GIF MTU parmeter is needed

[Hideki Yamamoto]

Hi,

Thank you for your information.
I understand what happened in my freebsd box.

As David and Julian said, "route change" command control MTU of gif, and the
initial value is set from gif interface MTU value when "route add"
command is executed.
The initial value is independent from the gif interface condition, up and down.

The following shows the result of my test:
---------------------------------------------------------------------
Even after creating gif interface and getting it up,
MTU value by route commamd is copied from interface MTU value
when "route add" is executed.
---------------------------------------------------------------------
h9# ifconfig gif5 create up
h9# route add -inet6 2003:98::/64 -interface gif5
add net 2003:98::/64: gateway gif5
h9# route get -inet6 2003:98::/64
   route to: 2003:98::
destination: 2003:98::
       mask: ffff:ffff:ffff:ffff::
  interface: gif5
      flags: <UP,DONE,STATIC>
 recvpipe  sendpipe  ssthresh  rtt,msec    mtu        weight    expire
       0         0         0         0      1280         1         0
h9# route delete -inet6 2003:98::/64
delete net 2003:98::/64
h9# ifconfig gif5 mtu 1500
h9# route add -inet6 2003:98::/64 -interface gif5
add net 2003:98::/64: gateway gif5
h9# route get -inet6 2003:98::/64
   route to: 2003:98::
destination: 2003:98::
       mask: ffff:ffff:ffff:ffff::
  interface: gif5
      flags: <UP,DONE,STATIC>
 recvpipe  sendpipe  ssthresh  rtt,msec    mtu        weight    expire
       0         0         0         0      1500         1         0
h9# ifconfig gif5 inet tunnel 192.168.1.8 192.168.1.51
h9# ( ./udpgw -o -s 1500 & tshark -V -i gif5 | grep Payload ) & sleep
2 ; killall udpgw
[4] 1236
[1] 1238
Let's send 1500 length packet to 2003:98::2
Capturing on gif5
    Payload length: 1456
    Payload length: 68
    Payload length: 1456
    Payload length: 68
    Payload length: 1456
    Payload length: 68
    Payload length: 1456
    Payload length: 1456
    Payload length: 68

---------------------------------------------------------------------
After changing MTU from 1500 to 1300 by ifconfig command,
MTU value by route commamd is changed to 1300 automatically.
---------------------------------------------------------------------
h9# ifconfig gif5 mtu 1300
h9# ( ./udpgw -o -s 1500 & tshark -V -i gif5 | grep Payload ) & sleep
2 ; killall udpgw
[5] 1281
[1] 1283
Let's send 1500 length packet to 2003:98::2
Capturing on gif5
    Payload length: 1256
    Payload length: 268
    Payload length: 1256
    Payload length: 268
    Payload length: 1256
    Payload length: 268
    Payload length: 1256
    Payload length: 268
    Payload length: 1256
    Payload length: 1256
    Payload length: 268
    Payload length: 1256
    Payload length: 268
    Payload length: 268
h9# route get -inet6 2003:98::/64
   route to: 2003:98::
destination: 2003:98::
       mask: ffff:ffff:ffff:ffff::
  interface: gif5
      flags: <UP,DONE,STATIC>
 recvpipe  sendpipe  ssthresh  rtt,msec    mtu        weight    expire
       0         0         0         0      1300         1         0

---------------------------------------------------------------------
However, when changing MTU from 1300 to 1500 by ifconfig command,
MTU value by route command is not changed to 1500 automatically.
---------------------------------------------------------------------

h9# ifconfig gif5 mtu 1500
h9# route get -inet6 2003:98::/64
   route to: 2003:98::
destination: 2003:98::
       mask: ffff:ffff:ffff:ffff::
  interface: gif5
      flags: <UP,DONE,STATIC>
 recvpipe  sendpipe  ssthresh  rtt,msec    mtu        weight    expire
       0         0         0         0      1300         1         0
h9#



2009/12/29 Julian Elischer <julian at elischer.org>:
> David DeSimone wrote:
>>
>> Hideki Yamamoto <hyama99 at gmail.com> wrote:
>>>
>>>    h99# ifconfig xl0 inet6 2001:99::1
>>>    h99# ifconfig gif0 create
>>>    h99# ifconfig gif0 inet tunnel 192.168.1.8 192.168.1.4
>>>    h99# route add -inet6 2001:98::/64 -interface gif0
>>>    add net 2001:98::/64: gateway gif0
>>>    h99# ifconfig  gif0 up
>>
>> I wonder if the problem you're seeing is due to the MTU attached to the
>> static route that you're adding rather than the MTU of the interface.
>>
>> Try different command sequences and perform a "route get" to find out
>> what MTU is being applied to the routes, to see if they change to what
>> you expect.
>>
>
>
> Actually the route mtu will over-ride the interface mtu I think..
> route change {address} mtu xxxx
>
>
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