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tunneling concept problem


in tunneling concept in general we have concepts of 

carrier , passenger , transport protocol

in case we are using 

GRE over IPSec


here GRE should be passenger and IPsec carrier

but in which case IPSec becomes as a Transport ?

in case of underlay protocol(infrastructure protocol) is IPSec so it can be transport

but mostly we have IP Protocol as a Transport 

my query is how possible IPSec is used as a Transport?

in other reference i see :

Generally, a logical interface that provides a way to encapsulate passesnger packets inside a transport protocol

but where we use term of carrier protocol




VIP Expert

here is the detailed good explanation in the cisco document for reference :



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Hello and thanks for your reply , Yes i see that link and one time i read it.
My Question is in my example GRE over IPSec
if IPSec is transport
GRE is carrier or passenger
and what will that traffic which is encapsulated by GRE !

but generaly tunnel related discussion X over Y means X is passenger and Y is Carrier



From my perspective and my understanding Transport means that protocol which is used to build network infrastructure, i know IP Security is Layer 3 protocol for encryption and also authentication of IP Packet being used !!!

but can purely uses IPsec for building Network infrastructure ....

if NO and from my side i also say NO

if IPSec is transport

GRE should be carrier in this case

because before configuring we can not figure it out what will be our traffic ?

and maybe i am confused with definition of Transport protocol in tunneling

maybe i should be revised 


one other idea is we take IPSec as a protocol which encrypting GRE packet and GRE also encapsulated our Data(pasenger protocol in it)

it is Transport for GRE not that things i mentioned before !


but still am i right IPsec ia not in format of network infrastructure protocol and is a protocol for carrying GRE ....




did my previous analysis was correct?

IPSec is transport the Carrier protocol (we can take this one idea)

but still we have like Layer 3 protocols is used for infrastructure (Routing ability)

mostly transport means (as i've understood) ability to gives a routing ;

IPsec although is a Layer 3 (in some reference is mentioned something else)

but how can is being used as a Transport individually

or maybe my description about Transport not correct !

transport for carrier

transport routing infrastructure