(Congestion Window Control)
Ashkan Paya
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 An argument for increasing TCP’s initial congestion window
Nandita Dukkipati, Tiziana Refice, Yuchung Cheng,
Jerry Chu, Tom Herbert, Amit Agarwal, Arvind Jain and Natalia Sutin. Google Inc.
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 Terminology
 TCP explanation and origin
 OSI model
 TCP header formats and fields
 TCP Flow Control
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 TCP: Transmission Control Protocol
 IP : Internet Protocol
 ACK : acknowledgement
 URG : Urgent Pointer field significance
 PSH : Push or send data to application
 SYN : Synchronize sequence numbers
 RST : Reset (abort) the connection
 FIN : No more data
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
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 cwnd : Congestion Window(an interval that sender choose a number of sending bytes from that) awnd : advertised window (how many more bytes the receiver is willing to accept beyond the last acknowledged byte) ss_thresh : defined constant as a critical value
RTT : Round Trip Time
BDP : Bandwidth Delay Product
Time Out : duration from sending a packet that will be lost and retransmission of that until receiving acknowledgement.
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 TCP is one of the main protocols in TCP/IP networks.
TCP enables two hosts to establish a connection and exchange streams of data (packets) and guarantees delivery of them in the same order which they were sent. In other words, TCP provides reliable, ordered delivery of a stream of bytes from a program on one computer to another program on another computer.
TCP is the protocol that major Internet applications such as the World Wide Web, email and file transfer rely on. Packet loss is one of the major concerns here.
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 On the other hand, other applications, which do not require reliable data stream service, may use the User
Datagram Protocol (UDP), which provides a datagram service that emphasizes reduced latency over reliability. Applications like Skype, ooVoo and almost all of the video/audio stream based applications using
UDP protocol. Packet loss is not important here.
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 Source and destination port : identifies the sending and receiving ports. (16 bits)
 Sequence Number : random number from (2^32 _ 1)
 ACK Number : A dedicated ACK or piggybacked ACK contains the sequence number of the next data segment the receiver expects. (= sequence number)
 Heather Length
 Reserved : reserved for future use. Must be zero
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 Flags : URG, ACK , PSH, RST, SYN and FIN. (6 bits)
 Window : flow control credit allocation, value of advertised window. (16 bits)
 Checksum : this field is used for error-checking of the header and data. (16 bits)
 Urgent Pointer : if the URG flag is set, then this 16-bit field is an offset from the sequence number indicating the last urgent data byte . Indicates data that is to be delivered as quickly as possible. This pointer specifies the position where urgent data ends.
 Options : Variables such that time stamping to augment seq.num in high speed connections
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 sender receiver
(Client)
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(server)
2
3
Three-way Hand shake Protocol
1. SYN, sequence number =x
2. SYN+ACK seq.num = y
Acknowledgement x+1
3. ACK, acknowledgement y+1
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client server
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1. FIN, seqNum = x
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2. acknowledgement = x+1
3. FIN, seqNum = y
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4. acknowledgement = y+1
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[1]
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 Prevent sender from “swamping” receiver with data e.g. a fast server sending to a slow client.
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 Prevent sender from “swamping” receiver with data e.g. a fast server sending to a slow client.
 Provide congestion control inside the network, e.g. in gateways or routers.
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 Prevent sender from “swamping” receiver with data e.g. a fast server sending to a slow client.
 Provide congestion control inside the network, e.g. in gateways or routers.
* In either case, a node can be forced to discard packets due to lack of buffer space.
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end-to-end flow control
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 1. Slow start phase initial_cwnd = 1
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 1. Slow start phase initial_cwnd = 1
 2. Congestion avoidance phase
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 1. Slow start phase initial_cwnd = 1
 2. Congestion avoidance phase
 3. Congestion detection and recovery a) Standard Recovery ( recovery by timeout) b) Fast retransmission and fast recovery
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 TCP initial_cwnd has remained unchanged since 2002.
 in 2009 one paper[2] indicated that average connection bandwidth is 1.7 Mbps with more than
50% of clients have bandwidth above 2 Mbps, while the usage of narrowband (<256 Kbps) has shrunk to about 5% of clients.
 Applications like IE8, Firefox 3 devised their own mechanism.
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 Reduce latency latency of a transfer completing in slow start without losses is:
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 Reduce latency latency of a transfer completing in slow start without losses is:
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 Keep up with growth in Web page sizes
The Internet average Web page size is 384 KB [2] including HTTP headers and composed resources .
Now web browsers using multiple TCP connection using slow start.
e.g. IE8 can open 180 connections, Firefox open more than 24 connections[4]
Therefore, increasing initial_cnwd mitigate that need.
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 Allow short transfers to compete fairly with bulk data traffic.
Internet measurements indicate that most of the network traffic are bulk data transfers ( such as video), while majority of connections are short-lived and transfer small amount of data.
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 I)Minimize average Web page download time,
 II)Minimize impact on tail latency due to increased packet loss,
 III)Maintain fairness with competing flows,
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 What is the best initial congestion window number?
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 What is the best initial congestion window number?
Based on experiments and charts presented in this paper, 10 segments (≈15KB)
* Ethernet and maximum size of segment is 1430 bytes.
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 The effect on retransmission rate
TCP’s retransmission rate represents an upper bound for the percentage of packets lost due to congestion.
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Ratio of retransmitted bytes to that of unique bytes transmitted, expressed as a percentage.
Initial_cwnd=10
Exp
Base
Diff
Exp
Base
Diff
All
2.29
1.98
0.31
All
4.21
3.54
0.67
Avg DC
Web search Maps
1.73
1.55
0.18
Slow DC
4.17
3.27
0.90
Web search Maps
3.50
5.79
2.98
0.52
3.94
1.85
Photos
2.64
2.25
0.39
Photos
6.10
4.97
1.12
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 Increasing initial congestion window is a small change with a significant positive impact on Web transfer latency.
 Mitigate the need for multiple TCP connections to increase download speed
 IETF trying to standardize the TCP’s initial congestion window to at least 10 segments
 Further work should focus on eliminating the initial congestion window as a manifest constant to scale even large network speeds and Web page sizes
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 An Argument for Incresing TCP’s Initial Congestion
Window google Inc
 Akamai. The state of the internet. 3 rd quarter 2009 http://www.akamai.com/stateofinternet
 S.Ramachandran and A. jain : size and number of resources http://code.google.com/speed/article/webmetrics.html
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