What OSI layer(s) does a stateful packet inpection firewall work at?

Greetings Experts,

I'm studying for the CompTIA Security+ exam and I'm reading material on SPI firewalls or Stateful Packet Inspection.  One aspect that I'm having a little of trouble grasping is at what layer(s) does SPI work at?  I went online and found Layer 3 but other sources say its Layer 3 and Layer 4.  I was wondering if someone could clarify and point me to some definite sources where I can find this information like Cisco's site or some other reputable networking company.  Thanks in advance experts.  
student_23Asked:
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lanboyoConnect With a Mentor Commented:
A stateful packet firewall would be inspecting at layer 4 and up.

Since the firewall is keeping track of the state of tcp sessions as they are traversing it, it is looking at ( for instance ) the tcp syn, ack bits as well as tcp source and destination ports.

If it was an IP source and destination only filter or access list this would be a level 3 OSI thing.

Best place to find this would be a security plus bran dump sad to say. These are security abstractions that relate to security abstractions.
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QlemoConnect With a Mentor Batchelor, Developer and EE Topic AdvisorCommented:
I agree to the above. Please keep in mind that TCP/IP is NOT working exactly with OSI Layers. Some of the protocols act on more than one layer, or in-midst of them. So an exact assignment isn't feasible in many cases.
About the "4 and up", a SPI firewall often even needs to inspect the payload, e.g. for FTP, to change private to public IPs, read port negotiations, and more. That corresponds to Layer 5 and up.

Implementing a good SPI firewall is quite complex because of that, and I don't think you will find any documentation readily composed for that reason.
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student_23Author Commented:
lanboyo and Qlemo,

Thank you very much for your valuable insight.  I did look in my book but there's isn't more information on SPI.
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