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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.
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
- 0 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.
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.
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
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