The biggest questions I have are:
- where to put this bit?
Right now, the *only* way an L2 with varied service levels can derive what service levels to use for best-effort traffic is to perform a layer violation. Continuing this tradition, the bit would be:
less_than_perfect_L2_error_detection = (ip.protocol == IPPROTO_UDPLITE)
Additionally the L3-L2 mapping can layer-violate up into the UDP-lite header and extract information about unequal error protection it may need i.e., apply more protection to data covered by the UDP-lite checksum than the uncovered data. Of course, the same thing can be done for unequal error detection (do apply good L2 error detection to the part covered by the UDP-lite checksum).
Ugly? Yes.
Unfortunately, the alternative is to make L3-L2 mappings a first-class citizen in the IP architecture and provide a way for an application to provide information that could help/influence this mapping. As RSVP has demonstrated, this is hairy stuff (even ignoring multicast).
- are there unintended consequences of doing this that we can forsee?
Some issues raised at the plenary were:
-- the old NFS-on-UDP-without-checksums story. Folks using UDP-lite for non-error-tolerant data get what they deserve.
-- a general feeling that nobody knows what will blow up when L2s do less error detection. "Allowing" this only for clearly defined cases (like ip.protocol == IPPROTO_UDPLITE) might help to allay these well-founded fears.
Gruesse, Carsten
PS.: those who don't know what all this is good for might want to read up on voice codecs like GSM 06.10 that have data fields for excitation energy -- generally, it is better to have a slightly corrupted packet with new values for this arrive than to error-conceal an erasure.
Of course, once this starts to get implemented, there are other possible applications, e.g. application-layer FEC.
