Exercise 3: Why There's No Distinct "Session Layer Header" in a Real Packet — Possible Solution
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Explanation:

Per the chapter's own warn-box, "TCP/IP's own encapsulation only
really produces four distinct header types -- link, internet,
transport, and application. Looking for a separate 'session layer'
header while examining a captured packet... will come up empty --
that concern, where it exists at all, is handled inside the
application-layer data itself." The colleague's expectation is built
on OSI's own seven-layer model, which describes Session as its own
distinct layer (layer 5) -- but per the chapter's earlier mapping,
"TCP/IP's Application layer absorbs OSI's Application, Presentation,
and Session (layers 5-7)." TCP/IP -- the model actually implemented
on real networks, per this chapter's own opening explanation of why it
won out over OSI -- was never built with a distinct header type
corresponding to OSI's own session concept in the first place.

Concretely, per the chapter's own encapsulation diagram, a real packet
only ever contains three added headers (Ethernet/link, IP/internet,
TCP-or-UDP/transport) wrapped around the original application data.
Whatever session-management behavior actually exists for a given
application -- keeping track of a login session, for example -- is
handled entirely within that application-layer data itself, by the
application protocol's own logic (e.g. a session cookie inside an HTTP
request), not by a separate header the way the transport or internet
layers have their own dedicated header fields.

This is a direct, concrete instance of net1-2's own warn-box, now made
literal at the level of actual bytes on the wire: OSI's seven layers
are a genuinely useful reference model for reasoning about
networking concepts, but they don't correspond to seven distinct
things that actually exist in a captured packet -- because the model
that was actually implemented (TCP/IP) only ever produces four header
types, not seven.

WHY THIS WORKS AS AN ANSWER
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This explains specifically why the colleague's expectation is
mismatched (OSI's session layer was never implemented as its own
distinct TCP/IP header), using the chapter's own warn-box and
encapsulation diagram, rather than just restating "OSI and TCP/IP are
different" without explaining the practical consequence for a real
packet capture.