Internet Engineering Task Force (IETF)
Request for Comments: 7982
Category: Standards Track
ISSN: 2070-1721
P. Martinsen
T. Reddy
D. Wing
V. Singh
September 2016

Measurement of Round-Trip Time and Fractional Loss

Using Session Traversal Utilities for NAT (STUN)


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

Copyright Notice

Copyright © 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 ( 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.


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:

  • 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].
  • The server that supports the TRANSACTION_TRANSMIT_COUNTER attribute MUST echo back the Req field in the response using a TRANSACTION_TRANSMIT_COUNTER attribute.
  • 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,
   [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,
   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
   [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,

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,


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


   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103


   Dan Wing
   Varun Singh
   Runeberginkatu 4c A 4
   Helsinki  00100