As a result of multicast-based soft handoff, the server will send duplicate encapsulated packets to the MH. To prevent the MH's TCP/IP stack and applications from receiving duplicates of the inner packet (i.e., original IP packet, see Figure 7), the ROAM proxy suppresses duplicates during multicast-based soft handoff.
The ROAM proxy maintains a small window of MD5  digests of recent packets. The proxy computes digests over the first 20 bytes of the IP header and the first 8 bytes of the transport header. The first 28 bytes of a packet are sufficient to differentiate non-identical packets in practice . To minimize duplicates, the window size must be sufficiently large so that a duplicated packet that arrives both via a very low latency link and via a very high latency link will be caught in the window. We use a window size of 1 second, which should be sufficient even if one path is very congested or contains a 500ms satellite link. Note that this scheme detects duplicates received on different addresses, which means that only duplicates generated by are eliminated, and not duplicate packets sent by the sender (e.g., TCP dup-ack). We show in Section 7 that a TCP bulk transfer flow using multicast-based soft-handoff achieves similar throughput to a flow without mobility.
Another implementation issue is when does the proxy stop using multicast. Our algorithm removes an address when a large fraction of its packets are duplicates as this indicates that the address is redundant. The ROAM proxy maintains a counter of duplicate packets received on both addresses () and a counter of packets received on each address (). When , we simply remove the address that has received fewer packets in the last window. The value is a constant indicating the fraction of an address's packets that must be duplicates before the address can be dropped. In addition, the proxy uses a timeout to prevent a newly added address with poor connectivity from being removed until the timeout expires. In our implementation, we use and s.