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have a look at this , a link (about encapsulation)

Discussion in 'Networks' started by kobem, Nov 7, 2007.

  1. kobem

    kobem Megabyte Poster

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    http://www.networkdictionary.com/networking/de.php

    "The physical layer in computer B receives the information unit and passes it to the data link layer. The data link layer in computer B then reads the control information contained in the header prepended by the data link layer in computer A. The header and the trailer are then removed, and the remainder of the information unit is passed to the network layer. Each layer performs the same actions: The layer reads the header and trailer from its peer layer, strips it off, and passes the remaining information unit to the next higher layer."

    above is a group of sentences in the link .

    now , "The header and the trailer are then removed" , WHICH HEADER AND TRAILER INFO ?

    YEAH I SUPPOSE I UNDERSTOOD!

    look : at layer 2 at the receiving end (for example assume router received the data)

    transmitting end layer 2 : layer2 header , ip header , tcp/udp header , upper layer headers , upper layer (application) data , layer 2 footer

    receiving end layer 2 (assume router) : layer 2 header and layer 2 footer ARE STRIPPED OFF
    then carried to network layer as :

    ip header , tcp/udp header , upper layer headers , upper layer
    (application) data


    and at network layer ip header is stripped off , so THERE IS STILL tcp/udp header , upper layer headers , upper layer (application) data


    now reliable data communication is possible!

    AM I RIGHT?
     
    Certifications: CCNA
  2. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    In the context of that quote - the data-link header and trailer.
    The router would not go as far as stripping the IP layer headers, as they contain important info required for the next hop. But otherwise correct.

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  3. Tinus1959

    Tinus1959 Gigabyte Poster

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    AFAIK they are replaced with next hop info. So they are stripped, rebuild and put on again.
     
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    WIP: MCSD, MCAD, CCNA, CCNP
  4. kobem

    kobem Megabyte Poster

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    hm if it is the way you mentioned , in network layer what should i strip off(receiving end)?
     
    Certifications: CCNA
  5. kobem

    kobem Megabyte Poster

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    no no ! as i said before , router will raise the packet up to 3th layer at most . In third layer
    there are "ip header , tcp/udp header , upper layer headers , upper layer
    (application) data"

    then router will use the ip header part to communicate with the sending host. AFTER ending
    the communication with its peer layer on the sending device
    IP HEADER IS STRIPPED OFF TO GIVE IT TO NEXT UPPER LAYER THE 4TH LAYER
     
    Certifications: CCNA
  6. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    In a simple case the only field you would change is the TTL surely? And then recalculate the checksum.

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  7. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    If a router stripped off the IP header it would lose the IP addresses for the source and destination - which isn't very useful! This is why I say that a router does *not* discard IP headers

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  8. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Not sure what you mean here. And what sort of device?

    A router lives at the network layer (usualy), so will remove all wrappings outside the network ones.

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
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  9. kobem

    kobem Megabyte Poster

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    i am getting idiot again, so , at that time in network layer what is stripped off?
     
    Certifications: CCNA
  10. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    In the case of a router - the layer 2 (data-link) header and trailer, as I've said before.

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  11. kobem

    kobem Megabyte Poster

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    layer 2 header and trailer is removed at layer 2 to give it to network layer , HM , IF PACKET
    IS REQUIRED TO BE TAKEN TO NEXT UPPER LAYER(i mean transport)
    , ip header is removed

    BUT when a router wants to send it to destination, this layer needs ip header and
    in this case because of no need the packet to be taken to transport layer


    contents of pdu remain below as:


    ip header , tcp/udp header , upper layer headers , upper layer
    (application) data"


    IS THIS OK NOW?
     
    Certifications: CCNA
  12. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Seems OK to me.

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  13. Tinus1959

    Tinus1959 Gigabyte Poster

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    In a simple case, yes, but cases are most off the time not simple. Just think of NAT...
     
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    WIP: MCSD, MCAD, CCNA, CCNP
  14. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Indeed - but I didn't want to add extra complication here! :biggrin

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
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