RE: [Caml-list] IPv6 zone-id

[David Allsopp <dra-news@metastack.com>]

Vincent B. wrote:
> On 16/07/2014 18:04, Francis Dupont wrote:
> > => I disagree: the zone-id is a member of the address, not an extra to
> > system calls using addresses.
> >
> > Regards
> 
> Agreed. My new proposition: make "type sockaddr" include flow_info and
> scope_id/zone_id
> 
> This would break existing code.

Two thoughts (one pure, one rather evil) on mechanisms which might not break existing code:

Given that the type inet_addr is abstract, and changing ADDR_INET is not desirable, a pure "trick" to maintain compatibility could be done with the type of inet_addr in Unix.ml where instead of being string it becomes string * int option * int option or something like that.

You can then add an extra function, ipv6_sockaddr with type inet_addr * int * int * int such that [ipv6_sockaddr address port flow scope] returns [ADDR_INET((#1 address, Some flow, Some scope), port)] and some kind of helper function ipv6_getsockaddr : sockaddr -> string * int option * int option.

That prevents existing code which expects ADDR_INET to have two values only from breaking but it does now mean that two inet_addr values will be non-equal based on a socket parameter which isn't great.

Another "solution" is to sacrifice one's soul on the altar of backwards compatibility and invoke Obj! Along the lines of:

type sockaddr = ...

type sockaddr_internal =
  ADDR_UNIX_INT of string
| ADDR_INET_INT of inet_addr * int * int * int

and then ipv6_sockaddr is defined as follows:

let ipv6_sockaddr inet_addr port flow scope =
  if domain_of_sockaddr inet_addr <> PF_INET6
  then invalid_arg "Unix.ipv6_sockaddr"
  else if flow = 0 && scope = 0
       then ADDR_INET(inet_addr, port)
       else (Obj.magic (ADDR_INET_INT(inet_addr, port, flow, scope)) : sockaddr)

let ipv6_getsockaddr sockaddr =
  match sockaddr with
    ADDR_INET(addr, port) ->
      if is_inet6_addr addr
      then if Obj.size (Obj.repr sockaddr) = 2
           then (addr, port, 0, 0)
           else let (addr, port, flow, scope) = (Obj.magic sockaddr : (inet_addr * int * int * int))
                in
                  (addr, port, flow, scope) 
     else invalid_arg "Unix.ipv6_getsockaddr"
  | _ ->
    invalid_arg "Unix.ipv6_getsockaddr"

Now you have a sockaddr type where differing flow and scope values will result in different sockaddr values (= good). The only thing (I think!) which this breaks, is C code which tests the block size of an ADDR_INET value instead of its tag... but that's a really odd thing to do. Obviously, a different runtime would also suffer - but the whole of Unix has to be rewritten for a different runtime anyway (JavaScript, HVLM, etc.) so that's maybe not so much of an issue.

The only weirdness is then needing a function to break apart the IPv6 parameters, but that seems to be unavoidable if your aim is backwards compatibility.

I should probably take a shower after writing that code, though...


David