Re: [Softwires] ietf-softwire: IPv4 + PSID primary key for lw4over6 binding

[<[e-mail]>]

Dear Andy,

Please see inline.

Cheers,
Med

> -----Message d'origine-----
> De : Softwires [mailto:[e-mail]] De la part de Andy
> Wingo
> Envoyé : mardi 12 juillet 2016 12:32
> À : [e-mail]
> Objet : [Softwires] ietf-softwire: IPv4 + PSID primary key for lw4over6
> binding
> 
> Hello list,
> 
> I have a change request for the draft-sun-softwire-yang-05 Internet
> Draft that defines a standard YANG model for lightweight 4-over-6
> binding tables.
> 
> This is my first post here, so allow me to introduce myself.  Together
> with some colleagues at Igalia we have made an open source
> implementation of the AFTR component of a lightweight 4-over-6
> deployment based on the Snabb toolkit for building software switches and
> other network functions.  This lwAFTR implementation is gradually
> wending its way upstream to https://github.com/snabbco/snabb.
> 
> To take a packet and look up a softwire in the binding table, the AFTR
> only has to look at one thing: the combination of the IPv4 address and
> port.  In the encapsulation direction you get this directly from the L3
> header.  In the decapsulation direction you get it from the encapsulated
> payload.  When decapsulating you also have to check that the B4 and BR
> addresses match the entries in the table, but you don't have to maintain
> a separate table that maps IPv6 B4 address to softwire: you just have
> the one IPv4+PSID-to-softwire table, along with a little side table that
> can map IPv4+port to PSID.
> 
> OK, cool.  Just one table, great.  However, draft-sun-softwire-yang-05
> specifies a different hierarchy:
> 
>   module: ietf-softwire
>      +--rw softwire-config
>         +--...
>         +--rw binding {binding}?
>            +--rw br {br}?
>               +--rw enable?                          boolean
>               +--rw br-instances
>                  +--rw br-instance* [id]
>                     +--rw binding-table-versioning
>                     |  +--rw binding-table-version?  uint64
>                     |  +--rw binding-table-date?     yang:date-and-time
>                     +--rw id                         uint32
>                     +--rw name?                      string
>                     +--rw softwire-num-threshold     uint32
>                     +--rw tunnel-payload-mtu         uint16
>                     +--rw tunnel-path-mru            uint16
>                     +--rw binding-table
>                        +--rw binding-entry* [binding-ipv6info]
>                           +--rw binding-ipv6info     union
>                           +--rw binding-ipv4-addr    inet:ipv4-address
>                           +--rw port-set
>                           |  +--rw psid-offset       uint8
>                           |  +--rw psid-len          uint8
>                           |  +--rw psid              uint16
>                           +--rw br-ipv6-addr         inet:ipv6-address
>                           +--rw lifetime?            uint32
> 
> This is figure 2 from section 5.2 (Lightweight 4over6 Tree Diagrams).
> This YANG schema would make it necessary to map from B4 address to
> softwire in some cases, which would be inefficient and not necessary
> from a data-plane point of view.

[Med] Actually, the data model allows to map a B4 to one or multiple softwires. 

The rationale for using binding-ipv6info as an index is to ease enforcing per-subscriber policies (e.g., limit the number of softwires per B4).

> 
> Additionally, this mapping prevents one B4 from having multiple
> softwires.

[Med] The data model in -05 allows for multiple softwires per B4 (distinct IPv6 addresses). This can even be controlled using softwire-num-threshold     

                     +--rw softwire-num-threshold     uint32

This design is relaxing the following restriction from RFC7596:

   Although it would be possible to extend lw4o6 to have more than one
   active lw4o6 tunnel configured simultaneously, this document is only
   concerned with the use of a single tunnel.

  It seems to me that one CPE could very well have multiple
> slices of IPv4 addresses.

[Med] That's possible with the current data model: distinct binding entries that belong to the same B4 may have distinct IPv4 addresses. Whether the same or distinct IPv4 addresses are bound to the same B4 is deployment-specific. IMHO, this should be considered with caution as it may lead to some applications failures e.g., RTP using IPv4@1 while companion RTCP flows are bound to another IPv4@2.

> 
> Lightweight 4-over-6 maps a part of the IPv4 space to a set of B4s in
> such a way that one IPv4+port pair will map to one B4, but the reverse
> of that is not necessarily true: one B4 may map to many IPv4+port
> pairs.  The natural way (to my mind) to implement a lwAFTR is to key
> your table by IPv4+PSID or IPv4+port, and I think that's probably the
> most natural way to manage it too -- IPv4 is after all the scarce
> resource.  Allowing one CPE to have multiple softwires can allow an
> operator to dynamically add capacity for a customer, on-demand.

[Med] As mentioned above, this doable with the current model. Policies are per-subscriber, not based on the IPv4+port set.

> 
> For all these reasons IMHO the binding-table subtree should look like:
> 
>            +--rw binding-table
>               +--rw binding-ipv4* [ipv4-addr]
>                  +--rw ipv4-addr            inet:ipv4-address
>                  +--rw psid-offset          uint8
>                  +--rw psid-len             uint8
>                  +--rw binding-entry* [psid]
>                     +--rw psid              uint16
>                     +--rw binding-ipv6info  union
>                     +--rw br-ipv6-addr      inet:ipv6-address
>                     +--rw lifetime?         uint32
> 
> OK, I drew it how I like it ;) This is an additional restriction where
> each IPv4 address corresponds in a one-to-one way with the "offset" and
> "len" PSID parameters.  If this restriction is feasible, it is certainly
> a simplification from the implementation point of view.  Otherwise if
> you allow each entry to have its own offset and len parameters, when you
> add a binding table entry it is difficult to validate that no other
> entry overlaps with that new PSID without doing a binding-table lookup
> for each port covered by that PSID.
> 
> Thoughts are very welcome :)
> 
> Regards,
> 
> Andy
> 
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