Distributed Systems Flashcards
What are the two ways a hard drive disk is split up?
Into:
- Sectors (like pizza slices)
- Tracks (concentric rings)
What does ‘block’ describe in terms of a hard drive disk?
The areas between sector boundaries and track boundaries
What is a cylinder in relation to hard disk drives?
A collection of tracks with the same index number
When does fragmentation occur in hard disk storage?
When storage is lost due to a block being underused and files are fit into leftover spaces
What happens when a hard disk is more fragmented?
The disk head spends more time moving back and forth and the read/write efficiency decreases
What memory does a solid state drive use?
Flash memory
How does a solid state drive differ from a hard disk?
Solid state drive has no moving parts so no read/write head and no platter
What is a solid state drive made of?
Transistors
What are the advantages of a solid state drive over a hard disk?
-Faster than hard drive
- Quieter, uses less power
- More tolerant to being moved
What is partitioning?
The traditional method where disks are divided and each partition is used for a specific purpose and can’t overflow into other sections
What do we use now instead of disk partitioning?
Volume management
What are the features of a logical volume?
- can span more than one physical hard drive
- several physical hard drives are seen as one big disk
- LVs can be resized easily
What does RAID mean?
Multiple physical disk drives forming a logical unit of storage
(redundant array of independent disks)
What are four benefits of RAID?
Reliability, performance, capacity, availability
What are index nodes (inodes)?
Structures that store information about each unix file
What does an inode store for regular files, directory files, and special files?
regular, directory: location of disk blocks
special file: identifies peripherals
How many inodes can a file have?
Always 1
What do inodes store for all file types?
- file type
- file permissions
- owner
- hard link count
- last modification time and last access time
What is a boot block?
The first logical block disk that contains executable code for when UNIX is first activated
What is a superblock?
Second logical block of a disk that contains info about the disk including bitmap of free blocks
What are the first two logical blocks of a disk?
1 bootblock
2 superblock
What is mounting?
File systems from other devices can be attached to the original directory hierarchy - can be done automatically when booting
What does mounting allow?
Allows users to access files seamlessly even when they’re on other devices
How are large unix systems usually stored?
Over many devices with each device holding a subtree
What are some features of NFS (network files system)?
- widely available
- clients mount partitions of a server as though the partitions were physically connected
What does NFS allow low cost computers to do?
Share the same high capacity disk
What are the two ways mounting can be set up for workstations using NFS?
- mounting at boot time
- mounting when referenced
How are files represented on NFS mounted filesystems?
file handles
What does it mean when an NFS file handle is opaque?
A client can’t interpret it
What does it mean for an NFS file handle to have meaning?
A server can interpret the contents of it
What are the three key parts of a UNIX file handle?
- file identifier
- file system identifier
- generation count (incremented each time a file is unlinked and recreated)
What do NFS file handles not include? and why?
A pathname, this can change while a file is being accessed
What two protocols is NFS based on?
MOUNT
NFS
What is the MOUNT protocol used for in NFS?
initial negotiation between NFS client and server
- allowing the client to know which file systems are available and what their file handles are
And used to mount remote directories to the client’s local file system.
What is the NFS protocol used for in NFS?
- allows users to perform file and directory operations
What does SAMBA do?
Allows UNIX filesystems to be shared to windows and makes the UNIX file systems look like windows filesystems
- and can allow UNIX systems to access windows filesystems
Describe the client-server model in simple terms.
The client (application on your machine) contacts the server which accepts instructions and returns data
What two pieces of software are involved in email
- email interface program
- mail transfer program
What are the 3 main categories of protocols used within email?
Transfer, access, representation
What does a transfer protocol do within email?
Protocol used to move a copy of an email message from one computer to another
What does an access protocol do within email?
Allows a user access to their mailbox and to send and receive messages
What does a representation protocol do within email?
Specifies the format of an email message when stored on disk
What does SMTP stand for? What is it?
Simple Mail Transfer Protocol, it’s the standard protocol used by mail transfer programs
What does SMTP do?
Transfers text, allows sender to specify recipients name and check them, sends only one copy of a given message
What actions are possible as part of mail access?
A user can view their mailbox, view headers, download, delete or send messages
Why do ISPs offer email services?
Because most users do not leave their computer running all the time and don’t know how to configure email
When ISPs run email services, what do they provide users with?
a mailbox for each user and an interface for them to view it with, with special purpose interface applications for mobile devices
What does POP stand for? What is it?
Post Office Protocol, a mail access protocol
What does IMAP stand for?
Internet Mail Access protocol
What basic services are offered by both POP and IMAP?
- users can access mailbox
- users can view headers, download, delete, send
- Email client runs on user’s personal computer
- server runs on computer where the mailbox is stored
What are the two ways of representing email messages?
RFC2822 mail message format
- MIME (multi-purpose internet mail extensions)
What does MIME stand for?
Multi-purpose Internet Mail Extensions
How are emails represented using RFC2822?
- Mail message represented as a text file consisting of:
header, blank line, body
How are email messages represented using MIME?
- extends SMTP so images and binary data can be represented
- users can divide messages into several parts so image attachment can be encoded differently from a spreadsheet
- emails are displayed as more than plain text
What does FTP stand for?
File transfer protocol
What are some characteristics of FTP?
- arbitrary file contents
- bidirectional transfer
- support for authentication and ownership
- ability to browse folders’
- textual control messages
What common action is FTP the common protocol for?
Users requesting a download from a browser
What are the two types of connection made by FTP?
Control connection AND data connection
What does DNS stand for?
Domain Name System
What is DNS for?
Translating between symbolic names and IP addresses
Who uses DNS?
Applications (which then becomes a client to the DNS server)
How are domain names
structured?
Hierarchically, most important on the right
e.g. marian.cs.nott.ac.uk
DNS database is a ____ system
distributed
When an application acts as a client, what resolution does it request?
recursive
When a server acts a client what resolution does it request?
iterative
What are the three main aspects of computer security?
Confidentiality
Integrity
Availability
What are the three things we are trying to achieve with computer security?
- Authentication
- Authorisation
- Non-repudiation
What does confidentiality mean? And what can it encompass?
Means keeping information secret from those who should not know it
encompasses: privacy and secrecy
What does integrity mean in computer security?
Ensuring things are not altered without permission
What does maintaining integrity require?
Only authorised users perform authorised actions on resources they are authorised to use
What does availability mean in computer security?
Ensuring that legitimate users of a system have reasonable and reliable access to their systems
What is cryptography?
The use of a key to encode/decode a message
What are the two types of key encryption?
Secret key encryption and public key encryption
What are three methods for maintaining data security?
- hash functions
- cryptography
- digital signatures and digital certificates
What is the purpose of digital signatures and certificates?
Verify integrity of data and identity of sender
What is a hash function?
An algorithm that transforms a message into a fixed length hash key (hash)
How does public key encryption work?
Messages are encrypted using public key
- messages are decoded with secret key
What is a common use of public key encryption?
Secure web purchases
What is a drawback of secret key encryption?
You need to keep the secret key secret and also send it to the other party for decoding
What is the purpose of a digital signature?
For the receiver to be certain that the message is from a particular sender
Summarise digital certificates.
- Used to authenticate a user, device, or server
- Issued by trusted third party, CA
- Contains: certified public keys, information about certificate holder
What is proven when digital signatures are used together with digital certificates?
- Sender is authentic, message has not been tampered with
What does VPN stand for?
Virtual Private Network
What are two methods of network security?
-VPNs
- Firewalls
What does a VPN do?
It’s a connection which makes a remote network appear as if its directly connected with encrypted traffic
What must VPNs ensure happens to packets on their path?
That once packets are encrypted by Ipsec they are not fragmented along the path
What does a firewall do?
Allow partial connected to the internet while retaining some isolation
Where are firewalls placed?
Between an organisation internal network and the external network
What do firewalls control and limit?
- control amount of traffic passing between internal and external network
- limits amount of damage
What can firewalls filter?
- filter packets from a particular source
- filter traffic for a particular service
- filter traffic from a particular source or port number
What are 2 types of firewalls?
Packet filters (chokes)
Proxies (gates)
What is a demilitarised zone in the context of a network firewall?
A section of devices offered limited protection
