RFC 5245
Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols
17. Example
The example is based on the simplified topology of Figure 8. +-----+ | | |STUN | | Srvr| +-----+ | +---------------------+ | | | Internet | | | | | +---------------------+ | | | | +---------+ | | NAT | | +---------+ | | | | | | | +-----+ +-----+ | | | | | L | | R | | | | | +-----+ +-----+ Figure 8: Example Topology Two agents, L and R, are using ICE. Both are full-mode ICE implementations and use aggressive nomination when they are controlling. Both agents have a single IPv4 address. For agent L, it is 10.0.1.1 in private address space [RFC1918], and for agent R, 192.0.2.1 on the public Internet. Both are configured with the same STUN server (shown in this example for simplicity, although in
practice the agents do not need to use the same STUN server), which
is listening for STUN Binding requests at an IP address of 192.0.2.2
and port 3478. TURN servers are not used in this example. Agent L
is behind a NAT, and agent R is on the public Internet. The NAT has
an endpoint independent mapping property and an address dependent
filtering property. The public side of the NAT has an IP address of
192.0.2.3.
To facilitate understanding, transport addresses are listed using
variables that have mnemonic names. The format of the name is
entity-type-seqno, where entity refers to the entity whose IP address
the transport address is on, and is one of "L", "R", "STUN", or
"NAT". The type is either "PUB" for transport addresses that are
public, and "PRIV" for transport addresses that are private.
Finally, seq-no is a sequence number that is different for each
transport address of the same type on a particular entity. Each
variable has an IP address and port, denoted by varname.IP and
varname.PORT, respectively, where varname is the name of the
variable.
The STUN server has advertised transport address STUN-PUB-1 (which is
192.0.2.2:3478).
In the call flow itself, STUN messages are annotated with several
attributes. The "S=" attribute indicates the source transport
address of the message. The "D=" attribute indicates the destination
transport address of the message. The "MA=" attribute is used in
STUN Binding response messages and refers to the mapped address.
"USE-CAND" implies the presence of the USE-CANDIDATE attribute.
The call flow examples omit STUN authentication operations and RTCP,
and focus on RTP for a single media stream between two full
implementations.
L NAT STUN R
|RTP STUN alloc. | |
|(1) STUN Req | | |
|S=$L-PRIV-1 | | |
|D=$STUN-PUB-1 | | |
|------------->| | |
| |(2) STUN Req | |
| |S=$NAT-PUB-1 | |
| |D=$STUN-PUB-1 | |
| |------------->| |
| |(3) STUN Res | |
| |S=$STUN-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |MA=$NAT-PUB-1 | |
| |<-------------| |
|(4) STUN Res | | |
|S=$STUN-PUB-1 | | |
|D=$L-PRIV-1 | | |
|MA=$NAT-PUB-1 | | |
|<-------------| | |
|(5) Offer | | |
|------------------------------------------->|
| | | |RTP STUN
alloc.
| | |(6) STUN Req |
| | |S=$R-PUB-1 |
| | |D=$STUN-PUB-1 |
| | |<-------------|
| | |(7) STUN Res |
| | |S=$STUN-PUB-1 |
| | |D=$R-PUB-1 |
| | |MA=$R-PUB-1 |
| | |------------->|
|(8) answer | | |
|<-------------------------------------------|
| |(9) Bind Req | |Begin
| |S=$R-PUB-1 | |Connectivity
| |D=L-PRIV-1 | |Checks
| |<----------------------------|
| |Dropped | |
|(10) Bind Req | | |
|S=$L-PRIV-1 | | |
|D=$R-PUB-1 | | |
|USE-CAND | | |
|------------->| | |
| |(11) Bind Req | |
| |S=$NAT-PUB-1 | |
| |D=$R-PUB-1 | |
| |USE-CAND | |
| |---------------------------->|
| |(12) Bind Res | |
| |S=$R-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |MA=$NAT-PUB-1 | |
| |<----------------------------|
|(13) Bind Res | | | |S=$R-PUB-1 | | | |D=$L-PRIV-1 | | | |MA=$NAT-PUB-1 | | | |<-------------| | | |RTP flows | | | | |(14) Bind Req | | | |S=$R-PUB-1 | | | |D=$NAT-PUB-1 | | | |<----------------------------| |(15) Bind Req | | | |S=$R-PUB-1 | | | |D=$L-PRIV-1 | | | |<-------------| | | |(16) Bind Res | | | |S=$L-PRIV-1 | | | |D=$R-PUB-1 | | | |MA=$R-PUB-1 | | | |------------->| | | | |(17) Bind Res | | | |S=$NAT-PUB-1 | | | |D=$R-PUB-1 | | | |MA=$R-PUB-1 | | | |---------------------------->| | | | |RTP flows Figure 9: Example Flow First, agent L obtains a host candidate from its local IP address (not shown), and from that, sends a STUN Binding request to the STUN server to get a server reflexive candidate (messages 1-4). Recall that the NAT has the address and port independent mapping property. Here, it creates a binding of NAT-PUB-1 for this UDP request, and this becomes the server reflexive candidate for RTP. Agent L sets a type preference of 126 for the host candidate and 100 for the server reflexive. The local preference is 65535. Based on this, the priority of the host candidate is 2130706431 and for the server reflexive candidate is 1694498815. The host candidate is assigned a foundation of 1, and the server reflexive, a foundation of 2. It chooses its server reflexive candidate as the default candidate, and encodes it into the m and c lines. The resulting offer (message 5) looks like (lines folded for clarity):
v=0
o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP
s=
c=IN IP4 $NAT-PUB-1.IP
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio $NAT-PUB-1.PORT RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 $L-PRIV-1.IP $L-PRIV-1.PORT typ
host
a=candidate:2 1 UDP 1694498815 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ
srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT
The offer, with the variables replaced with their values, will look
like (lines folded for clarity):
v=0
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
s=
c=IN IP4 192.0.2.3
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998
This offer is received at agent R. Agent R will obtain a host
candidate, and from it, obtain a server reflexive candidate (messages
6-7). Since R is not behind a NAT, this candidate is identical to
its host candidate, and they share the same base. It therefore
discards this redundant candidate and ends up with a single host
candidate. With identical type and local preferences as L, the
priority for this candidate is 2130706431. It chooses a foundation
of 1 for its single candidate. Its resulting answer looks like:
v=0
o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP
s=
c=IN IP4 $R-PUB-1.IP
t=0 0
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
m=audio $R-PUB-1.PORT RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host
With the variables filled in:
v=0
o=bob 2808844564 2808844564 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
m=audio 3478 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host
Since neither side indicated that it is lite, the agent that sent the
offer that began ICE processing (agent L) becomes the controlling
agent.
Agents L and R both pair up the candidates. They both initially have
two pairs. However, agent L will prune the pair containing its
server reflexive candidate, resulting in just one. At agent L, this
pair has a local candidate of $L_PRIV_1 and remote candidate of
$R_PUB_1, and has a candidate pair priority of 4.57566E+18 (note that
an implementation would represent this as a 64-bit integer so as not
to lose precision). At agent R, there are two pairs. The highest
priority has a local candidate of $R_PUB_1 and remote candidate of
$L_PRIV_1 and has a priority of 4.57566E+18, and the second has a
local candidate of $R_PUB_1 and remote candidate of $NAT_PUB_1 and
priority 3.63891E+18.
Agent R begins its connectivity check (message 9) for the first pair
(between the two host candidates). Since R is the controlled agent
for this session, the check omits the USE-CANDIDATE attribute. The
host candidate from agent L is private and behind a NAT, and thus this check won't be successful, because the packet cannot be routed from R to L. When agent L gets the answer, it performs its one and only connectivity check (messages 10-13). It implements the aggressive nomination algorithm, and thus includes a USE-CANDIDATE attribute in this check. Since the check succeeds, agent L creates a new pair, whose local candidate is from the mapped address in the Binding response (NAT-PUB-1 from message 13) and whose remote candidate is the destination of the request (R-PUB-1 from message 10). This is added to the valid list. In addition, it is marked as selected since the Binding request contained the USE-CANDIDATE attribute. Since there is a selected candidate in the Valid list for the one component of this media stream, ICE processing for this stream moves into the Completed state. Agent L can now send media if it so chooses. Soon after receipt of the STUN Binding request from agent L (message 11), agent R will generate its triggered check. This check happens to match the next one on its check list -- from its host candidate to agent L's server reflexive candidate. This check (messages 14-17) will succeed. Consequently, agent R constructs a new candidate pair using the mapped address from the response as the local candidate (R-PUB-1) and the destination of the request (NAT-PUB-1) as the remote candidate. This pair is added to the Valid list for that media stream. Since the check was generated in the reverse direction of a check that contained the USE-CANDIDATE attribute, the candidate pair is marked as selected. Consequently, processing for this stream moves into the Completed state, and agent R can also send media.18. Security Considerations
There are several types of attacks possible in an ICE system. This section considers these attacks and their countermeasures. These countermeasures include: o Using ICE in conjunction with secure signaling techniques, such as SIPS. o Limiting the total number of connectivity checks to 100, and optionally limiting the number of candidates they'll accept in an offer or answer.
18.1. Attacks on Connectivity Checks
An attacker might attempt to disrupt the STUN connectivity checks. Ultimately, all of these attacks fool an agent into thinking something incorrect about the results of the connectivity checks. The possible false conclusions an attacker can try and cause are: False Invalid: An attacker can fool a pair of agents into thinking a candidate pair is invalid, when it isn't. This can be used to cause an agent to prefer a different candidate (such as one injected by the attacker) or to disrupt a call by forcing all candidates to fail. False Valid: An attacker can fool a pair of agents into thinking a candidate pair is valid, when it isn't. This can cause an agent to proceed with a session, but then not be able to receive any media. False Peer Reflexive Candidate: An attacker can cause an agent to discover a new peer reflexive candidate, when it shouldn't have. This can be used to redirect media streams to a Denial-of-Service (DoS) target or to the attacker, for eavesdropping or other purposes. False Valid on False Candidate: An attacker has already convinced an agent that there is a candidate with an address that doesn't actually route to that agent (for example, by injecting a false peer reflexive candidate or false server reflexive candidate). It must then launch an attack that forces the agents to believe that this candidate is valid. If an attacker can cause a false peer reflexive candidate or false valid on a false candidate, it can launch any of the attacks described in [RFC5389]. To force the false invalid result, the attacker has to wait for the connectivity check from one of the agents to be sent. When it is, the attacker needs to inject a fake response with an unrecoverable error response, such as a 400. However, since the candidate is, in fact, valid, the original request may reach the peer agent, and result in a success response. The attacker needs to force this packet or its response to be dropped, through a DoS attack, layer 2 network disruption, or other technique. If it doesn't do this, the success response will also reach the originator, alerting it to a possible attack. Fortunately, this attack is mitigated completely through the STUN short-term credential mechanism. The attacker needs to inject a fake response, and in order for this response to be processed, the attacker needs the password. If the offer/answer
signaling is secured, the attacker will not have the password and its response will be discarded. Forcing the fake valid result works in a similar way. The agent needs to wait for the Binding request from each agent, and inject a fake success response. The attacker won't need to worry about disrupting the actual response since, if the candidate is not valid, it presumably wouldn't be received anyway. However, like the fake invalid attack, this attack is mitigated by the STUN short-term credential mechanism in conjunction with a secure offer/answer exchange. Forcing the false peer reflexive candidate result can be done either with fake requests or responses, or with replays. We consider the fake requests and responses case first. It requires the attacker to send a Binding request to one agent with a source IP address and port for the false candidate. In addition, the attacker must wait for a Binding request from the other agent, and generate a fake response with a XOR-MAPPED-ADDRESS attribute containing the false candidate. Like the other attacks described here, this attack is mitigated by the STUN message integrity mechanisms and secure offer/answer exchanges. Forcing the false peer reflexive candidate result with packet replays is different. The attacker waits until one of the agents sends a check. It intercepts this request, and replays it towards the other agent with a faked source IP address. It must also prevent the original request from reaching the remote agent, either by launching a DoS attack to cause the packet to be dropped, or forcing it to be dropped using layer 2 mechanisms. The replayed packet is received at the other agent, and accepted, since the integrity check passes (the integrity check cannot and does not cover the source IP address and port). It is then responded to. This response will contain a XOR- MAPPED-ADDRESS with the false candidate, and will be sent to that false candidate. The attacker must then receive it and relay it towards the originator. The other agent will then initiate a connectivity check towards that false candidate. This validation needs to succeed. This requires the attacker to force a false valid on a false candidate. Injecting of fake requests or responses to achieve this goal is prevented using the integrity mechanisms of STUN and the offer/answer exchange. Thus, this attack can only be launched through replays. To do that, the attacker must intercept the check towards this false candidate, and replay it towards the other agent. Then, it must intercept the response and replay that back as well.
This attack is very hard to launch unless the attacker is identified by the fake candidate. This is because it requires the attacker to intercept and replay packets sent by two different hosts. If both agents are on different networks (for example, across the public Internet), this attack can be hard to coordinate, since it needs to occur against two different endpoints on different parts of the network at the same time. If the attacker itself is identified by the fake candidate, the attack is easier to coordinate. However, if SRTP is used [RFC3711], the attacker will not be able to play the media packets, but will only be able to discard them, effectively disabling the media stream for the call. However, this attack requires the agent to disrupt packets in order to block the connectivity check from reaching the target. In that case, if the goal is to disrupt the media stream, it's much easier to just disrupt it with the same mechanism, rather than attack ICE.18.2. Attacks on Server Reflexive Address Gathering
ICE endpoints make use of STUN Binding requests for gathering server reflexive candidates from a STUN server. These requests are not authenticated in any way. As a consequence, there are numerous techniques an attacker can employ to provide the client with a false server reflexive candidate: o An attacker can compromise the DNS, causing DNS queries to return a rogue STUN server address. That server can provide the client with fake server reflexive candidates. This attack is mitigated by DNS security, though DNS-SEC is not required to address it. o An attacker that can observe STUN messages (such as an attacker on a shared network segment, like WiFi) can inject a fake response that is valid and will be accepted by the client. o An attacker can compromise a STUN server by means of a virus, and cause it to send responses with incorrect mapped addresses. A false mapped address learned by these attacks will be used as a server reflexive candidate in the ICE exchange. For this candidate to actually be used for media, the attacker must also attack the connectivity checks, and in particular, force a false valid on a false candidate. This attack is very hard to launch if the false address identifies a fourth party (neither the offerer, answerer, nor attacker), since it requires attacking the checks generated by each agent in the session, and is prevented by SRTP if it identifies the attacker themself.
If the attacker elects not to attack the connectivity checks, the worst it can do is prevent the server reflexive candidate from being used. However, if the peer agent has at least one candidate that is reachable by the agent under attack, the STUN connectivity checks themselves will provide a peer reflexive candidate that can be used for the exchange of media. Peer reflexive candidates are generally preferred over server reflexive candidates. As such, an attack solely on the STUN address gathering will normally have no impact on a session at all.18.3. Attacks on Relayed Candidate Gathering
An attacker might attempt to disrupt the gathering of relayed candidates, forcing the client to believe it has a false relayed candidate. Exchanges with the TURN server are authenticated using a long-term credential. Consequently, injection of fake responses or requests will not work. In addition, unlike Binding requests, Allocate requests are not susceptible to replay attacks with modified source IP addresses and ports, since the source IP address and port are not utilized to provide the client with its relayed candidate. However, TURN servers are susceptible to DNS attacks, or to viruses aimed at the TURN server, for purposes of turning it into a zombie or rogue server. These attacks can be mitigated by DNS-SEC and through good box and software security on TURN servers. Even if an attacker has caused the client to believe in a false relayed candidate, the connectivity checks cause such a candidate to be used only if they succeed. Thus, an attacker must launch a false valid on a false candidate, per above, which is a very difficult attack to coordinate.18.4. Attacks on the Offer/Answer Exchanges
An attacker that can modify or disrupt the offer/answer exchanges themselves can readily launch a variety of attacks with ICE. They could direct media to a target of a DoS attack, they could insert themselves into the media stream, and so on. These are similar to the general security considerations for offer/answer exchanges, and the security considerations in RFC 3264 [RFC3264] apply. These require techniques for message integrity and encryption for offers and answers, which are satisfied by the SIPS mechanism [RFC3261] when SIP is used. As such, the usage of SIPS with ICE is RECOMMENDED.
18.5. Insider Attacks
In addition to attacks where the attacker is a third party trying to insert fake offers, answers, or stun messages, there are several attacks possible with ICE when the attacker is an authenticated and valid participant in the ICE exchange.18.5.1. The Voice Hammer Attack
The voice hammer attack is an amplification attack. In this attack, the attacker initiates sessions to other agents, and maliciously includes the IP address and port of a DoS target as the destination for media traffic signaled in the SDP. This causes substantial amplification; a single offer/answer exchange can create a continuing flood of media packets, possibly at high rates (consider video sources). This attack is not specific to ICE, but ICE can help provide remediation. Specifically, if ICE is used, the agent receiving the malicious SDP will first perform connectivity checks to the target of media before sending media there. If this target is a third-party host, the checks will not succeed, and media is never sent. Unfortunately, ICE doesn't help if its not used, in which case an attacker could simply send the offer without the ICE parameters. However, in environments where the set of clients is known, and is limited to ones that support ICE, the server can reject any offers or answers that don't indicate ICE support.18.5.2. STUN Amplification Attack
The STUN amplification attack is similar to the voice hammer. However, instead of voice packets being directed to the target, STUN connectivity checks are directed to the target. The attacker sends an offer with a large number of candidates, say, 50. The answerer receives the offer, and starts its checks, which are directed at the target, and consequently, never generate a response. The answerer will start a new connectivity check every Ta ms (say, Ta=20ms). However, the retransmission timers are set to a large number due to the large number of candidates. As a consequence, packets will be sent at an interval of one every Ta milliseconds, and then with increasing intervals after that. Thus, STUN will not send packets at a rate faster than media would be sent, and the STUN packets persist only briefly, until ICE fails for the session. Nonetheless, this is an amplification mechanism. It is impossible to eliminate the amplification, but the volume can be reduced through a variety of heuristics. Agents SHOULD limit the
total number of connectivity checks they perform to 100.
Additionally, agents MAY limit the number of candidates they'll
accept in an offer or answer.
Frequently, protocols that wish to avoid these kinds of attacks force
the initiator to wait for a response prior to sending the next
message. However, in the case of ICE, this is not possible. It is
not possible to differentiate the following two cases:
o There was no response because the initiator is being used to
launch a DoS attack against an unsuspecting target that will not
respond.
o There was no response because the IP address and port are not
reachable by the initiator.
In the second case, another check should be sent at the next
opportunity, while in the former case, no further checks should be
sent.
18.6. Interactions with Application Layer Gateways and SIP
Application Layer Gateways (ALGs) are functions present in a NAT
device that inspect the contents of packets and modify them, in order
to facilitate NAT traversal for application protocols. Session
Border Controllers (SBCs) are close cousins of ALGs, but are less
transparent since they actually exist as application layer SIP
intermediaries. ICE has interactions with SBCs and ALGs.
If an ALG is SIP aware but not ICE aware, ICE will work through it as
long as the ALG correctly modifies the SDP. A correct ALG
implementation behaves as follows:
o The ALG does not modify the m and c lines or the rtcp attribute if
they contain external addresses.
o If the m and c lines contain internal addresses, the modification
depends on the state of the ALG:
If the ALG already has a binding established that maps an
external port to an internal IP address and port matching the
values in the m and c lines or rtcp attribute, the ALG uses
that binding instead of creating a new one.
If the ALG does not already have a binding, it creates a new
one and modifies the SDP, rewriting the m and c lines and rtcp
attribute.
Unfortunately, many ALGs are known to work poorly in these corner cases. ICE does not try to work around broken ALGs, as this is outside the scope of its functionality. ICE can help diagnose these conditions, which often show up as a mismatch between the set of candidates and the m and c lines and rtcp attributes. The ice- mismatch attribute is used for this purpose. ICE works best through ALGs when the signaling is run over TLS. This prevents the ALG from manipulating the SDP messages and interfering with ICE operation. Implementations that are expected to be deployed behind ALGs SHOULD provide for TLS transport of the SDP. If an SBC is SIP aware but not ICE aware, the result depends on the behavior of the SBC. If it is acting as a proper Back-to-Back User Agent (B2BUA), the SBC will remove any SDP attributes it doesn't understand, including the ICE attributes. Consequently, the call will appear to both endpoints as if the other side doesn't support ICE. This will result in ICE being disabled, and media flowing through the SBC, if the SBC has requested it. If, however, the SBC passes the ICE attributes without modification, yet modifies the default destination for media (contained in the m and c lines and rtcp attribute), this will be detected as an ICE mismatch, and ICE processing is aborted for the call. It is outside of the scope of ICE for it to act as a tool for "working around" SBCs. If one is present, ICE will not be used and the SBC techniques take precedence.19. STUN Extensions
19.1. New Attributes
This specification defines four new attributes, PRIORITY, USE- CANDIDATE, ICE-CONTROLLED, and ICE-CONTROLLING. The PRIORITY attribute indicates the priority that is to be associated with a peer reflexive candidate, should one be discovered by this check. It is a 32-bit unsigned integer, and has an attribute value of 0x0024. The USE-CANDIDATE attribute indicates that the candidate pair resulting from this check should be used for transmission of media. The attribute has no content (the Length field of the attribute is zero); it serves as a flag. It has an attribute value of 0x0025. The ICE-CONTROLLED attribute is present in a Binding request and indicates that the client believes it is currently in the controlled role. The content of the attribute is a 64-bit unsigned integer in network byte order, which contains a random number used for tie- breaking of role conflicts.
The ICE-CONTROLLING attribute is present in a Binding request and indicates that the client believes it is currently in the controlling role. The content of the attribute is a 64-bit unsigned integer in network byte order, which contains a random number used for tie- breaking of role conflicts.19.2. New Error Response Codes
This specification defines a single error response code: 487 (Role Conflict): The Binding request contained either the ICE- CONTROLLING or ICE-CONTROLLED attribute, indicating a role that conflicted with the server. The server ran a tie-breaker based on the tie-breaker value in the request and determined that the client needs to switch roles.20. Operational Considerations
This section discusses issues relevant to network operators looking to deploy ICE.20.1. NAT and Firewall Types
ICE was designed to work with existing NAT and firewall equipment. Consequently, it is not necessary to replace or reconfigure existing firewall and NAT equipment in order to facilitate deployment of ICE. Indeed, ICE was developed to be deployed in environments where the Voice over IP (VoIP) operator has no control over the IP network infrastructure, including firewalls and NAT. That said, ICE works best in environments where the NAT devices are "behave" compliant, meeting the recommendations defined in [RFC4787] and [RFC5766]. In networks with behave-compliant NAT, ICE will work without the need for a TURN server, thus improving voice quality, decreasing call setup times, and reducing the bandwidth demands on the network operator.20.2. Bandwidth Requirements
Deployment of ICE can have several interactions with available network capacity that operators should take into consideration.20.2.1. STUN and TURN Server Capacity Planning
First and foremost, ICE makes use of TURN and STUN servers, which would typically be located in the network operator's data centers. The STUN servers require relatively little bandwidth. For each component of each media stream, there will be one or more STUN
transactions from each client to the STUN server. In a basic voice- only IPv4 VoIP deployment, there will be four transactions per call (one for RTP and one for RTCP, for both caller and callee). Each transaction is a single request and a single response, the former being 20 bytes long, and the latter, 28. Consequently, if a system has N users, and each makes four calls in a busy hour, this would require N*1.7bps. For one million users, this is 1.7 Mbps, a very small number (relatively speaking). TURN traffic is more substantial. The TURN server will see traffic volume equal to the STUN volume (indeed, if TURN servers are deployed, there is no need for a separate STUN server), in addition to the traffic for the actual media traffic. The amount of calls requiring TURN for media relay is highly dependent on network topologies, and can and will vary over time. In a network with 100% behave-compliant NAT, it is exactly zero. At time of writing, large- scale consumer deployments were seeing between 5 and 10 percent of calls requiring TURN servers. Considering a voice-only deployment using G.711 (so 80 kbps in each direction), with .2 erlangs during the busy hour, this is N*3.2 kbps. For a population of one million users, this is 3.2 Gbps, assuming a 10% usage of TURN servers.20.2.2. Gathering and Connectivity Checks
The process of gathering of candidates and performing of connectivity checks can be bandwidth intensive. ICE has been designed to pace both of these processes. The gathering phase and the connectivity check phase are meant to generate traffic at roughly the same bandwidth as the media traffic itself. This was done to ensure that, if a network is designed to support multimedia traffic of a certain type (voice, video, or just text), it will have sufficient capacity to support the ICE checks for that media. Of course, the ICE checks will cause a marginal increase in the total utilization; however, this will typically be an extremely small increase. Congestion due to the gathering and check phases has proven to be a problem in deployments that did not utilize pacing. Typically, access links became congested as the endpoints flooded the network with checks as fast as they can send them. Consequently, network operators should make sure that their ICE implementations support the pacing feature. Though this pacing does increase call setup times, it makes ICE network friendly and easier to deploy.20.2.3. Keepalives
STUN keepalives (in the form of STUN Binding Indications) are sent in the middle of a media session. However, they are sent only in the absence of actual media traffic. In deployments that are not
utilizing Voice Activity Detection (VAD), the keepalives are never used and there is no increase in bandwidth usage. When VAD is being used, keepalives will be sent during silence periods. This involves a single packet every 15-20 seconds, far less than the packet every 20-30 ms that is sent when there is voice. Therefore, keepalives don't have any real impact on capacity planning.20.3. ICE and ICE-lite
Deployments utilizing a mix of ICE and ICE-lite interoperate perfectly. They have been explicitly designed to do so, without loss of function. However, ICE-lite can only be deployed in limited use cases. Those cases, and the caveats involved in doing so, are documented in Appendix A.20.4. Troubleshooting and Performance Management
ICE utilizes end-to-end connectivity checks, and places much of the processing in the endpoints. This introduces a challenge to the network operator -- how can they troubleshoot ICE deployments? How can they know how ICE is performing? ICE has built-in features to help deal with these problems. SIP servers on the signaling path, typically deployed in the data centers of the network operator, will see the contents of the offer/answer exchanges that convey the ICE parameters. These parameters include the type of each candidate (host, server reflexive, or relayed), along with their related addresses. Once ICE processing has completed, an updated offer/answer exchange takes place, signaling the selected address (and its type). This updated re-INVITE is performed exactly for the purposes of educating network equipment (such as a diagnostic tool attached to a SIP server) about the results of ICE processing. As a consequence, through the logs generated by the SIP server, a network operator can observe what types of candidates are being used for each call, and what address was selected by ICE. This is the primary information that helps evaluate how ICE is performing.20.5. Endpoint Configuration
ICE relies on several pieces of data being configured into the endpoints. This configuration data includes timers, credentials for TURN servers, and hostnames for STUN and TURN servers. ICE itself does not provide a mechanism for this configuration. Instead, it is assumed that this information is attached to whatever mechanism is
used to configure all of the other parameters in the endpoint. For SIP phones, standard solutions such as the configuration framework [SIP-UA-FRMWK] have been defined.21. IANA Considerations
This specification registers new SDP attributes, four new STUN attributes, and one new STUN error response.21.1. SDP Attributes
This specification defines seven new SDP attributes per the procedures of Section 8.2.4 of [RFC4566]. The required information for the registrations is included here.21.1.1. candidate Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Attribute Name: candidate Long Form: candidate Type of Attribute: media-level Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides one of many possible candidate addresses for communication. These addresses are validated with an end-to-end connectivity check using Session Traversal Utilities for NAT (STUN)). Appropriate Values: See Section 15 of RFC 5245.21.1.2. remote-candidates Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Attribute Name: remote-candidates Long Form: remote-candidates Type of Attribute: media-level Charset Considerations: The attribute is not subject to the charset attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the identity of the remote
candidates that the offerer wishes the answerer to use in its
answer.
Appropriate Values: See Section 15 of RFC 5245.
21.1.3. ice-lite Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-lite
Long Form: ice-lite
Type of Attribute: session-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent has the minimum
functionality required to support ICE inter-operation with a peer
that has a full implementation.
Appropriate Values: See Section 15 of RFC 5245.
21.1.4. ice-mismatch Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-mismatch
Long Form: ice-mismatch
Type of Attribute: session-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent is ICE capable,
but did not proceed with ICE due to a mismatch of candidates with
the default destination for media signaled in the SDP.
Appropriate Values: See Section 15 of RFC 5245.
21.1.5. ice-pwd Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Attribute Name: ice-pwd Long Form: ice-pwd Type of Attribute: session- or media-level Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides the password used to protect STUN connectivity checks. Appropriate Values: See Section 15 of RFC 5245.21.1.6. ice-ufrag Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Attribute Name: ice-ufrag Long Form: ice-ufrag Type of Attribute: session- or media-level Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides the fragments used to construct the username in STUN connectivity checks. Appropriate Values: See Section 15 of RFC 5245.21.1.7. ice-options Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Attribute Name: ice-options Long Form: ice-options Type of Attribute: session-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates the ICE options or extensions
used by the agent.
Appropriate Values: See Section 15 of RFC 5245.
21.2. STUN Attributes
This section registers four new STUN attributes per the procedures in
[RFC5389].
0x0024 PRIORITY
0x0025 USE-CANDIDATE
0x8029 ICE-CONTROLLED
0x802A ICE-CONTROLLING
21.3. STUN Error Responses
This section registers one new STUN error response code per the
procedures in [RFC5389].
487 Role Conflict: The client asserted an ICE role (controlling
or
controlled) that is in conflict with the role of the server.
22. IAB Considerations
The IAB has studied the problem of "Unilateral Self-Address Fixing",
which is the general process by which a agent attempts to determine
its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism [RFC3424]. ICE is an
example of a protocol that performs this type of function.
Interestingly, the process for ICE is not unilateral, but bilateral,
and the difference has a significant impact on the issues raised by
IAB. Indeed, ICE can be considered a B-SAF (Bilateral Self-Address
Fixing) protocol, rather than an UNSAF protocol. Regardless, the IAB
has mandated that any protocols developed for this purpose document a
specific set of considerations. This section meets those
requirements.
22.1. Problem Definition
>From RFC 3424, any UNSAF proposal must provide: Precise definition of a specific, limited-scope problem that is to be solved with the UNSAF proposal. A short-term fix should not be generalized to solve other problems; this is why "short-term fixes usually aren't". The specific problems being solved by ICE are: Provide a means for two peers to determine the set of transport addresses that can be used for communication. Provide a means for a agent to determine an address that is reachable by another peer with which it wishes to communicate.22.2. Exit Strategy
>From RFC 3424, any UNSAF proposal must provide: Description of an exit strategy/transition plan. The better short-term fixes are the ones that will naturally see less and less use as the appropriate technology is deployed. ICE itself doesn't easily get phased out. However, it is useful even in a globally connected Internet, to serve as a means for detecting whether a router failure has temporarily disrupted connectivity, for example. ICE also helps prevent certain security attacks that have nothing to do with NAT. However, what ICE does is help phase out other UNSAF mechanisms. ICE effectively selects amongst those mechanisms, prioritizing ones that are better, and deprioritizing ones that are worse. Local IPv6 addresses can be preferred. As NATs begin to dissipate as IPv6 is introduced, server reflexive and relayed candidates (both forms of UNSAF addresses) simply never get used, because higher-priority connectivity exists to the native host candidates. Therefore, the servers get used less and less, and can eventually be remove when their usage goes to zero. Indeed, ICE can assist in the transition from IPv4 to IPv6. It can be used to determine whether to use IPv6 or IPv4 when two dual-stack hosts communicate with SIP (IPv6 gets used). It can also allow a network with both 6to4 and native v6 connectivity to determine which address to use when communicating with a peer.
22.3. Brittleness Introduced by ICE
>From RFC 3424, any UNSAF proposal must provide: Discussion of specific issues that may render systems more "brittle". For example, approaches that involve using data at multiple network layers create more dependencies, increase debugging challenges, and make it harder to transition. ICE actually removes brittleness from existing UNSAF mechanisms. In particular, classic STUN (as described in RFC 3489 [RFC3489]) has several points of brittleness. One of them is the discovery process that requires an agent to try to classify the type of NAT it is behind. This process is error-prone. With ICE, that discovery process is simply not used. Rather than unilaterally assessing the validity of the address, its validity is dynamically determined by measuring connectivity to a peer. The process of determining connectivity is very robust. Another point of brittleness in classic STUN and any other unilateral mechanism is its absolute reliance on an additional server. ICE makes use of a server for allocating unilateral addresses, but allows agents to directly connect if possible. Therefore, in some cases, the failure of a STUN server would still allow for a call to progress when ICE is used. Another point of brittleness in classic STUN is that it assumes that the STUN server is on the public Internet. Interestingly, with ICE, that is not necessary. There can be a multitude of STUN servers in a variety of address realms. ICE will discover the one that has provided a usable address. The most troubling point of brittleness in classic STUN is that it doesn't work in all network topologies. In cases where there is a shared NAT between each agent and the STUN server, traditional STUN may not work. With ICE, that restriction is removed. Classic STUN also introduces some security considerations. Fortunately, those security considerations are also mitigated by ICE. Consequently, ICE serves to repair the brittleness introduced in classic STUN, and does not introduce any additional brittleness into the system. The penalty of these improvements is that ICE increases session establishment times.
22.4. Requirements for a Long-Term Solution
From RFC 3424, any UNSAF proposal must provide: ... requirements for longer term, sound technical solutions -- contribute to the process of finding the right longer term solution. Our conclusions from RFC 3489 remain unchanged. However, we feel ICE actually helps because we believe it can be part of the long-term solution.22.5. Issues with Existing NAPT Boxes
From RFC 3424, any UNSAF proposal must provide: Discussion of the impact of the noted practical issues with existing, deployed NA[P]Ts and experience reports. A number of NAT boxes are now being deployed into the market that try to provide "generic" ALG functionality. These generic ALGs hunt for IP addresses, either in text or binary form within a packet, and rewrite them if they match a binding. This interferes with classic STUN. However, the update to STUN [RFC5389] uses an encoding that hides these binary addresses from generic ALGs. Existing NAPT boxes have non-deterministic and typically short expiration times for UDP-based bindings. This requires implementations to send periodic keepalives to maintain those bindings. ICE uses a default of 15 s, which is a very conservative estimate. Eventually, over time, as NAT boxes become compliant to behave [RFC4787], this minimum keepalive will become deterministic and well-known, and the ICE timers can be adjusted. Having a way to discover and control the minimum keepalive interval would be far better still.23. Acknowledgements
The authors would like to thank Dan Wing, Eric Rescorla, Flemming Andreasen, Rohan Mahy, Dean Willis, Eric Cooper, Jason Fischl, Douglas Otis, Tim Moore, Jean-Francois Mule, Kevin Johns, Jonathan Lennox, and Francois Audet for their comments and input. A special thanks goes to Bill May, who suggested several of the concepts in this specification, Philip Matthews, who suggested many of the key performance optimizations in this specification, Eric Rescorla, who drafted the text in the introduction, and Magnus Westerlund, for doing several detailed reviews on the various revisions of this specification.
24. References
24.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, October 2003. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, July 2003. [RFC3312] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of Resource Management and Session Initiation Protocol (SIP)", RFC 3312, October 2002. [RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session Initiation Protocol (SIP) Preconditions Framework", RFC 4032, March 2005. [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC4091] Camarillo, G. and J. Rosenberg, "The Alternative Network Address Types (ANAT) Semantics for the Session Description Protocol (SDP) Grouping Framework", RFC 4091, June 2005. [RFC4092] Camarillo, G. and J. Rosenberg, "Usage of the Session Description Protocol (SDP) Alternative Network Address Types (ANAT) Semantics in the Session Initiation Protocol (SIP)", RFC 4092, June 2005.
[RFC3484] Draves, R., "Default Address Selection for Internet Protocol version 6 (IPv6)", RFC 3484, February 2003. [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, 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, April 2010. [RFC5768] Rosenberg, J., "Indicating Support for Interactive Connectivity Establishment (ICE) in the Session Initiation Protocol (SIP)", RFC 5768, April 2010.24.2. Informative References
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN - Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs)", RFC 3489, March 2003. [RFC3235] Senie, D., "Network Address Translator (NAT)-Friendly Application Design Guidelines", RFC 3235, January 2002. [RFC3303] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A. Rayhan, "Middlebox communication architecture and framework", RFC 3303, August 2002. [RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo, "Best Current Practices for Third Party Call Control (3pcc) in the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April 2004. [RFC3102] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro, "Realm Specific IP: Framework", RFC 3102, October 2001. [RFC3103] Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi, "Realm Specific IP: Protocol Specification", RFC 3103, October 2001. [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001. [RFC3389] Zopf, R., "Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)", RFC 3389, September 2002. [RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone Generation in the Session Initiation Protocol (SIP)", RFC 3960, December 2004. [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998. [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. [RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007. [SDP-PRECON] Andreasen, F., Camarillo, G., Oran, D., and D. Wing, "Connectivity Preconditions for Session Description Protocol Media Streams", Work in Progress, March 2010. [NO-OP-RTP] Andreasen, F., Oran, D., and D. Wing, "A No-Op Payload Format for RTP", Work in Progress, May 2007. [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and Control Packets on a Single Port", RFC 5761, April 2010. [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006. [RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text Conversation", RFC 4103, June 2005.
[RFC5626] Jennings, C., Mahy, R., and F. Audet, "Managing Client- Initiated Connections in the Session Initiation Protocol (SIP)", RFC 5626, October 2009. [RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, RFC 5382, October 2008. [SIP-UA-FRMWK] Petrie, D. and S. Channabasappa, Ed., "A Framework for Session Initiation Protocol User Agent Profile Delivery", Work in Progress, February 2010. [ICE-TCP] Perreault, S., Ed. and J. Rosenberg, "TCP Candidates with Interactive Connectivity Establishment (ICE)", Work in Progress, October 2009.
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