Network Standards Layered Architectures Chapter 2 Updated January 2009 XU Zhengchuan Fudan...

Network StandardsLayered Architectures

Chapter 2Updated January 2009

XU Zhengchuan

Fudan University

1.Message Standards (Protocols)

2-3

Figure 2-1: Standards Govern the Exchange of Messages

• Standards

– Rules of operation that allow two hardware or software processes to work together

– Even if they are from different vendors

• Standards Govern the Exchange of Messages

– Messages must be governed by strict rules

– Because computers are not intelligent

Message

2-4

Figure 2-1: Standards Govern the Exchange of Messages (Continued)

• Standards Govern Syntax– Syntax: the organization of the message

– Human example: “Susan thanked Tom”

– This sentence has a subject-verb-object syntax

• Standards Govern Semantics– Semantics: The meaning of the message

– Human example: “Susan thanked Tom”

– Humans understand the meaning of this easily

2-5

Figure 2-2: Hypertext Transfer Protocol (HTTP) Interactions

Client PC Webserver

Browser WebserverApplication

1.HTTP Request Message

Asking for a File

2.HTTP Response Message

delivering the File orgiving an error message

Semantics in HTTP, which governs the Web

2-6

Figure 2-3: Syntax of HTTP Request and Response Messages

• [CRLF]

– Carriage return and line feed (starts a new line)

• HTTP Request Message

– GET /reports/project1/final.htm HTTP/1.1[CRLF]• GET is the method (others exist)• Next comes the path to the file to be retrieved• Last comes the version of the HTTP standard

– Host: voyager.cba.Hawaii.edu[CRLF]

• The host to be sent the request message

2-7

Figure 2-3: Syntax of HTTP Request and Response Messages, Continued

• HTTP Response Message– HTTP/1.1 200 OK[CRLF]

– Date: Tuesday, 20-JAN-2006 18:32:15 GMT[CRLF]

– Server: name of server software[CRLF]

– MIME-version: 1.0[CRLF]

– Content-type: text/plain[CRLF]

– [CRLF]

– File to be downloaded (byte stream)

• Syntax of fields (lines) after first line:– Keyword : Content [CRLF]

Syntax isvery rigid

2-8

• Test Your Understanding

• P 100

2-9

Figure 2-1: Standards Govern the Exchange of Messages, Continued

• General Message Syntax (Organization)

– General Message Organization (Figure 2-4)

– Primary parts of messages

• Data Field (content to be delivered)

• Header (everything before the data field)

• Trailer (everything after the data field)

– The header and trailer act like a delivery envelope for the data field.

HeaderData FieldTrailer

2-10

Figure 2-1: Standards Govern the Exchange of Messages, Continued

• General Message Syntax (Organization)– Header and trailer are further divided into fields

Trailer Data Field Header

OtherHeader

FieldDestination

AddressField is

Used by Switches and RoutersLike the Address on an Envelope

Message withall three parts

2-11

Figure 2-4: General Message Organization, Continued

Data Field Header

OtherHeader

Field

DestinationAddress

Field

Message withouta trailer

Usually only data linklayer messages have trailers

2-12

Figure 2-4: General Message Organization, Continued

Header

OtherHeader

Field

DestinationAddress

FieldMessage withonly a header

e.g.TCP supervisory messages are pure headers

(there is no data field content to deliver)

2-13


• P 100

2.Reliability

2-15

Figure 2-5: Reliable Transmission Control Protocol (TCP) Session

• The Transmission Control Protocol (TCP) is an important standard in Internet transmission

• TCP

– Receiver acknowledges each correctly-received TCP segment.

– If an acknowledgments is not received by the sender, the sender retransmits the TCP message (called a TCP segment)

– This gives reliability: error detection and error correction

2-16

TCP Segment (Message) 4Carries an HTTP Request

Segment 5 Acknowledges It

There Is No Need to Resend

Figure 2-5: Reliable TCP Session, Continued

Client PCTCP Process

WebserverTCP Process

4. Data = HTTP Request

5. ACK (4)

6. Data = HTTP Response

7. ACK (6)

CarryHTTPReq &Resp

(4)

Request-ResponseCycle for Data Transfer

2-17

Figure 2-5: A TCP Session, Continued



CarryHTTPReq &Resp

(4)

8. Data = HTTP Request (Error)

9. Data = HTTP Request (No ACK so Retransmit)

10. ACK (9)

11. Data = HTTP Response

12. ACK (11)Error Handling

TCP Segment (Message) 8Is Lost in Transmission

There Is No Acknowledgment

So the Sender Retransmits It

2-18


• P 102

3.Connection-Oriented andConnectionless Protocols

2-20

Figure 2-6: Connection-Oriented and Connectionless Protocols

Message(No Sequence Number)

Connectionless Protocol

A B

Message 1 (Seq. Num = A1)

Message 2 (Seq. Num = A2)

Close Connection

Connection-Oriented Protocol

Open ConnectionA B

Message 3 (Seq. Num B1)

Connection-oriented protocols haveFormal openings and closings likeTelephone calls

Also have sequence numbersso that the receiver can putmessages in order

And so the receiver can sendAcknowledgments for specificmessages

2-21

Figure 2-6: Connection-Oriented and Connectionless Protocols, Continued

Client PCBrowser

WebserverApplication

HTTP Request

HTTP is connectionless

No OpeningsNo Closings

No Sequence NumbersNo Acknowledgments

2-22

Figure 2-6: Connection-Oriented and Connectionless Protocols, Continued



Connection-Opening Messages

Time

Connection-Closing Messages

Messages During the Connection

In TCP

2-23

Figure 2-7: Advantages and Disadvantages or Connection-Oriented Protocols

• Advantages

– Thanks to sequence numbers, the parties can tell if a message is lost.

– Error messages, such as ACKs can refer to specific messages.

– Long messages can be fragmented into many smaller messages that can fit inside packets.

• Fragmentation followed by reassembly on the destination host is an important concept in networking.

2-24

Figure 2-7: Advantages and Disadvantages or Connection-Oriented Protocols, Cont.

• Disadvantages

– The presence of many supervisory messages consumes existing bandwidth

– The processing of connection information places a heavy processing load on computers connected to the network

2-25


• P 105

4.The Hybrid TCP/IP-OSI Standards Architecture

2-27

Standards Architecture

• A Standards Architecture is a Broad Plan for Creating Standards

– Break the problem of effective communication into smaller pieces for ease of development

– Develop standards for the individual pieces

– Just as a building architect creating a general plan for a house before designing the individual rooms in detail

– The dominant architecture today is the hybrid TCP/IP-OSI standards architecture shown in the next slide

2-28

Figure 2-8: Hybrid TCP/IP-OSI Architecture

General Purpose(Core Later)

Layer Specific Layer Purpose

Application-application communication

Application (5) Application-application interworking

Transmission of a packet across an internet

Transport (4) Host-host communication

Internet (3) Packet delivery across an internet

Transmission of a frame across a single network (LAN or WAN)

Data Link (2) Frame delivery across a network

Physical (1) Device-device connection

2-29

Figure 2-8: Hybrid TCP/IP-OSI Architecture, Continued

• Physical and Data Link Layer Standards

– Govern Communication Through a Single Network

– LAN or WAN

2-30

Figure 2-9: Physical and Data Link Layer Standards in a Single Network

• Physical Layer

– Physical layer standards govern transmission between adjacent devices connected by a transmission medium

Switch X1

Physical LinkA-X1

Host A

Switch X2Physical LinkX1-X2

2-31

Figure 2-9: Physical and Data Link Layer Standards in a Single Network, Continued

• Data Link Layer

– Data link layer standards govern the transmission of frames across a single network—typically by sending them through several switches along the data link

Switch X1Host A

Switch X2

Host BData LinkA-B

Frame

2-32


• Data Link Layer

– Data link layer standards also govern

• Frame organization

• Switch operation

2-33


Host A

Mobile ClientStation

ServerStation

Switch

SwitchX2

Switch X1

Switch

Data LinkA-R1

Physical LinkA-X1

PhysicalLink

X1-X2

Router R1

PhysicalLink

X2-R1

3 Physical Links1 Data Link2 Switches

2-34


• P 107

2-35

Figure 2-10: Internet and Data Link Layers in an Internet

• Internet and Transport Layers

– An internet is a group of networks connected by routers so that any application on any host on any network can communicate with any application on any other host on any other network

– Internet and transport layer standards govern communication across an internet composed of two or more single networks

2-36

Figure 2-10: Internet and Data Link Layers in an Internet, Continued

• Internet Layer

– Internet layer standards govern the transmission of packets across an internet—typically by sending them through several routers along the route

– Messages at the internet layer are called packets

– Internet layer standards also govern packet organization and router operation

Router 1 Router 2

Packet

2-37


Host B

Host A

Network XNetwork Y

Network Z

R1

R2

Data Link A-R1

Data Link R3-B

DataLink

R1-R2Route A-B

3 Data Links: One per Network1 Route per Internet

2-38


Host A

Mobile ClientStation

ServerStation

Switch

SwitchX2

SwitchX1

Switch

Data LinkA-R1

Router R1

Packet

Frame X

Network X

RouteA-B

In Network X:Two Destination Addresses:

Packet: Host B (Destination Host)Frame: Router R1

2-39


Router R1

Router R2

Packet

Frame Y

ToNetwork X

ToNetwork Z

Network Y

Data LinkR1-R2

RouteA-B

In Network Y:Two Destination Addresses:

Packet: Host B (Destination Host)Frame: Router R2

2-40


Host B

Mobile ClientStations

SwitchZ1

SwitchX2

SwitchZ2

PacketFrame Z

Network Z

Router R2

Router

Data LinkR2-B

In Network Z:Two Destination Addresses:

Packet: Host B (Destination Host)Frame: Host B

2-41

Frames and Packets

• In an internet with hosts separated by N networks, there will be:

– 2 hosts

– One packet (going all the way between hosts)

– One route (between the two hosts)

– N frames (one in each network)

– There usually are many switches within single networks

– There usually are many physical links within networks

2-42

Figure 2-11: Internet and Transport Layer Standards

• Transport Layer

– Transport layer standards govern aspects of end-to-end communication between two end hosts that are not handled by the internet layer

– These standards allow hosts to work together even if the two computers are from different vendors and have different internal designs

2-43

Figure 2-11: Internet and Transport Layer Standards, Continued

2.Transport Layer

end-to-end (host-to-host)TCP is connection-oriented, reliable

UDP is connectionless and unreliable

1.Internet Layer

(usually IP)hop-by-hop (host-router or router-router)

connectionless, unreliable

Router 1 Router 2 Router 3

Client PCServer

2-44


• P 110

2-45

Figure 2-12: Application Layer Standards

• Application Layer

– The application layer governs how two applications work with each other, even if they are from different vendors

Webserver

Browser WebserverApplication

Client PC

2-46

Figure 2-12: Application Layer Standards

• There are more application layer standards than any other type of standard because there are many applications

– HTTP

– E-Mail

– Database

– Instant Messaging

– FTP

– Etc.

2-47


• P 111

2-48

Standards Layers: Recap

• Application (5)

• Transport (4)

• Internet (3)

• Data Link (2)

• Physical (1)

Be able to repeatthis in your sleep!

Be able to repeatthis in your sleep!

2-49

• Why Layered Architectures?

• Page 111

5.Syntax Examples for Some Layer Messages

2-51

Octets

• Field length may be measured in octets

• An octet is a group of eight bits

• In computer science, an octet is called a byte

Octet = 8 Bits10010111

2-52

Figure 2-14: Ethernet Frame

Preamble (7 octets) 10101010 …

Start of Frame Delimiter(1 octet) 10101011

Destination Ethernet (MAC) Address (48 bits)

Source Ethernet (MAC) Address (48 bits)

Length (2 octets) Length of Data Field

Header

The Ethernet frame has 48-bit destination and source address fields.

2-53

Figure 2-14: Ethernet Frame, Continued

Data Field(variablelength)

PAD (added if data field < 46 octets)

Frame Check Sequence (32 bits)

LLC Subheader(usually 8 octets)

UsuallyIP Packet

EncapsulatedPacket

The Ethernet frame’s data field contains a IP packet(preceded by an LLC subheader).

PAD is added if the data field is less than 46 octets longPAD length is set to keep the data field plus PAD 46 octets

DataField

2-54

Figure 2-14: Ethernet Frame, Continued

• Sender computes the frame check sequence field value based on contents of other fields– Receiver recomputes the field value

• If the values match, there have been no errors

• If the values do not match, there has been an error– The receiver simply discards the frame

• Unreliable: error detection but not error correction

Frame Check Sequence (32 bits)

2-55


• P 115 12-15

2-56

Figure 2-15: Internet Protocol (IP) Packet, Continued

Total Length(16 bits)

Version(4 bits)

Diff-Serv(8 bits)

HeaderLength(4 bits)

Identification(16 bits)

Flags(3 bits)

Fragment Offset(13 bits)

Header Checksum (16 bits)Protocol(8 bits)

Time to Live(8 bits)

Bit 0 Bit 31

Version is Bits 0-3

Header length is Bits 4-7

Diff Serv is Bits 8-15

Total Length is Bits 16-31

Identification is Bits 32-47

Time to live is Bits 48-55

The IP packet is drawn 32 bits to a line

2-57

Figure 2-15: Internet Protocol (IP) Packet

Total LengthVersion Diff-ServHeaderLength

Source IP Address (32 bits)

Identification Flags Fragment Offset

Header ChecksumProtocolTime to Live

Bit 0 Bit 31

Destination IP Address (32 bits)

Options (if any)Padding

(to 32-bit boundary)

Data Field(dozens, hundreds, or thousands of bits)

Often contains a TCP segment

2-58


• P 116-117, 16-18

2-59

Figure 2-16: TCP and UDP at the Transport Layer

• TCP is reliable

• Not all applications need reliability

– Voice over IP cannot wait for lost or damaged packets to be transmitted

– Network management protocols need to place as low a burden on the network as possible

– Both types of applications use the simpler User Datagram Protocol (UDP) instead of TCP

2-60

Figure 2-16: TCP and UDP at the Transport Layer, Continued

Protocol TCP UDP

Layer Transport Transport

Connection-Oriented? Yes No

Reliable? Yes No

Burden on the two hosts High Low

Burden on the network High Low

2-61

Why Make TCP Reliable?

• Two reasons:

• 1. The transport layer only involves processing on the two hosts.

– Reliability is a heavy process.

– It would be far more expensive to make the internet or data link layer reliable because this would require complex processing on many routers or switches, respectively.

• 2. TCP’s reliability fixes errors at the transport layer and all lower layers in the process. This allows the transport layer to give the application clean data.

2-62

Figure 2-17: A Complex Application Protocol: The Simple Mail Transfer Protocol (SMTP)

• Some application protocols are simple

– HTTP: Simple request-response message cycle shown in Figure 2-2

• Some application protocols are complex (Figure 2-17)

– Simple Mail Transfer Protocol (SMTP) for e-mail

– More than a dozen messages must be exchanged to send an e-mail message

2-63


• P 119-121, 19-22

6.Vertical Communication Between Layer Processes on the Same Host

2-65

Figure 2-18: Layered Communication on the Source Host

ApplicationProcess

HTTPMessage

TransportProcess

HTTPMessage

TCPHdr

Encapsulation of HTTP Messagein Data Field of TCP Segment

Passes MessageDown to Transport Process

The process begins when a browser creates an HTTP request message

2-66

Figure 2-18: Layered Communication on the Source Host, Continued

• When a layer process (N) creates a message, it passes it down to the next-lower-layer process (N-1) immediately

• The receiving process (N-1) will encapsulate the Layer N message, that is, place it in the data field of its own (N-1) message

2-67


TransportProcess

HTTPMessage

InternetProcess

HTTPMessage

TCPHdr

TCPHdr

IPHdr

Encapsulation of TCP Segmentin Data Field of IP Packet

2-68


InternetProcess

HTTPMessage

TCPHdr

IPHdr

Data LinkProcess

HTTPMessage

TCPHdr

IPHdr

EthHdr

EthTrlr

Encapsulation of IP Packetin Data Field of Ethernet Frame

2-69


Data LinkProcess

HTTPMessage

TCPHdr

IPHdr

EthHdr

EthTrlr

Physical Process

Physical Layer converts the bits of the frame into signals.

2-70


The following is the final frame for aan HTTP message on an Ethernet LAN

HTTPMessage

TCPHdr

IPHdr

EthHdr

EthTrlr

L5 L4 L3 L2L2

Notice the Pattern: From Right to Left: L2, L3, L4, L5, maybe L2

Start with the highest-layer message (in this case, 5)

Add headers for each lower layer (L4, L3, and L2, in this case)

Don’t forget the possible trailing L2 trailer

2-71


• P 124 23

2-72

Figure 2-19: Decapsulation on the Destination Host

HTTPMessage

TCPHdr

IPHdr

EthHdr

EthTrlr

Data LinkProcess

Physical Process

2-73

Figure 2-19: Decapsulation on the Destination Host, Continued

HTTPMessage

TCPHdr

IPHdr

EthHdr

EthTrlr

Data LinkProcess

InternetProcess

HTTPMessage

TCPHdr

IPHdr

Decapsulation of IP Packetfrom Data Field of Ethernet Frame

2-74


InternetProcess

HTTPMessage

TCPHdr

IPHdr

TransportProcess

HTTPMessage

TCPHdr

Decapsulation of TCP Segmentfrom Data Field of IP Packet

2-75


TransportProcess

HTTPMessage

TCPHdr

ApplicationProcess

HTTPMessage

Decapsulation of HTTP Messagefrom Data Field of TCP Segment

2-76


• P 126 24

2-77

Figure 2-20: Layered End-to-End Communication

Int

App

DL

Trans

Phy

SourceHost

DestinationHost

Switch1

Switch2

Router1

Switch3

Router2

Source andDestinationHosts Have

5 Layers

SwitchesHave Two

Layers---

Each SwitchPort

Has OneLayer (1)

RoutersHave Three

Layers---

Each RouterPort

Has TwoLayers (1&2)

2-78

Figure 2-21: Combining Horizontal and Vertical Communication

Int

App

DL

Trans

Phy

SourceHost

DestinationHost

Switch1

Switch2

Router1

Switch3

Router2

Hypertext Transfer Protocol

Transmission Control Protocol

Internet Protocol

2-79


• P 127 25-26

7.OSI, TCP/IP, and Other Standards Architectures

2-81

Figure 2-22: The Hybrid TCP/IP-OSI Architecture

TCP/IPOSIHybrid TCP/IP-OSIBroad Purpose

Application

Application

Presentation

Session

Application(Layer 5)

Communicationbetweenapplications

Transport

Internet

Transport

Network

Transport (Layer 4)

Internet (Layer 3)Internetworking

Use OSI Standards Here

Data Link

Physical

Data Link (Layer 2)

Physical (Layer 1)

Transmissionwithin a singleLAN or WAN

2-82

Figure 2-23: OSI and TCP/IP

OSI TCP/IP

StandardsAgency or Agencies

ISO (InternationalOrganization for Standardization)

ITU-T (InternationalTelecommunicationsUnion—TelecommunicationsStandards Sector)

IETF (InternetEngineering TaskForce)

2-83

Figure 2-23: OSI and TCP/IP, Continued

OSI TCP/IP

Dominance Nearly 100% dominant at physical and datalink layers

70%-80% dominantat the internet and transportlayers.

Documents areCalled

Various Mostly RFCs (requestsfor comments)

2-84

Figure 2-24: OSI Layers

• Layer 1: OSI Physical Layer Standards

– Nearly always used in the hybrid TCP/IP-OSI architecture

• Layer 2: OSI Data Link Layer Standards

– Nearly always used in the hybrid TCP/IP-OSI architecture

2-85

Figure 2-24: OSI Layers, Continued

• Layer 3: OSI Network Layer Standards– Same function as internet layer standards in TCP/IP

– But OSI network layer standards are incompatible with TCP/IP internet layer standards

– Rarely used

• Layer 4: OSI Transport Layer Standards– Same function as transport layer in TCP/IP

– But OSI transport layer standards are incompatible with TCP/IP transport layer standards

– Rarely used

2-86


• Layer 5: OSI Session Layer Standards

– Initiate and maintain a connection between application programs on different computers

– Nothing like this layer in TCP/IP

– Rarely used because OSI is rarely used above the data link layer and below the application layer

2-87


• Layer 6: OSI Presentation Layer Standards

– Designed to handle data formatting differences between the computers, data compression, and encryption.

• Rarely used this way because OSI standards are rarely used above the data link layer and below the application layer

– In practice, a category for general OSI file format standards used in multiple applications

• JPEG, etc.

• These standards are widely used

2-88


• Layer 7: OSI Application Layer

– For other application-specific matters

– Some OSI application layer standards are used

• Run over TCP/IP transport/internet layer processes

• Almost always without actual session and presentation layer processes

2-89

Figure 2-25: Other Major Standards Architectures

• IPX/SPX

– Used by older Novell NetWare file servers

– Popular option for newer Novell NetWare file servers

• SNA (Systems Network Architecture)

– Used by IBM mainframe computers

• AppleTalk

– Used by Apple Macintoshes

2-90

Figure 2-26: Characteristics of Protocols Discussed in the Chapter

Layer Protocol Connection-Oriented/Connectionless

Reliable/Unreliable

5 (App) HTTP Connectionless Unreliable

4 (Transport) TCPConnection-oriented

Reliable

3 (Internet) IP Connectionless Unreliable

2 (Data Link) Ethernet Connectionless Unreliable

Note: Only TCP is connection-oriented and reliable

4 (Transport) UDP Connectionless Unreliable

2-91

• Case study

• Chp 1 a page 42

• Chp 1c page 77

8.Topics Covered

2-93

Topics Covered

• Standards govern the semantics and syntax of messages

– HTTP: Text request and response messages

– Data field, header, and trailer

– Header and trailer subdivided into fields

• Reliability

– In TCP, receiver sends ACKs

– Senders retransmit non-acknowledged segments

2-94

Topics Covered

• Connection-oriented versus connectionless

– TCP is connection-oriented

– HTTP is connectionless

• Hybrid TCP/IP-OSI Architecture

– OSI is nearly 100% dominant at Layers 1 and 2

– TCP/IP is 70% to 80% dominant at Layers 3 and 4

– Situation at Layer 5 is complex

2-95

Topics Covered

• Hybrid TCP/IP-OSI Standards Architecture

– 5. Application layer (application-to-application)

– 4. Transport layer (host-to-host)

– 3. Internet layer (across an internet)

– 2. Data link layer (across a switched network)

– 1. Physical layer (between adjacent devices)

2-96

Topics Covered

• Ethernet

– Source and destination addresses are 48 bits long

– Switches forward packets by destination addresses

– Data field encapsulates an IP packet

– Unreliable: if detects an error, drops the frame

• Internet Protocol (IP)

– 32-bit addresses

– Show 32 bits on each line

– Unreliable: checks headers for errors but discards

2-97

Topics Covered

• Vertical Communication on the Source Host

– Layer process creates message and then sends the message to the next-lower layer

– Next-lower layer encapsulates the message in its own message

– This continues until the final frame at the data link layer

• Vertical Communication on the Destination Host– Decapsulation and passing up

2-98

Topics Covered

• Not All Devices Have All Layers

– Hosts have all five

– Routers have only the lowest three

– Switches have only the lowest two

2-99

Topics Covered

• OSI Architecture– Divides application layer into three layers

• Session• Presentation• Application

• Other Standards Architectures– IPX/SPX

– SNA

– AppleTalk

2-100

标准化组织1 ． ISO （ International Standards Organization ），国际标准化组织，

为大量科目制定标准；

2 ． ITU （ International Telecommunication Union ），国际电信联盟，主要有三个部门：无线通信部门、电信标准化部门、开发部门。任务是制定电话、电报和数据通信接口的技术建议。

3 ． IEEE （ Institute of Electrical and Electronics Engineers ），电气和电子工程师协会，是世界最大的电类专业组织。

4 ． NIST （ National Institute of Standards and Technology ），美国标准和技术协会，是美国商业部的一个办事机构，制定美国政府购买物品的强制标准。

5 ． IAB （ Internet Activities Board ），因特网活动委员会

6 ． IAB （ Internet Architecture Board ），因特网体系结构委员会

2-101

关于因特网的标准化工作

因特网协会 ISOC

因特网研究指导小组IRSG

因特网研究部 IRTF 因特网工程部 IETF

因特网工程指导小组IESG

…

RG WG… …RG…

领域领域

因特网体系结构研究委员会 IAB

WGWGWG

2-102

制订因特网的正式标准要经过以下的四个阶段

• 因特网草案 (Internet Draft) —— 在这个阶段还不是 RFC 文档。

• 建议标准 (Proposed Standard) —— 从这个阶段开始就成为 RFC 文档。

• 草案标准 (Draft Standard)

• 因特网标准 (Internet Standard)

2-103

各种 RFC 之间的关系

因特网草案

建议标准

草案标准

因特网标准

历史的 RFC

实验的 RFC 提供信息的 RFC

6 种 RFC

Network Standards Layered Architectures Chapter 2 Updated January 2009 XU Zhengchuan Fudan...

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