netdev - Re: [PATCH net-next v5 2/7] ipv6: Cleanup IPv6 TLV definitions

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Message-ID: <CALx6S35Z63WDMOkZLJ7PzR6BcEV4h5f7Jm-Okw9izcuFcygNoA@mail.gmail.com>
Date: Fri, 30 Jan 2026 09:22:43 -0800
From: Tom Herbert <tom@...bertland.com>
To: Justin Iurman <justin.iurman@...il.com>
Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>, davem@...emloft.net, kuba@...nel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next v5 2/7] ipv6: Cleanup IPv6 TLV definitions

On Thu, Jan 29, 2026 at 10:13 AM Justin Iurman <justin.iurman@...il.com> wrote:
>
> On 1/29/26 06:30, Willem de Bruijn wrote:
> > Tom Herbert wrote:
> >> Move IPV6_TLV_TNL_ENCAP_LIMIT to uapi/linux/in6.h to be with the rest
> >> of the TLV definitions. Label each of the TLV definitions as to whether
> >> they are a Hop-by-Hop option, Destination option, or both.
> >>
> >> Signed-off-by: Tom Herbert <tom@...bertland.com>
> >> ---
> >>   include/uapi/linux/in6.h        | 21 ++++++++++++++-------
> >>   include/uapi/linux/ip6_tunnel.h |  1 -
> >>   2 files changed, 14 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/include/uapi/linux/in6.h b/include/uapi/linux/in6.h
> >> index 5a47339ef7d7..438283dc5fde 100644
> >> --- a/include/uapi/linux/in6.h
> >> +++ b/include/uapi/linux/in6.h
> >> @@ -140,14 +140,21 @@ struct in6_flowlabel_req {
> >>
> >>   /*
> >>    * IPv6 TLV options.
> >> + *
> >> + *  Hop-by-Hop and Destination options share the same number space.
> >> + *  For each option below whether it is a Hop-by-Hop option or
> >> + *  a Destination option is indicated by HBH or DestOpt.
> >>    */
> >> -#define IPV6_TLV_PAD1               0
> >> -#define IPV6_TLV_PADN               1
> >> -#define IPV6_TLV_ROUTERALERT        5
> >> -#define IPV6_TLV_CALIPSO    7       /* RFC 5570 */
> >> -#define IPV6_TLV_IOAM               49      /* RFC 9486 */
> >> -#define IPV6_TLV_JUMBO              194
> >> -#define IPV6_TLV_HAO                201     /* home address option */
> >> +#define IPV6_TLV_PAD1               0       /* HBH or DestOpt */
> >> +#define IPV6_TLV_PADN               1       /* HBH or DestOpt */
> >> +#define IPV6_TLV_TNL_ENCAP_LIMIT 4  /* RFC 2473, DestOpt */
> >> +#define IPV6_TLV_ROUTERALERT        5       /* HBH */
> >> +#define IPV6_TLV_CALIPSO    7       /* RFC 5570, HBH */
> >> +#define IPV6_TLV_IOAM               49      /* RFC 9486, HBH or Destopt
> >> +                                     * IOAM sent and rcvd as HBH
> >
> > Explicit labeling with HBH or Destopt is quite informative.
> >
> > Does this mean that IPV6_TLV_IOAM should also be accepted in ip6_parse_tlv
> > in the Destopt branch? RFC 9486 indeed did reserve a number.
>
> Nope, not right now. The only IOAM option currently implemented in the
> kernel is the Pre-allocated Trace, which uses a Hop-by-Hop option. It
> wouldn't make sense to have it in a Destination option, although you
> could (i.e., it's not forbidden, just weird). Actually, the only IOAM
> option that would make sense to carry in a Destination Option is the
> Edge-to-Edge (E2E), but it's not implemented in the kernel. Should it be
> implemented at some point, then yes, you'd have IPV6_TLV_IOAM in the
> Destopt branch as well.

Justin,

Conceptually, someone could put IOAM in Destination Options before the
Routing Header. There's about 0% of that ever happening though.

Tom

>
> >> +                                     */
> >> +#define IPV6_TLV_JUMBO              194     /* HBH */
> >> +#define IPV6_TLV_HAO                201     /* home address option, DestOpt */
> >>
> >>   /*
> >>    * IPV6 socket options
> >> diff --git a/include/uapi/linux/ip6_tunnel.h b/include/uapi/linux/ip6_tunnel.h
> >> index 85182a839d42..35af4d9c35fb 100644
> >> --- a/include/uapi/linux/ip6_tunnel.h
> >> +++ b/include/uapi/linux/ip6_tunnel.h
> >> @@ -6,7 +6,6 @@
> >>   #include <linux/if.h>              /* For IFNAMSIZ. */
> >>   #include <linux/in6.h>             /* For struct in6_addr. */
> >>
> >> -#define IPV6_TLV_TNL_ENCAP_LIMIT 4
> >>   #define IPV6_DEFAULT_TNL_ENCAP_LIMIT 4
> >>
> >>   /* don't add encapsulation limit if one isn't present in inner packet */
> >> --
> >> 2.43.0
> >>
> >
> >
>