Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people, just like you, are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
Solved

What is a TCP Duplicate ACK?

Posted on 2008-09-29
1
29,681 Views
Last Modified: 2013-11-13
Hi,
  I'm writing a web service client and my wireshark output shows MANY TCP DUP ACKs.  What are they and does this indicate a problem somewhere?

Thanks!
0
Comment
Question by:pbenito
1 Comment
 
LVL 8

Accepted Solution

by:
rweil earned 500 total points
ID: 22601924
Apparently there is either transmisison problems (lost packets), orpakets being sent out of order.

from http://www.isi.edu/nsnam/DIRECTED_RESEARCH/DR_WANIDA/DR/JavisInActionFastRetransmitFrame.html

The idea of fast retransmit is straightforward. It only adds a tiny thing to the normal operation of TCP. Every time a packet with sequence number x arrives correctly at the receiver; the receiver acknowledges the packet #x by sending an ACK packet (containing the sequence number of another packet which it is waiting for - this number may or may not be "x+1") back to the sender. Therefore, when a packet arrives out of order (for example, 1 2 3 ... 5 : "4" is missing), TCP at the receiving side resends the last ACK packet to portray the expected packet again. This causes a duplicate ACK at the sending side. ("duplicate ACK" means the second, third, fourth, ... transmission of the same acknowledgement.) Fast retransmit plays an important role here. After receiving some numbers of duplicate ACKs, TCP at the sending side retransmits the missing packet without waiting for the timer to be expired. Moreover, receiving some numbers of duplicate ACKs means that the network congestion has been occured. Thus, TCP at the sending side resets cwnd to 1 and sets ssthresh to (old cwnd / 2) due to the congestion control algorithm; then starts slow-start again. In the practical TCP, the third duplicate ACKs triggers fast retransmit.

 

Note ! The reason that the sending side has to wait until the third duplicate ACK is described in RFC2001 as follows:

    " Since TCP does not know whether a duplicate ACK is caused by a lost segment or just a reordering of segments, it waits for a small number of duplicate ACKs to be received. It is assumed that if there is just a reordering of the segments, there will be only one or two duplicate ACKs before the reordered segment is processed, which will then generate a new ACK. If three or more duplicate ACKs are received in a row, it is a strong indication that a segment has been lost. "

0

Featured Post

Free Tool: ZipGrep

ZipGrep is a utility that can list and search zip (.war, .ear, .jar, etc) archives for text patterns, without the need to extract the archive's contents.

One of a set of tools we're offering as a way to say thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Article by: rfc1180
The Maximum Segment size (MSS) is an important consideration when troubleshooting connectivity via the Internet/Intranet. As the packets are routed via the Internet/Intranet, the packets must traverse through multiple routers in the path between two…
Network ports are the threads that hold network communication together. They are an essential part of networking that can be easily ignore or misunderstood, my goals is to show those who don't have a strong network foundation how network ports opera…
Viewers will learn how to properly install and use Secure Shell (SSH) to work on projects or homework remotely. Download Secure Shell: Follow basic installation instructions: Open Secure Shell and use "Quick Connect" to enter credentials includi…
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…

861 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question