Step(1:(A’s(Ini%al(SYN(Packet · 2 Step(2:(B’s(SYN9ACK(Packet 5 B’s port A’s port B’s...

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Star%ng and Ending a Connec%on: TCP Handshakes

1

Establishing a TCP Connec%on

•  Three-‐way handshake to establish connec%on – Host A sends a SYNchronize (open) to the host B – Host B returns a SYN ACKnowledgment (SYN ACK) – Host A sends an ACK to acknowledge the SYN ACK

2

SYN

SYN ACK

ACK Data

A B

Data

Each host tells its ISN to the other host.!

TCP Header

3

Source port Destination port

Sequence number

Acknowledgment

Advertised window HdrLen Flags 0

Checksum Urgent pointer

Options (variable)

Data

Flags: SYN FIN RST PSH URG ACK

Step 1: A’s Ini%al SYN Packet

4

A’s port B’s port

A’s Initial Sequence Number

Acknowledgment

Advertised window 20 Flags 0


Options (variable)


A tells B it wants to open a connec4on…

2

Step 2: B’s SYN-‐ACK Packet

5

B’s port A’s port

B’s Initial Sequence Number

A’s ISN plus 1



Options (variable)


B tells A it accepts, and is ready to hear the next byte…

… upon receiving this packet, A can start sending data

Step 3: A’s ACK of the SYN-‐ACK

6

A’s port B’s port

B’s ISN plus 1



Options (variable)


A tells B it is okay to start sending…!

Sequence number

… upon receiving this packet, B can start sending data!

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 %mer and wait for the SYN-‐ACK – … and retransmits the SYN if needed

•  How should the TCP sender set the %mer? – Sender has no idea how far away the receiver is – Hard to guess a reasonable length of %me to wait – Some TCPs use a default of 3 or 6 seconds

7

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 impa%ent – … and click the hyperlink again, or click “reload”

•  User triggers an “abort” of the “connect” –  Browser creates a new socket and does a “connect” –  Essen%ally, forces a faster send of a new SYN packet! –  Some%mes very effec%ve, and the page comes fast

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3

Tearing Down the Connec%on

•  Closing (each end of) the connec%on – 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

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SYN

SYN

ACK

ACK

D

ata

FIN

ACK

ACK

time A

B

FIN A

CK

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

•  Receiving a FIN: EOF –  Process is reading data from the socket

–  Eventually, the acempt to read returns an EOF

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11

Flow Control: TCP Sliding Window

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4

Mo%va%on for Sliding Window •  Stop-‐and-‐wait is inefficient

– Only one TCP segment is “in flight” at a %me – Esp. bad when delay-‐bandwidth product is high

•  Numerical example – 1.5 Mbps link with a 45 msec round-‐trip %me (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/seg / 45 Msec/seg 182 Kbps •  Just one-‐eighth of the 1.5 Mbps link capacity

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Sliding Window •  Allow a larger amount of data “in flight”

– Allow sender to get ahead of the receiver – … though not too far ahead

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Sending process! Receiving process!

Last byte ACKed!

Last byte sent!

TCP! TCP!

Next byte expected!

Last byte written! Last byte read!

Last byte received!

Receiver Buffering •  Window size

– Amount that can be sent without acknowledgment –  Receiver needs to be able to store this amount of data

•  Receiver adver%ses the window to the sender –  Tells the sender the amount of free space lel – … and the sender agrees not to exceed this amount

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Window Size

Outstanding Un-ack’d data

Data OK to send

Data not OK to send yet

Data ACK’d

TCP Header for Receiver Buffering

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Sequence number

Acknowledgment



Options (variable)

Data


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Conclusions

•  Transport protocols – Mul%plexing and demul%plexing – Checksum-‐based error detec%on – Sequence numbers – Retransmission – Window-‐based flow control

•  Next lecture – Conges%on control

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Conges%on Control Reading: Chapter 3

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Goals of Today’s Lecture

•  Conges%on in IP networks – Unavoidable due to best-‐effort service model –  IP philosophy: decentralized control at end hosts

•  Conges%on control by the TCP senders –  Infers conges%on is occurring (e.g., from packet losses) –  Slows down to alleviate conges%on, for the greater good

•  TCP conges%on-‐control algorithm – Addi%ve-‐increase, mul%plica%ve-‐decrease –  Slow start and slow-‐start restart

•  Ac%ve Queue Management (AQM) –  Random Early Detec%on (RED) –  Explicit Conges%on No%fica%on (ECN)

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No Problem Under Circuit Switching

•  Source establishes connec%on to des%na%on – Nodes reserve resources for the connec%on – Circuit rejected if the resources aren’t available – Cannot have more than the network can handle

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6

IP Best-‐Effort Design Philosophy

•  Best-‐effort delivery – Let everybody send – Network tries to deliver what it can – … and just drop the rest

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source destination

IP network

Conges%on is Unavoidable •  Two packets arrive at the same %me

– The node can only transmit one – … and either buffer or drop the other

•  If many packets arrive in short period of %me – The node cannot keep up with the arriving traffic – … and the buffer may eventually overflow

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The Problem of Conges%on

•  What is conges%on? – Load is higher than capacity

•  What do IP routers do? – Drop the excess packets

•  Why is this bad? – Wasted bandwidth for retransmissions

23 Load

Goodput “congestion! collapse”!

Increase in load that results in a decrease in

useful work done.

Ways to Deal With Conges%on •  Ignore the problem

– Many dropped (and retransmiced) packets –  Can cause conges%on collapse

•  Reserva%ons, like in circuit switching –  Pre-‐arrange bandwidth alloca%ons –  Requires nego%a%on before sending packets

•  Pricing – Don’t drop packets for the high-‐bidders –  Requires a payment model

•  Dynamic adjustment (TCP) –  Every sender infers the level of conges%on –  Each adapts its sending rate “for the greater good”

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7

Many Important Ques%ons

•  How does the sender know there is conges%on? –  Explicit feedback from the network? –  Inference based on network performance?

•  How should the sender adapt? –  Explicit sending rate computed by the network? –  End host coordinates with other hosts? –  End host thinks globally but acts locally?

•  What is the performance objec%ve? – Maximizing goodput, even if some users suffer more? –  Fairness? (Whatever the heck that means!)

•  How fast should new TCP senders send? 25

Inferring From Implicit Feedback

•  What does the end host see?

•  What can the end host change? 26

?

Where Conges%on Happens: Links •  Simple resource alloca%on: FIFO queue & drop-‐tail

•  Access to the bandwidth: first-‐in first-‐out queue – Packets transmiced in the order they arrive

•  Access to the buffer space: drop-‐tail queuing –  If the queue is full, drop the incoming packet

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How it Looks to the End Host

•  Packet delay – Packet experiences high delay

•  Packet loss – Packet gets dropped along the way

•  How does TCP sender learn this? – Delay

•  Round-‐trip %me es%mate

– Loss •  Timeout •  Duplicate acknowledgments

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8

What Can the End Host Do?

•  Upon detec%ng conges%on (well, packet loss) – Decrease the sending rate –  End host does its part to alleviate the conges%on

•  But, what if condi%ons change? –  Suppose there is more bandwidth available – Would be a shame to stay at a low sending rate

•  Upon not detec%ng conges%on –  Increase the sending rate, a licle at a %me – And see if the packets are successfully delivered

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TCP Conges%on Window

•  Each TCP sender maintains a conges%on window – Maximum number of bytes to have in transit –  I.e., number of bytes s%ll awai%ng acknowledgments

•  Adap%ng the conges%on window – Decrease upon losing a packet: backing off –  Increase upon success: op%mis%cally exploring – Always struggling to find the right transfer rate

•  Both good and bad –  Pro: avoids having explicit feedback from network –  Con: under-‐shoo%ng and over-‐shoo%ng the rate

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Addi%ve Increase, Mul%plica%ve Decrease (AIMD)

•  How much to increase and decrease? –  Increase linearly, decrease mul%plica%vely – A necessary condi%on for stability of TCP – Consequences of over-‐sized window are much worse than having an under-‐sized window •  Over-‐sized window: packets dropped and retransmiced •  Under-‐sized window: somewhat lower throughput

•  Mul%plica%ve decrease – On loss of packet, divide conges%on window in half

•  Addi%ve increase – On success for last window of data, increase linearly

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Leads to the TCP “Sawtooth”

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t

Window

halved

Loss

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Prac%cal Details

•  Conges%on window – Represented in bytes, not in packets (Why?)

– Packets have MSS (Maximum Segment Size) bytes

•  Increasing the conges%on window –  Increase by MSS on success for last window of data

•  Decreasing the conges%on window – Never drop conges%on window below 1 MSS

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Receiver Window vs. Conges%on Window

•  Flow control –  Keep a fast sender from overwhelming a slow receiver

•  Conges%on control –  Keep a set of senders from overloading the network

•  Different concepts, but similar mechanisms –  TCP flow control: receiver window –  TCP conges%on control: conges%on window –  TCP window: min { conges%on window, receiver window }

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How Should a New Flow Start

35

t

Window

But, could take a long time to get started!

Need to start with a small CWND to avoid overloading the network.

“Slow Start” Phase

•  Start with a small conges%on window –  Ini%ally, CWND is 1 Max Segment Size (MSS) – So, ini%al sending rate is MSS/RTT

•  That could be precy wasteful – Might be much less than the actual bandwidth – Linear increase takes a long %me to accelerate

•  Slow-‐start phase (really “fast start”) – Sender starts at a slow rate (hence the name) – … but increases the rate exponen%ally – … un%l the first loss event

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10

Slow Start in Ac%on

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Double CWND per round-trip time

D A D D A A D D

A A

D

A

Src

Dest

D

A

1 2 4 8

Slow Start and the TCP Sawtooth

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Loss

Exponential “slow start”

t

Window

Why is it called slow-start? Because TCP originally had no congestion control mechanism. The source would just start by sending a whole receiver window’s worth of data.

Two Kinds of Loss in TCP

•  Timeout –  Packet n is lost and detected via a %meout –  E.g., because all packets in flight were lost – Aler the %meout, blas%ng away for the en%re CWND – … would trigger a very large burst in traffic –  So, becer to start over with a low CWND

•  Triple duplicate ACK –  Packet n is lost, but packets n+1, n+2, etc. arrive –  Receiver sends duplicate acknowledgments – … and the sender retransmits packet n quickly – Do a mul%plica%ve decrease and keep going

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Repea%ng Slow Start Aler Timeout

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t

Window

Slow-start restart: Go back to CWND of 1, but take advantage of knowing the previous value of CWND.

Slow start in operation until it reaches half of

previous cwnd.

timeout

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Repea%ng Slow Start Aler Idle Period

•  Suppose a TCP connec%on goes idle for a while – E.g., Telnet session where you don’t type for an hour

•  Eventually, the network condi%ons change – Maybe many more flows are traversing the link – E.g., maybe everybody has come back from lunch!

•  Dangerous to start transmisng at the old rate – Previously-‐idle TCP sender might blast the network – … causing excessive conges%on and packet loss

•  So, some TCP implementa%ons repeat slow start – Slow-‐start restart aler an idle period

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TCP Achieves Some No%on of Fairness

•  Effec%ve u%liza%on is not the only goal – We also want to be fair to the various flows – … but what the heck does that mean?

•  Simple defini%on: equal shares of the bandwidth – N flows that each get 1/N of the bandwidth? –  But, what if the flows traverse different paths? –  E.g., bandwidth shared in propor%on to the RTT

42

What About Chea%ng?

•  Some folks are more fair than others –  Running mul%ple TCP connec%ons in parallel – Modifying the TCP implementa%on in the OS – Use the User Datagram Protocol

•  What is the impact – Good guys slow down to make room for you –  You get an unfair share of the bandwidth

•  Possible solu%ons? –  Routers detect chea%ng and drop excess packets? –  Peer pressure? –  ???

43

Queuing Mechanisms

Random Early Detec%on (RED) Explicit Conges%on No%fica%on (ECN)

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12

Bursty Loss From Drop-‐Tail Queuing

•  TCP depends on packet loss –  Packet loss is the indica%on of conges%on –  In fact, TCP drives the network into packet loss – … by con%nuing to increase the sending rate

•  Drop-‐tail queuing leads to bursty loss – When a link becomes congested… – … many arriving packets encounter a full queue – And, as a result, many flows divide sending rate in half – … and, many individual flows lose mul%ple packets

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Slow Feedback from Drop Tail •  Feedback comes when buffer is completely full

– … even though the buffer has been filling for a while •  Plus, the filling buffer is increasing RTT

– … and the variance in the RTT •  Might be becer to give early feedback

– Get 1-‐2 connec%ons to slow down, not all of them

– Get these connec%ons to slow down before it is too late

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Random Early Detec%on (RED) •  Basic idea of RED

–  Router no%ces that the queue is gesng backlogged – … and randomly drops packets to signal conges%on

•  Packet drop probability – Drop probability increases as queue length increases –  If buffer is below some level, don’t drop anything – … otherwise, set drop probability as func%on of queue

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Average Queue Length

Prob

abili

ty

Proper%es of RED

•  Drops packets before queue is full –  In the hope of reducing the rates of some flows

•  Drops packet in propor%on to each flow’s rate – High-‐rate flows have more packets – … and, hence, a higher chance of being selected

•  Drops are spaced out in %me – Which should help desynchronize the TCP senders

•  Tolerant of burs%ness in the traffic – By basing the decisions on average queue length

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13

Problems With RED

•  Hard to get the tunable parameters just right – How early to start dropping packets? – What slope for the increase in drop probability? – What %me scale for averaging the queue length?

•  Some%mes RED helps but some%mes not –  If the parameters aren’t set right, RED doesn’t help – And it is hard to know how to set the parameters

•  RED is implemented in prac%ce –  But, olen not used due to the challenges of tuning right

•  Many varia%ons in the research community – With cute names like “Blue” and “FRED”…

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Explicit Conges%on No%fica%on •  Early dropping of packets

– Good: gives early feedback –  Bad: has to drop the packet to give the feedback

•  Explicit Conges%on No%fica%on –  Router marks the packet with an ECN bit – … and sending host interprets as a sign of conges%on

•  Surmoun%ng the challenges – Must be supported by the end hosts and the routers –  Requires 2 bits in the IP header for detec%on (forward dir)

•  One for ECN mark; one to indicate ECN capability •  Solu%on: borrow 2 of Type-‐Of-‐Service bits in IPv4 header

– Also 2 bits in TCP header for signaling sender (reverse dir) 50

Other TCP Mechanisms

Nagle’s Algorithm and Delayed ACK

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Mo%va%on for Nagle’s Algorithm

•  Interac%ve applica%ons –  Telnet and rlogin – Generate many small packets (e.g., keystrokes)

•  Small packets are wasteful – Mostly header (e.g., 40 bytes of header, 1 of data)

•  Appealing to reduce the number of packets –  Could force every packet to have some minimum size – … but, what if the person doesn’t type more characters?

•  Need to balance compe%ng trade-‐offs –  Send larger packets – … but don’t introduce much delay by wai%ng

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14

Nagle’s Algorithm •  Wait if the amount of data is small

– Smaller than Maximum Segment Size (MSS) •  And some other packet is already in flight

–  I.e., s%ll awai%ng the ACKs for previous packets •  That is, send at most one small packet per RTT

– … by wai%ng un%l all outstanding ACKs have arrived

•  Influence on performance –  Interac%ve applica%ons: enables batching of bytes – Bulk transfer: transmits in MSS-‐sized packets anyway

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vs.

ACK

Nagle’s Algorithm •  Wait if the amount of data is small

– Smaller than Maximum Segment Size (MSS) •  And some other packet is already in flight

–  I.e., s%ll awai%ng the ACKs for previous packets •  That is, send at most one small packet per RTT

– … by wai%ng un%l all outstanding ACKs have arrived

•  Influence on performance –  Interac%ve applica%ons: enables batching of bytes – Bulk transfer: transmits in MSS-‐sized packets anyway

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vs.

ACK

Turning Nagle Off!void !

tcp_nodelay (int s) !{ !

!int n = 1; !!if (setsockopt (s, IPPROTO_TCP, TCP_NODELAY, !! (char *) &n, sizeof (n)) < 0) !

warn ("TCP_NODELAY: %m\n"); !} !

Mo%va%on for Delayed ACK

•  TCP traffic is olen bidirec%onal – Data traveling in both direc%ons – ACKs traveling in both direc%ons

•  ACK packets have high overhead – 40 bytes for the IP header and TCP header – … and zero data traffic

•  Piggybacking is appealing – Host B can send an ACK to host A – … as part of a data packet from B to A

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TCP Header Allows Piggybacking

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Sequence number

Acknowledgment



Options (variable)

Data


15

Example of Piggybacking

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Data

Data+ACK

Data

A B

ACK

Data

Data + ACK

B has data to send

A has data to send

B doesn’t have data to send

Increasing Likelihood of Piggybacking

•  Example: rlogin or telnet – Host A types characters at prompt – Host B receives the character and executes a command

– … and then data are generated – Would be nice if B could send the ACK with the new data

•  Increase piggybacking –  TCP allows the receiver to wait to send the ACK

– … in the hope that the host will have data to send

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Data

Data+ACK

Data

A B

ACK

Data

Data + ACK

Delayed ACK

•  Delay sending an ACK – Upon receiving a packet, the host B sets a %mer

•  Typically, 200 msec or 500 msec

–  If B’s applica%on generates data, go ahead and send •  And piggyback the ACK bit

–  If the %mer expires, send a (non-‐piggybacked) ACK

•  Limi%ng the wait – Timer of 200 msec or 500 msec – ACK every other full-‐sized packet

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Conclusions •  Conges%on is inevitable

–  Internet does not reserve resources in advance –  TCP ac%vely tries to push the envelope

•  Conges%on can be handled –  Addi%ve increase, mul%plica%ve decrease –  Slow start, and slow-‐start restart

•  Ac%ve Queue Management can help –  Random Early Detec%on (RED) –  Explicit Conges%on No%fica%on (ECN)

•  Fundamental tensions –  Feedback from the network? –  Enforcement of “TCP friendly” behavior?

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