openssl/doc/designs/quic-design/glossary.md

Glossary of QUIC Terms
======================

**ACKM:** ACK Manager. Responsible for tracking packets in flight and generating
notifications when they are lost, or when they are successfully delivered.

**Active Stream:** A stream which has data or control frames ready for
transmission. Active stream status is managed by the QSM.

**AEC:** Application error code. An error code provided by a local or remote
application to be signalled as an error code value by QUIC. See QUIC RFCs
(`STOP_SENDING`, `RESET_STREAM`, `CONNECTION_CLOSE`).

**APL:** API Personality Layer. The QUIC API Personality Layer lives in
`quic_impl.c` and implements the libssl API personality (`SSL_read`, etc.) in
terms of an underlying `QUIC_CHANNEL` object.

**Bidi:** Abbreviation of bidirectional, referring to a QUIC bidirectional
stream.

**CC:** Congestion controller. Estimates network channel capacity and imposes
limits on transmissions accordingly.

**CFQ:** Control Frame Queue. Considered part of the FIFM, this implements
the CFQ strategy for frame in flight management. For details, see FIFM
design document.

**Channel:** Core QUIC connection handling object and connection state machine
implementation. This is fused tightly with the RXDP.

**CID:** Connection ID.

**CMPL:** The maximum number of bytes left to serialize another QUIC packet into
the same datagram as one or more previous packets. This is just the MDPL minus
the total size of all previous packets already serialized into to the same
datagram.

**CMPPL:** The maximum number of payload bytes we can put in the payload of
another QUIC packet which is to be coalesced with one or more previous QUIC
packets and placed into the same datagram. Essentially, this is the room we have
left for another packet payload.

**CSM:** Connection state machine. Refers to some aspects of a QUIC channel. Not
implemented as an explicit state machine.

**DCID:** Destination Connection ID. Found in most QUIC packet headers.

**DEMUX:** The demuxer routes incoming packets to the correct connection QRX by
DCID.

**DGRAM:** (UDP) datagram.

**DISPATCH:** Refers to the QUIC-specific dispatch code in `ssl_lib.c`. This
dispatches calls to libssl public APIs to the APL.

**EL:** Encryption level. See RFC 9000.

**FC:** Flow control. Comprises TXFC and RXFC.

**FIFD:** Frame-in-flight dispatcher. Ties together the CFQ and TXPIM to handle
frame tracking and retransmission on loss.

**FIFM:** Frame-in-flight manager. Tracks frames in flight until their
containing packets are deemed delivered or lost, so that frames can be
retransmitted if necessary. Comprises the CFQ, TXPIM and FIFD.

**GCR:** Generic Control Frame Retransmission. A strategy for regenerating lost
frames. Stores raw frame in a queue so that it can be retransmitted if lost. See
FIFM design document for details.

**Key epoch:** Non-negative number designating a generation of QUIC keys used to
encrypt or decrypt packets, starting at 0. This increases by 1 when a QUIC key
update occurs.

**Key Phase:** Key phase bit in QUIC packet headers. See RFC 9000.

**Keyslot**: A set of cryptographic state used to encrypt or decrypt QUIC
packets by a QRX or QTX. Due to the QUIC key update mechanism, multiple keyslots
may be maintained at a given time. See `quic_record_rx.h` for details.

**KP:** See Key Phase.

**KS:** See Keyslot.

**Locally-initiated:** Refers to a QUIC stream which was initiated by the local
application rather than the remote peer.

**MDPL:** Maximum Datagram Payload Length. The maximum number of UDP payload
bytes we can put in a UDP packet. This is derived from the applicable PMTU. This
is also the maximum size of a single QUIC packet if we place only one packet in
a datagram. The MDPL may vary based on both local source IP and destination IP
due to different path MTUs.

**MinDPL:** In some cases we must ensure a datagram has a minimum size of a
certain number of bytes. This does not need to be accomplished with a single
packet, but we may need to add PADDING frames to the final packet added to a
datagram in this case.

**MinPL:** The minimum serialized packet length we are using while serializing a
given packet. May often be 0. Used to meet MinDPL requirements, and thus equal
to MinDPL minus the length of any packets we have already encoded into the
datagram.

**MinPPL:** The minimum number of bytes which must be placed into a packet
payload in order to meet the MinPL minimum size when the packet is encoded.

**MPL:** Maximum Packet Length. The maximum size of a fully encrypted and
serialized QUIC packet in bytes in some given context. Typically equal to the
MDPL and never greater than it.

**MPPL:** The maximum number of plaintext bytes we can put in the payload of a
QUIC packet. This is related to the MDPL by the size of the encoded header and
the size of any AEAD authentication tag which will be attached to the
ciphertext.

**MSMT:** Multi-stream multi-thread. Refers to a type of multi-stream QUIC usage
in which API calls can be made on different threads.

**MSST:** Multi-stream single-thread. Refers to a type of multi-stream QUIC
usage in which API calls must not be made concurrently.

**PN:** Packet number. Most QUIC packet types have a packet number (PN); see RFC
9000.

**PTO:** Probe timeout. See RFC 9000.

**QC:** See `QUIC_CONNECTION`.

**QCSO:** QUIC Connection SSL Object. This is an SSL object created using
`SSL_new` using a QUIC method.

**QCTX**: QUIC Context. This is a utility object defined within the QUIC APL
which helps to unwrap a SSL object pointer (a QCSO or QSSO) into the relevant
structure pointers such as `QUIC_CONNECTION` or `QUIC_XSO`.

**QRL:** QUIC record layer. Refers collectively to the QRX and QTX.

**QRX:** QUIC Record Layer RX. Receives incoming datagrams and decrypts the
packets contained in them. Manages decrypted packets in a queue pending
processing by upper layers.

**QS:** See `QUIC_STREAM`.

**QSM:** QUIC Streams Mapper. Manages internal `QUIC_STREAM` objects and maps
IDs to those objects. Allows iteration of active streams.

**QSO:** QUIC SSL Object. May be a QCSO or a QSSO.

**QSSO:** QUIC Stream SSL Object. This is an SSL object which is subsidiary to a
given QCSO, obtained using (for example) `SSL_new_stream` or
`SSL_accept_stream`.

**QTLS**, **QUIC_TLS**: Implements the QUIC handshake layer using TLS 1.3,
wrapping libssl TLS code to implement the QUIC-specific aspects of QUIC TLS.

**QTX:** QUIC Record Layer TX. Encrypts and sends packets in datagrams.

**QUIC_CONNECTION:** QUIC connection. This is the object representing a QUIC
connection in the APL.

**QUIC_STREAM**: Internal object tracking a QUIC stream. Unlike an XSO this is
not part of the APL. An XSO wraps a QUIC_STREAM once that stream is exposed as
an API object. As such, a `QUIC_CONNECTION` is to a `QUIC_CHANNEL` what a
`QUIC_XSO` is to a `QUIC_STREAM`.

**REGEN:** A strategy for regenerating lost frames. This strategy regenerates
the frame from canonical data sources without having to store a copy of the
frame which was transmitted. See FIFM design document for details.

**Remotely-initiated:** Refers to a QUIC stream which was initiated by the
remote peer, rather than by the local application.

**RSTREAM:** Receive stream. Internal receive buffer management object used to
store data which has been RX'd but not yet read by the application.

**RTT:** Round trip time. Time for a datagram to reach a given peer and a reply
to reach the local machine, assuming the peer responds immediately.

**RXDP:** RX depacketiser. Handles frames in packets which have been decrypted
by a QRX.

**RXE:** RX entry. Structure containing decrypted received packets awaiting
processing. Stored in a queue known as the RXL. These structures belong to a
QRX.

**RXFC:** RX flow control. This determines how much a peer may send to us and
provides indication of when flow control frames increasing a peer's flow control
budget should be generated. Exists in both connection-level and stream-level
instances.

**RXL:** RXE list. See RXE.

**RCMPPL:** The number of bytes left in a packet whose payload we are currently
forming. This is the CMPPL minus any bytes we have already put into the payload.

**SCID:** Source Connection ID. Found in some QUIC packet headers.

**SSTREAM:** Send stream. Internal send buffer management object used to store
data which has been passed to libssl for sending but which has not yet been
transmitted, or not yet been acknowledged.

**STATM:** Statistics manager. Measures estimated connection RTT.

**TA:** Thread assisted mode.

**TPARAM:** Transport parameter.  See RFC 9000.

**TSERVER:** Test server. Internal test server object built around a channel.

**TXE:** TX entry. Structure containing encrypted data pending transmission.
Owned by the QTX.

**TXFC:** TX flow control. This determines how much can be transmitted to the
peer. Exists in both connection-level and stream-level instances.

**TXL:** TXE list. See TXE.

**TXP:** TX packetiser. This is responsible for generating yet-unencrypted
packets and passing them to a QTX for encryption and transmission. It must
decide how to spend the space available in a datagram.

**TXPIM:** Transmitted Packet Information Manager. Stores information about
transmitted packets and the frames contained within them. This information
is needed to facilitate retransmission of frames if the packets they are in
are lost. Note that the ACKM records only receipt or loss of entire packets,
whereas TXPIM tracks information about individual frames in those packets.

**TX/RX v. Send/Receive:** The terms *TX*  and *RX* are used for *network-level*
communication, whereas *send* and *receive* are used for application-level
communication. An application *sends*  on a stream (causing data to be appended
to a *send stream buffer*, and that data is eventually TX'd by the TXP and QTX.)

**Uni:** Abbreviation of unidirectional, referring to a QUIC unidirectional
stream.

**URXE:** Unprocessed RX entry. Structure containing yet-undecrypted received
datagrams pending processing. Stored in a queue known as the URXL.
Ownership of URXEs is shared between DEMUX and QRX.

**URXL:** URXE list. See URXE.

**XSO:** External Stream Object. This is the API object representing a QUIC
stream in the APL. Internally, it is the `QUIC_XSO` structure, externally it is
a `SSL *` (and is a QSSO).