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Cisco Bug: CSCuq41530 - Access to websites fails if initial SYN/ACK has 0 window size

Last Modified

Nov 27, 2020

Products (1)

  • Cisco IronPort Web Security Appliance Software

Known Affected Releases

7.7.0-608 7.7.0-753 8.0.5-075 8.0.6-078

Description (partial) 

Symptom:
Access to certain websites will fail while going through the WSA. 

Packet capture would show that WSA receives a SYN/ACK from server with "0" window size followed a TCP Keep-alive with non-zero window size

Conditions:
The issue would only happen for websites which initlally respond with a "0" window size in SYN/ACK, followed by a TCP Keep-alive with non-zero window size

Related Community Discussions

<key>CSCuq41530</key> - Access to websites fails if initial SYN/ACK has 0 window size
Hi all, Maybe the picture is a little bit fuzzy but it does not hinder to see the details. The issue I am confronting is that, During the state of three-handshake, server sends a SYNACK segment with a zero receive window. In most case, server will send an ack with flag window_update to open its receive window and which enbales client to send the following data, every things works normal. However, if the server does not do like that, then client is forced to get into zero-window probe state after ...
Latest activity: Jan 22, 2015
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Bug Details Include

  • Full Description (including symptoms, conditions and workarounds)
  • Status
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