Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model??? - TCP-IP

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Thread: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

  1. Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    I have posted this topic at somewhere else,but nobody could gives me a
    definitive answer,so I post it here and hope someone make it clear for
    me

  2. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    iLRainyday writes:

    > I have posted this topic at somewhere else,but nobody could gives me a
    > definitive answer,so I post it here and hope someone make it clear for
    > me


    Short answer - it's both.

    TCP/IP has NOTHING to do with the OSI reference model.

    There's a reason why we use TCP/UDP/IP stack instead of OSI stack.
    The OSI reference model is fatally flawed.
    It's a classic example on how to design a protocol stack the wrong way.

    ARP crosses the layers in order to function. That's what real protocols do.

    As Padlipski says, "If you know what you are doing, three layers is
    enough; if you don't, even seventeen levels won't help"



    --
    Posted via a free Usenet account from http://www.teranews.com


  3. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    On Feb 4, 1:11*pm, Bruce Barnett
    wrote:
    > iLRainyday writes:
    > > I have posted this topic at somewhere else,but nobody could gives me a
    > > definitive answer,so I post it here and hope someone make it clear for
    > > me

    >
    > Short answer - it's both.
    >
    > TCP/IP has NOTHING to do with the OSI reference model.


    The original poster was not asking about the OSI stack, but only about
    the OSI 7-layer model.

    ARP resolves the L2 address from an L3 address, but ARP requests and
    replies are not routed. And the L3 address can be for any Layer 3
    protocol. ARP is not limited to responding only to IP devices.

    I don't think that anyone doubts the applicability of the ISO/OSI 7-
    layer model, AT LEAST with respect to layers 1-4. Maybe the usefulness
    of layers above 4 might be debated. The question in the subject line
    is that ARP operates at Layer 2, in the sense that it does not use IP,
    or other L3 headers, for routing purposes. It is broadcast at the L2
    layer, and replies are sent back using L2 the address of the
    requesting host.

    Bert

  4. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    Albert Manfredi wrote:
    > On Feb 4, 1:11?pm, Bruce Barnett
    > wrote:
    > > iLRainyday writes:
    > > > I have posted this topic at somewhere else,but nobody could gives me a
    > > > definitive answer,so I post it here and hope someone make it clear for
    > > > me

    > >
    > > Short answer - it's both.
    > >
    > > TCP/IP has NOTHING to do with the OSI reference model.


    > The original poster was not asking about the OSI stack, but only about
    > the OSI 7-layer model.


    > ARP resolves the L2 address from an L3 address, but ARP requests and
    > replies are not routed. And the L3 address can be for any Layer 3
    > protocol. ARP is not limited to responding only to IP devices.


    > I don't think that anyone doubts the applicability of the ISO/OSI 7-
    > layer model, AT LEAST with respect to layers 1-4. Maybe the usefulness
    > of layers above 4 might be debated. The question in the subject line
    > is that ARP operates at Layer 2, in the sense that it does not use IP,
    > or other L3 headers, for routing purposes. It is broadcast at the L2
    > layer, and replies are sent back using L2 the address of the
    > requesting host.


    You have left-out layers 8 and 9:

    https://secure.isc.org/index.pl?/store/t-shirt/



    rick jones
    --
    No need to believe in either side, or any side. There is no cause.
    There's only yourself. The belief is in your own precision. - Jobert
    these opinions are mine, all mine; HP might not want them anyway...
    feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

  5. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    On Feb 4, 12:11*pm, Bruce Barnett
    wrote:
    > iLRainyday writes:
    > > I have posted this topic at somewhere else,but nobody could gives me a
    > > definitive answer,so I post it here and hope someone make it clear for
    > > me

    >
    > Short answer - it's both.
    >
    > TCP/IP has NOTHING to do with the OSI reference model.


    Hmmm...I guess.

    Surely you will agree though, that one can see a bit of layering
    regarding what is located at each end of an abstract unit of data
    transmitted and received:

    1. interface to interface [link-layer frame]
    2. node to node [network-layer packet]
    3. port to port..[transport layer whatever]
    4.+..[There is some irregularity at this point and higher that
    researchers still have not figured out]

    > There's a reason why we use TCP/UDP/IP stack instead of OSI stack.
    > The OSI reference model is fatally flawed.
    > It's a classic example on how to design a protocol stack the wrong way.


    I definitely agree here. When I read the OSI "specification", it was
    like being locked in a closet that smelled like 500-year-old moth
    balls. If someone asked me to find the OSI specification within 5
    minutes using Google to save my life, I'd probably die.

    > ARP crosses the layers in order to function. That's what real protocols do..


    Yup. Primitives are still king. You do need those. But OSI put the
    cart before the horse. They took something that was good - the notion
    of layering - and presumed it would save the world. It might have been
    better to write an entire, working protocol stack, think and rethink
    it until one is confident that it is nearing its regular form, clean
    it up, then write about that. Instead, they created 500-page
    documents filled with nauseating vaguery that would make even the most
    patient technophile get eye-rot.

    > As Padlipski says, "If you know what you are doing, three layers is
    > enough; if you don't, even seventeen levels won't help"


    Hah! This is classic.

    -Le Chaud Lapin-

  6. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    Albert Manfredi writes:

    > ARP resolves the L2 address from an L3 address, but ARP requests and
    > replies are not routed. And the L3 address can be for any Layer 3
    > protocol. ARP is not limited to responding only to IP devices.


    Well, while ARP routes at layer 2, it needs information from layer 3.
    That, to my mind, violates the OSI concept of layering.

    > I don't think that anyone doubts the applicability of the ISO/OSI 7-
    > layer model, AT LEAST with respect to layers 1-4.


    Applicability? It's a conceptual model that lawnchair theorists like to use.
    Those that have tried to build protocols based on the concept
    failed, as it caused increased complexity and decreased efficiency.

    If the concept of layering is so wonderful, where are these layered
    protocols used?




    --
    Posted via a free Usenet account from http://www.teranews.com


  7. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    On Feb 5, 6:23*am, Bruce Barnett
    wrote:

    > Well, while ARP routes at layer 2, it needs information from layer 3.
    > That, to my mind, violates the OSI concept of layering.


    The layer 3 information in ARP frames, i.e. the L3 address, is nothing
    but data payload in ARP. So it really doesn't violate anything.

    > > I don't think that anyone doubts the applicability of the ISO/OSI 7-
    > > layer model, AT LEAST with respect to layers 1-4.

    >
    > Applicability? It's a conceptual model that lawnchair theorists like to use.
    > Those that have tried to build protocols based on the concept
    > failed, as it caused increased complexity and decreased efficiency.


    I've always found these arguments hard to parse.

    I've used this example before: If someone wants to learn how
    automobile drivelines work, he will probably be taught a generic
    model, which includes engine, transmission, differential, wheels.

    Now, if some car designs combine the transmission and differential in
    one unit called "transaxle," does this invalidate the model? Of course
    not.

    Same applies here. The 7-layer model doesn't HAVE to be treated as a
    straightjacket. But at the same time, someone who doesn't understand
    the difference between L1, L2, L3, and L4 (at the very least) is
    someone who is completely lost about the topic of digital networks.

    Bert

  8. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    On Feb 5, 3:23 am, Bruce Barnett
    wrote:

    > Well, while ARP routes at layer 2, it needs information from layer 3.
    > That, to my mind, violates the OSI concept of layering.


    Nonsense. ARP is built on top of layer 2, as a layer 3 protocol should
    be. ARP is a layer 3 protocol. ARP implements layer 3 services by
    using a layer 2 protocol. That is basically how layering is supposed
    to work. A layer N service provides layer N capabilities by using a
    layer N-1 service.

    > If the concept of layering is so wonderful, where are these layered
    > protocols used?


    Everywhere. Ethernet is a layer 2 protocol, and it is often helpful to
    understand it by thinking of it in that way.

    Just don't eat the recipe.

    DS

  9. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    On Feb 5, 7:06*pm, David Schwartz wrote:

    > Nonsense. ARP is built on top of layer 2, as a layer 3 protocol should
    > be. ARP is a layer 3 protocol. ARP implements layer 3 services by
    > using a layer 2 protocol. That is basically how layering is supposed
    > to work. A layer N service provides layer N capabilities by using a
    > layer N-1 service.
    >
    > > If the concept of layering is so wonderful, where are these layered
    > > protocols used?

    >
    > Everywhere. Ethernet is a layer 2 protocol, and it is often helpful to
    > understand it by thinking of it in that way.
    >
    > Just don't eat the recipe.


    I agree with all except the first part.

    IP implements L3 services by being layered over L2 protocols. By doing
    so, IP routes packets between L2 networks, which is the definition of
    a L3 service.

    But ARP does no such thing. It only carries the IP address within a L2
    network. In a somewhat similar way, IGMP and ICMP operate directly
    over IP, but they don't provide L4 services. They only enable a L3
    function to take place, e.g. multicast, and/or they provide L3
    troubleshooting tools. They don't provide L4 services in any way
    comparable with TCP or UDP.

    Anyway, Douglas Comer, while not being very specific, does say that
    ARP is a very low level protocol.

    Bert

  10. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    On Feb 5, 8:35*pm, Albert Manfredi wrote:
    > On Feb 5, 7:06*pm, David Schwartz wrote:
    >
    > > Nonsense. ARP is built on top of layer 2, as a layer 3 protocol should
    > > be.

    >
    > I agree with all except the first part.


    Perhaps the disconnect here is that I am responding to the question
    "at what layer does ARP operate?" Maybe "belong to" is more ambiguous,
    since ARP does carry L3 information as part of its payload.

    IP operates at L3. ARP operates at L2.

    Bert

  11. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    Split the difference - call ARP a Layer 2.5 protocol.

    rick jones
    --
    No need to believe in either side, or any side. There is no cause.
    There's only yourself. The belief is in your own precision. - Jobert
    these opinions are mine, all mine; HP might not want them anyway...
    feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

  12. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    On Feb 5, 8:02*pm, Rick Jones wrote:
    > Split the difference - call ARP a Layer 2.5 protocol. *


    Funny. That's the image I get when I think about ARP.

    I see both points being made, and I would have to slightly lean toward
    Albert's point of view, but OTOH, David is right about something I
    think is slightly different than what we are talking about - which is
    upper layer protocols employ lower layer protocols to achieve certain
    effects, but there is also a bootstrapping that goes on in protocol
    design that makes it impossible to regard a protocol stack as strictly
    conforming to layers.

    It would be nice if someone were to write a paper describing this
    bootstrapping phenomenon because it is prevalent in all kinds of
    systems, not just software.

    For example:

    1. If a mailman delivers mail to an area of a city that includes his
    own, does that means he delivers mail to himself? Does he then put it
    in his own mailbox and take it right back out as required by law or
    does he put it in his pocket?

    2. If a neurosurgeon feels tingling sensation in his frontal lobe that
    warns of impending danger, does he prescribe preemptive medication or
    does he see a doctor?

    3. The memory manager of an OS allocates memory to processes that need
    it. But the memory manager needs memory of its own to function. From
    where should it request memory?

    4. A sheriff in a northern USA state recently wrote himself a ticket
    for his role in minor traffic accident He imposed the appropriate
    fine, plead no-contest, and paid the fine, even though it was probably
    case where two individuals involved might have just settled it between
    themselves (http://tinyurl.com/3xxm99). .

    All of these examples demonstrate lateral-self-service principle of
    systems, whereby one layer of abstraction of a system will not only
    provide services to conceptually higher layers, but will also provide
    services to lateral portions of the system, including itself.

    Perhaps this is one of the points that David was making. If so, I
    agree. I think that the secret to finding a well-formed protocol
    stack (beyond TCP/IP) lies in acknowledging and embracing this notion,
    but not getting so carried away with it that one makes a mess.
    Minimalism is key, so that if you adhere to this philosophy with a
    minimalist approach, the areas of necessary applicability will become
    obvious and singular in their manifestation.

    -Le Chaud Lapin-

  13. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    On Feb 5, 5:35 pm, Albert Manfredi wrote:

    > IP implements L3 services by being layered over L2 protocols. By doing
    > so, IP routes packets between L2 networks, which is the definition of
    > a L3 service.


    Right.

    > But ARP does no such thing. It only carries the IP address within a L2
    > network.


    But this:

    1) Implemented using L2 services, and

    2) Needed to implement the IP L3 services.

    This makes it part of L3.

    > In a somewhat similar way, IGMP and ICMP operate directly
    > over IP, but they don't provide L4 services.


    I agree. They provide L3 services. They are part of the IP L3 suite.

    > They only enable a L3
    > function to take place, e.g. multicast, and/or they provide L3
    > troubleshooting tools. They don't provide L4 services in any way
    > comparable with TCP or UDP.


    I agree. IGMP and ICMP live entirely at L3.

    > Anyway, Douglas Comer, while not being very specific, does say that
    > ARP is a very low level protocol.


    It's a part of the IP L3 protocol.

    DS

  14. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    Hello,

    Albert Manfredi a écrit :
    >>On Feb 5, 7:06 pm, David Schwartz wrote:
    >>
    >>>Nonsense. ARP is built on top of layer 2, as a layer 3 protocol should
    >>>be.


    Not everything that is built on top of layer N is layer N+1 (and
    conversely). IIRC the protocol layer is defined by the services this
    protocol offers, not by which protocol layer it runs on.

    > Perhaps the disconnect here is that I am responding to the question
    > "at what layer does ARP operate?" Maybe "belong to" is more ambiguous,
    > since ARP does carry L3 information as part of its payload.
    >
    > IP operates at L3. ARP operates at L2.


    What about IPv6 neighbor discovery ? It does the same job as ARP and
    runs on top of ICMPv6.

  15. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    On Feb 6, 1:27 am, Pascal Hambourg
    wrote:

    > Not everything that is built on top of layer N is layer N+1 (and
    > conversely). IIRC the protocol layer is defined by the services this
    > protocol offers, not by which protocol layer it runs on.


    The service ARP provides is part of the L3 service. Don't think of ARP
    as a protocol by itself, think of it as part of the IP service.

    DS

  16. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???


    "David Schwartz" wrote in message
    news:39b660c7-4ef3-4cc8-9a63-57a05027f091@v4g2000hsf.googlegroups.com...
    > On Feb 6, 1:27 am, Pascal Hambourg
    > wrote:
    >
    >> Not everything that is built on top of layer N is layer N+1 (and
    >> conversely). IIRC the protocol layer is defined by the services this
    >> protocol offers, not by which protocol layer it runs on.

    >
    > The service ARP provides is part of the L3 service. Don't think of ARP
    > as a protocol by itself, think of it as part of the IP service.
    >
    > DS


    ARP is a layer 2 protocol
    http://tools.ietf.org/html/rfc826
    http://en.wikipedia.org/wiki/Address...ution_Protocol



  17. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    iLRainyday writes:
    > I have posted this topic at somewhere else,but nobody could gives me a
    > definitive answer,so I post it here and hope someone make it clear for
    > me


    osi layer 2 is point-to-point inferface

    osi layer 3 is network layer ... figuring out which point-to-point
    interface to use

    LANs, MACs, ARP, etc ... aren't covered by OSI reference model ... the
    physical interface has some of the characteristics of a point-to-point
    interface. however, the networking characteristics of LANs can be
    considered to place them in the middle of networking stuff defined in
    OSI layer 3.

    ISO compounded the problems with the OSI reference model by passing out
    an edict that ISO networking standards (and work by ISO nationally
    chartered bodies) could only be done on protocols that conformed to the
    OSI reference model (which was also a ISO standard).

    I was involved in trying to get HSP (high-speed protocol) work done in
    X3S3.3 (the US ISO chartered standards body responsible for standards
    work related to OSI layers 3&4).

    eventually HSP standards work couldn't be considered in x3s3.3 (and ISO)
    because it violated OSI reference model:

    1) went directly from transport interface to the mac interface bypassing
    network interface ... violating OSI reference model

    2) supported "internetworking" layer (i.e. IP) ... something that
    doesn't exist in the OSI reference model ... and violates the OSI
    reference model (aka may require sending traffice to a "gateway"
    responsible for internetworking between networks ... not having
    internetworking ... there are also no gateways).

    3) interfaced to LAN/MAC ... which doesn't exist in OSI reference model,
    and therefor violates the OSI reference model.

    ARP is part of the LAN/MAC genre not defined in OSI ... so it takes a
    lot of really hard squinting to pretend to see it fitting.

    misc. past posts
    http://www.garlic.com/~lynn/subnetwork.html#xtphsp

  18. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???



    eager wrote:

    > ARP is a layer 2 protocol


    ARP translates layer 3 addresses into layer 2 addresses. As such, it
    is part of the implementation of layer 3 using layer 2. I don't see
    any of those links claiming ARP is a layer 2 protocol, in fact, they
    claim the reverse.

    The layer 2 protocol will work just fine without ARP.

    DS

  19. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    "eager" wrote:
    > ARP is a layer 2 protocol
    > http://tools.ietf.org/html/rfc826
    > http://en.wikipedia.org/wiki/Address...ution_Protocol


    I disagree:

    Does Ethernet need ARP in order for it to deliver Ethernet frames?
    No.

    Does IP need ARP in order for it to deliver IP datagrams over Ethernet?
    Yes.

    The OSI Reference Model is just a specific way of doing classic functional
    decomposition of all the tasks needed for multiple computers to communicate
    in real world situations. The model adopted a sequential or layering
    approach. One of the reasons the OSI "reference model" was created was to
    aid in communication among humans working on networking software - after
    all, computers currently have no use for an abstract "model" or have any
    need to "reference" back to that model during their sending and receiving
    of packets.

    I would suggest that one way to determine the lowest layer at which a
    particular function or task is best categorized is to determine the lowest
    layer at which the following question can be answered yes:

    Does this OSI layer need X in order for it to accomplish any of the tasks
    required of it in the OSI Reference Model?

    If the reference model is sufficiently complete, I believe one should be
    able to fit all of TCP/IP functionality into the OSI Reference Model using
    that sort of mapping strategy. If the model is complete enough, the reverse
    mapping to an actual implementation like TCP/IP may not always be possible
    or relevant. The forward mapping is sometimes useful from both an
    educational standpoint and from an engineering standpoint when one needs to
    compare communications "stacks".

    All the above is in my humble opinion, of course.

  20. Re: Does ARP Belong to Layer 2 Or Layer 3 OSI Reference Model???

    David Schwartz wrote:
    > eager wrote:


    > > ARP is a layer 2 protocol


    > ARP translates layer 3 addresses into layer 2 addresses. As such, it
    > is part of the implementation of layer 3 using layer 2. I don't see
    > any of those links claiming ARP is a layer 2 protocol, in fact, they
    > claim the reverse.


    > The layer 2 protocol will work just fine without ARP.


    Layer 3 protocols can also work just fine without ARP. It might not
    be as convient, but it can work. ARP also isn't the only thing out
    there ever to translate IP to MAC addresses. "Back in the day" HP had
    something called Probe for its NS Transport which was used to resolve
    names to IPs, IPs to MACs and also exchange routing information..

    I suppose if folks cannot agree on whether it is a floor wax or a
    dessert topping they could just agree to call it Shimmer...

    rick jones
    --
    portable adj, code that compiles under more than one compiler
    these opinions are mine, all mine; HP might not want them anyway...
    feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

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