chapter 3: transport layer transport layer transport layer services transport layer services...

CHAPTER 3: TRANSPORT LAYER

• Transport Layer Services• Establishing/Releasing Connections• Multiplexing/Demultiplexing• User Datagram Protocol (UDP)• Reliable Data Transfer• Flow Control• Error Control• Transmission Control Protocol (TCP)• Congestion Control

TRANSPORT LAYER SERVICES

Chapter 3CS 447

Hardware

Network Interface

Internet

Transport

Application

OUTPUT PROCESS: FROM APPLICATION TO HARDWARE

Chapter 3CS 447

Hardware

Operating System

Operating System

Application Programs

An application calls system routines to have UDP allocate an IP datagram, fill in the proper destination address, encapsulate the UDP packet, and send it to the IP process for delivery.

An application calls a system routine to buffer data, and notifythe output process.

The output process segments the data stream, encapsulates the TCP packets, and sends them to the IP process for delivery.

UDP OutputUDP Output

TCPOutputProcess

TCPTimer

Process

Net2 Hardware

Net2 Device

Net1 Hardware

Net1 Device

Netn Hardware

Netn Device

IPProcess

ControlMessagesControl

Messages

TCPOutput

Port

TCPOutput

PortQueue ForDatagramsSent To IP

Queue ForDatagramsSent To IP

Queues ForOutgoingPackets

Queues ForOutgoingPackets

TCP OutputTCP Output

When the IP process enqueues an outgoing datagram, a device driver routine is called to start the hardware. When the output has occurred, the interrupt handler dequeues the datagram and restarts the hardware to send the next datagram.

Source: Internetworking with TCP/IP, Volume II by Douglas E. Comer and David L. Stevens

INPUT PROCESS: FROM HARDWARE TO APPLICATION

Chapter 3CS 447

Hardware

Operating System

Operating System

Application Programs

UDP packets are delivered to UDP ports, from which application programs read them.

IP demultiplexes packets and delivers them to appropriate transport protocol.

Incoming IP datagrams are queued for the IP process.

Source: Internetworking with TCP/IP, Volume II by Douglas E. Comer and David L. Stevens

Net2 Hardware

Net2 Device

Net1 Hardware

Net1 Device

Netn Hardware

Netn Device

IPProcess

Queues ForPackets

Sent To IP

Queues ForPackets

Sent To IP

TCP InputTCP Input UDP InputUDP Input

TCPInput

Process

Semaphore-Controlled

Buffers

Semaphore-Controlled

Buffers

Port ForSegments

Sent To TCP

Port ForSegments

Sent To TCP

UDPDatagram

Ports

UDPDatagram

Ports

TCP packets are delivered to TCP buffers, from which application programs read them.

ESTABLISHING CONNECTIONS

Chapter 3CS 447

In the Transport Layer, the common approach for establishing an end-to-end connection involves a three-way handshake.

This approach helps to determine whether connection requests or acknowledgements are disposable duplicates or not.

Connection Request(Sequence #A)

Acknowledgement(Seq #B/Ack #A)

Data Packet(Seq #C/Ack #B) OLD DUPLICATE

Connection Request(Sequence #A)

Acknowledgement(Seq #B/Ack #A)

REJECT(NAK #B)

OLD DUPLICATEConnection Request

(Sequence #A)Acknowledgement(Seq #B/Ack #A)

REJECT(NAK #B)

OLD DUPLICATEData Packet

(Seq #C/Ack #D)

RELEASING CONNECTIONS

Chapter 3CS 447

The three-way handshake is also used to release an end-to-end connection, since it facilitates the detection of lost disconnect

messages and/or ACKs.

Disconnect Request(Start Sender Timer)

Disconnect Request(Start Receiver Timer)

Disconnect ACK(Connection Released)



Disconnect ACK Connection Released

When Timer Expires

Disconnect Request(Start Sender Timer)Disconnect Request

(Start Receiver Timer)

Disconnect ACK(Connection Released)




Disconnect Request(Start Sender Timer)Disconnect Request(Start Sender Timer)Disconnect Request(Start Sender Timer)


Connection Released

When Timer Expires

Connection Released

When Timer Expires N

Times


TRANSPORT LAYER COMMUNICATION

Chapter 3CS 447

The Transport Layer only deals with logical end-to-end message transport, but those messages are physically relayed through

intermediary devices operating at lower layers.

INTERNET TRANSPORT-LAYER PROTOCOLS

Chapter 3CS 447

Transmission Control Protocol (TCP)

• Connection-Oriented (end-to-end session is set up in advance)

• Reliable, In-Order Delivery (packets are numbered and any missing packets are sent again)

• Flow Control (transmission rate adjusted to receiver’s capacity to receive)

• Congestion Control (throttles traffic to avoid bottlenecks)

User Datagram Protocol (UDP)

• Connectionless (no end-to-end session is set up)

• Unreliable, Unordered Delivery (depends on IP for “best delivery”)

• No Congestion Avoidance (could hog excessive bandwidth)

MULTIPLEXING

Chapter 3CS 447

Depending on the application layer service being used, a source host specifies a 16-bit source port number and a 16-bit destination port number, attaching a header to the outgoing application information

that includes these port numbers.

Transport Layer Segment

SourcePort #

Destination

Port #Other Header Fields Application Layer Payload

The endpoint in a Transport Layer communication is called a socket, with the port number and IP address comprising the socket address.

Any specific Transport Layer communication uses two sockets, the source port and IP address, and the destination port and IP address,

all of which together comprises a socket pair.

DEMULTIPLEXING (CONNECTIONLESS)

Chapter 3CS 447

In connectionless Transport Layer service (e.g., UDP), sockets are created with port numbers that are local to the host.

Process 1 Process

2Process

3

DatagramSocket clientSocketA = new DatagramSocket(8945) DatagramSocket clientSocketA = new DatagramSocket(9601)

DatagramSocket serverSocketC = new DatagramSocket(6734)

Port 8945 Port

9601Port 6734

8945 6734 payload,etc. 6734 8945 response,etc. 9601 6734 payload,etc.6734 9601 response,etc.

Note that IP datagrams with different source IP addresses and/or source port numbers may be directed to same socket.

DEMULTIPLEXING (CONNECTION-ORIENTED)

Chapter 3CS 447

In connection-oriented Transport Layer service (e.g., TCP), a receiving host uses all four values (the source and destination ports, and the

source and destination IP addresses) to direct a segment to the appropriate socket.

Process 1 Process

3

Process 5

Port 7530 Port

4378Port 8641

Process 4

Process 6 Process

2

Port 9256

7530 8641 payload,etc.E.F.G.H W.X.Y.Z9256 8641 payload,etc.L.M.N.O W.X.Y.Z4378 8641 payload,etc.L.M.N.O W.X.Y.Z

A server host may support several simultaneous TCP sockets (each socket identified by its own 4-tuple), and web servers have different

sockets for each connecting client.

USER DATAGRAM PROTOCOL

Chapter 3CS 447

Source Port Destination Port

Message Length Checksum

UDP provides an unreliable delivery service using IP to transport messages between machines.

As an unreliable service, it supports:

• No flow control and no congestion control

• No acknowledgements, no timers, and no retransmissions

• Error recovery, using the same checksums as TCP

Source Port: Application’s UDP port number at the source station.

Message Length: # of bytes in the UDP datagram, including header and data.

Destination Port: Application’s UDP port number at the destination station.


Message Length Checksum

Checksum: 1’s complement of 1’s complement sum of all 16-bit words in header, data, & “pseudoheader”.

The UDP Header

An application using UDP accepts full responsibility for handling reliability, including: message loss, duplication, delay, out-of-order

delivery, and loss of connectivity!

UDP/TCP PSEUDOHEADER

Chapter 3CS 447

Source Address

Transport Segment Length00000000

Destination Address

Protocol #

Destination Address: IP address of the destination station.

Protocol Number: IP’s identification of the protocol type being used (i.e., 6 for TCP, 17 for UDP).

Source Address: IP address of the source station.

00000000: Padding.

Segment Length: Number of bytes in the Transport segment, including header and data.

Source Address

Transport Segment Length00000000

Destination Address

Protocol #

When the destination receives an IP datagram, IP passes the source and destination IP addresses, the datagram’s total length, and its protocol ID, to

the Transport Layer, along with the Transport segment itself.

The Transport protocol formulates the pseudoheader, and then calculates and checks the checksum.

Note that the pseudoheader is not counted in the segment length (since it’s not part of the segment!) and is not transmitted!

The Transport protocol uses the pseudoheader as part of its checksum calculation in order to verify both the source and destination sockets when

the Transport segment is delivered.

RELIABLE DATA TRANSFER

Chapter 3CS 447

A Transport protocol is considered reliable if it delivers all bits uncorrupted and in the correct order, regardless of how unreliable the

underlying network is.

Common Reliability Concerns

w/Common Solutions

Error Control

Use ACKs, NAKs, Timeouts

ACK/NAK Loss Or Corruption

Resend Packet

Duplicate Packets Due To

ResendUse Sequence

Numbers

Duplicate Packets Due To

TimeoutUse Sequence

Numbers

Flow Control

Automatic Repeat Request (ARQ)

Receiver Burdened With Bookkeeping

Use Go-Back-N

Pipeline Burdened With

Retransmissions

Use Selective Repeat

Congestion Control

Vary Transmission Rates

Inability To Locate Precise

ProblemMultiplicative

Decrease Congestion Avoidance

Inability To Recognize RecoveryAdditive Increase

Slow-Start Recovery

Segment 0

ACK 1Segment 1

ACK 2Segment 2

ACK 3Segment 3

ACK4

Segment 0

ACK1

Segment 4

ACK5

Segment 1

ACK2

Segment 5 ACK 6Segment

6NAK 6Segment

6NAK 6Segment

6ACK 7Segment

7ACK 8Segment

8Segment

8Segment

8ACK 9Segment

9

FLOW CONTROL

Chapter 3CS 447

When an endstation transmits segments faster than the recipient can “digest” them, either due to a lack of memory capacity or a

preoccupation with other tasks, the recipient will usually just discard the excess frames.

The “Stop-and-Wait” approach to Transport Layer flow control has the recipient respond with an acknowledgement (ACK) whenever it has

finished dealing with the previous transport segment and is ready to receive the next segment.

• “Piggybacking” ACKs on segments going the opposite direction on the link, in order to reduce channel utilization.

• Sending negative acknowledgements (NAKs) whenever a segment is corrupted or not received in a timely fashion.

• Retransmitting a segment automatically whenever an ACK is not received in a timely fashion.

Variations on this approach include:

SLIDING WINDOWS

Chapter 3CS 447

By increasing the memory on each endstation and using a more sophisticated algorithm for keeping track of which Transport

segments have and haven’t been accepted, traffic can flow more smoothly between the endstations.

The “sliding windows” approach sets a particular window size for the total number of segments that the receiving station has either not received, not

acknowledged, or not delivered to the current application.

Every time the receiver transmits a segment to the sender, it includes a field advertising the number of slots in the window that are not

occupied by segments that have been received and acknowledged, but not yet delivered.

That number serves as a limit on how many more segments the sender can send to the receiver.

Each segment is numbered with a “sequence number” so lost or duplicate segments can be determined, and the sequence

numbers cycle back to zero to start repeating when the likelihood decreases that duplicate numbers will occur.

ERROR CONTROL

Chapter 3CS 447

When a Transport segment is determined to be corrupted, there are two principal response alternatives.

When a Transport segment is determined to be corrupted, there are two principal response alternatives.

Go-Back-NHave the sending station resend

the faulty segment and all subsequent segments.

Go-Back-NHave the sending station resend

the faulty segment and all subsequent segments.

• Simplifies processing on the endstations.

• Yields a lot of unnecessary retransmissions.

• Simplifies processing on the endstations.

• Yields a lot of unnecessary retransmissions.

Selective RepeatHave the sending station resend only the segment that was faulty.

Selective RepeatHave the sending station resend only the segment that was faulty.

• More productive use of bandwidth.

• More sophisticated (i.e., expensive) endstations.

• More productive use of bandwidth.

• More sophisticated (i.e., expensive) endstations.

Sliding Window Size

4

Current Window Gap

4

Last Sent Segment -

Last Received Segment -

Last Acknowledged Segment

-

Sliding Window Size

4

Current Window Gap

3

Last Sent Segment 0

Last Received Segment 0


-

Sliding Window Size

4

Current Window Gap

2

Last Sent Segment 1



-

Sliding Window Size

4

Current Window Gap

1

Last Sent Segment 2



-



1

Sliding Window Size

4

Current Window Gap

3

Last Sent Segment 2

Sliding Window Size

4

Current Window Gap

2

Last Sent Segment 3

Sliding Window Size

4

Current Window Gap

1

Last Sent Segment 4

Sliding Window Size

4

Current Window Gap

0

Last Sent Segment 5

Sliding Window Size

4

Current Window Gap

4

Last Sent Segment 2

Sliding Window Size

4

Current Window Gap

3

Last Sent Segment 3

Sliding Window Size

4

Current Window Gap

2

Last Sent Segment 4

Sliding Window Size

4

Current Window Gap

1

Last Sent Segment 5

Sliding Window Size

4

Current Window Gap

0

Last Sent Segment 6

Sliding Window Size

4

Current Window Gap

3

Last Sent Segment 6

Sliding Window Size

4

Current Window Gap

2

Last Sent Segment 7

Sliding Window Size

4

Current Window Gap

1

Last Sent Segment 0

Sliding Window Size

4

Current Window Gap

0

Last Sent Segment 1

Sliding Window Size

4

Current Window Gap

4

Last Sent Segment 7

Sliding Window Size

4

Current Window Gap

3

Last Sent Segment 0

Sliding Window Size

4

Current Window Gap

2

Last Sent Segment 1

Sliding Window Size

4

Current Window Gap

1

Last Sent Segment 2

Sliding Window Size

4

Current Window Gap

0

Last Sent Segment 3



1



1



1



2



2



2



2



2



5



5



5



5



7



7



7



7



7

Segment 0

ACK 2 Segment 1

Segment 2

Segment 3

Segment 4

Segment 5

Segment 3

Segment 4

Segment 5

NAK 3 Segment 6

Segment 7

Segment 0

ACK 6NAK 0 Segment 1

Segment 0

Segment 1

Segment 2

Segment 3

Sliding Window Size

4

Current Window Gap

4

Sent Segments -

Received Segments -


-

Segment 0

ACK 2 Segment 1

Segment 2

Segment 3

Segment 4

Segment 5

Segment 3

NAK 3 Segment 6

Segment 7

Segment 0

ACK 5NAK 0 Segment 1

Segment 0

Segment 2

Segment 3

Sliding Window Size

4

Current Window Gap

3

Sent Segments 0

Received Segments 0


-

Sliding Window Size

4

Current Window Gap

2

Sent Segments 0,1

Received Segments 0,1


-

Sliding Window Size

4

Current Window Gap

1

Sent Segments 0,1,2

Received Segments 0,1,2


-

Sliding Window Size

4

Current Window Gap

3

Sent Segments 2

Received Segments 2


1

Sliding Window Size

4

Current Window Gap

2

Sent Segments 2,3



1

Sliding Window Size

4

Current Window Gap

1

Sent Segments 2,3,4



1

Sliding Window Size

4

Current Window Gap

0

Sent Segments 2,3,4,5

Received Segments 2,3,4,5


1

Sliding Window Size

4

Current Window Gap

1

Sent Segments 4,5



2

Sliding Window Size

4

Current Window Gap

1

Sent Segments 4,5,3



2

Sliding Window Size

4

Current Window Gap

0




2

Sliding Window Size

4

Current Window Gap

2

Sent Segments 5,6



4

Sliding Window Size

4

Current Window Gap

1

Sent Segments 5,6,7



4

Sliding Window Size

4

Current Window Gap

0




4

ACK 7

Sliding Window Size

4

Current Window Gap

2

Sent Segments 7,0



6

Sliding Window Size

4

Current Window Gap

1

Sent Segments 7,0,1



6

Sliding Window Size

4

Current Window Gap

0




6

Sliding Window Size

4

Current Window Gap

1

Sent Segments 1,2



7

Sliding Window Size

4

Current Window Gap

1

Sent Segments 1,2,0



7

Sliding Window Size

4

Current Window Gap

0




7

TRANSMISSION CONTROL PROTOCOL

Chapter 3CS 447

To provide reliable Transport service, TCP must use a

much more elaborate header

than UDP.

Source Port: TCP port number of the application at the source station.

Acknowledgement Number: Sequence number of the next Transport segment expected from the destination.

Reserved: Not currently used.

Sequence Number: Position in the sender’s byte stream of this Transport segment.Destination Port: TCP port number of the application at the destination station.

Header Length: Total length of the header (in 32-bit multiples).

Window Size: Current sliding window size (in bytes) - for end-to-end flow control.

Urgent Pointer: Position in the Transport segment where urgent data ends.

Code Bits: 6 flags: URG (Is Urgent pointer field valid?), ACK (Is Ack. # Valid?), PSH (Push delivery w/o waiting for full buffer), RST (Reset connection), SYN (Synchronize sequence #s), FIN (End of sender’s byte stream)

Checksum: 1’s complement of 1’s complement sum of 16-bit words in header, data, & pseudoheader.

Options & Padding: For example, Maximum-Segment-Size, Selective-Repeat, Delay-ACKs.


Sequence Number

Window SizeHdrLen

Checksum

Options & Padding

Acknowledgement Number

Urgent Pointer

Reserved Code Bits


Sequence Number

Window SizeHdrLen

Checksum

Options & Padding

Acknowledgement Number

Urgent Pointer

Reserved Code Bits

TCP CONGESTION CONTROL

Chapter 3CS 447

Flow control deals with one pair of endstations adjusting transmission rates to accommodate the receiver’s capacity, but congestion control deals with a station’s perception that it might be contributing to an

overall network traffic problem.

How does a station “perceive” a congestion problem?

ACK#527

ACK#527

ACK#527

Duplicate AcknowledgementsWhile ACKs usually indicate that something is successfully getting through the network, duplicate ACK numbers indicate that the segment in question has been

lost or corrupted.

Acknowledgement TimeoutsIf no ACK is received in the

required time, then it is unknown whether anything is getting through the network

and a problem definitely exists.

MULTIPLICATIVE DECREASE CONGESTION AVOIDANCE

Chapter 3CS 447

Since endstations cannot pinpoint the cause of perceived congestion problems, TCP dictates that they respond dramatically, as if they are

the source of the problem.

The standard TCP response to a perceived congestion

problem is to have the endstation try to avoid

contributing to the problem by setting

ssthresh to ½cwnd and then resetting cwnd to

mss.

This prevents the station from having too many segments, without ACKs, and from returning to a high transmission

rate, until the traffic problem has dissipated.

TCP handles this response by means of three values:• The maximum TCP segment size (mss) that the receiving endstation can

accommodate.• The slow-start threshold (ssthresh) that prevents a station from

recovering too quickly after it has perceived congestion.• The congestion window (cwnd) that limits how much unacknowledged

traffic a station can have on the network.

old cwnd

old ssthres

h

new cwnd

new ssthres

h

ADDITIVE INCREASESLOW-START RECOVERY

Chapter 3CS 447

While TCP’s Congestion Avoidance scheme responds severely to bad news about congestion, TCP responds very cautiously to good news

about successful transmissions.

Whenever a positive ACK is received, the station increases cwnd by 1mss for each acknowledged segment until cwnd reaches ssthresh.

To prevent a recurrence of the congestion problem, once cwnd reaches ssthresh, cwnd only increases by 1 mss for every passage of rtt milliseconds

(where rtt is the estimated round-trip time for a segment to successfully travel across this connection to its destination and for an ACK to return).

chapter 3: transport layer transport layer transport layer services transport layer services...

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