Many Internet providers operate their own backbone; however, their customers dial in to the network over the access nodes provided by third-party telecommunications providers. The "two-stage" dial-in procedure can lead to problems with the realized data rate:
- When dialing into the nodes of Deutsche Telekom, for example, a device conducting a PPP negotiation agrees a permissible maximum transmission unit (MTU), which defines the greatest possible size of unfragmented data packet. The MTU results from the lesser of the MRU (Maximum Receive Unit) protocol parameters set on the local and remote devices. This MTU is then observed by the device.
- When the data packets are forwarded to the actual provider, an additional header is added which increases the size of the data packets again. For the data packets to meet with the permitted size, they must now be fragmented into smaller units. This additional fragmentation can cause losses in the data-transfer speeds.
This problem can be avoided by entering a fixed MTU for each remote site.
