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

2. 1. Message Standards (Protocols)

3. 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

4. 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

5. 2-5 Figure 2-2: Hypertext Transfer Protocol (HTTP) Interactions Client PC Webserver BrowserWebserver Application 1. HTTP Request Message Asking for a File 2. HTTP Response Message delivering the File or giving an error message Semantics in HTTP, which governs the Web

6. 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

7. 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 is very rigid

8. 2-8 Test Your Understanding P 100

9. 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

10. 2-10 Figure 2-1: Standards Govern the Exchange of Messages, Continued General Message Syntax (Organization) –Header and trailer are further divided into fields TrailerData FieldHeader Other Header Field Destination Address Field is Used by Switches and Routers Like the Address on an Envelope Message with all three parts

11. 2-11 Figure 2-4: General Message Organization, Continued Data FieldHeader Other Header Field Destination Address Field Message without a trailer Usually only data link layer messages have trailers

12. 2-12 Figure 2-4: General Message Organization, Continued Header Other Header Field Destination Address Field Message with only a header e.g. TCP supervisory messages are pure headers (there is no data field content to deliver)

13. 2-13 Test Your Understanding P 100

14. 2. Reliability

15. 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

16. 2-16 TCP Segment (Message) 4 Carries an HTTP Request Segment 5 Acknowledges It There Is No Need to Resend Figure 2-5: Reliable TCP Session, Continued Client PC TCP Process Webserver TCP Process 4. Data = HTTP Request 5. ACK (4) 6. Data = HTTP Response 7. ACK (6) Carry HTTP Req & Resp (4) Request-Response Cycle for Data Transfer

17. 2-17 Figure 2-5: A TCP Session, Continued Client PC TCP Process Webserver TCP Process Carry HTTP Req & 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) 8 Is Lost in Transmission There Is No Acknowledgment So the Sender Retransmits It

18. 2-18 Test Your Understanding P 102

19. 3. Connection-Oriented and Connectionless Protocols

20. 2-20 Figure 2-6: Connection-Oriented and Connectionless Protocols Message (No Sequence Number) Connectionless Protocol AB Message 1 (Seq. Num = A1) Message 2 (Seq. Num = A2) Close Connection Connection-Oriented Protocol Open Connection AB Message 3 (Seq. Num B1) Connection-oriented protocols have Formal openings and closings like Telephone calls Also have sequence numbers so that the receiver can put messages in order And so the receiver can send Acknowledgments for specific messages

21. 2-21 Figure 2-6: Connection-Oriented and Connectionless Protocols, Continued Client PC Browser Webserver Application HTTP Request HTTP is connectionless No Openings No Closings No Sequence Numbers No Acknowledgments

22. 2-22 Figure 2-6: Connection-Oriented and Connectionless Protocols, Continued Client PC TCP Process Webserver TCP Process Connection-Opening Messages Time Connection-Closing Messages Messages During the Connection In TCP

23. 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.

24. 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

25. 2-25 Test Your Understanding P 105

26. 4. The Hybrid TCP/IP-OSI Standards Architecture

27. 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

28. 2-28 Figure 2-8: Hybrid TCP/IP-OSI Architecture General Purpose (Core Later) LayerSpecific 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

29. 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

30. 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 Link A-X1 Host A Switch X2 Physical Link X1-X2

31. 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 X1 Host A Switch X2 Host B Data Link A-B Frame

32. 2-32 Figure 2-9: Physical and Data Link Layer Standards in a Single Network, Continued Data Link Layer –Data link layer standards also govern Frame organization Switch operation

33. 2-33 Figure 2-9: Physical and Data Link Layer Standards in a Single Network, Continued Host A Mobile Client Station Server Station Switch X2 Switch X1 Switch Data Link A-R1 Physical Link A-X1 Physical Link X1-X2 Router R1 Physical Link X2-R1 3 Physical Links 1 Data Link 2 Switches

34. 2-34 Test Your Understanding P 107

35. 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

36. 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 1Router 2 Packet

37. 2-37 Figure 2-10: Internet and Data Link Layers in an Internet, Continued Host B Host A Network X Network Y Network Z R1 R2 Data Link A-R1 Data Link R3-B Data Link R1-R2 Route A-B 3 Data Links: One per Network 1 Route per Internet

38. 2-38 Figure 2-10: Internet and Data Link Layers in an Internet, Continued Host A Mobile Client Station Server Station Switch X2 Switch X1 Switch Data Link A-R1 Router R1 Packet Frame X Network X Route A-B In Network X: Two Destination Addresses: Packet: Host B (Destination Host) Frame: Router R1

39. 2-39 Figure 2-10: Internet and Data Link Layers in an Internet, Continued Router R1 Router R2 Packet Frame Y To Network X To Network Z Network Y Data Link R1-R2 Route A-B In Network Y: Two Destination Addresses: Packet: Host B (Destination Host) Frame: Router R2

40. 2-40 Figure 2-10: Internet and Data Link Layers in an Internet, Continued Host B Mobile Client Stations Switch Z1 Switch X2 Switch Z2 Packet Frame Z Network Z Router R2 Router Data Link R2-B In Network Z: Two Destination Addresses: Packet: Host B (Destination Host) Frame: Host B

41. 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

42. 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

43. 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 1Router 2 Router 3 Client PC Server

44. 2-44 Test Your Understanding P 110

45. 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 BrowserWebserver Application Client PC

46. 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.

47. 2-47 Test Your Understanding P 111

48. 2-48 Standards Layers: Recap Application (5) Transport (4) Internet (3) Data Link (2) Physical (1) Be able to repeat this in your sleep! Be able to repeat this in your sleep!

49. 2-49 Why Layered Architectures? Page 111

50. 5. Syntax Examples for Some Layer Messages

51. 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 Bits 10010111

52. 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.

53. 2-53 Figure 2-14: Ethernet Frame, Continued Data Field (variable length) PAD (added if data field < 46 octets) Frame Check Sequence (32 bits) LLC Subheader (usually 8 octets) Usually IP Packet Encapsulated Packet 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 long PAD length is set to keep the data field plus PAD 46 octets Data Field

54. 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)

55. 2-55 Test Your Understanding P 115 12-15

56. 2-56 Figure 2-15: Internet Protocol (IP) Packet, Continued Total Length (16 bits) Version (4 bits) Diff-Serv (8 bits) Header Length (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 0Bit 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

57. 2-57 Figure 2-15: Internet Protocol (IP) Packet Total LengthVersionDiff-Serv Header Length Source IP Address (32 bits) IdentificationFlagsFragment Offset Header ChecksumProtocolTime to Live Bit 0Bit 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

58. 2-58 Test Your Understanding P 116-117, 16-18

59. 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

60. 2-60 Figure 2-16: TCP and UDP at the Transport Layer, Continued ProtocolTCPUDP LayerTransport Connection-Oriented?YesNo Reliable?YesNo Burden on the two hostsHighLow Burden on the networkHighLow

61. 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.

62. 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

63. 2-63 Test Your Understanding P 119-121, 19-22

64. 6. Vertical Communication Between Layer Processes on the Same Host

65. 2-65 Figure 2-18: Layered Communication on the Source Host Application Process HTTP Message Transport Process HTTP Message TCP Hdr Encapsulation of HTTP Message in Data Field of TCP Segment Passes Message Down to Transport Process The process begins when a browser creates an HTTP request message

66. 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

67. 2-67 Figure 2-18: Layered Communication on the Source Host, Continued Transport Process HTTP Message Internet Process HTTP Message TCP Hdr TCP Hdr IP Hdr Encapsulation of TCP Segment in Data Field of IP Packet

68. 2-68 Figure 2-18: Layered Communication on the Source Host, Continued Internet Process HTTP Message TCP Hdr IP Hdr Data Link Process HTTP Message TCP Hdr IP Hdr Eth Hdr Eth Trlr Encapsulation of IP Packet in Data Field of Ethernet Frame

69. 2-69 Figure 2-18: Layered Communication on the Source Host, Continued Data Link Process HTTP Message TCP Hdr IP Hdr Eth Hdr Eth Trlr Physical Process Physical Layer converts the bits of the frame into signals.

70. 2-70 Figure 2-18: Layered Communication on the Source Host, Continued The following is the final frame for a an HTTP message on an Ethernet LAN HTTP Message TCP Hdr IP Hdr Eth Hdr Eth Trlr L5L4L3L2 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

71. 2-71 Test Your Understanding P 124 23

72. 2-72 Figure 2-19: Decapsulation on the Destination Host HTTP Message TCP Hdr IP Hdr Eth Hdr Eth Trlr Data Link Process Physical Process

73. 2-73 Figure 2-19: Decapsulation on the Destination Host, Continued HTTP Message TCP Hdr IP Hdr Eth Hdr Eth Trlr Data Link Process Internet Process HTTP Message TCP Hdr IP Hdr Decapsulation of IP Packet from Data Field of Ethernet Frame

74. 2-74 Figure 2-19: Decapsulation on the Destination Host, Continued Internet Process HTTP Message TCP Hdr IP Hdr Transport Process HTTP Message TCP Hdr Decapsulation of TCP Segment from Data Field of IP Packet

75. 2-75 Figure 2-19: Decapsulation on the Destination Host, Continued Transport Process HTTP Message TCP Hdr Application Process HTTP Message Decapsulation of HTTP Message from Data Field of TCP Segment

76. 2-76 Test Your Understanding P 126 24

77. 2-77 Figure 2-20: Layered End-to-End Communication Int App DL Trans Phy Source Host Destination Host Switch 1 Switch 2 Router 1 Switch 3 Router 2 Source and Destination Hosts Have 5 Layers Switches Have Two Layers --- Each Switch Port Has One Layer (1) Routers Have Three Layers --- Each Router Port Has Two Layers (1&2)

78. 2-78 Figure 2-21: Combining Horizontal and Vertical Communication Int App DL Trans Phy Source Host Destination Host Switch 1 Switch 2 Router 1 Switch 3 Router 2 Hypertext Transfer Protocol Transmission Control Protocol Internet Protocol

79. 2-79 Test Your Understanding P 127 25-26

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

81. 2-81 Figure 2-22: The Hybrid TCP/IP-OSI Architecture TCP/IPOSIHybrid TCP/IP-OSIBroad Purpose Application Presentation Session Application (Layer 5) Communication between applications 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) Transmission within a single LAN or WAN

82. 2-82 Figure 2-23: OSI and TCP/IP OSITCP/IP Standards Agency or Agencies ISO (International Organization for Standardization) ITU-T (International Telecommunications Union— Telecommunications Standards Sector) IETF (Internet Engineering Task Force)

83. 2-83 Figure 2-23: OSI and TCP/IP, Continued OSITCP/IP DominanceNearly 100% dominant at physical and data link layers 70%-80% dominant at the internet and transport layers. Documents are Called VariousMostly RFCs (requests for comments)

84. 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

85. 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

86. 2-86 Figure 2-24: OSI Layers, Continued 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

87. 2-87 Figure 2-24: OSI Layers, Continued 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

88. 2-88 Figure 2-24: OSI Layers, Continued 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

89. 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

90. 2-90 Figure 2-26: Characteristics of Protocols Discussed in the Chapter LayerProtocolConnection- Oriented /Connectionless Reliable/ Unreliable 5 (App)HTTPConnectionlessUnreliable 4 (Transport)TCP Connection- oriented Reliable 3 (Internet)IPConnectionlessUnreliable 2 (Data Link)EthernetConnectionlessUnreliable Note: Only TCP is connection-oriented and reliable 4 (Transport)UDPConnectionlessUnreliable

91. 2-91 Case study Chp 1 a page 42 Chp 1c page 77

92. 8. Topics Covered

93. 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

94. 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

95. 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)

96. 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

97. 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

98. 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

99. 2-99 Topics Covered OSI Architecture –Divides application layer into three layers Session Presentation Application Other Standards Architectures –IPX/SPX –SNA –AppleTalk

100. 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

101. 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 因特网体系结构 研究委员会 IAB WG

102. 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)

103. 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