AVT A. Begen
Internet-Draft D. Wing
Intended status: Standards Track Cisco
Expires: May 22, 2011 T. VanCaenegem
Alcatel-Lucent
November 18, 2010
Token-Based Port Mapping Between Unicast and Multicast RTP Sessions
draft-ietf-avt-ports-for-ucast-mcast-rtp-03
Abstract
This document presents a port mapping solution that allows RTP
receivers to choose their own ports for an auxiliary unicast session
in RTP applications using both unicast and multicast services
(almost) without the need for retrieving pre-authorization.
Status of this Memo
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This Internet-Draft will expire on May 22, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5
3. Token-Based Port Mapping . . . . . . . . . . . . . . . . . . . 6
3.1. Token Request and Retrieval . . . . . . . . . . . . . . . 6
3.2. Unicast Session Establishment . . . . . . . . . . . . . . 6
4. The portmapping-req Attribute . . . . . . . . . . . . . . . . 11
5. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Port Mapping Request . . . . . . . . . . . . . . . . . . . 13
5.2. Port Mapping Response . . . . . . . . . . . . . . . . . . 13
5.3. Token Verification . . . . . . . . . . . . . . . . . . . . 14
6. Procedures for Token Construction . . . . . . . . . . . . . . 15
7. Validating Tokens . . . . . . . . . . . . . . . . . . . . . . 16
8. SDP Example . . . . . . . . . . . . . . . . . . . . . . . . . 17
9. Address Pooling NATs . . . . . . . . . . . . . . . . . . . . . 19
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
11.1. Registration of SDP Attributes . . . . . . . . . . . . . . 21
11.2. Registration of FMT Values . . . . . . . . . . . . . . . . 21
11.3. SFMT Values for Port Mapping Messages Registry . . . . . . 21
11.4. RAMS Response Code Space Registry . . . . . . . . . . . . 22
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
13.1. Normative References . . . . . . . . . . . . . . . . . . . 24
13.2. Informative References . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
In (any-source or source-specific) multicast RTP applications,
destination ports, i.e., the ports on which the multicast receivers
receive the RTP and RTCP packets, are defined declaratively. In
other words, the receivers cannot choose their receive ports and the
sender(s) use the pre-defined ports.
In unicast RTP applications, the receiving end needs to choose its
ports for RTP and RTCP since these ports are local resources and only
the receiving end can determine which ports are available to use.
The receiving may convey its request to the sending end through
different ways, one of which is the Offer/Answer Model [RFC3264] for
the Session Description Protocol (SDP) [RFC4566]. However, the
Offer/Answer Model requires offer/answer exchange(s) between the
endpoints, and the resulting delay may not be desirable in delay-
sensitive real-time applications. Furthermore, the Offer/Answer
Model may be burdensome for the endpoints that are concurrently
running a large number of unicast sessions with other endpoints.
In this specification, we consider an RTP application that uses one
or more unicast and multicast RTP sessions together. While the
declaration and selection of the ports are well defined and work well
for multicast and unicast RTP applications, respectively, the usage
of the ports introduces complications when a receiving end mixes
unicast and multicast RTP sessions within the same RTP application.
An example scenario is where the RTP packets are distributed through
source-specific multicast (SSM) and a receiver sends unicast RTCP
feedback to a local repair server (also functioning as a feedback
target) [RFC5760] asking for a retransmission of the packets it is
missing, and the local repair server sends the retransmission packets
over a unicast RTP session [RFC4588].
Another scenario is where a receiver wants to rapidly acquire a new
primary multicast RTP session and receives one or more RTP burst
packets over a unicast session before joining the SSM session
[I-D.ietf-avt-rapid-acquisition-for-rtp]. Similar scenarios exist in
applications where some part of the content is distributed through
multicast while the receivers get additional and/or auxiliary content
through one or more unicast connections, as sketched in Figure 1.
In this document, we discuss this problem and introduce a solution
that we refer to as Port Mapping. This solution allows receivers to
choose their desired UDP ports for RTP and RTCP in every unicast
session when they are running RTP applications using both unicast and
multicast services, and offer/answer exchange is not available. This
solution is not applicable in cases where TCP is used as the
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transport protocol in the unicast sessions. For such scenarios,
refer to [RFC4145].
-----------
| Unicast |................
| Source |............. :
| (Server) | : :
----------- : :
v v
----------- ---------- -----------
| Multicast |------->| Router |---------->|Client RTP |
| Source | | |..........>|Application|
----------- ---------- -----------
| :
| : -----------
| :..............>|Client RTP |
+---------------->|Application|
-----------
-------> Multicast RTP Flow
.......> Unicast RTP Flow
Figure 1: RTP applications simultaneously using both unicast and
multicast services
In the remainder of this document, we refer to the RTP endpoints that
serve other RTP endpoints over a unicast session as the Servers. The
receiving RTP endpoints are referred to as Clients.
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2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. Token-Based Port Mapping
Token-based Port Mapping consists of two steps: (i) Token request
and retrieval, and (ii) unicast session establishment. These are
described below.
3.1. Token Request and Retrieval
This first step is required to be completed only once. Once a Token
is retrieved from a particular server, it can be used for all the
unicast sessions the client will be running with this particular
server. By default, Tokens are server specific. However, the client
can use the same Token to communicate with different servers if these
servers are provided with the same key used to generate the Token.
The Token becomes invalid if client's public IP address changes or
when the server expires the Token. In these cases, the client has to
request a new Token.
The Token is essentially an opaque encapsulation that conveys
client's IP address information (as seen by the server) using a
reversible transform only known to the server. When a request is
received, the server creates a Token for this particular client, and
sends it back to the client. Later, when the client wants to
establish a unicast session, the Token will be validated by the
server, making sure that the IP address information matches. This is
effective against DoS attacks, e.g., an attacker cannot simply spoof
another client's IP address and start a unicast transmission towards
random clients.
3.2. Unicast Session Establishment
We illustrate the second step with an example. Consider an SSM
distribution network where a distribution source multicasts RTP
packets to a large number of clients, and one or more retransmission
servers function as feedback targets to collect unicast RTCP feedback
from these clients [RFC5760]. The retransmission servers also join
the multicast session to receive the multicast packets and cache them
for a certain time period. When a client detects missing packets in
the multicast session, it requests a retransmission from one of the
retransmission servers by using an RTCP NACK message [RFC4585]. The
retransmission server pulls the requested packet(s) out of the cache
and retransmits them to the requesting client [RFC4588].
The pertaining RTP and RTCP flows are sketched in Figure 2. Between
the client and server, there can be one or more Network Address Port
Translators (NAPT - hereafter simply called NAT) devices [RFC4787].
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-------------- --- ----------
| |-------------------------------| |-->|P1 |
| |-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-| |.->|P2 |
| | | | | |
| Distribution | ---------------- | | | |
| Source | | | | | | |
| |---->|P1 | | | | |
| |.-.->|P2 | | | | |
| | | | | | | |
-------------- | P3|<.=.=.=.| |=.=|*c0 |
| P3|<~~~~~~~| |~~~|*c1 |
MULTICAST RTP | | | | | |
SESSION with | | | | | |
UNICAST FEEDBACK | | | N | | |
| Retransmission | | A | | Client |
- - - - - - - - - - -| - - - - - - - -| - - - -| - |- -| - - - - -|-
| Server | | T | | |
| | | | | |
PORT MAPPING | PT|<~~~~~~~| |~~>|*cT |
| | | | | |
- - - - - - - - - - -| - - - - - - - -| - - - -| - |- -| - - - - -|-
| | | | | |
AUXILIARY UNICAST | | | | | |
RTP SESSION | | | | | |
| P3|........| |..>|*c1 |
| P3|=.=.=.=.| |=.>|*c1 |
| P4|<.=.=.=.| |=.=|*c2 |
| | | | | |
---------------- --- ----------
-------> Multicast RTP Flow
.-.-.-.> Multicast RTCP Flow
.=.=.=.> Unicast RTCP Reports
~~~~~~~> Unicast RTCP Feedback Messages
.......> Unicast RTP Flow
Figure 2: Example scenario showing an SSM distribution with support
for retransmissions from a server
In this figure, we have the following multicast and unicast ports:
o Ports P1 and P2 denote the destination RTP and RTCP ports in the
multicast session, respectively. The clients listen to these
ports to receive the multicast RTP and RTCP packets. Ports P1 and
P2 are defined declaratively.
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o Port P3 denotes the RTCP port on the feedback target running on
the retransmission server to collect the RTCP feedback messages,
and RTCP receiver and extended reports from the clients in the
multicast session. This is also the port that the retransmission
server uses to send the RTP packets and RTCP sender reports in the
unicast session. Port P3 is defined declaratively.
o Port P4 denotes the RTCP port on the retransmission server used to
collect the RTCP receiver and extended reports for the unicast
session. Port P4 is defined declaratively and MUST be different
from port P3.
o Ports *c0, *c1 and *c2 are chosen by the client. *c0 denotes the
port on the client used to send the RTCP reports for the multicast
session. *c1 denotes the port on the client used to send the
unicast RTCP feedback messages in the multicast session and to
receive the RTP packets and RTCP sender reports in the unicast
session. *c2 denotes the port on the client used to send the RTCP
receiver and extended reports in the unicast session. Ports c0,
c1 and c2 MAY be the same port or different ports. However, there
are two advantages of using the same port for both c0 and c1:
1. Some NATs only keep bindings active when a packet goes from
the inside to the outside of the NAT (See REQ-6 of Section 4.3
of [RFC4787]). When the retransmission server sends unicast
packets for a long period of time, this can exceed that
timeout. If c0=c1, the occasional (periodic) RTCP receiver
reports sent from port c0 (for the multicast session) will
ensure the NAT does not time out the public port associated
with the incoming unicast traffic to port c1.
2. Having c0=c1 conserves NAT port bindings.
Thus, it is strongly RECOMMENDED that the client uses the same
port for c0 and c1.
o Ports PT and cT denote the ports through which the Token request
and retrieval occur at the server and client sides, respectively.
Port PT is declared on a per unicast session basis, although its
value MAY be the same for all unicast sessions sourced by the
server. This way, a Token once requested and retrieved by a
client from port PT remains valid across different unicast
sessions. Port PT MAY be equal to port P3. Port cT MAY also be
equal to ports c0 and c1.
In addition to the ports, we use the following notation:
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o DS: IP address of the distribution source
o G: Destination multicast address
o S: IP address of the retransmission server
o C: IP address of the client
o C': Public IP address of the client (as seen by the server)
We assume that the information declaratively defined is available as
part of the session description information and is provided to the
clients. The Session Description Protocol (SDP) [RFC4566] and other
session description methods can be used for this purpose.
The following steps summarize the Token-based solution:
1. The client ascertains server address (S) and port numbers (P3 and
P4) from the session description.
2. The client determines its port numbers (*c0, *c1 and *c2).
3. If the client does not have a valid Token:
A. The client first sends a message to the server via a new RTCP
message, called Port Mapping Request to port PT. This
message is sent from port cT on the client side. The server
learns client's public IP address (C') from the received
message. The client can send this message anytime it wants
(e.g., during initialization), and does not normally ever
need to re-send this message (See Section 7).
B. The server generates an opaque encapsulation (i.e., the
Token) that conveys client's IP address information using a
reversible transform only known to the server. For details,
see Section 6.
C. The server sends the Token back to the client using a new
RTCP message, called Port Mapping Response. This message
MUST be sent from port PT to port cT.
4. The client provides the Token to the server using a new RTCP
message, called Token Verification, whenever the client sends an
RTCP feedback message for triggering or controlling a unicast
session. Note that the unicast session is only established after
the server has received a feedback message (along with a valid
Token) from the client for which it needs to react by sending
unicast data. Until a unicast session is established, neither
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the server nor the client needs to send RTCP reports for the
unicast session.
5. Normal flows ensue as shown in Figure 2. Note that in the
unicast session the RTP and RTCP packets MUST be multiplexed on
the (same) port c1. If the client uses the same port for both c0
and c1, the RTCP reports sent for the multicast session keep the
P3->c1(=c0) binding alive. If the client uses different ports
for c0 and c1, the client needs to periodically send an explicit
keep-alive message [I-D.ietf-avt-app-rtp-keepalive] to keep the
P3->c1 binding alive during the lifetime of the unicast session
if the unicast session's lifetime is likely to exceed the NAT's
timeout value.
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4. The portmapping-req Attribute
This new SDP attribute is used declaratively to indicate the port for
obtaining a Token. Its presence indicates that a Token MUST be
included in the feedback messages sent to the server triggering or
controlling a unicast session.
The formal description of the 'portmapping-req' attribute is defined
by the following ABNF [RFC5234] syntax:
portmapping-req-attribute = "a=portmapping-req:" port CRLF
Here, 'port' is defined as specified in Section 9 of [RFC4566]. The
'portmapping-req' attribute is used as a session-level or media-level
attribute.
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5. Message Formats
This section defines the formats of the RTCP transport-layer feedback
messages that are exchanged between a server and a client for the
purpose of Token-based port mapping. Three RTCP messages are
defined:
1. Port Mapping Request
2. Port Mapping Response
3. Token Verification
These are all payload-independent RTCP feedback messages with a
common format defined in Section 6.1 of [RFC4585], also sketched in
Figure 3.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| FMT | PT | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Feedback Control Information (FCI) :
: :
Figure 3: The common packet format for the RTCP feedback messages
Each feedback message has a fixed-length field for version, padding,
feedback message type (FMT), packet type (PT), length, SSRC of packet
sender, SSRC of media source as well as a variable-length field for
feedback control information (FCI).
In the new messages defined in this section, the PT field is set to
RTPFB (205) and the FMT field is set to Port Mapping (7). Individual
Port Mapping messages are identified by a sub-field called Sub
Feedback Message Type (SFMT). Any Reserved field SHALL be set to
zero and ignored.
Following the rules specified in [RFC3550], all integer fields in the
messages defined below are carried in network-byte order, that is,
most significant byte (octet) first, also known as big-endian.
Unless otherwise stated, numeric constants are in decimal (base 10).
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5.1. Port Mapping Request
The Port Mapping Request message is identified by SFMT=1. This
message is a unicast feedback message transmitted by the client to a
dedicated server port to request a Token. In the Port Mapping
Request message, the client MUST set both the packet sender SSRC and
media source SSRC fields to its own SSRC since the Port Mapping
Request message is not necessarily linked to any specific media
source. The FCI field has the structure depicted in Figure 4.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFMT=1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Random |
| Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: The FCI field of Port Mapping Request message
o Random Nonce (64 bits): Mandatory field that contains a random
nonce value generated by the client following the procedures of
[RFC4086]. This nonce MUST be taken into account by the server
when generating a Token for the client to enable better security
for clients that share the same IP address. If the Port Mapping
Request message is transmitted multiple times for redundancy
reasons, the random nonce value MUST remain the same in these
duplicated messages.
5.2. Port Mapping Response
The Port Mapping Response message is identified by SFMT=2. This
message is sent by the server and delivers the Token to the client.
In the Port Mapping Response message, the packet sender SSRC and
media sender SSRC fields are both set to the client's SSRC since the
Port Mapping Response message is not necessarily linked to any
specific media source. The FCI field has the structure depicted in
Figure 5.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFMT=2 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Token :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Expiry Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: FCI field syntax for the Port Mapping Response message
o Token (128 bits): Mandatory element that contains the Token
generated by the server.
o Expiry Time (32 bits): Mandatory element that contains the expiry
time of the Token. The expiry time is expressed in seconds from
the time the Token was generated. An expiry time of zero
indicates that the accompanying Token is not valid.
5.3. Token Verification
The Token Verification message is identified by SFMT=3. This message
contains the Token and MUST accompany any other RTCP feedback message
sent by the client to trigger or control a unicast session. Examples
include the RAMS-R and RAMS-T messages
[I-D.ietf-avt-rapid-acquisition-for-rtp] as well as the NACK messages
[RFC4585]. In the Token Verification message, the client MUST set
both the packet sender SSRC and media source SSRC fields to its own
SSRC since the media source SSRC may not be known. The client MUST
NOT send a Token Verification message with a Token that has expired.
The FCI field has the structure depicted in Figure 6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFMT=3 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Token :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: FCI field syntax for the Token Verification message
o Token (128 bits): Mandatory element that contains the Token
previously acquired by the client.
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6. Procedures for Token Construction
Editor's notes:
The Token SHOULD be calculated by the server by taking into account:
o Client's IP address as seen by the server
o The nonce generated by the client and inserted in the Port Mapping
Request message
o A timestamp to protect against replay attacks
o HMAC [RFC2104] of the above information (where only the server
knows the HMAC secret)
The server conveys the expiration time in the clear to the client in
the Port Mapping Response message. Thus, the client can request a
new Token before the current one expires.
Details are TBC.
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7. Validating Tokens
Upon receipt of an RTCP feedback message along with the Token
Verification message that contains a Token, the server MUST validate
the Token. The server considers a Token valid if the source IP
address of the RTCP feedback message matches the IP address in the
Token and the Token has not expired yet.
The IP address is encoded into the Token by the server, using an
algorithm known only to the server. This, combined with the
expiration time provides protection against DoS attacks so that a
client using a certain IP address cannot cause one or more RTP
packets to be sent to another client with a different IP address.
When the server detects that the Token is invalid, it SHOULD NOT
silently discard client's message since this adds an undesired delay.
Instead, it is RECOMMENDED that applications define an application-
specific error response. In applications that have not defined an
error response, the server MUST reply back to the client with a Port
Mapping Response message (that goes from port P3 on the server to
port c1 on the client) where the Token field carries the invalid
Token sent by the client and the Expiry Time field is set to zero
(indicating that the Token is invalid).
For applications using [I-D.ietf-avt-rapid-acquisition-for-rtp], this
draft defines a new 4xx-level response code in the RAMS Response Code
Space Registry.
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8. SDP Example
The declarative SDP describing the scenario given in Figure 2 is
written as:
v=0
o=ali 1122334455 1122334466 IN IP4 nack.example.com
s=Local Retransmissions
t=0 0
a=group:FID 1 2
a=rtcp-unicast:rsi
m=video 41000 RTP/AVPF 98
i=Multicast Stream
c=IN IP4 233.252.0.2/255
a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1 ; Note 1
a=rtpmap:98 MP2T/90000s
a=multicast-rtcp:41500 ; Note 1
a=rtcp:42000 IN IP4 192.0.2.1 ; Note 2
a=rtcp-fb:98 nack ; Note 2
a=mid:1
m=video 42000 RTP/AVPF 99 ; Note 3
i=Unicast Retransmission Stream
c=IN IP4 192.0.2.1
a=sendonly
a=rtpmap:99 rtx/90000
a=rtcp-mux ; Note 4
a=rtcp:42500 ; Note 5
a=fmtp:99 apt=98; rtx-time=5000
a=portmapping-req:30000 ; Note 6
a=mid:2
Figure 7: SDP describing an SSM distribution with support for
retransmissions from a local server
In this description, we highlight the following notes:
Note 1: The source stream is multicast from a distribution source
with a source IP address of 198.51.100.1 (DS) to the multicast
destination address of 233.252.0.2 (G) and port 41000 (P1). The
associated RTCP packets are multicast in the same group to port 41500
(P2).
Note 2: A retransmission server including feedback target
functionality with an IP address of 192.0.2.1 (S) and port of 42000
(P3) is specified with the 'rtcp' attribute. The feedback
functionality is enabled for the RTP stream with payload type 98
through the 'rtcp-fb' attribute [RFC4585].
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Note 3: The port specified in the second "m" line (for the unicast
stream) does not mean anything in this scenario as the client does
not send any RTP traffic back to the server.
Note 4: The server multiplexes RTP and RTCP packets on the same port
(c1 in Figure 2).
Note 5: The server uses port 42500 (P4) for the unicast sessions.
Note 6: The "a=portmapping-req" line indicates that a Token needs to
be retrieved first before a unicast session associated to the
multicast session can be established and that the Port Mapping
Request message needs to be sent to port 30000 (PT).
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9. Address Pooling NATs
Large-scale NAT (LSN) devices have a pool of public IPv4 addresses
and map internal hosts to one of those public IPv4 addresses. As
long as an internal host maintains an active mapping in the NAT, the
same IPv4 address is assigned to new connections. However, once all
of the host's mappings have been deleted (e.g., because of timeout),
it is possible that a new connection from that same host will be
assigned a different IPv4 address from the pool. When that occurs,
the Token will be considered invalid by the server, causing an
additional round trip for the client to acquire a fresh Token.
Any traffic from the host which traverses the NAT will prevent this
problem. As the host is sending RTCP receiver reports at least every
5 seconds (Section 6.2 of [RFC3550]) for the multicast session it is
receiving, those RTCP messages will be sufficient to prevent this
problem.
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10. Security Considerations
The Token, which is generated based on a client's IP address and
expiration date, provides protection against denial-of-service (DoS)
attacks. An attacker using a certain IP address cannot cause one or
more RTP packets to be sent to a victim client who has a different IP
address. However, if the attacker acquires a valid Token for a
victim and can spoof the victim's source address, this approach
becomes vulnerable to replay attacks.
Multicast is deployed on managed networks - not the Internet. These
managed networks will choose to enable network ingress filtering
[RFC2827] or not. If ingress filtering is enabled on a network, an
attacker attacker cannot spoof a victim's IP address to use a Token
to initiate an attack against a victim. However, if ingress
filtering is not enabled on a network, an attacker could obtain a
Token and spoof the victim's address, causing traffic to flood the
victim. On such a network, the server can reduce the time period for
such an attack by expiring a Token in a short period of time. In the
extreme case, the server can expire the Token immediately after its
first use. An expired Token forces a round trip from the client to
the server.
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11. IANA Considerations
The following contact information shall be used for all registrations
in this document:
Ali Begen
abegen@cisco.com
Note to the RFC Editor: In the following, please replace "XXXX" with
the number of this document prior to publication as an RFC.
11.1. Registration of SDP Attributes
This document registers a new attribute name in SDP.
SDP Attribute ("att-field"):
Attribute name: portmapping-req
Long form: Port for requesting Token
Type of name: att-field
Type of attribute: Either session or media level
Subject to charset: No
Purpose: See this document
Reference: [RFCXXXX]
Values: See this document
11.2. Registration of FMT Values
Within the RTPFB range, the following format (FMT) value is
registered:
Name: Port Mapping
Long name: Port Mapping Between Unicast and Multicast RTP Sessions
Value: 7
Reference: [RFCXXXX]
11.3. SFMT Values for Port Mapping Messages Registry
This document creates a new sub-registry for the sub-feedback message
type (SFMT) values to be used with the FMT value registered for Port
Mapping messages. The registry is called the SFMT Values for Port
Mapping Messages Registry. This registry is to be managed by the
IANA according to the Specification Required policy of [RFC5226].
The length of the SFMT field in the Port Mapping messages is a single
octet, allowing 256 values. The registry is initialized with the
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following entries:
Value Name Reference
----- -------------------------------------------------- -------------
0 Reserved [RFCXXXX]
1 Port Mapping Request [RFCXXXX]
2 Port Mapping Response [RFCXXXX]
3 Token Verification [RFCXXXX]
4-254 Assignable - Specification Required
255 Reserved [RFCXXXX]
The SFMT values 0 and 255 are reserved for future use.
Any registration for an unassigned SFMT value needs to contain the
following information:
o Contact information of the one doing the registration, including
at least name, address, and email.
o A detailed description of what the new SFMT represents and how it
shall be interpreted.
11.4. RAMS Response Code Space Registry
This document adds the following entry to the RAMS Response Code
Space Registry.
Code Description Reference
----- -------------------------------------------------- -------------
405 Invalid Token [RFCXXXX]
This response code is used when the Token included by the RTP_Rx in
the RAMS-R message is invalid.
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12. Acknowledgments
The approach presented in this document came out after discussions
with various individuals in the AVT and MMUSIC WGs, and the breakout
session held in the Anaheim meeting. We thank each of these
individuals.
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13. References
13.1. Normative References
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
Protocol (RTCP) Extensions for Single-Source Multicast
Sessions with Unicast Feedback", RFC 5760, February 2010.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
13.2. Informative References
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.
[I-D.ietf-avt-rapid-acquisition-for-rtp]
Steeg, B., Begen, A., Caenegem, T., and Z. Vax, "Unicast-
Based Rapid Acquisition of Multicast RTP Sessions",
draft-ietf-avt-rapid-acquisition-for-rtp-16 (work in
progress), October 2010.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
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[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[I-D.ietf-avt-app-rtp-keepalive]
Marjou, X. and A. Sollaud, "Application Mechanism for
keeping alive the Network Address Translator (NAT)
mappings associated to RTP flows.",
draft-ietf-avt-app-rtp-keepalive-09 (work in progress),
September 2010.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
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Authors' Addresses
Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
Canada
Email: abegen@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
USA
Email: dwing@cisco.com
Tom VanCaenegem
Alcatel-Lucent
Copernicuslaan 50
Antwerpen, 2018
Belgium
Email: Tom.Van_Caenegem@alcatel-lucent.com
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