本文发表在 rolia.net 枫下论坛There are many causes and various fixes or workarounds. Most relate to end users or the transit network running PPP with
other protocols thus causing the protocol overhead and payload to exceed the common MTU of 1500 bytes. PPPoE
typically requires an MTU of 1492 bytes. PPPoA typically requires an MTU of 1500 bytes. When either of these
protocols are used in conjunction with L2TP as a transfer protocol, problems can occur because the L2TP protocol adds an
extra protocol overhead thus pushing the total packet size of a PPPoA connection from 1500 to 1540 bytes, and PPPoE
from 1492 to 1532.
Since most network links inside provider networks run Ethernet(with a MTU of 1500 bytes), the provider must chop the
data into smaller pieces to fit through(fragmentation). This is generally not an issue unless other protocol safeguards
additionally fail. The first is ICMP type 3, code 4 or more commonly known as message ‘Fragmentation needed and Don’t
Fragment(DF) bit set’ which is sent back to the originator of the large packet to request a smaller packet that can fit
through. This is called Path MTU Discovery (PMTUD). If this important message is blocked at any stage in the end to end
data session, the packet size discovery phase is broken and all large packets are simply dropped, this is the where the end
user sees their download freeze. This is a common problem on Internet links and through providers who install blanket
ICMP filters to stop Denial of Service(DoS) attacks called Ping Floods therefore blocking all messaging traffic, good and
bad. A more intelligent way to implement this is by only blocking/limiting the bad traffic and allowing the important
messages through. This generally requires a good understanding of how the protocol works and how to apply the filters
properly.更多精彩文章及讨论，请光临枫下论坛 rolia.net
other protocols thus causing the protocol overhead and payload to exceed the common MTU of 1500 bytes. PPPoE
typically requires an MTU of 1492 bytes. PPPoA typically requires an MTU of 1500 bytes. When either of these
protocols are used in conjunction with L2TP as a transfer protocol, problems can occur because the L2TP protocol adds an
extra protocol overhead thus pushing the total packet size of a PPPoA connection from 1500 to 1540 bytes, and PPPoE
from 1492 to 1532.
Since most network links inside provider networks run Ethernet(with a MTU of 1500 bytes), the provider must chop the
data into smaller pieces to fit through(fragmentation). This is generally not an issue unless other protocol safeguards
additionally fail. The first is ICMP type 3, code 4 or more commonly known as message ‘Fragmentation needed and Don’t
Fragment(DF) bit set’ which is sent back to the originator of the large packet to request a smaller packet that can fit
through. This is called Path MTU Discovery (PMTUD). If this important message is blocked at any stage in the end to end
data session, the packet size discovery phase is broken and all large packets are simply dropped, this is the where the end
user sees their download freeze. This is a common problem on Internet links and through providers who install blanket
ICMP filters to stop Denial of Service(DoS) attacks called Ping Floods therefore blocking all messaging traffic, good and
bad. A more intelligent way to implement this is by only blocking/limiting the bad traffic and allowing the important
messages through. This generally requires a good understanding of how the protocol works and how to apply the filters
properly.更多精彩文章及讨论，请光临枫下论坛 rolia.net