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Failed to get authenticated using Windows XP, PPPoE.

Last response: in Windows XP
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January 12, 2005 4:37:01 AM

Archived from groups: microsoft.public.windowsxp.work_remotely (More info?)

Hi,

Could someone assist in diagnosing this problem?

The story goes like this: A certificate which is about 1700 bytes in size
had to tranverse through the link so that the user could be authenticated by
the server. However, with the use of sniffing tool on client side, only the
2nd packet was detected and the 1st packet was not seen(MTU is 1518 bytes for
Ethernet, therefore only 2 packets are sent, 1st is 1518, 2nd 182.) When used
with Aztech(www.aztech.com.sg) software which comes with modem known as
WinPOET(A PPPoE software), the both packets are sniffed.

How come?

Thanks in advance!

Regards,
Ron
Anonymous
January 12, 2005 1:51:22 PM

Archived from groups: microsoft.public.windowsxp.work_remotely (More info?)

Ron <Ron@discussions.microsoft.com> wrote:

> The story goes like this: A certificate which is about 1700 bytes in
> size had to tranverse through the link so that the user could be
> authenticated by the server. However, with the use of sniffing tool
> on client side, only the 2nd packet was detected and the 1st packet
> was not seen(MTU is 1518 bytes for Ethernet, therefore only 2 packets
> are sent, 1st is 1518, 2nd 182.) When used with
> Aztech(www.aztech.com.sg) software which comes with modem known as
> WinPOET(A PPPoE software), the both packets are sniffed.

The MTU for Ethernet is 1500, not 1518. The 18 bytes of MAC addresses, CRCs
and so on, do not count as part of the MTU. The 1500 bytes includes IP and
UDP/TCP headers, which further reduce the amount of user data which can be
carried. In the case of TCP, only 1460 bytes of user data will fit within a
single ethernet packet with MTU 1500.

However, in the case of PPPoE, the MTU is further reduced by the PPP
encapsulation, so that the MTU on a PPPoE link is typically 1492.

Your problem looks exactly like an MTU-misconfiguration problem: the first
packet is lost (never transmitted) because it is too large to be
encapsulated within a PPPoE packet.

You should ensure that MTU on the PPPoE connection is set to be no more than
1492 (leading to a TCP MSS of 1452).

--
Robin Walker [MVP Networking]
rdhw@cam.ac.uk
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