Encapsulation: can a higher protocol encapsulate a lower protocol?

Question

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 '20 at 18:23
Thanks for the question that allowed me to reach the 15K milestone ;) — JFL, May 12 '20 at 13:06

score 12 · Accepted Answer · edited Oct 07 '21 at 07:34

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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).

edited Oct 07 '21 at 07:34

Community

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answered May 11 '20 at 09:40

JFL

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

score 9 · Answer 2 · answered May 11 '20 at 12:27

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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.

answered May 11 '20 at 12:27

Ron Trunk

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So one could, for example, ship a toy UPS truck via DHL. – Jonathon Reinhart May 11 '20 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 '20 at 18:28
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@JoL Perhaps more realistic analogy would be like how Airbus might uses [Airbus Beluga](https://en.m.wikipedia.org/wiki/Airbus_Beluga) to carry another Airbus aircraft fuselage or whole wing section so they can be reassembled elsewhere. – Lie Ryan May 12 '20 at 02:54
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@JoL It's not a big truck. It's a series of tubes. – J... May 12 '20 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 '20 at 22:21

Encapsulation: can a higher protocol encapsulate a lower protocol?

2 Answers2