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Unformatted text preview: |/Documents%20and%20Settings/bigini/Docum...i/homenet2run/tcpip/tcp-ip-illustrated/tcp_fut.htm (1 of 20) [12/09/2001 14.47.33] Chapter 24. TCP Futures and Performance In Section 2.9 we described the concept of the path MTU. It is the minimum MTU on any
network that is currently in the path between two hosts. Path MTU discovery entails
setting the "don't fragment" (DF) bit in the IP header to discover if any router on the
current path needs to fragment IP datagrams that we send. In Section 11.6 we showed the
ICMP unreachable error returned by a router that is asked to forward an IP datagram with
the DF bit set when the MTU is less than the datagram size. In Section 11.7 we showed a
version of the traceroute program that used this mechanism to determine the path
MTU to a destination. In Section 11.8 we saw how UDP handled path MTU discovery.
In this section we'll examine how this mechanism is used by TCP, as specified by RFC
1191 [Mogul and Deering 1990].
Of the various systems used in this text (see the Preface) only Solaris 2.x supports path MTU discovery. TCP's path MTU discovery operates as follows. When a connection is established, TCP
uses the minimum of the MTU of the outgoing interface, or the MSS announced by the
other end, as the starting segment size. Path MTU discovery does not allow TCP to
exceed the MSS announced by the other end. If the other end does not specify an MSS, it
defaults to 536. It is also possible for an implementation to save path MTU information
on a per-route basis, as we mentioned in Section 21.9.
Once the initial segment size is chosen, all IP datagrams sent by TCP on that connection
have the DF bit set. If an intermediate router needs to fragment a datagram that has the
DF bit set, it discards the datagram and generates the ICMP "can't fragment" error we
described in Section 11.6.
If this ICMP error is received, TCP decreases the segment size and retransmits. If the
router generated the newer form of this ICMP error, the segment size can be set to the
next-hop MTU minus the sizes of the IP and T...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12