Skip to content
  • Lawrence Stewart's avatar
    This commit marks the first formal contribution of the "Five New TCP Congestion · dbc42409
    Lawrence Stewart authored
    Control Algorithms for FreeBSD" FreeBSD Foundation funded project. More details
    about the project are available at:
    - Add a KPI and supporting infrastructure to allow modular congestion control
      algorithms to be used in the net stack. Algorithms can maintain per-connection
      state if required, and connections maintain their own algorithm pointer, which
      allows different connections to concurrently use different algorithms. The
      TCP_CONGESTION socket option can be used with getsockopt()/setsockopt() to
      programmatically query or change the congestion control algorithm respectively
      from within an application at runtime.
    - Integrate the framework with the TCP stack in as least intrusive a manner as
      possible. Care was also taken to develop the framework in a way that should
      allow integration with other congestion aware transport protocols (e.g. SCTP)
      in the future. The hope is that we will one day be able to share a single set
      of congestion control algorithm modules between all congestion aware transport
    - Introduce a new congestion recovery (TF_CONGRECOVERY) state into the TCP stack
      and use it to decouple the meaning of recovery from a congestion event and
      recovery from packet loss (TF_FASTRECOVERY) a la RFC2581. ECN and delay based
      congestion control protocols don't generally need to recover from packet loss
      and need a different way to note a congestion recovery episode within the
    - Remove the net.inet.tcp.newreno sysctl, which simplifies some portions of code
      and ensures the stack always uses the appropriate mechanisms for recovering
      from packet loss during a congestion recovery episode.
    - Extract the NewReno congestion control algorithm from the TCP stack and
      massage it into module form. NewReno is always built into the kernel and will
      remain the default algorithm for the forseeable future. Implementations of
      additional different algorithms will become available in the near future.
    - Bump __FreeBSD_version to 900025 and note in UPDATING that rebuilding code
      that relies on the size of "struct tcpcb" is required.
    Many thanks go to the Cisco University Research Program Fund at Community
    Foundation Silicon Valley and the FreeBSD Foundation. Their support of our work
    at the Centre for Advanced Internet Architectures, Swinburne University of
    Technology is greatly appreciated.
    In collaboration with:	David Hayes <dahayes at swin edu au> and
    			Grenville Armitage <garmitage at swin edu au>
    Sponsored by:	Cisco URP, FreeBSD Foundation
    Reviewed by:	rpaulo
    Tested by:	David Hayes (and many others over the years)
    MFC after:	3 months