Optimizing NAT and Firewall Keepalives Using Port Control Protocol (PCP)
draft-ietf-pcp-optimize-keepalives-03

PCP                                                             T. Reddy
Internet-Draft                                                     Cisco
Intended status: Standards Track                              M. Isomaki
Expires: February 15, 2015                                         Nokia
                                                                 D. Wing
                                                                P. Patil
                                                                   Cisco
                                                         August 14, 2014

Optimizing NAT and Firewall Keepalives Using Port Control Protocol (PCP)
                 draft-ietf-pcp-optimize-keepalives-03

Abstract

   This document describes how Port Control Protocol is useful in
   reducing NAT and firewall keepalive messages for a variety of
   applications.

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 February 15, 2015.

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   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Overview of Operation . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Application Scenarios . . . . . . . . . . . . . . . . . .   3
     3.2.  NAT Topologies and Detection  . . . . . . . . . . . . . .   5
       3.2.1.  PCP based detection . . . . . . . . . . . . . . . . .   5
       3.2.2.  Application based detection . . . . . . . . . . . . .   6
     3.3.  Detection of PCP unaware firewalls  . . . . . . . . . . .   6
     3.4.  Keepalive Optimization  . . . . . . . . . . . . . . . . .   7
   4.  Keepalive Interval Determination Procedure when PCP unaware
       Firewall or NAT is detected . . . . . . . . . . . . . . . . .   8
   5.  Application-Specific Operation  . . . . . . . . . . . . . . .   9
     5.1.  SIP . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  HTTP  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Media and data channels with ICE  . . . . . . . . . . . .  10
     5.4.  Detecting Flow Failure  . . . . . . . . . . . . . . . . .  11
     5.5.  Firewalls . . . . . . . . . . . . . . . . . . . . . . . .  12
       5.5.1.  IPv6 Network with Firewalls . . . . . . . . . . . . .  12
       5.5.2.  Mobile Network with Firewalls . . . . . . . . . . . .  12
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Example PHP script . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   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 keepalives may be high.  An IPv4 or IPv6 host can
   use the Port Control Protocol (PCP)[RFC6887] 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 keepalive messages for both client-server and
   peer-to-peer type of communication.

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   The mechanism described in this document is especially useful in
   cellular mobile networks, where frequent keepalive messages make the
   radio transition between active and power-save states causing
   congestion in the signaling path.  The excessive time spent on the
   active state due to keepalives also greatly reduces the battery life
   of the cellular connected devices such as smartphones or tablets.
   According to requirement #14 in
   [I-D.binet-v6ops-cellular-host-reqs-rfc3316update] a cellular host
   SHOULD support PCP in order to save battery consumption exacerbated
   by keepalive messages.

2.  Notational Conventions

   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].

   This note uses terminology defined in [RFC5245] and [RFC6887].

3.  Overview of Operation

3.1.  Application Scenarios

   PCP can help both client-server and peer-to-peer applications to
   reduce their keepalive 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.

   A typical client-server scenario is depicted in Figure 1.  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)
   [RFC3261], Extensible Messaging and Presence Protocol (XMPP)
   [RFC3921], Internet Mail Application Protocol (IMAP) [RFC2177] with
   its IDLE command, the WebSocket protocol [RFC6455] 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 keepalive 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

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   application keepalives to keep the state on the Application Server/
   Peer as mentioned in Section 3.4.

        PCP          PCP
       Client       Server      __________
   +-----------+   +------+    /          \   +-----------+
   |Application|___| NAT/ |____| Internet |___|Application|
   |  Client   |   |  FW  |    |          |   |   Server  |
   +-----------+   +------+    \__________/   +-----------+
                  (multiple
                   layers)

          ------------> PCP

          ----------------------------------------->
                  Application keepalive

               Figure 1: PCP with Client-Server applications

   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 Figure 2.  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 WebRTC
   [I-D.ietf-rtcweb-overview].  Examples of the peer-to-peer protocols
   include RTP and WebRTC 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.  If there is no traffic flowing, the peers have to
   inject periodic keepalive packets to keep the NAT/FW mappings on both
   sides of the communication active.  Instead of application
   keepalives, both peers can use PCP to control the mappings on the
   NAT/FWs to reduce the keepalive frequency as explained in
   Section 3.4.

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        PCP          PCP                        PCP          PCP
       Client       Server      __________     Server       Client
   +-----------+   +------+    /          \   +------+   +-----------+
   |Application|___| NAT/ |____| Internet |___| NAT/ |___|Application|
   |   Peer    |   |  FW  |    |          |   |  FW  |   |    Peer   |
   +-----------+   +------+    \__________/   +------+   +-----------+
                  (multiple                  (multiple
                   layers)                    layers)

          ------------> PCP                   PCP <------------

          <--------------------------------------------------->
                          Application keepalive

               Figure 2: PCP with Peer-to-Peer applications

3.2.  NAT Topologies and Detection

   Before an application can reduce its keepalive 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 to the Internet.

3.2.1.  PCP based detection

   PCP itself is able to detect unexpected NATs between the PCP client
   and PCP server as depicted in Figure 3.  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 PCP server, and the server will return an
   error.  Unless the application has some other means (like UPnP) to
   control these PCP unaware NATs, it has to fall back to its default
   keepalive mechanism.

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        PCP           PCP       PCP
       Client       Unaware    Aware       __________
   +-----------+   +------+   +------+    /          \   +-----------+
   |Application|___| NAT  |___| NAT/ |____| Internet |___|Application|
   |  Client   |   |      |   |  FW  |    |          |   |   Server  |
   +-----------+   +------+   +------+    \__________/   +-----------+

         <-----------///---------->
             PCP based detection

        Figure 3: PCP unaware NAT between PCP client and PCP server

3.2.2.  Application based detection

   Figure 4 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 client must have the capability to request from its
   application server or peer what IP and transport address it sees.  If
   those differ from the IP and transport address given by the PCP aware
   NAT/FW then the application client can determine that there is at
   least one PCP unaware NAT on the path.  In this case, the application
   client has to fall back to its default keepalive mechanism.

        PCP          PCP        PCP
       Client       Aware     Unaware      __________
   +-----------+   +------+   +------+    /          \   +-----------+
   |Application|___| NAT/ |___| NAT  |____| Internet |___|Application|
   |  Client   |   |  FW  |   |      |    |          |   |   Server  |
   +-----------+   +------+   +------+    \__________/   +-----------+

         <------------>
               PCP

         <---------------------///--------------------------->
                    Application based detection

       Figure 4: PCP unaware NAT external to the last PCP aware NAT

3.3.  Detection of PCP unaware firewalls

   PCP and application based detection mechanisms explained in
   Section 3.2.1 and Section 3.2.2 are based on change in the address
   and will not detect PCP unaware firewalls.  In order to detect a PCP

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   unaware firewall, the application client sends a STUN [RFC5389]
   Binding request to the STUN server.  If STUN server supports the STUN
   extensions defined in [RFC5780] then it returns its alternate IP
   address and alternate port in OTHER-ADDRESS attribute in the STUN
   Binding response.  The client then uses PCP to send 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 there could be one or more on path PCP unaware firewall
   devices.  Application client will perform the tests separately for
   each transport protocol.  If no response is received, the client will
   then repeat the test at most three times for connectionless transport
   protocols.

      PCP            PCP        PCP
     Client         Aware     Unaware      __________
   +-----------+   +------+   +------+    /          \   +-----------+
   |Application|___| NAT/ |___| FW   |____| Internet |___|   STUN    |
   |  Client   |   | FW   |   |      |    |          |   |   Server  |
   +-----------+   +------+   +------+    \__________/   +-----------+

         <--------------------------------------------------->
               STUN

         <------------>
               PCP
                                 X<---------------------------
                    STUN based detection

                      Figure 5: PCP unaware firewall

   This procedure can be adopted by other protocols to detect PCP
   unaware firewalls.

3.4.  Keepalive Optimization

   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 keepalives to maintain the NAT/FW state.  It can use PCP PEER
   Request with the Requested Lifetime set to an appropriate value.  The

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   application may still send some application-specific heartbeat
   messages end-to-end.

   Processing the lifetime value of the PEER Opcode is described in
   Sections 10.3 and 15 of [RFC6887].  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 reduce
   power consumption by making PCP PEER message interval longer than
   what the application would normally use to the keep the middle box
   state alive, and strictly shorter than the server state refresh
   interval.

4.  Keepalive Interval Determination Procedure when PCP unaware Firewall
    or NAT is detected

   If PCP unaware NAT/firewall is detected then a client can use the
   following heuristics method to determine the keepalive interval:

   1.  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 [RFC5780].

   2.  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.

   3.  The Client will initially set the default keepalive interval for
       NAT/FW mappings to 60 seconds (FWa).

   4.  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.

   5.  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.

   6.  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 old FWa value as Keepalive interval
       to refresh FW/NAT mappings.

   The above procedure will be done separately for each transport
   protocol.  For connectionless transport protocols like UDP if timer

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   of 2 seconds elapses without response from the STUN server then the
   client will repeat step 4 at most three times to handle packet loss.

   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 [RFC4380]).

   Note: The keepalive interval learnt using the above described
   heuristics method can be inaccurate if a firewall is configured with
   application specific inactivity timeout.

   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.

5.  Application-Specific Operation

   This section describes how PCP is used with specific application
   protocols.

5.1.  SIP

   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
   [RFC5626].  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 Section 3.4.  There is no further need to send STUN
   Binding requests over the SIP flow to keep the NAT Binding alive.

   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
   [RFC5626].

   For connection-oriented transports, the UA sends a STUN Binding
   request multiplexed with SIP over the TCP connection.  STUN

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   multiplexed with other data over a TCP or TLS-over-TCP connection is
   explained in section 7.2.2 of [RFC5389].  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 Section 3.4.

   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 keepalive 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 [RFC5626].

5.2.  HTTP

   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 [I-D.isomaki-rtcweb-mobile].  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.

   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 Section 3.4.

   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
   Appendix A for an example server side PHP script to obtain the client
   source IP address.

   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.

5.3.  Media and data channels with ICE

   The ICE agent learns the External IP Addresses and Ports using the
   MAP opcode.  If server reflexive candidates learnt using STUN
   [RFC5389] and external IP addresses learnt using PCP are different
   then candidates learnt through both STUN and PCP are encoded in the

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   ICE offer and answer . When using the Recommended Formula explained
   in section 4.1.2.1 of [RFC5245] to compute priority for the candidate
   learnt through PCP, the ICE agent should use a preference value
   greater than the server reflexive candidate and hence tested before
   the server reflexive candidate.  The recommended type preference
   value is 105 for candidates discovered using PCP and is explained in
   section 4.2 of [RFC6544].

   The ICE agent, in addition to the ICE connectivity checks, performs
   the following:

   The ICE agent checks if the XOR-MAPPED-ADDRESS from the STUN Binding
   response received as part of ICE connectivity check matches the
   External IP address and Port provided by PCP MAP response.

   1.  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.

   2.  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 [RFC6263] to keep NAT bindings alive.  This means
       that multiple layers of NAT are involved and intermediate NATs
       are not PCP aware.

   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 Section 3.4.

5.4.  Detecting Flow Failure

   Using the Rapid Recovery technique in section 14 of [RFC6887] upon
   receiving a PCP ANNOUNCE from a PCP server, a PCP client becomes
   aware that the 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.

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5.5.  Firewalls

   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 in order to permit inbound traffic.  The endpoint can
   further use PCP to send keepalives to keep the firewall mappings
   alive.

5.5.1.  IPv6 Network with Firewalls

   For scenarios where the client uses the ICE Lite implementation
   explained in section 2.7 of [RFC5245], 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 and 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.  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).

5.5.2.  Mobile Network with Firewalls

   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
   [I-D.chen-pcp-mobile-deployment].  As described in Section 3.4, 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.  This would result in saving of radio and power
   consumption of the Mobile device while protecting it from attacks.

6.  IANA Considerations

   None

7.  Security Considerations

   The security considerations in [RFC5245] and [RFC6887] apply to this
   use.

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8.  Acknowledgements

   Authors would like to thank Dave Thaler, Basavaraj Patil, Anca
   Zamfir, Reinaldo Penno, Suresh Kumar and Dilipan Janarthanan for
   valuable inputs to the document.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April
              2010.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5626]  Jennings, C., Mahy, R., and F. Audet, "Managing Client-
              Initiated Connections in the Session Initiation Protocol
              (SIP)", RFC 5626, October 2009.

   [RFC5780]  MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
              Using Session Traversal Utilities for NAT (STUN)", RFC
              5780, May 2010.

   [RFC6263]  Marjou, X. and A. Sollaud, "Application Mechanism for
              Keeping Alive the NAT Mappings Associated with RTP / RTP
              Control Protocol (RTCP) Flows", RFC 6263, June 2011.

   [RFC6544]  Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
              "TCP Candidates with Interactive Connectivity
              Establishment (ICE)", RFC 6544, March 2012.

   [RFC6887]  Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
              Selkirk, "Port Control Protocol (PCP)", RFC 6887, April
              2013.

9.2.  Informative References

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   [I-D.binet-v6ops-cellular-host-reqs-rfc3316update]
              Binet, D., Boucadair, M., Ales, V., Byrne, C., and G.
              Chen, "Internet Protocol Version 6 (IPv6) for Cellular
              Hosts", draft-binet-v6ops-cellular-host-reqs-
              rfc3316update-03 (work in progress), October 2012.

   [I-D.chen-pcp-mobile-deployment]
              Chen, G., Cao, Z., Boucadair, M., Ales, V., and L.
              Thiebaut, "Analysis of Port Control Protocol in Mobile
              Network", draft-chen-pcp-mobile-deployment-04 (work in
              progress), July 2013.

   [I-D.ietf-rtcweb-overview]
              Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", draft-ietf-rtcweb-overview-10
              (work in progress), June 2014.

   [I-D.isomaki-rtcweb-mobile]
              Isomaki, M., "RTCweb Considerations for Mobile Devices",
              draft-isomaki-rtcweb-mobile-00 (work in progress), July
              2012.

   [RFC2177]  Leiba, B., "IMAP4 IDLE command", RFC 2177, June 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3921]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence", RFC
              3921, October 2004.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380, February
              2006.

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC
              6455, December 2011.

Appendix A.  Example PHP script

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   <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>

Authors' Addresses

   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103
   India

   Email: tireddy@cisco.com

   Markus Isomaki
   Nokia
   Keilalahdentie 2-4
   FI-02150 Espoo
   Finland

   Email: markus.isomaki@nokia.com

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134
   USA

   Email: dwing@cisco.com

   Prashanth Patil
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marthalli Outer Ring Road
   Bangalore, Karnataka  560103
   India

   Email: praspati@cisco.com

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