Transcript ppt

CIS 4100 Systems Performance and Evaluation
Lecture 3
by
Zornitza Genova Prodanoff
Lect3..ppt - 09/12/04
Lecture Outline
Chapter 2:
• Types of networks
• Network protocols
• Client/Server Model
• Peer-to-Peer Model
• Web services protocols
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Communication Protocols
• Communication is between two remote processes
• Protocol is a set of rules
– Addressing
– Routing
– Error detection
– Error recovery
– Sequence control
– Flow control
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Communication Protocols
(Continued)
• Connection oriented protocols (e.g. TCP)
– Brake the message into fragments
– Once connection is established no fragment routing occurs
– Ensure that all fragments are processed in original order
– Used for applications communicating through large messages
• Connectionless protocols (e.g. UDP)
– Each message is routed independently
– Used when messages are smaller than the max. fragment size
– Order of arrival is not important - a single fragment is sent
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Communication Protocols
(Continued)
Protocol specification
• Syntax: message format and fields (bits) designation
• Semantics: actions taken by each entity when events occur
• E.g. fragment loss, timeout, message arrival
• Complex design
– Solution: use layered approach
- Abstract from details (at each layer) and let the protocols at
other layers handle them
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Type of Networks
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(Continued)
Client/Server Performance
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(continued)
Communication Protocols: Internet Protocols
• The two most important protocols in the Internet are the
Internet Protocol (IP) and the Transmission Control Protocol
(TCP)
- Suite of protocols known as TCP/IP
• Figure 2.7 shows the layering of important TCP/IP-based
protocols
• IP is a network layer protocol on top of which we find two
transport layer protocols: TCP-a connection-oriented protocoland User Datagram Protocol (UDP)-a connectionless protocol
• Simple Mail Transfer Protocol (used to send e-mail), and Telnet
(an interactive login protocol) use TCP
• Network File System (NFS) [8], Domain Name Server (DNS),
and Simple Network Management Protocol (SNMP) are built on
top of UDP
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Client/Server Performance
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(continued)
Communication Protocols: IP
Internet Protocol:
•
•
•
•
Specifies formats of packets sent across the Internet
Connectionless protocol
Source to destination forwarding of packets (called datagrams)
Every host connected to the Internet has a unique address,
called IP Address
• Address representation is dotted decimal notation, e.g.
129.2.0.37
• Prefix (identifies a network) and suffix (identifies a host)
address bits
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Client/Server Performance
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(continued)
Communication Protocols: TCP
Transport Control Protocol:
• Connection oriented protocol
• TCP guarantees that all the packets of a message are received in
their original sequence
• TCP guarantees that data is delivered without loss or
modification of bits
• Packets are called segments
• Connection establishment through three-way-handshake
• Full-duplex communication
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Client/Server Performance
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(continued)
Communication Protocols: TCP
• TCP uses ACKs (acknowledgement messages), timeouts, and
retransmissions for error control
• ACK are cumulative
• Flow control is implemented by TCP through a sliding window
mechanism
- The window size is the maximum number of bytes that can be
sent before an acknowledgement is received
- Window size is limited by
 the buffer size at the receiver
 by network congestion as estimated by the sender
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Communication Protocols: TCP
•
•
•
•
•
•
B is limiting bandwidth
RTT is round trip time
MSS is maximum segment size
Wc is the current window size
Wm is the maximum window size (prevents buffer overflow)
Wc doubles every RTT but cannot exceed the receivers window
Wm
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Communication Protocols: TCP
“Sliding window” flow control
• The rate with which sender can send data is determined by the
rate of incoming ACKs for previous segments
• The rate of incoming ACKs is determined by the bottleneck in
the round trip path (destination or the network)
• Wm is advertised by the receiver
• If network congestion is detected, the sender will reduce its
window size Wc to reduce its transmission rate
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Client/Server Performance
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(continued)
Communication Protocols: TCP
(continued)
• Slow start phase and Congestion avoidance phase
• Consider Figure 2.10
• During slow start Wc is initialized to one segment and is
increased by one for every ACK received
• Once segments are sent an ACK is received after one RTT
unit
• Wc doubles every RTT unit
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Communication Protocols: TCP
(continued)
• Network congestion can be detected by TCP through
− the receipt of a duplicate ACK, which indicates that the
receiver received an out-of-sequence segment
− or by a timeout at the sender
• The current value of the window size is saved in a variable called
slow start threshold window size, Wssthr, and Wc, is reduced
• TCP Reno: If a duplicate ACK is received, W, is divided by
two and TCP enters congestion avoidance phase (described in
Figure 2.11)
• TCP Reno: If a timeout occurs, Wc is set to one and TCO
goes back to slow start
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Communication Protocols: TCP
•
(continued)
When Wc reaches Wssthr, TCP enters congestion avoidance
• During congestion avoidance the window size Wc is incremented
by one every RTT unit
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Communication Protocols: TCP
(continued)
• In the example of Fig. 2.11, a duplicate ACK is received at time
5. Then, half of the current value of Wc, i.e., 8, is stored in
Wssthr and the TCP connection enters a congestion avoidance
phase.
• Another duplicate ACK is received at time 20. The value of
Wssthr is set to 7 (14 / 2). If a timeout occurs, then Wc, is set
to one and TCP goes back to slow start
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Communication Protocols: TCP
(continued)
• In Fig. 2.11, a timeout occurred at time 10 and the TCP
connection enters a slow start phase
• When Wc, reaches Wsethr during slow start, TCP switches to
congestion avoidance. For example, at time 13 in Fig. 2.11, Wc,
reaches the current value of Wssthr, which is 8, and the
connection enters a congestion avoidance phase.
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Communication Protocols: TCP
(continued)
TCP and congestion control
• The throughput XTCP of a TCP connection, measured in segments
per second
− decreases with RTT,
− decreases with the probability p that packets are dropped
− increases with the receiver advertised window size Wr
measured in segments
− decreases with the value of the TCP timeout To
( Based on Padhye’s expression for XTCP for a TCP Reno connection)
• A Model is implemented in the TCPModel. XLS
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Communication Protocols: TCP
(Continued)
• Fig. 2.12 shows variation of the TCP throughput in KB/sec as a
function of p and for four values of Wm: 10, 20, 30, and 40
segments
• TCP timeout (To) = 2 seconds
• Maximum segment size of 1,460 bytes
• Limiting bandwidth of the connection between sender and
receiver equal to 12,500 KB/sec
• RTT = 0.04 seconds
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Client/Server Performance
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(continued)
Communication Protocols: TCP
(Continued)
• As Wm increases, the throughput increases
• The benefits of a larger window decrease as the packet loss
probability increases because more data has to be retransmitted
in case of packet losses
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Communication Protocols: TCP
(Continued)
• The best-case throughput of a TCP connection is obtained when
the window size Wc, reaches the receiver maximum window size
Wm and no congestion occurs
• The throughput is given by
or
(Wm / RTT )  MSS
Wm / RTT
segments per second
bytes/sec where MSS is the
maximum segment size
• The throughput of a TCP connection cannot exceed the limiting
bandwidth B, in bytes/sec, of the network connecting the sender
and receiver =>
X TCP  min{ B, (Wm / RTT )  MSS}
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Communication Protocols: TCP
(Continued)
• Then, the maximum throughput is achieved when the windows
size given in segments Wm , is equal to
Wm*
 B  RTT 


MSS


• The numerator is called the bandwidth-delay product.
• During slow start, TCP operates with windows that are below the
window size that would be achieved during congestion avoidance.
− problem for short-lived connections that may never achieve
the maximum throughput under an optimal window size
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Communication Protocols: TCP
(Continued)
• Table 2.1 shows the bandwidth-delay product, and the optimum
receiver window size, Wm* , in segments, assuming a segment size
of 1,460 bytes, for various types of networks.
• As shown by the table, high-bandwidth, low-latency networks
require large window sizes for the throughput of the TCP
connection to use as much of the network bandwidth as possible.
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Client/Server Communication
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