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"""

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    Transport-layer functionality.

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    This implements the core TCP/UDP/SCTP functionality, plus relevant transport/application layer stuff like TLS, SOCKS, SSH-channels, etc.

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    XXX: non-blocking interface?

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    Firstly, this defines a set of abstract interfaces that model the actual implementation as built on top of the

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    socket packages's network-layer implementation:

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    Interfaces:

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        Transport           - very abstract interface that embodies the notion of sending something somewhere

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            ConnectedTransport      - connection-oriented transport

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                Stream                  - connection-oriented full-duplex byte stream (reliable, sequenced)

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                    BufferedStream          - buffered version of the above

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                PacketStream            - connection-oriented full-duplex packet-oriented communication

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            UnconnectedTransport    - non-connection-oriented transport dealing with packets

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                Peer                    - connectionless packet-oriented communication

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        Endpoint        - some transport-level endpoint (local/remote address/socket)

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        Client          - connection-oriented endpoint that can establish a Transport to some remote Service

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        Service         - listening endpoint that recieves connections from remote Clients and represents those as Transports

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    Next, there are some mid-level helpers, mostly for dealing with 

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    Then there is are implementations of these interfaces for specific protocols, layering on top of the socket module,

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    or other application-level protocols.

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    TCP:

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        TCP implementation using a SOCK_STREAM socket.

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        Stream          - connected TCP socket with TCP's byte stream semantics

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        Service         - TCP socket accept()'ing Streams from remote Clients

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        Client          - TCP socket connect()'ing to a remote Service

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    UDP:

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        UDP implementation using a SOCK_DGRAM socket.

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        Peer            - bind()'d UDP socket without any connection

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        Client          - connect()'d UDP socket, representing a PacketStream

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    UNIX:

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        UNIX socket implementation support both SOCK_STREAM and SOCK_DGRAM.

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        XXX: verify that this is correct...

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        Stream          - connected AF_UNIX SOCK_STREAM

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        Service         - accept()'ing AF_UNIX socket of either SOCK_STREAM mode

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        Client          - connect()'ing AF_UNIX socket of SOCK_STREAM mode

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        Peer            - bind()'d AF_UNIX socket of SOCK_DGRAM mode

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    SCTP:

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        SCTP implementation offering both SOCK_SEQPACKET and SOCK_STREAM -based socket interfaces.

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        Stream          - a PacketStream carried by an Association

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        Association     - a Transport that represents a SCTP association between a local and remote Endpoint

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        Peer            - one-to-many style interface, with a single Peer holding multiple Associations

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        Service         - a one-to-one style listen()'ing interface which accepts Associations from remote Endpoints

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        Client          - a one-to-one style connect()'ing interface which establishes Associations with remote Endpoints

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    SSL:

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        SSL implementation using either OpenSSL or gnutls.

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        Stream          - tcp.Stream carrying an encrypted byte stream

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        Service         - tcp.Service accepting connections from SSL endpoints

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        Client          - tcp.Client connecting to an SSL service

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    SOCKS:

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        SOCKSv5 client implementing TCP client serviecs.

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        Stream          - tcp.Stream carried over a SOCKSv5 connections

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        Client          - tcp.Client connecting via a SOCKSv5 server

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    TODO:

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        * implement the above

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"""