NFS, by default, has a read and write block size of 8192, so a NFS IPv4/UDP datagram is approximately 8500 bytes (which includes NFS, UDP, and IPv4 headers). A sending station connected to an Ethernet (MTU 1500) has to fragment the 8500 byte datagram into six pieces; five 1500 byte fragments and one 1100 byte fragment.
2. Client -> Server DF- / fragmentation. In next scenario, a client uploads some data with a protocol other than TCP, which has the DF flag cleared. For example, this might be a user playing a game using UDP, or having a voice call. The big outbound packets might get fragmented at some point in the path. I want to enable UDP-Fragmentation-Offload on the interfaces connecting these servers. When I try to do it I get: # ethtool -K eno1 ufo on Unable to change UDP-Fragmentation-Offload. Unable to change any device features. When setting tso, it does can set, but it will output noise msg: # ethtool -K eth0 tso on Cannot get device udp-fragmentation-offload settings: Operation not supported Cannot get device udp-fragmentation-offload settings: Operation not supported # ethtool -k eth0 Features for eth0: Cannot get device udp-fragmentation-offload settings: Operation Fragmentation. Fragmentation occurs when a packet is sent that exceeds the MTU of a network interface. The TCP/IP stack will break the packet into smaller pieces (fragments) that conform to the interface's MTU. Fragmentation occurs at the IP layer and is independent of the underlying protocol (such as TCP). UDP Fragmentation Flood. It is another one of those cleverly masked DDoS attacks that are not easily detected. The activity generated by this attack resembles valid traffic and all of it is kept within limits. This version of the UDP Flood attack sends larger yet fragmented packets to exhaust more bandwidth by sending fewer fragmented UDP packets. Typically the attacker will use large UDP packets over 1500 bytes of data which forces fragmentation as ethernet MTU is 1500 bytes. This attack is a variation on a typical UDP flood but it enables more network bandwidth to be consumed with fewer packets. Fragmented UDP Flood (UDP Fragmentation, Nuke) This is one of the kinds of UDP Flood attacks. Its distinctive feature is using fragmented packets of the maximum size in order to fill the channel with as few packets as possible. Since these fragments of packets are fabricated and have nothing to do with real data, the victim server that receives
At the app level you don't have to worry about UDP packet fragmentation (send or receive): on send, if you exceed the OS MTU, the packets will be automatically fragmented. During receive, UDP packets will get dropped if there is a problem (timeout/checksum error, etc.): you'll never see a partial UDP packet (you may see a corrupted one, though
Any IP datagram can be fragmented if it is larger than the MTU. Whether it contains UDP, TCP, ICMP, etc. does not matter. Most Ethernet networks support a 1500 byte MTU. With the IPv4 header being 20 bytes and the UDP header being 8 bytes, the payload of a UDP packet should be no larger than 1500 - 20 - 8 = 1472 bytes to avoid fragmentation. UDP and ICMP fragmentation attacks - These attacks involve the transmission of fraudulent UDP or ICMP packets that are larger than the network's MTU, (usually ~1500 bytes). As these packets are fake, and are unable to be reassembled, the target server's resources are quickly consumed, resulting in server unavailability.
IP Datagram Fragmentation with Example Not all link-layer protocols can carry network-layer packets of the same size. Some protocols can carry big datagrams, whereas other protocols can carry only little packets. For example, Ethernet frames can carry upto 1,500 bytes of data, whereas frames for some wide-area links can carry no more than 576 bytes.
IP fragmentation is an Internet Protocol (IP) process that breaks packets into smaller pieces (fragments), so that the resulting pieces can pass through a link with a smaller maximum transmission unit (MTU) than the original packet size. The fragments are reassembled by the receiving host.. RFC 791 describes the procedure for IP fragmentation, and transmission and reassembly of IP packets. However, on the other hand, UDP being a message oriented protocol, it does not have a built-in reordering or retransmitting mechanism, so fragmentation should be avoided. Further, when your traffic flows through devices that you have no control over nor visibility on such as sending traffic over the internet, then this should be avoided at all