HTTP P. O'Hanlon
Internet-Draft J. Gruessing
Intended status: Standards Track British Broadcasting Corporation
Expires: May 20, 2020 November 17, 2019
The Transport-Info Response Header
draft-ohanlon-transport-info-header-00
Abstract
The Transport-Info header provides a mechanism to inform a client of
the last-mile server's view of the network transport related
information such as current delivery rate and round-trip time. 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
https://github.com/bbc/draft-ohanlon-transport-info-header [1].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 20, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The Transport-Info Response Header . . . . . . . . . . . . . 3
2.1. Utilisation of Transport-Info header metrics . . . . . . 5
3. Server side behaviour . . . . . . . . . . . . . . . . . . . . 6
4. Client side proxy considerations . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The Transport-Info header provides for relaying of transport protocol
related information from a last-mile edge server entity to a client
with the aim of informing the client of the server'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.
The information can also be utilised by a client to provide for
application level client oriented metric logging to back-end systems
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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
[I-D.ietf-quic-transport].
1.1. Motivation
This work is motivated in part by the fact that even in modern web
browsers web applications are not currently able to obtain such low
level information about their 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, but these are not always indicative of the performance
of the transport layer.
There exist W3C specifications such as the Network Information API
[network-info-api], which contains an attribute named "downlink" that
purports to provide measurement of effective bandwidth "across
recently active connections". However, in practice the downlink
measurement appears to be a very rough estimate which is of little
use for informing an application of dynamic network conditions.
Furthermore, it currently has limited browser support.
1.2. Notational 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.
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 Response Header
The Transport-Info header uses the proposed Structured Header draft
[I-D.ietf-httpbis-header-structure]
Transport-Info = sh-list
Each member of the parameterised list represents an entry that
contains a set of metrics reported by the edge server.
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The list members identify the server 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-1.example.com"), and
IP address, or a generated string.
Each member of the list can also have a number of parameters that
contain metrics. While all but one of these parameters are OPTIONAL,
edge servers are encouraged to provide as much information as
possible.
o Exactly one parameter whose name is "ts", and whose value is an
sh-float indicating the measurement timestamp in seconds since
UNIX epoch.
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 by the server for this connection.
o Optionally one parameter whose name is "cwnd", and whose value is
a sh-integer, conveying the size of the server's 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 server's 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 server's Maximum Segment
Size in bytes.
o Optionally one parameter whose name is "rtt", and whose value is
an sh-float, in milliseconds, indicating the server's 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 server's estimate
of the variation of the Round-Trip Time [RFC6298] from its
transport layer.
o Optionally one parameter whose name is "send_rate", and whose
value is a sh-float, in kilobits per second, conveying the
server's 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=1567176968.69; alpn="h2"; cwnd=24;
rtt=250; mss=1460; rttvar=10; dstport=12345
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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 header with multiple entries, utilising the
structured header inner-list type:
Transport-Info = "edge-1.example.com"; ts=1567176968.69; alpn="h2"; cwnd=24;
rtt=250; mss=1452; rttvar=10; dstport=123451,
"edge-1.example.com"; ts=1567176969.97; alpn="h2"; cwnd=23;
rtt=255; mss=1452; rttvar=12; dstport=123451
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 or OPTION * requests for a resource at the
same origin. This works with HTTP/2 and later as all requests to the
same origin utilise one TCP or QUIC connection. Whilst the HTTP
priorities can affect the allocation of capacity between streams, one
use-case for the Transport-Info header is for information regarding
sustained flows, such as media delivery, which tend consist of a
known limited number of flows to the same origin so the priorities
would not affect the calculations.
2.1. Utilisation of Transport-Info header metrics
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 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.
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.
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The calculation of the send rate maybe performed at the server, or
may be left to the client to calculate as and when required.
3. Server side 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 mile 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. Also
the Transport-Info MUST not be cached.
_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
tcp_info data structure which are available on Linux and BSD systems.
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
information.
4. Client side proxy considerations
In the case where a proxy services client requests, this proxy would
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 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
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o Applicable protocol: http
o Status: standard
o Author/Change Controller: IETF
o Specification document(s): [this document]
o Related information:
6. Security Considerations
The content of the Transport-Info is largely available through other
techniques such as packet capture so it should not lead to security
issues. Certain metrics, such as the cwnd, may be considered as less
visible but since they are part of the transport layer they can
inferred. Any metrics that may be considered private should not be
sent in the header, or sent only over an encrypted connection.
In the case where clients are connected via a proxy then
organisations may wish modify or drop the header if they consider the
it might reveal unwanted information to end clients.
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
[]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-14 (work in progress),
October 2019.
[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>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
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[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>.
[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>.
7.2. Informative References
[alpn-ids]
"Application-Layer Protocol Negotiation (ALPN) Protocol
ID", IANA , n.d., <http://www.iana.org/assignments/tls-
extensiontype-values>.
[exploring-mtu]
Custura, A., Fairhurst, G., and I. Learmonth, "Exploring
usable Path MTU in the Internet", Network Traffic
Measurement and Analysis Conference , April 2018,
<https://doi.org/10.23919/TMA.2018.8506538>.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-23 (work
in progress), September 2019.
[network-info-api]
Grigorik, I., "Network Information API", W3C , September
2019, <http://wicg.github.io/netinfo/>.
[RFC4898] Mathis, M., Heffner, J., and R. Raghunarayan, "TCP
Extended Statistics MIB", RFC 4898, DOI 10.17487/RFC4898,
May 2007, <https://www.rfc-editor.org/info/rfc4898>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[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,
<https://www.rfc-editor.org/info/rfc7230>.
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7.3. URIs
[1] https://github.com/bbc/draft-ohanlon-transport-info-header
Authors' Addresses
Piers O'Hanlon
British Broadcasting Corporation
Email: piers.ohanlon@bbc.co.uk
James Gruessing
British Broadcasting Corporation
Email: james.gruessing@bbc.co.uk
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