Discovery of path characteristics using STUN

The ICE mechanism uses a 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 further connectivity tests are stopped. This approach works well for an endpoint with a single interface, but is too simplistic for endpoints with multiple interfaces, wherein a candidate pair with a lower priority might infact have better path characteristics (e.g., round-trip time, loss, etc.). The ICE connectivity checks can assist in measuring the path characteristics, but as currently defined, the STUN responses to re-transmitted requests are indistinguishable from each other. This draft extends STUN to distinguish STUN responses to re-transmitted requests and this assists the client in determining the path characteristics like round-trip time (RTT) and packet loss in each direction between endpoints. These metrics can then be used by the controlling agent to influence the ICE candidate pair priorities. The PATH-CHARACTERISTICS attribute introduced in this specification can be used in ICE connectivity checks (STUN Binding request and response). When multiple TURN servers are discovered then this new attribute can also be used with Allocate request to determine the priority amongst the relayed candidates. This specification can be used with the regular nomination procedure defined in ICE, wherein ICE connectivity checks need to be performed on all or subset of the chosen candidate pairs. Finalizing an approporiate candidate pair based on the path characteristics depends on the number of candidate pairs, time interval for pacing ICE connectivity checks and the corresponding RTO values. By picking appropriate values the endpoints will not observe any noticeable impact to the media setup time. TODO: Add details of ICE continous nomination procedure discussed in mmusic WG which allows picking better candidate pairs as and when they appear. http://juberti.github.io/draughts/nombis/draft-uberti-mmusic-nombis.html explains simplifying and improving the procedures for candidate nomination in ICE to make dynamic decisions.

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 . This note uses terminology defined in ICE and STUN .

When multiple paths are available for communication, the endpoint sends ICE connectivity checks across each path (candidate pair) and perhaps chooses the path with the lowest round trip time. Choosing the path with the lowest round trip time is a reasonable approach, but re-transmits can cause an otherwise-good path to appear flawed. However, STUN's retransmission algorithm cannot determine the round-trip time (RTT) if a STUN request packet is re-transmitted, because each request and retransmission packet is identical. Further, several STUN requests may be sent before the connectivity between candidate pairs is ascertained (see Section 16 of ). To resolve the issue of identical request and response packets in a STUN transaction, this document changes that retransmission behavior for idempotent packets. In addition to determining RTT, it is also desirable to detect which path direction caused packet loss, described as "bi-directional path characteristics," below. This is achieved by defining a new STUN attribute and requires compliant STUN (TURN, ICE) endpoints to count request packets. This specification defines a new comprehension-optional STUN attribute PATH-CHARACTERISTIC. PATH-CHARACTERISTIC will have a STUN Type TBD-CA. This type is in the comprehension-optional range, which means that STUN agents can safely ignore the attribute if they do not understand it. If a client wishes to determine the path characteristics, it inserts the PATH-CHARACTERISTIC attribute in a STUN request. In the PATH-CHARACTERISTIC attribute client sends the number of times the STUN request is retransmitted with the same Transaction ID. The server would echo back the retransmission count in the response so that client can distinguish STUN responses from the re-transmitted 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 times it received the request with the same Transaction ID and the number of responses it has sent for the STUN request to the client. Further, this information enables the client to determine packet loss in each direction.

The PATH-CHARACTERISTIC attribute in a STUN request takes a 1-byte Value, which means that the Length is 1 and 3 bytes of padding are required after the value (Section 15 of ). When sending a STUN request, the PATH-CHARACTERISTIC attribute allows a client to indicate to the server that it wants to determine path characteristics. If the client receives a STUN response with error code 420 (Unknown Attribute) and PATH-CHARACTERISTIC is listed in the UNKNOWN-ATTRIBUTE attribute of the message, the client SHOULD retransmit the original request without the PATH-CHARACTERISTIC attribute. However this case is not expected to occur, due to the use of the comprehension-optional attribute type. This specification updates one the STUN message structuring rules explained in Section 6 of that resends of the same request reuse the same transaction ID and are bit-wise identical to the previous request. For idempotent packets the ReTransCnt in the PATH-CHARACTERISTIC attribute will be incremented by 1 for every re-transmission and the re-transmitted STUN request MUST be bit-wise identical to the previous request except for the ReTransCnt value. The format of the value in PATH-CHARACTERISTIC attribute in the request is:

The field is described below: Number of times request is re-transmitted with the same transaction ID to the server.

When a server receives a STUN request that includes a PATH-CHARACTERISTIC attribute, it processes the request as per the STUN specification plus the specific rules mentioned here. The server checks the following: If the PATH-CHARACTERISTIC 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 3.1 of . The server that supports PATH-CHARACTERISTIC attribute MUST echo back ReTransCnt in the response. If the server is stateless or does not want to remember the transaction ID then it would populate value 0 for the ReqTransCnt and RespTransCnt fields in PATH-CHARACTERISTIC attribute sent in the response .If the server is stateful then it populates ReqTransCnt with the number of times it received the STUN request with the same transaction ID and RespTransCnt with the number of responses it has sent for the STUN request.

The fields are described below: Copied from request. Number of times request is received from the client with the same transaction ID. Number of responses sent to the client for the same transaction ID.

The operation is described in . In the first case, all the requests and responses are received correctly. In the upstream loss case, the first request is lost, but the second one is received correctly, the client on receiving the response notes that while 2 requests were sent, only one was received by the server, also the server realizes that the RespTransCnt does not match the ReTransCnt, therefore 1 request was lost. This may also occur at startup in the presence firewalls or NATs that block unsolicited incoming traffic. In the downstream loss case, the responses get lost, client expecting multiple response notes that while the server responded to 3 requests but only 1 response was received. In the both loss case, requests and responses get lost in tandem, the server notes one request packet was not received, while the client expecting 3 responses received only one, it notes that one request and response packets were lost.

The STUN attribute defined in this specification can be used by applications in the following scenarios: When an endpoint has multiple interfaces (for example 3G, 4G, WiFi, VPN, etc.), an ICE agent can choose the interfaces for media streams according to the path characteristics. After STUN responses to STUN checks are received, the ICE agent using regular nomination can sort the ICE candidate pairs according to the path characteristics (loss and RTT) discovered using STUN. The controlling agent can then assign the highest priority to candidate pair which best fulfills the desired path characteristics. However, it should be noted that the path capacity or throughput is not determined by these STUN checks. If an endpoint needs to pick paths based on capacity, it would have to send media on those paths. When a host has multiple interfaces available an MPRTP application can choose the interfaces for the corresponding subflows according to the path characteristics discovered using STUN. For example, the scheduling algorithm described in uses path capacity, latency, and loss rate for choosing the most suitable subset of paths. The STUN extension proposed in this specification can also be used to choose a TURN server that provides the best user experience (section 3.1 of ).

[Paragraphs in braces should be removed by the RFC Editor upon publication] [The PATH-CHARACTERISTIC attribute requires that IANA allocate a value in the "STUN attributes Registry" from the comprehension-optional range (0x8000-0xFFFF), to be replaced for TBD-CA throughout this document] This document defines the PATH-CHARACTERISTIC STUN attribute, described in . IANA has allocated the comprehension-optional codepoint TBD-CA for this attribute.

Security considerations discussed in are to be taken into account. STUN requires the 96 bits transaction ID to 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 PATH-CHARACTERISTIC attribute to mislead the client and server, this attack can be mitigated using STUN authentication. As PATH-CHARACTERISTIC is expected to be used between peers using ICE, and ICE uses STUN short-term credential mechanism the risk of on-path attack influencing the messages is minimal. However, an attacker could corrupt, remove, or delay an ICE request or response, in order to discourage that path from being used. Unauthenticated STUN message MUST NOT include the PATH-CHARACTERISTIC attribute in order to prevent on-path attacker from influencing decision-making.

Thanks to Brandon Williams, Simon Perreault, Aijun Wang for valuable inputs and comments.