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Chapter 5: Transport LayerECE 610 Winter 2013
Dr. Mohamed Mahmoud
Department of Electrical and Computer Engineering
University of Waterloo
http://ece.uwaterloo.ca/~mmabdels/
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Outline
5.1 Introduction
5.2 Connectionless transport: UDP
5.3 Connection-oriented transport: TCP
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- Transport layer: end-to end layer not implemented in theintermediate routers
- At sender: receives message from application layer
encapsulate it in segment and send to network layer segmentation: breaks application messages into segments
(smaller pieces) and passes them to network layer
- At receiver: decapsulate segments - deliver messages to
application layer
Application Application
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- The transport layer is responsible for process-to-process
data delivery.
- Provide logical communication between application
processes running on different hosts
- Two processes communicate together - exchange messages
between applications (e.g., email, file transfer, the Web)
Types of data deliveries
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Duties of the transport layer
1- End-to-End Delivery
- The network layer treats each packet as an independent
entry, even those belonging to a single message.
- The transport layer oversees the end-to-end (source-to-
destination) delivery of an entire message maintains
communication streams between applications.
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2- Addressing
- MAC address: Data link level protocols need to know which
two computers within a network are communicating.
- IP address: Network level protocols need to know which
two computers within an internet are communicating.
- Port number: Transport level protocols need to know which
upper-layer protocols are communicating.
- Multiple processes may be run at a host each processmust have a unique identifier
3- Flow control- Flow control at this layer is performed end-to-end rather
than across a single link.
- Prevent the sender from overrunning the receiver
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1- Error Control
- Error detection and retransmission.
- Unlike error handling at the data link layer: this is end-to-end error control
2- Sequence Control
- At sender: An application layers messages is divided intosegments
4- Reliable Delivery:
- Aspects of reliable data transfer
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- At receiver: the original message is reassembled
- Segments should be properly reassembled at the receiver
- Transport layer adds a sequence number at each segment.
This number indicates the order for reassembly.
- Each segment carries a field that indicates whether it is thefinal segment or middle segment of a transmission.
3- Loss Control
- Sequence numbers allow the receivers transport layerprotocol to identify any missing segments and request
redelivery.
4- Duplication Control
- Sequence numbers allow the receiver to identify and discard
duplicate segments.
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5- Multiplexing
- Sender may send several segments destined to different
processes. Multiplexing: the segments should have enough
information to be distributed to the correct processes.
- Demultiplexing is needed at the receiver to distribute the
segments to the correct processes
5 - 7
Note:
- Handling process-to process communication cannot be
handled by the data link layer becausenodes would not know what happened two or more hops
ahead of them, e.g., packets dropped, machine crashed,
link failed Only ends can be sure of receipt/non-receipt ofa message
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Internet transport-layer protocols: TCP, UDP
1- TCP Transmission Control Protocol
- Reliable, in-order delivery - TCP uses acknowledgements
and retransmissions detect corrupted and missing data -
makes sure all packets are successfully received
- Congestion control: throttle sender when network
overloaded
- Flow control: sender wont overwhelm receiver
- Connection-oriented: establish connection before data
transfer
- Full-Duplex: data can flow in both directions simultaneously
- Segmentation and re-assemble segments in the right order
- Stream-of-bytes service: sends and receives a stream of
bytes not messages 5 - 8
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Motivation: IP layer provides a best-effort service - need to
build a reliable layer on top of IP
TCP Applications
- Applications require 100% reliable data transfer to beeffective all of the sent data arrive at the destination in
its original condition, in order for the data to be useful.
Any missing data could cause a corrupt communication
that is either incomplete or unreadable.
- Examples: WWW using HTTP, Electronic mail using SMTP,
File transfer using FTP
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2- UDP: User Datagram Protocol
- Unreliable (best-effort), unordered data transfer received
data can be out of order - corrupted duplicated missing, ..
- Connectionless: No connection establishment before data
transfer
- No flow control and no congestion control
- Full duplex
- UDP advantages
- UDP requires less overhead and offers faster performance
than TCP- No delay for connection establishment UDP sends data
without contacting the end host first
- No congestion control: UDP can blast away as fast asdesired 5 - 10
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- Small packet header size: only 8 bytes
- Simple: no connection state at sender and receiver No
allocation of buffers, parameters easier to handle many
clients at once
UDP Applications
1- Used for applications that can tolerate small amount of
packet loss and require low delay to be effective: Multimedia
applications, Internet telephony, real-time-video
conferencing, Audio
- For example, if one or two segments of a video stream fail
to arrive, it would only create a momentary disruption in the
stream. This may appear as distortion in the image but may
not even be noticeable to the user.5 - 11
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2- An application that sends just one message, like Domain
Name System (DNs).
- TCP sends several packets of overhead before one single
useful data message to establish a connection.
- This does not mean that applications that use UDP arealways unreliable. It simply means that these functions are
not provided by the Transport layer protocol and must be
implemented elsewhere if required.
- DNS will simply retry the request if it does not receive a
response, and therefore it does not need TCP to guarantee
the message delivery
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Creating a network application
-Write programs that run on end systems.
- Process: An instance of a program in execution.
- Processes on two hosts communicate over network by
Processes communicating across network
sending and receiving
messages, e.g., web serversoftware communicates with
browser software
- No need to write softwarefor network-core devices
application
- Network-core devices donot run user applications
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- Applications on end systems allows for rapid applications
development and propagation
- The process receives messages from, and sends messages
into the network through its socket
- A socket is the interface between the application layer and
the transport layer within a host.
- Sockets are the programming interface used to build
network applications over the internet.- Create sockets with port number 6600:
DatagramSocket mySocket1 = new DatagramSocket(6600);
- Socket API is a library in C functions.
- Socket is similar to a file descriptor controls a
communication end point
- Socket = API (application host or programming interface)5 - 14
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- Socket analogous to door
- Sending process shoves message out door and relies on
transport infrastructure on other side of door to transport
message to socket at receiving process
- Programmers can select which transport layer protocol (UDP
or TCP) to be used by the application and select few transport-
layer parameters (maximum buffer size, Maximum segment
size, starting sequence number of segment).
- Delivering the data: division of labor
- Network: Deliver data packet to the destination host based
on the destination IP address
- Operating system: Deliver data to the destination socket
based on the destination port number
- Application: Read data from and write data to the socket
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Addressing processes
- A single host may run several processes process must
have identifier
- A port address: A unique number assigned to an application
process as an address to receive or send data.- When a host receives a message, it needs to know which
process should receive the message.
- Port numbers are unique on each host cannot use sameport number twice with same address, otherwise, the
operating system cant demultiplex packets correctly
- Port number uniquely identifies the socket
- Operating system enforces uniqueness
- Port numbers: 16-bit integer
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Port numbers are divided in three ranges:
1- Well-known ports (numbers 0 to 1023)
Popular applications have well-known ports assigned by the
Internet Assigned Numbers Authority (IANA) See
http://www.iana.org/assignments/port-numbers
- e.g., port 80 for Web and port 25 for e-mail, 53 DNS, 21
FTP, 80 HTTP, 443 Secure HTTP(HTTPS)
- Client applications can be programmed to request aconnection to one of these ports and its associated service.
2- Registered Ports (numbers 1024 to 49151)
- Not controlled by IANA but registers and lists the uses of
these ports
e.g., 1812 RADIUS Authentication protocol, 1863 MSN
messengers5 - 17
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3- Dynamic or private ports (numbers 49152 to 65535)
Can be assigned dynamically to client applications when
initiating a connection. short-lived ports
Client picks an unused ephemeral (i.e., temporary) port
- Transport layer at the receiving host delivers data to thesocket
- There should be a unique identifier for each socket.
- Socket identifier is called socket address = IP address andport number, e.g., 192.168.2.12:80 identifies each
endpoint of a connection
- A socket pair for a TCP connection is the four-tuple thatdefines the a TCP connection (client IP, client port, server IP,
server port)
- A socket pair uniquely identifies the traffic between thehosts 5 - 18
While server processes have static port numbers assigned
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- While server processes have static port numbers assigned
to them, clients dynamically chooses a port number for each
conversation.
1) TCP server (ftp) with a passive open on port 21
2) Connection request from client to server
server{*:21, *:*}
10.19.0.115
listening socket
client
{10.3.3.137:49152,10.19.0.115:21}
10.3.3.137
server
{*:21, *:*}
10.19.0.115
listening socketconnection request to10.19.0.115, port 21
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3) Concurrent server has child handle client.
client
{10.3.3.137:49152, 10.19.0.115:21}
10.3.3.137
server
{*:21, *:*}
10.19.0.115
listening socket
serverchild
{10.19.0.115:21,10.3.3.137:49152}
connected socket
Main server spawns a child to handle each new connection
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4) Second client connection with same server.
client
{10.3.3.137:49152,10.19.0.115:21}
10.3.3.137
{*:21, *:*}
10.19.0.115
listening socket
server
child{10.19.0.115:21,10.3.3.137:49152}
client
{10.3.3.137:49153, 10.19.0.115:21}
server
child{10.19.0.115:21,10.3.3.137:49153}
connected socket
connected socket
server
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T i l li t
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Typical client program:-
1- Prepare to communicate: create a socket, determine
server address and port number, initiate the connection tothe server
2- Exchange data with the server: Write data to the socket,
read data from the socket
3- Close the socket
Typical server program:-
1- Prepare to communicate: create a socket, associate local
address and port with the socket, wait to hear from a client
(passive open), accept an incoming connection from a client2- Exchange data with the client over new socket: Receive
data from the socket, do stuff to handle the request (e.g.,
get a file), send data to the socket3- Close the socket 5 - 22
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Servers differ from clients
- Passive open- Prepare to accept connections
- but dont actually establish
- until hearing from a client
- Hearing from multiple clients
- Allowing a backlog of waiting clients- ... in case several try to communicate at once
- Create a socket for each client
- Upon accepting a new client
- create a new socket for the communication
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l l d d l l
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Multiplexing and demultiplexing
5 - 24
Multiplexing (at the sending node): the process of
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- Multiplexing (at the sending node): the process of
encapsulating messages from different applications sockets
with the header information and pass the segments to thenetwork layer
- The header information are later used for demultiplexing
- Demultiplexing (at the receiving node): The process of
delivering the received data segment to the correct
application (socket)
- Example: Suppose the following is running on the same
computer:
- Downloading a web page while transferring data throughFTP and two telnet sessions (provide a bidirectional
interactive text-oriented communication) are also running
- Transport layer receives packets from network layer for allfour processes 5 - 25
How demultiplexing works
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How demultiplexing works
- Each IP datagram has source IP address, destination IP
address, and one transport-layer segment. Each segment
has source and destination port numbers.
- Host uses IP addresses & port numbers to direct segment toappropriate socket
- TCP socket is identified by 4-tuple: (source IP address,
source port number, dest IP address, dest port number)
receiving host uses all four values to direct segment to
appropriate socket
- UDP: directs segment to socket with that the segments port
number IP datagrams with same destination port number,
but different source IP addresses and/or source portnumbers will be directed to same socket at destination 5 - 26
Connection-oriented demultiplexing: example
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transport
application
physical
linknetwork
P3transport
application
physical
link
P4
transport
application
physical
linknetwork
P2
source IP,port: A,9157
dest IP, port: B,80
source IP,port: B,80dest IP,port: A,9157
host: IP
address A
host: IP
address C
network
P6P5P3
source IP,port: C,5775dest IP,port: B,80
source IP,port: C,9157dest IP,port: B,80
server: IP
address B
Connection-oriented demultiplexing: example
- Three segments, all destined to IP address: B, destinationport: 80 are demultiplexed to different sockets
5 - 27
Connectionless demultiplexing: example
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transport
application
physical
link
network
P3transport
application
physical
link
network
P1
transport
application
physical
link
network
P4
source port: 9157
dest port: 6428
source port: 6428dest port: 9157
source port: 6428dest port: 9266
source port: 9266
dest port: 6428
- A server application that uses UDP serves only ONE request
at a time.- All other requests are stored in a queue waiting for service.
Connectionless demultiplexing: example
Port: 9157
Port: 6428
Port: 9266
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Outline
5.1 Introduction
5.2 Connectionless transport: UDP
5.3 Connection-oriented transport: TCP
UDP: User Datagram Protocol
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UDP: User Datagram Protocol
- Unreliable best effort service has no flow and error control
- A UDP segment can be lost, arrive out of order,
duplicated, or corrupted
- Reliable transfer over UDP: add reliability at application layer- Application writes a message to a UDP socket
- which is then encapsulated in a UDP datagram
- which is then sent to destination
- Connectionless: no handshaking between sender and
receiver - each segment is handled independently of others- It does not add anything to the services of IP except to
provide process-to-process communication.
- UDP runs up to 40% faster than TCP, under some conditions,because it does nothing 5 - 29
User datagram format
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User datagram format
- A fixed-size header of 8 bytes
- Source port: the sending port and the port to reply to if
needed. If not used, then it should be zero.
- Destination port: the destination port and is required.
- Total Length (in bytes) of UDP segment, including header. It
is between 8 and 65,535 bytes (8 byte header + 65527 bytesof data). 5 - 30
- Checksum: detect errors (e g flipped bits) in the entire
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Checksum: detect errors (e.g., flipped bits) in the entire
UDP segment.
Checked at receiver drop packet if error is detected
- Optional: The value sent for the checksum field is all 0s to
show that the checksum is not calculated.
Example: The following is a UDP header in hexadecimal
format: CB84000D001C001Ca. What is the source port number?
b. What is the destination port number?
c. What is the total length of the user datagram?
d. What is the length of the data?
e. Is the packet directed from a client to a server or viceversa? 5 - 31
Solution
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Solution
a. The source port number is the first four hexadecimal digits
(CB84)16 or 52100.
b. The destination port number is the second four hexadecimal
digits (000D)16
or 13.
c. The third four hexadecimal digits (001C)16 define the length of
the whole UDP packet as 28 bytes.
d. The length of the data is the length of the whole packetminus the length of the header, or 28 8 = 20 bytes.
e. Since the destination port number is 13 (well-known port), the
packet is from the client to the server.
5 - 32
O tli
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Outline
5.1 Introduction
5.2 Connectionless transport: UDP
5.3 Connection-oriented transport: TCP
- TCP is a connection-oriented protocol; it creates a virtual
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connection between two TCPs to send data.
- TCP uses flow and error control mechanisms at the transportlevel.
- Byte-stream: source application writes bytes - TCP sends
segments destination application reads bytes
- Each segment encapsulated in IP datagram
- Full duplex: bi-directional data flow in same connection
5 - 33
- Point-to-point one sender, one receiver
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p ,
- Reliable, in-order delivery of a stream of bytes
- Flow control: keep sender from overrunning receiver
Receiver limits the amount of data the sender can send at any
time
- Congestion control: Prevents the sender from overloading
network switches and routers
- Allows two application programs to form a connection, senddata in either direction, and then terminate the connection.
- TCP is reliable byte stream protocol that can create a
connection between any pair of processes on any pair ofInternet hosts
5 - 34
5.3.1 TCP segment format
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5.3.1 TCP segment format
5 - 35
- Source port (16 bits): identifies the sending port
- Destination port (16 bits): identifies the receiving port
- Sequence number field (32 bits):
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Sequence number field (32 bits):
- Contains a number that uniquely identifies the segment
- The identifier enables TCP receivers to identify when
parts of a communication stream are missing.
- Each TCP host self-assigns its own initial sequencenumber (ISN)
- ISN is set during connection establishment. It is the
sequence number of the actual first data byte.- For each segment, TCP host updates the sequence
numbers to point to the position of the segments first
data in the byte stream
- Acknowledgement number field (32 bits): indicates the
next expected sequence number from the other side of the
communications.5 - 36
H t A
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Host A
Host B
TCP Data
TCP Data
TCPHDR
TCPHDR
ISN (initial sequence number)
Sequencenumber = 1st
byte Ack sequencenumber = nextexpected byte
- Sequence Number/Acknowledgement Number Fields are
used to be sure each packet has arrived, place segments inorder, allow a specific segment to be acknowledged
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- Header length (Hlen): header length in 4-byte words - value
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ranges from 5 to 15
- Checksum (16 bits):
- The checksum is computed on the contents of the TCP
header and data
- Enables receiver to check if there is an error in the
segment.
- Window size (16 bits):
- Advertised window size in bytes. Space remaining in
receivers buffer and willing to accept. Number of bytes
sender can send before receiving an ACK.
- A window size of zero indicates that a sender should stop
transmitting the receivers TCP buffer is full
- Reserved (6 bits): for future use and should be set to zero5 - 38
Six 1-bit flags: play an important role in establishing,
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5 - 39
maintaining, and terminating a connection
- URG (urgent pointer): The urgent pointer should beexamined
- ACK(acknowledgment): ACK sequence number is valid
- PSH (push request): Do not delay delivery of data -
Inform receiver to send data to application as soon as
possible bypass buffering and pass data to upper layer.
- RST(Reset Request): Reset the connection (reject or
abnormal termination)
- SYN (Synchronization Request): Synchronize segmentfor setup - synchronize sequence numbers during
connection.
- FIN: Final segment for teardown - sender at end ofbyte stream - End of Transmission Marker
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5 - 40
- Urgent pointer (16 bits): if the URG flag is set, then this
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16-bit field is an offset from the sequence number indicating
the last urgent data byte.
- Options and padding: Rarely used - Padding (or dummy
bits) may be needed to bring options to 32-bit boundary - up
to 40 bytes of options.
- Maximum segment (MSS) Option: the maximum amount of
data receiver is willing to accept in each TCP segment
- Window Scale Option
- maximum window is 65,535 bytes (corresponding field in
TCP header occupies 16 bits)
-it can be scaled (left-shifted) by 0-14 bits providing a
maximum of 65,535 * 214 bytes (one gigabyte)
- Needed for high-speed connections or long delay paths5 - 41
5.3.2 Connection management
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- Connection-oriented data delivery has three stages:1- Connection establishment: ensure that both hosts are
ready and know the initial sequence numbers.
2- Data transfer
3- Connection termination: When data transfer is finished,
the hosts send signals to end the connection.
timeA
B
Connectionestablishment
Datatransfer
Connectiontermination
5 - 42
1- TCP Connection establishment
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- Three-way handshaking
Requesting computerResponding computer
1- Connection
request
2- Connectionconfirmation
3- Acknowledgementof confirmation
A B
5 - 43
1- Connection request message:
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- Objectives:
1- Ensure the destination host is available, has an active
service and is accepting requests on the destination port
number
2- Tells the destination that the initiator wants to open aconnection and its initial sequence number and port number
- Connection request from A to B, send a self-assigned initial
sequence number (e.g., 8000)
- SYN = 1 (remaining flags are 0)
- Source port (e.g., 2352), destination port (e.g., 80 for http)
2- Connection confirmation message:
- B tells A it accepts, and is ready to hear the next byte, Bs
Initial Sequence Number5 - 44
- Upon receiving this packet, A can start sending data with As
initi l eq en e n mbe + 1
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initial sequence number + 1
- ACK for message 1 (ACK flag = 1), ACK sequence= 8000+1
- Connection request from B to A (SYN = 1)
- Send initial sequence number (e.g., 1500)
- The acknowledgement number is that of the next segment
expected - it implicitly acknowledges all segments with
smaller numbers (cumulative ACK)
3- Acknowledgement of confirmation message:
- A tells B it is ok to start sending
- Upon receiving this message, B can start sending data with
Bs initial sequence number +1
- ACK for message 2 (ACK flag = 1), ACK sequence =1500+1
- The message may contain data5 - 45
- Client: connection initiator - user clicks on a hypertext link
- browser creates a socket and does a connect to trigger
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browser creates a socket and does a connect to trigger
the operating system to transmit a SYN
Socket clientSocket = new Socket("hostname", "port number");
- Server: contacted by client
Socket connectionSocket = welcomeSocket.accept();
- What if the SYN packet gets lost, e.g., packet is lost inside
the network, or server rejects the packet (e.g., listen queue
is full)?
No SYN-ACK arrives: sender sets a timer and wait for the
SYN-ACK and retransmits the SYN 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 adefault of 3 or 6 seconds 5 - 46
2- Data Transfer
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The next expectedbyte
The seq.number of the
first byte in thepacket
ACK the datareceiver fromthe server
5 - 47
- Sender: TCP transfers data as a continuous stream of
bytes
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bytes.
- Receiver: TCP checks that data has been received.
-Receiver: sends an acknowledgement giving the sequence
number of the byte that it expects next.
Segment sent when:
1. Segment full (MSS bytes, default 352),
2. Not full, but times out, or
3. Pushed by application.
5 - 48
3- Connection termination
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Using three-way handshaking
Disconnectionrequest
Disconnection
confirmation
Acknowledgementof confirmation
Requesting computerResponding computer
AB
closing
closing
closed
closed
5 - 49
1- Disconnection request message: (from A to B)
Cli l k b i ki li S k l ()
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- Client closes socket by invoking: clientSocket.close();
- Once client has sent all of the outstanding bytes, it sends
TCP FIN control segment to server with finish flag (FIN) =1
- The FIN segment consumes one sequence number if it does
not carry data.
- Finish (FIN) to close and receive remaining bytes
- Reset (RST) to close and not receive remaining bytes
2- Disconnection confirmation message: (from B to A)
- ACK = 1, FIN = 1- The segment consumes one sequence number if it does not
carry data.
3- Acknowledgement of confirmation: from B to A, ACK = 15 - 50
Half-close
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5 - 51- A sends all its data and closes the connection- B closes it later after finishing data transmission
Failure Recovery
- When the system that the transport entity is running on fails
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- When the system that the transport entity is running on fails
and subsequently restarts, the state information of all activeconnections is lost
- Affected connections become half open because the side that
did not fail does not realize the problem- Still active side of a half-open connection can close the
connection using a keep-alive timer
- In some cases, reset (RST) segments can be used to closeconnections quickly (an abnormal termination).
- Out of order segments could cause the FIN segment to
arrive before the last data segment a connection may beclosed before finishing data transfer
- To avoid this, the next sequence number after the last one is
assigned to FIN - each side must acknowledge the FIN of theother using this sequence number. 5 - 52
Netstat
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- Shows protocol, local address and port number, foreign
address and port number.
- Unexpected connections may mean there is a securityproblem. 5 - 53
5.3.3 TCP flow control
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Sliding window
- Flow control: sender wont overrun receivers buffer by
transmitting too much, too fast
- A sliding window is used to make data transmission more
efficient (pipelining see chapter 3) and to control the flow of
data so that the receiver does not become overwhelmed.
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Receivers buffer:
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- RcvBuffer: size of TCP Receive buffer
- RcvWindow: amount of spare room in buffer
- Application process may remove data from TCP socket buffer
slower than the TCP receiver is delivering (sender is sending)
- Or receiver host may be shared by many processes andcannot consume data received at the rate that sender host
sends.
- Speed-matching service: matching the sending rate to thereceiving applications drain rate
- Receiver: explicitly informs sender of (dynamically
changing) amount of free buffer space RcvWindow field in
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TCP ACK segment
- Sender: keeps the amount of transmitted and unACKed data
less than most recently received RcvWindow
- Guarantees receiving buffer doesnt overflow
- RcvWindow: Defines the amount of data that can be
transmitted without receiving an acknowledgement.
- Receiver can controls how many bytes are sent The
receiver can increase or decrease the size of the sender
window.
- Sliding window at the transport layer is byte oriented rather
than frame oriented.
- The initial window size is determined during the session
startup via the three-way handshake.5 - 56
- An acknowledgment can expand the size of the sender
window based on the sequence number of the acknowledged
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data segment.
- The senders window size is dynamic and controllable by
receiver.
- Flow control is achieved by forcing the sender to wait foran acknowledgment
Receiver Side
Advertised Window: Shrinks as data arrives and grows asthe application consumes data 5 - 57
Q: What is the value of the receiver window (RcvWindow)
for host A if the receiver, host B, has a buffer size of 5000
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bytes and 1000 bytes of received and unprocessed data?
A: The value of RcvWindow = 5000 1000 = 4000. Host B
can receive only 4000 bytes of data before overflowing its
buffer. Host B advertises this value in its next segment to A
Flow Control Congestion control
congestion control is about preventing too manylosses inside the network
Flow Control can improve reliability and performance of TCP
transmission
1- Prevent buffer overflow Reduce the chance of droppingsegments at destination if the buffer is full avoid waste of
resources
2- reduce the retransmissions avoid waste of resourcesin retransmissions
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- The receiver can temporarily shut down the window
- Window size = 0 receiver asks the sender to stoptransmitting data 5 - 59