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List: wireshark-users
Subject: Re: [Wireshark-users] Windows 7 Ignores TCP MSS?
From: Hansang Bae <for_list_hbae () nyc ! rr ! com>
Date: 2011-04-07 1:14:59
Message-ID: 4D9D1013.9010507 () nyc ! rr ! com
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On 3/23/2011 9:11 PM, Robinson, Eric wrote:
> If the NIC has offloading enabled, I guess the stack knows this and
> sends it bigger frames, knowing it will packetize them smaller?
> Because I went into the adapter properties and turned off all the
> offloading features, and now it is being reported as expected in
> WireShark. So I guess when the NIC's offload capabilities are
> disabled, the stack detects this and only sends it frames that are
> appropriate for the MTU?
(wow..sorry for the late reply - it's been hard to keep up on the list!)
Yes, I don't know how it breaks it, but TCP cheats by looking at what
the MTU is and subtracts the header appropriately to come up with MSS.
Theoretically, IP is supposed to do that, but I suppose it's one of
those things that started and no one ever deviated from it. I do recall
seeing a post by Rich Seifert on Usenet (when it actually was useful)
comp.dcom.lans.ethernet explaining how this came to be. But I don't
recall the gory details.
hsb
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On 3/23/2011 9:11 PM, Robinson, Eric wrote:
<blockquote
cite="mid:768FEA3F936926468F594E3CA7741FE802C74614@ipa-vault.nvipa.com"
type="cite">
<div dir="ltr" align="left"><span class="980250801-24032011"><font
color="#0000ff" face="Verdana" size="2"> </font></span></div>
<div dir="ltr" align="left"><span class="980250801-24032011"><font
color="#0000ff" face="Verdana" size="2">If the NIC has
offloading enabled, I guess the stack knows this and sends
it bigger frames, knowing it will packetize them smaller?
Because I went into the adapter properties and turned off
all the offloading features, and now it is being reported as
expected in WireShark. So I guess when the NIC's offload
capabilities are disabled, the stack detects this and only
sends it frames that are appropriate for the MTU?</font></span><br>
</div>
</blockquote>
<font color="#0000ff"><font size="2"><font face="Verdana"><br>
(wow..sorry for the late reply - it's been hard to keep up on
the list!)<br>
<br>
Yes, I don't know how it breaks it, but TCP cheats by looking
at what the MTU is and subtracts the header appropriately to
come up with MSS. Theoretically, IP is supposed to do that,
but I suppose it's one of those things that started and no one
ever deviated from it. I do recall seeing a post by Rich
Seifert on Usenet (when it actually was useful)
comp.dcom.lans.ethernet explaining how this came to be. But I
don't recall the gory details.<br>
<br>
hsb<br>
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