Lecture 15 – Transport Protocols Eric Anderson Fall Computer Networking

Outline Transport introduction Review: Error recovery & flow control TCP flow control Introducing congestion control 2

Transport Protocols Lowest level end-to-end protocol. Header generated by sender is interpreted only by the destination Routers view transport header as part of the payload Transport IP Datalink Physical Transport IP Datalink Physical IP router

Functionality Split Network provides best-effort delivery End-systems implement many functions Reliability In-order delivery Demultiplexing Message boundaries Connection abstraction Congestion control … 4

Transport Protocols UDP provides just integrity and demux TCP adds… Connection-oriented Reliable Ordered Point-to-point Byte-stream Full duplex Flow and congestion controlled 5

UDP: User Datagram Protocol [RFC 768] “No frills,” “bare bones” Internet transport protocol “Best effort” service, UDP segments may be: Lost Delivered out of order to app Connectionless: No handshaking between UDP sender, receiver Each UDP segment handled independently of others 6 Why is there a UDP? No connection establishment (which can add delay) Simple: no connection state at sender, receiver Small header No congestion control: UDP can blast away as fast as desired

UDP, cont. Often used for streaming multimedia apps Loss tolerant Rate sensitive Other UDP uses (why?): DNS, SNMP Reliable transfer over UDP Must be at application layer Application-specific error recovery 7 Source port #Dest port # 32 bits Application data (message) UDP segment format Length Checksum Length, in bytes of UDP segment, including header

UDP Checksum Sender: Treat segment contents as sequence of 16-bit integers Checksum: addition (1’s complement sum) of segment contents Sender puts checksum value into UDP checksum field Receiver: Compute checksum of received segment Check if computed checksum equals checksum field value: NO - error detected YES - no error detected But maybe errors nonethless? 8 Goal: detect “errors” (e.g., flipped bits) in transmitted segment – optional use!

High-Level TCP Characteristics Protocol implemented entirely at the ends Fate sharing Protocol has evolved over time and will continue to do so Nearly impossible to change the header Use options to add information to the header These do change sometimes Change processing at endpoints Backward compatibility is what makes it TCP 9

TCP Header 10 Source portDestination port Sequence number Acknowledgement Advertised windowHdrLen Flags 0 ChecksumUrgent pointer Options (variable) Data Flags: SYN FIN RESET PUSH URG ACK

Evolution of TCP TCP & IP RFC 793 & TCP described by Vint Cerf and Bob Kahn In IEEE Trans Comm 1983 BSD Unix 4.2 supports TCP/IP 1984 Nagel’s algorithm to reduce overhead of small packets; predicts congestion collapse 1987 Karn’s algorithm to better estimate round-trip time 1986 Congestion collapse observed 1988 Van Jacobson’s algorithms congestion avoidance and congestion control (most implemented in 4.3BSD Tahoe) BSD Reno fast retransmit delayed ACK’s 1975 Three-way handshake Raymond Tomlinson In SIGCOMM 75

TCP Through the 1990s ECN (Floyd) Explicit Congestion Notification 1993 TCP Vegas (Brakmo et al) delay-based congestion avoidance 1994 T/TCP (Braden) Transaction TCP 1996 SACK TCP (Floyd et al) Selective Acknowledgement 1996 Hoe NewReno startup and loss recovery 1996 FACK TCP (Mathis et al) extension to SACK

TCP Through the 2000s NewReno (Floyd et. al.) Partial ACK in Fast Recovery 2007 CUBIC Rhee, Xu, Ha Convex-Concave Response Fn Data Center TCP (too many authors) ECN, proportional window scaling 2011 Multi-Path TCP Barré, Bonaventure TCP over multiple subflows

Outline Transport introduction Error recovery & flow control TCP flow control Introducing congestion control 14

Review: Stop and Wait 15 Time Packet ACK Timeout ARQ Receiver sends acknowledgement (ACK) when it receives packet Sender waits for ACK and timeouts if it does not arrive within some time period Simplest ARQ protocol Send a packet, stop and wait until ACK arrives SenderReceiver

Recovering from Error 16 Packet ACK Timeout Packet ACK Timeout Packet Timeout Packet ACK Timeout Time Packet ACK Timeout Packet ACK Timeout ACK lostPacket lost Early timeout DUPLICATE PACKETS!!!

Problems with Stop and Wait How to recognize a duplicate Performance Can only send one packet per round trip 17

How to Recognize Resends? 18 Use sequence numbers both packets and acks Sequence # in packet is finite  How big should it be? For stop and wait? One bit – won’t send seq #1 until received ACK for seq #0 Pkt 0 ACK 0 Pkt 0 ACK 1 Pkt 1 ACK 0

How to Keep the Pipe Full? Send multiple packets without waiting for first to be acked Number of pkts in flight = window Reliable, unordered delivery Several parallel stop & waits Send new packet after each ack Sender keeps list of unack’ed packets; resends after timeout Receiver same as stop & wait How large a window is needed? Suppose 10Mbps link, 4ms delay, 500byte pkts 1? 10? 20?11020 Round trip delay * bandwidth = capacity of pipe 19

Sliding Window Reliable, ordered delivery Receiver has to hold onto a packet until all prior packets have arrived Why might this be difficult for just parallel stop & wait? Sender must prevent buffer overflow at receiver Circular buffer at sender and receiver Packets in transit  buffer size Advance when sender and receiver agree packets at beginning have been received 20

Sender/Receiver State 21 Receiver Sender …… Sent & AckedSent Not Acked OK to SendNot Usable …… Max acceptable Receiver window Max ACK receivedNext seqnum Received & AckedAcceptable Packet Not Usable Sender window Next expected

Sequence Numbers How large do sequence numbers need to be? Must be able to detect wrap-around Depends on sender/receiver window size E.g. Max seq = 7, send win=recv win=7 If pkts 0..6 are sent succesfully and all acks lost Receiver expects 7,0..5, sender retransmits old 0..6!!! Max sequence must be  send window + recv window 22

Window Sliding – Common Case On reception of new ACK (i.e. ACK for something that was not acked earlier) Increase sequence of max ACK received Send next packet On reception of new in-order data packet (next expected) Hand packet to application Send cumulative ACK – acknowledges reception of all packets up to sequence number Increase sequence of max acceptable packet 23

Loss Recovery On reception of out-of-order packet Send nothing (wait for source to timeout) Cumulative ACK (helps source identify loss) Timeout (Go-Back-N recovery) Set timer upon transmission of packet Retransmit all unacknowledged packets Performance during loss recovery No longer have an entire window in transit Can have much more clever loss recovery 24

Go-Back-N in Action 25

Selective Repeat Receiver individually acknowledges all correctly received pkts Buffers packets, as needed, for eventual in-order delivery to upper layer Sender only resends packets for which ACK not received Sender timer for each unACKed packet Sender window N consecutive seq #’s Again limits seq #s of sent, unACKed packets 26

Selective Repeat: Sender, Receiver Windows 27

Important Lessons Transport service UDP  mostly just IP service TCP  congestion controlled, reliable, byte stream Types of ARQ protocols Stop-and-wait  slow, simple Go-back-n  can keep link utilized (except w/ losses) Selective repeat  efficient loss recovery Sliding window flow control Addresses buffering issues and keeps link utilized 28

Next Lecture Congestion control TCP Reliability 29

The rest of the slides are FYI EXTRA SLIDES

Ponder This… A bus station is where a bus stops. A train station is where a train stops. A work station is where… Maybe that explains why it is so hard getting project 2 done …. ouch 31

Outline Transport introduction Error recovery & flow control TCP flow control Introducing congestion control 32

33 Good Ideas So Far… Flow control Stop & wait Sliding window Loss recovery Timeouts Acknowledgement-driven recovery Selective repeat versus go-back-N Cumulative acknowledgement

Window Flow Control Stop and wait flow control results in poor throughput for long-delay paths: packet size/ roundtrip-time. Solution: receiver provides sender with a window that it can fill with packets. The window is backed up by buffer space on receiver Receiver acknowledges the a packet every time a packet is consumed and a buffer is freed Sender Receiver 34

Bandwidth-Delay Product Sender Receiver Time Max Throughput = Window Size Roundtrip Time RTT 35

Automatic Repeat Request (ARQ) Sender retransmits packet after timeout Recognize retransmissions using sequence numbers both packets and acks How big should it be? For stop&wait, sliding window? Ack can acknowledge one packet or all earlier packets “Selective” vs cumulative Pkt 0 ACK 0 Pkt 0 ACK 1 Pkt 1 ACK 0 36

37 Sequence Numbers How large do sequence numbers need to be? Must be able to detect wrap-around Depends on sender/receiver window size Example Assume seq number space = 7, window = 7 If pkts 0..6 are sent successfully and all acks lost Receiver expects 7,0..5, sender retransmits old 0..6!!! Condition: max window size < size sequence number space

38 Sequence Numbers 32 Bits, Unsigned  for bytes not packets! Circular Comparison Why So Big? For sliding window, must have |Sequence Space| > |Window| No problem Also, want to guard against stray packets With IP, packets have maximum lifetime of 120s Sequence number would wrap around in this time at 286MB/s 0Max a b a < b 0Max b a b < a >a <a >a <a

39 TCP Flow Control TCP is a sliding window protocol For window size n, can send up to n bytes without receiving an acknowledgement When the data is acknowledged then the window slides forward Each packet advertises a window size Indicates number of bytes the receiver has space for Original TCP always sent entire window Congestion control now limits this

40 Window Flow Control: Send Side Sent but not ackedNot yet sent window Next to be sent Sent and acked

41 acknowledgedsentto be sentoutside window Source Port Dest. Port Sequence Number Acknowledgment HL/Flags Window D. Checksum Urgent Pointer Options… Source Port Dest. Port Sequence Number Acknowledgment HL/Flags Window D. Checksum Urgent Pointer Options... Packet Sent Packet Received App write Window Flow Control: Send Side

42 Acked but not delivered to user Not yet acked Receive buffer window Window Flow Control: Receive Side New What should receiver do?

43 Aside: TCP Persist What happens if window is 0? Receiver updates window when application reads data What if this update is lost? TCP Persist state Sender periodically sends 1 byte packets Receiver responds with ACK even if it can’t store the packet

44 Performance Considerations The window size can be controlled by receiving application Can change the socket buffer size from a default (e.g. 8Kbytes) to a maximum value (e.g. 64 Kbytes) The window size field in the TCP header limits the window that the receiver can advertise 16 bits  64 KBytes 10 msec RTT  51 Mbit/second 100 msec RTT  5 Mbit/second TCP options to get around 64KB limit  increases above limit

Outline Transport introduction Error recovery & flow control TCP flow control Introducing congestion control 45

46 Internet Pipes? How should you control the faucet?

47 Internet Pipes? How should you control the faucet? Too fast – sink overflows!

48 Internet Pipes? How should you control the faucet? Too fast – sink overflows! Too slow – what happens?

49 Internet Pipes? How should you control the faucet? Too fast – sink overflows Too slow – what happens? Goals Fill the bucket as quickly as possible Avoid overflowing the sink Solution – watch the sink

50 Plumbers Gone Wild! How do we prevent water loss? Know the size of the pipes?

51 Plumbers Gone Wild 2! Now what? Feedback from the bucket or the funnels?

52 Congestion Different sources compete for resources inside network Why is it a problem? Sources are unaware of current state of resource Sources are unaware of each other Manifestations: Lost packets (buffer overflow at routers) Long delays (queuing in router buffers) Can result in throughput less than bottleneck link (1.5Mbps for the above topology)  a.k.a. congestion collapse 10 Mbps 100 Mbps 1.5 Mbps

53 Causes & Costs of Congestion Four senders – multihop paths Timeout/retransmit Q: What happens as rate increases?

54 Causes & Costs of Congestion When packet dropped, any “upstream transmission capacity used for that packet was wasted! “ideal” max

55 Congestion Collapse Definition: Increase in network load results in decrease of useful work done Many possible causes Spurious retransmissions of packets still in flight Classical congestion collapse How can this happen with packet conservation Solution: better timers and TCP congestion control Undelivered packets Packets consume resources and are dropped elsewhere in network Solution: congestion control for ALL traffic

56 Congestion Control and Avoidance A mechanism that: Uses network resources efficiently Preserves fair network resource allocation Prevents or avoids collapse Congestion collapse is not just a theory Has been frequently observed in many networks

57 Approaches Towards Congestion Control End-end congestion control: No explicit feedback from network Congestion inferred from end- system observed loss, delay Approach taken by TCP Network-assisted congestion control: Routers provide feedback to end systems Single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) Explicit rate sender should send at Problem: makes routers complicated Two broad approaches towards congestion control:

58 TCP Congestion Control Very simple mechanisms in network FIFO scheduling with shared buffer pool Feedback through packet drops TCP interprets packet drops as signs of congestion and slows down This is an assumption: packet drops are not a sign of congestion in all networks E.g. wireless networks Periodically probes the network to check whether more bandwidth has become available.

59 Objectives Simple router behavior Distributedness Efficiency: X =  x i (t) Fairness: (  x i ) 2 /n(  x i 2 ) What are the important properties of this function? Convergence: control system must be stable

60 Basic Control Model Reduce speed when congestion is perceived How is congestion signaled? Either mark or drop packets How much to reduce? Increase speed otherwise Probe for available bandwidth – how?

61 Linear Control Many different possibilities for reaction to congestion and probing Examine simple linear controls Window(t + 1) = a + b Window(t) Different a i /b i for increase and a d /b d for decrease Supports various reaction to signals Increase/decrease additively Increased/decrease multiplicatively Which of the four combinations is optimal?

62 Phase Plots Simple way to visualize behavior of competing connections over time User 1’s Allocation x 1 User 2’s Allocation x 2

63 Phase Plots What are desirable properties? What if flows are not equal? Efficiency Line Fairness Line User 1’s Allocation x 1 User 2’s Allocation x 2 Optimal point Overload Underutilization

64 Additive Increase/Decrease T0T0 T1T1 Efficiency Line Fairness Line User 1’s Allocation x 1 User 2’s Allocation x 2 Both X 1 and X 2 increase/ decrease by the same amount over time Additive increase improves fairness and additive decrease reduces fairness

65 Muliplicative Increase/Decrease Both X 1 and X 2 increase by the same factor over time Extension from origin – constant fairness T0T0 T1T1 Efficiency Line Fairness Line User 1’s Allocation x 1 User 2’s Allocation x 2

66 Convergence to Efficiency xHxH Efficiency Line Fairness Line User 1’s Allocation x 1 User 2’s Allocation x 2

67 Distributed Convergence to Efficiency xHxH Efficiency Line Fairness Line User 1’s Allocation x 1 User 2’s Allocation x 2 a=0 b=1 a>0 & b<1 a 1 a<0 & b<1 a>0 & b>1

68 Convergence to Fairness xHxH Efficiency Line Fairness Line User 1’s Allocation x 1 User 2’s Allocation x 2 x H’

69 Convergence to Efficiency & Fairness Intersection of valid regions For decrease: a=0 & b < 1 xHxH Efficiency Line Fairness Line User 1’s Allocation x 1 User 2’s Allocation x 2 x H’

70 What is the Right Choice? Constraints limit us to AIMD Can have multiplicative term in increase (MAIMD) AIMD moves towards optimal point x0x0 x1x1 x2x2 Efficiency Line Fairness Line User 1’s Allocation x 1 User 2’s Allocation x 2

71 Important Lessons Transport service UDP  mostly just IP service TCP  congestion controlled, reliable, byte stream Types of ARQ protocols Stop-and-wait  slow, simple Go-back-n  can keep link utilized (except w/ losses) Selective repeat  efficient loss recovery Sliding window flow control TCP flow control Sliding window  mapping to packet headers 32bit sequence numbers (bytes)

72 Important Lessons Why is congestion control needed? How to evaluate congestion control algorithms? Why is AIMD the right choice for congestion control? TCP flow control Sliding window  mapping to packet headers 32bit sequence numbers (bytes)

73 Good Ideas So Far… Flow control Stop & wait Parallel stop & wait Sliding window Loss recovery Timeouts Acknowledgement-driven recovery Selective repeat versus go-back-N Cumulative acknowledgement Congestion control AIMD  fairness and efficiency Next Lecture: How does TCP actually implement these?