#MTU F5 VPN CLIENT WINDOWS#
These steps are based on the Windows ping command. By using the ping commands to send a packet at various sizes with the DF bit set we can see if the router sends back the correct ICMP message, what the PMTU is, and where the black hole actually is. Using the ping command we can troubleshoot and hopefully find the hop in which the blackhole exists. Of course this greatly increases the delay when trying open the web page, and can cause much confusion to the client and system admins. In this case the sending server doesn’t receive an acknowledgment for their sent packets so after a certain amount of retries it reduces its MTU (and in turn MSS) and tries resend the packets, in a hope that the reduced packets will make it through to the destination.
This can be down to black hole detection. The most common scenario that I see is where a page will load but it will take ages to do so. Such as the client finding they might be unable to access one site, this is normally an SSL based site due to the data payload overhead of SSL. The destination is still waiting for its packet, and the whole session falls down.īelow shows you the 2 scenarios, ICMP messages not being sent In this scenario the sender is waiting for an acknowledgment for its sent packet. This can be down to the router not sending the ICMP message or the ICMP message being blocked on the way back to the sender, MTU of next hop: 1400 What is a PMTU Black Hole ?Ī PMTU black hole is where the ICMP message doesn’t reach the sending host to inform it that it needs to adjust its MTU. Once the sender has received this ICMP message it can then adjust its MTU so that it can send a packet at the correct size so that the router is then able to pass it on.īelow shows an sample of the ICMP header and ‘ next-hop MTU field’ field, RFC 1911 expands this ICMP message to incorporate the MTU of the interface that is unable to fragment the packet (shown below). This message is stating that it needs to fragment the packet but it is unable to because of the DF bit being set.
#MTU F5 VPN CLIENT CODE#
If the DF bit is set, it is unable to fragment the packet so it discards the packet and sends a ICMP (Type 3 Code 4) message ‘Fragmentation needed and DF set’ message back to the sender.
When the networking node (router) receives the frame which is larger then the outgoing interfaces MTU it checks for the DF bit. Modern systems tend not to use fragmentation due to the overhead involved in sending multiple packets, not to mention the various security issues involved. Path MTU Discovery – If the DF (Don`t Fragment) bit is set the network device will send an ICMP packet back to the sending computer stating its MTU size.Fragmentation – If the sending computer has not set the DF (Don`t Fragment) bit, then the traffic will be fragmented.When a server sends its traffic across the network (internet), one of the network devices (routers, etc) may have an MTU smaller then the sending computer. What does this have to do with PMTU Discovery ? Leaving us with 1460 bytes for the date payload. The TCP header would use 20 bytes, with another 20 bytes used for the IP header. The default MTU would be 1500 which excludes the Ethernet headers and trailers. This (network interface) setting dictates the size of the largest frame it can send across the network.Įxample : A server is wanting to send an Ethernet Packet using TCP. When sending traffic across a network, computers use something called an MTU (Maximum Transmission Unit). What does this have to do with PMTU Discovery ?.