Date: Sun, 5 Jan 2003 08:17:28 -0800 (PST) From: Paolo M <bloom064@yahoo.com> To: freebsd-stable@freebsd.org Subject: Re: wi0 and mtu setting [bad idea] Message-ID: <20030105161728.87121.qmail@web14511.mail.yahoo.com>
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May be I am wrong but PPPoE is not incapsulated into
an Ethernet frame, it should happen the contrary, a
PPP frame incapsulated into an Ethernet frame, isn't
it?
- Paolo
On Saturday, January 4, 2003, at 05:25 PM, Evren
Yurtesen wrote:
I want to change MTU because I will use PPPoE on
wireless interfaces
So I need to set the MTU to 1508 so that 1500 byte
ethernet frames can be
encapsulated in the access point and sent to the
client. 1500byte ethernet
plus 8byte pppoe overhead makes a total of 1508 bytes
MTU which I need.
You showed what is the ifmtu default, but how does
ifconfig decide what is
the maximum MTU for this interface that a user can
set? or it doesnt care
about the interface and maximum that you can set is
1500?
Evren
On Sat, 4 Jan 2003, David Magda wrote:
On Sat, Jan 04, 2003 at 02:41:38PM +0200, Evren
Yurtesen wrote:
I just dont understand why FreeBSD people have to make
this wi driver so
tight in standarts even though there are wireless
cards which support
non-standard(and sometimes nice) things which can be
useful. I definetely
disagree that something should be in standard so you
will allow setting of
it!
The wi(4) driver is treated as an Ethernet device, so
it inherits many
of the characteristics of such devices. From
/sys/dev/wi/if_wi.c:
[...]
ifp->if_softc = sc;
ifp->if_unit = sc->wi_unit;
ifp->if_name = "wi";
ifp->if_mtu = ETHERMTU;
ifp->if_flags = IFF_BROADCAST | IFF_SIMPLEX |
IFF_MULTICAST;
ifp->if_ioctl = wi_ioctl;
ifp->if_output = ether_output;
ifp->if_start = wi_start;
ifp->if_watchdog = wi_watchdog;
ifp->if_init = wi_init;
ifp->if_baudrate = 10000000;
ifp->if_snd.ifq_maxlen = IFQ_MAXLEN;
[...]
And if you look at /usr/include/net/ethernet.h:
[...]
#define ETHERMTU
(ETHER_MAX_LEN-ETHER_HDR_LEN-ETHER_CRC_LEN)
#define ETHERMIN
(ETHER_MIN_LEN-ETHER_HDR_LEN-ETHER_CRC_LEN)
[...]
If you want to change this you'll have to hack code.
Since this is the
first I've heard about it, I don't think many people
share your concern
regarding the issue. Is there any reason why you need
this
functionality?
For example in /etc/defaults/rc.conf it says
-----------------------------------------------
tcp_drop_synfin="NO" # Set to YES to drop
TCP packets with SYN+FIN
# NOTE: this violates
the TCP specification
-----------------------------------------------
So why do you let this to violate TCP specifications
but you cant let user
to violate ethernet specifications?
Perhaps because this does not necessarily break
inter-operability
between different hosts, but changing the MTU could.
There are also issues with compatibility when talking
about Ethernet
(though someone mentioned that "WiFi" isn't truly
Ethernet). If the
number is less then 1536 most Ethernet drivers assume
one type of
encapsulation is used (RFC 894, for "Ethernet"
networks), while if it's
greater then 1536 another type is (RFC 1042, for "IEEE
802.3" networks).
Messing around with the MTU is fine if you can control
all the hosts on
the network, but things can get real messy real
quickly if any mistakes
are made.
--
David Magda <dmagda at ee.ryerson.ca>
Because the innovator has for enemies all those who
have done well under
the old conditions, and lukewarm defenders in those
who may do well
under the new. -- Niccolo Machiavelli, _The Prince_,
Chapter VI
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