3Com VCX V7122, Руководство пользователя

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V7122 Gateway User Guide
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No-Op Packets
The gateway's No-Op packet support can be used to verify Real-Time Transport Protocol
(RTP) and T.38 connectivity, and to keep NAT bindings and Firewall pinholes open. No-Op
packets are available for sending in RTP and T.38 formats.
Users can control the activation of No-Op packets by using the ini file parameter
NoOperationSendingMode. If No-Op packet transmission is activated, users can control the
time interval in which No-Op packets are sent in the case of silence (that is, no RTP or T.38
traffic). This is performed using the NoOpInterval
ini parameter.
Receipt of No-Op is always supported.
ƒ
RTP No-Op:
The RTP No-Op support complies with IETF’s draft-wing-avt-rtp-noop-03.txt (titled ‘A No-
Op Payload Format for RTP’). This IETF document defines a No-Op payload format for
RTP.
The draft defines the RTP payload type as dynamic. Users can control the payload type
with which the No-Op packets are sent. This is performed using the
RTPNoOpPayloadType
ini parameter. 3Com’s default payload type is 120.
ƒ T.38 No-Op:
T.38 No-Op packets are sent only while a T.38 session is activated. Sent packets are a
duplication of the previously sent frame (including duplication of the sequence number).
Point-to-Point Protocol over Ethernet (PPPoE)
PPPoE is a method of sending the Point-to-Point Protocol over Ethernet network.
Point-to-Point Protocol (PPP) Overview
Point-to-Point Protocol (PPP) provides a method of transmitting data over serial point-to-
point links. The protocol defines establishing, configuring and testing the data link connection
and the network protocol.
The PPP standard describes a state machine used to establish a valid connection between
two hosts over a serial connection. There are three major stages described, helping to
establish a network layer (such as an IP) connection over the point-to-point link: LCP (Link
Configuration Protocol) Authentication and NCP (Network Control Protocol). Once the
network protocol is configured, the two hosts can communicate, sending network layer
protocol (such as IP) over the PPP connection (a small PPP header is added at the
beginning of each packet).
At the initial phase, the hosts use LCP (link configuration protocol) to negotiate for link
characteristic and parameters. Packets sent in this phase have two octets of ‘PPP header’
followed by LCP message with variable length. Various parameters and options are