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NE40E V800R010C00 Feature Description - WAN Access 01

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
PPP Basic Concepts

PPP Basic Concepts

PPP Architecture

PPP works at the network access layer of the Transmission Control Protocol (TCP)/IP suite for point-to-point (P2P) data transmission over full-duplex synchronous and asynchronous links.

Figure 4-1  Location of PPP in the TCP/IP suite

PPP negotiation involves the following protocols:
  • Link Control Protocol (LCP): used to set up, monitor, and tear down data links.

  • Network Control Protocol (NCP): used to negotiate the formats and types of the data transmitted on data links.

  • (Optional) Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP): used to improve network security.

PPP Packet Format

Figure 4-2 shows the PPP packet format.

Figure 4-2  PPP packet format

A PPP packet contains the following fields:

  • Flag field

    The Flag field identifies the start and end of a physical frame and is always 0x7E.

  • Address field

    The Address field uniquely identifies a peer. PPP is used on P2P links, so two devices communicating using PPP do not need to know the link-layer address of each other. This field must be filled with a broadcast address of all 1s and is of no significance to PPP.

  • Control field

    The Control field value defaults to 0x03, indicating an unsequenced frame. By default, PPP does not use sequence numbers or acknowledgement mechanisms to ensure transmission reliability.

    The Address and Control fields together identify a PPP packet. That is, a PPP packet header is FF03 by default.

  • Protocol field

    The Protocol field identifies the protocol of the data encapsulated in the Information field of a PPP packet.

    The structure of this field complies with the International Organization for Standardization (ISO) 3309 extension mechanism for address fields. All Protocol field values must be odd. The least significant bit of the least significant byte must be "1". The least significant bit of the most significant byte must be "0".

    If a device receives a data packet that does not comply with these rules, the device considers the packet unrecognizable and sends a Protocol-Reject packet padded with the protocol code of the rejected packet to the sender.

    Table 4-1  Common protocol codes

    Protocol Code

    Protocol Type

    0021

    Internet Protocol

    002b

    Novell IPX

    002d

    Van Jacobson Compressed TCP/IP

    002f

    Van Jacobson Uncompressed TCP/IP

    8021

    Internet Protocol Control Protocol

    802b

    Novell IPX Control Protocol

    8031

    Bridging NC

    C021

    Link Control Protocol

    C023

    Password Authentication Protocol

    C223

    Challenge Handshake Authentication Protocol

  • Information field

    The Information field contains the data. The maximum length of the Information field, including the Padding content, is equivalent to the maximum receive unit (MRU) length. The MRU defaults to 1500 bytes and can be negotiated.

    In the Information field, the Padding content is optional. If data is padded, the communicating devices can communicate only when they can identify the padding information as well as the payload to be transmitted.

  • Frame check sequence (FCS) field

    The FCS field checks whether PPP packets contain errors.

    Some mechanisms used to ensure proper data transmission increase the transmission cost and cause delay in data exchange at the application layer.

LCP Packet Format

Figure 4-2 shows the LCP packet format.

Two devices exchange LCP packets to establish a PPP link. An LCP packet is encapsulated into the Information field of a PPP packet as the payload. The value of the Protocol field of a PPP packet is always 0xC021.

During the establishment of a PPP link, the Information field is variable and can contain various LCP packets, which are identified using the Code field.

The following describes the cold field in the Information field:

  • Code field

    The 1–byte-long Code field identifies the LCP packet type.

    If a receiver receives an LCP packet with an unknown Code field from a sender, the receiver sends a Code-Reject packet to the sender.

    Table 4-2  Code field values

    Code Value

    Packet Type

    0x01

    Configure-Request

    0x02

    Configure-Ack

    0x03

    Configure-Nak

    0x04

    Configure-Reject

    0x05

    Terminate-Request

    0x06

    Terminate-Ack

    0x07

    Code-Reject

    0x08

    Protocol-Reject

    0x09

    Echo-Request

    0x0A

    Echo-Reply

    0x0B

    Discard-Request

    0x0C

    Reserved

  • Identifier field

    The Identifier field is 1 byte long. It is used to match requests and replies. If a packet with an invalid Identifier field is received, the packet is discarded.

    The sequence number of a Configure-Request packet usually starts at 0x01 and increases by 1 each time the Configure-Request packet is sent. After a receiver receives a Configure-Request packet, it must send a reply packet with the same sequence number as the received Configure-Request packet.

  • Length field

    The Length field specifies the length of a negotiation packet, including the length of the Code, Identifier, Length, and Data fields.

    The Length field value cannot exceed the MRU of the link. Bytes outside the range of the Length field are treated as padding and are ignored after they are received.

  • Data field

    The Data field contains the contents of a negotiation packet and includes the following fields:
    • Type field: specifies the negotiation option type.

    • Length field: specifies the total length of the Data field.

    • Data field: contains the contents of the negotiation option.

    Table 4-3  Negotiation options in the Type field

    Negotiation Option Value

    Negotiation Packet Type

    0x01

    Maximum-Receive-Unit

    0x02

    Async-Control-Character-Map

    0x03

    Authentication-Protocol

    0x04

    Quality-Protocol

    0x05

    Magic-Number

    0x06

    RESERVED

    0x07

    Protocol-Field-Compression

    0x08

    Address-and-Control-Field-Compression

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Updated: 2018-07-04

Document ID: EDOC1100027168

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