Xpra Documentation

Xpra Protocol

Status of This Document

This document specifies the Xpra application protocol implemented by the master branch on 16 August 2026 (Xpra 6.6 development series). It is a normative implementer specification for the modern protocol only.

Historical packet names, encoders, compressors, capability aliases and packet layouts accepted when XPRA_BACKWARDS_COMPATIBLE=1 are deliberately excluded. An implementation conforming to this document MUST NOT depend on them.

The key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT and MAY are to be interpreted as described by RFC 2119 and RFC 8174.

1. Scope

This specification covers:

• the Xpra record header, packet encoding, compression and raw chunks;
• connection establishment, capability negotiation and authentication;
• normal interactive sessions and one-shot request connections;
• control, encoder, proxy registration and listener flows;
• the application packet registry used by the current client and server.

It begins after the selected transport has delivered an ordered byte stream. TCP, Unix-domain sockets, TLS, SSH, WebSocket and QUIC can provide that stream. Transport handshakes, HTTP/WebSocket framing, SSH, QUIC, RFB and HTTP services are outside this specification.

Xpra’s AES packet extension is also outside this specification. A peer MAY recognize protocol flag 0x02 as an extension marker, but a conforming implementation need not implement it and MUST NOT advertise it merely because it implements this RFC. Deployments SHOULD use TLS or SSH for confidentiality and peer authentication.

2. Terminology and Data Model

client is the endpoint initiating an Xpra session connection. server is the endpoint accepting it. Proxies act as a server on one connection and a client on another.

Packets are written in this document as:

[packet-type, argument-1, argument-2, ...]

The packet type MUST be a Unicode string. Packet arguments use these types:

Notation	Meaning
bool	Boolean, distinct from an integer in packet schemas
u8, u16, u32, u64	Non-negative integer fitting that width
i8, i16, i32, i64	Signed integer fitting that width
str	Unicode string
bytes	Arbitrary byte string
list<T>	Ordered sequence of T; a tuple is encoded as a list
map	String-keyed dictionary unless stated otherwise
T?	Optional trailing value of type T
T...	Zero or more trailing values of type T

Receivers MUST validate argument count, type, integer range and identifiers before using a packet. Unknown keys in an extensible map MUST be ignored unless the map’s definition says otherwise. Unknown packet types MUST be rejected or cause a clean connection close; they MUST NOT be interpreted as another type.

3. Record Layer

3.1 Header

Every Xpra record consists of an eight-byte header followed immediately by the number of payload bytes declared by that header.

Offset	Size	Field	Encoding
0	1	magic	ASCII P (0x50)
1	1	protocol flags	bit field, Section 3.2
2	1	compression	bit field, Section 3.3
3	1	chunk index	unsigned integer, Section 3.5
4	4	payload length	unsigned 32-bit, network byte order

The payload length does not include the header. A receiver MUST reject an incorrect magic byte, an invalid flag combination or an unacceptable payload length before allocating or reading the declared payload.

3.2 Protocol Flags

Value	Name	Meaning
0x04	YAML	main payload is YAML
0x08	flush	no packet is queued immediately after this record
0x10	rencodeplus	main payload is rencodeplus
0x02	cipher extension	outside this RFC

A main record (chunk index zero) MUST set exactly one of 0x04 and 0x10. Values 0x00 (bencode) and 0x01 (rencode) are not valid modern encoders. All unassigned bits MUST be zero.

A raw chunk MUST set neither encoder flag and MUST NOT set flush. The flush flag is only a scheduling hint; it does not change packet semantics.

3.3 Compression Byte

Zero means no record-layer compression. A non-zero byte has a four-bit compression level in its low nibble and exactly one compressor in its high nibble:

High bit	Compressor
0x10	LZ4 block
0x40	Brotli
0x80	Zstandard

For a compressed record, the level MUST be in 1..15 and exactly one listed compressor bit MUST be set. A level without a compressor, multiple compressor bits, or an unassigned bit is invalid. In particular, the former implicit zlib form is invalid.

Compression applies independently to each record, including raw chunks. After decompression the result is either an encoded main packet or the bytes of one raw chunk.

An LZ4 payload starts with the uncompressed size as an unsigned 32-bit little-endian integer, followed by one raw LZ4 block. Brotli and Zstandard payloads are ordinary frames for those formats.

Packet compression is independent of image, video and audio codecs carried inside packet arguments.

3.4 Limits

Before the peer’s hello is accepted, an implementation MUST impose a maximum declared payload length of 4 MiB (4 * 1024 * 1024). This limit applies before allocation and before reading the payload.

After capability processing, the normal per-record limit is 16 MiB. A subsystem MAY increase it when a negotiated operation requires a larger record. The default absolute declared-payload limit is 256 MiB; display negotiation MAY replace it with a dimension-derived bound large enough for the negotiated pixel area. No decompressed record may exceed 256 MiB. Implementations MAY configure smaller limits and reject the associated operation cleanly.

Senders normally avoid record compression below 378 bytes. This is an optimization, not a receiver requirement.

3.5 Raw Chunks

Raw chunks move large byte arguments without copying them through the packet encoder. Their chunk index is the zero-based position of the argument they replace in the decoded main packet.

• Index zero is reserved for the main packet.
• A raw chunk index MUST be in 1..15.
• At most three raw chunks MAY precede one main packet.
• Each raw chunk MUST precede its main packet.
• A main packet placeholder MUST exist at every received chunk index.
• A receiver MUST reject an index outside the decoded packet rather than grow or otherwise mutate the packet shape.
• On receipt of the main packet, each placeholder is replaced by the decompressed bytes of the matching chunk, then the complete packet is dispatched.

If the receiver advertises chunks=false, the sender MUST inline all arguments in the main packet.

4. Packet Encoders

4.1 Requirements

Servers conforming to this RFC MUST implement rencodeplus and LZ4. Clients MUST implement rencodeplus and SHOULD implement LZ4. YAML is OPTIONAL but RECOMMENDED. Zstandard and Brotli are OPTIONAL.

The initial hello MUST use rencodeplus because negotiation has not occurred. It MAY use LZ4, which is the required server-side compressor. Thereafter each sender chooses an encoder and compressor offered by the receiver, independently for each direction. Rencodeplus and LZ4 are preferred.

4.2 Rencodeplus

Rencodeplus is a self-delimiting binary representation. Multi-byte numeric values use network byte order. Its type codes are:

Code	Value
0..43	integer equal to the code
44	IEEE-754 binary64, 8 bytes
59	variable list, values followed by 127
60	variable dictionary, key/value pairs followed by 127
61	base-10 integer bytes followed by 127
62	signed 8-bit integer
63	signed 16-bit integer
64	signed 32-bit integer
65	signed 64-bit integer
66	IEEE-754 binary32, 4 bytes
67, 68, 69	true, false, null respectively
70..101	integers -1..-32 respectively
102..126	dictionaries containing 0..24 pairs
128..191	UTF-8 strings containing 0..63 bytes
192..255	lists containing 0..63 values

A longer UTF-8 string is encoded as ASCII decimal byte length, :, then that many bytes. A byte string is encoded as ASCII decimal byte length, /, then that many bytes. Lengths count bytes, not Unicode characters.

The packet encoder can represent null and floating-point values, but packet schemas in this RFC do not use null and SHOULD avoid floats. Senders MUST use a byte string, not a Unicode string, for opaque data.

4.3 YAML

A YAML main payload MUST decode to the same packet list data model. YAML tags or object construction outside strings, integers, booleans, lists, dictionaries and binary data MUST NOT be used. Receivers MUST use a safe loader.

5. Connection Establishment

5.1 State Machine

The ordinary sequence is:

transport established
  -> client hello
  -> [server challenge -> client challenge response]*
  -> server hello
  -> established application traffic
  -> connection-close or transport EOF

The client sends ["hello", capabilities:map]. The server either requests authentication, accepts the connection with its own hello, answers a one-shot request, offers an SSL upgrade, or closes the connection.

Except for hello, challenge, ssl-upgrade and connection-close, ordinary application packets MUST NOT be sent until the peer’s hello has been accepted. The established connection remains asymmetric: advertised receive capabilities govern what the other endpoint may send.

5.2 Core Capabilities

Key	Type	Semantics
version	str	Xpra version; REQUIRED in a normal hello
encoders	list<str>	accepted packet encoders; includes rencodeplus
compressors	list<str>	accepted record compressors
compression_level	u8	requested level; zero disables compression
chunks	bool	whether raw chunks can be reassembled; default true
packet-types	list<str>	packet types accepted by the sender of this hello
request	str	one-shot or special profile, Section 5.4
wants	list<str>	optional server capability groups requested
username	str	authentication identity
digest	list<str>	challenge digest algorithms accepted by client
hostname, uuid, machine_id	str	endpoint identity/advisory metadata
build, platform, network	map	extensible diagnostic metadata

packet-types is an allow-list when present. A sender SHOULD NOT send an optional packet absent from the peer’s list. Mandatory handshake packets remain valid even when omitted from that list.

5.3 Authentication and Upgrade Packets

Direction	Packet	Arguments
C -> S	hello	capabilities:map
S -> C	hello	capabilities:map
S -> C	challenge	salt:bytes, auth_caps:map, digest:str, salt_digest:str, prompt:str?
C -> S	hello	original capabilities plus challenge response fields
S -> C	ssl-upgrade	attributes:map
either	connection-close	reasons:list<str>

Challenge algorithms and transport TLS configuration are specified in the authentication and TLS documents.

5.4 Connection Profiles

A request capability selects a profile. Unless stated otherwise, the server answers with hello and closes after delivering the result.

Request	Additional client capability	Result
absent	session capabilities	normal interactive attach
connect_test	connect_test_request:str	echo in connect_test_response
version	full-version-request:bool?	version hello
id	none	server identity hello
info	subsystems:list<str>?	information hello or info-response
screenshot	none	display-screenshot
icon	none	display-icon
command	command_request:list<str>	control-command response hello
run	run:list<str>	process launch result in run_response
print	print:list	submit file data as a print job
encode	encoding:map	stateless encoder service
detach	none	disconnect attached clients as authorized
exit	none	terminate the server process as authorized
stop	none	stop the session as authorized
register	registration fields	register a session with a proxy registry

Authorization is profile-specific. A server MUST NOT grant a request merely because its name is recognized. Unknown requests MUST be rejected.

The encoder service is selected by connecting to an encoder server and uses the same handshake followed by the encode and context-* packets in Section 8.11. A listener or proxy may use register with uuid, session identity, display addresses and connection options, and MAY hand the authenticated connection to the selected session. Handover is an implementation action; the Xpra record stream continues unchanged.

6. Capability Namespaces

Capabilities describe what the sender can receive or what local service it is offering. Feature maps are extensible: omitted keys mean unsupported or their documented default.

Namespace/key	Interoperability fields
audio	send:bool, receive:bool, encoders:list<str>, decoders:list<str>, codec properties
bandwidth	limit:u64, detection:bool
clipboard	enabled, direction, selections, greedy, want_targets, preferred-targets, notification
command	start, start-child, accepted signals and commands
cursors / cursor	enabled state, encodings, size, default and maximum sizes
display keys	desktop_size, screen_sizes, monitors, dpi, screen-scaling, vrefresh
encoding	accepted picture encodings, RGB formats, quality/speed/scaling and codec profiles
file	enabled, ask, size-limit, chunks, open, open-url, ask-timeout
keyboard / keymap	enabled state, modifiers, repeat timing and keymap definition
mmap	read/write area descriptors including file, size and token validation
notifications	enabled state and supported actions
pointer	synchronization, initial position and double-click parameters
webcam	enabled state and accepted image encodings
window	metadata supported, restacking, focus and position synchronization

Pixel formats, codec option dictionaries, monitor records, keymaps, clipboard targets and memory-map layouts are defined in the matching subsystem documents. Diagnostic version keys and unrecognized feature properties MUST be treated as advisory and ignored.

7. Core and Session Packets

The tables below are the modern packet registry. Integer widths are receiver validation limits, not a separate on-wire integer representation.

7.1 Core, Status and Control

Direction	Packet	Arguments
either	ping	monotonic_ms:u64, sid:str?
either	ping-echo	monotonic_ms:u64, load1:u64, load5:u64, load15:u64, latency_ms:i64, sid:str?
S -> C	startup-complete	none
either	setting-change	name:str, value:any
S -> C	server-event	name:str, values:any...
S -> C	control	command:str, arguments:any...
C -> S	control-request	request_id:u64, command:str, arguments:any...
S -> C	info-response	information:map
C -> S	info-request	window_ids:list<u64>, categories:list<str>, subsystems:list<str>?
C -> S	shutdown-server	exit:bool, reason:str?
C -> S	suspend	suspended:bool
C -> S	bell-set	enabled:bool

Ping timestamps use the sender’s monotonic clock and are correlation values, not wall-clock time. Load values are scaled by 1000. A negative latency means unknown.

setting-change is bidirectional, but asymmetric. A server MAY send any setting. A server MUST apply an allow-list to the settings a client is permitted to change, and MUST ignore any setting outside it. That allow-list holds readonly:bool, and - on servers managing an X11 display - xsettings:map, which carries the client’s xsettings-blob and resource-manager values for the server to apply to its own display.

7.2 Logging, Shell and Commands

Direction	Packet	Arguments
either	logging-event	level:u8, message:bytes|list<str>, time_ms:u64
C -> S	logging-control	action:str (start or stop)
C -> S	shell-exec	code:str
S -> C	shell-reply	returncode:u8, output:str
C -> S	command-start	name:str, command:list<str>, ignore:bool, sharing:bool
C -> S	command-signal	pid:u32, signal:str

7.3 Sharing and Bandwidth

Direction	Packet	Arguments
C -> S	sharing-toggle	enabled:bool
C -> S	sharing-lock	locked:bool
C -> S	bandwidth-limit	bits_per_second:u64
C -> S	bandwidth-status	attributes:map

8. Subsystem Packets

8.1 Display

Direction	Packet	Arguments
C -> S	display-configure	configuration:map
C -> S	display-request-screenshot	none
C -> S	display-request-icon	none
S -> C	display-screenshot	width:u16, height:u16, encoding:str, rowstride:u32, data:bytes
S -> C	display-icon	width:u16, height:u16, encoding:str, rowstride:u32, data:bytes
S -> C	display-show-desktop	show:bool
S -> C	display-resized	width:u16, height:u16, max_width:u16, max_height:u16

display-configure carries the same monitor, desktop, DPI and scaling structures advertised in the display capabilities.

8.2 Windows and Drawing

Direction	Packet	Arguments
S -> C	window-create	wid:u64, x:i32, y:i32, w:u16, h:u16, metadata:map, client_properties:map
S -> C	window-metadata	wid:u64, metadata:map
S -> C	window-move-resize	wid:u64, x:i32, y:i32, w:u16, h:u16, resize_counter:u64
S -> C	window-resized	wid:u64, w:u16, h:u16, resize_counter:u64
S -> C	window-raise	wid:u64
S -> C	window-restack	wid:u64, detail:u8, sibling:u64
S -> C	window-initiate-moveresize	wid:u64, root_x:i32, root_y:i32, direction:u8, button:u8, source:u8
S -> C	window-destroy	wid:u64
S -> C	window-icon	wid:u64, w:u16, h:u16, encoding:str, data:bytes
S -> C	window-draw	wid:u64, x:i16, y:i16, w:u16, h:u16, encoding:str, data:bytes, sequence:u64, rowstride:u32, options:map
S -> C	window-eos	wid:u64
S -> C	window-bell	wid:u64, device:u16, percent:i8, pitch:i32, duration:i32, class:u32, id:u32, name:str
S -> C	window-grab	wid:u64
S -> C	window-ungrab	wid:u64
S -> C	window-stacking	wids:list<u64>
C -> S	window-map	wid:u64, x:i32, y:i32, w:u16, h:u16, client_properties:map, state:map?, monitor:i32?
C -> S	window-unmap	wid:u64, iconified:bool?, state:map?
C -> S	window-configure	wid:u64, configuration:map
C -> S	window-close	wid:u64
C -> S	window-focus	wid:u64, modifiers:list<str>?
C -> S	window-action	wid:u64, action:str, arguments:any...
C -> S	window-stacking	wids:list<u64>
C -> S	window-refresh	wid:u64, options:map
C -> S	window-ack	wid:u64, width:u16, height:u16, sequence:u64, decode_time_us:i32, message:str
C -> S	window-draw-ack	sequence:u64, wid:u64, width:u16, height:u16, decode_time_us:i32, message:str?

Window IDs are allocated by the server and remain valid until window-destroy. The encoding and options of window-draw MUST have been advertised by the client. A sequence acknowledgement MUST refer to the matching draw and window. window-stacking lists windows from bottom to top. Servers forward it to recording clients and clients advertising window.sync-stacking; this stream does not require a connection record or sync permission.

8.3 Encoding Control

Direction	Packet	Arguments
C -> S	encoding-set	encoding:str, window_ids:list<u64>?
C -> S	encoding-options	options:map
C -> S	encoding-config	configuration:map
S -> C	encoding-set	properties:map

The principal option keys are quality, min-quality, speed, min-speed, scaling, rgb_formats, full_csc_modes, codec profile maps and batch delay parameters. Values outside advertised ranges MUST be rejected or clamped.

8.4 Keyboard and Pointer

Direction	Packet	Arguments
C -> S	keyboard-event	wid:u64, keyname:str, pressed:bool, properties:map
C -> S	keyboard-config	configuration:map
C -> S	keyboard-sync	enabled:bool
either	pointer-motion	device:i64, sequence:u64, wid:u64, pointer:list<u16>, properties:map
either	pointer-button	device:i64, sequence:u64, wid:u64, button:u8, pressed:bool, pointer:list<u16>, properties:map
C -> S	pointer-wheel	wid:u64, axis:u8, delta:i64, pointer:list<u16>, modifiers:list<str>, buttons:list<u8>, properties:map
S -> C	pointer-wheel	wid:u64, axis:u8, delta:i64, pointer:list<u16>, modifiers:list<str>
S -> C	pointer-position	wid:u64, x:i32, y:i32, relative_x:i32, relative_y:i32

Keyboard event properties include modifier names, key value, text, hardware keycode and layout group. Pointer coordinates are [x, y]; extra device axes may follow only when advertised.

8.5 Cursor

Direction	Packet	Arguments
C -> S	cursor-set	enabled:bool
S -> C	cursor-data	encoding:str, w:u16, h:u16, hotspot_x:u16, hotspot_y:u16, serial:u64, data:bytes, name:str
S -> C	cursor-default	none

The cursor encoding MUST be in the client’s advertised cursor encodings.

8.6 Clipboard

Clipboard packets can flow in either direction when the negotiated direction permits it.

Packet	Arguments
clipboard-status	enabled:bool, reason:str?
clipboard-enable-selections	selections:list<str>
clipboard-data	selection:str, options:map
clipboard-request	request_id:u64, selection:str, target:str
clipboard-contents	request_id:u64, selection:str, data_type:str, data_format:u8, wire_encoding:str, data:any
clipboard-contents-none	request_id:u64, selection:str?
clipboard-pending-requests	count:u32

The clipboard-data options are claim:bool, greedy:bool, targets:list<str> and data:map. Each data entry maps a target name to [data_type:str, data_format:u8, wire_encoding:str, wire_data:any].

Request IDs correlate exactly one contents response. Receivers MUST enforce negotiated selection, target and size limits before exposing clipboard data.

8.7 Notifications

Direction	Packet	Arguments
S -> C	notification-show	dbus_id:str, id:u64, application:str, replaces:u64, icon_name:str, summary:str, body:str, expire_ms:i64, icon:any?, actions:list<str>?, hints:map?
either	notification-close	id:u64, reason:str?, text:str?
C -> S	notification-action	id:u64, action:str
C -> S	notification-status	enabled:bool

8.8 Audio

Direction	Packet	Arguments
either	audio-capabilities	capabilities:map
either	audio-control	command:str, arguments:any...
either	audio-data	codec:str, data:bytes, metadata:map, extra_data:bytes...
either	audio-keepalive	sequence:u64
S -> C	audio-level	sample:map
S -> C	audio-signal	present:bool

audio-data metadata marks stream start/end, sequence, timestamp and codec properties. Codec and direction MUST have been negotiated. Implementations MAY exchange audio level/signal packets advertised in packet-types; their payload is codec-backend metadata and is optional.

8.9 Webcam

Direction	Packet	Arguments
C -> S	webcam-start	device:i64, width:u16, height:u16
C -> S	webcam-frame	device:i64, frame:u64, encoding:str, width:u16, height:u16, data:bytes, options:map
S -> C	webcam-ack	device:i64, frame:u64, width:u16, height:u16
either	webcam-stop	device:i64, reason:str?

8.10 Files, URLs and Printers

File packets may flow in either direction when the corresponding file capability permits it. printer-devices and printer-file flow from client to server.

Packet	Arguments
file-send	filename:str, mimetype:str, print:bool, open:bool, size:u64, data:bytes, options:map, send_id:str?
file-send-chunk	chunk_id:str, chunk_no:u32, data:bytes, has_more:bool
file-ack-chunk	chunk_id:str, ok:bool, message:bytes|str, chunk_no:u32
file-data-request	type:str, send_id:str, url:str, mimetype:str, size:u64, print:bool, open:bool, options:map?
file-data-response	send_id:str, accept:bool
file-request	filename:str, open:bool, send_id:str
open-url	url:str
printer-devices	printers:map
printer-file	filename:str, data:bytes, mimetype:str?, source_uuid:str?, title:str?, printer:str?, copies:u16?, options:map|str?

file-send options carry digests and, for chunked transfers, file-chunk-id. A receiver MUST validate the declared size, negotiated maximum, authorization, basename, chunk order and digest. file-data-response authorizes only the exact pending send_id; it is not blanket permission to send data.

8.11 Encoder Service

Direction	Packet	Arguments
C -> S	encode	input_coding:str, pixel_format:str, pixels:bytes, width:u16, height:u16, rowstride:u32, options:map, metadata:map
C -> S	context-request	id:u64, codec_type:str, encoding:str, width:u16, height:u16, source_format:str, options:map
C -> S	context-compress	id:u64, metadata:map, pixels:bytes|list<bytes>, options:map, send_options:map
C -> S	context-close	id:u64, reason:str
S -> C	encodings	capabilities:map
S -> C	encode-response	encoding:str, data:bytes, options:map, width:u16, height:u16, rowstride:u32, bpp:u8, metadata:map
S -> C	context-response	id:u64, accepted:bool, message:str, properties:map
S -> C	context-data	id:u64, data:bytes, client_options:map, reply_options:map

Context IDs are client-selected and unique within one connection. Dimensions, formats and codec options MUST be supported by the encoder capabilities.

8.12 Miscellaneous Feature Packets

Direction	Packet	Arguments
C -> S	gsettings-update	settings:map

8.13 Recording Profile

A client advertising the recording capabilities receives normal window and input packets plus these wrappers:

Direction	Packet	Arguments
S -> C	keyboard-record	wid:u64, event:map
S -> C	clipboard-record	direction:str, embedded_type:str, embedded_arguments:any...

The keyboard event map contains press, name, keyval, keycode, modifiers, is-modifier and sync. The clipboard wrapper preserves the original modern clipboard packet after the direction field.

Optional platform-specific packets MUST be listed in packet-types and MUST use a subsystem prefix. A peer that does not advertise that feature must not receive them.

9. Ordering and Lifecycle Rules

• Packets on a connection are ordered as dispatched after chunk reassembly.
• A sender MUST NOT reuse a live window, request, transfer or encoder-context ID for a different object.
• A destroy, stop, close or end-of-stream packet terminates the corresponding object; later data for it MUST be ignored or rejected.
• An acknowledgement MUST not acknowledge data that has not been received and validated.
• A receiver MAY process explicitly thread-safe packets concurrently, but MUST preserve externally visible ordering for each object.
• connection-close SHOULD be sent before an orderly transport close. Transport EOF without it is an abnormal disconnect.

10. Error Handling

Malformed headers, invalid flags, oversized records, decompression failures, packet decoder failures, invalid raw-chunk indices and structurally invalid packets are protocol errors. The receiver MUST stop parsing the affected record stream and SHOULD send connection-close when it can do so safely.

Before a stream has been identified as Xpra, a server MAY return a short plain-text diagnostic for an HTTP request or another recognized protocol. Once an Xpra header has been accepted, errors MUST use Xpra framing.

Application errors such as an unavailable codec, denied file, unknown window or failed command SHOULD use the subsystem’s response or a clean close and MUST NOT be treated as permission to relax record validation.

11. Extensibility

New optional capabilities belong in a namespaced map. New packets SHOULD use a stable subsystem prefix and MUST be advertised in packet-types before use. Adding optional trailing fields is permitted only when the receiver can identify and ignore them safely. Changing the meaning, order or type of an existing field requires a new packet type or an explicitly negotiated capability.

Protocol and compressor flag bits, header fields and packet names in this document are not implicitly extensible: unrecognized values are invalid unless a future specification explicitly negotiates them.

12. Security Considerations

Xpra packet framing provides no confidentiality, integrity or authentication. Implementations SHOULD use TLS or SSH on untrusted networks and MUST authenticate before enabling privileged profiles or application packets.

All declared lengths, decompressed sizes, collection counts, raw chunk indices, IDs, paths, URLs, codec dimensions and option maps are attacker-controlled. Receivers MUST bound them before allocation or external side effects. File open, URL open, printing, command execution, shell, shutdown, detach, registration, clipboard and device forwarding require explicit local policy; capability advertisement alone is not authorization.

YAML MUST be decoded without object construction. Compressed data and media codecs MUST be subject to output and resource limits. Sensitive diagnostic metadata SHOULD be omitted unless requested by an authenticated peer.

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