Understanding TCP/IP Model Internals and Interfaces

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1508 Understanding TCP/IP Model Internals and Interfaces Lab 2: TCP Flavor Comparison Background TCP detects a packet loss when: • Retransmission timer expires, or • n-th duplicate acknowledgement is received for the connection. TCP flavors differ in how they respond to packet loss. While all TCP implementations reset the congestion window after retransmission timeout expiration to one maximum segment size (MSS), they may proceed differently after duplicate ACKs are received. The missing segment is always resent immediately, but transmission of new or unacknowledged data depends on the selected flavor. We will compare the following flavors: • Tahoe—Fast retransmit followed by slow start • Reno—Fast retransmit followed by fast recovery • New Reno—Similar to Reno, but does not halve congestion window multiple times during recovery process • Selective acknowledgement (SACK)—Selective retransmission based on received selective acknowledgements • Reno with Explicit Congestion Notification (ECN) – similar to Reno, but has ability to process congestion notification from network upon which it performs fast recovery Objective We will study TCP behavior after three segments are dropped from the same window of data. The following graph shows the sequence numbers of sent and dropped segments. Dropped segments CONFIDENTIAL – RESTRICTED ACCESS: This information may not be disclosed, copied, or transmitted in any format without the prior written consent of OPNET Technologies, Inc. © 2010 OPNET Technologies, Inc. 12
1508 Understanding TCP/IP Model Internals and Interfaces For each of the modeled TCP flavors, we will examine: • Segment retransmission process (the order in which segments are retransmitted) • The reason why segments get retransmitted—fast recovery vs. expiration of retransmission timeout timer • Congestion window after drops We will compare: • Application response time Network Model Network Description The workstation downloads a 30,000-byte file from the server. TCP Maximum Segment Size is 536 Bytes. The packet analyzer is used to drop selected segments and collect statistics. Main Configuration Steps • Examine network configuration • Configure TCP Reno on hosts, run the simulation, collect and analyze results • Configure TCP SACK on hosts, run the simulation, collect and analyze results • Run simulation for TCP Tahoe, TCP New Reno and TCP Reno with ECN • Compare response time across scenarios CONFIDENTIAL – RESTRICTED ACCESS: This information may not be disclosed, copied, or transmitted in any format without the prior written consent of OPNET Technologies, Inc. © 2010 OPNET Technologies, Inc. 13
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Understanding TCP/IP Model Internals and Interfaces

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