This specification defines a standard for representing and propagating a set of user-defined properties associated with a distributed request.
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This is the First Public Working Draft (FPWD). There are a few implementations of this protocol available. Experimental interoperability scenarios were run and have demonstrated promising results. The specification will be progressed into Candidate Recommendation stage after that, drafts for binary, AMQP and MQTT protocols will be written to make sure the concepts and structure defined in this specification can be ported to other protocols.
This document was published by the Distributed Tracing Working Group as a Working Draft using the Recommendation track.
Publication as a Working Draft does not imply endorsement by W3C and its Members.
This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
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This document is governed by the 2 November 2021 W3C Process Document.
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words MAY, MUST, MUST NOT, and SHOULD in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
baggage header represents a set of user-defined properties associated with a distributed request. Libraries and platforms SHOULD propagate this header.
baggage header is used to propagate user-supplied key-value pairs through a distributed request.
A received header MAY be altered to change or add information and it SHOULD be passed on to all downstream requests.
baggage headers are allowed. Values can be combined in a single header according to RFC 7230.
In order to increase interoperability across multiple protocols and encourage successful integration, implementations SHOULD keep the header name lowercase.
This section uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234].
baggage-string = list-member 0*179( OWS "," OWS list-member ) list-member = key OWS "=" OWS value *( OWS ";" OWS property ) property = key OWS "=" OWS value property =/ key OWS key = token ; as defined in RFC 7230, Section 3.2.6 value = *baggage-octet baggage-octet = %x21 / %x23-2B / %x2D-3A / %x3C-5B / %x5D-7E ; US-ASCII characters excluding CTLs, ; whitespace, DQUOTE, comma, semicolon, ; and backslash OWS = *( SP / HTAB ) ; optional white space, as defined in RFC 7230, Section 3.2.3
token is defined in [RFC7230], Section 3.2.6: https://tools.ietf.org/html/rfc7230#section-3.2.6
The definition of
OWS is taken from [RFC7230], Section 3.2.3: https://tools.ietf.org/html/rfc7230#section-3.2.3
list-members with optional properties attached.
Uniqueness of keys between multiple
list-members in a
baggage-string is not guaranteed.
The order of duplicate entries SHOULD be preserved when mutating the list.
Producers SHOULD try to produce a
baggage-string without any
list-members which duplicate the
key of another list member.
token which identifies a
value in the
token is defined in RFC7230, Section 3.2.6.
Leading and trailing whitespaces (
OWS) are allowed and are not considered to be a part of the key.
A value contains a string whose character encoding MUST be UTF-8 [Encoding].
Any characters outside of the
baggage-octet range of characters MUST be percent-encoded.
Characters which are not required to be percent-encoded MAY be percent-encoded.
Percent-encoding is defined in [RFC3986], Section 2.1: https://datatracker.ietf.org/doc/html/rfc3986#section-2.1.
When decoding the value, percent-encoded octet sequences that do not match the UTF-8 encoding scheme MUST be replaced with the replacement character (U+FFFD).
Leading and trailing whitespaces (
OWS) are allowed and are not considered to be a part of the value.
value MAY contain any number of the equal sign (
=) characters. Parsers
MUST NOT assume that the equal sign is only used to separate
Additional metadata MAY be appended to values in the form of property set, represented as semi-colon
; delimited list of keys and/or key-value pairs, e.g.
Property keys and values are given no specific meaning by this specification.
Leading and trailing
OWS is allowed and is not considered to be a part of the property key or value.
Note that the following limits are minimum requirements to comply with the specification.
An implementor or platform MAY define higher limits and SHOULD propagate as much baggage information as is reasonable within their requirements.
If a platform cannot propagate all baggage, it MUST NOT propagate any partial
If there are multiple
baggage headers, all limits apply to the combination of all
baggage headers and not each header individually.
list-members up to at least 64
list-members including any
list-members added by the platform.
list-members including any
list-members added by the platform if the resulting
baggage-stringwould be 8192 bytes or less. If the resulting
baggage-stringwould be greater than 8192 bytes, some
list-members MAY be dropped until the resulting
baggage-stringis 8192 characters or less.
If a platform does not propagate all
list-members, it is left to the implementer to decide which
list-members to propagate.
The following example header contains 3
baggage-string contained in the header contains 86 bytes.
82 bytes come from the
list-members and 4 bytes come from commas and optional whitespace.
baggage: key1=value1;property1;property2, key2 = value2, key3=value3; propertyKey=propertyValue
key2 = value2
Assume we want to propagate these entries:
Here is one more example where values with characters outside of the
baggage-octet range of characters are percent-encoded. Consider the entry:
Context might be split into multiple headers:
baggage: userId=alice baggage: serverNode=DF%2028,isProduction=false
Values and names might begin and end with spaces:
baggage: userId = alice baggage: serverNode = DF%2028, isProduction = false
For example, if all of your data needs to be sent to a single node, you could propagate a property indicating that.
For example, if you need to annotate logs with some request-specific information, you could propagate a property using the baggage header.
For example, if you have non-production requests that flow through the same services as production requests.
A system receiving a
baggage request header SHOULD send it to outgoing requests.
A system MAY mutate the value of this header before passing it on.
Because baggage entry keys, values, and metadata are not specified here, producers and consumers MAY agree on any set of mutation rules that don't violate the specification. For example, keys may be deduplicated by keeping the first entry, keeping the last entry, or concatenating values together.
The following mutations are allowed:
If a system receiving or updating a
baggage request header determines that the number of baggage entries exceeds the limit defined in the limits section above, it MAY drop or truncate certain baggage entries in any order chosen by the implementation.
If a system determines that the value of a baggage entry is not in the format defined in this specification, it MAY remove that entry before propagating the baggage header as part of outgoing requests.
Systems relying on the
baggage headers should also follow all best practices for parsing potentially malicious data, including checking for header length and content of header values.
These practices help to avoid buffer overflow, HTML injection, and other types of attacks.
As mentioned in the privacy section,
baggage may carry sensitive information.
Application owners should either ensure that no proprietary or confidential information is stored in
baggage, or they should ensure that
baggage isn't present in requests that cross trust-boundaries.
Application owners need to make sure to test all code paths leading to the sending of the
baggage header. For example, in single page browser applications, it is typical to make cross-origin requests. If one of these code paths leads to
baggage headers being sent by cross-origin calls that are restricted using
Access-Control-Allow-Headers [FETCH], it may fail.
Requirements to propagate headers to downstream services, as well as storing values of these headers, open up potential privacy concerns. Using proprietary ways of context propagation, vendors and application developers could always encode information that contains user identifiable data. This standard makes it possible for systems to operate on a known, standardized header to restrict propagation of sensitive data in the baggage when crossing trust boundaries.
Systems MUST assess the risk of header abuse. This section provides some considerations and initial assessment of the risk associated with storing and propagating this header. Systems may choose to inspect and remove sensitive information from the fields before processing or propagating the received data. All mutations should, however, conform to the list of mutations defined in this specification.
The main purpose of this header is to provide additional system-specific information to other systems within the same trust-boundary.
baggage header may contain any value in any of the keys.
As such, the
baggage header can contain user-identifiable data, however no key or its value or properties is given semantic meaning by this specification.
Systems MUST ensure that the
baggage header does not leak beyond defined trust boundaries and they MUST ensure that the channel that is used to transport potentially user-identifiable data is secured.