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[lwip-devel] [bug #50476] active open, ssthresh is really low with auto-

From: Joel Cunningham
Subject: [lwip-devel] [bug #50476] active open, ssthresh is really low with auto-tuning receiver
Date: Tue, 7 Mar 2017 00:54:24 -0500 (EST)
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:51.0) Gecko/20100101 Firefox/51.0


                 Summary: active open, ssthresh is really low with auto-tuning
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: jcunningham
            Submitted on: Tue 07 Mar 2017 05:54:23 AM UTC
                Category: TCP
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: git head



I've found through testing for task #14128, that when LwIP performs an active
open with a Windows 7 host using auto-tuning receive window, LwIP ends up
getting stuck with a 8192 value for ssthresh.

>From wireshark captures, the following exchange shows that Windows 7 first
advertises a 8192 window in the SYN-ACK, but then bumps up to 65536 (I have
window scaling enabled in LwIP) in the first ACK following 3-way handshake. 
This leaves LwIP with an extremely lower slow-start threshold.

What's also interesting, is that in the passive open case, LwIP ends up with
sstresh as 65536.  Windows 7 reports a window of 8192 in the SYN and then a
window of 65536 in the ACK for LwIP's SYN-ACK

I'm not 100% sure this is buggy behavior, but I do think it warrants
investigation since active open ends up with a really low ssthresh in this

See the files in task #14128 to see the different ssthresh values for active
vs passive open


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