tcp/ip performance comt 429. © hans kruse, ohio university 2 protocol overview ethernet, x.25, hdlc...
TRANSCRIPT
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TCP/IP Performance
COMT 429
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© Hans Kruse, Ohio University 2
Protocol Overview
Ethernet, X.25, HDLC etc.
IP ICMP ARP RARP (Auxiliary Services)
TCP UDP
E-Mail HTTP (WWW) Remote LoginFile Transfer
ATM
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© Hans Kruse, Ohio University 3
Connection Types in TCP/IP
Data Link Layer and Physical Network
Network Layer
Transport Layer TCP: Connection Oriented
UDP: Connection-less
Connection-less
Depends on the network
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© Hans Kruse, Ohio University 4
Real Networks
• Include many different types of circuits– Different speeds– Some LAN, some Wide-Area connections
• Rely on routers to connect the different sub-networks
• Routers are not expected to have detailed knowledge about the traffic flows they are handling
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© Hans Kruse, Ohio University 5
Network Knowledge and Lack Thereof
• End Systems– Know the applications they are running– Often know the network capacity they would like to
have– Do not know the actual network capacity available– Do not know the “competition”, i.e. other network
users’ traffic
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Network Knowledge and Lack Thereof
• Routers– Know the capacity of the links they are attached to– Do not know much about the network farther away
from them– Do not know the complete path taken by the
packets handled in the router– Do not know (from the network traffic itself) what
the applications’ needs are
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Routers
• Must cope with packet flows that may exceed the available capacity on their outbound route– Short-term this indicates randomness in the traffic
and we need to deal with it– If the overload persists long-term we call it
congestion, and we would like for it to go away
• Routers use queues to handle the short-term variations
• Long-term overload??
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Applications
• Should and often can adapt to the available capacity
• Should be fair in their use of resources, or• Should identify themselves as high-capacity
users (and compensate the network operator accordingly)
• Need information about the network and the capacity is can deliver
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In the ideal case
• Use a control protocol to communicate this information between applications and “the network”– Standard procedure in circuit switched and virtual
circuit networks• Telephone network• Frame Relay and ATM
– Increases overall complexity– Can provide a wide range of services really well
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The case of the Internet
• Successful because a transparent network encourages application development and deployment
• Because the network elements are simple– Reasonably low complexity– Great flexibility– Not much capability to communicate network
information to applications
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Performance Issues
• Long term– Increase complexity and add QoS protocol layers– Throw capacity at the network faster than
applications require it (good luck...)
• Short term– Implicit communication of congestion in the TCP
protocol– Network performs many different functions, some
better than others
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Application View
• Network attachment over which I dispatch my packets -- known
• Intermediate network– Contains many links and queues
• Application sees an overall “latency”, or delay between packet dispatch and receipt
• More precisely, applications can discover Round Trip Times
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Sliding Window
1 2 3 M…
1 2 3 M…
1 2 3 M…
Idle Time
One “Cycle”
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In practice
• How is the sliding window mechanism used in TCP
• What control do we have over performance parameters
• Starting with a quick TCP review...
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UDP Header
Source Port Destination Port
Length Checksum
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TCP Header
Source Port Destination Port
Sequence Number
Acknowledgement Number
Window (flow cntrl)misc Flags
Checksum Urgent
Options
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TCP Connection Setup
• “Three-Way Handshake”– Send SYN packet– Wait for peer to return a SYN/ACK packet– Acknowledge the SYN/ACK packet
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TCP Connection Termination
• Send a FIN packet• Wait to receive acknowledgement of FIN
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TCP Data Exchange
• Sequence Numbers - Sliding Window– Arbitrary initial setting– Labels the first byte of the segment
• Acknowledgements– Indicate the next byte the receiver is looking for, all
previous bytes have been received.
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TCP Segment Size
• Originally Unlimited– IP fragments segments that are too large– Turned out to be very inefficient
• SYN packet can carry the MSS (Maximum Segment Size) option– Must be approved in the SYN/ACK– Default used if the option is not present
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TCP Sliding Window Operation
Sender
Receiver
snd.una snd.nxt snd.una+snd.wnd
rcv.nxt rcv.nxt+rcv.wnd
snd.wnd (local to the Sender)
rcv.wnd (Must tell the sender this value)
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Slow Start Congestion Control
1
1 1 2
2
Idle Time
1
Idle Time
1 2 3 4
Window doubles in each “cycle”Note: recent TCP amendments permit more than 1 initial segment
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The Congestion Collapse Problem
• Original TCP specs used the window for flow control, and retransmission after 2 round trip times
• Congestion of a link causes the timers to “go off” before an ack can be returned
• The network goes into steady state congestion where every segment is transmitted about three times
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Congestion Issues
• Slow Start - New Connection– Set send window to n*MSS (n <= 4)– Increase the window by MSS for each ack
received– Exponential increase in send window size
• What is the limit?– Window size reached before full utilization– Path is overloaded and an intermediate router
discards one or more packets
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Congestion Issues cont...
• Packet loss– may occur due to actual errors or congestion– TCP equates loss with congestion
• Congestion Avoidance, Timer Back-Off– Reduce send window to 1/2 of previous size for
each retransmit (exponential back-off)– After a segment is retransmitted, set the new RTO
timer for that segment to 2*RTO, up to a hard upper bound (2*MSL, Maximum Segment Life)(RTO = Retransmit Time-Out)
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Congestion Issues cont...
• Slow Start - After retransmission– Exponential slow-start up to 1/2 of the original
window size– Increase the window by MSS for each send
window ack’ed without loss– Linear increase in send window size
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What can we control
• Vendors– TCP implementation needs to follow most recent
guidelines– TCP window size should be configurable
• Users– Control the TCP window– For each application (rare)– For the entire workstation (more likely)
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Tuning, cont.
• Network Administrator– Router Queues
In
Out
In
Out
In
Out
In
Out
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Optional
Slides on TCP window operation
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Example ...
Sender
Receiver
1001 2001 2501 5001
1001 50011501
Received and acked; not yet picked up by client
Available receive window space
Sent but no ack received yetNext segment to send
Available window for further sends
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Segment Dispatch
• Dispatch segment to IP• Set RTO (Retransmit Time Out) timer
– Proportional to the Round Trip Time (RTT)
Sender
1001 2001 2501 5001
Sent but no ack received yetNext segment to send
Available window for further sends
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Segment Receipt withPickup
• Send Ack segment with Ack=2001• Window = 4000
Receiver
1001 50011501
Received and picked up by client
Available receive window space
2001 6001
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Segment Receipt w/oPickup
• Send Ack packet with Ack = 2001• Window = 3500
Receiver
1001 50011501
Received and picked up by client
Available receive window space
2001 5501
Received but not picked up by client
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Acknowledgement Receipt
• Seg received with Ack=2001, Win=3500– Left window edge to 2001– Right window edge to 5501
Sender
1001 2001 2501 5001before
2001 2501 5001after
5501
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Segment Receipt AfterSegment Loss
• Send a “duplicate” acknowledgement– Send Ack packet with Ack = 2001– Window = 3500
Receiver
1001 50011501
Received and picked up by client
2001 5501
Received but not picked up by client
Last segment receivedMissing segment
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Retransmission
• Highest Ack Number received is 2001– Duplicate Ack=2001 may have been received
• RTO timer for segment 2001 expires and 2001 is retransmitted– Trigger congestion avoidance algorithm– We really want to avoid this because RTO is large
Sender
2001 2501 5001 5501
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Retransmit Timing andWindow Size - Single Error
• BDP (Bandwidth Delay Product)– Ethernet: 1ms * 10Mbps = 1250 bytes– Satcom T1: 500ms * 1.5Mbps = 94 kbytes
• Assume window size = BDP– RTO > 2*RTT– “Recovery Ack” after retransmit needs 1 RTT– Channel idles for length of RTO (“drained pipe”)
2001 2501 5000 5501
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Retransmission TimerImplementation
• Running estimate (based on Acks) of– Average RTT– RTT variance factor
• Exclude retransmissions• Set RTO to RTT times RTT variance factor
(with a hard upper bound)– Around 2 RTT for lightly loaded links– As high as 16 RTT for congested links
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Window Scaling
• 16 bit window field in the TCP header allows a maximum of 64 kbytes for the window.
• RFC 1323 defines the window scaling option:– Syn segment suggests a “scaling” factor– Ack/Syn approves– All window advertisements are scaled by that
factor prior to use in TCP
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Window Scaling cont...
• Large windows cause an adjunct problem: sequence number reuse– RFC 1323 limits the window to about 1Gbyte to fit
within the sequence number space– OC-12 will use all sequence numbers in about 28
sec.– Segements can “live” in the network for 120 sec
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Fast Retransmit
• Duplicate Ack=2001 have been received• Re-send segment 2001 before RTO expires
– “Guess” that 2001 was lost– Wait for >=3 dup acks (segements could just have
arrived out-of-order)– Enter congestion avoidance with allowance for
duplicate acks
Sender
2001 2501 5000 5501
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Selective Acknowledgement
• Enabled during Syn and Syn/Ack• Receiver send segment with
– Ack = 2001, Window = 3500– SACK option: block start=2501, end=2600
Receiver
1001 50001501 2001 5501
Last segment received
Missing Segment
2501 2601