One document matched: draft-ietf-pcp-optimize-keepalives-00.xml
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<rfc category="std" docName="draft-ietf-pcp-optimize-keepalives-00"
ipr="trust200902">
<front>
<title abbrev="Optimizing Keepalives with PCP">Optimizing NAT and Firewall
Keepalives Using Port Control Protocol (PCP)</title>
<author fullname="Tirumaleswar Reddy" initials="T." surname="Reddy">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>Cessna Business Park, Varthur Hobli</street>
<street>Sarjapur Marathalli Outer Ring Road</street>
<city>Bangalore</city>
<region>Karnataka</region>
<code>560103</code>
<country>India</country>
</postal>
<email>tireddy@cisco.com</email>
</address>
</author>
<author fullname="Markus Isomaki" initials="M." surname="Isomaki">
<organization>Nokia</organization>
<address>
<postal>
<street>Keilalahdentie 2-4</street>
<city>FI-02150 Espoo</city>
<country>Finland</country>
</postal>
<email>markus.isomaki@nokia.com</email>
</address>
</author>
<author fullname="Dan Wing" initials="D." surname="Wing">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>170 West Tasman Drive</street>
<city>San Jose</city>
<region>California</region>
<code>95134</code>
<country>USA</country>
</postal>
<email>dwing@cisco.com</email>
</address>
</author>
<author fullname="Prashanth Patil" initials="P." surname="Patil">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>Cessna Business Park, Varthur Hobli</street>
<street>Sarjapur Marthalli Outer Ring Road</street>
<city>Bangalore</city>
<region>Karnataka</region>
<code>560103</code>
<country>India</country>
</postal>
<email>praspati@cisco.com</email>
</address>
</author>
<date />
<workgroup>PCP</workgroup>
<abstract>
<t>This document describes how Port Control Protocol is useful to reduce
NAT and firewall keepalive messages for a variety of applications.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>Many types of applications need to keep their Network Address
Translator (NAT) and Firewall (FW) mappings alive for long periods of
time, even when they are otherwise not sending or receiving any traffic.
This is typically done by sending periodic keep-alive messages just to
prevent the mappings from expiring. As NAT/FW mapping timers may be
short and unknown to the endpoint, the frequency of these keep-alives
may be high. An IPv4 or IPv6 host can use the Port Control Protocol
(PCP)<xref target="RFC6877"></xref> to flexibly manage the IP address
and port mapping information on NATs and FWs to facilitate
communications with remote hosts. This document describes how PCP can be
used to reduce keep-alive messages for both client-server and
peer-to-peer type of communication.</t>
<t>The mechanism described in this document is especially useful in
cellular mobile networks, where frequent keep-alive messages make the
radio transition between active and power-save states causing signaling
congestion. The excessive time spent on the active state due to
keep-alives also greatly reduces the battery life of the cellular
connected devices such as smartphones or tablets. Requirement #14 in
<xref target="I-D.binet-v6ops-cellular-host-reqs-rfc3316update"></xref>
explains that cellular host SHOULD support of PCP as a driver to save
battery consumption exacerbated by keepalive messages.</t>
</section>
<section anchor="notation" title="Notational Conventions">
<t>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 <xref
target="RFC2119"></xref>.</t>
<t>This note uses terminology defined in <xref target="RFC5245"></xref>
and <xref target="RFC6877"></xref> .</t>
</section>
<section title="Overview of Operation">
<section title="Application Scenarios">
<t>PCP can help both client-server and peer-to-peer applications to
reduce their keep-alive rate. The relevant applications are the ones
that need to keep their NAT/FW mappings alive for long periods of
time, for instance to be able to send or receive application messages
in both directions at any time.</t>
<t>A typical client-server scenario is depicted in <xref
target="figure1"></xref>. A client, who may reside behind one or
multiple layers of NATs/FWs, opens a connection to a globally
reachable server, and keeps it open to be able to receive messages
from the server at any time. The connection may be a
connection-oriented transport protocol such as TCP or SCTP or
connection-less transport protocol such as UDP. Protocols operating in
this manner include Session Initiation Protocol (SIP) <xref
target="RFC3261"></xref>, Extensible Messaging and Presence Protocol
(XMPP) <xref target="RFC3921"></xref>, Internet Mail Application
Protocol (IMAP) <xref target="RFC2177"></xref> with its IDLE command,
the WebSocket protocol and the various HTTP long-polling protocols.
There are also a number of proprietary instant messaging, Voice over
IP, e-mail and notification delivery protocols that belong in this
category. All of these protocols aim to keep the client-server
connection alive for as long as the application is running. When the
application has otherwise no traffic to send, specific keep-alive
messages are sent periodically to ensure that the NAT/FW state in the
middle does not expire. The client can use PCP to keep the required
mapping at the NAT/FW and use application keep-alives to keep the
state on the Application Server/Peer as mentioned in <xref
target="optimize"></xref>.</t>
<t><figure anchor="figure1"
title="PCP with Client-Server applications">
<artwork><![CDATA[
PCP PCP
Client Server __________
+-----------+ +------+ / \ +-----------+
|Application|___| NAT/ |____| Internet |___|Application|
| Client | | FW | | | | Server |
+-----------+ +------+ \__________/ +-----------+
(multiple
layers)
------------> PCP
----------------------------------------->
Application keep-alive
]]></artwork>
</figure></t>
<t>There are also scenarios where the long-term communication
association is between two peers, both of whom may reside behind one
or more layers of NAT/FW. This is depicted in <xref
target="figure2"></xref>. The initiation of the association may have
happened using mechanisms such as Interactive Communications
Establishment (ICE), perhaps first triggered by a "signaling" protocol
such as SIP or XMPP or RTCWeb. Examples of the peer-to-peer protocols
include RTP and RTCWeb data channel. A number of proprietary VoIP or
video call or streaming or file transfer protocols also exist in this
category. Typically the communication is based on UDP, but TCP or SCTP
may be used. Unless there is no traffic flowing otherwise, the peers
have to inject periodic keep-alive packets to keep the NAT/FW mappings
on both sides of the communication active. Instead of application
keep-alives, both peers can use PCP to control the mappings on the
NAT/FWs to reduce the keep-alive frequency as explained in <xref
target="optimize"></xref>.</t>
<t><figure anchor="figure2" title="PCP with Peer-to-Peer applications">
<artwork><![CDATA[
PCP PCP PCP PCP
Client Server __________ Server Client
+-----------+ +------+ / \ +------+ +-----------+
|Application|___| NAT/ |____| Internet |___| NAT/ |___|Application|
| Peer | | FW | | | | FW | | Peer |
+-----------+ +------+ \__________/ +------+ +-----------+
(multiple (multiple
layers) layers)
------------> PCP PCP <------------
<--------------------------------------------------->
Application keep-alive
]]></artwork>
</figure></t>
</section>
<section title="NAT and Firewall Topologies and Detection">
<t>Before an application can reduce its keep-alive rate, it has to
make sure it has all of the NATs and Firewalls on its path under
control. This means it has to detect the presence of any PCP-unaware
NATs and Firewalls on its path. PCP itself is able to detect
unexpected NATs between the PCP client and server as depicted in <xref
target="figure3"></xref>. The PCP client includes its own IP address
and UDP port within the PCP request. The PCP server compares them to
the source IP address and UDP port it sees on the packet. If they
differ, there are one or more additional NATs between the PCP client
and server, and the server will return an error. Unless the
application has some other means to control these PCP unaware NATs, it
has to fall back to its default keep-alive mechanism.</t>
<t><figure anchor="figure3"
title="PCP unaware NAT/FW between PCP client and server">
<artwork><![CDATA[
PCP PCP PCP
Client Unaware Aware __________
+-----------+ +------+ +------+ / \ +-----------+
|Application|___| NAT/ |___| NAT/ |____| Internet |___|Application|
| Client | | FW | | FW | | | | Server |
+-----------+ +------+ +------+ \__________/ +-----------+
<-----------///---------->
PCP based detection
]]></artwork>
</figure></t>
<t><xref target="figure4"></xref> shows a topology where one or more
PCP unaware NATs are deployed on the exterior of the PCP capable
NAT/FWs. To detect this, the application must have the capability to
request from its server or peer what IP and transport address it sees.
If those differ from the IP and transport address given to the
application by the out most PCP aware NAT/FW, the application can
detect that there is at least one more PCP unaware NAT on the path. In
this case, the application has to fall back to its default keep-alive
mechanism.</t>
<t><figure anchor="figure4"
title="PCP unaware NAT external to the last PCP aware NAT">
<artwork><![CDATA[
PCP PCP PCP
Client Aware Unaware __________
+-----------+ +------+ +------+ / \ +-----------+
|Application|___| NAT/ |___| NAT |____| Internet |___|Application|
| Client | | FW | | | | | | Server |
+-----------+ +------+ +------+ \__________/ +-----------+
<------------>
PCP
<---------------------///--------------------------->
Application based detection
]]></artwork>
</figure></t>
<t><xref target="apps"></xref> describes how the detection works in a
number of real application protocols.</t>
<t>The caveat is that Firewalls can not be detected this way. The
client will have to use the alternative procedure explained in <xref
target="Detect"></xref> to detect PCP unaware Firewalls.</t>
</section>
<section anchor="Detect" title="Detect PCP Unaware Firewalls">
<t>The client sends a STUN Binding Request to the STUN server. STUN
server will return its alternate IP address and alternate port in
OTHER-ADDRESS in the binding response <xref target="RFC5780"></xref>.
The client then sends MAP request with FILTER option to PCP server to
permit STUN server to reach the client using the STUN servers
alternate IP address and alternate port. The client then sends a
binding request to the primary address of the STUN server with the
CHANGE-REQUEST attribute set to change-port and change-IP. This will
cause the server to send its response from its alternate IP address
and alternate port. If the client receives a response then the client
is aware that on path Firewall devices are PCP aware. If the client
does not receive a response then the client is aware that could be one
or more on path PCP unaware Firewall devices. PCP client will perform
the tests separately for each transport protocol. If no response is
received, the client will then repeat the test atmost three times for
connectionless transport protocols.</t>
<t>If the STUN server does not support OTHER-ADDRESS then this test
cannot be run. This procedure can be adopted by other protocols to
detect PCP unaware Firewalls.</t>
</section>
<section anchor="optimize" title="Keepalive Optimization">
<t>If the application determines that all NATs and Firewalls on its
path to the Internet support PCP, it can start using PCP instead of
its default keep-alives to maintain the NAT/FW state. It can use PCP
PEER Request with the Requested Lifetime set to an appropriate value.
The application may still send some application-specific heartbeat
messages end-to-end.</t>
<t>Processing the lifetime value of the PEER Opcode is described in
Sections 10.3 and 15 of <xref target="RFC6877"></xref>. Sending a PEER
request with a very short Requested Lifetime can be used to query the
lifetime of an existing mapping. PCP recommends that lifetimes of
mapping created or lengthened with PEER be longer than the lifetimes
of implicitly-created NAT and Firewall mappings. Thus PCP can be used
to save battery consumption by making PCP PEER message interval longer
than what the application would normally use the keep middle box state
alive, and strictly shorter than the server state refresh
interval.</t>
</section>
</section>
<section anchor="heuristics"
title="Keepalive Interval Determination Procedure when PCP unaware Firewall or NAT is detected ">
<t>If PCP unaware NAT/Firewall is detected then a client can use the
following heuristics method to determine the keepalive interval :</t>
<t><list style="numbers">
<t>The client sends a STUN Binding Request to the STUN server. This
connection is called the Primary Channel. STUN server will return
its alternate IP address and alternate port in OTHER-ADDRESS in the
binding response <xref target="RFC5780"></xref>.</t>
<t>The client then sends STUN Binding Request to the STUN server
using alternate IP address and alternate port. This connection is
called the Secondary Channel.</t>
<t>The Client will initially set the default keepalive interval for
NAT/FW mappings to 60 seconds (FWa).</t>
<t>After FWa seconds the Client will send a binding request to the
STUN server using the Primary Channel with the CHANGE-REQUEST
attribute set to change-port and change-IP. This will cause the STUN
server to send its response from the Secondary channel.</t>
<t>If the client receives response from the server then it will
increase the keepalive interval value FWa = (old FWa) + (old FWa)/2.
This indicates that NAT/FW mappings are alive.</t>
<t>Steps 4 and 5 will be repeated until there is no response from
the STUN server. If there is no response from the STUN server then
the client will use the FWa value as Keepalive interval to refresh
FW/NAT mappings.</t>
</list></t>
<t>The above procedure will be done separately for each transport
protocol. For connectionless transport protocols like UDP if timer of 2
seconds elapses without response from the STUN server then the client
will repeat step 4 atmost three times to handle packet loss.</t>
<t>This procedure can be adopted by other protocols to use Primary and
Secondary channels, so that the client can determine the keepalive
interval to refresh FW/NAT mapping. This procedure only serves as a
guideline and if applications already use some other heuristics method
to determine keepalive, they can continue with the existing logic. For
example Teredo determines Refresh interval using the procedure in
"Optional Refresh Interval Determination Procedure" (Section 5.2.7 of
<xref target="RFC4380"></xref>).</t>
<t>To improve reliability, applications SHOULD continue to use PCP to
lengthen the FW/NAT mappings even if the above described mechanism is
used to detect PCP unaware NAT/Firewall. This ensures that PCP aware
FW/NAT do not close old mappings with no packet exchange when there is a
resource-crunch situation.</t>
</section>
<section anchor="apps" title="Application-Specific Operation">
<t>This section describes how PCP is used with specific application
protocols.</t>
<section title="SIP">
<t>For connection-less transports the User Agent (UA) sends a STUN
Binding Request over the SIP flow as described in section 4.4.2 of
<xref target="RFC5626"></xref>. The UA then learns the External IP
Address and Port using a PEER request/response. If the
XOR-MAPPED-ADDRESS in the STUN Binding Response matches the external
address and port provided by PCP PEER response then the UA optimizes
the keepalive traffic as described in <xref target="optimize"></xref>.
There is no further need to send STUN Binding Requests over the SIP
flow to keep the NAT binding alive.</t>
<t>If the XOR-MAPPED-ADDRESS in the STUN Binding Response does not
match the external address and port provided by the PCP PEER response
then PCP will not be used to keep the NAT bindings alive for the flow
that is being used for the SIP traffic. This means that multiple
layers of NAT are involved and intermediate NATs are not PCP aware. In
this case the UA will continue to use the technique in section 4.4.2
of <xref target="RFC5626"></xref>.</t>
<t>For connection-oriented transports, the UA sends a STUN Binding
Request multiplexed with SIP over the TCP connection. STUN multiplexed
with other data over a TCP or TLS-over-TCP connection is explained in
section 7.2.2 of <xref target="RFC5389"></xref>. The UA then learns
the External IP address and port using a PEER request/response. If the
XOR-MAPPED-ADDRESS in the STUN Binding Response matches the external
address and port provided by PCP PEER response then the UA optimizes
the keepalive traffic as described in <xref
target="optimize"></xref>.</t>
<t>If the XOR-MAPPED-ADDRESS in the STUN Binding Response does not
match the external address and port provided by PCP PEER response then
PCP will not be used to keep the NAT bindings alive. In this case the
UA performs a keep-alive check by sending a double-CRLF (the "ping")
then waits to receive a single CRLF (the "pong") using the technique
in section 4.4.1 of <xref target="RFC5626"></xref>.</t>
</section>
<section title="HTTP">
<t>Web Applications that require persistent connections use techniques
such as HTTP long polling and Websockets for session keep alive as
explained in section 3.1 of <xref
target="I-D.isomaki-rtcweb-mobile"></xref>. In such scenarios, after
the client establishes a connection with the HTTP server, it can
execute server side scripts such as PHP residing on the server to
provide the transport address and port of the HTTP client seen at the
HTTP server. In addition, the HTTP client also learns the external IP
Address and port using the PCP PEER request/response.</t>
<t>If the IP address and port learned from the server matches the
external address and port provided by PCP PEER response then the HTTP
client optimizes keepalive traffic as described in <xref
target="optimize"></xref>.</t>
<t>If the IP address and port do not match then PCP will not be used
to keep the NAT bindings alive for the flow that is being used for the
HTTP traffic. This means that there are NATs or HTTP proxies between
the PCP server and the HTTP server. The HTTP client will have to
resort to use existing techniques for keep alive. Please see <xref
target="example"></xref> for an example server side PHP script to
obtain the client source IP address.</t>
<t>HTTP protocol allows intermediaries like transparent proxies to be
involved and there is no way for the client to know that
request/response is relayed through a proxy.</t>
</section>
<section title="Media and data channels with ICE">
<t>The ICE agent learns the External IP Addresss and Port using the
MAP opcode. This candidate learnt through PCP is encoded in the ICE
offer and answer just like the server reflexive candidate, If the
server reflexive candidate and External IP address learnt using PCP
are different. When using the Recommended Formula in section 4.1.2.1
of <xref target="RFC5245"></xref> to compute priority for the
candidates learnt through PCP, the ICE agent should use a preference
value greater than the server reflexive candidate and hence they are
tested before the server reflexive candidates. The recommended type
preference value is 105 for candidates discovered using PCP and is
explained in section 4.2 of <xref target="RFC6544"></xref>.</t>
<t>The ICE agent in addition to ICE connectivity checks and performs
the following :</t>
<t>The ICE agent checks if the XOR-MAPPED-ADDRESS from the STUN <xref
target="RFC5389"></xref> Binding response received as part of ICE
connectivity check matches the External IP address and Port provided
by PCP MAP response.</t>
<t><list style="numbers">
<t>If the match is successful then PCP will be used to keep the
NAT bindings alive. The ICE agent optimizes keepalive traffic by
refreshing the mapping via a new PCP MAP request containing
information from the earlier PCP response.</t>
<t>If the match is not successful then PCP will not be used for
keep NAT binding alive. The ICE agent will use the technique in
section 4.4 of <xref target="RFC6263"></xref> to keep NAT bindings
alive. This means that multiple layers of NAT are involved and
intermediate NATs are not PCP aware.</t>
</list></t>
<t>Some network operators deploying a PCP Server may allow PEER but
not MAP. In such cases the ICE agent learns the external IP address
and port using a STUN binding request/response during ICE connectivity
checks. The ICE agent also learns the external IP Address and port
using a PCP PEER request/response. If the IP address and port learned
from the STUN binding response matches the external address and port
provided by the PCP PEER response then the ICE agent optimizes
keepalive traffic as described in <xref target="optimize"></xref>.</t>
</section>
<section title="Detecting Flow Failure">
<t>Using the Rapid Recovery technique in section 14 of <xref
target="RFC6877"></xref> PCP client upon receiving a PCP ANNOUNCE from
a PCP server becomes aware that PCP server has rebooted or lost its
mapping state. The PCP client issues new PCP requests to recreate any
lost mapping state and thus reconstructs lost mappings fast enough
that existing media, HTTP and SIP flows do not break. If the NAT state
cannot be recovered the endpoint will find the new external address
and port as part of the Rapid Recovery technique in PCP itself and
reestablish a connection with the peer.</t>
</section>
<section title="Firewalls">
<t>PCP allows applications to communicate with Firewall devices with
PCP functionality to create mappings for incoming connections. In such
cases PCP can be used by the endpoint to create an explicit mapping on
Firewall to permit inbound traffic and further use PCP to send
keep-alives to keep the Firewall mappings alive.</t>
<section title="IPv6 Network with Firewalls">
<t>For scenarios where the client uses ICE Lite implementation
explained in section 2.7 of <xref target="RFC5245"></xref>, the ICE
Lite endpoint will not generate its own ICE connectivity checks, by
definition. As part of the call setup, the ICE Lite endpoint would
gather its host candidates and relayed candidate from a TURN server,
send the candidates in the offer to the peer endpoint. On receiving
the answer from the peer endpoint, the ICE Lite endpoint sends a PCP
MAP request with FILTER opcode to create a dynamic mapping in
Firewall to permit ICE connectivity checks and subsequent media
traffic from the remote peer. In this way, the ICE Lite endpoint and
its network are protected from unsolicited incoming UDP traffic, and
can still operate using ICE Lite (rather than full ICE).</t>
</section>
<section title="Mobile Network with Firewalls">
<t>Mobile Networks are also making use of a Firewall to protect
their customers from various attacks like downloading malicious
content. The Firewall is usually configured to block all unknown
inbound connections as explained in section 2.1 of <xref
target="I-D.chen-pcp-mobile-deployment"></xref>. In such cases PCP
can be used by Mobile devices to create an explicit mapping on the
Firewall to permit inbound traffic and optimize the keepalive
traffic as described in <xref target="optimize"></xref>. This would
result in saving of radio and power consumption of the Mobile device
while protecting it from attacks.</t>
</section>
</section>
</section>
<section title="IANA Considerations">
<t>None</t>
</section>
<section title="Security Considerations">
<t>The security considerations in <xref target="RFC5245"></xref><xref
target="RFC6877"> and </xref> apply to this use.</t>
</section>
<section anchor="ack" title="Acknowledgements">
<t>Authors would like to thank Dave Thaler, Basavaraj Patil for valuable
inputs to the document.</t>
</section>
<section title="Change History">
<t>[Note to RFC Editor: Please remove this section prior to
publication.]</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"
?>
<?rfc include="reference.RFC.5245"?>
<?rfc include="reference.RFC.5389"?>
<?rfc include='reference.RFC.6877'?>
<?rfc include="reference.RFC.6263"?>
<?rfc include="reference.RFC.5626"
?>
<?rfc include="reference.RFC.5780"?>
<?rfc include="reference.RFC.6544"?>
<?rfc ?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.3261"
?>
<?rfc include="reference.RFC.3921"
?>
<?rfc include="reference.RFC.2177"?>
<?rfc include='reference.I-D.chen-pcp-mobile-deployment'?>
<?rfc include='reference.I-D.isomaki-rtcweb-mobile'
?>
<?rfc include='reference.I-D.binet-v6ops-cellular-host-reqs-rfc3316update'?>
<?rfc include="reference.RFC.4380"?>
<?rfc ?>
<!---->
</references>
<section anchor="example" title="Example PHP script">
<figure>
<artwork><![CDATA[<html>
Connected to <?PHP echo gethostname(); ?> on port <?PHP echo
getenv(SERVER_PORT) ?> on <?PHP echo date("d-M-Y H:i:s"); ?> Pacific Time
<p>
Your IP address is: <?PHP echo getenv(REMOTE_ADDR); ?>,
port <?PHP echo getenv(REMOTE_PORT); ?>
</p>;
</html>]]></artwork>
</figure>
</section>
</back>
</rfc>
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