SPPP(4)

💖 SPPP(4) 💖

the sppp page, from the Kernel Interfaces Manual — okay bestie, sit down, we're talking about the point to point protocol network layer for synchronous lines ✨

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🌸 NAME 🌸

sppp — the point to point protocol network layer for synchronous lines. that's it, that's the whole vibe 🦋

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✨ SYNOPSIS ✨

device sppp

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🎀 DESCRIPTION 🎀

okay so the sppp network layer is the girl who implements the state machine and the Link Control Protocol (LCP) of the point to point protocol (PPP), exactly as described in RFC 1661. iconic behavior honestly 💅

but bestie, listen — this layer does not provide network interfaces of its own. she's not like that. she is rather intended to be layered on top of drivers providing a synchronous point-to-point connection that wish to run a PPP stack over it. the corresponding network interfaces have to be provided by those hardware drivers. she's the brains, they bring the face card 😌

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👑 the three basic modes of operation 👑

mode 1 — the default (no special flags set): create the PPP connection (administrative Open event to the LCP layer) as soon as the interface is taken up with the ifconfig(8) command. taking the interface down again will terminate the LCP layer and thus all other layers on top. the link will also terminate itself as soon as no Network Control Protocol (NCP) is open anymore, indicating that the lower layers are no longer needed. self-care fr fr 🌸

mode 2 — link0: setting the link-level flag link0 with ifconfig(8) will cause the respective network interface to go into passive mode. this means, the administrative Open event to the LCP layer will be delayed until after the lower layers signals an Up event (rise of “carrier”). lower layers can use this to support a dialin connection where the physical layer is not available immediately at startup, but only after some external event arrives. receipt of a Down event from the lower layer will not take the interface completely down in this case. she's patient, she's chill, she waits for the signal 🫶

mode 3 — link1: setting the flag link1 will cause the interface to operate in dial-on-demand mode. this is also only useful if the lower layer supports the notion of a carrier. upon configuring the respective interface, it will delay the administrative Open event to the LCP layer until either an outbound network packet arrives, or until the lower layer signals an Up event, indicating an inbound connection. as with passive mode, receipt of a Down event (loss of carrier) will not automatically take the interface down, thus it remains available for further connections. no ghosting the interface, she stays available 💀

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🔥 the debug flag 🔥

the sppp layer supports the debug interface flag that can be set with ifconfig(8). if this flag is set, the various control protocol packets being exchanged as well as the option negotiation between both ends of the link will be logged at level LOG_DEBUG. this can be helpful to examine configuration problems during the first attempts to set up a new configuration. very much the "let me read the receipts" energy 🍓

without this flag being set, only the major phase transitions will be logged at level LOG_INFO. so like, the highlight reel only 📸

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💜 the IP address negotiation bit 💜

it is possible to leave the local interface IP address open for negotiation by setting it to 0.0.0.0. this requires that the remote peer can correctly supply a value for it based on the identity of the caller, or on the remote address supplied by this side.

⚠️ but heads up bestie: due to the way the IPCP option negotiation works, this address is being supplied late during the negotiation, which might cause the remote peer to make wrong assumptions. not her fault, that's just the timeline 😭

in a similar spirit the remote address can be set to the magical value 0.0.0.* which means that we do not care what address the remote side will use, as long as it is not 0.0.0.0. this is useful if your ISP has several dial-in servers. you can of course route add something_or_other 0.0.0.* and it will do exactly what you would want it to. manifesting, but with routing tables ✨

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🌈 authentication & compression 🌈

the PAP and CHAP authentication protocols as described in RFC 1334, and RFC 1994 resp., are also implemented. their parameters are being controlled by the spppcontrol(8) utility.

VJ header compression is implemented, and enabled by default. it can be disabled using spppcontrol(8). she comes pre-slay 💅

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💀 DIAGNOSTICS 💀

• <ifname><ifnum>: <proto> illegal <event> in state <statename> — an event happened that should not happen for the current state the respective control protocol is in. see RFC 1661 for a description of the state automaton. wrong place, wrong time, bestie 🙈
• <ifname><ifnum>: loopback — the state automaton detected a line loopback (that is, it was talking with itself). the interface will be temporarily disabled. she was texting herself, it happens 😭
• <ifname><ifnum>: up — the LCP layer is running again, after a line loopback had previously been detected. we love a comeback story 👑
• <ifname><ifnum>: down — the keepalive facility detected the line being unresponsive. keepalive must be explicitly requested by the lower layers in order to take place. no vibes detected, no response 🌧️

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🦋 SEE ALSO 🦋

• inet(4)
• intro(4)
• ifconfig(8)
• spppcontrol(8)

and the required reading, because she cited her sources 💖

• W. Simpson, Editor, The Point-to-Point Protocol (PPP), RFC 1661.
• G. McGregor, The PPP Internet Protocol Control Protocol (IPCP), RFC 1332.
• B. Lloyd and W. Simpson, PPP Authentication Protocols, RFC 1334.
• W. Simpson, PPP Challenge Handshake Authentication Protocol (CHAP), RFC 1994.

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💅 AUTHORS 💅

the original implementation of sppp was written in 1994 at Cronyx Ltd., Moscow by Serge Vakulenko <vak@cronyx.ru>. 🎀

Jörg Wunsch <joerg_wunsch@uriah.heep.sax.de> rewrote a large part in 1997 in order to fully implement the state machine as described in RFC 1661, so it could also be used for dialup lines. he also wrote this man page. author AND documentarian, main character energy fr 👑

Serge later on wrote a basic implementation for PAP and CHAP, which served as the base for the current implementation, done again by Jörg Wunsch. the collaboration arc we deserved 🤝

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🙈 BUGS 🙈

• Many. yes, literally just "Many." she said what she said 💀
• Currently, only the IPCP control protocol and ip(4) network protocol is supported. more NCPs should be implemented, as well as other control protocols for authentication and link quality reporting.
• Negotiation loop avoidance is not fully implemented. if the negotiation does not converge, this can cause an endless loop. not a cute loop, bestie, an endless one 😭
• the various parameters that should be adjustable per RFC 1661 are currently hard-coded into the kernel, and should be made accessible through spppcontrol(8).
• Passive mode has not been tested extensively. so be nice to her 🫶
• Link-level compression protocols should be supported.

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and that's the whole page, bestie 💖

this one shipped with Debian and it's dated May 25, 2008 — so yes, she's vintage, and yes, vintage is a vibe. stan sppp(4). periodt ✨💅🌸