About BEST and MAX MTU with PPPOE + PPTP in the router.
I have Has been setup WAN to PPPOE and MTU is 1492, because 1464 is maxed on my ISP. and I have connected PPTP on the same router, and MTU is 1942 max is 1372, so need I change WAN MTU to 1400?
Router – About BEST and MAX MTU with PPPOE + PPTP
mturouterwanwan-optimizer
Related Solutions
If a PC receives an ICMP Fragmentation required message, the PC must resend any traffic. Normally only TCP handles retransmission (including loss from MTU issues); TCP also includes support for Path MTU discovery.
In case of non-TCP applications which are written to support PMTU discovery and retransmission, the PC could know which packets to resend because ICMP errors include the context of the packet which generated the error... this is a dump from tshark -Vi eth0 icmp... Look below MTU of next hop, it shows that this ICMP message was received in response to a UDP packet to 172.16.255.1 (with DF set), which was too large.
Internet Protocol, Src: 172.16.1.1 (172.16.1.1), Dst: 172.16.1.5 (172.16.1.5)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0xc0 (DSCP 0x30: Class Selector 6; ECN: 0x00)
1100 00.. = Differentiated Services Codepoint: Class Selector 6 (0x30)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 56
Identification: 0x6297 (25239)
Flags: 0x00
0.. = Reserved bit: Not Set
.0. = Don't fragment: Not Set
..0 = More fragments: Not Set
Fragment offset: 0
Time to live: 255
Protocol: ICMP (0x01)
Header checksum: 0xfe46 [correct]
[Good: True]
[Bad : False]
Source: 172.16.1.1 (172.16.1.1)
Destination: 172.16.1.5 (172.16.1.5)
Internet Control Message Protocol
Type: 3 (Destination unreachable)
Code: 4 (Fragmentation needed)
Checksum: 0x8813 [correct]
MTU of next hop: 1420
Internet Protocol, Src: 172.16.1.5 (172.16.1.5), Dst: 172.16.255.1 (172.16.255.1)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 1500
Identification: 0x3aff (15103)
Flags: 0x02 (Don't Fragment)
0.. = Reserved bit: Not Set
.1. = Don't fragment: Set
..0 = More fragments: Not Set
Fragment offset: 0
Time to live: 1
[Expert Info (Note/Sequence): "Time To Live" only 1]
[Message: "Time To Live" only 1]
[Severity level: Note]
[Group: Sequence]
Protocol: UDP (0x11)
Header checksum: 0xe0ea [correct]
[Good: True]
[Bad : False]
Source: 172.16.1.5 (172.16.1.5)
Destination: 172.16.255.1 (172.16.255.1)
User Datagram Protocol, Src Port: 60742 (60742), Dst Port: 33437 (33437)
Source port: 60742 (60742)
Destination port: 33437 (33437)
Length: 1480
Checksum: 0xf73f [unchecked, not all data available]
[Good Checksum: False]
[Bad Checksum: False]
When a router should fragment but it receives the IP datagram with DF flag set, it sends ICMP (type should fragment but DF is set) to the source. Then the source adjust the MTU and it send again the datagram. This is how PMTUD works.
What happens here is that after you receives the ICMP from the router adjust the MTU to 500 for the path so the following Ping Requests you are sending have the DF flag set, but they are fragmented at source (meaning that your machine is sending the Ping Request fragmented in several datagrams).
Best Answer
The M in MTU stands for maximum (transmission unit), so there's no "max MTU".
When determining the MTU in a tunnel you need to know the expected overheads from the wrapping protocols:
The MTU outside the tunnel minus the summed overhead is the MTU inside the tunnel (unless the tunnel streams packets end-to-end, then the MTU depends on the tunnel parameters).