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Re: ppp/ipcp curiosities

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Nice figuring Lew, thanks. The question of why so many occurrances does 
remain though.

Hul

Lew Pitcher <lew.pitcher@digitalfreehold.ca> wrote:
> On Tuesday 01 July 2014 11:22, in comp.os.linux.networking, "Hul Tytus"
> <ht@panix.com> wrote:

> > comp.os.linux.networking
> > ppp/ipcp curiosities
> > 
> > Does anyone know the purpose of a dial-up service sending their ip address
> > as an option in a request? The lines below, starting with the ppp header,
> > show an example (in hex) of a sucsessful ipcp exchange with a national US
> > "dial up" service. The 40 88 a7 a2 is the services ip address,
> > globalpop.com, if memory serves.

> A cursory read through RFC 1332 (the IPCP RFC) and others suggests that this
> is the accepted way for a link to request that the peer supply the local
> side IP address.

> Specifically, it looks like we are interested in the IP Address
> Configuration option of IPCP, which starts with the 03 06. It reads:
>   03 06 40 88 A7 A2
> and breaks out as
>   03 - IP-Address Configuration Option
>   06 - 6 bytes in option (header + payload)
>   40 8B A7 A 2 - IP address

> To quote the RFC:
>       This Configuration Option provides a way to negotiate the IP
>       address to be used on the local end of the link.  It allows the
>       sender of the Configure-Request to state which IP-address is
>       desired, or to request that the peer provide the information.  The
>       peer can provide this information by NAKing the option, and
>       returning a valid IP-address.
>   
> Note the process described:
>  1) The local end sends an IP-Address request containing an IP address. I
>  2) The peer responds with an ACK or a NAK
> 3a) If the response was an ACK, the local end uses that address
> 3b) If the response was a NAK, the peer will send a valid IP address to use

> So, the local end has to ask "Can I use this address?", and the peer has to
> say "No, use /this one/ instead."

> It makes sense that, if the local end has to send an IP address, that it use
> an IP address that the peer knows is not available. And thus, it seems
> likely that the ISP might want the local end to ask for the ISP's IP
> address, just to make the request obvious.

> > Hul
> > 
> > 
> > ppp send: unadorned 16 byte packet:
> > FF 03 80 21 01 01 00 0A 03 06 00 00 00 48 5F E6                       1
> > ppp recv: unadorned 22 byte packet:
> > FF 03 80 21 01 01 00 10 02 06 00 2D 0F 01 03 06 40 88 A7 A2 5B 21             2
> > ppp send: unadorned 16 byte packet:
> > FF 03 80 21 04 01 00 0A 02 06 00 2D 0F 01 86 DE                       3
> > ppp recv: unadorned 16 byte packet:
> > FF 03 80 21 03 01 00 0A 03 06 48 FB 20 E0 9D E2                       4
> > ppp send: unadorned 16 byte packet:
> > FF 03 80 21 01 02 00 0A 03 06 48 FB 20 E0 D4 6C                       5
> > ppp recv: unadorned 16 byte packet:
> > FF 03 80 21 01 02 00 0A 03 06 40 88 A7 A2 62 46                       6
> > ppp send: unadorned 16 byte packet:
> > FF 03 80 21 02 02 00 0A 03 06 40 88 A7 A2 0B 32                       7
> > ppp recv: unadorned 16 byte packet:
> > FF 03 80 21 02 02 00 0A 03 06 48 FB 20 E0 BD 18                       8


> -- 
> Lew Pitcher
> "In Skills, We Trust"
> PGP public key available upon request

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ppp/ipcp curiosities Hul Tytus <ht@panix.com> - 2014-07-01 15:22 +0000
  Re: ppp/ipcp curiosities Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2014-07-01 12:44 -0400
    Re: ppp/ipcp curiosities Hul Tytus <ht@panix.com> - 2014-07-01 22:18 +0000
    Re: ppp/ipcp curiosities Moe Trin <ibuprofin@painkiller.example.tld.invalid> - 2014-07-02 03:28 +0000

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