Address Resolution w/ PPP and HDLC

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Hello all, quick question. As we know, a host will use ARP to resolve unknown MAC to known IP on Ethernet. We know basically the same thing happens for frame relay using DLCIs and ATM using PVCs or NSAP addresses. How does address resolution work with PPP or HDLC?

Nate--IRL--

Seems to be IPCP? Internet Protocol Control Protocol - Wikipedia, the free encyclopedia It seems this is used by the layer 2 PPP NCP sub-layer to negotiate IP over PPP.

Nate

chX

Correct me if I'm wrong, but I believe PPP doesn't use Layer 2 addressing in the same way that Ethernet would. As far as I can tell, the address field in PPP frames is the broadcast address, or "the other side of this link".

The IT Guy

chX,
That is correct. It is also my understanding that address resolution is not neccessary in poin-to-point links for obviouse reasons (directly connected) with only two host on any given ppp link. The PPP header only has one address field and will not behave like ARP/RARP anyway. My point is that you dont need address resolution for two routers sending packets to each other over a PPP link. You will need adress resolution in an ethernet environment where you have many routers sending packets over the same segment. I beleive Odom explains this better in one of his older cisco press books.

MrBrian

I haven't looked into this extensively, but yea I don't think PPP or HDLC need any address resolution mechanisms, like some of the above posters said.

It has to do with the multi-access nature of the Layer 2 protocol such as Ethernet and Frame-Relay.. whereas PPP and HDLC are point to point protocols, so they don't really have anything to resolve. Although I'm not sure what the exact header format is for PPP or HDLC.. so I'm not sure what they're capable of doing other than pap/chap for PPP. If anyone had any further clarification I'd listen

phoeneous

MrBrian wrote: »

I haven't looked into this extensively, but yea I don't think PPP or HDLC need any address resolution mechanisms, like some of the above posters said.

It has to do with the multi-access nature of the Layer 2 protocol such as Ethernet and Frame-Relay.. whereas PPP and HDLC are point to point protocols, so they don't really have anything to resolve. Although I'm not sure what the exact header format is for PPP or HDLC.. so I'm not sure what they're capable of doing other than pap/chap for PPP. If anyone had any further clarification I'd listen

According to RFC 1661, PAP and CHAP are the only two authentication methods available. Although speaking from experience, I've never setup ppp authentication when connecting a single serial link or multilink with an isp. They just never ask for it.

Authentication-Protocol


The Authentication-Protocol field is two octets, and indicates the
authentication protocol desired. Values for this field are always
the same as the PPP Protocol field values for that same
authentication protocol.


Up-to-date values of the Authentication-Protocol field are
specified in the most recent "Assigned Numbers" RFC [2]. Current
values are assigned as follows:


Value (in hex) Protocol


c023 Password Authentication Protocol
c223 Challenge Handshake Authentication Protocol

And if you look at a PPP frame, it uses NCP during the Network phase and includes layer 3 info. So I would assume that network addressing on some kind is needed.

drkat

PPP doesnt do any layer 2 mapping - this is correct. It puts the frame on the wire and wishes it the best of luck.