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RFC 8445

Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal

Pages: 100
Proposed STD
Obsoletes:  5245
Part 6 of 6 – Pages 89 to 100
First   Prev   None

Top   ToC   Page 89   prevText
Appendix A.  Lite and Full Implementations

   ICE allows for two types of implementations.  A full implementation
   supports the controlling and controlled roles in a session and can
   also perform address gathering.  In contrast, a lite implementation
   is a minimalist implementation that does little but respond to STUN
   checks, and it only supports the controlled role in a session.

   Because ICE requires both endpoints to support it in order to bring
   benefits to either endpoint, incremental deployment of ICE in a
   network is more complicated.  Many sessions involve an endpoint that
   is, by itself, not behind a NAT and not one that would worry about
   NAT traversal.  A very common case is to have one endpoint that
   requires NAT traversal (such as a VoIP hard phone or soft phone) make
   a call to one of these devices.  Even if the phone supports a full
   ICE implementation, ICE won't be used at all if the other device
   doesn't support it.  The lite implementation allows for a low-cost
   entry point for these devices.  Once they support the lite
   implementation, full implementations can connect to them and get the
   full benefits of ICE.

   Consequently, a lite implementation is only appropriate for devices
   that will *always* be connected to the public Internet and have a
   public IP address at which it can receive packets from any
   correspondent.  ICE will not function when a lite implementation is
   placed behind a NAT.

   ICE allows a lite implementation to have a single IPv4 host candidate
   and several IPv6 addresses.  In that case, candidate pairs are
   selected by the controlling agent using a static algorithm, such as
   the one in RFC 6724, which is recommended by this specification.
   However, static mechanisms for address selection are always prone to
   error, since they can never reflect the actual topology or provide
   actual guarantees on connectivity.  They are always heuristics.
   Consequently, if an ICE agent is implementing ICE just to select
   between its IPv4 and IPv6 addresses, and none of its IP addresses are
   behind NAT, usage of full ICE is still RECOMMENDED in order to
   provide the most robust form of address selection possible.

   It is important to note that the lite implementation was added to
   this specification to provide a stepping stone to full
   implementation.  Even for devices that are always connected to the
   public Internet with just a single IPv4 address, a full
   implementation is preferable if achievable.  Full implementations
   also obtain the security benefits of ICE unrelated to NAT traversal.
   Finally, it is often the case that a device that finds itself with a
   public address today will be placed in a network tomorrow where it
   will be behind a NAT.  It is difficult to definitively know, over the
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   lifetime of a device or product, if it will always be used on the
   public Internet.  Full implementation provides assurance that
   communications will always work.

Appendix B.  Design Motivations

   ICE contains a number of normative behaviors that may themselves be
   simple but derive from complicated or non-obvious thinking or use
   cases that merit further discussion.  Since these design motivations
   are not necessary to understand for purposes of implementation, they
   are discussed here.  This appendix is non-normative.

B.1.  Pacing of STUN Transactions

   STUN transactions used to gather candidates and to verify
   connectivity are paced out at an approximate rate of one new
   transaction every Ta milliseconds.  Each transaction, in turn, has a
   retransmission timer RTO that is a function of Ta as well.  Why are
   these transactions paced, and why are these formulas used?

   Sending of these STUN requests will often have the effect of creating
   bindings on NAT devices between the client and the STUN servers.
   Experience has shown that many NAT devices have upper limits on the
   rate at which they will create new bindings.  Discussions in the IETF
   ICE WG during the work on this specification concluded that once
   every 5 ms is well supported.  This is why Ta has a lower bound of
   5 ms.  Furthermore, transmission of these packets on the network
   makes use of bandwidth and needs to be rate limited by the ICE agent.
   Deployments based on earlier draft versions of [RFC5245] tended to
   overload rate-constrained access links and perform poorly overall, in
   addition to negatively impacting the network.  As a consequence, the
   pacing ensures that the NAT device does not get overloaded and that
   traffic is kept at a reasonable rate.

   The definition of a "reasonable" rate is that STUN MUST NOT use more
   bandwidth than the RTP itself will use, once data starts flowing.
   The formula for Ta is designed so that, if a STUN packet were sent
   every Ta seconds, it would consume the same amount of bandwidth as
   RTP packets, summed across all data streams.  Of course, STUN has
   retransmits, and the desire is to pace those as well.  For this
   reason, RTO is set such that the first retransmit on the first
   transaction happens just as the first STUN request on the last
   transaction occurs.  Pictorially:
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              First Packets              Retransmits

                    |                        |
                    |                        |
             -------+------           -------+------
            /               \        /               \
           /                 \      /                 \

           +--+    +--+    +--+    +--+    +--+    +--+
           |A1|    |B1|    |C1|    |A2|    |B2|    |C2|
           +--+    +--+    +--+    +--+    +--+    +--+

        ---+-------+-------+-------+-------+-------+------------ Time
           0       Ta      2Ta     3Ta     4Ta     5Ta

   In this picture, there are three transactions that will be sent (for
   example, in the case of candidate gathering, there are three host
   candidate/STUN server pairs).  These are transactions A, B, and C.
   The retransmit timer is set so that the first retransmission on the
   first transaction (packet A2) is sent at time 3Ta.

   Subsequent retransmits after the first will occur even less
   frequently than Ta milliseconds apart, since STUN uses an exponential
   backoff on its retransmissions.

   This mechanism of a global minimum pacing interval of 5 ms is not
   generally applicable to transport protocols, but it is applicable to
   ICE based on the following reasoning.

   o  Start with the following rules that would be generally applicable
      to transport protocols:

      1.  Let MaxBytes be the maximum number of bytes allowed to be
          outstanding in the network at startup, which SHOULD be 14600,
          as defined in Section 2 of [RFC6928].

      2.  Let HTO be the transaction timeout, which SHOULD be 2*RTT if
          RTT is known or 500 ms otherwise.  This is based on the RTO
          for STUN messages from [RFC5389] and the TCP initial RTO,
          which is 1 sec in [RFC6298].

      3.  Let MinPacing be the minimum pacing interval between
          transactions, which is 5 ms (see above).
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   o  Observe that agents typically do not know the RTT for ICE
      transactions (connectivity checks in particular), meaning that HTO
      will almost always be 500 ms.

   o  Observe that a MinPacing of 5 ms and HTO of 500 ms gives at most
      100 packets/HTO, which for a typical ICE check of less than 120
      bytes means a maximum of 12000 outstanding bytes in the network,
      which is less than the maximum expressed by rule 1.

   o  Thus, for ICE, the rule set reduces to just the MinPacing rule,
      which is equivalent to having a global Ta value.
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B.2.  Candidates with Multiple Bases

   Section 5.1.3 talks about eliminating candidates that have the same
   transport address and base.  However, candidates with the same
   transport addresses but different bases are not redundant.  When can
   an ICE agent have two candidates that have the same IP address and
   port but different bases?  Consider the topology of Figure 11:

          | STUN Srvr|
           //     \\
          |         |
         |  B:net10  |
          |         |
           \\     //
          |   NAT    |
           //     \\
          |    A    |
         |192.168/16 |
          |         |
           \\     //
               |      -----
          +----------+           //     \\             +----------+
          |          |          |         |            |          |
          | Initiator|---------|  C:net10  |-----------| Responder|
          |          ||         | |          |
          +----------+           \\     //             +----------+

           Figure 11: Identical Candidates with Different Bases
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   In this case, the initiating agent is multihomed.  It has one IP
   address,, on network C, which is a net 10 private network.
   The responding agent is on this same network.  The initiating agent
   is also connected to network A, which is 192.168/16, and has an IP
   address of  There is a NAT on this network, natting
   into network B, which is another net 10 private network, but it is
   not connected to network C.  There is a STUN server on network B.

   The initiating agent obtains a host candidate on its IP address on
   network C ( and a host candidate on its IP address on
   network A (  It performs a STUN query to its
   configured STUN server from  This query passes
   through the NAT, which happens to assign the binding
   The STUN server reflects this in the STUN Binding response.  Now, the
   initiating agent has obtained a server-reflexive candidate with a
   transport address that is identical to a host candidate
   (  However, the server-reflexive candidate has a
   base of, and the host candidate has a base of

B.3.  Purpose of the Related-Address and Related-Port Attributes

   The candidate attribute contains two values that are not used at all
   by ICE itself -- related address and related port.  Why are they

   There are two motivations for its inclusion.  The first is
   diagnostic.  It is very useful to know the relationship between the
   different types of candidates.  By including it, an ICE agent can
   know which relayed candidate is associated with which reflexive
   candidate, which in turn is associated with a specific host
   candidate.  When checks for one candidate succeed but not for others,
   this provides useful diagnostics on what is going on in the network.

   The second reason has to do with off-path Quality-of-Service (QoS)
   mechanisms.  When ICE is used in environments such as PacketCable
   2.0, proxies will, in addition to performing normal SIP operations,
   inspect the SDP in SIP messages and extract the IP address and port
   for data traffic.  They can then interact, through policy servers,
   with access routers in the network, to establish guaranteed QoS for
   the data flows.  This QoS is provided by classifying the RTP traffic
   based on 5-tuple and then providing it a guaranteed rate, or marking
   its DSCP appropriately.  When a residential NAT is present, and a
   relayed candidate gets selected for data, this relayed candidate will
   be a transport address on an actual TURN server.  That address says
   nothing about the actual transport address in the access router that
   would be used to classify packets for QoS treatment.  Rather, the
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   server-reflexive candidate towards the TURN server is needed.  By
   carrying the translation in the SDP, the proxy can use that transport
   address to request QoS from the access router.

B.4.  Importance of the STUN Username

   ICE requires the usage of message integrity with STUN using its
   short-term credential functionality.  The actual short-term
   credential is formed by exchanging username fragments in the
   candidate exchange.  The need for this mechanism goes beyond just
   security; it is actually required for correct operation of ICE in the
   first place.

   Consider ICE agents L, R, and Z.  L and R are within private
   enterprise 1, which is using  Z is within private
   enterprise 2, which is also using  As it turns out, R and
   Z both have IP address  L sends candidates to Z.  Z
   responds to L with its host candidates.  In this case, those
   candidates are and  As it turns out, R
   is in a session at that same time and is also using and as host candidates.  This means that R is prepared to
   accept STUN messages on those ports, just as Z is.  L will send a
   STUN request to and another to  However,
   these do not go to Z as expected.  Instead, they go to R!  If R just
   replied to them, L would believe it has connectivity to Z, when in
   fact it has connectivity to a completely different user, R.  To fix
   this, STUN short-term credential mechanisms are used.  The username
   fragments are sufficiently random; thus it is highly unlikely that R
   would be using the same values as Z.  Consequently, R would reject
   the STUN request since the credentials were invalid.  In essence, the
   STUN username fragments provide a form of transient host identifiers,
   bound to a particular session established as part of the candidate

   An unfortunate consequence of the non-uniqueness of IP addresses is
   that, in the above example, R might not even be an ICE agent.  It
   could be any host, and the port to which the STUN packet is directed
   could be any ephemeral port on that host.  If there is an application
   listening on this socket for packets, and it is not prepared to
   handle malformed packets for whatever protocol is in use, the
   operation of that application could be affected.  Fortunately, since
   the ports exchanged are ephemeral and usually drawn from the dynamic
   or registered range, the odds are good that the port is not used to
   run a server on host R, but rather is the agent side of some
   protocol.  This decreases the probability of hitting an allocated
   port, due to the transient nature of port usage in this range.
   However, the possibility of a problem does exist, and network
   deployers need to be prepared for it.  Note that this is not a
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   problem specific to ICE; stray packets can arrive at a port at any
   time for any type of protocol, especially ones on the public
   Internet.  As such, this requirement is just restating a general
   design guideline for Internet applications -- be prepared for unknown
   packets on any port.

B.5.  The Candidate Pair Priority Formula

   The priority for a candidate pair has an odd form.  It is:

      pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0)

   Why is this?  When the candidate pairs are sorted based on this
   value, the resulting sorting has the MAX/MIN property.  This means
   that the pairs are first sorted based on decreasing value of the
   minimum of the two priorities.  For pairs that have the same value of
   the minimum priority, the maximum priority is used to sort amongst
   them.  If the max and the min priorities are the same, the
   controlling agent's priority is used as the tiebreaker in the last
   part of the expression.  The factor of 2*32 is used since the
   priority of a single candidate is always less than 2*32, resulting in
   the pair priority being a "concatenation" of the two component
   priorities.  This creates the MAX/MIN sorting.  MAX/MIN ensures that,
   for a particular ICE agent, a lower-priority candidate is never used
   until all higher-priority candidates have been tried.

B.6.  Why Are Keepalives Needed?

   Once data begins flowing on a candidate pair, it is still necessary
   to keep the bindings alive at intermediate NATs for the duration of
   the session.  Normally, the data stream packets themselves (e.g.,
   RTP) meet this objective.  However, several cases merit further
   discussion.  Firstly, in some RTP usages, such as SIP, the data
   streams can be "put on hold".  This is accomplished by using the SDP
   "sendonly" or "inactive" attributes, as defined in RFC 3264
   [RFC3264].  RFC 3264 directs implementations to cease transmission of
   data in these cases.  However, doing so may cause NAT bindings to
   time out, and data won't be able to come off hold.

   Secondly, some RTP payload formats, such as the payload format for
   text conversation [RFC4103], may send packets so infrequently that
   the interval exceeds the NAT binding timeouts.

   Thirdly, if silence suppression is in use, long periods of silence
   may cause data transmission to cease sufficiently long for NAT
   bindings to time out.
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   For these reasons, the data packets themselves cannot be relied upon.
   ICE defines a simple periodic keepalive utilizing STUN Binding
   Indications.  This makes its bandwidth requirements highly
   predictable and thus amenable to QoS reservations.

B.7.  Why Prefer Peer-Reflexive Candidates?

   Section 5.1.2 describes procedures for computing the priority of a
   candidate based on its type and local preferences.  That section
   requires that the type preference for peer-reflexive candidates
   always be higher than server reflexive.  Why is that?  The reason has
   to do with the security considerations in Section 19.  It is much
   easier for an attacker to cause an ICE agent to use a false server-
   reflexive candidate rather than a false peer-reflexive candidate.
   Consequently, attacks against address gathering with Binding requests
   are thwarted by ICE by preferring the peer-reflexive candidates.

B.8.  Why Are Binding Indications Used for Keepalives?

   Data keepalives are described in Section 11.  These keepalives make
   use of STUN when both endpoints are ICE capable.  However, rather
   than using a Binding request transaction (which generates a
   response), the keepalives use an Indication.  Why is that?

   The primary reason has to do with network QoS mechanisms.  Once data
   begins flowing, network elements will assume that the data stream has
   a fairly regular structure, making use of periodic packets at fixed
   intervals, with the possibility of jitter.  If an ICE agent is
   sending data packets, and then receives a Binding request, it would
   need to generate a response packet along with its data packets.  This
   will increase the actual bandwidth requirements for the 5-tuple
   carrying the data packets and introduce jitter in the delivery of
   those packets.  Analysis has shown that this is a concern in certain
   Layer 2 access networks that use fairly tight packet schedulers for

   Additionally, using a Binding Indication allows integrity to be
   disabled, which may result in better performance.  This is useful for
   large-scale endpoints, such as Public Switched Telephone Network
   (PSTN) gateways and Session Border Controllers (SBCs).

B.9.  Selecting Candidate Type Preference

   One criterion for selecting type and local preference values is the
   use of a data intermediary, such as a TURN server, a tunnel service
   such as a VPN server, or NAT.  With a data intermediary, if data is
   sent to that candidate, it will first transit the data intermediary
   before being received.  One type of candidate that involves a data
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   intermediary is the relayed candidate.  Another type is the host
   candidate, which is obtained from a VPN interface.  When data is
   transited through a data intermediary, it can have a positive or
   negative effect on the latency between transmission and reception.
   It may or may not increase the packet losses, because of the
   additional router hops that may be taken.  It may increase the cost
   of providing service, since data will be routed in and right back out
   of a data intermediary run by a provider.  If these concerns are
   important, the type preference for relayed candidates needs to be
   carefully chosen.

   Another criterion for selecting preferences is the IP address family.
   ICE works with both IPv4 and IPv6.  It provides a transition
   mechanism that allows dual-stack hosts to prefer connectivity over
   IPv6 but to fall back to IPv4 in case the v6 networks are
   disconnected.  Implementation SHOULD follow the guidelines from
   [RFC8421] to avoid excessive delays in the connectivity-check phase
   if broken paths exist.

   Another criterion for selecting preferences is topological awareness.
   This is beneficial for candidates that make use of intermediaries.
   In those cases, if an ICE agent has preconfigured or dynamically
   discovered knowledge of the topological proximity of the
   intermediaries to itself, it can use that to assign higher local
   preferences to candidates obtained from closer intermediaries.

   Another criterion for selecting preferences might be security or
   privacy.  If a user is a telecommuter, and therefore connected to a
   corporate network and a local home network, the user may prefer their
   voice traffic to be routed over the VPN or similar tunnel in order to
   keep it on the corporate network when communicating within the
   enterprise but may use the local network when communicating with
   users outside of the enterprise.  In such a case, a VPN address would
   have a higher local preference than any other address.
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Appendix C.  Connectivity-Check Bandwidth

   The tables below show, for IPv4 and IPv6, the bandwidth required for
   performing connectivity checks, using different Ta values (given in
   ms) and different ufrag sizes (given in bytes).

   The results were provided by Jusin Uberti (Google) on 11 April 2016.

                     IP version: IPv4
                     Packet len (bytes): 108 + ufrag
                       ms |     4     8    12    16
                      500 | 1.86k 1.98k 2.11k 2.24k
                      200 | 4.64k 4.96k 5.28k  5.6k
                      100 | 9.28k 9.92k 10.6k 11.2k
                       50 | 18.6k 19.8k 21.1k 22.4k
                       20 | 46.4k 49.6k 52.8k 56.0k
                       10 | 92.8k 99.2k  105k  112k
                        5 |  185k  198k  211k  224k
                        2 |  464k  496k  528k  560k
                        1 |  928k  992k 1.06M 1.12M

                     IP version: IPv6
                     Packet len (bytes): 128 + ufrag
                       ms |     4     8    12    16
                      500 | 2.18k  2.3k 2.43k 2.56k
                      200 | 5.44k 5.76k 6.08k  6.4k
                      100 | 10.9k 11.5k 12.2k 12.8k
                       50 | 21.8k 23.0k 24.3k 25.6k
                       20 | 54.4k 57.6k 60.8k 64.0k
                       10 |  108k  115k  121k  128k
                        5 |  217k  230k  243k  256k
                        2 |  544k  576k  608k  640k
                        1 | 1.09M 1.15M 1.22M 1.28M

                  Figure 12: Connectivity-Check Bandwidth
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   Most of the text in this document comes from the original ICE
   specification, RFC 5245.  The authors would like to thank everyone
   who has contributed to that document.  For additional contributions
   to this revision of the specification, we would like to thank Emil
   Ivov, Paul Kyzivat, Pal-Erik Martinsen, Simon Perrault, Eric
   Rescorla, Thomas Stach, Peter Thatcher, Martin Thomson, Justin
   Uberti, Suhas Nandakumar, Taylor Brandstetter, Peter Saint-Andre,
   Harald Alvestrand, and Roman Shpount.  Ben Campbell did the AD
   review.  Stephen Farrell did the sec-dir review.  Stewart Bryant did
   the gen-art review.  Qin We did the ops-dir review.  Magnus
   Westerlund did the tsv-art review.

Authors' Addresses

   Ari Keranen
   Hirsalantie 11
   02420 Jorvas


   Christer Holmberg
   Hirsalantie 11
   02420 Jorvas


   Jonathan Rosenberg
   Monmouth, NJ
   United States of America