The question asks to identify a protocol used by a Virtual Private Network (VPN) specifically for tunneling. Layer 2 Tunneling Protocol (L2TP) is designed for this exact purpose. Its primary function is to create a tunnel across an untrusted network to encapsulate Layer 2 Point-to-Point Protocol (PPP) frames. While L2TP does not provide confidentiality or integrity on its own and is almost always paired with IPSec for security, its fundamental role is that of a tunneling protocol. Therefore, it is the most direct and accurate answer to the question posed.

B. HTTPS: This is an application-layer protocol for secure web communication (HTTP over TLS/SSL). It is not a general-purpose network tunneling protocol for site-to-site VPNs.
C. SSL: Secure Sockets Layer (and its successor, TLS) is a cryptographic protocol that provides security for communications. While used in SSL VPNs, its primary role is encryption, not tunneling itself.
D. IPSec: IPSec is a security protocol suite that operates at the network layer. While its "tunnel mode" creates a secure tunnel, its primary purpose is security (confidentiality, integrity, authentication), not solely tunneling.

1. Kaufman

C.

Hoffman

P.

& Eronen

P. (2013). Network Security: Private Communication in a Public World (2nd ed.). Pearson Education. In Chapter 17

which discusses VPNs

L2TP is explicitly defined as a tunneling protocol that encapsulates PPP packets for transport over an IP network. It is distinguished from IPSec

which is presented as the security mechanism.

2. Townsley

W.

Valencia

A.

Rubens

A.

Pall

G.

Zorn

G.

& Palter

B. (1999). RFC 2661: Layer Two Tunneling Protocol "L2TP". IETF. The abstract states

"This document describes a protocol that allows the Point-to-Point Protocol (PPP) to be tunneled through an IP network." This foundational document defines L2TP's primary purpose as tunneling.

3. Frankel

S.

& Krishnan

S. (2011). NIST Special Publication 800-113: Guide to SSL VPNs. National Institute of Standards and Technology. Section 2.1 explains that SSL VPNs use the SSL protocol and its successor

TLS

to provide remote access

distinguishing its typical use case from the site-to-site scenario described in the question.

4. Kurose

J. F.

& Ross

K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. In Section 8.7

"Securing TCP Connections: SSL

" and Section 8.8

"Network Layer Security: IPsec and Virtual Private Networks

" the text differentiates the roles. It notes that L2TP is a tunneling protocol that "does not provide any encryption or confidentiality on its own" and is often paired with IPSec

which provides the security. This supports the distinction that L2TP's specific role is tunneling.