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Hello Everyone,

In case of DSL Modem , normally service providers employ PPPoE connection between Modem and BRAS (BNG). We all know that when Modem dials , it goes through PPPoE stages namely 1 ) PADI 2) PADO 3) PADR and 4) PADS.

After PPPoE session is established , PPP session starts establishing whereby it went through PPP stages namely 1) LCP 2) PAP or CHAP and 3) NCP.

After PPP session is established , Modem gets dynamic IP from BRAS and user starts browsing. However for session maintenance / house keeping , PPP keepalives are exchanged between Modem and BRAS after pre defined interval.

Please note that this is vendor independent query and is related to standard implementation which usually all vendors follow.

The question is Who initiates the PPP keepalives packets? 

Below are the three possibilities

  1. Is it like that the modem initiates and BRAS answers or BRAS initiates and modem answers?

  2. Is it like both modem and BRAS send PPP keepalives packets independently to each other? BRAS sends PPP keepalives packets to modem and wait for answer. Likewise Modem sends PPP keepalive packets to BRAS and wait for answer.

If possible , please share documented proof (standards) which supports argument.

Many Thanks

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RFC 1661 state:

"5.8. Echo-Request and Echo-Reply

Description

  LCP includes Echo-Request and Echo-Reply Codes in order to provide
  a Data Link Layer loopback mechanism for use in exercising both
  directions of the link.  This is useful as an aid in debugging,
  link quality determination, performance testing, and for numerous
  other functions.

  Upon reception of an Echo-Request in the LCP Opened state, an
  Echo-Reply MUST be transmitted.

  Echo-Request and Echo-Reply packets MUST only be sent in the LCP
  Opened state.  Echo-Request and Echo-Reply packets received in any
  state other than the LCP Opened state SHOULD be silently
  discarded. "

There is no industry standard who initiates echo request and on what intervals. It's depended on hardware/software/configuration variable. Some modems/routers check every 30s some every 60s as default. It is initiated from both sides and run independent intervals of each other. Some use smart algorithms for a check like interface data counters.

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