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<pre>Network Working Group J. T. Korb
Request for Comments: 877 Purdue University
September 1983
<span class="h1">A Standard for the Transmission of IP Datagrams</span>
<span class="h1">Over</span>
<span class="h1">Public Data Networks</span>
This RFC specifies a standard adopted by CSNET, the VAN gateway, and
other organizations for the transmission of IP datagrams over the
X.25-based public data networks.
An X.25 virtual circuit is opened on demand when a datagram arrives at
the network interface for transmission. A virtual circuit is closed
after some period of inactivity (the length of the period depends on
the cost associated with an open virtual circuit). A virtual circuit
may also be closed if the interface runs out of virtual circuits. An
algorithm for managing virtual circuits during peak demand is given
in [<a href="#ref-1" title=""CSNET Protocol Software: The IP-to-X.25 Interface"">1</a>].
STANDARDS
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a> The first octet in the Call User Data Field (the first data octet</span>
<span class="h3"> in the Call Request packet) is used for protocol demultiplexing.</span>
The value hex CC (binary 11001100, decimal 204) is used to mean
INTERNET PROTOCOL.
<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a> IP datagrams are sent as X.25 "complete packet sequences". That is,</span>
<span class="h3"> datagrams begin on packet boundaries and the M bit ("more data") is</span>
used for datagrams that are larger than one packet. There are no
additional headers or other data in the packets.
<span class="h3"><a class="selflink" id="section-1.3" href="#section-1.3">1.3</a> Unless a larger packet size is negotiated, the maximum size of an</span>
<span class="h3"> IP datagram transmitted over X.25 is 576 octets. If two sites</span>
negotiate a large X.25 packet size (for example, 1024 octets), an
IP datagram of that size is allowed.
<span class="h3"><a class="selflink" id="section-1.4" href="#section-1.4">1.4</a> Either site may close a virtual circuit. If the virtual circuit is</span>
<span class="h3"> closed or reset while a datagram is being transmitted, the datagram</span>
is lost.
GENERAL REMARKS
<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a> Protocols above IP, such as TCP, do not affect this standard. In</span>
<span class="h3"> particular, no attempt is made to open X.25 virtual circuits</span>
corresponding to TCP connections.
<span class="grey">Korb [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc877">RFC 877</a> September 1983</span>
Transmission of IP Datagrams Over Public Data Networks
<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a> Features of X.25 not discussed in this document are not used.</span>
<span class="h3"> For example, interrupt packets and the D bit (indicating</span>
end-to-end significance) are not used.
<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a> Negotiable features (facilities) of X.25 are allowed. For</span>
<span class="h3"> example, sites are free to negotiate larger packet and window</span>
sizes.
<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a> Some sites, such as CSNET sites, may attempt to open multiple</span>
<span class="h3"> virtual circuits to a single site. Sites should attempt to</span>
handle such incoming calls gracefully: transmit on the
additional circuits if possible and accept incoming datagrams
from them, but do not accept the CALL REQUEST, only to
immediately close the connection or ignore datagrams
transmitted on such circuits.
REFERENCE
[<a id="ref-1">1</a>] Comer, D.E. and Korb, J.T., "CSNET Protocol Software: The
IP-to-X.25 Interface", SIGCOMM Symposium on Communications
Architectures and Protocols, March 1983.
Korb [Page 2]
</pre>
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