Re: [mpls] [tsvwg] on UDP encapsulation

["Black, David" <[e-mail]>]

Lloyd,

> > I finally noticed that RFC7510, on MPLS in UDP encap, has
> > been published. Do take a look at all of section 3, which
> > laboriously goes through the nuances of when to choose
> > between UDP checksums, for safety, and turning them off,
> > for speed, at slight risk. That section and the RFCs it
> > refers to are quite the complex read. And that's just on
> > UDP checksums and whether to choose to turn them off. (It
> > gives dull security documents a run for their money.)

To complete the picture, UDP checksums come up again in the
final paragraph of the security considerations section ;-).

With respect to UDP-Lite, Tom described the primary consideration
for MPLS/UDP:

> Also, one of primary reasons for UDP encapsulation (e.g. MPLS/UDP,
> GRE/UDP) is to get ECMP in the network for non-TCP, non-UDP protocols.

That consideration ruled out UDP-Lite for the specific MPLS/UDP encap.
However, a discussion of the applicability and merits of UDP-Lite for
this sort of encap would be germane to the UDP Usage Guidelines draft,
draft-ietf-tsvwg-rfc5405bis:

	http://datatracker.ietf.org/doc/draft-ietf-tsvwg-rfc5405bis/

Could I ask you to take a look at that draft?  TSVWG plans to send
it to the IESG in the near future.

Thanks,
--David

> -----Original Message-----
> From: tsvwg [mailto:[e-mail]] On Behalf Of Tom Herbert
> Sent: Saturday, November 14, 2015 1:06 PM
> To: [e-mail]
> Cc: [e-mail]; [e-mail]
> Subject: Re: [tsvwg] on UDP encapsulation
> 
> On Fri, Nov 13, 2015 at 7:10 PM,  <[e-mail]> wrote:
> > https://tools.ietf.org/html/rfc7510
> >
> > I finally noticed that RFC7510, on MPLS in UDP encap, has
> > been published. Do take a look at all of section 3, which
> > laboriously goes through the nuances of when to choose
> > between UDP checksums, for safety, and turning them off,
> > for speed, at slight risk. That section and the RFCs it
> > refers to are quite the complex read. And that's just on
> > UDP checksums and whether to choose to turn them off. (It
> > gives dull security documents a run for their money.)
> >
> > What's odd is that this is standards track, and the third
> > option, use standards-track UDP-Lite to provide checksum
> > protection of the UDP/IP pseudoheader to avoid zero-checksum
> > port pollution on the host (on IPv4) or anywhere pollution
> > (IPv6) is not mentioned as an alternative - particularly as
> > UDP-Lite's partial payload coverage can cover the MPLS stack
> > s well.
> >
> > In some cases the Lite checksum is just a precomputed value
> > across a generated header that doesn't change, at no encode
> > cost and little decode cost, and within corporate private
> > networks the barriers to running UDP-Lite are less.
> >
> > It's unfortunate that UDP-Lite is the ideal approach for
> > this, yet doesn't deserve consideration. Going back further
> > in time, one might argue it's unfortunate that UDP-Lite was
> > pushed to a separate protocol number, dooming it to oblivion.
> > And, further back, that it was unfortunate that designers of
> > IPv6 thought leaving the pseudo-header demux check up to
> > separate transports that could then skip it, while insisting
> > UDP always have its checksum on, which was always going
> > to be relaxed, was a good idea for consistent error-detecting
> > demux...
> >
> 
> Lloyd, the potential for UDP-Lite is mentioned in Encapsulation
> Considerations draft (draft-ietf-rtgwg-dt-encap-00):
> 
> "Conceptually, the ideal solution to this problem is a checksum that
>    covers only the newly added headers of interest.  There is little
>    value in the portion of the UDP checksum that covers the encapsulated
>    packet because that would generally be protected by other checksums
>    and this is the expensive portion to compute.  In fact, this solution
>    already exists in the form of UDP-Lite and UDP based encapsulations
>    could be easily ported to run on top of it.  Unfortunately, the main
>    value in using UDP as part of the encapsulation header is that it is
>    recognized by already deployed equipment for the purposes of ECMP,
>    RSS, and middlebox operations.  As UDP-Lite uses a different protocol
>    number than UDP and it is not widely implemented in middleboxes, this
>    value is lost.  A possible solution is to incorporate the same
>    partial-checksum concept as UDP-Lite or other header checksum
>    protection into the encapsulation header and continue using UDP as
>    the outer protocol.  One potential challenge with this approach is
>    the use of NAT or other form of translation on the outer header will
>    result in an invalid checksum as the translator will not know to
>    update the encapsulation header."
> 
> IMO, the allowance that nodes may send a zero UDP checksum is a
> concession being made for devices that are unable to perform the
> computation. These are precisely devices in the network that before
> the advent of UDP tunneling never had any reason to source UDP
> packets. At end hosts we've had checksum offload capabilities in NICs
> for years, so there is no reason a host should send zero UDP checksums
> for IPv6 (except if it knows that the receiver is an aforementioned
> device that can't do the checksum calculation).
> 
> Also, one of primary reasons for UDP encapsulation (e.g. MPLS/UDP,
> GRE/UDP) is to get ECMP in the network for non-TCP, non-UDP protocols.
> For IPv6 we can use the flow label to provide for this in lieu of
> devices parsing the transport layer (RFC6438). We do need switch
> support for this and there does seem to be good progress on this. So
> the use of the flow label for ECMP should obviate the motivation to
> use UDP encapsulation in the first place (for many use cases).
> 
> Tom