AVT A. Begen
Internet-Draft D. Wing
Intended status: Standards Track Cisco
Expires: June 14, 2011 T. VanCaenegem
Alcatel-Lucent
December 11, 2010
Port Mapping Between Unicast and Multicast RTP Sessions
draft-ietf-avt-ports-for-ucast-mcast-rtp-08
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. The
solution provides protection against denial-of-service attacks that
could be used to cause one or more RTP packets to be sent to a victim
client.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 14, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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
3.2.1. Motivating Scenario . . . . . . . . . . . . . . . . . 6
3.2.2. Normative Behavior and Requirements . . . . . . . . . 9
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Port Mapping Request . . . . . . . . . . . . . . . . . . . 11
4.2. Port Mapping Response . . . . . . . . . . . . . . . . . . 12
4.3. Token Verification Request . . . . . . . . . . . . . . . . 14
4.4. Token Verification Failure . . . . . . . . . . . . . . . . 15
5. Procedures for Token Construction . . . . . . . . . . . . . . 17
6. Validating Tokens . . . . . . . . . . . . . . . . . . . . . . 19
7. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. The portmapping-req Attribute . . . . . . . . . . . . . . 20
7.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 21
7.3. Example and Discussion . . . . . . . . . . . . . . . . . . 21
8. Address Pooling NATs . . . . . . . . . . . . . . . . . . . . . 24
9. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9.1. Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . 25
9.2. The portmapping-req Attribute . . . . . . . . . . . . . . 25
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
10.1. Registration of SDP Attributes . . . . . . . . . . . . . . 27
10.2. Registration of RTCP Control Packet Types . . . . . . . . 27
10.3. SMT Values for TOKEN Packet Type Registry . . . . . . . . 27
10.4. RAMS Response Code Space Registry . . . . . . . . . . . . 28
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
12.1. Normative References . . . . . . . . . . . . . . . . . . . 30
12.2. Informative References . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
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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. In
addition, Network Address Port Translators (NAPT - hereafter simply
called NAT) devices are commonly deployed in networks, thus, static
port assignments cannot be used. 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
NACK feedback to a local repair server (also functioning as a unicast
RTCP 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
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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
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 secret key and algorithm used to
generate the Token and are at least loosely clock-synchronized. 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 is based on
client's IP address (as seen by 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
The second step is the unicast session establishment. We illustrate
this step with an example. First, we describe the motivation
scenario and then define the normative behavior and requirements.
3.2.1. Motivating Scenario
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].
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The RTP and RTCP flows pertaining to the scenario described above are
sketched in Figure 2. Between the client and server, there can be
one or more NAT devices [RFC4787].
-------------- --- ----------
| |-------------------------------| |-->|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 Figure 2, we have the following multicast and unicast ports:
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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.
o Port P3 denotes the RTCP port on the feedback target running on
the retransmission server to collect any RTCP packet sent by the
clients including feedback messages, and RTCP receiver and
extended reports. 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.
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 could be the same port or different ports. 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 gap between the packets sent from the
client to the server is long, this can exceed that timeout.
If c0=c1, the occasional (periodic) RTCP receiver reports sent
from port c0 (for the multicast session's RTCP port P3) 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.
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 the
same port could be used for two or more unicast sessions sourced
by the server. A Token once requested and retrieved by a client
from port PT remains valid until its expiration time.
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.
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3.2.2. Normative Behavior and Requirements
In this section, we describe the normative behavior and requirements.
To simplify the presentation, we refer to the port numbers described
in the example presented in Figure 2. However, the behavior and
requirements described here are not specific to that particular
example.
The following steps summarize the Token-based solution:
1. The client ascertains server address and port numbers (P3, P4 and
PT) from the session description. Port P4 MUST be different from
port P3. Port PT MAY be equal to port P3.
2. The client selects its local port numbers (*c0, *c1, *c2 and
*cT). It is strongly RECOMMENDED that the client uses the same
port for c0 and c1. Port cT MAY be equal to ports c0 and c1.
A client cannot keep using the same receive port for different
unicast sessions since there could be packet leakage when
switching from one unicast session to another unless each
received unicast stream has its own distinct Synchronization
Source (SSRC) identifier to allow the client to filter out the
undesired packets. Unless this is guaranteed (which is not often
easy), a client SHOULD use separate receive ports for subsequent
unicast sessions. After a sufficient time, a previously used
receive port could be used again.
3. If the client does not have a Token (or the existing Token has
expired):
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 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 6).
B. The server generates an opaque encapsulation (i.e., the
Token) based on certain information including client's IP
address.
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.
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4. The client needs to provide the Token to the server using a new
RTCP message, called Token Verification Request, whenever the
client sends an RTCP feedback message for triggering or
controlling a unicast session (See Section 4.3). 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 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, traffic from the server to the client (i.e.,
both the RTP and RTCP packets sent from port P3 to port c1) 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. Message Formats
This section defines the formats of the RTCP messages that are
exchanged between a server and a client for the purpose of port
mapping. A new RTCP control packet type is introduced and four port
mapping messages using this control packet are defined:
1. Port Mapping Request
2. Port Mapping Response
3. Token Verification Request
4. Token Verification Failure
Each message has a fixed-length field for version (V), padding (P),
sub-message type (SMT), packet type (PT), length and SSRC of packet
sender. Messages have other fields as defined below. In all
messages defined in this section, the PT field is set to TOKEN (210).
Individual messages are identified by the SMT field. The length
field indicates the message size in 32-bit words minus one, including
the header and any padding. This definition is in line with the
definition of the length field used in RTCP sender and receiver
reports. In all messages, 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).
Note that RTCP is not a timely or reliable protocol. The RTCP
packets might get lost or re-ordered in the network. When sending a
new Port Mapping Request message, the scheduling rules that apply to
sending initial RTCP messages [RFC4585] apply. When a client sends a
Port Mapping Request or Token Verification Request message but it
does not receive a response back from the server (either a Port
Mapping Response or Token Verification Failure message), it MAY
resend its request by following the timer rules defined for RTCP
feedback messages in Section 3.5 of [RFC4585] as a good practice.
When sending an RTCP (feedback) message bundled with a Token
Verification Request message, the timer rules of [RFC4585] apply as
usual.
4.1. Port Mapping Request
The Port Mapping Request message is identified by SMT=1. This
message is transmitted by the client to a dedicated server port (and
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possibly an address) to request a Token. In the Port Mapping Request
message, the packet sender SSRC is set to the client's SSRC. The
packet format has the structure depicted 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| SMT=1 | PT=TOKEN=210 | Length=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Packet Sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Random |
| Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Packet format for the 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 is 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. However, the client MUST generate a new
random nonce for every new Port Mapping Request message.
4.2. Port Mapping Response
The Port Mapping Response message is identified by SMT=2. This
message is sent by the server and delivers the Token to the client as
a response to the Port Mapping Request message. In the Port Mapping
Response message, the packet sender SSRC is set to the server's SSRC.
The packet format has the structure depicted in Figure 4.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| SMT=2 | PT=TOKEN=210 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Packet Sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Requesting Client |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Associated |
| Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Token Element :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Absolute |
| Expiration Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Relative Expiration Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Packet format for the Port Mapping Response message
o SSRC of Requesting Client (32 bits): Mandatory field that
contains the SSRC of the client who sent the request.
o Associated Nonce (64 bits): Mandatory field that contains the
nonce received in the Port Mapping Request message and used in
Token construction.
o Token Element (Variable size): Mandatory element that is used to
carry the Token generated by the server. This element is a
Length-Value element. The Length field, which is 8 bits,
indicates the length (in octets) of the Value field that follows
the Length field. The Value field carries the Token (or more
accurately, the output of the encoding process on the server).
o Absolute Expiration Time (64 bits): Mandatory field that contains
the absolute expiration time of the Token. The absolute
expiration time is expressed as a Network Time Protocol (NTP)
timestamp value in seconds since year 1900 [RFC5905]. The client
does not need to use this element directly, thus, does not need to
synchronize its clock with the server. However, the client needs
to send this element back to the server along with the associated
nonce in the Token Verification Request message, thus, needs to
keep it associated with the Token.
o Relative Expiration Time (32 bits): Mandatory field that contains
the relative expiration time of the Token. The relative
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expiration time is expressed in seconds from the time the Token
was generated. A relative expiration time of zero indicates that
the accompanying Token is not valid.
The server conveys the relative expiration time in the clear to
the client to allow the client to request a new Token well before
the expiration time.
4.3. Token Verification Request
The Token Verification Request message is identified by SMT=3. This
message contains the Token and accompanies any RTCP message that
would trigger a new or control an existing unicast session.
Currently, the following RTCP messages are REQUIRED to be accompanied
by a Token Verification Request message:
o Messages that trigger a new unicast session:
* NACK messages [RFC4585]
* RAMS-R messages [I-D.ietf-avt-rapid-acquisition-for-rtp]
o Messages that control an existing unicast session associated with
a multicast session:
* BYE messages [RFC3550]
* RAMS-T messages [I-D.ietf-avt-rapid-acquisition-for-rtp]
* CCM messages [RFC5104]
Other RTCP messages defined in the future, which could be abused to
cause packet amplification attacks, SHOULD also be authenticated
using the mechanism described in this document. The Token
Verification Request message might also be bundled with packets
carrying RTCP receiver or extended reports. While such packets do
not have a strong security impact, a specific application might
desire to have a more controlled reporting scheme from the clients.
In the Token Verification Request message, the packet sender SSRC is
set to the client's SSRC. The client MUST NOT send a Token
Verification Request message with a Token that has expired. The
packet format 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| SMT=3 | PT=TOKEN=210 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Packet Sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Associated |
| Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Token Element :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Associated Absolute |
| Expiration Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Packet format for the Token Verification Request message
o Associated Nonce (64 bits): Mandatory field that contains the
nonce associated with the Token above.
o Token Element (Variable size): Mandatory Token element that was
previously received in the Port Mapping Response message.
o Associated Absolute Expiration Time (64 bits): Mandatory field
that contains the absolute expiration time associated with the
Token above.
4.4. Token Verification Failure
The Token Verification Failure message is identified by SMT=4. This
message is sent by the server and notifies the client that the Token
was invalid or that the client did not include a Token Verification
Request message in the RTCP packet although it was supposed to. In
the Token Verification Failure message, the packet sender SSRC is set
to the server's SSRC. The packet format has the structure depicted
in Figure 6.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| SMT=4 | PT=TOKEN=210 | Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Packet Sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Requesting Client |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Associated |
| Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Packet format for the Token Verification Failure message
o SSRC of Requesting Client (32 bits): Mandatory field that
contains the SSRC of the client who sent the verification request.
o Associated Nonce (64 bits): Mandatory field that contains the
nonce received in the Token Verification Request message. If
there was no Token Verification Request message included by the
client, this field is set to 0.
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5. Procedures for Token Construction
The Token encoding is known to the server but opaque to the client.
Implementations MUST encode the following information into the Token
as a minimum, in order to provide adequate security:
o Client's IP address as seen by the server (32/128 bits for IPv4/
IPv6 addresses)
o The nonce generated and inserted in the Port Mapping Request
message by the client (64 bits)
o The absolute expiration time chosen by the server indicated as an
NTP timestamp value in seconds since year 1900 [RFC5905] (64 bits,
to protect against replay attacks)
An example way for constructing Tokens is to perform HMAC-SHA1
[RFC2104] on the concatenated values of the information listed above.
The HMAC key should be at least 160 bits long, and generated using a
cryptographically secure random source [RFC4086]. However,
implementations MAY adopt different approaches and are encouraged to
encode whatever additional information is deemed necessary or useful.
For example, key rollover is simplified by encoding a key-id into the
Token. As another example, a cluster of anycast servers could find
advantage by encoding a server identifier into the Token. As another
example, if HMAC-SHA1 has been compromised, a replacement HMAC
algorithm could be used instead (e.g., HMAC-SHA256).
To protect from offline attacks, the server SHOULD occasionally
choose a new HMAC key. To ease implementation, a key-id can be
assigned to each HMAC key. This can be encoded as simply as one bit
(where the new key is X (e.g., 1) and the old key is the inverted
value of X (e.g., 0)), or if several keys are supported at once could
be encoded into several bits. As the encoding of the Token is
entirely private to the server and opaque to the clients, any
encoding can be used. By encoding the key-id into the Token element,
the server can reject an old key without bothering to do HMAC
validation (saving CPU cycles). The key-id can be encoded into the
Value field of the Token element by simply concatenating the
(plaintext) key-id with the hashed information (i.e., the Token
itself).
For example, the Value field in the Token element can be computed as:
key-id || hash-alg (client-ip | nonce | abs-expiration)
During Token construction, the expiration time has to be chosen
carefully based on the intended service duration. Tokens that are
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valid for an unnecessarily long period of time (e.g., several hours)
might impose security risks. Depending on the application and use
cases, a reasonable value needs to be chosen by the server. Note
that using shorter lifetimes requires the clients to acquire Tokens
more frequently. However, since a client can acquire a new Token
well before it will need to use it, the client will not necessarily
be penalized for the acquisition delay.
Finally, be aware that NTP timestamps will wrap around in year 2036
and implementations might need to handle this eventually. Refer to
Section 6 of [RFC5905] for further details.
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6. Validating Tokens
Upon receipt of an RTCP feedback message along with the Token
Verification Request message that contains a Token, nonce and
absolute expiration time, the server MUST validate the Token.
The server first applies the its own procedure for constructing the
Tokens by using client's IP address from the received Token
Verification Request message, and the nonce and absolute expiration
time values reported in the received Token Verification Request
message. The server then compares the resulting output with the
Token sent by the client in the Token Verification Request message.
If they match and the absolute expiration time has not passed yet,
the server declares that the Token is valid.
Note that if the client's IP address changes, the Token will not
validate. Similarly, if the client inserts an incorrect nonce or
absolute expiration time value in the Token Verification Request
message, validation will fail. It is also possible that the server
wants to expire the Token prematurely. In these cases, the server
MUST reply back to the client with a Token Verification Failure
message (that goes from port P3 on the server to port c1 on the
client).
In addition to the Token Verification Failure message, it is
RECOMMENDED that applications define an application-specific error
response to be sent by the server when the server detects that the
Token is invalid. For applications using
[I-D.ietf-avt-rapid-acquisition-for-rtp], this document defines a new
4xx-level response code in the RAMS Response Code Space Registry. A
client that received a Token Verification Failure message can request
a new Token from the server.
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7. SDP Signaling
7.1. The portmapping-req Attribute
This new SDP attribute is used declaratively to indicate the port and
optionally the address for obtaining a Token. Its presence indicates
that (i) a Token MUST be included in the feedback messages sent to
the server triggering or controlling a unicast session (See
Section 4.3 for details), and (ii) the client MUST receive the
unicast session's RTP and RTCP packets from the server on the port
from which it sent the RTCP message triggering or controlling the
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 [SP nettype SP
addrtype SP connection-address]] CRLF
Here, 'port', 'nettype', 'addrtype' and 'connection-address' are
defined as specified in Section 9 of [RFC4566]. The 'portmapping-
req' attribute is used as a session-level or media-level attribute.
If used at a media level, the attribute MUST be used in a unicast
media block.
Note: This does not imply that Token Verification Request
messages need to be sent in the unicast session. Token
Verification Request messages accompany RTCP messages that trigger
or control this unicast session, and are sent either in the
multicast session or the unicast session, depending on the RTCP
message (See Section 4.3).
In the optional address value, only unicast addresses are allowed;
multicast addresses SHOULD NOT be used without evaluating the
additional security risks such as non-legit servers generating fake
Tokens. If the address is not specified, the (source) address in the
"c" line corresponding to the unicast media stream is implied.
When using this SDP attribute in SDP offer/answer [RFC3264], the
following needs to be considered. This attribute is used
declaratively. If included at session level, this applies to all
media lines that uses RTP. If included at media level, it applies to
the RTCP feedback messages declared by this media block.
An offerer that desires the answerer to use Tokens in any RTCP
message sent to the offerer, i.e., received by the offerer, the
attribute is included. In case an offerer desires to declare support
for using Tokens as defined in this specification but do not need
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Tokens to be included for any RTCP messages to be received by the
offerer, it can include the 'portmapping-req' attribute without any
parameters, neither port nor address, either at a session or media
level.
An answerer receiving an SDP offer with the "a=portmapping-req" line
with a port number SHALL use that port number and the address, either
explicitly provided in the attribute or implicitly provided by the
"c" line, for any needed Token request. If the "a=portmapping-req"
line attribute does not contain a port, the answer SHALL take note of
the capability.
When sending an answer, if the 'portmapping-req' attribute has been
present in the offer including a port number and the answerer
supports this specification, then the answerer MUST include the
attribute in its answer. The answer may or may not include a port
and address. This depends on the application and the desire of the
answerer. The answerer includes a port and possibly an address when
it requires to receive Tokens in RTCP messages. If it only supports
this specification but does not need Tokens to be included, the
attribute is included without any port or address.
7.2. Requirements
The use of SDP for the port mapping solution normatively requires the
support for:
o The SDP grouping framework and flow identification (FID) semantics
[RFC5888]
o The RTP/AVPF profile [RFC4585]
o The RTCP extensions for SSM sessions with unicast feedback
[RFC5760]
o The 'multicast-rtcp' attribute [I-D.ietf-avt-rtcp-port-for-ssm]
o Multiplexing RTP and RTCP on a single port on both endpoints in
the unicast session [RFC5761]
7.3. Example and Discussion
The declarative SDP describing the scenario given in Figure 2 is
written as:
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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/90000
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 to the multicast destination
address of 233.252.0.2 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 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].
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).
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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). Since there is
no address indiciated in this line, the client needs to retrieve the
Token from the address specified in the "c" line.
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8. Address Pooling NATs
Large-scale NAT 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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9. Security Considerations
9.1. Tokens
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. This is especially easy if the
attacker and victim are behind a large-scale NAT and share the same
IP address.
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 in such a short period
of time, such that the client will have to acquire a new Token
immediately before using it in a Token Verification Request message.
HMAC-SHA1 provides a level of security that is widely regarded as
being more than sufficient for providing message authentication. It
is believed that the economic cost of breaking that algorithm is
significantly higher than the cost of more direct approaches to
violating system security, e.g., theft, bribery, wiretapping, and
other forms of malfeasance. HMAC-SHA1 is secure against all known
cryptanalytic attacks that use computational resources that are
currently economically feasible.
9.2. The portmapping-req Attribute
The 'portmapping-req' attribute is not believed to introduce any
significant security risk to multimedia applications. A malevolent
third party could use this attribute to redirect the Port Mapping
Request messages by altering the port number or cause the unicast
session establishment to fail by removing it from the SDP
description. But, this requires intercepting and rewriting the
packets carrying the SDP description; and if an interceptor can do
that, many more attacks are possible, including a wholesale change of
the addresses and port numbers at which the media will be sent.
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In order to avoid attacks of this sort, the SDP description needs to
be integrity protected and provided with source authentication. This
can, for example, be achieved on an end-to-end basis using S/MIME
[RFC5652] when SDP is used in a signaling packet using MIME types
(application/sdp). Alternatively, HTTPS [RFC2818] or the
authentication method in the Session Announcement Protocol (SAP)
[RFC2974] could be used as well.
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10. 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.
10.1. 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
10.2. Registration of RTCP Control Packet Types
In accordance with Section 15 of [RFC3550], this specification adds
the following value to the RTCP Control Packet types sub-registry of
the Real-Time Transport Protocol (RTP) Parameters registry:
Value Abbrev. Name Reference
-------- --------- --------------------------------------- ---------
210 TOKEN Port Mapping [RFCXXXX]
10.3. SMT Values for TOKEN Packet Type Registry
This document creates a new sub-registry for the sub-message type
(SMT) values to be used with the TOKEN packet type. The registry is
called the SMT Values for TOKEN Packet Type Registry. This registry
is to be managed by the IANA according to the Specification Required
policy of [RFC5226].
The length of the SMT field is five bits, allowing 32 values. The
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registry is initialized with the following entries:
Value Name Reference
----- -------------------------------------------------- -------------
0 Reserved [RFCXXXX]
1 Port Mapping Request [RFCXXXX]
2 Port Mapping Response [RFCXXXX]
3 Token Verification Request [RFCXXXX]
4 Token Verification Failure [RFCXXXX]
5-30 Assignable - Specification Required
31 Reserved [RFCXXXX]
The SMT values 0 and 31 are reserved for future use.
Any registration for an unassigned SMT value needs to contain the
following information:
o Contact information of the one doing the registration, including
at least name and email.
o A detailed description of what the new SMT represents and how it
shall be interpreted.
10.4. 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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11. 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, in particular to Magnus Westerlund and Colin Perkins.
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12. References
12.1. 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.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[I-D.ietf-avt-rtcp-port-for-ssm]
Begen, A., "RTP Control Protocol (RTCP) Port for Source-
Specific Multicast (SSM) Sessions",
draft-ietf-avt-rtcp-port-for-ssm-03 (work in progress),
October 2010.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
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12.2. 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-17 (work in
progress), November 2010.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[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.
[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.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, February 2008.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
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[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
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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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