netdev - Re: discrepancy in ip(7) wrt. IP DF flag for UDP sockets

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Message-ID: <20110920143109.GB16323@hmsreliant.think-freely.org>
Date:	Tue, 20 Sep 2011 10:31:09 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Benjamin Poirier <benjamin.poirier@...il.com>
Cc:	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-man@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: discrepancy in ip(7) wrt. IP DF flag for UDP sockets

On Tue, Sep 20, 2011 at 10:12:34AM -0400, Benjamin Poirier wrote:
> On 11-09-20 09:38, Neil Horman wrote:
> > On Tue, Sep 20, 2011 at 09:29:54AM -0400, Benjamin Poirier wrote:
> > > On 11-09-20 08:14, Michael Kerrisk wrote:
> > > > Hello Benjamin, Neil,
> > > > 
> [snip]
> > > > 
> > > > Could you describe the required change in terms of how the man page
> > > > text should look--i.e., rewrite the passage as you think it should
> > > > look?
> > > 
> > > How about changing it to:
> > > 	IP_MTU_DISCOVER (since Linux 2.2)
> > > 	Set or receive the Path MTU Discovery setting for a socket. When
> > > 	enabled, the don't-fragment flag is set on all outgoing packets.
> > > 	Linux will perform Path MTU Discovery as defined in RFC 1191 on
> > > 	SOCK_STREAM sockets. For non-SOCK_STREAM sockets, it is the
> > > 	user's responsibility to packetize the data in MTU sized chunks
> > > 	and to do the retransmits if necessary. The kernel will reject
> > > 	(with EMSGSIZE) datagrams that are bigger than the known path
> > > 	MTU. The system-wide default is controlled by the
> > > 	/proc/sys/net/ipv4/ip_no_pmtu_disc file. 
> > > 
> > > 	Path MTU discovery flags   Meaning
> > > 	[...]
> > > 
> > > There are some differences between _DO and _WANT that are glossed over
> > > in this description, but I suppose there's only so much detail you can
> > > put in...
> > > 
> > > Thanks,
> > > -Ben
> > > 
> > Yeah, I think thats close, but its only the users responsibility to package
> > datagrams in mtu sized chunks if they force the dont fragment bit on.  If they
> > go wtih the default, the stack will fragment a datagram is it sees fit according
> > to the mtu of the path it traverses
> 
> Exactly. To get into this level of detail, I think we have to mention
> the option value, not just enabled/disabled. Let's try like this:
> 
>  	IP_MTU_DISCOVER (since Linux 2.2)
> 	Set or receive the Path MTU Discovery setting for a socket. When
> 	enabled, Linux will perform Path MTU Discovery as defined in RFC
> 	1191 on SOCK_STREAM sockets.  For non-SOCK_STREAM sockets,
> 	IP_PMTUDISC_DO forces the don't-fragment flag to be set on all
> 	outgoing packets.  It is the user's responsibility to packetize
> 	the data in MTU sized chunks and to do the retransmits if
> 	necessary.  The kernel will reject (with EMSGSIZE) datagrams
> 	that are bigger than the known path MTU.  IP_PMTUDISC_WANT will
> 	fragment a datagram if needed according to the path MTU or will
> 	set the don't-fragment flag otherwise.
> 
> 	The system-wide default can be toggled between IP_PMTUDISC_WANT
> 	and IP_PMTUDISC_DONT by writting to the
> 	/proc/sys/net/ipv4/ip_no_pmtu_disc file.
> 
Yes, that sounds good to me.  Thanks for doing this!
Acked-by: Neil Horman <nhorman@...driver.com>

Neil

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