6

I've heard someone say that a protocol higher up the osi model can encapsulate a lower protocol.

As far as I know, personally, a protocol can only encapsulate a same-level protocol or one at a higher level, i.e. you couldn't have Ethernet carrying ip, which in turn has another Ethernet frame encapsulated (and then IP, and the whole stack).

Could some clear the air?

  • I have seen, as a proof-of-concept, IP encapsulated in MIME, attached to emails, delivered over SMTP. – Vatine May 11 at 18:23
  • Thanks for the question that allowed me to reach the 15K milestone ;) – JFL May 12 at 13:06
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    SSL VPNs are a well known example of this. – Barmar May 12 at 17:03
12

Yes, encapsulation hide the details of what is encapsulated and doesn't really care about the payload nature.

VxLAN is a sensible example of this, with layer2 (VLAN) being encapsulated in layer 4 (UDP).

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    Some other examples: IP over DNS; or IP over PPP over SSH; or Ethernet over HTTPS. – david May 11 at 20:28
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    You may also hear this concept called tunnelling. – Michael Hampton May 11 at 23:51
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    It sucks that you can't encapsulate PoE over TCP/IP though... – Dai May 12 at 17:09
  • @Dai yeah 802.3at over IP over 802.11... – JFL May 12 at 19:10
  • You can, but the frame size limitations are killer in that particular application. – William May 12 at 20:22
9

Consider a package delivery service, like UPS or DHL. They don't care what's inside the box - they just make sure it gets to its destination. Similarly, the protocol doesn't care what the payload is. It doesn't have to be a higher layer.

The idea of a layered protocol model is that the "payload" of a layer can be anything. @JFL gave one example. IPSec VPN, MPLS, GRE, L2TP, Geneve, are others.

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    So one could, for example, ship a toy UPS truck via DHL. – Jonathon Reinhart May 11 at 18:06
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    @JonathonReinhart The VPN example would be UPS truck delivering a (real, perhaps smaller) UPS truck with packages inside. – JoL May 11 at 18:28
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    @JoL Perhaps more realistic analogy would be like how Airbus might uses Airbus Beluga to carry another Airbus aircraft fuselage or whole wing section so they can be reassembled elsewhere. – Lie Ryan May 12 at 2:54
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    @JoL It's not a big truck. It's a series of tubes. – J... May 12 at 19:49
  • @J... But the bandwidth of a UPS truck full of hard disks slowly rolling across the country is way higher than even a decent amount of fibre lines – Hagen von Eitzen May 12 at 22:21

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