[Taps] MTU / equivalent at the transport layer

[Michael Welzl <[e-mail]>]

Hi all,

I have a problem with one particular primitive, or lack of it, in UDP, UDP-Lite and SCTP. It's something I just don't get.

Consider this text from draft-fairhurst-taps-transports-usage-udp:

"GET_INTERFACE_MTU:  The GET_INTERFACE_MTU function a network-layer
      function that indicates the largest unfragmented IP packet that
      may be sent."

Indeed, this is a network-layer function. It's about the interface, not about UDP. Does that mean that, to decide how many bytes fit in the payload of a packet, the programmer needs to know if it's IPv4 or IPv6, with or without options, and do the calculation?
If so, isn't it extremely odd that UDP doesn't offer a primitive that provides a more useful number: the available space in its payload, in bytes?

I also have the same question for SCTP.  For TCP, it's obvious that the application shouldn't bother, but not for UDP or SCTP.
At the last meeting, knowing the MTU was mentioned as one of the needs that latency-critical protocols have. I understand that - but I didn't include this primitive in the last version of the usage draft because it is a network-layer primitive... now I don't know how to approach this.

Thoughts? Suggestions?

Cheers,
Michael