STUN RTT & Loss: Measurement of Round-Trip Time and Fractional Loss Using Session Traversal Utilities for NAT (STUN)
A small STUN attribute that counts retransmissions so endpoints can measure round-trip time and see which direction lost packets.
Martinsen, P., Reddy, T., Wing, D., and V. Singh, "Measurement of Round-Trip Time and Fractional Loss Using Session Traversal Utilities for NAT (STUN)", RFC 7982, DOI 10.17487/RFC7982, September 2016, <https://www.rfc-editor.org/info/rfc7982>.
@misc{rfc7982,
series = {Request for Comments},
number = 7982,
howpublished = {RFC 7982},
publisher = {RFC Editor},
doi = {10.17487/RFC7982},
url = {https://www.rfc-editor.org/info/rfc7982},
author = {P. Martinsen and T. Reddy and D. Wing and V. Singh},
title = {{Measurement of Round-Trip Time and Fractional Loss Using Session Traversal Utilities for NAT (STUN)}},
pagetotal = 10,
year = 2016,
month = sep,
} RFC text
Contents
Internet Engineering Task Force (IETF) P. Martinsen
Request for Comments: 7982 T. Reddy
Category: Standards Track Cisco
ISSN: 2070-1721 D. Wing
V. Singh
callstats.io
September 2016
Measurement of Round-Trip Time and Fractional Loss
Using Session Traversal Utilities for NAT (STUN)
Abstract
A host with multiple interfaces needs to choose the best interface
for communication. Oftentimes, this decision is based on a static
configuration and does not consider the path characteristics, which
may affect the user experience.
This document describes a mechanism for an endpoint to measure the
path characteristics fractional loss and RTT using Session Traversal
Utilities for NAT (STUN) messages.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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
http://www.rfc-editor.org/info/rfc7982.
Copyright Notice
Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 4
3. Measuring RTT and Fractional Loss . . . . . . . . . . . . . . 4
3.1. TRANSACTION_TRANSMIT_COUNTER Attribute . . . . . . . . . 4
3.2. Usage in Requests . . . . . . . . . . . . . . . . . . . . 5
3.3. Usage in Responses . . . . . . . . . . . . . . . . . . . 5
3.4. Example Operation . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
This document extends STUN [RFC5389] to make it possible to correlate
STUN responses to specific requests when retransmits occur. This
assists the client in determining path characteristics like round-
trip time (RTT) and fractional packet loss.
The TRANSACTION_TRANSMIT_COUNTER attribute introduced in Section 3.1
can be used in Interactive Connectivity Establishment (ICE) [RFC5245]
connectivity checks (STUN Binding request and response). It can also
be used with Traversal Using Relays around NAT (TURN) [RFC5766] by
adding this attribute to Allocate requests and responses to measure
loss and RTT between the client and the respective TURN server.
ICE is a mechanism commonly used in Voice over IP (VoIP) applications
to traverse NATs, and it uses a static prioritization formula to
order the candidate pairs and perform connectivity checks, in which
the most preferred address pairs are tested first, and when a
sufficiently good pair is discovered, that pair is used for
communications and then further connectivity tests are stopped.
When multiple paths are available for communication, the endpoint
sends ICE connectivity checks across each path (candidate pair).
Choosing the path with the lowest round-trip time is a reasonable
approach, but retransmits can cause an otherwise good path to appear
flawed. However, STUN's retransmission algorithm [RFC5389] cannot
determine the round-trip time (RTT) if a STUN request packet is
retransmitted because each request and retransmission packet is
identical. Further, several STUN requests may be sent before the
connectivity between candidate pairs are ascertained (see Section 16
of [RFC5245]). To resolve the issue of identical request and
response packets in a STUN transaction, this document changes the
retransmission behavior for idempotent packets. Using the mechanism
described herein, a client can determine RTT as well as get a hint
regarding which path direction caused packet loss. This is achieved
by defining a new STUN attribute and requires compliant STUN (TURN
and ICE) endpoints to count request packets.
The mechanisms described in this document can be used by the
controlling agent to influence the ICE candidate pair selection. How
ICE will actually use this information to improve the active
candidate pair selection is outside the scope of this document.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification uses terminology defined in ICE [RFC5245] and STUN
[RFC5389].
3. Measuring RTT and Fractional Loss
This document defines a new comprehension-optional STUN attribute
TRANSACTION_TRANSMIT_COUNTER with a STUN Type 0x8025. This type is
in the comprehension-optional range, which means that STUN agents can
safely ignore the attribute. If ICE is in use, it will fall back to
normal procedures.
If a client wishes to measure RTT, it inserts the
TRANSACTION_TRANSMIT_COUNTER attribute in a STUN request. In this
attribute, the client sends the number of times the STUN request is
transmitted with the same transaction ID. The server would echo back
the transmission count in the response so that the client can
distinguish between STUN responses coming from retransmitted
requests. Hence, the endpoint can use the STUN requests and
responses to determine the round-trip time (RTT). The server may
also convey the number of responses it has sent for the STUN request
to the client. Further, this information enables the client to get a
hint regarding in which direction the packet loss occurred. In some
cases, it is impossible to distinguish between packet reordering and
packet loss. However, if this information is collected as network
metrics from several clients over a longer time period, it will be
easier to detect a pattern that can provide useful information.
3.1. TRANSACTION_TRANSMIT_COUNTER Attribute
The TRANSACTION_TRANSMIT_COUNTER attribute in a STUN request takes a
32-bit value. This document updates one of the STUN message
structuring rules explained in Section 6 of [RFC5389] wherein
retransmission of the same request reuses the same transaction ID and
is bit-wise identical to the previous request. For idempotent
packets, the Req and Resp fields in the TRANSACTION_TRANSMIT_COUNTER
attribute will be incremented by 1 by the client or server for every
transmission with the same transaction ID. Any retransmitted STUN
request MUST be bit-wise identical to the previous request except for
the values in the TRANSACTION_TRANSMIT_COUNTER attribute.
The IANA-assigned STUN type for the new attribute is 0x8025.
The format of the value in the TRANSACTION_TRANSMIT_COUNTER attribute
in the request is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved (Padding) | Req | Resp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: TRANSACTION_TRANSMIT_COUNTER Attribute in Request
The fields are described below:
Req: Number of times the request is transmitted with the same
transaction ID to the server.
Resp: Number of times a response with the same transaction ID is
sent from the server. MUST be set to zero in requests and ignored
by the receiver.
The padding is necessary to hit the 32-bit boundary needed for STUN
attributes. The padding bits are ignored, but to allow for future
reuse of these bits, they MUST be set to zero.
3.2. Usage in Requests
When sending a STUN request, the TRANSACTION_TRANSMIT_COUNTER
Attribute allows a client to indicate to the server that it wants to
measure RTT and get a hint about the direction of any packet loss.
The client MUST populate the Req value in the
TRANSACTION_TRANSMIT_COUNTER. This value MUST reflect the number of
requests that have been transmitted to the server. Therefore, the
initial value for the first request sent is 1. The first retransmit
will set the value to 2 and so on.
The Resp field in the attribute MUST be set to zero in the request.
3.3. Usage in Responses
When a server receives a STUN request that includes a
TRANSACTION_TRANSMIT_COUNTER attribute, it processes the request as
per the STUN protocol [RFC5389] plus the specific rules mentioned
here. The server checks the following:
o If the TRANSACTION_TRANSMIT_COUNTER attribute is not recognized,
ignore the attribute because its type indicates that it is
comprehension-optional. This should be the existing behavior as
explained in Section 7.3 of [RFC5389].
o The server that supports the TRANSACTION_TRANSMIT_COUNTER
attribute MUST echo back the Req field in the response using a
TRANSACTION_TRANSMIT_COUNTER attribute.
o If the server is stateless or does not want to remember the
transaction ID, then it populates value 0 for the Resp field in
the TRANSACTION_TRANSMIT_COUNTER attribute sent in the response.
If the server is stateful, then it populates the Resp field with
the number of responses it has sent for the STUN request.
A client that receives a STUN response with a
TRANSACTION_TRANSMIT_COUNTER can check the values in the Req field to
accurately calculate the RTT if retransmits are occurring.
If the server sending the STUN response is stateless, the value of
the Resp field will always be 0. If the server keeps state of the
numbers of STUN requests with that same transaction ID, the value
will reflect how many packets the server has seen and responded to.
This gives the client a hint about in which direction loss occurred.
See Section 3.4 for more details.
3.4. Example Operation
An example operation, when a server is stateful, is described in
Figure 2. In the first case, all the requests and responses are
received correctly.
In the case of upstream loss, the first request is lost, but the
second one is received correctly. The client, upon receiving the
response, notes that while two requests were sent, only one was
received by the server. The server also realizes that the value in
the Req field does not match the number of received requests,
therefore one request was lost. This may also occur at startup in
the presence of firewalls or NATs that block unsolicited incoming
traffic.
In the case of downstream loss, the responses get lost, the client
expecting multiple responses notes that, while the server responded
to three requests, only one response was received.
In the case of loss in both directions, requests and responses get
lost in tandem, the server notes that one request packet was not
received, while the client expecting three responses received only
one, and then it notes that one request and response packet were
lost.
| Normal | Upstream loss | Downstream loss | Both upstream &|
| | | | downstream loss|
| Client Server | Client Server | Client Server | Client Server |
|+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|
| 1 1,1 | 1 x | 1 1,1 | 1 x |
| 1,1 | | x | |
| | 2 2,1 | 2 2,2 | 2 2,1 |
| | 2,1 | x | x |
| | | 3 3,3 | 3 3,2 |
| | | 3,3 | 3,2 |
Figure 2: Retransmit Operation between Client and Server
Another example is when the client sends two requests but the second
request arrives at the server before the first request because of
out-of-order delivery. In this case, the stateful server populates
value 1 for the Resp field in the TRANSACTION_TRANSMIT_COUNTER
attribute sent in response to the second request and value 2 for the
Resp field in the TRANSACTION_TRANSMIT_COUNTER attribute sent in
response to the first request.
The intention with this mechanism is not to carry out comprehensive
and accurate measurements regarding in what direction loss is
occurring. In some cases, it might not be able to distinguish the
difference between downstream loss and packet reordering.
4. IANA Considerations
This document defines the TRANSACTION_TRANSMIT_COUNTER STUN
attribute, described in Section 3. IANA has allocated the
comprehension-optional codepoint 0x8025 for this attribute.
5. Security Considerations
Security considerations discussed in [RFC5389] are to be taken into
account. STUN requires that the 96-bit transaction ID be uniformly
and randomly chosen from the interval 0 .. 2**96-1, and be
cryptographically strong. This is good enough security against an
off-path attacker. An on-path attacker can either inject a fake
response or modify the values in the TRANSACTION_TRANSMIT_COUNTER
attribute to mislead the client and server. This attack can be
mitigated using STUN authentication. As the
TRANSACTION_TRANSMIT_COUNTER is expected to be used between peers
using ICE, and ICE uses a STUN short-term credential mechanism, the
risk of an on-path attack influencing the messages is minimal. If
the TRANSACTION_TRANSMIT_COUNTER is used with an Allocate request,
one of the following mechanisms can be used to prevent attackers from
trying to impersonate a TURN server and sending a bogus
TRANSACTION_TRANSMIT_COUNTER attribute in the Allocate response:
1) the STUN long-term credential mechanism, 2) the STUN Extension for
Third-Party Authorization [RFC7635], or 3) a TLS or DTLS connection
between the TURN client and the TURN server. However, an attacker
could corrupt, remove, or delay an ICE request or response, in order
to discourage that path from being used.
If not encrypted, the information sent in any STUN packet can
potentially be observed passively and used for reconnaissance and
later attacks.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
DOI 10.17487/RFC5245, April 2010,
<http://www.rfc-editor.org/info/rfc5245>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<http://www.rfc-editor.org/info/rfc5389>.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, April 2010,
<http://www.rfc-editor.org/info/rfc5766>.
6.2. Informative References
[RFC7635] Reddy, T., Patil, P., Ravindranath, R., and J. Uberti,
"Session Traversal Utilities for NAT (STUN) Extension for
Third-Party Authorization", RFC 7635,
DOI 10.17487/RFC7635, August 2015,
<http://www.rfc-editor.org/info/rfc7635>.
Acknowledgements
Thanks to Brandon Williams, Simon Perreault, Aijun Wang, Martin
Thomson, Oleg Moskalenko, Ram Mohan Ravindranath, Spencer Dawkins,
Suresh Krishnan, Ben Campbell, Mirja Kuehlewind, Lionel Morand,
Kathleen Moriarty, and Alissa Cooper for their valuable input and
comments.
Authors' Addresses
Paal-Erik Martinsen
Cisco Systems, Inc.
Philip Pedersens vei 22
Lysaker, Akershus 1325
Norway
Email: palmarti@cisco.com
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
Dan Wing
Email: dwing-ietf@fuggles.com
Varun Singh
CALLSTATS I/O Oy
Runeberginkatu 4c A 4
Helsinki 00100
Finland
Email: varun@callstats.io
URI: https://www.callstats.io/about
AI ELI5
On this page
§1Why STUN Needs Transmit Counters
RFC 7982 extends STUN (RFC 5389) so that a client can correlate STUN responses with specific requests when retransmits occur. This helps the client work out path characteristics such as round-trip time (RTT) and fractional packet loss.
The motivation is ICE (RFC 5245), which sends connectivity checks over each candidate pair and uses a static prioritization formula. Choosing the path with the lowest RTT is reasonable, but retransmits can make an otherwise good path look flawed. STUN's retransmission algorithm cannot determine RTT when a request is retransmitted, because each request and retransmission is identical.
The attribute can be used in ICE connectivity checks (STUN Binding request and response) and in TURN Allocate requests and responses (RFC 5766) to measure loss and RTT to a TURN server. How ICE uses the numbers to improve candidate pair selection is outside the scope of the document.
When a message is sent again and again, every copy looks the same, so you cannot tell which copy got the reply. This document puts a counter on each copy so you can.
§2The TRANSACTION_TRANSMIT_COUNTER Attribute
The document defines a new comprehension-optional STUN attribute, TRANSACTION_TRANSMIT_COUNTER, with STUN type 0x8025. Because the type is in the comprehension-optional range, STUN agents that do not know it can safely ignore it, and ICE falls back to normal procedures.
The attribute value is 32 bits. A 16-bit Reserved (Padding) field is followed by two 8-bit counters:
Req: the number of times the request has been transmitted with the same transaction ID.Resp: the number of times a response with the same transaction ID has been sent by the server. It MUST be set to zero in requests and is ignored by the receiver.
The padding exists to reach the 32-bit boundary needed for STUN attributes. Receivers ignore the padding bits, but senders MUST set them to zero so the bits can be reused later.
This changes one STUN message structuring rule from RFC 5389, under which a retransmission is bit-wise identical to the previous request. Now a retransmitted request MUST be bit-wise identical except for the values in this attribute.
The attribute is a tiny tag with two numbers on it: how many times the question was asked, and how many times the answer was given. A few spare bits are left empty on purpose.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
RESERVED | Reserved (Padding) | 2 bytes | Ignored by receivers; MUST be set to zero. | |
REQ | Req | 1 byte | Times the request has been sent with the same transaction ID. | |
RESP | Resp | 1 byte | Responses sent by the server; MUST be zero in requests. |
§3Client and Server Rules
A client that wants to measure RTT adds the attribute to its STUN request. It MUST fill in Req with the number of requests transmitted to the server so far, so the first request carries 1, the first retransmit carries 2, and so on. Resp MUST be zero.
A server that supports the attribute processes the request as usual under RFC 5389 and MUST echo the Req field back in a TRANSACTION_TRANSMIT_COUNTER attribute in the response. A server that does not recognize the attribute ignores it, which is the existing comprehension-optional behavior.
The Resp field depends on whether the server keeps state:
- A stateless server, or one that does not want to remember the transaction ID, sets
Respto 0. - A stateful server sets
Respto the number of responses it has sent for that request.
Using the echoed Req, the client can calculate RTT accurately even when retransmits occur. A nonzero Resp also tells the client how many requests the server has seen and answered, which hints at the direction in which loss occurred.
The asker numbers every try. The answerer repeats the number back, so the asker knows exactly which try the answer is for. A careful answerer also says how many answers it has given.
§4Example: Spotting Where Packets Are Lost
The RFC walks through four cases with a stateful server. In the normal case every request and response arrives. In the other cases the two counters disagree in a telling way:
- Upstream loss: the first request is lost and the second arrives. The client sent two requests but the response shows the server saw only one.
- Downstream loss: the server responded to three requests, but the client received only one response.
- Loss in both directions: the server notes that one request never arrived, and the client expecting three responses received only one.
Upstream loss may also occur at startup when firewalls or NATs block unsolicited incoming traffic. In a reordering example, the second request reaches the server first, so the stateful server answers it with Resp 1 and answers the first request with Resp 2.
The RFC says the goal is not comprehensive or accurate loss measurement. In some cases downstream loss cannot be told apart from reordering, but collecting the metrics from several clients over a longer period makes patterns easier to see.
Counting how many tries were sent and how many were heard lets you guess whether your voice did not carry or the reply got lost on the way back.
- Request, Req=1 (lost)The first request never reaches the server.
- Request, Req=2The retransmit arrives.
- Response, Req=2 Resp=1The server has seen and answered one request, though two were sent.
§5Security and IANA Considerations
IANA has allocated the comprehension-optional codepoint 0x8025 for the attribute. For security, the considerations of RFC 5389 apply. STUN requires the 96-bit transaction ID to be uniformly random and cryptographically strong, which is good enough against an off-path attacker.
An on-path attacker can inject a fake response or modify the counter values to mislead the client and server. STUN authentication mitigates this, and since ICE uses the STUN short-term credential mechanism, the risk is judged minimal for ICE peers.
If the attribute is used with a TURN Allocate request, impersonation of the server can be prevented with the STUN long-term credential mechanism, third-party authorization (RFC 7635), or a TLS or DTLS connection. An attacker could still corrupt, remove or delay an ICE request or response to discourage a path. Unencrypted STUN packets can be observed passively and used for reconnaissance.
Someone sitting in the middle could scribble on the counters to fool both sides, so the messages should be signed or sent privately.