RFC 1973

PPP in Frame Relay

📅 published June 1996, from ietf

RFC 1973: PPP in Frame Relay 🎀

okay so bestie, this is RFC 1973, and it's called PPP in Frame Relay 💖 It's from the Network Working Group, written by W. Simpson of Daydreamer, dated June 1996. It's on the Standards Track ✨ and it doesn't list any RFCs it obsoletes or updates, so there's no lineage drama here 💅

Status of this Memo

ok so this one is an Internet standards track protocol for the Internet community, and it's asking for discussion and suggestions for improvements 🦋 For the current standardization state and status, you check the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 🌈

Abstract

the Point-to-Point Protocol (PPP) [1] is the standard way to shuttle multi-protocol datagrams over point-to-point links 🚀

this document explains how to use Frame Relay as the framing for PPP encapsulated packets ✨

Applicability

this spec is for implementations that want the PPP goodies, like the Link Control Protocol, Network-layer Control Protocols, authentication, and compression 💜 Those features need a point-to-point relationship between peers. They are not built for multi-point or multi-access environments 🙈

  1.     Introduction ..........................................    1

  2.     Physical Layer Requirements ...........................    1

  3.     The Data Link Layer ...................................    2
     3.1       Frame Format ....................................    2
     3.2       Modification of the Basic Frame .................    3

  4.     In-Band Protocol Demultiplexing .......................    4

  5.     Out-of-Band signaling .................................    5

  6.     Configuration Details .................................    5

  SECURITY CONSIDERATIONS ......................................    7

  REFERENCES ...................................................    7

  ACKNOWLEDGEMENTS .............................................    7

  CHAIR'S ADDRESS ..............................................    8

  AUTHOR'S ADDRESS .............................................    8

1. Introduction ✨

okay so Frame Relay [2] is a relative newcomer to the serial link scene 🌸 Like X.25, it was built to give you virtual circuits between stations on the same Frame Relay network. The glow-up over X.25 is that Q.922 only delivers packets. It skips sequencing and flow control, which makes the whole service way simpler 💅

PPP uses ISO 3309 HDLC as the basis for its framing [3] 🧚

when Frame Relay is set up as a point-to-point circuit, PPP can use it as a framing mechanism and just ignore its other features 🎀 This is the same idea used to carry SNAP headers over Frame Relay [4], so it's not a new vibe.

at one point people hoped PPP in HDLC-like frames and Frame Relay could coexist on the same links 😭 Sadly, Q.922's method for growing the address from 1 to 2 to 4 octets doesn't look like the ISO 3309 method, because of how its Data Link Connection Identifier (DLCI) subfields are structured. So coexistence is off the table 💀

2. Physical Layer Requirements 🦋

PPP treats Frame Relay framing like a bit-synchronous link. The link MUST be full-duplex, but it MAY be either dedicated (permanent) or switched 🌈

Interface Format

PPP shows up to the physical layer as an octet interface 💖 There's no way for sub-octets to be supplied or accepted, so don't even try 🙅

Transmission Rate

PPP doesn't put any limits on transmission rate of its own. Whatever the particular Frame Relay interface can do is the only cap 🚀

Control Signals

Frame Relay needs control signals that tell you when the link has connected or disconnected. Those signals feed the Up and Down events into the LCP state machine ✨

because PPP doesn't normally need control signals, the failure of those signals MUST NOT affect correct operation of PPP 🫶 The implications are covered in [2].

Encoding

the encodings themselves are the job of the DTE/DCE equipment in use. That's outside the scope of this spec 🍓

PPP works no matter how the bit stream is represented underneath, but Frame Relay requires NRZ encoding 💜

3. The Data Link Layer 🌸

this spec uses the principles, terminology, and frame structure from "Multiprotocol Interconnect over Frame Relay" [4] ✨

the point isn't to re-document what's already standardized in [4] 🙄 Instead, this doc gives a concise summary and calls out the specific options and features PPP uses 💅

3.1. Frame Format 🎀

as described in [4], the Q.922 header address and control fields get combined with the Network Layer Protocol Identifier (NLPID), which tells you what encapsulation follows. Everything is 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
+-+-+-+-+-+-+-+-+
|  Flag (0x7e)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Q.922 Address         |    Control    |  NLPID(0xcf)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         PPP Protocol          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

the PPP Protocol field and the Information and Padding fields after it are described in the Point-to-Point Protocol Encapsulation [1] 🦋

3.2. Modification of the Basic Frame 💖

the Link Control Protocol can negotiate changes to the basic frame structure 🧚 Modified frames will always be clearly different from standard frames, so you can tell them apart 💅

Address-and-Control-Field-Compression

the Address and Control field values aren't constant. They get modified as the frame moves through the network switching fabric 😭 Because of that, Address-and-Control-Field-Compression MUST NOT be negotiated. Period 🙅

Protocol-Field-Compression

heads up: unlike PPP in HDLC-like framing, Frame Relay framing does not line up the Information field on a 32-bit boundary 👀 Alignment to 32 bits only happens when the NLPID is removed and the Protocol field gets squished down to a single octet. When that boost actually improves throughput, Protocol-Field-Compression SHOULD be negotiated 🚀

References ✨

Okay bestie, here's where the receipts live 📚💅 Every source this spec leans on is listed below, with all the details.

[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. 🦋
[2] CCITT Recommendation Q.922, "ISDN Data Link Layer Specification for Frame Mode Bearer Services", International Telegraph and Telephone Consultative Committee, 1992. 🌸
[3] Simpson, W., Editor, "PPP in HDLC-like Framing", STD 51, RFC 1662, July 1994. 💜
[4] Bradley, T., Brown, C., and A. Malis, "Multiprotocol Interconnect over Frame Relay", RFC 1490, July 1993. 🧚
[5] ISO/IEC TR 9577:1990(E), "Information technology - Telecommunications and Information exchange between systems - Protocol Identification in the network layer", 1990-10-15. 🎀

Acknowledgments 🫶

Shoutout to the design's inspo, the paper "Parameter Negotiation for the Multiprotocol Interconnect" by Keith Sklower and Clifford Frost from the University of California, Berkeley. It's from 1992 and it's unpublished, but it's giving main character energy for the whole idea 🌈✨

Chair's Address 👑

If you want to reach the working group, the current chair is the move:

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

EMail: karl@ascend.com

Author's Address 💖

Got questions about this memo? Slide into the author's inbox 🙈 Here's where William Allen Simpson can be found:

William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan  48071

    wsimpson@UMich.edu
    wsimpson@GreenDragon.com (preferred)

📦 where this page comes from

ietf: rfc1973 1996-06
💖 girlified by claude-haiku-5-5 on 2026-10-17: a language model's rewrite of the original, under its terms