RFC 1979

PPP Deflate Protocol

📅 published August 1996, from ietf

PPP Deflate Protocol 🦋

okay bestie, this one is RFC 1979 and it's giving PPP compression realness 💖 It was written by J. Woods at Proteon, Inc. 🎀 It's filed under the Network Working Group, Request for Comments: 1979, with the category Informational and a date of August 1996 ✨ The header doesn't list anything it obsoletes or updates, so there's no drama there 💅

Status of This Memo 📝

this memo is info for the Internet community, not a spec. It does not define an Internet standard of any kind 🙈 Distribution is unlimited, so share it with whoever you want 🌸

Abstract 🌈

okay so PPP [1] is the protocol that gives you a standard way to shove multi-protocol datagrams across point-to-point links 🧚

The PPP Compression Control Protocol [2] is the part that lets you negotiate and use compression protocols over PPP encapsulated links. It's the matchmaker of the whole operation 💜

this document is all about the PPP Deflate compression protocol and how it compresses PPP encapsulated packets. Let's go 🔥

Table of Contents 🧚

     1.     Introduction ......................................    2
        1.1       Licensing ...................................    2
     2.     PPP Deflate Packets ...............................    3
        2.1       Packet Format ...............................    6
     3.     Configuration Option Format .......................    8
     SECURITY CONSIDERATIONS ..................................    9
     REFERENCES ...............................................    9
     ACKNOWLEDGEMENTS .........................................    9
     CHAIR'S ADDRESS ..........................................   10
     AUTHOR'S ADDRESS .........................................   10

1. Introduction ✨

okay this is the tea ☕ The 'deflate' compression format [3] is the one PKZIP and gzip use, and it's also in the freely and widely distributed zlib [4] library source code. Here's why it's so iconic:

an apparently unencumbered encoding and compression algorithm, with an open and publically-available specification.
low-overhead escape mechanism for incompressible data. The PPP Deflate specification offers options to reduce that overhead further.
heavily used for many years in networks, on modem and other point-to-point links to transfer files for personal computers and workstations.
easily achieves 2:1 compression on the Calgary corpus [5] using less than 64KBytes of memory on both sender and receive.

Honestly it's giving main character energy 👑🫶

1.1. Licensing 📜

ngl the legal part matters, so here's the zlib copyright and license text, copied exactly as written so nothing gets lost in translation 🙈

   The zlib source is widely and freely available, subject to the
   following copyright:

      (C) 1995 Jean-Loup Gailly and Mark Adler

       This software is provided 'as-is', without any express or implied
       warranty.  In no event will the authors be held liable for any
       damages arising from the use of this software.

       Permission is granted to anyone to use this software for any
       purpose, including commercial applications, and to alter it and
       redistribute it freely, subject to the following restrictions:

       1. The origin of this software must not be misrepresented; you
          must not claim that you wrote the original software. If you
          use this software in a product, an acknowledgment in the
          product documentation would be appreciated but is not
          required.

       2. Altered source versions must be plainly marked as such, and
          must not be misrepresented as being the original software.

       3. This notice may not be removed or altered from any source
          distribution.

       Jean-Loup Gailly        Mark Adler
       gzip@prep.ai.mit.edu    madler@alumni.caltech.edu

      If you use the zlib library in a product, we would appreciate
      *not* receiving lengthy legal documents to sign. The sources are
      provided for free but without warranty of any kind.  The library
      has been entirely written by Jean-Loup Gailly and Mark Adler; it
      does not include third-party code.

The deflate format and compression algorithm are based on Lempel-Ziv LZ77 compression. The GNU Project has done extensive research on it, and the Portable Network Graphics working group backs its patent free status 🌸

2. PPP Deflate Packets 🦋

okay bestie, let's talk about how the packets actually work 💅

Before any PPP Deflate packets may be communicated, PPP must reach the Network-Layer Protocol phase, and the CCP Control Protocol must reach the Opened state. no skipping the setup, babe 🙈

Exactly one PPP Deflate datagram is encapsulated in the PPP Information field, where the PPP Protocol field contains 0xFD or 0xFB. 0xFD is used when the PPP multilink protocol is not used or "above" multilink. 0xFB is used "below" multilink, to compress independently on individual links of a multilink bundle 🔥

The maximum length of the PPP Deflate datagram transmitted over a PPP link is the same as the maximum length of the Information field of a PPP encapsulated packet. ✨

Only packets with PPP Protocol numbers in the range 0x0000 to 0x3FFF and neither 0xFD nor 0xFB are compressed. Other PPP packets are always sent uncompressed. Control packets are infrequent and should not be compressed for robustness 🧚

Padding 🎀

PPP Deflate packets require the previous negotiation of the Self-Describing-Padding Configuration Option [6] if padding is added to packets. If no padding is added, then Self-Describing-Padding is not required. so you only need it if you're actually padding stuff 💖

Reliability and Sequencing 🔐

PPP Deflate requires the packets to be delivered in sequence. It relies on Reset-Request and Reset-Ack LCP packets, or on renegotiation of the Compression Control Protocol [2], to indicate loss of synchronization between the transmitter and receiver. The LCP FCS detects corrupted packets and the normal mechanisms discard them. Missing or out of order packets are detected by the sequence number in each packet. The packet sequence number ought to be checked before decoding the packet 😭

Instead of transmitting a Reset-Request packet when detecting a sequence error, the receiver MAY momentarily force CCP to drop out of the Opened state by transmitting a new CCP Configure-Request. This method is more expensive than using Reset-Requests 💀

When the receiver first encounters an unexpected sequence number, it SHOULD send a Reset-Request LCP packet as defined in the Compression Control Protocol. When the transmitter sends the Reset-Ack, or when the receiver receives a Reset-ACK, they must reset the sequence number to zero, clear the compression dictionary, and resume sending and receiving compressed packets. The receiver MUST discard all compressed packets after detecting an error and until it receives a Reset-Ack. This strategy can be thought of as abandoning the transmission of one "file" and starting the transmission of a new "file." fresh start, new file, new era 🌈

The receiver MUST discard all compressed packets after detecting an error and until it receives a Reset-Ack. Don't skip this one, it's the whole point of the reset dance 🫶

The transmitter must clear its compression history and respond with a Reset-Ack each time it receives a Reset-Request, because it cannot know if previous Reset-Acks reached the receiver. The receiver need not do anything to its history when it receives a Reset-Ack, because the transmitter will simply not refer to any prior history ('deflate' is a sliding-window compressor) 🦋

When the link is busy, one decompression error is usually followed by several more before the Reset-Ack can be received. It is undesirable to transmit Reset-Requests more frequently than the round-trip-time of the link, because redundant Reset-Requests cause unnecessary compression dictionary clearing 🙄 The receiver MAY transmit an additional Reset-Request each time it receives a compressed or uncompressed packet until it finally receives a Reset-Ack, but the receiver ought not transmit another Reset-Request until the Reset-Ack for the previous one is late. The receiver MUST transmit enough Reset-Request packets to ensure that the transmitter receives at least one. For example, the receiver might choose to not transmit another Reset-Request until after one second (or, of course, a Reset-Ack has been received and decompression resumed) ⏳

Data Expansion 💥

'Deflate', as used in this standard, expands incompressible data by approximately 14-18 bytes. That's 8 bytes worst-case at the 'deflate' level, plus two further bytes for the 'deflate' end-of-block and the zero-length synchronization block header, plus two bytes of sequence number, plus two bytes to account for adding the PPP Protocol Field to the transmitted data unit 🧮

The BSD Compress draft proposal [7] describes an escape mechanism for incompressible data. It trades off a layering violation for the irritating complications of variable and potentially unpredictable effective MRU lengths. That direct escape mechanism (and much of the text of its description) is used here as well 🌷

If an incompressible data packet does not fit within the MRU of the link, the packet MUST be sent in its original form without CCP encapsulation. PPP packets with significant data expansion that do not exceed the MRU of the link SHOULD be sent in their original form without CCP encapsulation. In both of these cases, the transmitter must increment the sequence number, as future encapsulated packets will depend on the correct reception of some number of unencapsulated packets 🫶

When a PPP packet is received with PPP Protocol numbers in the range 0x0000 to 0x3FFF (except, of course, 0xFD and 0xFB), it is assumed that the packet would have caused expansion. The packet is locally added to the compression history. (Given the definition of the 'deflate' format, a convenient method of doing this is to locally "decompress" a stored-block header of the appropriate length, followed by the actual data block; or the data can simply be appended to the receiver's history, depending on implementation details.) 🧠

Sending incompressible packets in their native encapsulation avoids maximum transmission unit complications. If uncompressed packets could be larger than their native form, then it would be necessary for the upper layers of an implementation to treat the PPP link as if it had a smaller MTU, to ensure that compressed incompressible packets are never larger than the negotiated PPP MTU. so native encapsulation keeps the MTU drama away 🙌

Using native encapsulation for incompressible packets complicates the implementation, not gonna lie 😩 The transmitter and the receiver must start putting information into the compression dictionary starting with the same packets, without relying upon seeing a compressed packet for synchronization. The first few packets after clearing the dictionary are usually incompressible, so they're likely to be sent in their native encapsulation, just like packets before compression is turned on. If CCP or LCP packets are handled separately from Network-Layer packets (e.g. a "daemon" for control packets and "kernel code" for data packets), care must be taken to ensure that the transmitter synchronizes clearing the dictionary with the transmission of the configure-ACK or Reset-Ack that starts compression, and the receiver must similarly ensure that its dictionary is cleared before it processes the next packet. tiny timing details, big consequences 🥹

2.1. Packet Format 🎀

okay so here's the PPP Deflate packet format at a glance, bestie 🦋 the fields get sent left to right, no cap.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         PPP Protocol          |           Sequence            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Data ...
   +-+-+-+-+-+-+-+-+

PPP Protocol

the PPP Protocol field is explained in the Point-to-Point Protocol Encapsulation [1] 💅 when the PPP Deflate compression protocol gets successfully negotiated through the PPP Compression Control Protocol [2], the protocol field is 0xFD or 0xFB. This value MAY be compressed when Protocol-Field-Compression is negotiated. ✨

Sequence

the sequence number gets sent most significant octet first 🧚 it starts at 0 when the dictionary is cleared, and goes up by 1 for each packet, including the uncompressed ones. after 65535 it wraps back to zero, the usual wrap-around, periodt 🌈

ngl this is the whole vibe check for lost or out of order packets 🫶 the sequence number makes sure they don't knock the compression databases of the peers out of sync. when an unexpected sequence number shows up, the dictionaries must be resynchronized with a CCP Reset-Request or Configure-Request. the packet sequence number can be checked before a compressed packet is decoded.

Data

the compressed PPP encapsulated packet follows, made of the Protocol and Data fields from the original, uncompressed packet 💖

Protocol field compression MUST be applied to the protocol field in the original packet before the sequence number is computed or the entire packet is compressed, regardless of whether the PPP protocol field compression has been negotiated. so if the original protocol number was less than 0x100, it must be compressed to a single byte 👑

the basic format of the compressed data is precisely described by the 'Deflate' Compressed Data Format Specification[3]. each transmitted packet must begin at a 'deflate' block boundary, so everything stays synced when incompressible data resets the transmitter's state. to make that happen, each transmitted packet must be terminated with a zero-length 'deflate' non-compressed block (BTYPE of 00). that means the last four bytes of the compressed format must be 0x00 0x00 0xFF 0xFF. These bytes MUST be removed before transmission; the receiver can reinsert them if required by the implementation. 🔥

3. Configuration Option Format ✨

Description

the CCP PPP Deflate Configuration Option is how you negotiate PPP Deflate on the link 🎀 by default, or with ultimate disagreement, no compression is used 🙈

a summary of the PPP Deflate Configuration Option format is below, fields sent left to right 🦋

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |Window | Method|    MBZ    |Chk|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

26 for PPP Deflate 💅

Length

3 🧚

Window

this is the max window size the decompressor is willing to allocate 🌸 it's written as the base-2 logarithm of the LZ77 window size, minus 8. 'Deflate' compliant decompressors must be willing to accept the maximum 32KB window size, which is a value of 7. a 'deflate' compliant compressor is free to use a reduced window size, so a PPP Deflate compressor MUST either honor the restriction requested or reject the option. 💀

Method

the value must be the binary number 1000. it's the 'zlib' Compression Method identifier of '8' for 'deflate' compression with up to 32K window size 🍓

MBZ

must be all 0 bits 🫧

Chk

must be 00 to specify the sequence number check method 💜

Security Considerations

Security issues are not discussed in this memo. 💀 plot twist of the century, but that's literally what it says.

References

[1] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994.

[2] Rand, D., "The PPP Compression Control Protocol (CCP)", RFC 1962, June 1996.

[3] Deutsch, L.P., "'Deflate' Compressed Data Format Specification", draft available in ftp.uu.net:/pub/archiving/zip/doc/deflate-1.1.doc.

[4] Gailly, J.-L., "Zlib 0.95 beta".

[5] Bell, T.C., Cleary, G. G. and Witten, I.H., "Text Compression", Prentice_Hall, Englewood Cliffs NJ, 1990. The compression corpus itself can be found in ftp.uu.net:/pub/archiving/zip/.

[6] Simpson, W., "PPP LCP Extensions", RFC 1570, January 1994.

[7] Schryver, V., "PPP BSD Compression Protocol", RFC 1977, August 1996.

Acknowledgments

William Simpson came in with the super valuable idea of not using any additional header bytes for incompressible packets 🙌🫶

Chair's Address

The working group can be contacted via the current chair:

Karl Fox Ascend Communications 3518 Riverside Drive, Suite 101 Columbus, Ohio 43221

EMail: karl@ascend.com

Author's Address

Questions about this memo can also be directed to:

John Woods Proteon, Inc. 9 Technology Drive Westborough MA 01581-1799

(508) 898-2800 ext. 2651 EMail: jfw@funhouse.com 🧚💖

📦 where this page comes from

ietf: rfc1979 1996-08
💖 girlified by claude-haiku-5-5 on 2026-10-15: a language model's rewrite of the original, under its terms