Transcript chapter_08_1 - Homework Market

Chapter Eight
The Transport Layer
Objectives
• Take a look at some other forms of logical
addressing.
• Examine the process of encapsulation.
• Take a closer look at flow control.
• Examine error correction/detection in detail.
• See how the Transport layer controls
congestion on the network.
The Transport Layer
• The responsibilities of the Transport Layer are:
– Handle end-to-end addressing
– Repackage long message into smaller segments for
transmission
– At the receiving end, rebuild packets into the original
message
– Monitor flow control of data
– Handle end-to-end error detection and recovery
– Handle congestion control on the network
The Transport Layer
• Why do we need transport
layer?
– Network layer is focused on
the routers. It provides
logical communication
between hosts.
– Transport layer runs on enduser devices. It provides
logical communication
between processes
Household analogy:
12 kids sending letters
to 12 kids
processes = kids
app messages = letters
in envelopes
hosts = houses
transport protocol =
John and Bill
network -layer protocol
= postal service
Addressing in the Transport Layer
• Ports and sockets can tell the OS what data is
intended for what applications.
– Ports are 16-bit numbers that identify applications
or processes.
– Sockets are a logical address consisting of a
combination of a port and an IP address.
Ports
• Well-known ports
– Assigned by Internet Assigned Number Authority
(IANA)
– Occupy ports 0 through 1023
• Ephemeral ports
– Used by the client software to establish a link
between applications
– Generally assigned by the application when it
launches
Some Commonly Used Ports
Port
Protocol
20
FTP, File Transfer Protocol, data
21
FTP, File Transfer Protocol, control
23
Telnet
25
SMTP, Simple Mail Transfer Protocol
80
HTTP, HyperText Transfer Protocol
109
POP, Post Office Protocol, version 2
110
POP, Post Office Protocol, version 3
666
Doom, ID software
Transport Layer Connections
• Connectionless connections
– No virtual connection is created.
– Data is basically thrown out onto the wire and the
transmitting workstation assumes it will arrive safely.
– The UDP is an example of a connectionless service
• Connection-oriented connections
– A virtual connection is created.
– For every packet transmitted, either an ACK or a NACK
must be returned.
– The TCP is an example of connection-oriented service
UDP
• often used for
streaming
multimedia apps
▸loss tolerant
▸rate sensitive
• other UDP uses
32 bits
Length, in
bytes of UDP
segment,
including
header
source port #
dest port #
checksum
length
data
▸DNS
UDP segment format
The Real-Time Transport Protocol
• RFC 1889
• Basic function of RTP is to Multiplex several realtime data streams onto a single UDP stream
• (a) The position of RTP in the protocol stack. (b)
Packet nesting.
TCP segment structure
32 bits
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
Connection’ estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)
source port #
dest port #
sequence number
acknowledgement number
head not
UA P R S F
len used
checksum
Receive window
Urg data pnter
Options (variable length)
application
data
(variable length)
counting
by bytes
of data
(not segments!)
# bytes
rcvr willing
to accept
TCP segment structure
– A 32-bit sequence number keeps packets in order.
– A 32-bit acknowledgement number is used to verify the
packet.
– 4-bit Header Length – Indicate the size of the entire TCP
header the receiver
– URG – 0 or 1. When set to 1, this bit indicate the urgent
pointer field is valid and should be considered.
– ACK – 0 or 1. When set to 1, this bit indicates that
acknowledgement number field is valid and being used
TCP segment structure
• A window sized field dictates how many packets will be
sent before waiting for ACKS.
– PSH – 0 or 1. When set to 1, this bit tells the receiver to
pass all data received at the point to the receiving
application immediately.
– RST – 0 or 1. This bit indicates an error condition has been
detected and notify the receiver to reset the connection
TCP segment structure
– SYN – 0 or 1. This bit synchronizes the sequence numbers
in order to establish a connection
– 16 bit TCP checksum – ensure that the TCP header has not
been modified in transmit
– 16-bit Urgent Pointer – This pointer is added to the
sequence number field to yield the sequence number of
urgent data.
Flow Control
socket
door
application
writes data
application
reads data
TCP
send buffer
TCP
receive buffer
socket
door
segment
• Buffer overflow
– Memory fills; transmission stops
• Stop and wait
– Send a frame and wait for the reply
• Neither methods very useful for busy
networks
• Rarely used
Advanced Flow Control
• Static window
– A fixed number of frames are transmitted.
– The transmitting station waits for the replies.
– No adjustments in transmission speed can be made.
• Sliding window
– It starts with a higher number of frames.
– As failures occur, the number of frames transmitted drops.
– If a frame is dropped, that frame and all frames following it
will get retransmitted.
MORE Flow Control
• Selectively repeat
– A number of
frames are
transmitted.
– If a failure occurs,
only the bad
packets need to be
transmitted.
MORE Flow Control
• Go Back N
– It is similar to sliding
window except that
a single ACK is sent
for all frames in a
window.
– If a failure occurs,
the protocol counts
back the correct
number of frames
and retransmits all.
Error Control in Transport
• The error correction in Data Link was bit-level
error correction.
– If user data was corrupted, the error was detected
and, if possible, fixed.
• Transport layer error correction is end-to-end.
– There may have error during encapsulation
– If a packet is lost or corrupted, the error is fixed.
Error Control in Transport
• Packet level errors can include packet loss, packet
corruption, and packet duplication. The network uses
– three-way handshake
– sequence number
– time-out for each packet
TCP Connection Establishment
• Recall: TCP sender,
receiver establish
connection before
exchanging data
segments
• initialize TCP variables:
▸seq. #s
▸buffers, flow control info
(e.g. RcvWindow)
Three way handshake:
Step 1: client host sends TCP
SYN segment to server
specifies initial seq #
no data
Step 2: server host receives
SYN, replies with SYNACK
segment
server allocates buffers
specifies server initial
seq. #
Step 3: client receives SYNACK,
replies with ACK segment,
which may contain data
TCP Connection Establishment
• (a) TCP connection establishment in the normal
case.
• (b) Call collision. – only one connection is
established
TCP Connection Close
client
server
close
timed wait
• Closing a connection:
• Step 1: client end system
sends TCP FIN control
segment to server
• Step 2: server receives FIN,
replies with ACK. Closes
connection, sends FIN.
• Step 3: client receives FIN,
replies with ACK.
• Step 4: server, receives
ACK. Connection closed.
closed
close
Principles of Congestion Control
• Congestion:
– informally: too many sources sending too much data
too fast for network to handle
– different from flow control!
• manifestations:
▸lost packets (buffer overflow at routers)
▸long delays (queueing in router buffers)
• a top-10 problem!
Congestion Control
• (a) A fast network feeding a low capacity receiver.
• (b) A slow network feeding a high-capacity
receiver.
Congestion Control
• No single device can control overall network
congestion.
• Therefore, Transport does what it can to make
sure THIS DEVICE does not contribute to
congestion.
• Connections requiring excessive
retransmission of data are dropped.
Approaches towards congestion
control
Two broad approaches towards congestion control:
• End-end congestion control:
▸no explicit feedback from network
▸congestion inferred from end-system observed loss,
delay
▸approach taken by TCP
• Network-assisted congestion control:
▸routers provide feedback to end systems
▸single bit indicating congestion (SNA, DECbit, TCP/IP
ECN, ATM)
▸explicit rate sender should send at
TCP Congestion Control
• Slow start (Jacobson 1998)
– Start with the maximum segment size
– If this is acknowledge then double the window size
– Send two maximum segemnt size
– Repeat
• When the CongWin = threshold, increase linearly.
– Threshold = 1/2 of CongWin value before timeout.
– Initially 64KB in addition to receiver flow control and congestion
control window
• When timeout occur
– reduce threshold to half of the congestion window
– Congestion window is reset to 1 segment
Slow Start
Host A
Host B
RTT
• When connection
begins, increase rate
exponentially until
threshold:
• double CongWin every
RTT
• done by incrementing
CongWin for every ACK
received
• Summary: initial rate is
slow but ramps up
exponentially fast
time
TCP Congestion Control (2)
• An example of the Internet congestion algorithm.
Summary: TCP Congestion Control
• When CongWin is below Threshold, sender in
slow-start phase, window grows exponentially.
• When CongWin is above Threshold, sender is in
congestion-avoidance phase, window grows
linearly.
• When timeout occurs, Threshold set to
CongWin/2 and CongWin is set to 1 MSS.
TCP Round Trip Time and Timeout
•Q: how to set TCP timeout value?
▸longer than RTT
▸but RTT varies
▸too short: premature timeout
– unnecessary retransmissions
▸too long: slow reaction to segment loss
▸Q: how to estimate RTT?
▸SampleRTT: measured time from segment transmission until ACK
receipt
▸ignore retransmissions
▸SampleRTT will vary, want estimated RTT smoother
▸average several recent measurements, not just current
SampleRTT
TCP Timer Management
• Variable Retransmission based on RTT
• Timeout based on Round Trip Time (RTT)
– RTT = αRTT + (1-α)M
– M is the time the ack received
– α is smoothing factor typically 7/8
• A better estimate
– Timeout = RTT + 4xD
– D = αD + (1-α)|RTT-M|
• Karn’s Algorithm
– Do not use RTT if retransmission happens
– Time out is doubled on every failure
Example RTT estimation:
RTT: gaia.cs.umass.edu to fantasia.eurecom.fr
RTT (milliseconds)
350
300
250
200
150
100
1
8
15
22
29
36
43
50
57
64
time (seconnds)
SampleRTT
Estimated RTT
71
78
85
92
99
106