![]() needed and DF set unreachable sent to 10.1.1.1īelow shows the format of ICMP header of a “fragmentation needed and DF set” “Destination Unreachable” message. Here is an example of an ICMP “fragmentation needed and DF set” message that you might see on a router after the debug ip icmp command is turned on: ICMP: dst (10.10.10.10) frag. When the source station receives the ICMP message, it will lower the send MSS, and when TCP retransmits the segment, it will use the smaller segment size. If a router tries to forward an IP datagram, with the DF bit set, onto a link that has a lower MTU than the size of the packet, the router will drop the packet and return an Internet Control Message Protocol (ICMP) “Destination Unreachable” message to the source of this IP datagram, with the code that indicates “fragmentation needed and DF set” (type 3, code 4). A host usually “remembers” the MTU value for a destination since it creates a “host” (/32) entry in its routing table with this MTU value. When a host sends a full MSS data packet with the DF bit set, PMTUD reduces the send MSS value for the connection if it receives information that the packet would require fragmentation. If PMTUD is enabled on a host, and it almost always is, all TCP/IP or UDP packets from the host will have the DF bit set. Note: PMTUD is only supported by TCP and UDP. It is used to dynamically determine the lowest MTU along the path from a packet’s source to its destination. PMTUD was developed in order to avoid fragmentation in the path between the endpoints. TCP MSS takes care of fragmentation at the two endpoints of a TCP connection, but it does not handle the case where there is a smaller MTU link in the middle between these two endpoints. ![]() The bandwidth-delay product is a measurement of the maximum number of bits that can be on a network segment at any one time, and it is calculated by multiplying the segment’s bandwidth (in bits/sec) by the latency packets experience as they cross the segment (in sec).Įxample, a network segment with a bandwidth of 768 kbps and an end-to-end latency of 100 ms would have a bandwidth-delay product of 76,800 bits: ( 768,000 * 0.1 = 76,800). The G.114 recommendation states that the one-way latency for VoIP traffic should not exceed 150 ms.Latency is a factor in the calculation of the bandwidth-delay product. Note: Latency is the time required for a packet to travel from its source to destination. Some applications, such as Voice over IP (VoIP), are latency sensitive, meaning that they do not perform satisfactorily if the latency of their packets is too high. ![]() However, if you are sending data over slower link speeds, large MTU values could cause delay for latency-sensitive traffic. Smaller MTU sizes result in more overhead, because more packets (and therefore more headers) are required to transmit the same amount of data. ![]() A Maximum Transmission Unit (MTU), typically refers to the largest packet size supported on a router interface (1500 bytes). ![]()
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