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On which OSI layer does PPP operate?

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8 years 3 days ago #38706 by Hisham Fouad
Hello Guys,

I need some help with this questions ...

Usually, we refer to PPP as (Layer 2 Wan Protocol) ... But by giving a closer look, we can see that it uses NCP -Network Control Protocol- to negotiate Layer 3 addressing.
I thought lower layer protocols can't negotiate higher level protocols and addressing ?
Shouldn't Layer 2 protocols -such as HDLC and PPP- focus only on encapsulating Layer 3 packets and forward them without changing or negotiation their Layer 3 addresses ?

Or shall we call PPP a Multi-layer Protocol rather than just L2 protocol ?

Regards,
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8 years 3 days ago #38707 by Chris
Hisham,

PPP is in fact a Layer 2 protocol as you corrected noted, however it does include Network Control Protocols (NCP) to allow higher level protocols to be supported. In fact there is one NCP for every higher-layer protocol supported by PPP and this was defined by the RFC 1661 ( tools.ietf.org/html/rfc1661 ).

In short, PPP is a Layer 2 protocol that provides support for higher layer protocols through NCP. NCP allows the negotiation of higher-level protocol parameters such as IP address, however this alone is not considered enough to classify PPP as a multi-level protocol.

You might also want to read through the Wikipedia for PPP which might provide an additional insight:
en.wikipedia.org/wiki/Point-to-Point_Protocol

Hope this helps.

Chris.

Chris Partsenidis.
Founder & Editor-in-Chief
www.Firewall.cx
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8 years 3 days ago - 8 years 3 days ago #38708 by Hisham Fouad
Hello Chris,

I think there's some kinda argument about when,how or if we should classify PPP as a multi-layer protocol.

I've previously opened such discussion on Cisco forums and one of the founding members -Mr Richard Burts- replied to me suggesting that PPP is a multi-layer protocol and the assumption of limiting such protocol within a single layer may be wrong one.

Here's our discussion link if you're interested to hear his opinion:
supportforums.cisco.com/discussion/13018...-layer-ppp-functions

I'm a little bit confused honestly, there's some kinda foggy area between OSI layers and how to classify protocols around them, and networks experts seem to have a contradictory points of view about it too.
Last edit: 8 years 3 days ago by Hisham Fouad.
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