Measurement of Round Trip
Time and Fractional Loss Using STUNCisco Systems, Inc.Philip Pedersens vei 22LysakerAkershus1325Norwaypalmarti@cisco.comCisco Systems, Inc.Cessna Business Park, Varthur HobliSarjapur Marathalli Outer Ring RoadBangaloreKarnataka560103Indiatireddy@cisco.comCisco Systems, Inc.170 West Tasman DriveSan JoseCalifornia95134USAdwing@cisco.com
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TRAM
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.
This document extends STUN to
make it possible to correlate STUN responses to specific
request when re-transmits 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 can be used in ICE connectivity checks (STUN Binding
request and response). It can also be used with TURN by adding this attribute to
Allocate requests and responses to measure loss and RTT
between the client and respective TURN server.
ICE is a mechanism commonly used in 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 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 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 are
ascertained (see Section 16 of ). To resolve the issue of identical
request and response packets in a STUN transaction, this
document changes the retransmission behavior for idempotent
packets. In addition to determining RTT, it is also possible
to get a hint regarding which path direction caused packet
loss. This is achieved by defining a new STUN attribute and
requires compliant STUN (TURN, 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 actually will use this information to
improve the active candidate pair selection is outside the
scope of this document.
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 specification uses terminology defined in ICE and STUN .
This document defines a new comprehension-optional STUN
attribute TRANSACTION_TRANSMIT_COUNTER with a STUN Type
TBD-CA. 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 fallback 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 client can distinguish between STUN responses coming
from 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 responses it
has sent for the STUN request to the client. Further, this
information enables the client to get a hint regarding what
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 wherein retransmit of the same request
reuse the same transaction ID and are 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
re-transmitted 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 TBD-CA.
The format of the value in TRANSACTION_TRANSMIT_COUNTER
attribute in the request is:
The fields is described below:
Number of times request is
transmitted with the same transaction ID to the
server.
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 0.
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 of 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. Initial value for the first request sent is
therefore 1. The first re-transmit will set the value to 2
and so on.
The Resp filed in the attribute MUST be set to zero in the
request.
When a server receives a STUN request that includes a
TRANSACTION_TRANSMIT_COUNTER attribute, it processes the
request as per the STUN
protocol 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
3.1 of .
The server that supports 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 would populate value 0 for
the Resp field in 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 request with that same
transaction id the value will reflect how many packets the
server have seen and responded to. This gives the client a
hint of which direction loss occurred. See section for more details.
Example operation, when a server is stateful, 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. The server also
realizes that the value in the Req field does not match the
number of received requests, 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 responses, 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.
Another example could be 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 TRANSACTION_TRANSMIT_COUNTER attribute sent in response
to the second request and value 2 for the Resp field
in 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.
[Paragraphs in braces should be removed by the RFC Editor
upon publication][The TRANSACTION_TRANSMIT_COUNTER attribute requires that IANA
allocate a value in the "STUN attributes Registry" from the
comprehension-optional range (0x8000-0xBFFF), to be replaced for
TBD-CA throughout this document]This document defines the TRANSACTION_TRANSMIT_COUNTER 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
TRANSACTION_TRANSMIT_COUNTER attribute to mislead the client
and server. This attack can be mitigated using STUN
authentication. As TRANSACTION_TRANSMIT_COUNTER 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. If TRANSACTION_TRANSMIT_COUNTER is
used with Allocate request then STUN long-term credential
mechanism or STUN Extension for Third-Party Authorization
or (D)TLS connection can be
used between the TURN client and the TURN server to prevent
attackers from trying to impersonate a TURN server and sending
bogus TRANSACTION_TRANSMIT_COUNTER attribute in the Allocate
response. However, an attacker could corrupt, remove, or delay
an ICE request or response, in order to discourage that path
from being used.
The information sent in any STUN packet if not encrypted can
potentially be observed passively and used for reconnaissance
and later attacks.
Thanks to Brandon Williams, Simon Perreault, Aijun Wang,
Martin Thomson, Oleg Moskalenko, Ram Mohan R, Spencer Dawkins,
Suresh Krishnan, Ben Campbell, Mirja Kuhlewind, Lionel Morand, Kathleen Moriarty
and Alissa Cooper for valuable inputs and comments.