As I've been pointed out by Andrew Gallatin, there are some

network interface cards smart (or twisted?) enough to be able
to calculate a TCP/UDP checksum for a packet fragmented by the
host CPU.  Therefore the paragraph on the case has been revised.

1 files changed, 12 insertions, 4 deletions

diff --git a/share/man/man9/mbuf.9 b/share/man/man9/mbuf.9
index de2519a..0ab55b1 100644
--- a/share/man/man9/mbuf.9
+++ b/share/man/man9/mbuf.9
@@ -857,11 +857,19 @@ of fragments contained in its
 field.
 .El
 .Pp
-It is obvious that computing the TCP or UDP checksum cannot
-be offloaded for a packet fragmented by the host CPU.
-Consequently, there is no ambiguity in the meaning of
+The last rule for fragmented packets takes precedence over the one
+for a TCP or UDP checksum.
+Nevertheless, offloading a TCP or UDP checksum is possible for a
+fragmented packet if the flag
+.Dv CSUM_IP_FRAGS
+is set in the field
+.Va ifnet.if_data.ifi_hwassist
+associated with the network interface.
+However, in this case the interface is expected to figure out
+the location of the checksum field within the sequence of fragments
+by itself because
 .Va csum_data
-for an outgoing packet.
+contains a fragment count instead of a checksum offset value.
 .Pp
 On input, an interface indicates the actions it has performed
 on a packet by setting one or more of the following flags in