10.7 ip_slowtimo Function

Team-Fly     TCP/IP Illustrated, Volume 2: The Implementation By Gary R. Wright, W. Richard Stevens Table of Contents Chapter 10.  IP Fragmentation and Reassembly 10.7 ip_slowtimo Function As shown in Section 7.4, each protocol in Net/3 may specify a function to be called every 500 ms. For IP, that function is ip_slowtimo, shown in Figure 10.30, which times out the fragments on the reassembly list. Figure 10.30. ip_slowtimo function. 515-534 ip_slowtimo traverses the list of partial datagrams and decrements the reassembly TTL field. ip_freef is called if the field drops to 0 to discard the fragments associated with the datagram. ip_slowtimo runs at splnet to prevent the lists from being modified by incoming packets. ip_freef is shown in Figure 10.31. Figure 10.31. ip_freef function. 470-486 ip_freef removes and releases every fragment on the list pointed to by fp and then releases the list itself. ip_drain Function In Figure 7.14 we showed that IP defines ip_drain as the function to be called when the kernel needs additional memory. This usually occurs during mbuf allocation, which we described with Figure 2.13. ip_drain is shown in Figure 10.32. Figure 10.32. ip_drain function. 538-545 The simplest way for IP to release memory is to discard all the IP fragments on the reassembly list. For IP fragments that belong to a TCP segment, TCP eventually retransmits the data. IP fragments that belong to a UDP datagram are lost and UDP-based protocols must handle this at the application layer. Team-Fly     Top