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Method to improve multicast path MTU discovery Disclosure Number: IPCOM000031804D
Original Publication Date: 2004-Oct-11
Included in the Prior Art Database: 2004-Oct-11

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When a sender S sends a multicast packet to multicast group G with path MTU discovery turned on, it might get ICMP error messages back from a router indicating that an outgoing link has a smaller MTU than the current estimate. The sender S then lowers the path MTU estimate and starts sending out smaller datagrams. Consider the following network shown below: S is the sender of multicast datagrams. R1-R9 are multicast routers that forward multicast datagrams. In this case sender S initially sends 1500 byte multicast datagrams. However router R7 will send it an ICMP error message indicating that there is a smaller MTU of 1000 bytes. Sender S will then fragment the multicast datagrams and send them out. Notice that in this case, every other link in the multicast tree can support 1500 byte datagrams. They will all be penalized because of the one small link between R7 and R9.