I spoke too soon.  The file I copied with cp got 4471808 bytes across,
then activity stopped.

ping from the sun side fails.  ping from the netbsd side fails for
lack of mbufs.  Ah well.  But clearly NFS is using up all of the buffer
pool.
 
woodside% netstat -m
105 mbufs in use:
        76 mbufs allocated to data
        17 mbufs allocated to protocol control blocks
        10 mbufs allocated to socket names and addresses
        2 mbufs allocated to socket options
10/14 mapped pages in use
41 Kbytes allocated to network (80% in use)
0 requests for memory denied
0 requests for memory delayed
0 calls to protocol drain routines
woodside% ping euclid
PING euclid (192.232.2.78): 56 data bytes
ping: sendto: No buffer space available
ping: wrote euclid 64 chars, ret=-1
ping: sendto: No buffer space available