ECTE364-Lect9

– that no piece of received data is a duplicate
Duplication control
Rules for Generating ACK (1)
1. When one end sends a data segment to the other end, it must include an ACK. That gives the next sequence number it expects to receive. 2. The receiver needs to delay sending (until another segment arrives or 500ms) an ACK segment if there is only one outstanding inorder segment. It prevents ACK segments from creating extra traffic. 3. There should not be more than 2 in-order unacknowledged segments at any time. It prevent the unnecessary retransmission
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Example 1
Example 2
Lost Segment
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Loss Control
Host A
Host B Host A Host B
X loss
time
lost ACK scenario
time
Seq=100 timeout Seq=92 timeout
timeout
premature timeout, cumulative ACKs
sequence number
acknowledgement number
header length
Aegments) Number of bytes receiver willing to accept
checksum
not U A PR S F used
TCP Round-Trip Time and Timeout
RTT: to fantasia.eurecom.fr
350
300
RTT (milliseconds)
250
200
150
100 1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106 time (seconnds) SampleRTT Estimated RTT
TCP Round Trip Time and Timeout
EstimatedRTT = (1-x)EstimatedRTT + xSampleRTT
Eth frames
IP Service Model
• There are risks associated with the Best Effort Service Model:
– – – – – – Unreliable service Packets may be lost Packets may be delivered out of order Duplicate copies of a packet may be delivered Packets can be delayed for a long time Maximum packet size along a given route
receiver buffering
TCP rcvr. window–size management in TCP
Data Transfer
Sender Side
Window size = minimum(rwnd, cwnd) …… n-1 n Closing n+1 …… m-1 m m+1 Opening ……
The Internet
H1 H8 TCP End-to-End TCP
R1
R2
R3
IP IP datagram
ETH ETH
IP
IP
IP
IP IP datagram
IP datagram
FDDI FDDI
IP datagram
PPP PPP
ETH
ETH
Eth frames
FDDI frames
PPP frames
rcvr window size
pointer to urgent data
PSH:
Options (variable length)
RST: reset connection SYN: used for connection setup
application data (variable length)
FIN: connection release
Connection Setup
3-way Handshake
seq: 8000
UAPRS F SYN
seq: 15000 ack: 8001 nd: 5000 U A P R S F rw SYN + ACK
seq: 8000 ack: 15001 UAPRS F rwnd: 10000 ACK
15,001 14,001 13,001 12,001 11,001 10,001
1000 Bytes
TCP segment structure
32 bits
source port number dest. port number
URG: Urgent data field (generally not used) Acknowledgement field push data now (generally not used)
Means “no data” ! seq: 8001 if piggybacking
Connection Termination
TCP Flow Control
flow control
sender won’t overrun receiver’s buffers by transmitting too much, too fast
Data Transfer
Receiver Side
TCP Overview
Error control
Sequence control – segmentation of application layer data – sequence numbers used to order & re-assemble data Loss control
TCP Round Trip Time and Timeout
Q: How to set TCP timeout value?
• Longer than RTT – note: RTT will vary • Too short: premature timeout – unnecessary retransmissions • Too long: slow reaction to segment loss
Transmission Process
Segment N
Segment 1
H
H
• Stream oriented – processes connected by an imaginary ‘tube’ to receive bytes • IP layer -- needs packets TCP creates segments • Numbering • Starting sequence number initialize at connection setup
RcvBuffer = size or TCP Receive Buffer RcvWindow = amount of spare room in Buffer
receiver: explicitly informs sender of (dynamically changing) amount of free buffer space – RcvWindow field in TCP segment sender: keeps the amount of transmitted, unACKed data less than most recently received RcvWindow
• It’s the transport layer’s problem!
TCP Overview
Congestion?
Delay?
Number of packets lost? Bandwidth?
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Wireless errors?
Network End-host (receiver)
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Out of order packets
INTERNET
`
`
The Transport Layer
Packets from Applications
Transport Layer
Connection Setup/Tear Down
Congestion control Flow control Reliability
Network Layer
Data Link Layer
Physical Layer
6
Service Access Points
TCP endpoints: <source IP+port number, destination IP+port number> UDP endpoints: <destination IP, port number>