Transcript TCP (Part 1) - Reliable Stream Transport Service

TCP
(Part 1)
- Reliable Stream Transport Service
D.E. Comer “Internetworking with TCP/IP:
Principles, Protocols and Architectures”, Ch.
13, Prentice Hall, 2000
Presented by Ming Su
[email protected]
Content
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TCP Introduction
Issue in TCP --- Flow Control
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Sliding window protocol
TCP acknowledgement scheme
TCP segment
TCP timeout
Summary
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TCP Introduction
Introduction - TCP
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TCP
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Transmission Control Protocol
TCP is not a software
Purpose
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Providing reliable stream delivery
Isolating application programs from the details of
networking
4
TCP Conceptual Layering
Application
Reliable Stream (TCP) User Datagram (UDP)
Internet (IP), ICMP
Network Interface
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Properties of the reliable delivery
service
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Stream Orientation
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Virtual Circuit Connection
Buffered Transfer
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data as a stream of bits, divided into 8-bit octets
transfer more efficiently & minimize network traffic
Unstructured Stream
Full Duplex Connection
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Two-way connection
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Issues in TCP
Two issues in TCP
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Flow Control
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End-to-end flow control problem
Congestion control problem
Virtual Circuit Connection
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Flow Control
- Sliding window protocol
End-to-end flow control
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Problem
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Sender can send more traffic that receiver can
handle. (Too fast)
Solution
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variable sliding window protocol
each acknowledgement, which specifies how
many octets have been received, contains a
window advertisement that specifies how many
additional octets receiver are prepared to accept.
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Variable Window Size
…
…
Window Advertisement
Receiver
Transmitter
Transmitter Window Size
Value of
Window Advertisement
Free space in buffer to fill
increase
bigger
increase
decrease
smaller
decrease
Stop transmissions
0
full
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Sliding window protocol in TCP
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TCP allows the window size to vary over time.
Window size changes at the time it slides
forward.
Advantage: it provides flow control as well as
reliable transfer.
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Flow Control
- TCP acknowledgement scheme
Acknowledgements and
Retransmission
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Cumulative acknowledgement scheme is used
in TCP.
A TCP ACK specifies “the sequence number
of the next octet that the receiver expects to
receive”.
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Acknowledgements and
Retransmission
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Adv.
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Easy to generate and unambiguous.
The lost ACKs don’t force retransmission.
Disadv.
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No information about all successful transmissions
for the sender, but only a single position in the
stream.
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Flow Control
- TCP segment
TCP segment format
0
4
16
24
31
Source Port
Destination Port
Sequence Number
Acknowledge Number
HLEN Reserved Code Bits
Window
Checksum
Urgent Pointer
Options (if any)
Padding
Data
…
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Flow Control
- TCP timeout
Timeout and Retransmission
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Question
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How to determine timeout?
Is the timeout always a constant?
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Timeout and Retransmission
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An adaptive retransmission algorithm is used
in TCP.
TCP monitors the performance of each
connection and adjust its timeout parameter
accordingly
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Timeout value may change.
Timeout is adjusted when a new round trip
sample ( RTT ) is obtained.
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Timeout and Retransmission
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RTT =
(α * Old_RTT) + ((1 – α) * new_RTT_sample )
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0<α<1
α close to 1 => no change in a short time
α close to 0 => RTT changes too quickly
Timeout = β * RTT
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β >1
Recommended setting, β= 2
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Accurate Measurement of Round Trip
Samples
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TCP acknowledgements are ambiguous.
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It is caused by the cumulative acknowledgement scheme.
It happens when retransmission.
It causes the question that the first received ACK does
correspond the original datagram or the retransmitted
datagram.
RTT couldn’t be measured accurately if the above
question cannot be answered.
How to do?
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Accurate Measurement of Round Trip Samples -- Karn’s Algorithm and Timer Backoff
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Karn’s algorithm: when computing the round trip
estimate, ignore samples that correspond to
retransmitted segments, but use a back-off strategy,
and retain the timeout value from a retransmitted
packet for subsequent packets until a valid sample is
obtained.
Timer back-off strategy:
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New_timeout = γ * timeout
( typically, γ =2 )
Each time timer expires (retransmit happens), TCP
increases timeout value.
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Karn’s Algorithm
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Use RTT to compute Timeout
When retransmission happens, Timeout increases in
γtimes continuously, until transfer successfully.
[Backoff strategy]
Use the timeout in the final turn of the last step to
send next segment. [Backoff strategy]
When an acknowledgement arrives corresponding to
a segment that did not require retransmission, then
TCP re-computes the RTT and reset the timeout
accordingly
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Responding to High Variance in Delay
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Queuing theory: variation in round trip delay
is proportional to 1/(1-L), where L is the
current network load, 0<L<1
( Timeout = β * RTT ) & (β=2 ) => L < 30%
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Not efficient
1989 TCP specification requires to use
estimated variance in place of β
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New RTT and Timeout Algorithm
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DIFF = sample – old_RTT
Smoothed_RTT = old_RTT + d * DIFF
DEV = old_DEV + p (|DIFF| - old_DEV)
Timeout = Smoothed_RTT + g * DEV
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DEV estimated mean deviation
d, a fraction between 0 and 1 to control how quickly the new sample
affects the weighted average
p, a fraction between 0 and 1 to control how quickly the new sample
affects mean deviation
g, a factor controls how much deviation affects round trip timeout
Research suggests: d=1/8, p=1/4 and g=4
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Summary
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Purpose of TCP
End-end Flow Control issue
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Variable Sliding window protocol in TCP
TCP acknowledgement scheme
TCP segment
TCP timeout
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RTT-Timeout Calculation and Karn’s Algorithm
New RTT and Timeout Algorithm
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Thanks