On 19/06/20 8:28 pm, @lbutlr wrote:
On 19 Jun 2020, at 02:18, Nick Tait <n...@tait.net.nz> wrote:
1. My server was using the default MTU of 1500 bytes.
2. My connection to my ISP uses PPPoE, which adds an 8-byte header onto all
packets travelling between my home to my ISP, effectively reducing the maximum
packet size from 1500 bytes down to 1492 bytes. They did have MSS clamping set
up, but it turned out that it wasn't working for incoming TCP connections (i.e.
from the Internet to me).
This would be likely to affect ALL incoming mail, not just mail from BT,
wouldn’t it?
(I thought of this, but discarded it because the OP reported only problems with
BT)
The first 'too big' packet that the SMTP client transmits will get as
far as the ISP, and (because it has the 'DF' flag set in the IP header)
the ISP will discard it and send an ICMP 'Destination Unreachable'
packet back to the sender. If the sender receives the ICMP packet, they
will reduce their packet size (MSS) accordingly, and then retransmit the
smaller packet which will get through. The reduced packet size will also
be used for all subsequent transmissions from the sender on the same TCP
connection.
The problem I described only happens if the ICMP 'Destination
Unreachable' packet gets blocked and doesn't make it back to the SMTP
client.
Nick.
