Download CSC 335 Data Communications and Networking I

CSC 336
Data Communications
and
Networking
Lecture 10: Transport Layer
Dr. Cheer-Sun Yang
Spring 2001
TOPICS
• OSI Transport Services, design, protocols
• Example Protocols: TCP, UDP
• Client-Server Model and Socket
Programming
Comparison with Data Link
Layer
• Similarity: both layers are focusing on how
information is exchanged between two entities
• Difference:Data link layer defines
communications between stations with a physical
connection, whereas transport layer protocols
define communications between sites with a
logical connection.
• Two kinds of transport layer protocols:
connection-oriented and connection-less.
Transport Layer Characteristics
• Reliable: flow control and error recovery are
provided
• Two kinds: connection-oriented or connection-less
• Example: Transmission Control Protocol(TCP),
User Datagram Protocol(UDP)
• Transport layer is the lowest layer which provides
end-to-end services. The lower three protocols
defines how network operates.
Transport Layer Functions
• Logical connection establishment – the transport
layer provides the “connection” the user perceives.
• A user can log on to computers at remote sites,
giving them the impression that they are
connected.
• But the connection is not a physical one as exists
when connecting wires or making phone calls
(using circuit switching).
Transport Layer Functions(cont’d)
• It is similar to a secretary whose function is to
place calls in behalf of an executive. The secretary
gets the executive’s request, makes the call, and
reaches the desired person, thus making the
connection.
• The executive then proceeds to have the
conversation independent of the trouble that the
secretary may have had in finding the desired
person.
Transport Layer Functions(cont’d)
• The connection management defines the
rules that allow two users to begin talking
with one another as if they were connected
directly. The function of defining and
setting up the connection is referred to as
handshaking.
Transport Layer Functions(cont’d)
• Graceful connection termination
• The secretary may have to finish the
connection by taking down some important
information such as client’s address,
checking the executive’s schedule for
making a future appointment.
• There are other functions.
Connection Oriented Transport
Protocol Mechanisms
• Example: Transmission Control
Protocol(TCP)
Connection-less Transport
Protocol Mechanisms
• No connection-establishment
• Datagram delivery
• User Datagram Protocol(UDP)
Motivations
• Why do we still need transport layer
running on top of network layer?
– They have similar connection-oriented and
connection-less services.
– They both provide addressing and flow-control
The answers are…
• What happens if the network layer provides
connection-oriented but unreliable service?
Suppose that it frequently loses packets? What
happens if routers crash all the time?
• Users have no control over the subnet, so they
cannot solve the problem of poor service by using
better routers or putting more error handling in the
data link layer.
• So another layer is added to provide better quality
of service(QoS).
Error Detection at the Transport Level
Transport Layer Functions
• Establishment of connectionless or connectionoriented communication
• Addressing
• Flow Control (transport layer)
• Error detection (transport layer)
• Interface with upper layers
• Multiplexing
• Quality of Service (QoS)
In general, a transport layer protocol must provide
reliable communications between end users.
Reliable Sequencing Network
Service
• Assume arbitrary length message
• Assume virtually 100% reliable delivery by
network service
– e.g. reliable packet switched network using X.25
– e.g. frame relay using LAPF control protocol
– e.g. IEEE 802.3 using connection oriented LLC service
• Transport service is end to end protocol between
two systems on same network
Reliable Sequencing Network
Service
• It is important because IP or other network
layer protocols do no guarantee reliable
service. Transport protocols must provide
acknowledgements and timers to make sure
that all of a user’s data are sent and
received.
TCP is not the
OSI Transport Layer Protocol
• TCP is designed and developed by the DoD
to run on top of IP for providing
connection-oriented transport layer services.
• OSI transport layer protocol is a generic
redesign of transport layer protocol which
includes more functions than TCP.
OSI vs. TCP
• OSI transport services include a more complete set
of services
• TCP is not identical to OSI transport protocol in
terms of the PDU format, and even some terms.
For example, TCP calls its PDU a segment; OSI
calls its PDU a TPDU; TCP identifies its
application using a port number, OSI uses a
Transport Service Access Point(TSAP). We will
summarize the comparison at the end of this unit
of slides.
Issues in OSI Transport Protocols
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•
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•
•
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Establishing a Connection
Releasing a connection
Addressing
Quality of Service (QoS)
Multiplexing
Flow Control and Buffering
Crash Recovery
Addressing
• Target user specified by:
– User identification: Transport Service Access
Point (TSAP)
– Machine identification: Network layer address,
such as IP address, identifies a host
Finding Addresses
• Four methods
– Know address ahead of time
• e.g. collection of network device stats
– Well known addresses
– Name server
– Sending process request to well known address
QoS
• Another way to look at the transport service is to
regard its primary function as enhancing the QoS
provided by the network layer.
• If the network layer is impeccable, the transport
layer has an easy job.
• If the network layer is unreliable, the transport
layer has to bridge the gap between what the user
wants and the network layer provides.
OSI Transport Service Types
• TP0: no error control, no resynch, no multiplexing
• TP1: no error control, can resynche, no
multiplexing
• TP2: no error control, multiplexing, no resynch
• TP3: no error control, can resynch, multiplexing,
flow control
• TP4: runs on top of un-reliable network services
such as IP; provides error control, resynch,
multiplexing, and flow control. OSI TP4 services
are similar to TCP services.
QoS
• What is QoS? It is characterized by a list of
QoS parameters which can be negotiated at
the connection establishment time.
• It is specified by users at the user layer.
• It is up to the transport layer to examine
them and determine whether or not it can
provide the required service.
QoS Parameters
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•
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Connection establishment delay
Connection establishment failure probability
Throughput
Transit delay
Residual error ratio
Protection
Priority
Resilience
QoS Parameters
• Connection establishment delay - the amount of
time elapsing between a transport connection
being requested and the confirmation being
provided by the user of the transport services
• Connection establishment failure probability - the
chance of a connection not being established
within the maximum establishment delay time, for
example, due to network congestion, lack of table
space somewhere, or other internal problems.
QoS Parameters
• Throughput – measures the number of bytes of
user data transferred per second.
• Transit delay – measures the time between a
message being sent by the transport user on the
source and its being received by the transport user
on the destination.
• Residual error ratio – measures the number of lost
or garbled messages as a fraction of the total sent.
QoS Parameters
• Protection – provides a way for the transport user to
specify interest in having the transport layer provide
protection against unauthorized third party reading or
modifying the transmitted data.
• Priority – specifies the priority of a connection. It is
used in the event of congestion to make sure that higher
priority connections get serviced before the lowerpriority connections.
Multiplexing
• Multiple users employ same transport protocol
• User identified by transport service access point
(TSAP)
• Multiple transport connections are connected to a
single network connection – upward multiplexing
– e.g. multiplexing a single virtual X.25 circuit to a
number of transport service user
• X.25 charges per virtual circuit connection time
Multiplexing
• Multiplexing can also be useful in the
transport layer for another reason.
• If the physical connection is a high-speed
physical connection and the user layer
application also generates data very fast, but
the network layer limits the window size to
a small constant, several transport layer
connection can be established simultaneous.
Layer
Layer
4
4
3
3
2
2
1
1
Upward Multiplexing
Downward Multiplexing
Connection Termination
• Entity in CLOSE WAIT state sends last data
segment, followed by FIN
• FIN arrives before last data segment
• Receiver accepts FIN
– Closes connection
– Loses last data segment
• Associate sequence number with FIN
• Receiver waits for all segments before FIN
sequence number
• Loss of segments and obsolete segments
– Must explicitly ACK FIN
Graceful Close
• Send FIN i and receive AN i
• Receive FIN j and send AN j
• Wait twice maximum expected segment
lifetime
Crash Recovery
• After restart all state info is lost
• Connection is half open
– Side that did not crash still thinks it is connected
• Close connection using persistence timer
– Wait for ACK for (time out) * (number of retries)
– When expired, close connection and inform user
• Send RST i in response to any i segment arriving
• User must decide whether to reconnect
– Problems with lost or duplicate data
RFCs regarding TCP & UDP
• Transmission Control Protocol
– Connection oriented
– RFC 793
• User Datagram Protocol (UDP)
– Connectionless
– RFC 768
TCP Mechanisms (1)
• Connection establishment
– Three way handshake
– Between pairs of ports
– One port can connect to multiple destinations
TCP Mechanisms (2)
• Data transfer
– Logical stream of octets
– Octets numbered modulo 223
– Flow control by credit allocation of number of
octets
– Data buffered at transmitter and receiver
– Congestion control
TCP Mechanisms (3)
• Connection termination
– Graceful close
– TCP users issues CLOSE primitive
– Transport entity sets FIN flag on last segment
sent
– Abrupt termination by ABORT primitive
• Entity abandons all attempts to send or receive data
• RST segment transmitted
TCP Header
TCP Services
• Reliable communication between pairs of
processes
• Across variety of reliable and unreliable networks
and internets
• Two labeling facilities (part of flags)
– Data stream push
• TCP user can require transmission of all data up to push flag
• Receiver will deliver in same manner
• Avoids waiting for full buffers
– Urgent data signal
• Indicates urgent data is upcoming in stream
• User decides how to handle it
TCP Connection Management
• What exactly is a connection?
Establishment and Termination
• Allow each end to now the other exists
• Negotiation of optional parameters
• Triggers allocation of transport entity
resources
• By mutual agreement
Connection Establishment
• Two way handshake
– A send SYN, B replies with SYN
– Lost SYN handled by re-transmission
• Can lead to duplicate SYNs
– Ignore duplicate SYNs once connected
• Lost or delayed data segments can cause
connection problems
– Segment from old connections
– Start segment numbers fare removed from previous
connection
• Use SYN i
• Need ACK to include i
• Three Way Handshake
Two Way
Handshake
:
Obsolete
Data
Segment
Two Way Handshake:
Obsolete SYN Segment
Three Way
Handshake:
Examples
Three Way
Handshake
:
State
Diagram
Flow Control
• Credit Mechanism
• A credit, stored in the segment’s window
field, specifies the maximum number of
bytes the entity (node) sending this segment
can receive and buffer from the other entity
(node). See Fig. 7.46.
Congestion Control
• There are problems that the flow control
mechanism cannot solve.
• Assume that the previous discussion showed that
the window sizes (credits) were adjusted based
only on what A or B can handle. It didn’t take into
account what might be in between.
• What happens that A and B both are connected to
others with T-1 links but use a link capable to
transmit 64 kbps between A and B?
Congestion Window
• Due to Jacobson [1988]- Jacobson’s
algorithm
• TCP is enhanced to allow a sending entity
to respond to congestion links and to alter
the number of segments it can send.
Congestion Window
• We will focus on the transmission from A to B.
• A maintains a congestion window that specifies
the number of bytes it thinks it can send without
causing or adding to congestion.
• If the congestion window’s capacity is larger than
A’s credit then A will still not send more than the
credit allows.
• Otherwise, A uses the congestion window’s value
to determine how many segments to send.
Congestion Window
• How can A determine when congestion
exists? – Timeout mechanism
• How does A respond to congestion? –
reduce the size of the congestion window by
half; resend; if timeout occurs again, the
window size is reduced by half again.
Congestion Window
• If the congestion is alleviated, A will
increase the congestion window size and
recalculate the sending window size.
• Consequently, A will reduce the congestion
window much more quickly than it will
increase it.
• A remaining question…
Congestion Window
• How is the initial congestion window size
determined?
• It is similar to the recovery procedure after
congestion.
Initial Value
• A will reduce the congestion window much
more quickly than it will increase it.
• The startup procedure is called a slow start.
Window Management
• Slow start
– Actual window= MIN[credit, congested window]
– Start connection with congested window size=1
– Increment congested window(cwnd) at each ACK, to
some max
• Dynamic windows sizing on congestion
– When a timeout occurs
– Set slow start threshold to half current congestion
window
• ssthresh=cwnd/2
– Set cwnd = 1 and slow start until cwnd=ssthresh
• Increasing cwnd by 1 for every ACK
– For cwnd >=ssthresh, increase cwnd by 1 for each RTT
Congestion Control
• RFC 1122, Requirements for Internet hosts
• Retransmission timer management
– To control a lost or discard segment, TCP employs a
retransmission timer which handles the retransmission
time, the waiting time for an ACK of a segment.
– For each connection, TCP maintains a variable, RTT,
that is the best estimate of the current round trip time to
the destination in question. When a segment is sent, a
timer is started.
Congestion Control
• When a timer is created, two situations can
occur:
– If an ACK is received for this particular
segment before the timer goes off, the timer is
destroyed.
– If the timer goes off before the ACK is
received, the segment is retransmitted and the
timer is reset.
Calculation of the
Retransmission Time
• Retransmission = 2 * RTT
• RTT: estimated Round-Trip Time
Calculation of RTT
• RTT =  * previous RTT + (1 - ) * current
RTT
•  is usually set to 90%.
Karn’s Algorithm
• Suppose that a segment is not
acknowledged during the retransmission
period and it is therefore retransmitted.
When the sending TCP receives an ACK for
this segment, it does not know if the ACK is
for the original segment or for the
retransmitted one. The value of the new
RTT therefore must be calculated based on
the departure of the segment.
Karn’s Algorithm
• Do not consider the RTT of a retransmitted
segment in the calculation of the new RTT.
• Do not update the value of the RTT until
you send a segment and receive an ACK
without the need for retransmission.
Karn’s Algorithm
• If a segment is re-transmitted, the ACK arriving
may be:
– For the first copy of the segment
• RTT longer than expected
– For second copy
•
•
•
•
No way to tell
Do not measure RTT for re-transmitted segments
Calculate backoff when re-transmission occurs
Use backoff RTO until ACK arrives for segment
that has not been re-transmitted
Conceptual TCP Primitives
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Open - request
Send - request
Deliver - indication
Accept - indication
Terminate – confirm
Etc.
Send
• If no push or close TCP entity transmits at
its own convenience
• Data buffered at transmit buffer
• May construct segment per data batch
• May wait for certain amount of data
Deliver
• In absence of push, deliver data at own
convenience
• May deliver as each in order segment
received
• May buffer data from more than one
segment
Accept
• Segments may arrive out of order
• In order
– Only accept segments in order
– Discard out of order segments
• In windows
– Accept all segments within receive window
Retransmit
• TCP maintains queue of segments
transmitted but not acknowledged
• TCP will retransmit if not ACKed in given
time
– First only
– Batch
– Individual
Acknowledgement
• Immediate
• Cumulative
UDP
• User datagram protocol (UDP) runs on top of IP.
• RFC 768
• Connectionless service for application level
procedures
– Unreliable
– Delivery and duplication control not guaranteed
• Reduced overhead
• There is no formal mechanism for acknowledging
errors or a provision for flow control or segment
sequencing.
UDP Uses
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Inward data collection
Outward data dissemination
Request-Response
Real time application
UDP Header
OSI vs. TCP
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Segment Types
Important Data
Graceful Termination
Piggyback acknowledgement
Sequencing
Flow Control
Socket Programming
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Sockets
Client/Server Model
Socket Data Structure
Socket Commands
Examples: Client Program, Server Program
Sockets
• A socket is a UNIX construct and is the
basis for UNIX networking services.
• A socket is similar to an envelop in which
information can be stored.
Client/Server Model
An example of file transfer:
• User requests a file.
• Client sends request to the server on behalf of the
user.
• Server receives a request from a client and
analyzes it.
• Server copies a file from its auxiliary storage.
• Server transmits contents of the file back to the
client.
• Client gets files’s contents from the server and
make it accessible to the user.
Socket Data Structures
Socket Data Structures
Socket Data Structures
Socket Data Structures
Suggested Reading
• Shay: Section 7.5, 7.6
• RFC793 (TCP) 768 (UDP) 1112 (Host
Extensions for Multicasting)