kern/54553: Successful TFTP boot ends with error packet

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         54553
>Category:       kern
>Synopsis:       Successful TFTP netboot ends with error packet
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Sep 17 09:00:01 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date 2019.09.09.22.01.23
>Organization:

>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

While debugging an unrelated issue by examining tcpdump output from a
network boot of a NetBSD-current/amd64 system, I noticed that when
pxeboot loads the kernel over TFTP, the last TFTP packet sent by the
booting system is an error packet, even in a successful boot:

  12:09:50.941416 98:4b:e1:67:68:98 > e4:11:5b:d3:13:fe, ethertype IPv4 (0x0800), length 65: 10.169.0.2.2707 > 10.169.0.1.69:  23 RRQ "netbsd-install" octet
  12:09:50.950317 e4:11:5b:d3:13:fe > 98:4b:e1:67:68:98, ethertype IPv4 (0x0800), length 558: 10.169.0.1.65405 > 10.169.0.2.2707:  516 DATA block 1
  12:09:50.950391 98:4b:e1:67:68:98 > e4:11:5b:d3:13:fe, ethertype IPv4 (0x0800), length 60: 10.169.0.2.2707 > 10.169.0.1.65405:  4 ACK block 1
  12:09:50.950426 e4:11:5b:d3:13:fe > 98:4b:e1:67:68:98, ethertype IPv4 (0x0800), length 558: 10.169.0.1.65405 > 10.169.0.2.2707:  516 DATA block 2
  12:09:50.950499 98:4b:e1:67:68:98 > e4:11:5b:d3:13:fe, ethertype IPv4 (0x0800), length 60: 10.169.0.2.2707 > 10.169.0.1.65405:  4 ACK block 2
  12:09:50.950531 e4:11:5b:d3:13:fe > 98:4b:e1:67:68:98, ethertype IPv4 (0x0800), length 558: 10.169.0.1.65405 > 10.169.0.2.2707:  516 DATA block 3
  [...many packets omitted for brevity...]
  12:09:56.152705 e4:11:5b:d3:13:fe > 98:4b:e1:67:68:98, ethertype IPv4 (0x0800), length 558: 10.169.0.1.65405 > 10.169.0.2.2707:  516 DATA block 49137
  12:09:56.152774 98:4b:e1:67:68:98 > e4:11:5b:d3:13:fe, ethertype IPv4 (0x0800), length 60: 10.169.0.2.2707 > 10.169.0.1.65405:  4 ACK block 49137
  12:09:56.152805 e4:11:5b:d3:13:fe > 98:4b:e1:67:68:98, ethertype IPv4 (0x0800), length 558: 10.169.0.1.65405 > 10.169.0.2.2707:  516 DATA block 49138
  12:09:56.152877 98:4b:e1:67:68:98 > e4:11:5b:d3:13:fe, ethertype IPv4 (0x0800), length 60: 10.169.0.2.2707 > 10.169.0.1.65405:  4 ACK block 49138
  12:09:56.152908 e4:11:5b:d3:13:fe > 98:4b:e1:67:68:98, ethertype IPv4 (0x0800), length 558: 10.169.0.1.65405 > 10.169.0.2.2707:  516 DATA block 49139
  12:09:56.152966 98:4b:e1:67:68:98 > e4:11:5b:d3:13:fe, ethertype IPv4 (0x0800), length 60: 10.169.0.2.2707 > 10.169.0.1.65405:  5 ERROR ENOSPACE ""

Although this is probably harmless in itself, it makes it harder to
debug netboot failures by distracting attention from the actual cause.

>How-To-Repeat:

>Fix: