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Part I: Networking Fundamentals

The TCP/IP and OSI Networking Models

Open Systems Interconnection (OSI) Взаимосвязи Открытых Систем

7ой – уровень: Приложение > Сервисы
6ой – уровень: Представление > Сервисы
5ый – уровень: Сессия > Связь
4ый – уровень: Транспортный > Связь
3ий – уровень: Сетевой > Связь
2ой – уровень: Данные > Физические соединения
1ый – уровень: Физический > Физические соединения

TCP/IP Networking Model (Transmission Control Protocol/Internet Protocol)

#TCP/IP Original TCP/IP Updated Примеры протоколов
4Application (Прикладной)Application (Прикладной)HTTP, FTP, SSH
3Transport (Транспортный)Transport (Транспортный)TCP, UDP
2Internet (Сетевой)Network (Сетевой)IP
1Link (Канальный)Data Link (Канальный)
Pysical (Физический)
Ethernet, Wireless

adjacent-layer interaction (взаимодействие соседних уровней на одном сетевом устройстве)
The general topic of how on one computer, two adjacent layers in a networking architectural model work together, with the lower layer providing services to the higher layer.

same-layer interaction (взаимодействие одинаковых уровней на разных сетевых устройствах)
The communication between two networking devices for the purposes of the functions defined at a particular layer of a networking model, with that communication happening by using a header defined by that layer of the model. The two devices set values in the header, send the header and encapsulated data, with the receiving device(s) interpreting the header to decide what action to take.

TCP/IP Link Layer (Data Link Plus Physical)
Step 1. Comp encapsulates the IP packet between an Ethernet header and Ethernet trailer, creating an Ethernet frame.
Step 2. Physically transmits the bits of this Ethernet frame, using electricity flowing over the Ethernet cabling.
Step 3. Router physically receives the electrical signal over a cable, and re-creates the same bits by interpreting the meaning of the electrical signals.
Step 4. Router deencapsulates the IP packet from the Ethernet frame by removing and discarding the Ethernet header and trailer.

Protocols define both headers and trailers for the same general reason, but headers exist at the beginning of the message and trailers exist at the end.


Step 1. Create and encapsulate the application data with any required application layer headers. For example, the HTTP OK message can be returned in an HTTP header, followed by part of the contents of a web page.
Step 2. Encapsulate the data supplied by the application layer inside a transport layer header. For end-user applications, a TCP or UDP header is typically used.
Step 3. Encapsulate the data supplied by the transport layer inside a network layer (IP) header. IP defines the IP addresses that uniquely identify each computer.
Step 4. Encapsulate the data supplied by the network layer inside a data link layer header and trailer. This layer uses both a header and a trailer.
Step 5. Transmit the bits. The physical layer encodes a signal onto the medium to transmit the frame.

In TCP, a term used to describe a TCP header and its encapsulated data (also called an L4PDU). Also in TCP, the process of accepting a large chunk of data from the application layer and breaking it into smaller pieces that fit into TCP segments. In Ethernet, a segment is either a single Ethernet cable or a single collision domain (no matter how many cables are used).

A logical grouping of bytes that includes the network layer header and encapsulated data, but specifically does not include any headers and trailers below the network layer.

A term referring to a data link header and trailer, plus the data encapsulated between the header and trailer.


Fundamentals of Ethernet LANs

A series of LAN standards defined by the IEEE, originally invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation.

The term Ethernet refers to a family of LAN standards that together define the physical and data link layers of the world’s most popular wired LAN technology. The standards, defined by the Institute of Electrical and Electronics Engineers (IEEE), defines the cabling, the connectors on the ends of the cables, the protocol rules, and everything else required to create an Ethernet LAN.

So, what is an Ethernet LAN? It is a combination of user devices, LAN switches, and different kinds of cabling. Each link can use different types of cables, at different speeds. However, they all work together to deliver Ethernet frames from the one device on the LAN to some other device.

Examples of Types of Ethernet

SpeedCommon NameInformal IEEE Standard NameFormal IEEE Standard NameCable Type, Maximum Length
10 MbpsEthernet10BASE-T802.3Copper, 100 m
100 MbpsFast Ethernet100BASE-T802.3uCopper, 100 m
1000 MbpsGigabit Ethernet1000BASE-LX802.3zFiber, 5000m
1000 MbpsGigabit Ethernet1000BASE-T802.3abCopper, 100m
10000 Mbps10 Gig Ethernet10GBASE-T802.3anCopper, 100m

FieldField Length in BytesDescription
Start Frame Delimiter (SFD)1Signifies that the next byte begins the Destination MAC Addres field
Destination MAC Address6Identifies the sender of this frame
Source MAC Addres6Identifies the sender of this frame
Type2Defines the type of protokol listed inside the frame; today, most likely identifies IP version 4 or IP version 6
Data and Pad46-1500Holds data from a higher layer, typically an L3PDU
Frame Check Sequence (FCS)4Provides a method for the receiving NIC to determine whether the frame experienced transmission errors

Half-duplex: Logic in which a port sends data only when it is not also receiving data; in other words, it cannot send and receive at same time.
Full-duplex: The absence of the half-duplex restriction.

Fundamentals of WANs

Customer Premises Equipment (CPE) оборудование на стороне заказчика.
The physical link requires a function called a channel service unit/data service unit (CSU/DSU). The CSU/DSU can either be integrated into the serial interface card in the router or sit outside the router as an external device.

HDLC High-Level Data Link Control. A bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO).

leased line A serial communications circuit between two points, provided by some service provider, typically a telephone company (telco). Because the telco does not sell a physical cable between the two endpoints, instead charging a monthly fee for the ability to send bits between the two sites, the service is considered to be a leased service.

serial interface A type of interface on a router, used to connect to some types of WAN links, particularly leased lines and Frame Relay access links.

DSL Digital subscriber line. Public network technology that delivers high bandwidth over conventional telco local-loop copper wiring at limited distances. Typically used as an Internet access technology, connecting a user to an ISP.

Стандартные скорости передачи данных в распределенных сетях.

DS064 Кбит/с
DS1(T1)1,544 Мбит/с (24 DS0 + 1 канал перегрузки на 8 Кбит/с)
DS3(T3)44,763 Мбит/с (28 DS1 + 1 дополнительный канал управления)
E12,048 Мбит/с (32 DS0)
E334,064 Мбит/с (16 E1 + 1 дополнительный канал управления)

Fundamentals of IPv4 Addressing and Routing

Коммутаторы работают на 2 уровне с фреймами, т.е. канальном и руководствуются mac адресами.
Маршрутизаторы работают на 3 уровне пакетами, т.е. сетевом и руководствуются ip адресами.

IP addresses consist of a 32-bit number, usually written in dotted-decimal notation (DDN).
All IP addresses in the same group must not be separated from each other by a router.
IP addresses separated from each other by a router must be in different groups.

Классовая адресация

Network IDs

ConceptClassNetwork ID
All addresses that begin with 8A8.0.0.0
All addresses that begin with 130.4B130.4.0.0
All addresses that begin with 199.1.1C199.1.1.0

All Possible Valid Network Numbers

ClassFirst Octet RangeValid Network Numbers
A1 to 1261.0.0.0 to
B128 to 191128.0.0.0 to
C192 to 223192.0.0.0 to

subnetting The process of subdividing a Class A, B, or C network into smaller groups called subnets.

• One group of the 254 addresses that begin with 150.9.1
• One group of the 254 addresses that begin with 150.9.2
• One group of the 254 addresses that begin with 150.9.3

IPv4 Routing

Hosts actually use some simple routing logic when choosing where to send a packet. If you assume that the design uses subnets (which is typical), this two-step logic is as follows:
Step 1. If the destination IP address is in the same IP subnet as I am, send the packet directly to that destination host.
Step 2. Otherwise, send the packet to my default gateway, also known as a default router. (This router has an interface on the same subnet as the host.)

First, when a router receives a data link frame addressed to that router’s data link address, the router needs to think about processing the contents of the frame. When such a frame arrives, the router uses the following logic on the data link frame:
Step 1. Use the data link Frame Check Sequence (FCS) field to ensure that the frame had no errors; if errors occurred, discard the frame.
Step 2. Assuming that the frame was not discarded at Step 1, discard the old data link header and trailer, leaving the IP packet.
Step 3. Compare the IP packet’s destination IP address to the routing table, and find the route that best matches the destination address. This route identifies the outgoing interface of the router, and possibly the next-hop router IP address.
Step 4. Encapsulate the IP packet inside a new data link header and trailer, appropriate for the outgoing interface, and forward the frame.

First, consider the goals of a routing protocol, regardless of how the routing protocol works:
• To dynamically learn and fill the routing table with a route to each subnet in the internetwork.
• If more than one route to a subnet is available, to place the best route in the routing table.
• To notice when routes in the table are no longer valid, and to remove them from the routing table.
• If a route is removed from the routing table and another route through another neighboring router is available, to add the route to the routing table. (Many people view this goal and the preceding one as a single goal.)
• To work quickly when adding new routes or replacing lost routes. (The time between losing the route and finding a working replacement route is called convergence time.)
• To prevent routing loops.

TCP/IP Layer 4 Protocols: TCP and UDP

TCP/IP Transport Layer Features

Multiplexing using portsProcess of numbering and acknowledging data with Sequence and Acknowledgment header fields
Error recoveryProcess that uses window sizes to protect buffer space and routing devices from begin overloaded with traffic
Connection establishment and terminationProcess used to initialize port number and Sequence and Acknowledgment fields
Order data transfer and data segmentationContinuous stream of bytes from an upper-layer proccess that is «segmented» for transmission and delivered to upper-layer processes at the receiving device, with the bytes in same order.

Transmission Control Protocol

Multiplexing Using TCP Port Numbers
TCP and UDP both use a concept called multiplexing.
Multiplexing relies on a concept called a socket. A socket consists of three things:
■ An IP address
■ A transport protocol
■ A port number

Popular TCP/IP Applications

Port Number Protocol Application
20 TCP FTP data
21 TCP FTP control
23 TCP Telnet
67, 68 UDP DHCP
110 TCP POP3

Connection Establishment and Termination
User Datagram Protocol
However, UDP provides some functions of TCP, such as data transfer and multiplexing using port numbers, and it does so with fewer bytes of overhead and less processing required than TCP.

Building Ethernet LANs with Switches

Switching Method Description
Store-and-forward The switch fully receives all bits in the frame (store) before forwarding the frame (forward). This allows the switch to check the FCS before forwarding the frame.
Cut-through The switch forwards the frame as soon as it can. This reduces latency but does not allow the switch to discard frames that fail the FCS check.
Fragment-free The switch forwards the frame after receiving the first 64 bytes of the frame, thereby avoiding forwarding frames that were errored because of a collision.


icnd1.txt · Последние изменения: 2015/03/31 08:57 — admin