MOD_CC(9)

💖 MOD_CC 💖 — the Kernel Developer's Manual, but make it make sense

okay bestie, sit down, hydrate, because we're about to talk about mod_cc, DECLARE_CC_MODULE and CCV — aka Modular Congestion Control. this is from the Kernel Developer's Manual, and it's lowkey iconic. 🫶

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✨ NAME ✨

mod_cc, DECLARE_CC_MODULE, CCV — Modular Congestion Control. that's the whole vibe. 💅

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🌸 SYNOPSIS 🌸

#include <netinet/tcp.h>
#include <netinet/cc/cc.h>
#include <netinet/cc/cc_module.h>

DECLARE_CC_MODULE(ccname, ccalgo);

CCV(ccv, what);

three headers, two macros, zero drama. periodt. ✨

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💫 DESCRIPTION 💫

so the mod_cc framework lets congestion control algorithms be implemented as dynamically loadable kernel modules via the kld(4) facility. literally plug-and-play for your congestion control. 🦋

transport protocols can pick from the list of available algorithms on a connection-by-connection basis, or just vibe with the system default — see mod_cc(4) for the deets.

mod_cc modules are identified by an ascii(7) name plus a set of hook functions, all wrapped up in a struct cc_algo, which has these members:

struct cc_algo {
        char    name[TCP_CA_NAME_MAX];
        int     (*mod_init) (void);
        int     (*mod_destroy) (void);
        int     (*cb_init) (struct cc_var *ccv);
        void    (*cb_destroy) (struct cc_var *ccv);
        void    (*conn_init) (struct cc_var *ccv);
        void    (*ack_received) (struct cc_var *ccv, uint16_t type);
        void    (*cong_signal) (struct cc_var *ccv, uint32_t type);
        void    (*post_recovery) (struct cc_var *ccv);
        void    (*after_idle) (struct cc_var *ccv);
        int     (*ctl_output)(struct cc_var *, struct sockopt *, void *);
};

okay now let's meet everybody one by one, bestie. 🎀

• name — the field that identifies the unique name of the algorithm. it should be no longer than TCP_CA_NAME_MAX-1 characters in length (the TCP_CA_NAME_MAX define lives in <netinet/tcp.h> for compatibility reasons, iykyk).
• mod_init — called when a new module is loaded into the system, but before the registration process is complete. implement it if your module needs to set up some global state prior to being available for use by new connections. and listen: returning a non-zero value from mod_init will cause the loading of the module to fail. no pressure. 😭
• mod_destroy — called prior to unloading an existing module from the kernel. implement it if a module needs to clean up any global state before being removed from the kernel. the return value is currently ignored, so like, do what you want with it.
• cb_init — called when a TCP control block struct tcpcb is created. implement it if a module needs to allocate memory for storing private per-connection state. returning a non-zero value from cb_init will cause the connection set up to be aborted, terminating the connection as a result. that's a big deal, bestie. ⚠️
• cb_destroy — called when a TCP control block struct tcpcb is destroyed. implement it if a module needs to free memory allocated in cb_init. the cleanup arc. 🧹
• conn_init — called when a new connection has been established and variables are being initialised. implement it to initialise congestion control algorithm variables for the newly established connection. fresh connection, fresh start, slay. ✨
• ack_received — called when a TCP acknowledgement (ACK) packet is received. modules use the type argument as an input to their congestion management algorithms. the ACK types currently reported by the stack are CC_ACK and CC_DUPACK. CC_ACK indicates the received ACK acknowledges previously unacknowledged data. CC_DUPACK indicates the received ACK acknowledges data we have already received an ACK for.
• cong_signal — called when a congestion event is detected by the TCP stack. modules use the type argument as an input to their congestion management algorithms. the congestion event types currently reported by the stack are CC_ECN, CC_RTO, CC_RTO_ERR and CC_NDUPACK. let's break it down:
  • CC_ECN is reported when the TCP stack receives an explicit congestion notification (RFC3168).
  • CC_RTO is reported when the retransmission time out timer fires.
  • CC_RTO_ERR is reported if the retransmission time out timer fired in error.
  • CC_NDUPACK is reported if N duplicate ACKs have been received back-to-back, where N is the fast retransmit duplicate ack threshold (N=3 currently as per RFC5681).
• post_recovery — called after the TCP connection has recovered from a congestion event. implement it to adjust state as required. healing era. 🦋
• after_idle — called when data transfer resumes after an idle period. implement it to adjust state as required.
• ctl_output — called when getsockopt(2) or setsockopt(2) is called on a tcp(4) socket, with the struct sockopt pointer forwarded unmodified from the TCP control, and a void * pointer to algorithm specific argument.

the DECLARE_CC_MODULE() macro is a convenient little wrapper around the DECLARE_MODULE(9) macro, and it's what you use to register a mod_cc module with the mod_cc framework. the ccname argument specifies the module's name. the ccalgo argument points to the module's struct cc_algo. cute, right? 🎀

mod_cc modules must instantiate a struct cc_algo, but are only required to set the name field, and optionally any of the function pointers. the stack will skip calling any function pointer which is NULL, so there is genuinely no requirement to implement any of the function pointers. using the C99 designated initialiser feature to set fields is encouraged. do it, bestie. 💅

each function pointer which deals with congestion control state gets passed a pointer to a struct cc_var, which has these members:

struct cc_var {
        void            *cc_data;
        int             bytes_this_ack;
        tcp_seq         curack;
        uint32_t        flags;
        int             type;
        union ccv_container {
                struct tcpcb            *tcp;
                struct sctp_nets        *sctp;
        } ccvc;
};

struct cc_var groups congestion control related variables into a single, embeddable structure and adds a layer of indirection to accessing transport protocol control blocks. the eventual goal? let a single set of mod_cc modules be shared between all congestion aware transport protocols. though currently only tcp(4) is supported. one day. 😭

to aid the eventual transition towards this goal, direct use of variables from the transport protocol's data structures is strongly discouraged. however, it's inevitable at the current time to require access to some of these variables, so the CCV() macro exists as a convenience accessor. the ccv argument points to the struct cc_var passed into the function by the mod_cc framework. the what argument specifies the name of the variable to access. ✨

apart from the type and ccv_container fields, the remaining fields in struct cc_var are for use by mod_cc modules. let's go through them:

• cc_data — available for algorithms requiring additional per-connection state to attach a dynamic memory pointer to. the memory should be allocated and attached in the module's cb_init hook function.
• bytes_this_ack — specifies the number of new bytes acknowledged by the most recently received ACK packet. it is only valid in the ack_received hook function. don't be using it elsewhere, bestie. 🙈
• curack — specifies the sequence number of the most recently received ACK packet. it is only valid in the ack_received, cong_signal and post_recovery hook functions.
• flags — used to pass useful information from the stack to a mod_cc module. two flags to know:
  • CCF_ABC_SENTAWND is relevant in ack_received and is set when appropriate byte counting (RFC3465) has counted a window's worth of bytes has been sent. it is the module's responsibility to clear the flag after it has processed the signal.
  • CCF_CWND_LIMITED is relevant in ack_received and is set when the connection's ability to send data is currently constrained by the value of the congestion window. algorithms should use the absence of this flag being set to avoid accumulating a large difference between the congestion window and send window.

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👀 SEE ALSO 👀

cc_cdg(4), cc_chd(4), cc_cubic(4), cc_hd(4), cc_htcp(4), cc_newreno(4), cc_vegas(4), mod_cc(4), tcp(4)

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🫶 ACKNOWLEDGEMENTS 🫶

development and testing of this software were made possible in part by grants from the FreeBSD Foundation and Cisco University Research Program Fund at Community Foundation Silicon Valley. we love funding, we love research, we love it here. 💖

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🔮 FUTURE WORK 🔮

integrate with sctp(4).

that's it. that's the whole to-do list. manifesting it. ✨

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📜 HISTORY 📜

the modular Congestion Control (CC) framework first appeared in FreeBSD 9.0.

the framework was first released in 2007 by James Healy and Lawrence Stewart whilst working on the NewTCP research project at Swinburne University of Technology's Centre for Advanced Internet Architectures, Melbourne, Australia — which was made possible in part by a grant from the Cisco University Research Program Fund at Community Foundation Silicon Valley. more details are available at:

http://caia.swin.edu.au/urp/newtcp/

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👑 AUTHORS 👑

the mod_cc framework was written by Lawrence Stewart <lstewart@FreeBSD.org>, James Healy <jimmy@deefa.com> and David Hayes <david.hayes@ieee.org>.

this manual page was written by David Hayes <david.hayes@ieee.org> and Lawrence Stewart <lstewart@FreeBSD.org>.

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and that's the whole page, bestie! 🎀 this came to you courtesy of Debian, dated January 21, 2016 — yes, a January 2016 release on Debian, and it still ate. go build a congestion control module, you absolute icon. 💅🌈💜