Download 9) Ch7 Transport Layer

The TCP/IP Model
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An exchange using the TCP/IP model
Application
Datagram
Frame
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Type of Addresses in TCP/IP
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Relationship of layers and addresses in TCP/IP
Port number:
Well-known port no.: 0 – 1023
Dynamic port no.: 1024 – 65535
IP Address:
192.168.200.4
MAC Address:
31:5C:BB:63:2A:D1
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Some well known port numbers
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IP Packet (L3) and Ethernet Frame (L2)
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Port numbers (L4), IP Packet (L3) and
Ethernet Frame (L2)
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Transport Layer
Process-to-Process Delivery:
UDP & TCP
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Transport layer (TL)
a) Transport layer: Process-to-Process delivery. (A process is an
application/message program running on a host).
b) Network layer: source-to-destination delivery (of individual packets to
be treated independently). No relationship btw those packets.
c) Transport layer ensures that packets belong to an application arrive
intact and in order, overseeing both error control and flow control.
d) A transport layer protocol can be either:
• Connectionless: treats each segment as independent packet and
delivery to TL at destination host. (UDP)
• Connection-oriented: TL makes connection with destination host
prior to packet delivery. (TCP, SCTP – not covered here)
e) A message is usually divided into several segments. UDP treat each
segment separately (non-related), while TCP creates a relationship
between the segments using sequence numbers.
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f) Flow and error control in TL is performed end to end.
Process-to-process Communication
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Position of UDP, TCP, and SCTP in TCP/IP suite
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Transport Layer Addressing using Port Number
a) Client/Server paradigm: A process from Client (local host) needs a service
from a process running on Server (remote host).
b) Since local or remote host can run several processes, need to define:
• Local host/Local process & Remote host/remote process
c) TL addressing using Port number (16 bit: 0 to 65535)
• To choose among multiple processes running on destination host.
• destination port number – delivery / Source port number – for reply.
d) Client port number can be randomly assigned (e.g. ephemarel port number
= 52000), but Server port number must be fixed for a server process: (e.g. 12
well-known port number = 13).
Multiplexing / Demutiplexing
a) At sender site, there may be several processes that need to send packets,
however, there is only one transport protocol at any time. (Many-to-1).
b) Multiplexing at TL accepts packets from different processes, differentiated
them by port numbers. After adding the header, TL passes the packet to NL.
c) At the receiver site, the relationship is the opposite (1-to-many) that requires
Demultiplexing process.
d) After error checking and dropping of header, TL delivers each message to
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appropriate process based on port number.
Reliable service using Error control
• Reliability at the data link layer is between two nodes (pink links)
• Reliability at transport layer ensures end-to-end reliability.
• Network layer is unreliable (best effort delivery) because it only concerns
about routing to appointed address.
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User Datagram Protocol
(UDP)
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User Datagram Protocol (UDP)
a) The User Datagram Protocol (UDP) is called a connectionless,
unreliable transport protocol. It does not add anything to the services
of IP except to provide process-to-process communication instead of
host-to-host communication. Very limited error checking.
b) Why needed UDP if so unreliable? It’s a simple protocol using
minimum overhead, hence fast delivery. e.g. a process wants to send a
small message & doesn’t care about it reliability.
c) UDP packets have a fixed-size header of 8 bytes.
UDP length = IP length – IP header’s length
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Encapsulation and decapsulation
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Multiplexing and demultiplexing
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Transmission Control
Protocol (TCP)
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Transmission Control Protocol (TCP)
a) The Transmission Control Protocol (TCP) is called a connectionoriented, reliable transport protocol.
b) Like UDP it is a process-to-process protocol that uses port numbers.
c) Unlike UDP, TCP creates a virtual connection between two TCPs to
send data. In addition, it also uses flow and error control (Reliable).
d) TCP is also reliable for stream-oriented protocol: allows sending and
receiving streams of bytes that are related in a ‘virtual tube’.
e) The segments of packet are related using a sequence number.
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TCP
a) TCP offers full duplex services, in which data can flow in both
direction at the same time.
b) Each TCP then has a sending and receiving buffer and segments
move in both directions. (mostly for flow and congestion control)
c) When site A wants to send and receive data from another site B:
• The two TCPs establish a connection between them
• Data are exchanged in both directions
• The connection is terminated when finish
d) The connection is virtual, not physical.
e) TCP keeps track of the segments by sequence and ACK numbers.
f) Some TCP segments can carry a combination of data and control
information (Piggy-backing), using a sequence number and ACK.
g) These segments are used for connection establishment, termination
or abortion.
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Sequence Number
The bytes of data being transferred in each connection are
numbered by TCP to establish the relationship between data
bytes/segments being sent.
The numbering starts with a randomly generated number btw
0 to 232-1. (not necessarily from zero).
The sequence number is actually a range of bytes.
The value in the sequence number field of a segment defines
the number of the first data byte contained in that segment.
The byte numbering is also used for flow and error control.
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Example
Suppose a TCP is transferring a file of 5000 bytes. The 1st
byes is 10,001. What are the sequence number for each
segment if data are sent in 5 equal segments?
Solution
The following shows the sequence number for each segment:
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Example
Imagine a TCP connection is transferring a file of 6000
bytes. The first byte is numbered 10010. What are the
sequence numbers for each segment if data is sent in five
segments with the first four segments carrying 1,000
bytes and the last segment carrying 2,000 bytes?
Solution
The following shows the sequence number for each segment:
Segment 1  10,010 (10,010 to 11,009)
Segment 2  11,010 (11,010 to 12,009)
Segment 3  12,010 (12,010 to 13,009)
Segment 4  13,010 (13,010 to 14,009)
Segment 5  14,010
(14,010 to 16,009)
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ACK Number
a) TCP is full-duplex where two parties/sites can send and receive
data at the same time.
b) Each party/site numbers the bytes (usual with a different starting
byte/sequence number).
c) The sequence number in each direction shows the number of the
1st byte carried by this segment.
d) Each party also uses acknowledgement number defines the
number to confirm the bytes it has received.
e) The value of the acknowledgment field in a segment defines the
number of the next byte a party expects to receive.
f) The acknowledgment number is cumulative, which means that the
party takes the number of the last byte that it has received safely
and adds ‘1’ to it and announces this sum as the ACK number.
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TCP header format
20 (if no option) to 60 bytes header
btw 5 & 15 (*4)
(Receiving)
6 different Control fields
For urgent data
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Control field
Description of flags in the control field
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TCP Connection
a) TCP is connection-oriented transport protocol that establishes a
virtual path between source and destination. i.e. Using a single
virtual pathway for entire message, facilitates the ACK process and
retransmission of damaged or lost frames or frames arrive out of
order.
b) Although TCP uses service of IP (connectionless) to deliver
individual segments, it has full control of the connection. IP is
unaware of any lost or retransmission of packets but TCP does. TCP
can holds it until the missing segments arrive.
c) In TCP, connection-oriented transmission requires 3 phases:
• Connection Establishment
• Data Transfer
• Connection Termination
d) Usually involves request and acknowledge procedures in all above
phases.
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TCP Flow Control
a) Unlike UDP, TCP provides the flow control mechanism.
b) The receiver controls the amount of data that are to be sent by the
sender, to prevent overflowing at destination (by announcing the
value of window size in the window size field of the TCP header).
c) Similar to data link layer, TCP uses sliding window (SW) and
numbering system to handle flow control to make transmission more
efficient as well as to control the flow of data.
d) However, this is done on end-to-end basis. The SW protocol is
something btw Go-Back-N and Select Repeat.
e) Two differences from the Data Link Layer:
a) SW in TCP is byte oriented, but SW in DL is frame oriented
b) TCP SW is variable size, but DL SW is fixed size.
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Sliding window
moving right
wall to the left
moving left wall
to the right
moving right
wall to the right
The 3 activities: opening, closing & shrinking are in the control of the
receiver (depending on congestion in the network), not the sender.
Sender must obey the commands of the receiver in this matter.
Opening allows more new bytes in the buffer that are eligible for sending.
Closing means some bytes have been acknowledged and the sender need
not to worry about then anymore.
Shrinking means shorten the size of window for congestion control purpose30
TCP header format
Window size field defines the size of the window in bytes, that the other party
must maintain. 16 bits can allow upt0 65535 bytes. This is normally refer to the
receiver window (rwnd) and is determined by the receiver.
The sender must obey the dictation of the receiver in this matter
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TCP Error Control
• Similar to UDP, TCP ensure reliable delivery using error control.
• This means that an application program that delivers a stream of data
to TCP relies on TCP to deliver the entire stream to the other side, in
orderly manner, without any error, lost or duplication.
• Error control includes mechanism for detecting corrupted segments,
lost segments, out-of-order segments and duplicated segments.
• It also includes mechanism for correcting errors after they are
detected, which is achieved through the use of:
• Checksum: to check for corrupted segment and discard it.
• Acknowledgement (ACK): to confirm the receipt of segments.
• Time-out (TO): Timer set for retransmission of segments.
• The heart of error control mechanism is Retransmission.
• The retransmission happens when the TO timer is expired: meaning
no receipt of ACK segment from opposite party.
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• No retransmission and timer set for an ACK segment.
TCP header format
Error Control fields: Checksum, Acknowledgement number
Lost and corrupted segments are treated the same way by the receiver. Both are
treated as lost!!! (Lost segments discarded somewhere in network by some
routers and corrupted segments discarder by receiver itself).
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Congestion Control &
Quality of Service
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Congestion Control & QoS
a) Congestion control & QoS are two issues that bound together
closely; improving one means improving the other or ignoring
one also ignoring the other.
b) Not only related to just Transport layer but all 3 layers
involved:
a) Data Link layer
b) Network layer
c) Transport layer
c) Need to think of them as co-operated property.
d) Involve directly with managing data traffic
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DATA TRAFFIC
• The main focus of congestion control and quality of
service is data traffic.
• In congestion control we try to avoid traffic congestion.
• In quality of service, we try to create an appropriate
environment for the traffic.
• So, before talking about congestion control and quality
of service, we discuss the data traffic itself.
Traffic Descriptor
Traffic Profiles
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Traffic descriptors
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Three traffic profiles
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CONGESTION
Congestion in a network may occur if the load on the
network—the number of packets sent to the network—
is greater than the capacity of the network—the number
of packets a network can handle.
Congestion control refers to the mechanisms and
techniques to control the congestion and keep the load
below the capacity.
Congestion happens in any system that involves waiting
or queuing.
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Queues in a router
• Two issues:
• If the rate of packet arrival is higher than the packet processing rate,
the input queues become longer and longer
• If the packer departure rate is lesser than the packet processing, the
output queues become longer and longer.
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Packet performance:
Packet delay and throughput as functions of load
• Congestion Control involves two factors that measure the performance
of a network:
• Delay: queuing due to processing delay and propagation delay
• Throughput: number of packet passing thru in a unit of time
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TCP assumes that the cause of
a lost segment is due to
congestion in the network.
If the cause of the lost segment
is congestion,
retransmission of the segment
not only does not remove the cause,
it aggravates it.
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CONGESTION CONTROL
• Congestion control refers to techniques and mechanisms
that can either prevent congestion, before it happens, or
remove congestion, after it has happened.
• In general, we can divide congestion control mechanisms
into two broad categories:
•Open-loop congestion control (prevention) and
•Closed-loop congestion control (removal).
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Congestion control categories
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Congestion Control in TCP
a) So far, we assume that it is only receiver can dictate to the sender the
size of its window and network entity has been ignored. If the network
cannot deliver the data as fast as they are created by the sender, it
must tell the sender to slow down.
b) In addition to the receiver, the network is a second entity that
determines the size of the sender’s window. Hence, the sender’s
window size is determined not only by the receiver but also by
congestion in the network. Window size = min(rwnd,cwnd)
c) TCP’s general policy for handling congestion is based on 3 phases:
• Slow start: Exponential Increase
• Congestion avoidance: Additive Increase
• Congestion detection: Multiplicative Decrease
d) Sender starts with a slow rate but then increase the rate rapidly until a
threshold, then reduce it to a linear rate to avoid congestion. Finally, if
congestion is detected, the sender goes back to the slow-start phase. 49
1. Slow start, exponential increase
In the slow-start algorithm, the size of the congestion window
increases exponentially until it reaches a threshold.
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2. Congestion avoidance, additive increase
In the congestion avoidance algorithm, the size of the congestion
window increases additively until congestion is detected.
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Congestion example
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QUALITY OF SERVICE (QoS)
• The main focus of congestion control & QoS is traffic.
• In congestion control we try to avoid traffic congestion.
• In quality of service, we try to create an appropriate
environment for the traffic.
• QoS is an internetworking issue that has been discussed
more than defined. We can informally define quality of
service as something a flow seeks to attain.
Flow Characteristics
Flow Classes
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Flow characteristics
• Reliability: vital characteristic that a flow needs. Losing reliability means
losing a packet or ACK, which entails retransmission. Some application
needs reliability more than others; e.g. email, file transfer and Internet
access require reliable transmission more than audio conferencing.
• Delay: degree of tolerance for later packet. Audio conferencing needs
minimum delay but delay in file transfer or email is less crucial.
• Jitter: variation in delay for packets belonging to the same flow. High
jitter means the difference between delay is large. Low jitter means low
variation of delay between packets.
• Bandwidth: Different application needs different bandwidth. Video
transmission needs millions of bps to refresh the screen while email
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needs minimum bandwidth to send a file.
Techniques to improve QoS
There are four common methods:
1.scheduling,
2.traffic shaping,
3.resource reservation
4.admission control
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