TCP IP Illustrated

1 interpretation of subnetsarelocal kernel variable

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Unformatted text preview: agrams are not forwarded. If 1, forwarding is always enabled. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/append_e.htm (1 of 16) [12/09/2001 14.48.06] Appendix E: Configurable Options GATEWAY If defined, causes IPFORWARDING to be set to 1. Additionally, defining this constant causes certain system tables (the ARP cache and the routing table) to be larger. SUBNETSARELOCAL The value of this constant initializes the kernel variable subnetsarelocal. If 1 (default), a destination IP address with the same network ID as the sending host but a different subnet ID is considered local. If 0, only destination IP addresses on an attached subnet are considered local. This is summarized in Figure E.1. Network IDs Subnet IDs subnetsarelocal 1 same same different same different - 0 local local nonlocal local nonlocal nonlocal Comment always local depends on configuration always nonlocal Figure E.1 Interpretation of subnetsarelocal kernel variable. This affects the MSS selected by TCP. When sending to local destinations, TCP chooses the MSS based on the MTU of the outgoing interface. When sending to nonlocal destinations, TCP uses the variable tcp_mssdflt as the MSS. IPSENDREDIRECTS The value of this constant initializes the kernel variable ipsendredirects. If 1 (default), the host will send ICMP redirects when forwarding IP datagrams. If 0, ICMP redirects are not sent. DIRECTED_BROADCAST If 1 (default), received datagrams whose destination address is the directed broadcast address of an attached interface are forwarded as a link-layer broadcast. If 0, these datagrams are silently discarded. The following variables can also be modified. These variables are spread throughout different files in the /usr/src/sys/netinet directory. tcprexmtthresh The number of consecutive ACKs that triggers the fast retransmit and fast recovery algorithm. The default value is 3. tcp_ttl The default value for the TTL field for TCP segments. Default value is 60. tcp_mssdflt The default TCP MSS for nonlocal destinations. Default value is 512. file:///D|/Documents%20and%20Settings...
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