wdiff rfc9440v4.txt rfc9440.txt

Internet Engineering Task Force (IETF) B. Campbell
Request for Comments: 9440 Ping Identity
Category: Informational M. Bishop, Ed.
ISSN: 2070-1721 Akamai
July 2023

Client-Cert HTTP Header Field

Abstract

This document describes HTTP extension header fields that allow a TLS
terminating reverse proxy (TTRP) to convey the client certificate
information of a mutually authenticated TLS connection to the origin
server in a common and predictable manner.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9440.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Requirements Notation and Conventions
1.2. Terminology and Applicability
2. HTTP Header Fields and Processing Rules
2.1. Encoding
2.2. Client-Cert HTTP Header Field
2.3. Client-Cert-Chain HTTP Header Field
2.4. Processing Rules
3. Deployment Considerations
3.1. Header Field Compression
3.2. Message Header Size
3.3. TLS Session Resumption
4. Security Considerations
5. IANA Considerations
5.1. HTTP Field Name Registrations
6. References
6.1. Normative References
6.2. Informative References
Appendix A. Example
Appendix B. Select Design Considerations
B.1. Field Injection
B.2. The Forwarded HTTP Extension
B.3. The Whole Certificate and Certificate Chain
Acknowledgements
Authors' Addresses

1. Introduction

A fairly common deployment pattern for HTTPS applications is to have
the origin HTTP application servers sit behind a reverse proxy that
terminates TLS connections from clients. The proxy is accessible to
the Internet and dispatches client requests to the appropriate origin
server within a private or protected network. The origin servers are
not directly accessible by clients and are only reachable through the
reverse proxy. The backend details of this type of deployment are
typically opaque to clients who make requests to the proxy server and
see responses as though they originated from the proxy server itself.
Although HTTPS is also usually employed between the proxy and the
origin server, the TLS connection that the client establishes for
HTTPS is only between itself and the reverse proxy server.

The deployment pattern is found in a number of varieties such as
n-tier architectures, content delivery networks, application load-
balancing services, and ingress controllers.

Although not exceedingly prevalent, TLS client certificate
authentication is sometimes employed, and in such cases the origin
server often requires information about the client certificate for
its application logic. Such logic might include access control
decisions, audit logging, and binding issued tokens or cookies to a
certificate, including the respective validation of such bindings.
The specific details needed from the certificate also vary with the
application requirements. In order for these types of application
deployments to work in practice, the reverse proxy needs to convey
information about the client certificate to the origin application
server. At the time of writing, a common way this information is
conveyed is by using non-standard fields to carry the certificate (in
some encoding) or individual parts thereof in the HTTP request that
is dispatched to the origin server. This solution works, but
interoperability between independently developed components can be
cumbersome or even impossible depending on the implementation choices
respectively made (like what field names are used or are
configurable, which parts of the certificate are exposed, or how the
certificate is encoded). A well-known predictable approach to this
commonly occurring functionality could improve and simplify
interoperability between independent implementations.

The scope of this document is to describe existing practice while
codifying specific details sufficient to facilitate improved and
lower-touch interoperability. As such, this document describes two
HTTP header fields, "Client-Cert" and "Client-Cert-Chain", which a
TLS terminating reverse proxy (TTRP) adds to requests sent to the
backend origin servers. The Client-Cert field value contains the
end-entity client certificate from the mutually authenticated TLS
connection between the originating client and the TTRP. Optionally,
the Client-Cert-Chain field value contains the certificate chain used
for validation of the end-entity certificate. This enables the
backend origin server to utilize the client certificate information
in its application logic. While there may be additional proxies or
hops between the TTRP and the origin server (potentially even with
mutually authenticated TLS connections between them), the scope of
the Client-Cert header field is intentionally limited to exposing to
the origin server the certificate that was presented by the
originating client in its connection to the TTRP.

1.1. Requirements Notation and Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" 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.

1.2. Terminology and Applicability

This document uses the following terminology from Section 3 of
[STRUCTURED-FIELDS] to specify syntax and parsing: List and Byte
Sequence.

Phrases like "TLS client certificate authentication" or "mutually
authenticated TLS" are used throughout this document to refer to the
process whereby, in addition to the normal TLS server authentication
with a certificate, a client presents its X.509 certificate [RFC5280]
and proves possession of the corresponding private key to a server
when negotiating a TLS connection or the resumption of such a
connection. In contemporary versions of TLS [TLS] [TLS1.2], mutual
authentication requires the client to send the Certificate and
CertificateVerify messages during the handshake and the server to
verify the CertificateVerify and Finished messages.

HTTP/2 restricts TLS 1.2 renegotiation (Section 9.2.1 of [HTTP/2])
and prohibits TLS 1.3 post-handshake authentication (Section 9.2.3 of
[HTTP/2]). However, they are sometimes used to implement reactive
client certificate authentication in HTTP/1.1 [HTTP/1.1] where the
server decides whether to request a client certificate based on the
HTTP request. HTTP application data sent on such a connection after
receipt and verification of the client certificate is also mutually
authenticated and thus suitable for the mechanisms described in this
document. With post-handshake authentication, there is also the
possibility, though unlikely in practice, of multiple certificates
and certificate chains from the client on a connection. In this
case, only the certificate and chain of the last post-handshake
authentication are to be utilized for the header fields described
herein.

2. HTTP Header Fields and Processing Rules

This document designates the following headers, defined further in
Sections 2.2 and 2.3, respectively, to carry the client certificate
information of a mutually authenticated TLS connection. The headers
convey the information from the reverse proxy to the origin server.

Client-Cert:
The end-entity certificate used by the client in the TLS handshake
with the reverse proxy.

Client-Cert-Chain:
The certificate chain used for validation of the end-entity
certificate provided by the client in the TLS handshake with the
reverse proxy.

2.1. Encoding

The headers in this document encode certificates as Byte Sequences
(Section 3.3.5 of [STRUCTURED-FIELDS]) where the value of the binary
data is a DER-encoded [ITU.X690] X.509 certificate [RFC5280]. In
effect, this means that the binary DER certificate is encoded using
base64 (without line breaks, spaces, or other characters outside the
base64 alphabet) and delimited with colons on either side.

Note that certificates are often stored in an encoded textual format,
such as the one described in Section 5.1 of [RFC7468], which is
already nearly compatible with a Byte Sequence. If certificates are
encoded as such, it will be sufficient to replace "---(BEGIN|END)
CERTIFICATE---" with ":" and remove line breaks in order to generate
an appropriate item.

2.2. Client-Cert HTTP Header Field

In the context of a TLS terminating reverse proxy deployment, the
proxy makes the TLS client certificate available to the backend
application with the Client-Cert HTTP header field. This field
contains the end-entity certificate used by the client in the TLS
handshake.

Client-Cert is a Byte Sequence with the value of the header encoded
as described in Section 2.1.

The Client-Cert header field is only for use in HTTP requests and
MUST NOT be used in HTTP responses. It is a singleton header field
value as defined in Section 5.5 of [HTTP], which MUST NOT have a list
of values or occur multiple times in a request.

Figure 2 in Appendix A has an example of the Client-Cert header
field.

2.3. Client-Cert-Chain HTTP Header Field

In the context of a TLS terminating reverse proxy deployment, the
proxy MAY make the certificate chain available to the backend
application with the Client-Cert-Chain HTTP header field.

Client-Cert-Chain is a List (Section 3.1 of [STRUCTURED-FIELDS]).
Each item in the List MUST be a Byte Sequence encoded as described in
Section 2.1. The order is the same as the ordering in TLS (as
described in Section 4.4.2 of [TLS]).

Client-Cert-Chain MUST NOT appear unless Client-Cert is also present,
and it does not itself include the end-entity certificate that is
already present in Client-Cert. The root certificate MAY be omitted
from Client-Cert-Chain, provided that the target origin server is
known to possess the omitted trust anchor.

The Client-Cert-Chain header field is only for use in HTTP requests
and MUST NOT be used in HTTP responses. It MAY have a list of values
or occur multiple times in a request. For header compression
purposes, it might be advantageous to split lists into multiple
instances.

Figure 3 in Appendix A has an example of the Client-Cert-Chain header
field.

2.4. Processing Rules

This section outlines the applicable processing rules for a TTRP that
has negotiated a mutually authenticated TLS connection to convey the
client certificate from that connection to the backend origin
servers. This technique is to be used as a configuration or
deployment option, and the processing rules described herein are for
servers operating with that option enabled.

A TTRP negotiates the use of a mutually authenticated TLS connection
with the client, such as is described in [TLS] or [TLS1.2], and
validates the client certificate per its policy and trusted
certificate authorities. Each HTTP request on the underlying TLS
connection is dispatched to the origin server with the following
modifications:

1. The client certificate is placed in the Client-Cert header field
of the dispatched request, as described in Section 2.2.

2. If so configured, the validation chain of the client certificate
is placed in the Client-Cert-Chain header field of the request,
as described in Section 2.3.

3. Any occurrence of the Client-Cert or Client-Cert-Chain header
fields in the original incoming request MUST be removed or
overwritten before forwarding the request. An incoming request
that has a Client-Cert or Client-Cert-Chain header field MAY be
rejected with an HTTP 400 response.

Requests to the TTRP made over a TLS connection where the use of
client certificate authentication was not negotiated MUST be
sanitized by removing any and all occurrences of the Client-Cert and
Client-Cert-Chain header fields prior to dispatching the request to
the backend server.

Backend origin servers may then use the Client-Cert header field of
the request to determine if the connection from the client to the
TTRP was mutually authenticated and, if so, the certificate thereby
presented by the client. Access control decisions based on the
client certificate (or lack thereof) can be conveyed by selecting
response content as appropriate or with an HTTP 403 response, if the
certificate is deemed unacceptable for the given context. Note that
TLS clients that rely on error indications at the TLS layer for an
unacceptable certificate will not receive those signals.

When the value of the Client-Cert request header field is used to
select a response (e.g., the response content is access-controlled),
the response MUST either be uncacheable (e.g., by sending Cache-
Control: no-store) or be designated for selective reuse only for
subsequent requests with the same Client-Cert header field value by
sending a "Vary: Client-Cert" response header. If a TTRP encounters
a response with a client-cert field name Client-Cert or Client-Cert-Chain in the "Vary" Vary header field,
field (Section 12.5.5 of [HTTP]), it SHOULD prevent the user agent
from caching the response by transforming the value of the Vary
response header field to "*".

Forward proxies and other intermediaries MUST NOT add the Client-Cert
or Client-Cert-Chain header fields to requests or modify an existing
Client-Cert or Client-Cert-Chain header field. Similarly, clients
MUST NOT employ the Client-Cert or Client-Cert-Chain header field in
requests.

3. Deployment Considerations

3.1. Header Field Compression

If the connection between the TTRP and origin is capable of field
compression (e.g., HPACK [HPACK] or QPACK [QPACK]), and the TTRP
multiplexes more than one client's requests into that connection, the
size and variation of Client-Cert and Client-Cert-Chain field values
can reduce compression efficiency significantly. An origin could
mitigate the efficiency loss by increasing the size of the dynamic
table. If the TTRP determines that the origin dynamic table is not
sufficiently large, it may find it beneficial to always send the
field value as a literal rather than entering it into the table.

3.2. Message Header Size

A server in receipt of a larger message header than it is willing to
handle can send an HTTP 431 (Request Header Fields Too Large) status
code per Section 5 of [RFC6585]. Due to the typical size of the
field values containing certificate data, recipients may need to be
configured to allow for a larger maximum header size. An
intermediary generating client certificate header fields on
connections that allow for advertising the maximum acceptable header
size (e.g., HTTP/2 [HTTP/2] or HTTP/3 [HTTP/3]) should account for
the additional size of the header of the requests it sends, versus
the requests it receives, by advertising a value to its clients that
is sufficiently smaller so as to allow for the addition of
certificate data.

3.3. TLS Session Resumption

Some TLS implementations do not retain client certificate information
when resuming. Providing inconsistent values of Client-Cert and
Client-Cert-Chain when resuming might lead to errors, so
implementations that are unable to provide these values SHOULD either
disable resumption for connections with client certificates or
initially omit a Client-Cert or Client-Cert-Chain field if it might
not be available after resuming.

4. Security Considerations

The header fields described herein enable a TTRP and backend or
origin server to function together as though, from the client's
perspective, they are a single logical server-side deployment of
HTTPS over a mutually authenticated TLS connection. However, use of
the header fields outside that intended use case may undermine the
protections afforded by TLS client certificate authentication.
Therefore, steps such as those described below need to be taken to
prevent unintended use, both in sending the header field and in
relying on its value.

Producing and consuming the Client-Cert and Client-Cert-Chain header
fields SHOULD be configurable options, respectively, in a TTRP and
backend server (or in an individual application in that server). The
default configuration for both should be to not use the header
fields, thus requiring an "opt-in" to the functionality.

In order to prevent field injection, backend servers MUST only accept
the Client-Cert and Client-Cert-Chain header fields from a trusted
TTRP (or other proxy in a trusted path from the TTRP). A TTRP MUST
sanitize the incoming request before forwarding it on by removing or
overwriting any existing instances of the fields. Otherwise,
arbitrary clients can control the field values as seen and used by
the backend server. It is important to note that neglecting to
prevent field injection does not "fail safe" in that the nominal
functionality will still work as expected even when malicious actions
are possible. As such, extra care is recommended in ensuring that
proper field sanitation is in place.

The communication between a TTRP and backend server needs to be
secured against eavesdropping and modification by unintended parties.

The configuration options and request sanitization are necessary
functionalities of the respective servers. The other requirements
can be met in a number of ways, which will vary based on specific
deployments. The communication between a TTRP and backend or origin
server, for example, might be authenticated in some way with the
insertion and consumption of the Client-Cert and Client-Cert-Chain
header fields occurring only on that connection. Appendix B.3 of
[HTTPSIG] gives one example of this with an application of HTTP
Message Signatures. Alternatively, the network topology might
dictate a private network such that the backend application is only
able to accept requests from the TTRP and the proxy can only make
requests to that server. Other deployments that meet the
requirements set forth herein are also possible.

5. IANA Considerations

5.1. HTTP Field Name Registrations

IANA has registered the following entries in the "Hypertext Transfer
Protocol (HTTP) Field Name Registry" defined by "HTTP Semantics"
[HTTP]:

Table 1: Hypertext Transfer Protocol (HTTP) Field
Name Registry

6. References

6.1. Normative References

[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.

[ITU.X690] ITU-T, "Information technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ITU-T Recommendation X.690, February 2021,
<https://www.itu.int/rec/T-REC-X.690/en>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/rfc/rfc8941>.

6.2. Informative References

[HPACK] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/info/rfc7541>.

[HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
June 2022, <https://www.rfc-editor.org/info/rfc9112>.

[HTTP/2] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>.

[HTTP/3] Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
June 2022, <https://www.rfc-editor.org/info/rfc9114>.

[HTTPSIG] Backman, A., Ed., Richer, J., Ed., and M. Sporny, "HTTP
Message Signatures", Work in Progress, Internet-Draft,
draft-ietf-httpbis-message-signatures-17, 2 May 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
message-signatures-17>.

[QPACK] Krasic, C., Bishop, M., and A. Frindell, Ed., "QPACK:
Field Compression for HTTP/3", RFC 9204,
DOI 10.17487/RFC9204, June 2022,
<https://www.rfc-editor.org/info/rfc9204>.

[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status
Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
<https://www.rfc-editor.org/info/rfc6585>.

[RFC7239] Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
RFC 7239, DOI 10.17487/RFC7239, June 2014,
<https://www.rfc-editor.org/info/rfc7239>.

[RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
April 2015, <https://www.rfc-editor.org/info/rfc7468>.

[RFC8705] Campbell, B., Bradley, J., Sakimura, N., and T.
Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
and Certificate-Bound Access Tokens", RFC 8705,
DOI 10.17487/RFC8705, February 2020,
<https://www.rfc-editor.org/info/rfc8705>.

[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[TLS1.2] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

Appendix A. Example

In a hypothetical example where a TLS client would present the client
and intermediate certificate from Figure 1 when establishing a
mutually authenticated TLS connection with the TTRP, the proxy would
send the Client-Cert field shown in Figure 2 to the backend. Note
that line breaks and extra spaces have been added to the field value
in Figures 2 and 3 for display and formatting purposes only.

Figure 1: Certificate Chain (with Client Certificate First)

Client-Cert: :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKDBJ
MZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQTAeFw0
yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFkwEwYHKoZ
Izj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmckC8vdgJ1p5Be
5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDVR0TBAIwADAfBgN
VHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf8EBAMCBsAwEwYDVR0
lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV4YW1wbGUuY29tMAoGCCq
GSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6bMjeSkC3dFCOOB8TAiEAx/k
HSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:

Figure 2: Header Field in HTTP Request to Origin Server

If the proxy were configured to also include the certificate chain,
it would also include the Client-Cert-Chain header field. Note that
while the following example does illustrate the TTRP inserting the
root certificate, many deployments will opt to omit the trust anchor.

Client-Cert-Chain: :MIIB5jCCAYugAwIBAgIBFjAKBggqhkjOPQQDAjBWMQsw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:, :MIICBjCCAaygAw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:

Figure 3: Certificate Chain in HTTP Request to Origin Server

Appendix B. Select Design Considerations

B.1. Field Injection

This document requires that the TTRP sanitize the fields of the
incoming request by removing or overwriting any existing instances of
the Client-Cert and Client-Cert-Chain header fields before
dispatching that request to the backend application. Otherwise, a
client could inject its own values that would appear to the backend
to have come from the TTRP. Although numerous other methods of
detecting and preventing field injection are possible, such as the
use of a unique secret value as part of the field name or value or
the application of a signature, HMAC, or AEAD, there is no common
general mechanism. The potential problem of client field injection
is not at all unique to the functionality of this document;
therefore, it would be inappropriate for this document to define a
one-off solution. Since a generic common solution does not currently
exist, stripping and sanitizing the fields is the de facto means of
protecting against field injection in practice. Sanitizing the
fields is sufficient when properly implemented and is a normative
requirement of Section 4.

B.2. The Forwarded HTTP Extension

The "Forwarded" Forwarded HTTP header field defined in [RFC7239] allows proxy
components to disclose information lost in the proxying process. The
TLS client certificate information of concern to this document could
have been communicated with an extension parameter to the Forwarded
field; however, doing so would have had some disadvantages that this
document endeavored to avoid. The Forwarded field syntax allows for
information about a full chain of proxied HTTP requests, whereas the
Client-Cert and Client-Cert-Chain header fields of this document are
concerned only with conveying information about the certificate
presented by the originating client on the TLS connection to the TTRP
(which appears as the server from that client's perspective) to
backend applications. The multi-hop syntax of the Forwarded field is
expressive but also more complicated, which would make processing it
more cumbersome and, more importantly, would make properly sanitizing
its content, as required by Section 4 to prevent field injection,
considerably more difficult and error-prone. Thus, this document
opted for a flatter and more straightforward structure.

B.3. The Whole Certificate and Certificate Chain

Different applications will have varying requirements about what
information from the client certificate is needed, such as the
subject and/or issuer distinguished name, subject alternative
name(s), serial number, subject public key info, fingerprint, etc.
Furthermore, some applications, such as that described in [RFC8705],
make use of the entire certificate. In order to accommodate the
latter and ensure wide applicability by not trying to cherry-pick
particular certificate information, this document opted to pass the
full, encoded certificate as the value of the Client-Cert field.

The validation of the client certificate and chain of the mutually
authenticated TLS connection is typically performed by the TTRP
during the handshake. With the responsibility of certificate
validation falling on the TTRP, the end-entity certificate is
oftentimes sufficient for the needs of the origin server. The
separate Client-Cert-Chain field can convey the certificate chain for
origin server deployments that require this additional information.

Acknowledgements

The authors would like to thank the following individuals who have
contributed to this document in various ways, ranging from just being
generally supportive of bringing forth the document to providing
specific feedback or content:

* Evan Anderson

* Annabelle Backman

* Alan Frindell

* Rory Hewitt

* Fredrik Jeansson

* Benjamin Kaduk

* Torsten Lodderstedt

* Kathleen Moriarty

* Mark Nottingham

* Erik Nygren

* Mike Ounsworth

* Lucas Pardue

* Matt Peterson

* Eric Rescorla

* Justin Richer

* Michael Richardson

* Joe Salowey

* Rich Salz

* Mohit Sethi

* Rifaat Shekh-Yusef

* Travis Spencer

* Nick Sullivan

* Willy Tarreau

* Martin Thomson

* Peter Wu

* Hans Zandbelt

Authors' Addresses

Brian Campbell
Ping Identity
Email: bcampbell@pingidentity.com

Mike Bishop (editor)
Akamai
Email: mbishop@evequefou.be