TCP IP Illustrated

The mss lets a host limit the size of datagrams that

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Unformatted text preview: and a destination whose IP address has a totally different network ID from ours is nonlocal, a destination with the same network ID but a different subnet ID could be either local or nonlocal. Most implementations provide a configuration option (Appendix E and Figure E.1) that lets the system administrator specify whether different subnets are local or nonlocal. The setting of this option determines whether the announced MSS is as large as possible (up to the outgoing interface's MTU) or the default of 536. The MSS lets a host limit the size of datagrams that the other end sends it. When combined with the fact that a host can also limit the size of the datagrams that it sends, this lets a host avoid fragmentation when the host is connected to a network with a small MTU. Consider our host slip, which has a SLIP link with an MTU of 296 to the router bsdi. Figure 18.8 shows these systems and the host sun. Figure 18.8 TCP connection from sun to slip showing MSS values. We initiate a TCP connection from sun to slip and watch the segments using tcpdump. Figure 18.9 shows only the connection establishment (with the window size advertisements removed). file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_conn.htm (9 of 37) [12/09/2001 14.47.16] Chapter 18. TCP Connection Establishment and Termination 1 0.0 2 0.10 (0.00) 3 0.10 (0.00) sun.1093 > slip.discard: S 517312000:517312000 (0) <mss 1460> slip.discard > sun.1093: S 509556225:509556225 (0) ack 517312001 <mss 256> sun.1093 > slip.discard: . ack 1 Figure 18.9 tcpdump output for connection establishment from sun to slip. The important fact here is that sun cannot send a segment with more than 256 bytes of data, since it received an MSS option of 256 (line 2). Furthermore, since slip knows that the outgoing interface's MTU is 296, even though sun announced an MSS of 1460, it will never send more than 256 bytes of data, to avoid fragmentation. It's OK for a system to send less than the MSS announ...
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