Gateway - can make two totally different layer 3 or 4 protocols to communicate with each other? - TCP-IP

This is a discussion on Gateway - can make two totally different layer 3 or 4 protocols to communicate with each other? - TCP-IP ; hello The job of router is to forward packets and it usually operates at layer 3 of OSI model. And I realize that many routers are also gateways. But the definition of gateway really got me confused: My textbook says ...

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Thread: Gateway - can make two totally different layer 3 or 4 protocols to communicate with each other?

  1. Gateway - can make two totally different layer 3 or 4 protocols to communicate with each other?

    hello


    The job of router is to forward packets and it usually operates at
    layer 3 of OSI model. And I realize that many routers are also
    gateways.

    But the definition of gateway really got me confused: My textbook says
    that gateway can connect networks with different architecture and
    protocols. It does this by taking the incoming packet and removing all
    of its headers ( including header created in layer 4 of the OSI
    model ) and replacing the removed headeres with headers created by
    protocols used on network this packet is destined to.

    Now as little as I know about networking, I always assumed that the
    two protocols ( residing on the two communicating computers ) must be
    the same --> I'm talking about protocols at layer 3 or layer 4 of OSI
    model?
    Thus if one PC uses IP protocol at layer 3, then the other PC must
    also use IP?! Right?
    Or if one PC uses TCP, then other must also use TCP in order for the
    two PCs to be able to communicate with eachother?!
    BTW - I realize that routers can pass packets between networks with
    different data link and physical layers

    So anyways, according to my book ( as I understood it ) the TCP
    protocol can establish a connection with some protocol that is
    totally different ( in terms of its packet structure and services it
    provides) than TCP, just as long as we have a good gateway between the
    two end systems?! In other words, one PC could be using TCP to
    communicate with PC which uses UDP ( I know this is far fetched, I'm
    just making an example using only two protocols I know of at layer 4
    of OSI model )?


    thank you


  2. Re: Gateway - can make two totally different layer 3 or 4 protocols to communicate with each other?

    On Jun 20, 5:13 pm, kaja_love...@yahoo.com wrote:
    > hello
    >
    > The job of router is to forward packets and it usually operates at
    > layer 3 of OSI model. And I realize that many routers are also
    > gateways.
    >
    > But the definition of gateway really got me confused: My textbook says
    > that gateway can connect networks with different architecture and
    > protocols. It does this by taking the incoming packet and removing all
    > of its headers ( including header created in layer 4 of the OSI
    > model ) and replacing the removed headeres with headers created by
    > protocols used on network this packet is destined to.
    >
    > Now as little as I know about networking, I always assumed that the
    > two protocols ( residing on the two communicating computers ) must be
    > the same --> I'm talking about protocols at layer 3 or layer 4 of OSI
    > model?
    > Thus if one PC uses IP protocol at layer 3, then the other PC must
    > also use IP?! Right?
    > Or if one PC uses TCP, then other must also use TCP in order for the
    > two PCs to be able to communicate with eachother?!
    > BTW - I realize that routers can pass packets between networks with
    > different data link and physical layers
    >
    > So anyways, according to my book ( as I understood it ) the TCP
    > protocol can establish a connection with some protocol that is
    > totally different ( in terms of its packet structure and services it
    > provides) than TCP, just as long as we have a good gateway between the
    > two end systems?! In other words, one PC could be using TCP to
    > communicate with PC which uses UDP ( I know this is far fetched, I'm
    > just making an example using only two protocols I know of at layer 4
    > of OSI model )?



    Part of the confusion is that historically IP Routers were called
    "gateways." But most routers are most certainly not gateways in the
    modern sense.

    But that aside, there are all sorts of gateways in the world, and
    these tend to connect networks at some higher level than OSI 3/4.
    These are also usually application specific, and not generic, since
    typically the low level protocol are different enough to make a simple
    translation impossible. For example, I have a mainframe terminal
    session on my desktop running TN2370/Telnet/TCP/IP to a gateway which
    has a 3270/LU2/SNA connection to the actual mainframe. Back in the
    days when mail was not exclusively handled via SMTP, there were scores
    of email gateways about.

    While most gateways involve higher level functions, it's not
    completely unheard for lower level gateways to exist. For example, I
    saw a TCP to (OSI) TP4 gateway at one point, although it had somewhat
    limited functionality on both sides, but managed the basic byte stream
    connection OK.


  3. Re: Gateway - can make two totally different layer 3 or 4 protocols to communicate with each other?

    wrote:

    > The job of router is to forward packets and it usually operates at
    > layer 3 of OSI model. And I realize that many routers are also
    > gateways.


    Actually, if you want to be rigorous with terms, no. Routers are layer 3
    devices which use a specific layer 3 protocol. It is an unfortunate
    circumstance that routers were often called "gateway" is early RFCs.
    This is now confusing. So just be aware of this oddity.

    > But the definition of gateway really got me confused: My textbook says
    > that gateway can connect networks with different architecture and
    > protocols. It does this by taking the incoming packet and removing all
    > of its headers ( including header created in layer 4 of the OSI
    > model ) and replacing the removed headeres with headers created by
    > protocols used on network this packet is destined to.


    Yes. Gateways come in many varieties. Some operate all the way up to
    layer 7, and were used some years ago to tie together, for example, IBM
    SNA networks with DECnet or IP networks. For example, they would be used
    to do file transfers or send e-mail among these different nets. By
    definition, these two examples would require changes all the way up to
    layer 7.

    > Now as little as I know about networking, I always assumed that the
    > two protocols ( residing on the two communicating computers ) must be
    > the same --> I'm talking about protocols at layer 3 or layer 4 of OSI
    > model?
    > Thus if one PC uses IP protocol at layer 3, then the other PC must
    > also use IP?! Right?
    > Or if one PC uses TCP, then other must also use TCP in order for the
    > two PCs to be able to communicate with eachother?!


    That's right. So what do you do to allow two PCs to communicate, when
    they are on different types of network? You install a gateway.

    > BTW - I realize that routers can pass packets between networks with
    > different data link and physical layers


    Same idea, moved up the protocol stack. Bridges can sometimes tie
    together different link layer protocols, but routers cannot tie together
    different network layer protocols. And what is the device that can tie
    together different application layer protocols, e.g. SMTP to VMSmail?
    "Gateway" is a rather ambiguous terms that describes this magic box that
    does this sort of thing. They used to be very popular in days before IP
    became the global standard for digital networks.

    > So anyways, according to my book ( as I understood it ) the TCP
    > protocol can establish a connection with some protocol that is
    > totally different ( in terms of its packet structure and services it
    > provides) than TCP, just as long as we have a good gateway between the
    > two end systems?! In other words, one PC could be using TCP to
    > communicate with PC which uses UDP ( I know this is far fetched, I'm
    > just making an example using only two protocols I know of at layer 4
    > of OSI model )?


    I think the SMTP to VMSmail is perhaps a better example, and was just
    the sort of thing these gateways were used for, back in the 1980s and
    early to mid 1990s. But yes, in principle, a gateway would also be the
    box that permits a TCP session one one side to become UDP on the other
    end.

    As far as I know, gateways are not in widespread use these days. I could
    be wrong, though.

    Bert


  4. Re: Gateway - can make two totally different layer 3 or 4 protocols to communicate with each other?

    hello

    Thanx to you guys it's starting to make some sense

    My text book also claims that gateways are used for the purpose of
    connecting main frame computers to LAN in such a way, that mainframes
    don't realize they are connected with LAN

    1)
    So do gateways connect mainframe PC to LAN in such a way, that other
    PCs in LAN perceive this mainframe as being member of this LAN ( thus
    it also gets broadcast messages etc )


    2)
    What is the purpose of connecting mainframe in such a way, that it
    doesn't realize it is connected to LAN why shouldn't mainframe know
    this?

    3)
    Main point I don't understand is why needs a gateway to translate
    between protocols LAN uses and protocols mainframe uses ( is this
    translation between protocols at application layer or network layer )?
    Meaning, does mainframe always uses some special protocols which LANs
    never use?


    thank you


  5. Re: Gateway - can make two totally different layer 3 or 4 protocols to communicate with each other?

    On Jun 22, 7:06 am, kaja_love...@yahoo.com wrote:
    > hello
    >
    > Thanx to you guys it's starting to make some sense
    >
    > My text book also claims that gateways are used for the purpose of
    > connecting main frame computers to LAN in such a way, that mainframes
    > don't realize they are connected with LAN
    >
    > 1)
    > So do gateways connect mainframe PC to LAN in such a way, that other
    > PCs in LAN perceive this mainframe as being member of this LAN ( thus
    > it also gets broadcast messages etc )
    >
    > 2)
    > What is the purpose of connecting mainframe in such a way, that it
    > doesn't realize it is connected to LAN why shouldn't mainframe know
    > this?
    >
    > 3)
    > Main point I don't understand is why needs a gateway to translate
    > between protocols LAN uses and protocols mainframe uses ( is this
    > translation between protocols at application layer or network layer )?
    > Meaning, does mainframe always uses some special protocols which LANs
    > never use?



    Most mainframes these days speak TCP/IP quite well, so you often don't
    need an external gateway, but you might have one for cost or
    performance reasons.

    Mainframes historically have used SNA for networking, which is very
    different from TCP/IP. While direct SNA connections all the way out
    to workstation are perfectly possible, and can run over "normal" LANs
    like Ethernet or Token Ring, it's clumsy and expensive. Not only do
    you need to SNA stack on the PC, you need the correct network
    infrastructure. Older sub-area SNA is not routable in the same sense
    that TCP/IP is, so you need to have routers that can also bridge the
    SNA traffic in addition to routing the IP stuff, which certainly
    complicates management of the network. Or if you had APPN, that's
    routable, but the APPN routing functions required pretty expensive
    high end routing gear (not for technical reasons, just because of the
    limited market).

    So even if you need much of the SNA functionality out to the
    workstation or near there, a gateway provides a single, much more
    manageable point where you can do the translation.

    And there are still some reasons for running SNA out to various
    servers, but it's pretty strongly avoided at the workstation level
    these days.

    Internally much of the traffic on the mainframe (and inside a cluster)
    is still SNA, and some of what's happened is that the gateway is part
    of the standard mainframe TCP/IP stack. For example, 3270 terminal
    traffic is almost all SNA internally, and the mainframe TCP/IP stack
    provides the TN3270/TELNET gateway function. That's actually a common
    function that's offloaded to an external gateway (and then it's low-
    overhead SNA traffic between the mainframe and the gateway, and you're
    not burning expensive CPU cycles on mundane terminal handling).

    Certainly the need for special purpose gateways in the mainframe world
    has diminished greatly over the years.



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