During upgrade to UTM 9.405, Sophos fixed som MTU issues for Amazon, this was about getting the MTU values from the ISP DHCP server, problem is that some ISP’s use a value of 576, which is wrong, as it should be 1500 in most cases. Until the release of todays 9.407-3,they introduced the fix “NUTM-4992”, which introduces a new confd parameter:

I believe the MTU of 576 was an expectation (standard) that any IP media would at least support that. So, IP, if it doesn't know otherwise, it assumes a MTU of 576. As for MSS being 536, that's assuming that the MTU is 576 and also assuming the IP and TCP headers are their minimal sizes of 20 bytes each this allow a TCP MSS of 536 (576 - 40). You probably have an MTU/fragmentation problem. For each network interface on both client and server set the MTU to 576, eg ifconfig eth0 mtu 576. If the problem goes away, read on. Long Answer Long answer: At each routing hop, IP packets bigger than the outgoing interface's Maximum Transmission Unit (MTU) get fragmented. Ogata Yasuhiko @yogata. RFC1812にIPv4の最小MTUが576と書いているって見たんだけれど、そうじゃないような。MTUを576にしてパケットフラグメントを防ぐテクニックがあるよ、とは書いているけれど、最小MTUサイズは576バイトと明示してはいないなぁ。 When using a broadband connection the mtu's are defaulted to size 1500 mtu(for general uses and speed, which this is the best setting for ). But for gaming the best setting is 576 mtu, (which is also the best all round setting for dial-up). May 28, 2015 · 1) Goto the client & check MTU of all LAN cards MTU. MAke sure that "LAN" MTU should be 1500. 2) VPN MTU is by default 1400 . 3) goto cmd & type netsh int ipv4 set subint "xxxxxxxxx" mtu=1200 store=persistent ( where xx= name of interface which you want to change the MTU" 4) after changing checked by cmd netsh int ipv4 sh int Since it is the minimum size specified in the IP standard, 576 bytes has become a common default MTU value used for IP datagrams. Even if a host is connected over a local network with an MTU larger than 576, it may choose to use an MTU value of 576 anyway, to ensure that no further fragmentation will be required by intermediate routers. They send an MTU of 576 via DHCP (I'll come back to this later) when we know the Layer 2 can and does carry 1500, case in point: # ping google.com -M do -s 1472 PING google.com (172.217.1.110

Forum discussion: I note most ppl state MTU of 1488 is best, but I note throughput drops when I set everything to 1488, and goes back to the better throughput when I reset back to 1500. Is

3. Our MTU size is 1500 which is the default MTU size on most systems. Change Windows MTU Size. 1. Open a Command Prompt CMD (Right Click CMD -> Run Ad Administrator) 2. Type the following commands in order. netsh interface ipv4. 3. Your command window will now be at the prompt to change MTU using the next command below. 4. , powered by the Localist Community Event Platform. Log in with your Michigan Tech ISO username and password. This is the number of bytes that FOR SURE will not cause fragmentation, because the minimum MTU size that an host can set is 576 and IP header max size can be 60 bytes (508 = 576 MTU - 60 IP - 8 UDP) By the way i'd try to go with 1472 bytes because 1500 is a standard-enough value.

For example, the MTU of many PPP connections is 576, so if you connect to the Internet via PPP, you might want to set your machine's MTU to 576 too. Most Ethernetnetworks, on the other hand, have an MTU of 1500, which is the default MTU setting for Windows 95.

I believe the MTU of 576 was an expectation (standard) that any IP media would at least support that. So, IP, if it doesn't know otherwise, it assumes a MTU of 576. As for MSS being 536, that's assuming that the MTU is 576 and also assuming the IP and TCP headers are their minimal sizes of 20 bytes each this allow a TCP MSS of 536 (576 - 40).