HTTP                                                         P. O'Hanlon
Internet-Draft                                              J. Gruessing
Intended status: Standards Track        British Broadcasting Corporation
Expires: September 4, 2020                                 March 3, 2020

                     The Transport-Info HTTP Header


   The Transport-Info header provides a mechanism to transmit network
   transport related information such as current delivery rate and
   round-trip time, from a server or a client.  This information has a
   wide range of uses such as client monitoring and diagnostics, or
   allowing a client to adapt to current network conditions.

Note to Readers

   _RFC Editor: please remove this section before publication_

   Source code and issues for this draft can be found at [1].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on September 4, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  The Transport-Info HTTP Header  . . . . . . . . . . . . . . .   4
     2.1.  Utilisation of Transport-Info header metrics  . . . . . .   6
   3.  Server based behaviour  . . . . . . . . . . . . . . . . . . .   7
   4.  Client based behaviour  . . . . . . . . . . . . . . . . . . .   7
     4.1.  Client side proxy considerations  . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     6.1.  Privacy Considerations  . . . . . . . . . . . . . . . . .   8
     6.2.  Information control . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Appendix B.  Changes  . . . . . . . . . . . . . . . . . . . . . .  11
     B.1.  Since -00 . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The Transport-Info header provides for relaying of transport protocol
   related information from either a server or client entity with the
   aim of informing the sender's view on the transport state.  The state
   of a connection is dependent upon information based upon packet
   exchanges during the transport processes.  Firstly, there is
   information that is common to both client and server, such as the
   calculated round-trip time (RTT), although it may be measured using
   different packets at each end.  Secondly, there is state information
   that exists only at each endpoint, such as the size of the
   congestion, and receive windows.  Thus certain transport state
   information is only available at the server which can be useful to
   the client, for example, to calculate the current transport rate.
   This information may then be used to better inform a client of the
   state of the network path and make appropriate adaptations.

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   The information can also be utilised by a client to provide for
   application level client oriented metric logging to back-end systems
   for monitoring and analysis purposes.  Such data could be utilised in
   a manner not unlike that proposed in [RFC4898].

   This approach is directly applicable to TCP but also can be utilised
   with other related transport protocols, such as QUIC

1.1.  Motivation

   This work is motivated, in part, by the fact that even modern web
   browser-based web applications are not currently able to obtain such
   low level information about specific connections.  Additionally, some
   information is only available at the server, such as the size of the
   server congestion window.  As a result clients often resort to
   application level measurements, to infer such things as the current
   delivery rate.  However, these are not always indicative of the
   performance of the transport layer, and may not be sufficiently
   precise due to a couple of issues; Firstly, browser based timing is
   limited by the granularity of the JavaScript timers, which were
   reduced in the light of timing based side-channel attacks, although
   due to new mitigations such timer limits are currently of the order
   5us-1ms.  These limits can be an issue for higher rate connections
   and/or those with smaller transactions.  Secondly, with flows where
   the content-length is unknown, such as with chunked transfer
   encoding, it is currently difficult to correctly measure the
   bandwidth in the browser as the even the fetch/streams APIs do not
   provide for sufficient information.

   There exist W3C specifications such as the Network Information API
   [network-info-api], which provides estimates of metrics, including
   downlink rate and RTT, that are measured "across recently active
   connections", but are platform and browser dependent, with limited
   cross-browser support.  In practice the downlink measurement is is
   generally of low accuracy and of little use for informing an
   application of dynamic network conditions, and the RTT measurement is
   also of low accuracy.  However, it is implemented in Chrome and the
   utilisation of the API is now seen in a large proportion of websites,
   mainly due to adoption of the API by widely used libraries.

   This information is already being sent by servers and clients so this
   document specifies a standard way for entities to encode and
   transport such information.

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1.2.  Use Cases

   The header can be used to provide sender specific transport
   information that can inform a range of functions:

   o  Assist or drive the media quality selection algorithms for
      streaming media.
   o  Inform initial rate selection.
   o  Provide better bandwidth information for shorter requests (e.g.
      gRPC, audio) which are harder to measure.

      *  Could be used to drive scheduling of different flows in systems
         such as Traefik.
   o  The RTT values are useful for informing the operation of latency
      sensitive applications.
   o  The RTTVAR could be used to provide an estimate of 'reliability'
      of rtt and bandwidth estimates.
   o  Inform client/browser media/data caching strategies.
   o  Use by intermediate nodes for traffic analytics and control.

1.3.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

   This document uses the Augmented Backus-Naur Form (ABNF) notation of
   [RFC5234] with the list rule extension defined in [RFC7230],
   Appendix B.  It includes by reference the DIGIT rule from [RFC5234]
   and the OWS and field-name rules from [RFC7230].

2.  The Transport-Info HTTP Header

   The Transport-Info header uses the proposed Structured Header draft

           Transport-Info = sh-list

   Each member of the parameterised list represents an entry that
   contains a set of metrics reported.

   The list members identify either the server or client that inserted
   the value, and MUST have a type of either sh-string or sh-token.
   Depending on the deployment, this might be a product or service name
   (e.g., ExampleEdge or "Example CDN"), a hostname ("edge-"), and IP address, or a generated string.

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   Each member of the list can also have a number of parameters that
   contain metrics.  While all but one of these parameters are OPTIONAL,
   implementations are encouraged to only provide as much information as

   o  Exactly one parameter whose name is "ts", and whose value is an
      sh-string indicating the measurement timestamp in [RFC3339]
   o  Optionally one parameter whose name is "alpn", and whose value is
      an sh-string representing the ALPN protocol identifier [alpn-ids].
   o  Optionally one parameter whose name is "cc_algo", and whose value
      is sh-string, conveying the name of congestion control algorithm
      used for this connection.
   o  Optionally one parameter whose name is "cwnd", and whose value is
      a sh-integer, conveying the size of the congestion window
      [RFC5681] in packets.
   o  Optionally one parameter whose name is "rcv_space", and whose
      value is a sh-integer, conveying the size of the receiver's window
      in bytes.
   o  Optionally one parameter whose name is "dstport", and whose value
      is a sh-integer, conveying the destination port of this connection
      for correlation of measurements between requests.
   o  Optionally one parameter whose name is "mss", and whose value is a
      sh-integer, conveying the size of the Maximum Segment Size in
   o  Optionally one parameter whose name is "rtt", and whose value is
      an sh-float, in milliseconds, indicating the estimate of the
      Round-Trip Time from its transport layer.
   o  Optionally one parameter whose name is "rttvar", and whose value
      is an sh-float, in milliseconds, indicating the estimate of the
      variation of the Round-Trip Time [RFC6298] from its transport
   o  Optionally one parameter whose name is "send_rate", and whose
      value is a sh-float, in kilobits per second, conveying the
      calculation of the sending rate for this connection.

   Here is an example of a header with a single set of metrics:

   Transport-Info = ExampleEdge; ts="2019-08-30T14:56:08.069Z";
                     alpn="h2"; send_rate="5100"

   Whilst it is understood that such metrics may only provide an
   instantaneous view on the transport state, the Transport-Info header
   is designed to allow for delivery of multiple timestamped entries in
   a single header.

   Here is an example of the header with multiple entries, utilising the
   structured header inner-list type:

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   Transport-Info = ""; ts="2019-08-30T14:56:08Z";
                     cwnd=24; rtt=50; mss=1452; rttvar=10; dstport=8065,
                    ""; ts="2019-08-30T14:57:08Z";
                     cwnd=23; rtt=55; mss=1452; rttvar=12; dstport=8065

   If the end points support HTTP/2, and later, another technique to
   increase temporal coverage for an ongoing session is for the client
   to issue additional HEAD requests for the resource at the same
   origin.  This works with HTTP/2, and later, as all requests to the
   same origin usually utilise one TCP or QUIC connection.  Whilst the
   HTTP priorities can affect the allocation of capacity between streams
   the header will still provide an estimate of the maximum available
   capacity.  Likewise, in some cases with HTTP/2, and later, there may
   be multiple flows traversing the same transport connection to
   different origins if connection reuse (Section 9.1.1 of [RFC7540]) is
   utilised, which could have a similar effect on interpretation of the
   metrics to HTTP priorities, but may have privacy implications which
   are addressed in the privacy section Section 6.1.

2.1.  Utilisation of Transport-Info header metrics

   The metrics may be used directly to inform entities that receive the
   header.  The calculation of the send rate maybe performed by the
   sender of the header and included in the send_rate parameter, or the
   receiver may calculate it as described below.  The decision may
   depend upon a variety of factors including the privacy consideration
   of transporting any required parameters.

   In the case of TCP, calculation of the transport transmission rate is
   possible using the cwnd and rtt, and knowledge of the mss.  The
   equation being as follows:

       send_rate = 8 * send_window / rtt

       Where send_window = min (cwnd * mss, rcv_space)

   If the mss is not available then it is possible to perform the
   calculation using an estimate of the mss, or a common value such as
   1460 for IPv4.  It is understood there can be some variation for
   different network and tunnelled paths (e.g. 1452 for IPv4 PPPoE) as
   can been seen in recent studies [exploring-mtu], although the large
   proportion of mss values fall within a range 1220-1460.  The
   send_window is preferably calculated using a minimum of the cwnd and
   rcv_space, but if the rcv_space is not available it may be
   approximated by just using the cwnd.

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   This equation maybe applied for other related window based transport
   protocols (e.g.  QUIC [I-D.ietf-quic-transport]) with similar
   information, although it may need some modification.

3.  Server based behaviour

   With most web server deployments an origin server sits behind some
   form of CDN, with varying levels of fan-out to a point where an edge
   server is connected on the last hop to clients.  The Transport-Info
   header SHOULD only be inserted into an HTTP stream by the last hop
   edge server that is connected to clients so that it conveys
   information pertinent to the client's direct transport path.  The
   Transport-Info header MUST not be cached.

   With respect to use in CORS [cors] enabled environments access to the
   header will be subject to restrictions in cross domain requests,
   which may be controlled through the inclusion of the Transport-Info
   header in the Access-Control-Request-Headers header.

   The use of the header is expected to comply with data minimisation
   approaches where servers only send the necessary information on
   relevant flows.

   _RFC Editor: please remove this section before publication_

   The provision of the Transport-Info header is possible using a number
   of existing server systems that already provide support for such
   metrics, which currently utilise operating system support for the
   "tcp_info" data structure which is available on Linux and BSD based

   In terms of current implementations there is in-built support in
   Nginx/Openresty using its variables "var.tcpinfo_rtt" etc.  Apache
   Traffic Server provides support using the TCPInfo plugin.  Varnish
   provides access to "tcp_info" using their "vmod_tcp" module.  Node.js
   has libraries such as "nodejs_tcpinfo" which provide support.  Whilst
   most of the implementations do not provide access to the TCP MSS it
   is available via the underlying kernel "tcp_info" data structure so
   it would be fairly straightforward to provide access to such

4.  Client based behaviour

   The use of the header by a client is envisaged as a less common use-
   case since such information is generally less readily available on
   clients, and its general use might have privacy implications,
   although servers will be aware of most transport state already.  We
   propose that use of the header could be controlled through the use of

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   the Allow-CH header [I-D.ietf-httpbis-client-hints].  The header can
   enable the server to make better informed decisions based upon client
   based transport information.  In the case of non-browser clients
   which have access to transport information directly through operating
   system interfaces, this information can be relayed using the header.
   Whilst with browser based clients such information could be obtained
   through the use of the JavaScript Network Information API.

4.1.  Client side proxy considerations

   In the case where a client is configured to utilise a proxy directly,
   or through the use of the HTTP CONNECT pseudo-method, this proxy
   should be configured according to local policy as to whether it
   passes through, modifies, or drops the Transport-Info header.  This
   decision can depend on a number of factors, including whether the
   flows are encrypted, the utility of the header given local network
   configuration, and also whether the header might reveal unwanted
   information to end clients, since the Transport-Info header would
   relate to the connection between the edge CDN node and the proxy.

5.  IANA Considerations

   This specification registers the following entry in the Permanent
   Message Header Field Names registry established by [RFC3864]:

   o  Header field name: Transport-Info
   o  Applicable protocol: http
   o  Status: standard
   o  Author/Change Controller: IETF
   o  Specification document(s): [this document]
   o  Related information:

6.  Security Considerations

6.1.  Privacy Considerations

   The Transport-Info header provides information about a senders view
   of its network transport metrics, such as bandwidth and latency, to
   its receiver.  This information may potentially be abused for such
   purposes as fingerprinting a user through their particular network
   metrics or a time series thereof.  In some situations it might also
   be possible to infer location of users.  This may also apply in the
   case where multiple users, or user identities, share a connection
   through the use of connection reuse mechanisms or otherwise.

   However, these issues are not new and such information is already
   being shared by some servers and clients to arbitrary levels of
   accuracy.  Furthermore, there are a number of other ways an attacker

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   can obtain such information.  In the client side, in a browser, there
   exist a number JavaScript based techniques to measure the bandwidth
   and latency through existing network APIs such as the Network
   Information, the Resource Timing, and WebRTC.  On the server side, or
   a non-browser client, there is no limit to the techniques that may be
   applied to obtain information about network flows.

6.2.  Information control

   Whilst such information may be available through other mechanisms we
   recommend that implementers minimise any potential privacy issues
   through the application of the following approaches: - The principle
   of data minimisation should be applied to any use of the header such
   that only information required for the purposes of the application be
   shared.  - Any metrics deemed sensitive should apply an appropriate
   level of quantisation and noise to the values to a level that
   provides privacy whilst allowing for actual utility of the values. -
   Consideration of limits to the temporal update frequency of the
   metric values. - Any metrics that may be considered private should
   not be sent in the header, or should be appropriately protected. -
   Metrics should be sent over an encrypted connection.

   If the header is delivered over a transport protocol whose content
   can be modified without detection then parties should be aware that
   the header could be maliciously modified to alter the metrics values
   which could result in the client making incorrect adaptations.

7.  References

7.1.  Normative References

              Grigorik, I. and Y. Weiss, "HTTP Client Hints", draft-
              ietf-httpbis-client-hints-10 (work in progress), February

              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-15 (work in progress),
              January 2020.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

7.2.  Informative References

              "Application-Layer Protocol Negotiation (ALPN) Protocol
              ID", IANA , n.d., <

   [cors]     van Kesteren, A., "Cross-Origin Resource Sharing", W3C ,
              January 2014,

              Custura, A., Fairhurst, G., and I. Learmonth, "Exploring
              usable Path MTU in the Internet", Network Traffic
              Measurement and Analysis Conference , April 2018,

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-27 (work
              in progress), February 2020.

              Grigorik, I., "Network Information API", W3C , September
              2019, <>.

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   [RFC4898]  Mathis, M., Heffner, J., and R. Raghunarayan, "TCP
              Extended Statistics MIB", RFC 4898, DOI 10.17487/RFC4898,
              May 2007, <>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

7.3.  URIs


Appendix A.  Acknowledgements

   The authors would like to thank Craig Taylor, Lucas Pardue, Patrick
   McManus, and the IETF HTTP Working Group for feedback on the
   development of this document.

Appendix B.  Changes

B.1.  Since -00

   o  Issue 1 (HTTP Tunnels): Added text regarding the use of HTTP
   o  Issue 3 (Is sub-second resolution appropriate?): Changed from UNIC
      Epoch to RFC3339 time format.
   o  Issue 4 (Could this be used for both request and response?):
      Updated text to describe both server and client use, and their
   o  Issue 5 (Privacy Implications): Added new Privacy Considerations
      section and updated security section
   o  Issue 9 (CORS considerations): Added text to address CORS usage.
   o  Issue 10 (Provide additional use-cases): Updated motivation and
      added use-cases section.

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Authors' Addresses

   Piers O'Hanlon
   British Broadcasting Corporation


   James Gruessing
   British Broadcasting Corporation


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