Re: [dhcwg] I-D Action: draft-ietf-dhc-rfc3315bis-09.txt - questions about Solicit Prefix Delegation

Alexandre Petrescu <alexandre.petrescu@gmail.com> Sat, 15 July 2017 08:44 UTC

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To: Ted Lemon <mellon@fugue.com>
Cc: =?UTF-8?B?VsOtemRhbCBBbGXFoQ==?= <ales.vizdal@t-mobile.cz>, dhcwg <dhcwg@ietf.org>, "Bernie Volz (volz)" <volz@cisco.com>
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From: Alexandre Petrescu <alexandre.petrescu@gmail.com>
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Date: Sat, 15 Jul 2017 10:44:06 +0200
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Subject: Re: [dhcwg] I-D Action: draft-ietf-dhc-rfc3315bis-09.txt - questions about Solicit Prefix Delegation
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Le 15/07/2017 à 06:36, Ted Lemon a écrit :
> Alexandre, it appears to me that you are asking the DHC working group to 
> make a change to the DHCPv6 spec in order to solve a problem you don't 
> know exists in an IPv6 layer 2 implementation.   I think that this is 
> wrong-headed on two fronts.   First, you are speculating that the 
> problem exists.  Find out whether the problem exists, don't speculate.   
> Second, the problem is not in DHCP, if it does exist--it is in an IP 
> implementation.   So fix the implementation--don't ask for a change to 
> the DHCP spec.

In a sense I agree with you.  I speculate on the problem existence with GTP.

However, three different DHCP clients set two different values in the 
Hop Limit (a Cisco client, and the ISC set it to 255, whereas the 
odhcp6c sets it, at 1).  Maybe that is not a problem either, but looks 
like to me like one would like all DHCP clients to set same value in 
that field.  Just like all ND implementations set it at one single 
value.   What do you think?

Alex

> 
> On Fri, Jul 14, 2017 at 9:50 PM, Alexandre Petrescu 
> <alexandre.petrescu@gmail.com <mailto:alexandre.petrescu@gmail.com>> wrote:
> 
> 
> 
>     Le 14/07/2017 à 21:26, Vízdal Aleš a écrit :
> 
> 
> 
>             On 14 Jul 2017, at 21:15, Alexandre Petrescu
>             <alexandre.petrescu@gmail.com
>             <mailto:alexandre.petrescu@gmail.com>> wrote:
> 
>                 Le 14/07/2017 à 20:35, Ted Lemon a écrit : So are the
>                 DHCP clients you are talking about setting the IP header
>                 hop count to
>                   0/1, or the DHCP header hop-count field to 0/1?   That
>                 is, what
>                   is the behavior you are concerned about, and why do
>                 you think it
>                   might cause a problem in this case?
> 
> 
>             Some clients I am talking about issue DHCP Solicit with Hop
>             Limit field in the IPv6 base header (not DHCP UDP header)
>             with value 1.
> 
>             This Solicit is sent on a cellular network.  The cellular
>             network encapsulates at some point in IPv4, and further
>             decapsulates.  The
>               encapsulation protocol is called "GTPU" by some
>             non-wireshark packet dump format, with fields like "TEID",
>             "GTP_TPDU_MSG".  This
>               cellular network does not offer IPv4 access to end user,
>             it only offers IPv6.
> 
>             There is no GTP RFC.
> 
> 
>         GTP aka GPRS Tunnelling Protocol has been specified by 3GPP.
> 
>         It starts at the eNodeB and terminates at the PGW where the DHCP
>         server/relay would sit.
> 
> 
>     You see, I am also told that my "MT" ("Mobile Terminal"?) terminates
>     GTP.  I guess both are possible (terminate some times at eNodeB and
>     other times at MT).
> 
>     Or otherwise the expression "terminate at" may have different meanings
>     to different people.
> 
>             There is an RFC for "Generic Packet Tunnelling in IPv6". 
>             This RFC
>               says that encap/decap decrements the Hop Limit.
> 
>             This raises a potential speculation that the network drops
>             an incoming packet that has Hop Limit 1.
> 
>             It may be that the GTP encapsulation (no RFC) does not
>             decrement the Hop Limit of a packet-to-be-encapsulated.  In
>             this case there is no problem with DHCP Solicit having Hop
>             Limit 1.
> 
> 
>         It shall keep the packet as-is, since it is not visible from
>         user/data-plane point of view.
> 
> 
>     I dont understand that.  Not sure what you mean by user-data planes.
> 
>     The IP Hop Limit is in the IP header.  Some IP tunnelling mechanisms do
>     touch the Hop Limit of the encapsulated packet.  Why would GTP be
>     different than the other IP tunnelling mechanisms?
> 
>         I suggest a deep dive into 3GPP specs ...
> 
> 
>     It is certainly advantageous to understand the 3GPP specs.
> 
>     But I'd rather consider putting that into an Internet Draft.
> 
>     Alex
> 
> 
> 
>             Alex
> 
> 
> 
>                 On Fri, Jul 14, 2017 at 8:32 PM, Alexandre Petrescu
>                 <alexandre.petrescu@gmail.com
>                 <mailto:alexandre.petrescu@gmail.com>
>                 <mailto:alexandre.petrescu@gmail.com
>                 <mailto:alexandre.petrescu@gmail.com>>> wrote: Le
>                 13/07/2017 à 23:14, Ted Lemon a écrit : On Jul 13, 2017
>                 16:01, "Alexandre Petrescu"
>                 <alexandre.petrescu@gmail.com
>                 <mailto:alexandre.petrescu@gmail.com>
>                 <mailto:alexandre.petrescu@gmail.com
>                 <mailto:alexandre.petrescu@gmail.com>>
>                 <mailto:alexandre.petrescu@gmail.com
>                 <mailto:alexandre.petrescu@gmail.com>
>                 <mailto:alexandre.petrescu@gmail.com
>                 <mailto:alexandre.petrescu@gmail.com>>>> wrote: My
>                 oppinion is to
>                   make DHCP spec Hop Limit > 1.  In order to make sure
>                 that the encap/decap of DHCP Solicit in IPv4 GTP
>                 happening on a cellular link does not drop it to 0 upon
>                 decap. If a link local sourced multicast with a hop
>                 limit of one is dropped between sender and receiver, ip
>                 is broken on that link, ne c'est pas? If that link is a
>                 real link then yes - ip is broken on that link. But if
>                 the link is a virtual link - like when on a tunnel -
>                 then it may be that tunnel works or no. Alex
> 
> 
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> 
> 
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>         <http://www.t-mobile.cz/dcpublic/Zasady_komunikace_pri_sjednavani_smluv_cz.pdf>>.
> 
> 
> 
>     Není-li v zásadách uvedeno jinak, nepředstavuje tato zpráva konečný
> 
>         návrh na uzavření či změnu smlouvy ani přijetí takového návrhu.
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>         applies when negotiating contracts are defined
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>         <http://www.t-mobile.cz/dcpublic/Zasady_komunikace_pri_sjednavani_smluv_en.pdf>>.
> 
> 
> 
>     Unless otherwise stated in the principles, this message does not
> 
>         constitute the final offer to contract or an amendment of a contract
>           or acceptance of such offer.
> 
>