netdev - Re: [net-next PATCH 0/2] GENEVE/VXLAN: Enable outer Tx checksum by default

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Message-ID: <CALx6S35ghjUf0TFMNqa8Va9BZmM9nXGy5frOv9n9ojQSfnWnEQ@mail.gmail.com>
Date:	Tue, 23 Feb 2016 10:11:31 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Edward Cree <ecree@...arflare.com>
Cc:	Rick Jones <rick.jones2@....com>, Jesse Gross <jesse@...nel.org>,
	Alex Duyck <aduyck@...antis.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [net-next PATCH 0/2] GENEVE/VXLAN: Enable outer Tx checksum by default

On Tue, Feb 23, 2016 at 9:38 AM, Edward Cree <ecree@...arflare.com> wrote:
> On 23/02/16 17:20, Rick Jones wrote:
>> On 02/23/2016 08:47 AM, Tom Herbert wrote:
>>> Right, GRO should probably not coalesce packets with non-zero IP
>>> identifiers due to the loss of information. Besides that, RFC6848 says
>>> the IP identifier should only be set for fragmentation anyway so there
>>> shouldn't be any issue and really no need for HW TSO (or LRO) to
>>> support that.
>>
>> You sure that is RFC 6848 "Specifying Civic Address Extensions in the Presence Information Data Format Location Object (PIDF-LO)" ?
> PossiblyRFC 6864 "Updated Specification of the IPv4 ID Field".

Yes RFC6864.

>> In whichever RFC that may be, is it a SHOULD or a MUST, and just how many "other" stacks might be setting a non-zero IP ID on fragments with DF set?
> "The IPv4 ID field MUST NOT be used for purposes other than fragmentation
>  and reassembly."(§4.1)
> "Originating sources MAY set the IPv4 ID field of atomic datagrams to any
>  value."(§4.1)
> "All devices that examine IPv4 headers MUST ignore the IPv4 ID field of
>  atomic datagrams."(§4.1)
> Atomic datagrams are defined by "(DF==1)&&(MF==0)&&(frag_offset==0)" (§4).
>
> So it's OK to coalesce packets with different identifiers, as long as they
> have DFset (and aren't fragmented already).  Also, the RFC takes pains to
> point out that it "does not reserve any IPv4 ID values, including 0, as
> distinguished" (§4.1), so one cannot rely on the ID always being zero.

Right, receive side is straightforward, just ignore IP IDs. Transmit
is more interesting. Operative code is in ip_select_ident_segs. The
comment as to why non-zero IDs are sent is:

          /* This is only to work around buggy Windows95/2000
           * VJ compression implementations.  If the ID field
           * does not change, they drop every other packet in
           * a TCP stream using header compression.
           */

> --
> -Ed