New Status codes and their usage with RTSP methods

Code	Reason	Method
150	Server still working on ICE connectivity checks	PLAY
480	ICE Connectivity check failure	PLAY, SETUP

4.5.1. 150 ICE Connectivity Checks in Progress

The 150 response code indicates that ICE connectivity checks are still in progress and haven't concluded. This response SHALL be sent within 200 milliseconds of receiving a PLAY request that currently can't be fulfilled because ICE connectivity checks are still running. Subsequently, every 3 seconds after the previous one was sent, a 150 reply SHALL be sent until the ICE connectivity checks conclude either successfully or in failure, and a final response for the request can be provided.

4.5.2. 480 ICE Processing Failed

The 480 client error response code is used in cases when the request can't be fulfilled due to a failure in the ICE processing, such as all the connectivity checks have timed out. This error message can appear either in response to a SETUP request to indicate that no candidate pair can be constructed, or in response to a PLAY request to indicate that the server's connectivity checks resulted in failure.

4.6. New Reason for PLAY_NOTIFY

A new value used in the PLAY_NOTIFY methods Notify-Reason header is defined: "ice-restart". This reason indicates that an ICE restart needs to happen on the identified resource and session.

Notify-Reas-val =/ "ice-restart"
 

4.7. Server Side SDP Attribute for ICE Support

If the server supports the media NAT traversal for RTSP controlled sessions as described in this RFC, then the server SHOULD include the "a=rtsp-ice-d-m" SDP attribute in any SDP (if used) describing content served by the server. This is a session-level only attribute, see [RFC4566].

The ABNF [RFC5234] for the "rtsp-ice-d-m" attribute is:

rtsp-ice-d-m-attr = "a=" "rtsp-ice-d-m"
 

5. ICE-RTSP

This section discusses differences between the regular ICE usage defined in [RFC5245] and ICE-RTSP. The reasons for the differences relate to the clearer client/server roles that RTSP provides and how the RTSP Session establishment signalling occurs within RTSP compared to SIP/SDP Offer/Answer.

5.1. ICE Features Not Required

A number of ICE signalling features are not needed with RTSP and are discussed below.

5.1.1. ICE-Lite

The ICE-Lite attribute SHALL NOT be used in the context of RTSP. The ICE specification describes two implementations of ICE: Full and Lite, where hosts that are not behind a NAT are allowed to implement only Lite. For RTSP, the Lite implementation is insufficient because it does not cause the media server to send a connectivity check, which is used to protect against making the RTSP server a denial of service tool.

5.1.2. ICE-Mismatch

The ice-mismatch parameter indicates that the offer arrived with a default destination for a media component that didn't have a corresponding candidate attribute. This is not needed for RTSP as the ICE based lower layer transport specification either is supported or another alternative transport is used. This is always explicitly indicated in the SETUP request and response.

5.1.3. ICE Remote Candidate Transport Header Parameter

The Remote candidate attribute is not needed for RTSP for the following reasons. Each SETUP results in an independent ICE processing chain which either fails or results in nominating a single candidate pair to usage. If a new SETUP request for the same media is sent, this needs to use a new username fragment and password to avoid any race conditions or uncertainty about which round of processing the STUN requests relate to.

5.2. High-Reachability Configuration

ICE-RTSP contains a high-reachability configuration when the RTSP servers are not behind NATs. Please note that "not behind NATs" may apply in some special cases also for RTSP servers behind NATs given that they are in an address space that has reachability for all the RTSP clients intended to able to reach the server. The high-reachability configuration is similar to ICE-Lite as it allows for some reduction in the server's burden. However, due to the need to still verify that the client is actually present and wants to receive the media stream, the server must also initiate binding requests and await binding responses. The reduction for the high-reachability configuration of ICE-RTSP is that they don't need to initiate their own checks, and instead rely on triggered checks for verification. This also removes a denial of service threat where a RTSP SETUP request will trigger large amount of STUN connectivity checks towards provided candidate addresses.

6. Detailed Solution

This section describes in detail how the interaction and flow of ICE works with RTSP messages.

6.1. Session description and RTSP DESCRIBE (optional)

The RTSP server is RECOMMENDED to indicate it has support for ICE by sending the "a=rtsp-ice-d-m" SDP attribute in the response to the RTSP DESCRIBE message if SDP is used. This allows RTSP clients to only send the new ICE exchanges with servers that support ICE thereby limiting the overhead on current non-ICE supporting RTSP servers. When not using RTSP DESCRIBE it is still RECOMMENDED to use the SDP attribute for the session description.

A client can also use the DESCRIBE request to determine explicitly if both server and any proxies support ICE. The client includes the "Supported" header with its supported feature tags, including "setup.ice-d-m". Any proxy upon seeing the "Supported" header will include the "Proxy-Supported" header with the feature tags it supports. The server will echo back the "Proxy-Supported" header and its own version of the Supported header so enabling a client to determine if all involved parties support ICE or not. Note that even if a proxy is present in the chain that doesn't indicate support for ICE, it may still work (see Section 7).

For example:
     C->S: DESCRIBE rtsp://server.example.com/fizzle/foo RTSP/2.0
           CSeq: 312
           User-Agent: PhonyClient 1.2
           Accept: application/sdp, application/example
           Supported: setup.ice-d-m, setup.rtp.rtcp.mux

     S->C: RTSP/2.0 200 OK
           CSeq: 312
           Date: 23 Jan 1997 15:35:06 GMT
           Server: PhonyServer 1.1
           Content-Type: application/sdp
           Content-Length: 367
           Supported: setup.ice-d-m, setup.rtp.rtcp.mux

           v=0
           o=mhandley 2890844526 2890842807 IN IP4 192.0.2.46
           s=SDP Seminar
           i=A Seminar on the session description protocol
           u=http://www.example.com/lectures/sdp.ps
           e=seminar@example.com (Seminar Management)
           t=2873397496 2873404696
           a=recvonly
           a=rtsp-ice-d-m
           a=control: *           
           m=audio 3456 RTP/AVP 0
           a=control: /audio
           m=video 2232 RTP/AVP 31
           a=control: /video
 

6.2. Setting up the Media Streams

The RTSP client reviews the session description returned, for example by an RTSP DESCRIBE message, to determine what media resources need to be setup. For each of these media streams where the transport protocol supports ICE connectivity checks, the client SHALL gather candidate addresses for UDP transport as described in section 4.1.1 in ICE [RFC5245] according to standard ICE rather than the ICE-Lite implementation and according to section 5 of ICE TCP [RFC6544] for TCP based candidates.

6.3. RTSP SETUP Request

The RTSP client will then send at least one SETUP request per media stream to establish the media streams required for the desired session. For each media stream where it desires to use ICE it MUST include a transport specification with "D-ICE" as the lower layer, and each media stream SHALL have its own unique combination of ICE candidates and ICE-ufrag. This transport specification SHOULD be placed first in the list to give it highest priority. It is RECOMMENDED that additional transport specifications are provided as a fallback in case of non-ICE supporting proxies. The RTSP client will be initiating and thus the controlling party in the ICE processing. For example (Note that some lines are broken in contradiction with the defined syntax due to space restrictions in the documenting format):

C->S: SETUP rtsp://server.example.com/fizzle/foo/audio RTSP/2.0
      CSeq: 313
      Transport: RTP/AVP/D-ICE; unicast; ICE-ufrag=8hhY;
                ICE-Password=asd88fgpdd777uzjYhagZg; candidates="
                1 1 UDP 2130706431 10.0.1.17 8998 typ host;
                2 1 UDP 1694498815 192.0.2.3 45664 typ srflx 
                         raddr 10.0.1.17 rport 8998"; RTCP-mux,
             RTP/AVP/UDP; unicast; dest_addr=":6970"/":6971",
             RTP/AVP/TCP;unicast;interleaved=0-1
      Accept-Ranges: NPT, UTC
      User-Agent: PhonyClient/1.2
      Supported: setup.ice-d-m, setup.rtp.rtcp.mux
 

6.4. Gathering Candidates

Upon receiving a SETUP request the server can determine what media resource should be delivered and which transport alternatives the client supports. If one based on D-ICE is on the list of supported transports and preferred among the supported, the below applies.

The transport specification will indicate which media protocol is to be used and based on this and the client's candidates, the server determines the protocol and if it supports ICE with that protocol. The server SHALL then gather its UDP candidates according to section 4.1.1 in ICE [RFC5245] and any TCP based ones according to section 5 of ICE TCP [RFC6544].

Servers that have an address that is generally reachable by any client within the address scope the server intends to serve MAY be specially configured (high-reachability configuration). This special configuration has the goal of reducing the server side candidate to preferably a single one per (address family, media stream, media component) tuple. Instead of gathering all possible addresses including relayed and server reflexive addresses, the server uses a single address per address family that the server knows should be reachable by a client behind one or more NATs. The reason for this special configuration is twofold: Firstly, it reduces the load on the server in address gathering and in ICE processing during the connectivity checks. Secondly, it will reduce the number of permutations for candidate pairs significantly thus potentially speeding up the conclusion of the ICE processing. Note however that using this option on a server that doesn't fulfill the requirement of being reachable is counter-productive and it is important that this is correctly configured.

The above general consideration for servers applies also for TCP based candidates. A general implementation should support several candidate collection techniques and connection types. For TCP based candidates a high-reachability configured server is recommended to only offer Host candidates. In addition to passive connection types the server can select to provide active or simultaneous-open (S-O) connection types to match the client's candidates.

6.5. RTSP Server Response

The server determines if the SETUP request is successful from the other perspectives and if so returns a 200 OK response; otherwise it returns an error code. At that point the server, having selected a transport specification using the "D-ICE" lower layer, will need to include that transport specification in the response message. The transport specification SHALL include the candidates gathered in Section 6.4 in the "candidates" transport header parameter as well as the server's ICE username fragment and password. In the case that there are no valid candidate pairs with the combination of the client and server candidates, a 480 (ICE Processing Failed) error response SHALL be returned which MUST include the server's candidates. The return of a 480 error allows both the server and client to release their candidates.

Example of a successful response to the request in Section 6.3.

S->C: RTSP/2.0 200 OK 
      CSeq: 313
      Session: 12345678
      Transport: RTP/AVP/D-ICE; unicast; RTCP-mux; ICE-ufrag=MkQ3;
                ICE-Password=pos12Dgp9FcAjpq82ppaF; candidates="
                 1 1 UDP 2130706431 192.0.2.56 50234 typ host"
      Accept-Ranges: NPT
      Date: 23 Jan 1997 15:35:06 GMT
      Server: PhonyServer 1.1
      Supported: setup.ice-d-m, setup.rtp.rtcp.mux
 

6.6. Server to Client ICE Connectivity Checks

The server SHALL start the connectivity checks following the procedures described in Section 5.7 and 5.8 of ICE [RFC5245] unless it is configured to use the high-reachability option. If it is then it MAY suppress its own checks until the server's checks are triggered by the client's connectivity checks.

Please note that ICE [RFC5245] section 5.8 does specify that the initiation of the checks are paced and new ones are only started every Ta milliseconds. The motivation for this is documented in Appendix B.1 of ICE [RFC5245] as for SIP/SDP all media streams within an offer/answer dialog are running using the same queue. To ensure the same behavior with RTSP, the server SHALL use a single pacer queue for all media streams within each RTSP session.

The values for the pacing of STUN and TURN transactions Ta and RTO can be configured but have the same minimum values defined in the ICE specification.

When a connectivity check from the client reaches the server it will result in a triggered check from the server as specified in Section 7.2.1.4 of ICE [RFC5245]. This is why servers with a high-reachability address can wait until this triggered check to send out any checks for itself so saving resources and mitigating the DDoS potential.

6.7. Client to Server ICE Connectivity Check

The client receives the SETUP response and learns the candidate addresses to use for the connectivity checks. The client SHALL initiate its connectivity check(s), following the procedures in Section 6 of ICE [RFC5245]. The pacing of STUN transactions (Section B.1 of [RFC5245]) SHALL be used across all media streams that are part of the same RTSP session.

Aggressive nomination SHOULD be used with RTSP during initial SETUP for a resource. This doesn't have all the negative impact that it has in offer/answer as media playing only starts after issuing a PLAY request. Thus the issue with a change of the media path being used for delivery can be avoided by not issuing a PLAY request while STUN connectivity checks are still outstanding. Aggressive nomination can result in multiple candidate pairs having their nominated flag set but according to Section 8.1.1.2 of ICE [RFC5245] when the PLAY request is sent the media will arrive on the pair with the highest priority. Note, different media resources may still end up with different foundations.

The above does not change ICE and its handling of aggressive nomination. When using aggressive nomination, a higher priority candidate pair with an outstanding connectivity check message can move into the Succeded state and the candidate pair will have its Nominated flag set. This results in the higher priority candidate pair being used instead of the previous pair, which is also in the Succeded state.

To avoid this occurring during actual media transport, the RTSP client can add additional logic when the ICE processing overall is completed to indicate if there is still higher priority connectivity checks outstanding. If some check is still outstanding, the implementation can choose to wait until some additional timeout triggers or the outstanding checks completes before progressing with a PLAY request. An alternative is to accept the risk for a path change during media delivery and start playing immediately.

RTSP clients that want to ensure that each media resource uses the same path can use regular nomination where both the ICE processing completion criteria can be controlled in addition to which media streams being nominated for use. This does not affect the RTSP server, as its role is the one of being controlled.

6.8. Client Connectivity Checks Complete

When the client has concluded all of its connectivity checks and has nominated its desired candidate pair for a particular media stream, it MAY issue a PLAY request for that stream. Note, that due to the aggressive nomination, there is a risk that any outstanding check may nominate another pair than what was already nominated. The candidate pair with the highest priority will be used for the media. If the client has locally determined that its checks have failed it may try providing an extended set of candidates and update the server candidate list by issuing a new SETUP request for the media stream.

If the client concluded its connectivity checks successfully and therefore sent a PLAY request but the server cannot conclude successfully, the server will respond with a 480 (ICE Processing Failed). Upon receiving the 480 (ICE Processing Failed) response, the client may send a new SETUP request assuming it has any new information that can be included in the candidate list. If the server is still performing the checks when receiving the PLAY request it will respond with a 150 (ICE connectivity checks in progress) response to indicate this.

6.9. Server Connectivity Checks Complete

When the RTSP server receives a PLAY request, it checks to see that the connectivity checks have concluded successfully and only then will it play the stream. If the PLAY request is for a particular media stream, the server only needs to check that the connectivity checks for that stream completed successfully. If the server has not concluded its connectivity checks, the server indicates that by sending the 150 (ICE connectivity checks in progress) [sec-sc-150]. If there is a problem with the checks, then the server sends a 480 response to indicate a failure of the checks. If the checks are successful then the server sends a 200 OK response and starts delivering media.

6.10. Freeing Candidates

Both server and client MAY free its non selected candidates as soon as a 200 PLAY response has been issued/received and no outstanding connectivity checks exist.

Clients and servers MAY free all their gathered candidates after having received or sent respectively a 480 response to a SETUP request. Client's will likely free their candidates first after having tried any additional actions that may resolve the issue, e.g. verifying the address gathering, or use additional STUN or TURN servers. Thus a server will have to weigh the cost of doing address gathering versus maintaining the gathered address for some time to allow any new SETUP request to be issued by the client.

If the 480 response is received or sent in response to a PLAY request, the server MUST NOT free its gathered candidates. Instead it will have to wait for additional actions from the client, or to terminate the RTSP session due to inactivity.

6.11. Steady State

The client and server SHALL use STUN to send keep-alive messages for the nominated candidate pair(s) following the rules of Section 10 of ICE [RFC5245]. This is important as normally RTSP play mode sessions only contain traffic from the server to the client so the bindings in the NAT need to be refreshed by the client to server traffic provided by the STUN keep-alive.

6.12. Re-SETUP

A client that decides to change any parameters related to the media stream setup will send a new SETUP request. In this new SETUP request the client MAY include a new different ICE username fragment and password to use in the ICE processing. New ICE username and password SHALL cause the ICE processing to start from the beginning again, i.e., an ICE restart (Section 9.1.1.1 of [RFC5245]). The client SHALL in case of ICE restart gather candidates and include the candidates in the transport specification for D-ICE.

ICE restarts may be triggered due to changes of client or server attachment to the network, i.e., changes to the media streams destination or source address or port. Most RTSP parameter changes would not require an ICE restart; instead existing mechanisms in RTSP for indicating from where in the RTP stream they apply is used. These include: Performing a pause prior to the parameter change and then resume; or assuming the server supports using SETUP during the PLAY state, using the RTP-Info header (Section 18.45 of [I-D.ietf-mmusic-rfc2326bis]) to indicate from where in the media stream the change shall apply.

Even if otherwise not supporting SETUP during PLAY state for other purposes, the server SHALL support SETUP requests in PLAY state, as long as the SETUP changes only the ICE parameters, which are: ICE-Password, ICE-ufrag and the content of ICE candidates.

If the RTSP session is in playing state at the time of sending the SETUP request requiring ICE restart, then the ICE connectivity checks SHALL use Regular nomination. Any ongoing media delivery continues on the previously nominated candidate pairs until the new pairs have been nominated for the individual media stream. Once the nomination of the new candidate pair has completed, all unused candidates may be released. If the ICE processing fails and no new candidate pairs are nominated for use, then the media stream MAY continue to use the previously nominated candidate pairs while they still function. If they appear to fail to transport media packets anymore then the client can select between two actions. First, if it can attempt any actions available that might make ICE work, like trying another STUN/TURN server, or change the transport parameters. In that case, the client modifies the RTSP session, and if ICE is still to be used, the client restarts ICE once more. If the client is unable to modify the transport or ICE parameters, it MUST NOT restart the ICE processing, and it SHOULD terminate the RTSP session.

6.13. Server Side Changes After Steady State

A Server may require an ICE restart because of server side load balancing or a failure resulting in an IP address and a port number change. In that case the server SHALL use the PLAY_NOTIFY method to inform the client (Section 13.5 [I-D.ietf-mmusic-rfc2326bis]) with a new Notify-Reason header: ice-restart. The server will identify if the change is for a single media or for the complete session by including the corresponding URI in the PLAY_NOTIFY request.

Upon receiving and responding to this PLAY_NOTIFY with ice-restart reason the client SHALL gather new ICE candidates, and send SETUP requests for each media stream part of the session. The server provides its candidates in the SETUP response the same way as for the first time ICE processing. Both server and client SHALL provide new ICE user names and passwords. The client MAY issue the SETUP request while the session is in PLAYING state.

If the RTSP session is in PLAYING state when the client issues the SETUP request, the client SHALL use regular nomination. If not the client will use the same procedures as for when first creating the session.

Note that keepalive messages on the previous set of candidate pairs SHOULD continue until all new candidate pairs have been nominated. After having nominated a new set of candidate pairs, the client may continue to receive media for some additional time. Even if the server stops delivering media over that candidate pair at the time of nomination, media may arrive for up to one maximum segment lifetime as defined in TCP (2 minutes). Unfortunately, if the RTSP server is divided into a separate controller and media stream, a failure may result in continued media delivery for a longer time than the maximum segment lifetime, thus source filtering is RECOMMENDED.

For example:
     
S->C: PLAY_NOTIFY rtsp://example.com/fizzle/foo RTSP/2.0
      CSeq: 854
      Notify-Reason: ice-restart
      Session: uZ3ci0K+Ld
      Server: PhonyServer 1.1

C->S: RTSP/2.0 200 OK
      CSeq: 854
      User-Agent: PhonyClient/1.2

C->S: SETUP rtsp://server.example.com/fizzle/foo/audio RTSP/2.0
      CSeq: 314
      Session: uZ3ci0K+Ld
      Transport: RTP/AVP/D-ICE; unicast; ICE-ufrag=Kl1C;
                 ICE-Password=H4sICGjBsEcCA3Rlc3RzLX; candidates="
                 1 1 UDP 2130706431 10.0.1.17 8998 typ host;
                 2 1 UDP 1694498815 192.0.2.3 51456 typ srflx 
                         raddr 10.0.1.17 rport 9002"; RTCP-mux,
                 RTP/AVP/UDP; unicast; dest_addr=":6970"/":6971",
                 RTP/AVP/TCP;unicast;interleaved=0-1
      Accept-Ranges: NPT, UTC
      User-Agent: PhonyClient/1.2

C->S: SETUP rtsp://server.example.com/fizzle/foo/video RTSP/2.0
      CSeq: 315
      Session: uZ3ci0K+Ld
      Transport: RTP/AVP/D-ICE; unicast; ICE-ufrag=hZv9;
                 ICE-Password=JAhA9myMHETTFNCrPtg+kJ; candidates="
                 1 1 UDP 2130706431 10.0.1.17 9000 typ host;
                 2 1 UDP 1694498815 192.0.2.3 51576 typ srflx 
                         raddr 10.0.1.17 rport 9000"; RTCP-mux,
                 RTP/AVP/UDP; unicast; dest_addr=":6972"/":6973",
                 RTP/AVP/TCP;unicast;interleaved=0-1
      Accept-Ranges: NPT, UTC
      User-Agent: PhonyClient/1.2

S->C: RTSP/2.0 200 OK 
      CSeq: 314
      Session: uZ3ci0K+Ld
      Transport: RTP/AVP/D-ICE; unicast; RTCP-mux; ICE-ufrag=CbDm;
                 ICE-Password=OfdXHws9XX0eBr6j2zz9Ak; candidates="
                 1 1 UDP 2130706431 192.0.2.56 50234 typ host"
      Accept-Ranges: NPT
      Date: 11 March 2011 13:17:46 GMT
      Server: PhonyServer 1.1

S->C: RTSP/2.0 200 OK 
      CSeq: 315
      Session: uZ3ci0K+Ld
      Transport: RTP/AVP/D-ICE; unicast; RTCP-mux; ICE-ufrag=jigs;
                 ICE-Password=Dgx6fPj2lsa2WI8b7oJ7+s; candidates="
                 1 1 UDP 2130706431 192.0.2.56 47233 typ host"
      Accept-Ranges: NPT
      Date: 11 March 2011 13:17:47 GMT
      Server: PhonyServer 1.1
           

7. ICE and Proxies

RTSP allows for proxies which can be of two fundamental types depending on whether they relay and potentially cache the media or not. Their differing impact on the RTSP NAT traversal solution, including backwards compatibility, is explained below.

7.1. Media Handling Proxies

An RTSP proxy that relays or caches the media stream for a particular media session can be considered to split the media transport into two parts: A media transport between the server and the proxy according to the proxy's need, and delivery from the proxy to the client. This split means that the NAT traversal solution will need to be run on each individual media leg according to need.

It is RECOMMENDED that any media handling proxy support the media NAT traversal defined within this specification. This is for two reasons: Firstly, to enable clients to perform NAT traversal for the media between the proxy and itself, and secondly to allow the proxy to be topology independent to support performing NAT traversal (to the server) for non-NAT traversal capable clients present in the same address domain as the proxy.

For a proxy to support the media NAT traversal defined in this specification a proxy will need to implement the solution fully and be able to act as both a controlling and a controlled ICE peer. The proxy also SHALL include the "setup.ice-d-m" feature tag in any applicable capability negotiation headers, such as "Proxy-Supported".

7.2. Signalling Only Proxies

A signalling only proxy handles only the RTSP signalling and does not have the media relayed through proxy functions. This type of proxy is not likely to work unless the media NAT traversal solution is in place between the client and the server, because the Denial of Service (DoS) protection measures, as discussed in Section 21.2.1 of RTSP 2.0 [I-D.ietf-mmusic-rfc2326bis], usually prevent media delivery to addresses other than from where the RTSP signalling arrives at the server.

The solution for the Signalling Only proxy is that it must forward the RTSP SETUP requests including any transport specification with the "D-ICE" lower layer and the related transport parameters. A proxy supporting this functionality SHOULD indicate its capability by always including the "setup.ice-d-m" feature tag in the "Proxy-Supported" header.

7.3. Non-supporting Proxies

A media handling proxy that doesn't support the ICE media NAT traversal specified here is assumed to remove the transport specification and use any of the lower prioritized transport specifications if provided by the requester. The specification of such a non ICE transport enables the negotiation to complete, although with a less preferred method since a NAT between the proxy and the client may result in failure of the media path.

A non-media handling proxy is expected to ignore and simply forward all unknown transport specifications, however, this can only be guaranteed for proxies following the RTSP 2.0 specification [I-D.ietf-mmusic-rfc2326bis].

Unfortunately, the usage of the "setup.ice-d-m" feature tag in the Proxy-Require will have contradicting results. For a non ICE supporting but media handling proxy, the inclusion of the feature tag will result in aborting the setup and indicating that it isn't supported, which is desirable if you want to provide other fallbacks or other transport configurations to handle the situation. For non-supporting non-media handling proxies the result will also result in aborting the setup, however, setup might have worked if the feature tag wasn't present in the Proxy-Require header. This variance in results is the reason we don't recommend the usage of the Proxy-Require header. Instead we recommend the usage of the Supported header to force proxies to include the feature tags for the intersection of what the proxy chain support in the Proxy-Supported header. This will provide a positive indication when all proxies in the chain between the client and server support the functionality.

If a proxy doesn't support the setup.ice-d-m feature, but that proxy is not a media handling proxy, the ICE based setup could still work, since such a proxy may do passthrough on any transport parameters. Unfortunately the Proxy-Require and Proxy-Supported RTSP headers failed to provide that information. The only way of finding if this is the case or not is to try perform an SETUP including a Transport header with transport specifications using ICE.

8. RTP and RTCP Multiplexing

"Multiplexing RTP Data and Control Packets on a Single Port" [RFC5761] specifies how and when RTP and RTCP can be multiplexed on the same port. This multiplexing SHALL be combined with ICE as it makes RTP and RTCP need only a single component per media stream instead of two, so reducing the load on the connectivity checks. For details on how to negotiate RTP and RTCP multiplexing, see Appendix C of RTSP 2.0 [I-D.ietf-mmusic-rfc2326bis].

Multiplexing RTP and RTCP has the benefit that it avoids the need for handling two components per media stream when RTP is used as the media transport protocol. This eliminates at least one STUN check per media stream and will also reduce the time needed to complete the ICE processing by at least the time it takes to pace out the additional STUN checks of up to one complete round trip time for a single media stream. In addition to the protocol performance improvements, the server and client side complexities are reduced as multiplexing halves the total number of STUN instances and holding the associated state. Multiplexing will also reduce the combinations and length of the list of possible candidates.

The implementation of RTP and RTCP multiplexing is additional work required for this solution. However, when implementing the ICE solution a server or client will need to implement a de-multiplexer between the STUN, and RTP or RTCP packets below the RTP/RTCP implementation anyway, so the additional work of one new demultiplexing point directly connected to the STUN and RTP/RTCP seems small relative to the benefits provided.

Due to the above mentioned benefits, RTSP servers and clients that support "D-ICE" lower layer transport in combination with RTP SHALL also implement RTP and RTCP multiplexing as specified in this section and [RFC5761].

9. Fallback and Using Partial ICE functionality to improve NAT/Firewall traversal

The need for fallback from ICE in RTSP should be less than for SIP using ICE in SDP offer/answer where a default destination candidate is very important to enable interworking with non-ICE capable endpoints. In RTSP, capability determination for ICE can happen prior to the RTSP SETUP request. This means a client should normally not need to include fallback alternatives when offering ICE, as the capability for ICE will already be determined. However, as described in this section, clients may wish to use part of the ICE functionality to improve NAT/Firewall traversal where the server is non-ICE capable.

Section 4.1.4 of the ICE [RFC5245] specification does recommend that the default destination, i.e. what is used as fallback if the peer isn't ICE capable, is a candidate of relayed type to maximize the likelihood of successful transport of media. This is based on the peer in SIP using SDP offer/answer is almost as likely as the RTSP client to be behind a NAT. For RTSP the deployment of servers are much more heavily weighted towards deployment with public reachability. In fact since publicly reachable servers behind NAT either need to support ICE or have static configurations that allow traversal, one can assume that the server will have a public address or support ICE. Thus, the selection of the default destination address for RTSP can be differently prioritized.

As an ICE enabled client behind a NAT needs to be configured with a STUN server address to be able to gather candidates successfully, this can be used to derive a server reflexive candidate for the client's port. How useful this is for a NAT'ed RTSP client as a default candidate depends on the properties of the NAT. As long as the NAT uses an address independent mapping, then using a STUN derived reflexive candidate is likely to be successful. This is however brittle in several ways, and the main reason why the original specification of STUN [RFC3489] and direct usage for NAT traversal was obsoleted. First, if the NATs behavior is attempted to be determined using STUN as described in [RFC3489], the determined behavior might not be representative of the behavior encountered in another mapping. Secondly, filter state towards the ports used by the server needs to be established. This requires that the server actually includes both address and ports in its response to the SETUP request. Thirdly messages need to be sent to these ports for keep-alive at a regular interval. How a server reacts to such unsolicited traffic is unknown. This brittleness may be accepted in fallback due to lack of support on the server side.

To maximize the likelihood that an RTSP client is capable of receiving media a relay based address should be chosen as the default fallback address. However, for RTSP clients lacking a relay server, like a TURN server, or where usage of such a server has significant cost associated with it the usage of a STUN derived server reflexive address as client default has a reasonable likelihood of functioning and may be used as an alternative.

Fallback addresses need to be provided in their own transport specification using a specifier that does not include the "D-ICE" lower layer transport. Instead the selected protocol, e.g. UDP needs to be explicitly or implicitly indicated. Secondly the selected default candidate needs to be included in the SETUP request. If this candidate is server reflexive or relayed the aspect of keep-alive needs to be ensured.

10. IANA Considerations

This document requests registration in a number of registries, both for RTSP and SDP. For all the below registrations the contact person on behalf of the IETF WG MMUSIC is Magnus Westerlund; Postal address: Farogatan 6, 164 80 Stockholm, Sweden; Email: magnus.westerlund@ericsson.com.

RFC-Editor Note: Please replace any occurrence of RFCXXXX in the below with the RFC number this specification is assigned.

10.1. RTSP Feature Tags

This document request that one RTSP 2.0 feature tag is registered in the "RTSP 2.0 Feature-tags" registry:

setup.ice-d-m

A feature tag representing the support of the ICE based establishment of datagram media transport that is capable of transport establishment through NAT and Firewalls. This feature tag applies to clients, servers and proxies and indicates support of all the mandatory functions of this specification.

10.2. Transport Protocol Specifications

This document needs to register a number of transport protocol combinations in the RTSP 2.0 "Transport Protocol Specifications" registry.

"RTP/AVP/D-ICE"

RTP using the AVP profile over an ICE established datagram flow.

"RTP/AVPF/D-ICE"

RTP using the AVPF profile over an ICE established datagram flow.

"RTP/SAVP/D-ICE"

RTP using the SAVP profile over an ICE established datagram flow.

"RTP/SAVPF/D-ICE"

RTP using the SAVPF profile over an ICE established datagram flow.

10.3. RTSP Transport Parameters

This document requests that 3 transport parameters are registered in the RTSP 2.0's "Transport Parameters" registry:

"candidates":

Listing the properties of one or more ICE candidate. See Section 4.2 of RFCXXXX.

"ICE-Password":

The ICE password used to authenticate the STUN binding request in the ICE connectivity checks. See Section 4.3 of RFCXXXX.

"ICE-ufrag":

The ICE username fragment used to authenticate the STUN binding requests in the ICE connectivity checks. See Section 4.3 of RFCXXXX.

10.4. RTSP Status Codes

This document requests that 2 assignments are done in the "RTSP 2.0 Status Codes" registry. The values are:

150:

The 150 response code indicates that ICE connectivity checks are still in progress and haven't concluded. This response SHALL be sent within 200 milliseconds of receiving a PLAY request that currently can't be fulfilled because ICE connectivity checks are still running. Subsequently, every 3 seconds after the previous sent one, a 150 reply SHALL be sent until the ICE connectivity checks conclude either successfully or in failure, and a final response for the request can be provided.

480:

The 480 response code is used in cases when the request can't be fulfilled due to a failure in the ICE processing, such as all the connectivity checks have timed out. This error message can appear either in response to a SETUP request to indicate that no candidate pair can be constructed or to a PLAY request that the server's connectivity checks resulted in failure.

10.5. Notify-Reason value

This document requests that one assignment is done in the RTSP 2.0 Notify-Reason header value registry. The defined value is:

ice-restart:

Server notifying the client about the need for an ICE restart. See Section 4.6.

10.6. SDP Attribute

The registration of one SDP attribute is requested:

   SDP Attribute ("att-field"):

     Attribute name:     rtsp-ice-d-m
     Long form:          ICE for RTSP datagram media NAT traversal 
     Type of attribute:  Session-level only
     Subject to charset: No
     Purpose:            RFC XXXX,  Section 4.7
     Values:             No values defined
     Contact:            Magnus Westerlund
                         E-mail: magnus.westerlund@ericsson.com
                         phone: +46 10 714 82 87
 

11. Security Considerations

ICE [RFC5245] and ICE TCP [RFC6544] provide an extensive discussion on security considerations which apply here as well.

11.1. ICE and RTSP

A long-standing risk with transmitting a packet stream over UDP is that the host may not be interested in receiving the stream. On today's Internet, many hosts are behind NATs or operate host firewalls which do not respond to unsolicited packets with an ICMP port unreachable error. Thus, an attacker can construct RTSP SETUP requests with a victim's IP address and cause a flood of media packets to be sent to a victim. The addition of ICE, as described in this document, provides protection from the attack described above. By performing the ICE connectivity check, the media server receives confirmation that the RTSP client wants the media. While this protection could also be implemented by requiring the IP addresses in the SDP match the IP address of the RTSP signaling packet, such a mechanism does not protect other hosts with the same IP address (such as behind the same NAT), and such a mechanism would prohibit separating the RTSP controller from the media play-out device (e.g., an IP-enabled remote control and an IP-enabled television); it also forces RTSP proxies to relay the media streams through them, even if they would otherwise be only signalling proxies.

To protect against the attacks in ICE based on signalling information RTSP signalling SHOULD be protected using TLS to prevent eavesdropping and modification of information.

The STUN amplification attack described in Section 18.5.2 in ICE [RFC5245] needs consideration. Servers that are able to run according to the high-reachability option have good mitigation against this attack as they only send connectivity checks towards an address and port pair they have received an incoming connectivity check from. This means an attacker requires both the capability to spoof source addresses and to signal the RTSP server a set of ICE candidates. Independently an ICE agent needs to implement the mitigation to reduce the volume of the amplification attack as described in the ICE specification.

12. Acknowledgements

The authors would like to thank Rémi Denis-Courmont for suggesting the method of integrating ICE in RTSP signalling, Dan Wing for help with the security section and numerous other issues, Ari Keranen for review of the document and its ICE details. Flemming Andreasen for a thourough review. In addition Bill Atwood have provided comments and suggestions for improvements.

13. References

13.1. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986]	Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
[RFC4566]	Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.
[RFC5234]	Crocker, D. 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.
[I-D.ietf-mmusic-rfc2326bis]	Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M. and M. Stiemerling, "Real Time Streaming Protocol 2.0 (RTSP)", Internet-Draft draft-ietf-mmusic-rfc2326bis-39, January 2014.
[RFC5245]	Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, April 2010.
[RFC5761]	Perkins, C. and M. Westerlund, "Multiplexing RTP Data and Control Packets on a Single Port", RFC 5761, April 2010.
[RFC6544]	Rosenberg, J., Keranen, A., Lowekamp, B.B. and A.B. Roach, "TCP Candidates with Interactive Connectivity Establishment (ICE)", RFC 6544, March 2012.

13.2. Informative References

[I-D.ietf-mmusic-rtsp-nat-evaluation]	Westerlund, M. and T. Zeng, "The Evaluation of Different Network Address Translator (NAT) Traversal Techniques for Media Controlled by Real-time Streaming Protocol (RTSP)", Internet-Draft draft-ietf-mmusic-rtsp-nat-evaluation-12, January 2014.
[RFC3022]	Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001.
[RFC2326]	Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998.
[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.
[RFC4340]	Kohler, E., Handley, M. and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[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.

Authors' Addresses