This specification defines standard HTTP headers and a value format to propagate context information that enables distributed tracing scenarios. The specification standardizes how context information is sent and modified between services. Context information uniquely identifies individual requests in a distributed system and also defines a means to add and propagate provider-specific context information.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.tszxgm.tw/TR/.
This document was published by the Distributed Tracing Working Group as a Proposed Recommendation. This document is intended to become a W3C Recommendation.
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The W3C Membership and other interested parties are invited to review the document and send comments to [email protected] (subscribe, archives) through 10 January 2020. Advisory Committee Representatives should consult their WBS questionnaires. Note that substantive technical comments were expected during the Candidate Recommendation review period that ended 08 October 2019.
Please see the Working Group's implementation report. See also changes since the previous publication.
Publication as a Proposed Recommendation does not imply endorsement by the W3C Membership. 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 1 March 2019 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, SHOULD, and SHOULD NOT 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.
Distributed tracing is a methodology implemented by tracing tools to follow, analyze and debug a transaction across multiple software components. Typically, a distributed trace traverses more than one component which requires it to be uniquely identifiable across all participating systems. Trace context propagation passes along this unique identification. Today, trace context propagation is implemented individually by each tracing vendor. In multi-vendor environments, this causes interoperability problems, like:
In the past, these problems did not have a significant impact as most applications were monitored by a single tracing vendor and stayed within the boundaries of a single platform provider. Today, an increasing number of applications are highly distributed and leverage multiple middleware services and cloud platforms.
This transformation of modern applications calls for a distributed tracing context propagation standard.
The trace context specification defines a universally agreed-upon format for the exchange of trace context propagation data - referred to as trace context. Trace context solves the problems described above by
A unified approach for propagating trace data improves visibility into the behavior of distributed applications, facilitating problem and performance analysis. The interoperability provided by trace context is a prerequisite to manage modern micro-service based applications.
Trace context is split into two individual propagation fields supporting interoperability and vendor-specific extensibility:
traceparentdescribes the position of the incoming request in its trace graph in a portable, fixed-length format. Its design focuses on fast parsing. Every tracing tool MUST properly set
traceparenteven when it only relies on vendor-specific information in
traceparentwith vendor-specific data represented by a set of name/value pairs. Storing information in
Tracing tools can provide two levels of compliant behavior interacting with trace context:
tracestateheaders and guarantee traces are not broken. This behavior is also referred to as forwarding a trace.
traceparentheader and relevant parts of the
tracestateheader containing their proprietary information. This is also referred to as participating in a trace.
A tracing tool can choose to change this behavior for each individual request to a component it is monitoring.
This section describes the binding of the distributed trace context to
tracestate HTTP headers.
traceparent header represents the incoming request in a tracing system in a common format, understood by all vendors. Here’s an example of a
tracestate header includes the parent in a potentially vendor-specific format:
For example, say a client and server in a system use different tracing vendors: Congo and Rojo. A client traced in the Congo system adds the following headers to an outbound HTTP request.
traceparent: 00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01 tracestate: congo=t61rcWkgMzE
Note: In this case, the
t61rcWkgMzE is the result of Base64 encoding the parent ID (
b7ad6b7169203331), though such manipulations are not required.
The receiving server, traced in the Rojo tracing system, carries over the
tracestate it received and adds a new entry to the left.
traceparent: 00-0af7651916cd43dd8448eb211c80319c-00f067aa0ba902b7-01 tracestate: rojo=00f067aa0ba902b7,congo=t61rcWkgMzE
You'll notice that the Rojo system reuses the value of its
traceparent for its entry in
tracestate. This means it is a generic tracing system (no proprietary information is being passed). Otherwise,
tracestate entries are opaque and can be vendor-specific.
If the next receiving server uses Congo, it carries over the
tracestate from Rojo and adds a new entry for the parent to the left of the previous entry.
traceparent: 00-0af7651916cd43dd8448eb211c80319c-b9c7c989f97918e1-01 tracestate: congo=ucfJifl5GOE,rojo=00f067aa0ba902b7
ucfJifl5GOE is the Base64 encoded parent ID
Notice when Congo wrote its
traceparent entry, it is not encoded, which helps in consistency for those doing correlation. However, the value of its entry
tracestate is encoded and different from
traceparent. This is ok.
Finally, you'll see
tracestate retains an entry for Rojo exactly as it was, except pushed to the right. The left-most position lets the next server know which tracing system corresponds with
traceparent. In this case, since Congo wrote
tracestate entry should be left-most.
traceparent HTTP header field identifies the incoming request in a tracing system. It has four fields:
In order to increase interoperability across multiple protocols and encourage successful integration, by default vendors SHOULD keep the header name lowercase. The header name is a single word without any delimiters, for example, a hyphen (
Vendors MUST expect the header name in any case (upper, lower, mixed), and SHOULD send the header name in lowercase.
This section uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234], including the DIGIT rule from that document. The
DIGIT rule defines a single number character
HEXDIGLC = DIGIT / "a" / "b" / "c" / "d" / "e" / "f" ; lowercase hex character value = version "-" version-format
The dash (
-) character is used as a delimiter between fields.
version = 2HEXDIGLC ; this document assumes version 00. Version 255 is forbidden
The value is US-ASCII encoded (which is UTF-8 compliant).
version) is 1 byte representing an 8-bit unsigned integer. Version
255 is invalid. The current specification assumes the
version is set to
version-format definition is used for version
version-format = trace-id "-" parent-id "-" trace-flags trace-id = 32HEXDIGLC ; 16 bytes array identifier. All zeroes forbidden parent-id = 16HEXDIGLC ; 8 bytes array identifier. All zeroes forbidden trace-flags = 2HEXDIGLC ; 8 bit flags. Currently, only one bit is used. See below for details
This is the ID of the whole trace forest and is used to uniquely identify a distributed trace through a system. It is represented as a 16-byte array, for example,
4bf92f3577b34da6a3ce929d0e0e4736. All bytes as zero (
00000000000000000000000000000000) is considered an invalid value.
trace-id value is invalid (for example if it contains non-allowed characters or all zeros), vendors MUST ignore the
See considerations for trace-id field
generation for recommendations
on how to operate with
This is the ID of this request as known by the caller (in some tracing systems, this is known as the
span-id, where a
span is the execution of a client request). It is represented as an 8-byte array, for example,
00f067aa0ba902b7. All bytes as zero (
0000000000000000) is considered an invalid value.
Vendors MUST ignore the
traceparent when the
parent-id is invalid (for example, if it contains non-lowercase hex characters).
An 8-bit field that controls tracing flags such as sampling, trace level, etc. These flags are recommendations given by the caller rather than strict rules to follow for three reasons:
You can find more in the section Security considerations of this specification.
Like other fields,
trace-flags is hex-encoded. For example, all
8 flags set would be
ff and no flags set would be
As this is a bit field, you cannot interpret flags by decoding the hex value and looking at the resulting number. For example, a flag
00000001 could be encoded as
01 in hex, or
09 in hex if present with the flag
00001000. A common mistake in bit fields is forgetting to mask when interpreting flags.
Here is an example of properly handling trace flags:
static final byte FLAG_SAMPLED = 1; // 00000001 ... boolean sampled = (traceFlags & FLAG_SAMPLED) == FLAG_SAMPLED;
The current version of this specification (
00) only supports a single flag called
When set, the least significant bit (right-most), denotes that the caller may have recorded trace data. When unset, the caller did not record trace data out-of-band.
There are a number of recording scenarios that may break distributed tracing:
Because of these issues, tracing vendors make their own recording decisions, and there is no consensus on what is the best algorithm for this job.
Various techniques include:
How these techniques are implemented can be tracing vendor-specific or application-defined.
tracestate field is designed to handle the variety of techniques for making recording decisions (or other specific information) specific for a given vendor. The
sampled flag provides better interoperability between vendors. It allows vendors to communicate recording decisions and enable a better experience for the customer.
For example, when a SaaS service participates in a distributed trace, this service has no knowledge of the tracing vendor used by its caller. This service may produce records of incoming requests for monitoring or troubleshooting purposes. The
sampled flag can be used to ensure that information about requests that were marked for recording by the caller will also be recorded by SaaS service downstream so that the caller can troubleshoot the behavior of every recorded request.
sampled flag has no restriction on its mutations except that it can only be mutated when parent-id is updated.
The following are a set of suggestions that vendors SHOULD use to increase vendor interoperability.
sampledflag value. Security considerations SHOULD be applied to protect from abusive or malicious use of this flag.
sampledflag should be propagated unchanged. It should be set to
0as the default option when the trace is initiated by this component.
There are two additional options that vendors MAY follow:
1for a subset of requests.
1for the subset of requests.
The behavior of other flags, such as (
00000100) is not defined and is reserved for future use. Vendors MUST set those to zero.
Valid traceparent when caller sampled this request:
Value = 00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01 base16(version) = 00 base16(trace-id) = 4bf92f3577b34da6a3ce929d0e0e4736 base16(parent-id) = 00f067aa0ba902b7 base16(trace-flags) = 01 // sampled
Valid traceparent when caller didn’t sample this request:
Value = 00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-00 base16(version) = 00 base16(trace-id) = 4bf92f3577b34da6a3ce929d0e0e4736 base16(parent-id) = 00f067aa0ba902b7 base16(trace-flags) = 00 // not sampled
This specification is opinionated about future versions of trace context. The current version of this specification assumes that future versions of the
traceparent header will be additive to the current one.
Vendors MUST follow these rules when parsing headers with an unexpected format:
Pass-through services should not analyze the version. They should expect that headers may have larger size limits in the future and only disallow prohibitively large headers.
When the version prefix cannot be parsed (it's not 2 hex characters followed by a dash (
-)), the implementation should restart the trace.
If a higher version is detected, the implementation SHOULD try to parse it by trying the following:
trace-id(from the first dash through the next 32 characters). Vendors MUST check that the 32 characters are hex, and that they are followed by a dash (
parent-id(from the second dash at the 35th position through the next 16 characters). Vendors MUST check that the 16 characters are hex and followed by a dash.
flags(2 characters from the third dash). Vendors MUST check that the 2 characters are either the end of the string or a dash.
If all three values were parsed successfully, the vendor should use them.
Vendors MUST NOT parse or assume anything about unknown fields for this version. Vendors MUST use these fields to construct the new
traceparent field according to the highest version of the specification known to the implementation (in this specification it is
The main purpose of the
tracestate HTTP header is to provide additional vendor-specific trace identification information across different distributed tracing systems and is a companion header for the
traceparent field. It also conveys information about the request’s position in multiple distributed tracing graphs.
If the vendor failed to parse
traceparent, it MUST NOT attempt to parse
tracestate. Note that the opposite is not true: failure to parse
tracestate MUST NOT affect the parsing of
In order to increase interoperability across multiple protocols and encourage successful integration, by default you SHOULD keep the header name lowercase. The header name is a single word without any delimiters, for example, a hyphen (
Vendors MUST expect the header name in any case (upper, lower, mixed), and SHOULD send the header name in lowercase.
tracestate field may contain any opaque value in any of the keys. Tracestate MAY be sent or received as multiple header fields. Multiple tracestate header fields MUST be handled as specified by RFC7230 Section 3.2.2 Field Order. The
tracestate header SHOULD be sent as a single field when possible, but MAY be split into multiple header fields. When sending
tracestate as multiple header fields, it MUST be split according to RFC7230. When receiving multiple
tracestate header fields, they MUST be combined into a single header according to RFC7230.
This section uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234], including the DIGIT rule in appendix B.1 for RFC5234. It also includes the
OWS rule from RFC7230 section 3.2.3.
DIGIT rule defines numbers
OWS rule defines an optional whitespace character. To improve readability, it is used where zero or more whitespace characters might appear.
The caller SHOULD generate the optional whitespace as a single space; otherwise, a caller SHOULD NOT generate optional whitespace. See details in the corresponding RFC.
tracestate field value is a
list-members separated by commas (
list-member is a key/value pair separated by an equals sign (
=). Spaces and horizontal tabs surrounding
list-members are ignored. There can be a maximum of 32
list-members in a
Empty and whitespace-only list members are allowed. Vendors MUST accept empty
tracestate headers but SHOULD avoid sending them. Empty list members are allowed in
tracestate because it is difficult for a vendor to recognize the empty value when multiple
tracestate headers are sent. Whitespace characters are allowed for a similar reason, as some vendors automatically inject whitespace after a comma separator, even in the case of an empty header.
A simple example of a
list with two
list-members might look like:
list = list-member 0*31( OWS "," OWS list-member ) list-member = key "=" value list-member = OWS
Identifiers for a
list are short (up to 256 characters) textual identifiers.
list-member contains a key/value pair.
The key is an identifier that describes the vendor.
key = lcalpha 0*255( lcalpha / DIGIT / "_" / "-"/ "*" / "/" ) key = ( lcalpha / DIGIT ) 0*240( lcalpha / DIGIT / "_" / "-"/ "*" / "/" ) "@" lcalpha 0*13( lcalpha / DIGIT / "_" / "-"/ "*" / "/" ) lcalpha = %x61-7A ; a-z
Note: Identifiers MUST begin with a lowercase letter or a digit, and can only contain lowercase letters (
z), digits (
9), underscores (
_), dashes (
-), asterisks (
*), and forward slashes (
For multi-tenant vendor scenarios, an at sign (
@) can be used to prefix the vendor name. Vendors SHOULD set the tenant ID at the beginning of the key. For example,
[email protected] -
fw529a3039 is a tenant ID and
@dt is a vendor name. Searching for
@dt= is more robust for parsing (for example, searching for all a vendor's keys).
The value is an opaque string up to 256 characters printable ASCII [RFC0020] characters (i.e., the range 0x20 to 0x7E) except comma (
,) and (
=). Note that this also excludes tabs, newlines, carriage returns, etc.
value = 0*255(chr) nblk-chr nblk-chr = %x21-2B / %x2D-3C / %x3E-7E chr = %x20 / nblk-chr
tracestate value is the concatenation of trace graph key/value pairs
Only one entry per key is allowed because the entry represents that last position in the trace. Hence vendors must overwrite their entry upon reentry to their tracing system.
For example, if a vendor name is Congo and a trace started in their system and then went through a system named Rojo and later returned to Congo, the
tracestate value would not be:
Instead, the entry would be rewritten to only include the most recent position:
Vendors SHOULD propagate at least 512 characters of a combined header. This length includes commas required to separate list items and optional white space (
There are systems where propagating of 512 characters of
tracestate may be expensive. In this case, the maximum size of the propagated
tracestate header SHOULD be documented and explained. The cost of propagating
tracestate SHOULD be weighted against the value of monitoring scenarios enabled for the end users.
In a situation where
tracestate needs to be truncated due to size limitations, the vendor MUST truncate whole entries. Entries larger than
128 characters long SHOULD be removed first. Then entries SHOULD be removed starting from the end of
tracestate. Note that other truncation strategies like safe list entries, blocked list entries, or size-based truncation MAY be used, but are highly discouraged. Those strategies decrease the interoperability of various tracing vendors.
Single tracing system (generic format):
Multiple tracing systems (with different formatting):
The version of
tracestate is defined by the version prefix of
traceparent header. Vendors need to attempt to parse
tracestate if a higher version is detected, to the best of its ability. It is the vendor’s decision whether to use partially-parsed
tracestate key/value pairs or not.
A vendor receiving a
traceparent request header MUST send it to outgoing requests. It MAY mutate the value of this header before passing it to outgoing requests.
If the value of the
traceparent field wasn't changed before propagation,
tracestate MUST NOT be modified as well. Unmodified header propagation is typically implemented in pass-through services like proxies. This behavior may also be implemented in a service which currently does not collect distributed tracing information.
Following is the list of allowed mutations:
parent-id: The value of the parent-id field can be set to the new value representing the ID of the current operation. This is the most typical mutation and should be considered a default.
sampled: The value of the sampled field reflects the caller's recording behavior: either trace data was dropped or may have been recorded out-of-band. This can be indicated by toggling the flag in both directions. This mutation gives the downstream vendor information about the likelihood that its parent's information was recorded. The
parent-idfield MUST be set to a new value with the
trace-flags) are regenerated. This mutation is used in services that are defined as a front gate into secure networks and eliminates a potential denial-of-service attack surface. Vendors SHOULD clean up
traceparentrestart. There are rare cases when the original
tracestateentries must be preserved after a restart. This typically happens when the
trace-idis reverted back at some point of the trace flow, for instance, when it leaves the secure network. However, it SHOULD be an explicit decision, and not the default behavior.
00) defines the behavior for a vendor that receives a
traceparentheader of a higher version. In this case, the first mutation is to downgrade the version of the header. Other mutations are allowed in combination with this one.
Vendors MUST NOT make any other mutations to the
Vendors receiving a
tracestate request header MUST send it to outgoing requests. It MAY mutate the value of this header before passing to outgoing requests. When mutating
tracestate, the order of unmodified key/value pairs MUST be preserved. Modified keys MUST be moved to the beginning (left) of the list.
Following are allowed mutations:
tracestatekeys for privacy and security concerns. The second scenario is a truncation of long
This section is non-normative.
This section provides a step-by-step example of a tracing vendor receiving a request with trace context headers, processing the request and then potentially forwarding it. This description can be used as a reference when implementing a trace context-compliant tracing system, middleware (like a proxy or messaging bus), or a cloud service.
This processing model describes the behavior of a vendor that modifies and forwards trace context headers. How the model works depends on whether or not a
traceparent header is received.
If no traceparent header is received:
traceparentheader is received, the vendor creates a new
parent-idthat represents the current request.
tracestateheader is received without an accompanying
traceparentheader, it is invalid and MUST be discarded.
tracestateheader and add a new key/value pair.
tracestateheader for the outgoing request.
traceparent header is received:
The vendor checks an incoming request for a
traceparent and a
traceparent header is present, the vendor tries to parse the version of the
If the version cannot be parsed, the vendor creates a new
traceparent header and deletes
If the version number is higher than supported by the tracer, the vendor uses the format defined in this specification (
00) to parse
The vendor will only parse the
trace-flags values supported by this version of this specification and ignore all other values. If parsing fails, the vendor creates a new
traceparent header and deletes the
tracestate. Vendors will set all unparsed / unknown
trace-flags to 0 on outgoing requests.
If the vendor supports the version number, it validates
parent-id. If either
trace-flags are invalid, the vendor creates a new
traceparent header and deletes
The vendor MAY validate the
tracestate header. If the
tracestate header cannot be parsed the vendor MAY discard the entire header. Invalid
tracestate entries MAY also be discarded.
For each outgoing request the vendor performs the following steps:
The vendor MUST modify the
parent-id: The value of property
parent-idMUST be set to a value representing the ID of the current operation.
sampled: The value of
sampledreflects the caller's recording behavior. The value of the
trace-flagsMAY be set to
1if the trace data is likely to be recorded or to
0otherwise. Setting the flag is no guarantee that the trace will be recorded but increases the likeliness of end-to-end recorded traces.
The vendor MAY modify the
The vendor sets the
tracestate header for the outgoing request.
The processing model above describes the complete set of steps for processing trace context headers. There are, however, situations when a vendor might only support a subset of the steps described above. Proxies or messaging middleware MAY decide not to modify the
traceparent headers but remove invalid headers or add additional information to
While trace context is defined for HTTP, the authors acknowledge it is also relevant for other communication protocols. Extensions of this specification, as well as specifications produced by external organizations, define the format of trace context serialization and deserialization for other protocols. Note that these extensions may be at a different maturity level than this specification.
Please refer to the [trace-context-protocols-registry] for the details of trace context implementation for other protocols.
Requirements to propagate headers to downstream services, as well as storing values of these headers, open up potential privacy concerns. Tracing vendors MUST NOT use
tracestate fields for any personally identifiable or otherwise sensitive information. The only purpose of these fields is to enable trace correlation.
Vendors MUST assess the risk of header abuse. This section provides some considerations and initial assessment of the risk associated with storing and propagating these headers. Tracing vendors may choose to inspect and remove sensitive information from the fields before allowing the tracing system to execute code that can potentially propagate or store these fields. All mutations should, however, conform to the list of mutations defined in this specification.
traceparent field is comprised of randomly-generated numbers. If a random number generator leverages any user identifiable information like IP address as seed state, this information may be exposed. Random number generators MUST NOT rely on any information that can potentially be user-identifiable.
Another privacy risk of the
traceparent field is the ability to correlate requests made as part of a single transaction. A downstream service may track and correlate two or more requests made in a single transaction and may make assumptions about the identity of the caller of a request based on information from another request.
Note that these privacy concerns of the
traceparent field are theoretical rather than practical. Some services initiating or receiving a request MAY choose to restart a
traceparent field to eliminate those risks completely. Vendors SHOULD find a way to minimize the number of distributed trace restarts to promote interoperability of tracing vendors. Instead of restarts, different techniques may be used. For example, services may define trust boundaries of upstream and downstream connections and the level of exposure that any requests may bring. For instance, a vendor might only restart
traceparent for authentication requests from or to external services.
Services may also define an algorithm and audit mechanism to validate the randomness of incoming or outgoing random numbers in the
traceparent field. Note that this algorithm is services-specific and not a part of this specification. One example might be a temporal algorithm where a reversible hash function is applied to the current clock time. The receiver can validate that the time is within agreed upon boundaries, meaning the random number was generated with the required algorithm and in fact doesn't contain any personally identifiable information.
tracestate field may contain any opaque value in any of the keys. The main purpose of this header is to provide additional vendor-specific trace-identification information across different distributed tracing systems.
Vendors MUST NOT include any personally identifiable information in the
Vendors extremely sensitive to personal information exposure MAY implement selective removal of values corresponding to the unknown keys. Vendors SHOULD NOT mutate the
tracestate field, as it defeats the purpose of allowing multiple tracing systems to collaborate.
When vendors include
tracestate headers in responses, these values may inadvertently be passed to cross-origin callers. Vendors should ensure that they include only these response headers when responding to systems that participated in the trace.
There are two types of potential security risks associated with this specification: information exposure and denial-of-service attacks against the vendor.
Vendors relying on
tracestate headers should also follow all best practices for parsing potentially malicious headers, including checking for header length and content of header values. These practices help to avoid buffer overflow and HTML injection attacks.
As mentioned in the privacy section, information in the
tracestate headers may carry information that can be considered sensitive. For example,
traceparent may allow one request to be correlated to the data sent with another requeest, or the
tracestate header may imply the version of monitoring software used by the caller. This information could potentially be used to create a larger attack.
Application owners should either ensure that no proprietary or confidential information is stored in
tracestate, or they should ensure that
tracestate isn't present in requests to external systems.
When distributed tracing is enabled on a service with a public API and naively continues any trace with the
sampled flag set, a malicious attacker could overwhelm an application with tracing overhead, forge
trace-id collisions that make monitoring data unusable, or run up your tracing bill with your SaaS tracing vendor.
Tracing vendors and platforms should account for these situations and make sure that checks and balances are in place to protect denial of monitoring by malicious or badly authored callers.
One example of such protection may be different tracing behavior for authenticated and unauthenticated requests. Various rate limiters for data recording can also be implemented.
Application owners need to make sure to test all code paths leading to the sending of
tracestate headers. For example, in single page browser applications, it is typical to make cross-origin requests. If one of these code paths leads to
tracestate headers being sent by cross-origin calls that are restricted using
Access-Control-Allow-Headers [FETCH], it may fail.
This section is non-normative.
This section suggests some best practices to consider when platform or tracing
trace-id generation and propagation algorithms. These
practices will ensure better interoperability of different systems.
The value of
trace-id SHOULD be globally unique. This field is typically used
for unique identification of a distributed trace. It is common for
distributed traces to span various components, including, for example,
cloud services. Cloud services tend to serve variety of clients and have a very
high throughput of requests. So global uniqueness of
trace-id is important,
even when local uniqueness might seem like a good solution.
Randomly generated value of
trace-id SHOULD be preferred over other
algorithms of generating a globally unique identifiers. Randomness of
addresses some security and privacy
concerns of exposing unwanted information. Randomness
also allows tracing vendors to base sampling decisions on
trace-id field value
and avoid propagating an additional sampling context.
As shown in the next section, it is important for
trace-id to carry
"uniqueness" and "randomness" in the right part of the
trace-id, for better
inter-operability with some existing systems.
trace-idfor complaint platforms with shorter internal identifiers
There are tracing systems which use a
trace-id that is shorter than 16 bytes,
which are still willing to adopt this specification.
If such a system is capable of propagating a fully compliant
while still requiring a shorter, non-complaint identifier for internal purposes,
the system is encouraged to utilize the
tracestate header to propagate the
additional internal identifier. However, if a system would instead prefer to use
the internal identifier as the basis for a fully complaint
trace-id, it SHOULD
be incorporated at the as rightmost part of a
trace-id. For example, tracing
system may receive
234a5bcd543ef3fa53ce929d0e0e4736 as a
internally it will use
53ce929d0e0e4736 as an identifier.
There are tracing systems which are not capable of propagating the entire 16
bytes of a
trace-id. For better interoperability between a fully compliant
systems with these existing systems, the following practices are recommended:
trace-idfrom a shorter identifier, it SHOULD left pad the original identifier with zeroes. For example, the identifier
53ce929d0e0e4736, SHOULD be converted to
trace-idto a shorter identifier, the rightmost part of
trace-idSHOULD be used as this identifier. For instance, if the the value of
234a5bcd543ef3fa53ce929d0e0e4736on an incoming request, tracing system SHOULD use identifier with the value of
Similar transformations are expected when tracing system converts other
distributed trace context propagation formats to W3C Trace Context. Shorter
identifiers SHOULD be left padded with zeros when converted to 16 bytes
trace-id and rightmost part of
trace-id SHOULD be used as a shorter
Note, many existing systems that are not capable of propagating the whole
trace-id will not propagate
tracestate header either. However, such system
can still use
tracestate header to propagate additional data that is known by
this system. For example, some systems use two flags indicating whether
distributed trace needs to be recorded or not. In this case one flag can be send
sampled flag of
traceparent header and
tracestate can be used to send
and receive an additional flag. Compliant systems will propagate this flag along
all other key/value pairs. Existing systems which are not capable of
tracestate propagation will truncate all additional values from
and only pass along that flag.
Thanks to Adrian Cole, Christoph Neumüller, Daniel Khan, Erika Arnold, Fabian Lange, Matthew Wear, Reiley Yang, Ted Young, Tyler Benson, Victor Soares for their contributions to this work.
This section is non-normative.