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[Aaron Rosen <arosen@xxxxxxxxxx>]

Thanks Rick that makes sense. I've never run into this myself. I just
assumed that a host would by default just set packets at it's MTU size
and the network would handle the fragmentation for you. Good to know.

Aaron


On Fri, Mar 8, 2013 at 11:56 AM, Rick Jones <rick.jones2@xxxxxx> wrote:
> On 03/08/2013 11:49 AM, Aaron Rosen wrote:
>>
>> Hi Rick,
>>
>> You are right. I just ran curl to test for myself and it does set the
>> DF bit. Why is this? Any ideas why it specifies that the packet cannot
>> be fragmented?
>
>
> Because most, if not virtually all TCP stacks going back to the mid 1990s
> (RFC 1191 is from 1990 and I figured a couple years to propagate) enable
> Path MTU discovery by default for TCP.  At least those with which I have
> come into contact.
>
> I doubt that curl itself asked for it.  I suspect you will find the DF bit
> set in the IP datagrams carrying the TCP segments of any application on your
> system using TCP - even netperf :)  PathMTU discovery, for TCP at least, and
> perhaps other "reliable" transports, is considered a Best Practice (tm) and
> so enabled by default.  Where it may not be enabled by default is for UDP.
>
> rick