Services vs. Protocols - TCP-IP

This is a discussion on Services vs. Protocols - TCP-IP ; Hi, everyone. Tanenbaum said in his book Computer Network about the differences between services and protocols as follows: "Services and protocols are distinct concepts, although they are frequently confused. This distinction is so important, however, that we emphasize it again ...

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Thread: Services vs. Protocols

  1. Services vs. Protocols

    Hi, everyone. Tanenbaum said in his book Computer Network about the
    differences between services and protocols as follows:

    "Services and protocols are distinct concepts, although they are
    frequently confused. This distinction is so important, however, that
    we emphasize it again here. A service is a set of primitives
    (operations) that a layer provides to the layer above it. The service
    defines what operations the layer is prepared to perform on behalf of
    its users, but it says nothing at all about how these operations are
    implemented. A service relates to an interface between two layers,
    with the lower layer being the service provider and the upper layer
    being the service user.

    A protocol, in contrast, is a set of rules governing the format and
    meaning of the packets, or messages that are exchanged by the peer
    entities within a layer. Entities use protocols to implement their
    service definitions. They are free to change their protocols at will,
    provided they do not change the service visible to their users. In
    this way, the service and the protocol are completely decoupled.

    Everything is fine for me, except I can't get why "Entities use
    protocols to implement their service definitions", because he also
    said that:

    "In other words, services relate to the interfaces *between layers*,
    as illustrated in Fig. 1-19. In contrast, protocols relate to the
    packets sent between peer entities on *different machines*."

    I hope someone will help me answer this question. Thanks in advance.

  2. Re: Services vs. Protocols

    On Sep 8, 6:44*am, Nguyen Minh Hai
    wrote:

    > Everything is fine for me, except I can't get why "Entities use
    > protocols to implement their service definitions", because he also
    > said that:


    In other words, if one computer offers a file access service to
    another computer, it might use the FTP protocol to implement that
    service. But it could also implement the same service using HTTP, NFS,
    or CIFS. The service is what you can do. The protocol is how you tell
    the other side to do it.

    > "In other words, services relate to the interfaces *between layers*,
    > as illustrated in Fig. 1-19. In contrast, protocols relate to the
    > packets sent between peer entities on *different machines*."


    Right, a "service" is the conceptual interface between the layers. Web
    servers offer a document retrieval service to browsers. However, if
    you look on the wire between a web server and a browser, you will see
    the HTTP protocol, because that's how the retrieval service is
    implemented.

    > I hope someone will help me answer this question. Thanks in advance.


    It's not entirely clear exactly what is confusing you. The distinction
    is that a service is a logical capability. A protocol is the actual
    information exchange to implement the capability.

    DS

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