In the Network File System (NFS) architecture, what is the main purpose of the Mount protocol between an NFS client and an NFS server?

Introduction / Context:
Network File System, or NFS, is a widely used protocol that allows a computer to access files over a network almost as if they were on local disks. Within the NFS architecture, several supporting protocols work together. One of these is the Mount protocol. This question tests your understanding of what the Mount protocol actually does and how it participates in providing network file access between client and server machines.

Given Data / Assumptions:

- We are working in the context of NFS client and NFS server communication.- We assume a Unix like or similar environment where remote directories can be mounted into a local directory hierarchy.- The Mount protocol is a distinct protocol from the core NFS read and write operations.

Concept / Approach:
The Mount protocol is used when an NFS client wants to attach a remote directory exported by an NFS server into its own local file system tree. The client sends a mount request, specifying the remote directory it wants to access. The server checks export rules, permissions, and host based access control. If the request is allowed, the server returns a file handle that uniquely identifies the remote file system. The client then uses this file handle in subsequent NFS operations like read, write, and lookup. The Mount protocol therefore acts as a control and negotiation step to establish access, rather than handling the data transfer itself.

Step-by-Step Solution:
Step 1: Recall that NFS is used to share directories from a server so that clients can mount them and use them like local directories.Step 2: Understand that the Mount protocol runs when the client performs a mount operation, which integrates the remote directory into the client file system.Step 3: During this process, the client needs to obtain a file system handle from the server and the server needs to verify that the client is allowed to access the export.Step 4: The protocol messages exchanged during this step are part of the Mount protocol, not the core read and write NFS operations.Step 5: Option A states that the Mount protocol allows an NFS client to request and establish access so that the remote file system can be mounted into the local directory tree, which matches the correct description.Step 6: Options B, C, D, and E describe functions of routing protocols, security protocols, transport protocols, or DNS, none of which are related to NFS mounting.

Verification / Alternative check:
Documentation for NFS implementations describes two main components: NFS itself, which handles file operations such as read and write, and the Mount protocol, which handles mount requests and export tables. Administrative commands like exportfs on Unix configure which directories can be mounted, while the mount command on the client triggers a Mount protocol exchange. The system log may show mount request entries that clearly demonstrate this separation of roles, confirming that the Mount protocol is about establishing access to remote file systems.

Why Other Options Are Wrong:
Option B refers to routing protocols like OSPF or BGP, not to NFS. Option C suggests that the Mount protocol encrypts user passwords, which is more related to authentication systems such as Kerberos or SSH. Option D talks about reliable transport for streaming video, which may involve TCP or specialised protocols, not NFS Mount. Option E describes DNS, which resolves domain names to IP addresses and is unrelated to mounting remote file systems.

Common Pitfalls:
Students sometimes assume that every protocol mentioned in the context of NFS must be responsible for file transfer itself. In reality, distributed file systems often have multiple control and data protocols. Another pitfall is to confuse Mount protocol operations with operating system level mount commands without understanding the underlying network messages. Remember that the Mount protocol is the negotiation step that grants access and returns a handle, while NFS handles the actual file operations once access is established.

Final Answer:
The Mount protocol is used so that an NFS client can request and establish access to a remote file system and mount it into the local directory tree, as described in option A.