Re: [multipathtcp] Multipath Address Family

[William Herrin <[e-mail]>]

On Mon, Nov 23, 2009 at 4:40 PM, Pasi Sarolahti <[e-mail]> wrote:
> I think a new address family would be a minimal needed change to the current
> socket API, without extending the API with new operations, but yet provide
> an interface to handle multiple addresses in a socket. One potentially
> useful characteristic would be that this would allow name resolver to return
> multiple IPv4 and/or IPv6 addresses from the name server as a single
> sockaddr structure, that could be directly given as a parameter to a connect
> call, which would lead to trying which of the addresses are reachable for
> the MPTCP session.

Hi Pasi,

It seems like you want to preserve the connect() call's parameters by
altering the definition of sockaddr so that it can contain multiple
addresses.

But functionally, what we really want to do is pass an array of
sockaddrs to connect().

Stupid question time: what makes redefining sockaddr superior to an
extended API call that supplants connect() and accepts an array of
sockaddrs?

Poking around linux's include files (netinet/in.h, bits/socket.h) I
notice that they go to a lot of trouble to make all sockaddrs come out
to exactly the same number of bytes. Won't a variable length sockaddr
that holds a variable number of addresses make a mess of that?


>> SOCK_MULTISTREAM or using a combination socket type like SOCK_STREAM |
>> SOCK_MULTIPATH
>> from the following reasons.

Found a quote I think bears on the topic at
http://lkml.indiana.edu/hypermail/linux/net/9904.2/0091.html :

>The AF_ prefix stands for "address family" and the PF_prefix stands for
>"protocol family". Historicly, the intent was that a single protocol family
>might support multiple address families and that the PF_ value was used to
>create the socket and the AF_ value was used in socket address structures.
>But in actuality, a protocol family supporting multiple address families has
>never been supported and the <sys/socket.h> header defines the PF_ value
>for a given protocol to be equal to the AF_ value for that protocol.

I think when we discuss allowing multipath TCP to use IPv4 and IPv6
addresses interchangeably, we're talking about one protocol family
using multiple address families, are we not?

Regards,
Bill Herrin



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