Network technologies

Lecturer:Rokovyi Oleksandr Petrovych

Contents

• Network technologies
  • UNIX and Linux systems
    • Filesystem hierarcy standard
    • Permission management
    • Processes
  • The Open Systems Interconnection model
    • OSI layers functions
  • TCP/IP protocol stack
  • IP Protocol
  • Control Protocols
    • ICMP
    • ARP (Address Resolution Protocol)

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UNIX and Linux systems

Примітка

further UNIX/Linux denotes all Linux, UNIX, Linux-like and UNIX-like systems

• There are two types of resources: files and processes. However file entity is much wider than one in Windows systems. Hardware devices, IPC mechanisms, etc are represented as files. There are 6 types of files:

  • Regular file

  • Directory

  • Device These are so called interface files for real devices. They do not store used data but provide an interface for operating system to work with underlying hardware devices.

    • Block device (data storages like HDD or SDD). Data exchange with such devices is done in chunks called blocks. In case of HDDs and SDDs size of block is 512B.
    • Char device. These devices are mostly used for I/O. System reads and writes data to char devices byte by byte.

  • Socket allows to exchange data between processes which are run on different physical devices. Sockets can be used for IPC on local machine but this is not efficient due to network stack it uses.

  • Pipe (IPC mechanism). In order to perform some data exchange between processes pipes are used. One process opens pipe for reading and other one for writing.

    This is like socket but the processes are run on the same physical node

    —© Roman Statkevych

  • Link

    • Hardlink is a one-way link between filename and its metadata in the inode. This means that filename knows where the metadata is but metadata knows nothing about filename.

      However metadata is aware about the number of file names which point to it.

      Hardlinks work only within the same filesystem and can be applied only for regular files

    • Symlink is a file which contains the path to other file. Symlinks do not work with file metadata and they can work among different filesystems as well as they can work with directories.

  This allows to manage everything in one way: by changing permissions on the filesystem level.

  Примітка

  further the word file denotes file of any of 6 types

• Filesystem hierarcy is important. It is represented as a tree, which starts from root (/). The mechanism behind the hierarcy is simple directories. Pay attention that directories do not store user data, but only a list of files which are located in that directory.

  There is a filesystem struture standard which defines where the parts of operating system are located, where which kinds of files are located, etc.

There are two ways to reference files in UNIX/Linux systems:

• absolute filepaths (those are started with / like /home/username)
• relative filepaths (username/foo/bar)

Примітка

In order to get help on any command system manual pages should be used. There is a special command man for that. Details on this command usage is available on the man page for man command

Main utility to navigate filesystems is cd command, reference for which can be found in man 1 cd. Here are some examples:

cd /home/user  # navigate to `/home/user` directory
cd user  # navigate to directory `user` which is located in current directory
cd ./user  # the same as above, `.` denotes current directory
cd ../  # navigate one level up in the directory tree
cd ../mydir  # navigate to the directory mydir which is on the same level as
# current dir
cd ~  # navigate to the current user's home directory
cd ~/foo # navigate to the directory `foo` which is the home directory

There are other useful utilities:

• cp – copies files
• rm – deletes (removes) files
• touch – create empty file (actually change file timestamps)
• pwd print working directory
• ls list files
• mv move files
• find search for files in directory tree

Filesystem hierarcy standard

Примітка

See man hier for more details

• /
  • boot/ – stores boot files. It usually contains kernel, initramfs image, bootloader cofiguration files, etc
  • etc/ – stores configuration files for both operating system and applications.
  • bin/ – stores executable files (which have the highest important for operating system).
  • sbin/ – like /bin holds less important executables for operating system start.
  • lib/ – holds shared libraries that are necessary to boot the system and to run the commands in the root filesystem.
  • root/ – home directory for root user
  • usr/ – (UNIX System resources) holds all other resources, which are not included in other directories
    • bin/ – primary directory for executable programs
  • var/ – holds temporary operating system files like logs, pid files and temporary sockets.
  • tmp/ – holds user temporary data
  • mnt/ – mountpoints
  • home/ – holds home directories for users
  • media/ – mountpoints for external storages like usb sticks
  • opt/ – additional appliations are installed here
  • srv/ – storing network services files
  • proc/ – virtual filesystem which holds info about running processes
  • sys/ – virtual filesystem which grants access to system
  • dev/ – stores device files for hardware devices

Permission management

There are three subjects for which permissions are set in UNIX/Linux systems for every file:

• u – owner user
• g – owner group
• o – other users

Main permissions are

• r – read permission
• w – write permission
• x – execute permission

$ ls -l
total 73
-r--r--r--   1 root  wheel  6199 Jul 21 02:11 COPYRIGHT
drwxr-xr-x   2 root  wheel  1024 Jul 21 02:09 bin
drwxr-xr-x   8 root  wheel  1536 Sep 13 02:39 boot
dr-xr-xr-x  12 root  wheel   512 Sep 13 02:39 dev
-rw-------   1 root  wheel  4096 Sep 13 02:40 entropy
drwxr-xr-x  27 root  wheel  2560 Sep 13 02:57 etc
lrwxr-xr-x   1 root  wheel     8 Sep 13 02:05 home -> usr/home
drwxr-xr-x   4 root  wheel  1536 Jul 21 02:09 lib
drwxr-xr-x   3 root  wheel   512 Sep 13 02:03 libexec
drwxr-xr-x   2 root  wheel   512 Jul 21 02:08 media
drwxr-xr-x   2 root  wheel   512 Jul 21 02:08 mnt
drwxr-xr-x   2 root  wheel   512 Jul 21 02:08 net
dr-xr-xr-x   2 root  wheel   512 Jul 21 02:08 proc
drwxr-xr-x   2 root  wheel  2560 Jul 21 02:09 rescue
drwxr-xr-x   2 root  wheel   512 Sep 13 02:57 root
drwxr-xr-x   2 root  wheel  2560 Jul 21 02:10 sbin
lrwxr-xr-x   1 root  wheel    11 Jul 21 02:11 sys -> usr/src/sys
drwxrwxrwt   7 root  wheel   512 Sep 13 03:11 tmp
drwxr-xr-x  17 root  wheel   512 Sep 13 02:06 usr
drwxr-xr-x  25 root  wheel   512 Sep 13 02:40 var

First triplet is used for owner user permissions, second one is for owner group permissions and the last one is for other users permissions.

# owner can read and execute,
# owner group can only read,
# others can do nothing
r-xr-----  ... file1

There is chmod command which is used for permissions management.

# add write permission for group:
chmod g+w file.txt
# add read and execute permissions for owner and group
chmod ug+rx file.txt
# add read permission for all
chmod ugo+r file.txt
# ugo can be replaced with a
chmod a+r file.txt  # same as above

# add write for user, read for others
chmod u+w,o+r file.txt

Every tiplet can be represented as a bitmask:

u  g  o
rwx------ ... file.txt
421421421

Thus in order to allow read and execute for owner and read for group we can do the following:

chmod u+rx,g+r file.txt
chmod 540 file.txt

For directories x permission means the ability to enter the directory.

Processes

Process is some code which is sored in the RAM and is being executed. Most of processes are run in the background. In order to move process to foreground there is fg command. To move process to background there is bg command. There is a possibility to start process in the background explicitly: all you have to do is to add ampersand character at the end of command:

# note the ampersand character at the end of the command
run_my_script --arg1 --arg2=foo &

Widely used utilities to manage processes in UNIX/Linux systems are:

• jobs – lists jobs running in the current terminal
• nohup – runs the process which will not die after terminal is detached
• kill – sends signals to the processes. Signals which are used to stop the processes are SIGTERM and SIGKILL. kill utility works with processes ids (PIDs)
• killall – similar to kill, but works with process name.
• top – displays Linux/UNIX processes.
• ps – reports a snapshot of the current processes. ps is usually invoked with aux keys.
• pstree – displays a tree of processes
• uptime – tells how long the system has been running.
• free – displays amount of free and used memory in the system.

The Open Systems Interconnection model

Protocol:is the set of rules and conventions which are used for interaction between the same layer of different nodes. Interface:is the set of rules which are used for interaction between adjacent layers of the same node Encapsulation:is the inclusion of the data from the upper layer into the special data structure of the lower layer. Decapsulation is the reverse procedure.

OSI model descibes 7 layers of network organization and the purpose of each layer. Protocols are not described in the OSI model.

№	Name
7	Application
6	Presentation
5	Session
4	Transport
3	Network
2	Data link
1	Physical

OSI layers functions

Physical layer

Bit transfer via physicall channels.

• forming electric sugnals
• data encoding
• synchronization
• modulation
• physical topology
• physical characteristics of interfaces

All of these are implemented in the hardware.

Data link layer

Reliable data frame delivery between two adjacent stations in the network with any topology or between any two stations in the network with typical topology.

• environment availability checks
• grouping data into frames
• computation and verification of the checksum
• physical addressing (MAC addresses)
• data flow management

These functions are implemented both in hardware and programmatically.

Network layer

Packet delivery

• between any two nodes of the network with any kind of topology
• between any two networks in the composite network
• logical addressing
• package data size coordination

Network:is the set of computers which use single network technology for data exchange Route:is the sequence of routers which are passed by the data package in the composite network

Transort layer

Data delivery with the determined reliability between any two processes in the network.

• splitting message into chunks and their numeration
• buffering
• ordering of the incoming packages
• application processes addressing
• flow management
• integrity check

Примітка

The unique identifier of process in the node in terms of TCP/IP terminology is called port

Session layer

Примітка

Explicit implementation of this layer is missing in most of TCP/IP stack implementation. Thus only ideas of session layer are described here.

Managing application layer objects’ dialog. (Synchronization between processes on different nodes).

• Sets message exchange methods (duplex/half-duplex)
• Message exchange synchronization
• Dialog check-points

Protocol data unit of session layer is message.

Presentation layer

Примітка

As well as session layer, presentation layer is not explicitly implemented in most TCP/IP implementations.

Coordination of data presentation for interprocess communication.

• Translates data from external format into internal format.
• Encrypts and decrypts data
• Data compression

Application layer

Set of all network services which can be used by the end users.

• User authentication and authorization
• File services, email, remote access, printing, etc

TCP/IP protocol stack

TCP/IP protocol stack is the implementation of the model. It includes only 4 layers:

• Application layer
• Transport layer
• Network layer
• Network interfaces layer

There are no network interfaces layer protocols in TCP/IP stack. There are only interfaces between network and network interfaces layers.

Layer	Protocols and standards
Application	HTTP, SMTP, DNS, FTP
Transport	TCP, UDP
Network	IP, ICMP
Network interfaces	Ethernet, WiFi

IP Protocol

IP (Internet Protocol) is a internetwork protocol described in RFC 791.

internet:is a joint network subnet:is a part of the network internetworking:  networks union Internet:name of the most well-known network

Internet Protocol is the base on which Internet is built.

IP functions:

• routing
• logical addressing
• coordinating MTUs
• uniting networks
• quality of service

Header format:

Control Protocols

In order to debug network layer and IP (internet protocol) such protocols as ICMP (Internet Control Message Protocol) and ARP (Address Resolution Protocol) were introduced. Please note that ARP works between Network and Network Interfaces layers.

There is also DHCP protocol which works on Application layer but is a auxiliary protocol for Network Layer.

ICMP

Protocol specification can be found in `RFC 792 <https://tools.ietf.org/html/rfc792`_. ICMP is mostly used as diagnostic tool to determine whether Network layer works correctly, hovewer the original purpose was much wider.

All information about package delivery (by IP) can be sent to the sender using ICMP.

Earlier ICMP was used to manage network nodes (get address resolution table, set up routing, etc), however better tools appeared later thus deprecating such purpose of ICMP.

Примітка

ICMP packet is icluded into the IP packet

ICMP cannot be used to debuug ICMP error messages, as the node which determines issue with the ICMP error message must not generate ICMP message.

ICMP does not fix problems and does not retry sending messages.

Header format:

ICMP message types:

Type	Description
0	Echo response
3	Destination host unreachable
4	Source suppression
5	Route redirection (Informing that better route to host exists) Disabled in modern systems due to security concerns
8	Echo request
11	Packet TTL expired
12	Malformed parameters
13	Timestamp request (Currently this is NTP functionality)
14	Timestamp response (Currently this is NTP functionality)
17	Mask request
18	Mask response

ICMP Error types are different for each message type.

Most widely used tools for network diagnostics (which use ICMP) are:

• ping

  This is a very simple network diagnostics tool.

  Uses ICMP message types 0 and 8. ping is used to determine concrete host availability in the network.

• traceroute

  Shows the route to the destination host. Please note that IP protocol does not have built-in utilities for determining routes, thus tools like traceroute cannot be 100% precise. For example, traceroute does it by sequentially sending packets with increasing ttl thus generating ttl expired error on every host in the route.

ARP (Address Resolution Protocol)

ARP is used to map IP addresses (logical addressing) to MAC addresses (physical addressing). The protocol is described in RFC 826