
  178. 178. Results from our previous work (FAST08) Network Level Solutions Increase Switch Buffer Size Results / Conclusions  Delays throughput collapse Throughput collapse inevitable Expensive
  179. 179. Results from our previous work (FAST08) Network Level Solutions Increase Switch Buffer Size Alternate TCP Implementations (avoiding timeouts, aggressive datadriven recovery, disable slow start) Results / Conclusions  Delays throughput collapse Throughput collapse inevitable Expensive Throughput collapse inevitable because timeouts are inevitable (complete window loss a common case)
  180. 180. Results from our previous work (FAST08) Network Level Solutions Increase Switch Buffer Size Alternate TCP Implementations (avoiding timeouts, aggressive datadriven recovery, disable slow start) Ethernet Flow Control Results / Conclusions  Delays throughput collapse Throughput collapse inevitable Expensive Throughput collapse inevitable because timeouts are inevitable (complete window loss a common case)  Effective Limited effectiveness (works for simple topologies) head-of-line blocking
  181. 181. Results from our previous work (FAST08) Network Level Solutions Increase Switch Buffer Size Alternate TCP Implementations (avoiding timeouts, aggressive datadriven recovery, disable slow start) Ethernet Flow Control Reducing minRTO (in simulation) Results / Conclusions  Delays throughput collapse Throughput collapse inevitable Expensive Throughput collapse inevitable because timeouts are inevitable (complete window loss a common case)  Effective Limited effectiveness (works for simple topologies) head-of-line blocking  Very effective Implementation concerns (µs timers for OS, TCP) Safety concerns
  182. 182. Outline • Overview • Why are TCP timeouts expensive? • How do coarse-grained timeouts affect apps? • Solution: Microsecond TCP Retransmissions – and eliminate minRTO • Is the solution safe?
  183. 183. µsecond Retransmission Timeouts (RTO) RTO = max( minRTO, f(RTT) ) 200ms RTT tracked in milliseconds 200µs? Track RTT in µsecond 0?
  184. 184. Lowering minRTO to 1ms • Lower minRTO to as low a value as possible without changing timers/TCP impl. • Simple one-line change to Linux • Uses low-resolution 1ms kernel timers
  185. 185. Default minRTO: Throughput Collapse Unmodified TCP (200ms minRTO)
  186. 186. Lowering minRTO to 1ms helps 1ms minRTO Unmodified TCP (200ms minRTO) Millisecond retransmissions are not enough
  187. 187. Requirements for µsecond RTO • TCP must track RTT in microseconds – Modify internal data structures – Reuse timestamp option • Efficient high-resolution kernel timers – Use HPET for efficient interrupt signaling
  188. 188. Solution: µsecond TCP + no minRTO microsecond TCP + no minRTO 1ms minRTO more servers • High throughput for up to 47 servers Unmodified TCP (200ms minRTO)
  189. 189. Simulation: Scaling to thousands Block Size = 80MB, Buffer = 32KB, RTT = 20us
  190. 190. Synchronized Retransmissions At Scale Simultaneous retransmissions  successive timeouts Successive RTO = RTO * 2backoff
  191. 191. Simulation: Scaling to thousands Desynchronize retransmissions to scale further Successive RTO = (RTO + (rand(0.5)*RTO) ) * 2backoff For use within datacenters only
  192. 192. • Overview Outline • Why are TCP timeouts expensive? • The Incast Workload • Solution: Microsecond TCP Retransmissions • Is the solution safe? – Interaction with Delayed-ACK within datacenters – Performance in the wide-area
  193. 193. Delayed-ACK (for RTO > 40ms) Seq # Seq # Seq # 1 2 1 1 2 Ack 2 Ack 0 40ms Ack 1 Sender Receiver Sender Receiver Sender Receiver Delayed-Ack: Optimization to reduce #ACKs sent
  194. 194. µsecond RTO and Delayed-ACK RTO < 40ms RTO > 40ms Seq # Seq # 1 1 1 40ms Timeout Retransmit packet Ack 1 Ack 1 Sender Receiver Sender Receiver Premature Timeout RTO on sender triggers before Delayed-ACK on receiver
  195. 195. Impact of Delayed-ACK
  196. 196. Is it safe for the wide-area? • Stability: Could we cause congestion collapse? – No: Wide-area RTOs are in 10s, 100s of ms – No: Timeouts result in rediscovering link capacity (slow down the rate of transfer) • Performance: Do we timeout unnecessarily? – [Allman99] Reducing minRTO increases the chance of premature timeouts • Premature timeouts slow transfer rate – Today: detect and recover from premature timeouts – Wide-area experiments to determine performance impact
  197. 197. Wide-area Experiment BitTorrent Seeds BitTorrent Clients Microsecond TCP + No minRTO Standard TCP Do microsecond timeouts harm wide-area throughput?
  198. 198. Wide-area Experiment: Results No noticeable difference in throughput
  199. 199. Best Effort Service Model – scheduling and policy 2.199
  200. 200. Question to the Class? 5 Mbps A 10 Mbps B C D Cross Traffic E F • Flow AD requires b/w, delay, loss guarantees • Cross traffic is unpredictable • Can IP provide this? • What modifications are necessary to accomplish this? 200
  201. 201. Limitations of IP • IP provides only best effort service • IP does not participate in resource management – Cannot provide service guarantees on a per flow basis – Cannot provide service differentiation among traffic aggregates • Early efforts – Tenet group at Berkeley – ATM • IETF efforts – Integrated services initiative – Differentiated services initiative 201
  202. 202. So, what is required? • Flow differentiation – Simple FIFO scheduling will not work! • Admission control • Resource reservation • Flow specification 202
  203. 203. Integrated Services Internet • Enhance IP’s service model – Old model: single best-effort service class – New model: multiple service classes, including best-effort and QoS classes • Create protocols and algorithms to support new service models – Old model: no resource management at IP level – New model: explicit resource management at IP level • Key architecture difference – Old model: stateless – New model: per flow state maintained at routers • used for admission control and scheduling • set up by signaling protocol 203
  204. 204. Integrated Services Network • Flow or session as QoS abstractions • Each flow has a fixed or stable path • Routers along the path maintain the state of the flow 204
  205. 205. Integrated Services Example • Achieve per-flow bandwidth and delay guarantees – Example: guarantee 1MBps and < 100 ms delay to a flow Receiver Sender 205
  206. 206. Integrated Services Example • Allocate resources - perform per-flow admission control Receiver Sender 206
  207. 207. Integrated Services Example • Install per-flow state Receiver Sender 207
  208. 208. Integrated Services Example • Install per flow state Receiver Sender 208
  209. 209. Integrated Services Example: Data Path • Per-flow classification Receiver Sender 209
  210. 210. Integrated Services Example: Data Path • Per-flow buffer management Receiver Sender 210
  211. 211. Integrated Services Example • Per-flow scheduling Receiver Sender 211
  212. 212. How Things Fit Together RSVP Admission Control Forwarding Table Per Flow QoS Table Control Plane Routing RSVP messages Data Plane Routing Messages Data In Route Lookup Classifier Scheduler Data Out 212
  213. 213. Service Classes • Service can be viewed as a contract between network and communication client – end-to-end service – other service scopes possible • Three common services – best-effort (“elastic” applications) – hard real-time (“real-time” applications) – soft real-time (“tolerant” applications) 213
  214. 214. Hard Real Time: Guaranteed Services • Service contract – network to client: guarantee a deterministic upper bound on delay for each packet in a session – client to network: the session does not send more than it specifies • Algorithm support – admission control based on worst-case analysis – per flow classification/scheduling at routers 214
  215. 215. Soft Real Time: Controlled Load Service • Service contract: – network to client: similar performance as an unloaded besteffort network – client to network: the session does not send more than it specifies • Algorithm Support – admission control based on measurement of aggregates – scheduling for aggregate possible 215
  216. 216. Improving QOS in IP Networks Thus far: “making the best of best effort” Future: next generation Internet with QoS guarantees – RSVP: signaling for resource reservations – Differentiated Services: differential guarantees – Integrated Services: firm guarantees • simple model for sharing and congestion studies:
  217. 217. Principles for QOS Guarantees • Example: 1MbpsI P phone, FTP share 1.5 Mbps link. – bursts of FTP can congest router, cause audio loss – want to give priority to audio over FTP Principle 1 packet marking needed for router to distinguish between different classes; and new router policy to treat packets accordingly
  218. 218. Principles for QOS Guarantees (more) • what if applications misbehave (audio sends higher than declared rate) – policing: force source adherence to bandwidth allocations • marking and policing at network edge: – similar to ATM UNI (User Network Interface) Principle 2 provide protection (isolation) for one class from others
  219. 219. Principles for QOS Guarantees (more) • Allocating fixed (non-sharable) bandwidth to flow: inefficient use of bandwidth if flows doesn’t use its allocation Principle 3 While providing isolation, it is desirable to use resources as efficiently as possible
  220. 220. Principles for QOS Guarantees (more) • Basic fact of life: can not support traffic demands beyond link capacity Principle 4 Call Admission: flow declares its needs, network may block call (e.g., busy signal) if it cannot meet needs
  221. 221. Summary of QoS Principles Let’s next look at mechanisms for achieving this ….
  222. 222. Scheduling And Policing Mechanisms • scheduling: choose next packet to send on link; allocate link capacity and output queue buffers to each connection (or connections aggregated into classes) • FIFO (first in first out) scheduling: send in order of arrival to queue – discard policy: if packet arrives to full queue: who to discard? • Tail drop: drop arriving packet • priority: drop/remove on priority basis • random: drop/remove randomly
  223. 223. Need for a Scheduling Discipline • Why do we need a non-trivial scheduling discipline? • Per-connection delay, bandwidth, and loss are determined by the scheduling discipline – The NE can allocate different mean delays to different connections by its choice of service order – it can allocate different bandwidths to connections by serving at least a certain number of packets from a particular connection in a given time interval – Finally, it can allocate different loss rates to connections by giving them more or fewer buffers
  224. 224. FIFO Scheduling • Disadvantage with strict FIFO scheduling is that the scheduler cannot differentiate among connections -- it cannot explicitly allocate some connections lower mean delays than others • A more sophisticated scheduling discipline can achieve this objective (but at a cost) • The conservation law – “the sum of the mean queueing delays received by the set of multiplexed connections, weighted by their fair share of the link’s load, is independent of the scheduling discipline”
  225. 225. Requirements • A scheduling discipline must satisfy four requirements: – Ease of implementation -- pick a packet every few microsecs; a scheduler that takes O(1) and not O(N) time – Fairness and Protection (for best-effort connections) -- FIFO does not offer any protection because a misbehaving connection can increase the mean delay of all other connections. Round-robin scheduling? – Performance bounds -- deterministic or statistical; common performance parameters: bandwidth, delay (worst-case, average), delay-jitter, loss – Ease and efficiency of admission control -- to decide given the current set of connections and the descriptor for a new connection, whether it is possible to meet the new connection’s performance bounds without jeopardizing the performance of existing connections
  226. 226. Schedulable Region
  227. 227. Designing a scheduling discipline • Four principal degrees of freedom: – the number of priority levels – whether each level is work-conserving or non-work-conserving – the degree of aggregation of connections within a level – service order within a level • Each feature comes at some cost – for a small LAN switch -- a single priority FCFS scheduler or at most 2-priority scheduler may be sufficient – for a heavily loaded wide-area public switch with possibly noncooperative users, a more sophisticated scheduling discipline may be required.
  228. 228. Work conserving and non-work conserving disciplines • A work-conserving scheduler is idle only when there is no packet awaiting service • A non-work-conserving scheduler may be idle even if it has packets to serve – makes the traffic arriving at downstream switches more predictable – reduces buffer size necessary at output queues and the delay jitter experienced by a connection – allows the switch to send a packet only when the packet is eligible – for example, if the (k+1)th packet on connection A becomes eligible for service only i seconds after the service of the kth packet, the downstream swicth receives packets on A no faster than one every i secs.
  229. 229. Eligibility times • By choosing eligibility times carefully, the output from a switch can be mode more predictable (so that bursts won’t build up in the n/w) • Two approaches: rate-jitter and delay-jitter • rate-jitter: peak rate guarantee for a connection – E(1) = A(1); E(k+1) = max(E(k) + Xmin, A(k+1)) where Xmin is the time taken to serve a fixed-sized packet at peak rate) • delay-jitter: at every switch, the input arrival pattern is fully reconstructed – E(0,k) = A (0,k); E(i+1, k) = E(i,k) + D + L where D is the delay bound at the previous switch and L is the largest possible delay on the link between switch i and i+1
  230. 230. Pros and Cons • Reduces delay jitter: Con -- we can remove jitter at endpoints with an elasticity buffer; Pro--reduces buffers(expensive) at the switches • Increases mean delay, problem?: pro--for playback applications, which delay packets until the delay-jitter bound, increasing mean delay does not affect the perceived performance • Wasted bandwidth, problem?: pro--It can serve best-effort packets when there are no eligible packets to serve • Needs accurate source descriptors -- no rebuttal from the non-work conserving camp
  231. 231. Priority Scheduling transmit highest priority queued packet • multiple classes, with different priorities – class may depend on marking or other header info, e.g. IP source/dest, port numbers, etc..
  232. 232. Priority Scheduling • The scheduler serves a packet from priority level k only if there are no packets awaiting service in levels k+1, k+2, …, n • at least 3 levels of priority in an integrated services network? • Starvation? Appropriate admission control and policing to restrict service rates from all but the lowest priority level • Simple implementation
  233. 233. Round Robin Scheduling • multiple classes • cyclically scan class queues, serving one from each class (if available) • provides protection against misbehaving sources (also guarantees a minimum bandwidth to every connection)
  234. 234. Max-Min Fair Share • Fair Resource allocation to best-effort connections? • Fair share allocates a user with a “small” demand what it wants, and evenly distributes unused resources to the “big” users. • Maximize the minimum share of a source whose demand is not fully satisfied. – Resources are allocated in order of increasing demand – no source gets a resource share larger than its demand – sources with unsatisfied demand s get an equal share of resource • A Generalized Processor Sharing (GPS) server will implement max-min fair share
  235. 235. Weighted Fair Queueing • generalized Round Robin (offers differential service to each connection/class) • each class gets weighted amount of service in each cycle
  236. 236. Policing Mechanisms Goal: limit traffic to not exceed declared parameters Three common-used criteria: • (Long term) Average Rate: how many pkts can be sent per unit time (in the long run) – crucial question: what is the interval length: 100 packets per sec or 6000 packets per min have same average! • Peak Rate: e.g., 6000 pkts per min. (ppm) avg.; 1500 ppm peak rate • (Max.) Burst Size: max. number of pkts sent consecutively (with no intervening idle)
  237. 237. Traffic Regulators • Leaky bucket controllers • Token bucket controllers
  238. 238. Policing Mechanisms Token Bucket: limit input to specified Burst Size and Average Rate. • bucket can hold b tokens • tokens generated at rate r token/sec unless bucket full • over interval of length t: number of packets admitted less than or equal to (r t + b).
  239. 239. Policing Mechanisms (more) • token bucket, WFQ combine to provide guaranteed upper bound on delay, i.e., QoS guarantee! arriving token rate, r traffic bucket size, b per-flow rate, R WFQ D = b/R max
  240. 240. Queries
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