Telindus 1031, Руководство пользователя и справочник

Страница: 130 / 394

Telindus 1031 Router Chapter 7
User manual Configuring the router
120
7.4.4 How does PAT work?
Again consider the network topology as depicted in 7.4.3 - Configuring PAT on page 119 .
The following two paragraphs explain how the Telindus 1031 Router treats the outgoing and incoming
traffic when PAT is applied.
Outgoing traffic (to the Internet)
The Telindus 1031 Router replaces the source address by its PAT address in all the traffic coming from
the Ethernet and destined for the Internet. Depending on the IP transport protocol and the number of
simultaneous users accessing the Internet, the Telindus 1031 Router takes different actions:
Protocol
TCP Description This is a connection-oriented protocol: two devices communicating with the
TCP protocol build a session before exchanging user data. When they have
finished exchanging user data, the session is closed.
Examples of such applications are Telnet, HTTP and FTP. The TCP header
contains a
port field indicating the higher-layer protocol.
Action When a session is started, a specific port number is assigned to this ses-
sion. All traffic from this session is assigned this specific port number.
The specific port number is freed within 5 minutes after the TCP session is
closed (i.e. after TCP Reset or TCP Finish is seen). If the session has not
been properly closed, the port number is freed 24 hours after the last ses-
sion traffic. This time is configurable (refer to
telindus1031/router/defaultNat/tcp-
SocketTimeOut on page 217 ).
UDP Description This is a connection-less protocol: user data can be sent without first build-
ing a session.
Examples of such applications are SNMP and TFTP. Although TFTP is ses-
sion-oriented, it builds the session at a higher level and uses UDP for its
simplicity as transport protocol. The UDP header contains a
port field indi-
cating the higher-layer protocol.
Action The Source Port Number is replaced by a specific port number. All traffic
from this source IP address / port number pair is assigned this specific port
number.
If there is no traffic for 5 to 10 minutes, the specific port number is freed.
ICMP Description This is a connection-less protocol: user data can be sent without first build-
ing a session.
An example of such an application is ping. These protocols do not have port
numbers.
Action Each ICMP packet is forwarded towards the Internet. Each ICMP packet is
considered as a new session.
If there is no traffic for 5 to 10 minutes, the session is closed.
The fact that it is possible to open a total of 2048 simultaneous sessions
and that each ICMP packet is considered as a new session, implies that for
instance a continuous series of ping requests at a rate of one per second
will allocate between 300 and 600 sessions.