• Linas Vepstas's avatar
    spidernet: Fix problem sending IP fragments · 33bdeec8
    Linas Vepstas authored
    The basic structure of "normal" UDP/IP/Ethernet
    frames (that actually work):
     - It starts with the Ethernet header (dest MAC, src MAC, etc.)
     - The next part is occupied by the IP header (version info, length of
    packet, id=0, fragment offset=0, checksum, from / to address, etc.)
     - Then comes the UDP header (src / dest port, length, checksum)
     - Actual payload
     - Ethernet checksum
    Now what's different for IP fragment:
     - The IP header has id set to some value (same for all fragments),
    offset is set appropriately (i.e. 0 for first fragment, following
    according to size of other fragments), size is the length of the frame.
     - UDP header is unchanged. I.e. length is according to full UDP
    datagram, not just the part within the actual frame! But this is only
    true within the first frame: all following frames don't have a valid
    UDP-header at all.
    The spidernet silicon seems to be quite intelligent: It's able to
    compute (IP / UDP / Ethernet) checksums on the fly and tests if frames
    are conforming to RFC -- at least conforming to RFC on complete frames.
    But IP fragments are different as explained above:
    I.e. for IP fragments containing part of a UDP datagram it sees
    incompatible length in the headers for IP and UDP in the first frame
    and, thus, skips this frame. But the content *is* correct for IP
    fragments. For all following frames it finds (most probably) no valid
    UDP header at all. But this *is* also correct for IP fragments.
    The Linux IP-stack seems to be clever in this point. It expects the
    spidernet to calculate the checksum (since the module claims to be able
    to do so) and marks the skb's for "normal" frames accordingly
    (ip_summed set to CHECKSUM_HW).
    But for the IP fragments it does not expect the driver to be capable to
    handle the frames appropriately. Thus all checksums are allready
    computed. This is also flaged within the skb (ip_summed set to
    Unfortunately the spidernet driver ignores that hints. It tries to send
    the IP fragments of UDP datagrams as normal UDP/IP frames. Since they
    have different structure the silicon detects them the be not
    "well-formed" and skips them.
    The following one-liner against 2.6.21-rc2 changes this behavior. If the
    IP-stack claims to have done the checksumming, the driver should not
    try to checksum (and analyze) the frame but send it as is.
    Signed-off-by: default avatarNorbert Eicker <n.eicker@fz-juelich.de>
    Signed-off-by: default avatarLinas Vepstas <linas@austin.ibm.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarJeff Garzik <jeff@garzik.org>
spider_net.c 67.2 KB