PPP Point-to-Point Frame Utility

PPP Frame Decoder

Decode raw PPP frames and inspect HDLC-like flags, optional Address and Control fields, PPP Protocol field, payload, protocol name and optional CRC-16/X.25 frame check sequence.

✓ IPv4 / IPv6 ✓ LCP ✓ PAP / CHAP ✓ IPCP / IPv6CP ✓ ACFC / PFC ✓ CRC-16 FCS

PPP

PPP Frame Decode

● Ready

PPP Frame Hex Bytes Enter one reconstructed PPP frame. Opening and closing 0x7E flags are optional. Spaces, commas, colons, hyphens, continuous hex and 0x-prefixed bytes are accepted.

Flag Handling

FCS Handling

Address/Control Handling PPP can omit FF 03 when Address-and-Control-Field-Compression is negotiated.

Protocol Field

PPP framing: a common asynchronous PPP frame appears as 7E FF 03 Protocol Information FCS 7E. Address/Control compression can remove FF 03, while Protocol-Field-Compression can reduce certain two-byte protocol values to one byte.

PPP Frame Decode Result Frame Decoded

Decoded PPP Frame —

Address —

Control —

ACFC —

Protocol —

Protocol Name —

Protocol Length —

Payload Length —

Total Input —

FCS Received —

FCS Calculated —

FCS Status —

Flags —

Protocol Decode —

Information / Payload —

Frame Breakdown —

Validation —

What Is a PPP Frame?

PPP, or Point-to-Point Protocol, carries network-layer and control protocols over point-to-point links. On asynchronous links it commonly uses an HDLC-like framing format with 0x7E flags, optional address and control fields, a protocol field, information data and a frame check sequence.

Flag
Address
Control
Protocol
Information
FCS
Flag

PPP Address and Control Fields

The traditional PPP frame contains an Address field of 0xFF and a Control field of 0x03.

Address:
FF

Control:
03

If Address-and-Control-Field-Compression is negotiated, both bytes can be omitted.

PPP Protocol Field

The Protocol field identifies the payload carried by PPP. It normally occupies two bytes, although Protocol-Field-Compression can reduce eligible protocol values to one byte.

Protocol	Value
IPv4	0x0021
IPv6	0x0057
IPCP	0x8021
IPv6CP	0x8057
LCP	0xC021
PAP	0xC023
CHAP	0xC223

PPP LCP

Link Control Protocol uses PPP protocol value 0xC021. LCP establishes, configures, tests and terminates the data-link connection.

LCP Header:

Code
Identifier
Length
Options...

PPP LCP Codes

Code	Meaning
1	Configure-Request
2	Configure-Ack
3	Configure-Nak
4	Configure-Reject
5	Terminate-Request
6	Terminate-Ack
9	Echo-Request
10	Echo-Reply

PPP PAP

Password Authentication Protocol uses protocol value 0xC023. PAP packets include a Code, Identifier and Length, followed by authentication-specific fields.

PPP CHAP

Challenge Handshake Authentication Protocol uses protocol value 0xC223. CHAP exchanges challenge, response, success and failure packets rather than sending the password in the same form as PAP.

PPP Protocol Field Compression

When Protocol-Field-Compression is negotiated, protocol values whose first octet would be zero can be transmitted using only the low-order octet.

Normal IPv4 Protocol:
00 21

Compressed:
21

PPP Frame Check Sequence

Asynchronous PPP commonly uses a 16-bit FCS based on the same reflected CRC-16/X.25 algorithm associated with HDLC framing.

Polynomial:
0x1021

Reflected polynomial:
0x8408

Initial:
0xFFFF

Final XOR:
0xFFFF

PPP Frame Decoder FAQs

What are FF 03 in a PPP frame?

0xFF is the traditional PPP Address field and 0x03 is the traditional Control field. Both may be omitted when Address-and-Control-Field-Compression is active.

What is PPP protocol 0x0021?

0x0021 identifies an IPv4 datagram carried inside PPP.

What is PPP protocol 0x0057?

0x0057 identifies IPv6.

What is PPP protocol 0xC021?

0xC021 identifies the Link Control Protocol, or LCP.

What is PPP protocol 0xC023?

0xC023 identifies Password Authentication Protocol, or PAP.

What does PPP Protocol-Field-Compression do?

It allows certain two-byte protocol fields with a zero first octet to be sent using only their low-order octet.

Does this decoder process escaped PPP bytes?

No. Enter reconstructed PPP octets after asynchronous control-character escaping has already been reversed.