The benefits include:
1. Simplify the implementation
2. Support Ctrl+C automatically
3. Support poll automatically
4. Call pm_activity automatically
5. Save one polling thread
* ld doesn't have --start-groupi/--end-group things. As far as I know,
it works that way by default.
* objcopy with Mach-O support is not widely available.
(GNU binutils seem to claim the support but it didn't actually work
for me. llvm-objcopy --redefine-syms explicitly rejects Mach-O.)
Instead, use -unexported_symbols_list linker flag to hide symbols
to avoid conflicts with host symbols.
This makes the user interface a little hostile. People thing of an MTU of 1500 bytes, but the corresponding packet is really 1514 bytes (including the 14 byte Ethernet header). A more friendly solution would configure the MTU (as before), but then derive the packet buffer size by adding the MAC header length. Instead, we define the packet buffer size then derive the MTU.
The MTU is not common currency in networking. On the wire, the only real issue is the MSS which is derived from MTU by subtracting the IP header and TCP header sizes (for the case of TCP). Now it is derived for the PKTSIZE by subtracting the IP header, the TCP header, and the MAC header sizes. So we should be all good and without the recurring 14 byte error in MTU's and MSS's.
Squashed commit of the following:
Trivial update to fix some spacing issues.
net/: Rename several macros containing _MTU to _PKTSIZE.
net/: Rename CONFIG_NET_SLIP_MTU to CONFIG_NET_SLIP_PKTSIZE and similarly for CONFIG_NET_TUN_MTU. These are not the MTU which does not include the size of the link layer header. These are the full size of the packet buffer memory (minus any GUARD bytes).
net/: Rename CONFIG_NET_6LOWPAN_MTU to CONFIG_NET_6LOWPAN_PKTSIZE and similarly for CONFIG_NET_TUN_MTU. These are not the MTU which does not include the size of the link layer header. These are the full size of the packet buffer memory (minus any GUARD bytes).
net/: Rename CONFIG_NET_ETH_MTU to CONFIG_NET_ETH_PKTSIZE. This is not the MTU which does not include the size of the link layer header. This is the full size of the packet buffer memory (minus any GUARD bytes).
net/: Rename the file d_mtu in the network driver structure to d_pktsize. That value saved there is not the MTU. The packetsize is the memory large enough to hold the maximum packet PLUS the size of the link layer header. The MTU does not include the link layer header.
-------
This patch enhances networking support for the simulation under Linux.
Includes updated support for Linux TUN/TAP, and the addition of support for
Linux bridge devices.
CHANGES
-------
o Check to see if the d_txavail callback is present before calling it in
the arp send code. This prevents a segfault when simulating the telnetd
daemon with arp send enabled.
o Adjust the simulation's netdriver_loop() so it will detect and respond to
ARP requests.
o Do not attempt to take the tap device's hardware address for use by the
simulation. That hardware address belongs to the host end of the link,
not the simulation end. Generate a randomized MAC address instead.
o Do not assign an IP address to the interface on the host side of the TAP
link.
+ Provide two modes: "host route" and "bridge".
+ In host route mode, maintain a host route that points any traffic for the
simulation's IP address to the tap device. In this mode, so long as the
simulation's IP is a free address in the same subnet as the host, no
additional configuration will be required to talk to it from the host.
Note that address changes are handled automatically if they follow the
rule of if-down/set-address/if-up, which everything seems to.
+ In bridge mode, add the tap device to the specified bridge instance. See
configs/sim/NETWORK-LINUX.txt for information and usage examples. This
enables much more flexible configurations (with fewer headaches), such as
running multiple simulations on a single host, all of which can access
the network the host is connected to.
o Refresh configurations in configs/sim where CONFIG_NET=y. They default
to "host route" mode.
o Add configs/sim/NETWORK-LINUX.txt
CAVEATS
-------
- The MAC address generation code is extremely simplistic, and does not
check for potential conflicts on the network. Probably not an issue, but
something to be aware of.
- I was careful to leave it in a state where Cygwin/pcap should still work,
but I don't have a Windows environment to test in. This should be
checked.
- I don't know if this was ever intended to work with OS X. I didn't even
try to test it there.
NOTES
-----
- Was able to get telnetd working and simulate nsh over telnet, but only so
long as listen backlogs were disabled.
There appears to be a bug in the backlog code where sockets are being
returned in SYN_RCVD state instead of waiting until they're ESTABLISHED;
if you perform an immediate send after accepting the connection, it will
confuse the stack and the send will hang; additionally, the connection
will never reach ESTABLISHED state.
Can be worked around by adding a sleep(1) after the accept in telnetd. I
don't have the necessary knowledge of the IP stack to know what the
correct fix is.
