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Unformatted text preview: It depends on the route being used at
any time. Also, routing need not be symmetric (the route from A to B may not be the reverse of file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm (9 of 11) [12/09/2001 14.46.33] file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm the route from B to A), hence the path MTU need not be the same in the two directions.
RFC 1191 [Mogul and Deering 1990] specifies the "path MTU discovery mechanism," a way to
determine the path MTU at any time. We'll see how this mechanism operates after we've described
ICMP and IP fragmentation. In Section 11.6 we'll examine the ICMP unreachable error that is
used with this discovery mechanism and in Section 11.7 we'll show a version of the
traceroute program that uses this mechanism to determine the path MTU to a destination.
Sections 11.8 and 24.2 show how UDP and TCP operate when the implementation supports path
MTU discovery. 2.10 Serial Line Throughput Calculations
If the line speed is 9600 bits/sec, with 8 bits per byte, plus I start bit and I stop bit, the line speed is
960 bytes/sec. Transferring a 1024-byte packet at this speed takes 1066 ms. If we're using the
SLIP link for an interactive application, along with an application such as FTP that sends or
receives 1024-byte packets, we have to wait, on the average, half of this time (533 ms) to send our
This assumes that our interactive packet will be sent across the link before any further "big"
packets. Most SLIP implementations do provide this type-of-service queuing, placing interactive
traffic ahead of bulk data traffic. The interactive traffic is normally Telnet, Rlogin, and the control
portion (the user commands, not the data) of FTP.
This type of service queuing is imperfect. It cannot affect noninteractive traffic that is already
queued downstream (e.g., at the serial driver). Also newer modems have large buffers so
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- Spring '12