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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Note: The ATMARP server mechanism requires that each client be<br />

administratively configured with the ATM address of the ATMARP server.<br />

ARP table add/update algorithm<br />

Consider the following points:<br />

► If the ATMARP server receives a new <strong>IP</strong> address in an InATMARP reply, the<br />

<strong>IP</strong> address is added to the ATMARP table.<br />

► If the InATMARP <strong>IP</strong> address duplicates a table entry <strong>IP</strong> address <strong>and</strong> the<br />

InATMARP ATM address does not match the table entry ATM address, <strong>and</strong><br />

there is an open VC associated with that table entry, the InATMARP<br />

information is discarded <strong>and</strong> no modifications to the table are made.<br />

► When the server receives an ATMARP request over a VC, where the source<br />

<strong>IP</strong> <strong>and</strong> ATM address match the association already in the ATMARP table <strong>and</strong><br />

the ATM address matches that associated with the VC, the server updates<br />

the timeout on the source ATMARP table entry. For example, if the client is<br />

sending ATMARP requests to the server over the same VC that it used to<br />

register its ATMARP entry, the server notes that the client is still “alive” <strong>and</strong><br />

updates the timeout on the client's ATMARP table entry.<br />

► When the server receives an ARP_REQUEST over a VC, it examines the<br />

source information. If there is no <strong>IP</strong> address associated with the VC over<br />

which the ATMARP request was received <strong>and</strong> if the source <strong>IP</strong> address is not<br />

associated with any other connection, the server adds this station to its<br />

ATMARP table. This is not the normal way because, as mentioned earlier, it<br />

is the responsibility of the client to register at the ATMARP server.<br />

ATMARP table aging<br />

ATMARP table entries are valid:<br />

► In clients for a maximum time of 15 minutes<br />

► In servers for a minimum time of 20 minutes<br />

Prior to aging an ATMARP table entry, the ATMARP server generates an<br />

InARP_REQUEST on any open VC associated with that entry <strong>and</strong> decides what<br />

to do according to the following rules:<br />

► If an InARP_REPLY is received, that table entry is updated <strong>and</strong> not deleted.<br />

► If there is no open VC associated with the table entry, the entry is deleted.<br />

Therefore, if the client does not maintain an open VC to the server, the client<br />

must refresh its ATMARP information with the server at least once every 20<br />

minutes. This is done by opening a VC to the server <strong>and</strong> exchanging the initial<br />

InATMARP packets.<br />

Chapter 2. Network interfaces 49

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