Transcript Lecture - 13

Overview
This Lecture
» Internet Protocols (4)
» Source: chapter 15
This Lecture
» TCP/UDP (1)
» Source: chapter 17
Next Lecture
» TCP/UDP (2)
» Source: chapter 17
TELE202 Lecture 13 TCP/UDP (1)
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Lecturer Dr Z. Huang
Transport layer
The transport layer is at the heart of
the whole protocol hierarchy
» It is the first layer which does not involve
intermediate nodes. It just involves end-toend communication.
» Its task is to provide a reliable, cost-effective
data transport from the source machine to
the destination machine, independent of the
physical networks in use
» There are two types of transport services:
connection-oriented and connectionless,
similar to the corresponding network services
TELE202 Lecture 13 TCP/UDP (1)
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Lecturer Dr Z. Huang
Transport layer
Functions of transport layer
» Connection management
– set-up and release connections
» Flow and error control
» Error detection
Why error detection is needed in
transport layer, given the data link
layer has done it hop by hop?
» Intermediate nodes/routers may cause errors
while processing packets
The software within the transport
layer that does the work is called the
transport entity
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Lecturer Dr Z. Huang
Transport layer
Why are there two distinct layers,
i.e., network and transport layers?
» Network layer is part of the communication
subnet and is run by the carriers
» The network layer may offer connectionoriented service which may be unreliable
» The users have no control over the subnet, so
the only possibility to improve the quality of
service is to put another layer on top of the
network layer
» The transport service interface for the higher
layer can be designed to be independent of
the network service interface, which may
vary considerably from network to network
» The primary function of the transport layer
can be regarded as enhancing the quality of
service provided by the network layer
Transport Protocol Data Unit
(TPDU) is used to name the messages
sent between transport entities
TELE202 Lecture 13 TCP/UDP (1)
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Lecturer Dr Z. Huang
Transport layer
Transport layer uses primitives,
provided by network layer, to send
and receive data
» Transport entities use NL_send(TPDU,
NL_address) and NL_recv(TPDU) to
send/receive TPDU
» TPDU is put in the payload of network packet
which is put in the payload of data link frame
» TPDU in TCP is called TCP segment, packet
in IP is called IP packet
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Lecturer Dr Z. Huang
QoS
Quality of Service (QoS) should be
supported in the transport layer
QoS can be charaterized by a
number of parameters, such as the
parameters in flow specification
» Throughput, transit delay, error ratio
The transport service allows the user
to specify preferred and minimum
unacceptable values for these
parameters at the time a connection
is set up
Some of the parameters also apply to
connectionless transport
It is up to the transport layer to
examine these parameters, and
depending on the kind of network
services available to it, determine
whether it can provide the required
service
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Lecturer Dr Z. Huang
QoS negotiation
The transport user makes a proposal
on some QoS parameters
The transport layer may immediately
realize that some of them are
unacceptable and report failure
(together with the reasons) to the
users
If the transport layer can not achieve
the desired goal (e.g. 600 Mbps
throughput), but it can achieve a
lower but still acceptable one
(150Mbps), it then sends the lower
rate and the minimum acceptable
rate to the destination
The destination machine may change
the QoS to an acceptable level or
reject the connection attempt
Finally the originating transport user
is informed of the result
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Lecturer Dr Z. Huang
Transport service
Two main differences between the
transport service and the network
service
» The network service is intended to model the
service offered by real (unreliable) networks,
while the (connection-oriented) transport
service is reliable
» The network service is used only by the
transport entities, while the transport service
is used by application programs directly and
must be convenient and easy to use
The transport service provides
(interface) primitives to allow
application programs to access the
transport service
»
»
»
»
»
LISTEN: wait for a connection
CONNECT: initiate a connection
SEND: send data
RECEIVE: get data or wait for data
DISCONNECT: initiate a release
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Lecturer Dr Z. Huang
Example
Example
» consider an application with a server and a
number of remote clients
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Lecturer Dr Z. Huang
Transport layer operation
Connection establishment
» The server executes a LISTEN primitive,
which blocks the server until a client turns up
» A client executes a CONNECT primitive,
which blocks the client , and send a TPDU
(encapsulating the information of the
connection request) to the server via the
underlying network layer
» When the TPDU arrives at the server side,
the transport entity checks to see that the
server is blocked on a LISTEN (i.e. interested
in handling a connection request).
» It then unblocks the server and sends a
CONNECTION ACCEPTED TPDU back to
the client
» When this TPDU arrives at the client side, the
client is unblocked and the connection is
established
Addressing
» When an application process wishes to set up
a connection to a remote application process,
it must specify which one to connect to
» The method normally used is to define
transport addresses to which processes can
listen for connection requests
» In Internet, transport addresses are (IP
address, local port) pairs
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Lecturer Dr Z. Huang
Transport layer operation
Data exchange
» Either party can do a (blocking) RECEIVE to
wait for the other party to do a SEND
» When the DATA TPDU arrives, the receiver
is unblocked
» As long as both sides can keep track of whose
turn it is to send, this scheme works fine
» Each TPDU sent will be (eventually)
acknowledged. These acknowledgements,
timers, and re-transmissions are managed by
the transport entities using the network layer
protocol and are not visible to the transport
users
Connection release
» Asymmetric disconnection: either transport
user can issue a DISCONNECT primitive,
which results in a DISCONNECT TPDU
being sent to the remote transport entiry.
Upon arrival, the connection is released
» Symmetric disconnection: when one side does
a DISCONNECT, that means it has no more
data to send, but is still willing to accept data
from its partner. A connection is released
when both sides have done a DISCONNECT
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Lecturer Dr Z. Huang
Problems in connection setup
Simple connection (two-way
handshake)
» Entity 1 send a TPDU to entity 2, saying
“good morning, I would like to talk with a
process with port number pn”
» Entity 2 receives the TPDU and ask the
process if it accept the request
» If the process agrees, entity 2 sends a TPDU
saying “ok, you can talk now”, and
connection is established
Problem with establishing a
connection occurs when the subnet
can lose, store, and duplicate packets
Consider the following scenario
» A user establishes a connection with a bank
» Sends messages telling the bank to transfer a
large amount of money to the account of a not
entirely trustworthy person
» And then releases the connection
What happens if all packets in the
above process are duplicated and
stored in the subnet?
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Lecturer Dr Z. Huang
Solution for connection problem
Unique sequence number
» To avoid the problem, each TPDU uses a
sequence number and is acknowledged
» The sequence number is determined using a
counter and ensured to be different from the
sequence number or acknowledged number
of existing TPDU wandering in the network
Three-way handshake protocol
» Transport entity A transmits a connection
request (in a TPDU) which is accompanied by
different sequence number x
» When entity B receives the request, it sends A
a connection acceptance TPDU
acknowledging the sequence number x and
initiates a new sequence number y
» Entity A acknowledges the acceptance with
the sequence number y
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Lecturer Dr Z. Huang
Examples
How three-way handshaking can
prevent false connection requests
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Lecturer Dr Z. Huang
Disconnect
Asymmetric release is abrupt and
may result in data loss
Symmetric release
» One way to avoid data loss is to use
symmetric release, in which each direction is
released independently of the other
» A host can continue to receive data even after
it has sent a DISCONNECT TPDU
The symmetric release acts as below
» A says:”I am done. Are you done too?”
» If B responds:”I am done too. Bye.”
» Then the connection can be safely released
This way does not always work
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Lecturer Dr Z. Huang
Disconnect
The two-army problem
» A white army is encamped in a valley
» On both of the surrounding hillsides are blue
armies
» The white army is larger than either of the
blue armies alone, but together they are
larger than the white army
» If either blue army attacks by itself, it will be
defeated, but if the two blue armies attack
simultaneously, they will be victorious
» The communication medium between the two
blue armies is to send messengers on foot
down into the valley, where they might be
captured and the message lost
» The question is, does a protocol exist that
allows the blue armies to win absolutely?
– The answer is that NO.
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Lecturer Dr Z. Huang
Disconnect
The same answer applies to the
absolutely safe disconnection
» Just substitute “disconnect” for “attack”. If
neither side is prepared to disconnect until it
is convinced that the other side is prepared to
disconnect too, the disconnection will never
happen
In practice, one is usually prepared
to take more risks when releasing
connections than attacking white
armies, so the situation is not entirely
hopeless
TELE202 Lecture 13 TCP/UDP (1)
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Lecturer Dr Z. Huang
Disconnect
Three-way handshake combined with
a timer
» Timer is used: if there is no TPDU from the
other party for some time, disconnect anyway
» The protocol can fail if the initial DR and n
retransmissions are all lost: the sender will
give up and delete the connection, while the
other side knows nothing about the attempts
to disconnect and is still fully active
» This situation is called half-open connection
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Lecturer Dr Z. Huang
Summary
Concepts
»
»
»
»
»
»
Transport layer
Transport entity
Transport address
TPDU
QoS
Two-army problem
Differences between transport layer
and network layer
Connection establishment and
release
» three-way handshake protocol for connection
establishment
» Three-way handshake combined with timer
for connection release
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Lecturer Dr Z. Huang