Movatterモバイル変換

Independent Submission                                       S. CheshireRequest for Comments: 6886                                   M. KrochmalCategory: Informational                                       Apple Inc.ISSN: 2070-1721                                               April 2013NAT Port Mapping Protocol (NAT-PMP)Abstract   This document describes a protocol for automating the process of   creating Network Address Translation (NAT) port mappings.  Included   in the protocol is a method for retrieving the external IPv4 address   of a NAT gateway, thus allowing a client to make its external IPv4   address and port known to peers that may wish to communicate with it.   From 2005 onwards, this protocol was implemented in Apple products   including Mac OS X, Bonjour for Windows, and AirPort wireless base   stations.  In 2013, NAT Port Mapping Protocol (NAT-PMP) was   superseded by the IETF Standards Track RFC "Port Control Protocol   (PCP)", which builds on NAT-PMP and uses a compatible packet format,   but adds a number of significant enhancements.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This is a contribution to the RFC Series, independently of any other   RFC stream.  The RFC Editor has chosen to publish this document at   its discretion and makes no statement about its value for   implementation or deployment.  Documents approved for publication by   the RFC Editor are not a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6886.Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.Cheshire & Krochmal           Informational                     [Page 1]

RFC 6886                         NAT-PMP                      April 2013Table of Contents1. Introduction ....................................................31.1. Transition to Port Control Protocol ........................42. Conventions and Terminology Used in This Document ...............53. Protocol and Packet Format ......................................53.1. Requests and Responses .....................................63.2. Determining the External Address ...........................73.3. Requesting a Mapping ......................................103.4. Destroying a Mapping ......................................133.5. Result Codes ..............................................143.6. Seconds Since Start of Epoch ..............................163.7. Recreating Mappings on NAT Gateway Reboot .................163.8. NAT Gateways with NAT Function Disabled ...................183.9. All Mappings Are Bidirectional ............................194. UNSAF Considerations ...........................................204.1. Scope .....................................................204.2. Transition Plan ...........................................204.3. Failure Cases .............................................214.4. Long-Term Solution ........................................234.5. Existing Deployed NATs ....................................235. Security Considerations ........................................236. IANA Considerations ............................................247. Acknowledgments ................................................248. Deployment History .............................................259. Noteworthy Features of NAT Port Mapping Protocol and PCP .......269.1. Simplicity ................................................279.2. Focused Scope .............................................279.3. Efficiency ................................................279.4. Atomic Allocation Operations ..............................299.5. Garbage Collection ........................................299.6. State Change Announcements ................................309.7. Soft State Recovery .......................................319.8. On-Path NAT Discovery .....................................3110. References ....................................................3210.1. Normative References .....................................3210.2. Informative References ...................................32Cheshire & Krochmal           Informational                     [Page 2]

RFC 6886                         NAT-PMP                      April 20131.  Introduction   Network Address Translation (NAT) is a method of sharing one public   Internet address with a number of devices.  This document is focused   on devices that are formally classified as "NAPTs" (Network   Address/Port Translators) [RFC2663].  A full description of NAT is   beyond the scope of this document.  The following brief overview will   cover the aspects relevant to this port mapping protocol.  For more   information on NAT, see "Traditional IP Network Address Translator   (Traditional NAT)" [RFC3022].   NATs have one or more external IP addresses.  A private network is   set up behind the NAT.  Client devices on that private network behind   the NAT are assigned private addresses, and those client devices use   the private address of the NAT device as their default gateway.   When a packet from any device behind the NAT is sent to an address on   the public Internet, the packet first passes through the NAT box.   The NAT box looks at the source port and address.  In some cases, a   NAT will also keep track of the destination port and address.  The   NAT then creates a mapping from the internal address and internal   port to an external address and external port if a mapping does not   already exist.   The NAT box replaces the internal address and port in the packet with   the external entries from the mapping and sends the packet on to the   next gateway.   When a packet from any address on the Internet is received on the   NAT's external side, the NAT will look up the destination address and   port (external address and port) in the list of mappings.  If an   entry is found, it will contain the internal address and port to   which the packet should be sent.  The NAT gateway will then rewrite   the destination address and port with those from the mapping.  The   packet will then be forwarded to the new destination addresses.  If   the packet did not match any mapping, the packet will most likely be   dropped.  Various NATs implement different strategies to handle this.   The important thing to note is that if there is no mapping, the NAT   does not know to which internal address the packet should be sent.   Mappings are usually created automatically as a result of observing   outbound packets.  There are a few exceptions.  Some NATs may allow   manually created permanent mappings that map an external port to a   specific internal IP address and port.  Such a mapping allows   incoming connections to the device with that internal address.  Some   NATs also implement a default mapping where any inbound packet thatCheshire & Krochmal           Informational                     [Page 3]

RFC 6886                         NAT-PMP                      April 2013   does not match any other more specific mapping will be forwarded to a   specified internal address.  Both types of mappings are usually set   up manually through some configuration tool.  Such manual   configuration of port mappings is unreasonably onerous for most   residential NAT users.   Without these manually created inbound port mappings, clients behind   the NAT would be unable to receive inbound connections, which   represents a loss of connectivity when compared to the original   Internet architecture [ETEAISD].  For those who view this loss of   connectivity as a bad thing, NAT-PMP allows clients to operate more   like a host directly connected to the unrestricted public Internet,   with an unrestricted public IPv4 address.  NAT-PMP allows client   hosts to communicate with the NAT gateway to request the creation of   inbound mappings on demand.  Having created a NAT mapping to allow   inbound connections, the client can then record its external IPv4   address and external port in a public registry (e.g., the worldwide   Domain Name System) or otherwise make it accessible to peers that   wish to communicate with it.1.1.  Transition to Port Control Protocol   NAT-PMP enjoyed almost a decade of useful service, and operational   experience with NAT-PMP informed the design of its IETF Standards   Track successor, Port Control Protocol (PCP) [RFC6887].  PCP builds   on NAT-PMP, using the same UDP ports 5350 and 5351, and a compatible   packet format.  PCP also adds significant enhancements, including   IPv6 support, management of outbound mappings, management of firewall   rules, full compatibility with large-scale NATs with a pool of   external addresses, error lifetimes, and an extension mechanism to   enable future enhancements.   Because of the significant enhancements in PCP, all existing NAT-PMP   implementations are encouraged to migrate to PCP.  The version number   in the packet header is 0 for NAT-PMP and 2 for PCP, so the packets   are easily distinguished.  (Version number 1 was used by a vendor   that shipped products that use a protocol that is incompatible with   the IETF Standard.  PCP implementations MUST NOT use version   number 1.)   For NAT-PMP servers, adding PCP support is simple.  When packets are   received, if the version number is 2, then the packet is interpreted   as a PCP request, the request is handled, and replies and updates   pertaining to that mapping are sent using PCP format.  If the version   number is 0, then the existing code handles the request exactly as it   already does, and replies and updates pertaining to that mapping areCheshire & Krochmal           Informational                     [Page 4]

RFC 6886                         NAT-PMP                      April 2013   sent using NAT-PMP format.  If the version number is 1 or any other   unsupported version, then result code 1 (Unsupported Version) is   returned.   NAT-PMP clients should add PCP support, and should default to sending   requests using PCP format, which will cause clients to prefer the   newer format when possible.  If a PCP request is sent to an old   NAT-PMP server that doesn't understand the new PCP format, then it   will return result code 1 (Unsupported Version), and the client   should then immediately retry the same request using NAT-PMP format.   The presence of the Unsupported Version reply allows fast fail-over   to NAT-PMP format, without waiting for timeouts, retransmissions, or   other arbitrary delays.  It is recommended that clients always try   PCP first for every new request, retransmission, and renewal, and   only try NAT-PMP in response to an "Unsupported Version" error.   Clients SHOULD NOT record that a given server only speaks NAT-PMP and   subsequently default to NAT-PMP for that server, since NAT firmware   gets updated, and even a NAT gateway that speaks only NAT-PMP today,   may be updated to speak PCP tomorrow.2.  Conventions and Terminology Used in This Document   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 "Key words for use in   RFCs to Indicate Requirement Levels" [RFC2119].3.  Protocol and Packet Format   The NAT Port Mapping Protocol runs over UDP.  Every packet starts   with an 8-bit version followed by an 8-bit operation code.   All numeric quantities in NAT-PMP larger than a single byte (e.g.,   error values, Seconds Since Start of Epoch, and mapping lifetime) are   transmitted in the traditional IETF network byte order (i.e., most   significant byte first).   Non-numeric quantities in NAT-PMP larger than a single byte (e.g.,   the NAT gateway's external IP address) are transmitted in the natural   byte order, with no byte swapping.   This document specifies version 0 of the protocol.  Any NAT-PMP   gateway implementing this version of the protocol, receiving a   request with a version number other than 0, MUST return result code 1   (Unsupported Version), indicating the highest version number it does   support (i.e., 0) in the version field of the response.Cheshire & Krochmal           Informational                     [Page 5]

RFC 6886                         NAT-PMP                      April 2013   Opcodes between 0 and 127 are client requests.  Opcodes from 128 to   255 are corresponding server responses.  Responses always contain a   16-bit result code in network byte order.  A result code of zero   indicates success.  Responses also contain a 32-bit unsigned integer   corresponding to the number of seconds since the NAT gateway was   rebooted or since its port mapping state was otherwise reset.   This protocol SHOULD only be used when the client determines that its   primary IPv4 address is in one of the private IPv4 address ranges   defined in "Address Allocation for Private Internets" [RFC1918].   This includes the address ranges 10/8, 172.16/12, and 192.168/16.   Clients always send their NAT-PMP requests to their default gateway,   as learned via DHCP [RFC2131], or similar means.  This protocol is   designed for small home networks, with a single logical link (subnet)   where the client's default gateway is also the NAT for that network.   For more complicated networks where the NAT is some device other than   the client's default gateway, this protocol is not appropriate.3.1.  Requests and Responses   NAT gateways are often low-cost devices, with limited memory and CPU   speed.  For this reason, to avoid making excessive demands on the NAT   gateway, clients SHOULD NOT issue multiple concurrent requests.  If a   client needs to perform multiple requests (e.g., on boot, wake from   sleep, network connection, etc.), it SHOULD queue them and issue them   serially, one at a time.  Once the NAT gateway responds to one   request the client machine may issue the next.  In the case of a fast   NAT gateway, the client may be able to complete requests at a rate of   hundreds per second.  In the case of a slow NAT gateway that takes   perhaps half a second to respond to a NAT-PMP request, the client   SHOULD respect this and allow the NAT gateway to operate at the pace   it can manage, and not overload it by issuing requests faster than   the rate it's answering them.   To determine the external IPv4 address, or to request a port mapping,   a NAT-PMP client sends its request packet to port 5351 of its   configured gateway address, and waits 250 ms for a response.  If no   NAT-PMP response is received from the gateway after 250 ms, the   client retransmits its request and waits 500 ms.  The client SHOULD   repeat this process with the interval between attempts doubling each   time.  If, after sending its ninth attempt (and then waiting for 64   seconds), the client has still received no response, then it SHOULD   conclude that this gateway does not support NAT Port Mapping Protocol   and MAY log an error message indicating this fact.  In addition, if   the NAT-PMP client receives an "ICMP Port Unreachable" message fromCheshire & Krochmal           Informational                     [Page 6]

RFC 6886                         NAT-PMP                      April 2013   the gateway for port 5351, then it can skip any remaining   retransmissions and conclude immediately that the gateway does not   support NAT-PMP.   As a performance optimization the client MAY record this information   and use it to suppress further attempts to use NAT-PMP, but the   client should not retain this information for too long.  In   particular, any event that may indicate a potential change of gateway   or a change in gateway configuration (hardware link change   indication, change of gateway MAC address, acquisition of new DHCP   lease, receipt of NAT-PMP announcement packet from gateway, etc.)   should cause the client to discard its previous information regarding   the gateway's lack of NAT-PMP support, and send its next NAT-PMP   request packet normally.   When deleting a port mapping, the client uses the same initial 250 ms   timeout, doubling on each successive interval, except that clients   may choose not to try the full nine times before giving up.  This is   because mapping deletion requests are in some sense advisory.  They   are useful for efficiency, but not required for correctness; it is   always possible for client software to crash, or for power to fail,   or for a client device to be physically unplugged from the network   before it gets a chance to send its mapping deletion request(s), so   NAT gateways already need to cope with this case.  Because of this,   it may be acceptable for a client to retry only once or twice before   giving up on deleting its port mapping(s), but a client SHOULD always   send at least one deletion request whenever possible, to reduce the   amount of stale state that accumulates on NAT gateways.   A client need not continue trying to delete a port mapping after the   time when that mapping would naturally have expired anyway.3.2.  Determining the External Address   To determine the external address, the client behind the NAT sends   the following UDP payload to port 5351 of the configured gateway   address:    0                   1    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Vers = 0      | OP = 0        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Cheshire & Krochmal           Informational                     [Page 7]

RFC 6886                         NAT-PMP                      April 2013   A compatible NAT gateway MUST generate a response with the following   format:    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Vers = 0      | OP = 128 + 0  | Result Code (net byte order)  |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Seconds Since Start of Epoch (in network byte order)          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | External IPv4 Address (a.b.c.d)                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   This response indicates that the NAT gateway implements this version   of the protocol, and returns the external IPv4 address of the NAT   gateway.  If the result code is non-zero, the value of the External   IPv4 Address field is undefined (MUST be set to zero on transmission,   and MUST be ignored on reception).   The NAT gateway MUST fill in the Seconds Since Start of Epoch field   with the time elapsed since its port mapping table was initialized on   startup, or reset for any other reason (seeSection 3.6, "Seconds   Since Start of Epoch").   Upon receiving a response packet, the client MUST check the source IP   address, and silently discard the packet if the address is not the   address of the gateway to which the request was sent.3.2.1.  Announcing Address Changes   Upon boot, acquisition of an external IPv4 address, subsequent change   of the external IPv4 address, reboot, or any other event that may   indicate possible loss or change of NAT mapping state, the NAT   gateway MUST send a gratuitous response to the link-local multicast   address 224.0.0.1, port 5350, with the packet format above, to notify   clients of the external IPv4 address and Seconds Since Start of   Epoch.   To accommodate packet loss, the NAT gateway SHOULD multicast 10   address notifications.  The interval between the first two   notifications SHOULD be 250 ms, and the interval between each   subsequent notification SHOULD double.  The Seconds Since Start of   Epoch field in each transmission MUST be updated appropriately to   reflect the passage of time, so as not to trigger unnecessary   additional mapping renewals (seeSection 3.7, "Recreating Mappings on   NAT Gateway Reboot").Cheshire & Krochmal           Informational                     [Page 8]

RFC 6886                         NAT-PMP                      April 2013   Upon receiving a gratuitous address announcement packet, the client   MUST check the source IP address, and silently discard the packet if   the address is not the address of the client's current configured   gateway.  This is to guard against inadvertent misconfigurations   where there may be more than one NAT gateway active on the network.   If the source IP address is correct, then the Seconds Since Start of   Epoch field is checked as described inSection 3.6, and if the value   is outside the expected plausible range, indicating that a NAT   gateway state loss has occurred, then the NAT-PMP client promptly   recreates all its active port mapping leases, as described inSection3.7, "Recreating Mappings on NAT Gateway Reboot".   IMPLEMENTATION NOTE: Earlier implementations of NAT-PMP used UDP port   5351 as the destination both for client requests (address and port   mapping) and for address announcements.  NAT-PMP servers would listen   on UDP 5351 for client requests, and NAT-PMP clients would listen on   UDP 5351 for server announcements.  However, implementers encountered   difficulties when a single device is acting in both roles, for   example, a home computer with Internet Sharing enabled.  This   computer is acting in the role of NAT-PMP server to its DHCP clients,   yet, at the same time, it has to act in the role of NAT-PMP client in   order to determine whether it is, itself, behind another NAT gateway.   While in principle it might be possible on some operating systems for   two processes to coordinate sharing of a single UDP port, on many   platforms this is difficult or even impossible, so, for pragmatic   engineering reasons, it is convenient to have clients listen on UDP   5350 and servers listen on UDP 5351.   IMPLEMENTATION NOTE: A given host may have more than one independent   NAT-PMP client running at the same time, and address announcements   need to be available to all of them.  Clients should therefore set   the SO_REUSEPORT option or equivalent in order to allow other   processes to also listen on port 5350.  Additionally, implementers   have encountered issues when one or more processes on the same device   listen to port 5350 on *all* addresses.  Clients should therefore   bind specifically to 224.0.0.1:5350, not to 0.0.0.0:5350.Cheshire & Krochmal           Informational                     [Page 9]

RFC 6886                         NAT-PMP                      April 20133.3.  Requesting a Mapping   To create a mapping, the client sends a UDP packet to port 5351 of   the gateway's internal IP address with the following format:    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Vers = 0      | OP = x        | Reserved                      |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Internal Port                 | Suggested External Port       |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Requested Port Mapping Lifetime in Seconds                    |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Opcodes supported:   1 - Map UDP   2 - Map TCP   The Reserved field MUST be set to zero on transmission and MUST be   ignored on reception.   The Ports and Lifetime are transmitted in the traditional network   byte order (i.e., most significant byte first).   The Internal Port is set to the local port on which the client is   listening.   If the client would prefer to have a high-numbered "anonymous"   external port assigned, then it should set the Suggested External   Port to zero, which indicates to the gateway that it should allocate   a high-numbered port of its choosing.  If the client would prefer   instead to have the mapped external port be the same as its local   internal port if possible (e.g., a web server listening on port 80   that would ideally like to have external port 80), then it should set   the Suggested External Port to the desired value.  However, the   gateway is not obliged to assign the port suggested, and may choose   not to, either for policy reasons (e.g., port 80 is reserved and   clients may not request it) or because that port has already been   assigned to some other client.  Because of this, some product   developers have questioned the value of having the Suggested External   Port field at all.  The reason is for failure recovery.  Most low-   cost home NAT gateways do not record temporary port mappings in   persistent storage, so if the gateway crashes or is rebooted, all the   mappings are lost.  A renewal packet is formatted identically to an   initial mapping request packet, except that for renewals the client   sets the Suggested External Port field to the port the gateway   actually assigned, rather than the port the client originally wanted.Cheshire & Krochmal           Informational                    [Page 10]

RFC 6886                         NAT-PMP                      April 2013   When a freshly rebooted NAT gateway receives a renewal packet from a   client, it appears to the gateway just like an ordinary initial   request for a port mapping, except that in this case the Suggested   External Port is likely to be one that the NAT gateway *is* willing   to allocate (it allocated it to this client right before the reboot,   so it should presumably be willing to allocate it again).  This   improves the stability of external ports across NAT gateway restarts.   The RECOMMENDED Port Mapping Lifetime is 7200 seconds (two hours).   After sending the port mapping request, the client then waits for the   NAT gateway to respond.  If after 250 ms the client hasn't received a   response from the gateway, the client SHOULD reissue its request as   described above inSection 3.1, "Requests and Responses".   The NAT gateway responds with the following packet format:    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Vers = 0      | OP = 128 + x  | Result Code                   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Seconds Since Start of Epoch                                  |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Internal Port                 | Mapped External Port          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Port Mapping Lifetime in Seconds                              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   The epoch time, ports, and lifetime are transmitted in the   traditional network byte order (i.e., most significant byte first).   The 'x' in the OP field MUST match what the client requested.  Some   NAT gateways are incapable of creating a UDP port mapping without   also creating a corresponding TCP port mapping, and vice versa, and   these gateways MUST NOT implement NAT Port Mapping Protocol until   this deficiency is fixed.  A NAT gateway that implements this   protocol MUST be able to create TCP-only and UDP-only port mappings.   If a NAT gateway silently creates a pair of mappings for a client   that only requested one mapping, then it may expose that client to   receiving inbound UDP packets or inbound TCP connection requests that   it did not ask for and does not want.   While a NAT gateway MUST NOT automatically create mappings for TCP   when the client requests UDP, and vice versa, the NAT gateway MUST   reserve the companion port so the same client can choose to map it in   the future.  For example, if a client requests to map TCP port 80,Cheshire & Krochmal           Informational                    [Page 11]

RFC 6886                         NAT-PMP                      April 2013   as long as the client maintains the lease for that TCP port mapping,   another client with a different internal IP address MUST NOT be able   to successfully acquire the mapping for UDP port 80.   The client normally requests the external port matching the internal   port.  If that external port is not available, the NAT gateway MUST   return an available external port if possible, or return an error   code if no external ports are available.   The source address of the packet MUST be used for the internal   address in the mapping.  This protocol is not intended to facilitate   one device behind a NAT creating mappings for other devices.  If   there are legacy devices that require inbound mappings, permanent   mappings can be created manually by the user through an   administrative interface, just as they are today.   If a mapping already exists for a given internal address and port   (whether that mapping was created explicitly using NAT-PMP,   implicitly as a result of an outgoing TCP SYN packet, or manually by   a human administrator) and that client requests another mapping for   the same internal port (possibly requesting a different external   port), then the mapping request should succeed, returning the   already-assigned external port.  This is necessary to handle the case   where a client requests a mapping with suggested external port X, and   is granted a mapping with actual external port Y, but the   acknowledgment packet gets lost.  When the client retransmits its   mapping request, it should get back the same positive acknowledgment   as was sent (and lost) the first time.   The NAT gateway MUST NOT accept mapping requests destined to the NAT   gateway's external IP address or received on its external network   interface.  Only packets received on the internal interface(s) with a   destination address matching the internal address(es) of the NAT   gateway should be allowed.   The NAT gateway MUST fill in the Seconds Since Start of Epoch field   with the time elapsed since its port mapping table was initialized on   startup or reset for any other reason (seeSection 3.6, "Seconds   Since Start of Epoch").   The Port Mapping Lifetime is an unsigned integer in seconds.  The NAT   gateway MAY reduce the lifetime from what the client requested.  The   NAT gateway SHOULD NOT offer a lease lifetime greater than that   requested by the client.   Upon receiving the response packet, the client MUST check the source   IP address, and silently discard the packet if the address is not the   address of the gateway to which the request was sent.Cheshire & Krochmal           Informational                    [Page 12]

RFC 6886                         NAT-PMP                      April 2013   The client SHOULD begin trying to renew the mapping halfway to expiry   time, like DHCP.  The renewal packet should look exactly the same as   a request packet, except that the client SHOULD set the Suggested   External Port to what the NAT gateway previously mapped, not what the   client originally suggested.  As described above, this enables the   gateway to automatically recover its mapping state after a crash or   reboot.3.4.  Destroying a Mapping   A mapping may be destroyed in a variety of ways.  If a client fails   to renew a mapping, then at the time its lifetime expires, the   mapping MUST be automatically deleted.  In the common case where the   gateway device is a combined DHCP server and NAT gateway, when a   client's DHCP address lease expires, the gateway device MAY   automatically delete any mappings belonging to that client.   Otherwise, a new client being assigned the same IP address could   receive unexpected inbound UDP packets or inbound TCP connection   requests that it did not ask for and does not want.   A client MAY also send an explicit packet to request deletion of a   mapping that is no longer needed.  A client requests explicit   deletion of a mapping by sending a message to the NAT gateway   requesting the mapping, with the Requested Lifetime in Seconds set to   zero.  The Suggested External Port MUST be set to zero by the client   on sending, and MUST be ignored by the gateway on reception.   When a mapping is destroyed successfully as a result of the client   explicitly requesting the deletion, the NAT gateway MUST send a   response packet that is formatted as defined inSection 3.3,   "Requesting a Mapping".  The response MUST contain a result code of   0, the internal port as indicated in the deletion request, an   external port of 0, and a lifetime of 0.  The NAT gateway MUST   respond to a request to destroy a mapping that does not exist as if   the request were successful.  This is because of the case where the   acknowledgment is lost, and the client retransmits its request to   delete the mapping.  In this case, the second request to delete the   mapping MUST return the same response packet as the first request.   If the deletion request was unsuccessful, the response MUST contain a   non-zero result code and the requested mapping; the lifetime is   undefined (MUST be set to zero on transmission, and MUST be ignored   on reception).  If the client attempts to delete a port mapping that   was manually assigned by some kind of configuration tool, the NAT   gateway MUST respond with a "Not Authorized" error, result code 2.Cheshire & Krochmal           Informational                    [Page 13]

RFC 6886                         NAT-PMP                      April 2013   When a mapping is destroyed as a result of its lifetime expiring or   for any other reason, if the NAT gateway's internal state indicates   that there are still active TCP connections traversing that now-   defunct mapping, then the NAT gateway SHOULD send appropriately   constructed TCP RST (reset) packets both to the local client and to   the remote peer on the Internet to terminate that TCP connection.   A client can request the explicit deletion of all its UDP or TCP   mappings by sending the same deletion request to the NAT gateway with   the external port, internal port, and lifetime set to zero.  A client   MAY choose to do this when it first acquires a new IP address in   order to protect itself from port mappings that were performed by a   previous owner of the IP address.  After receiving such a deletion   request, the gateway MUST delete all its UDP or TCP port mappings   (depending on the opcode).  The gateway responds to such a deletion   request with a response as described above, with the internal port   set to zero.  If the gateway is unable to delete a port mapping, for   example, because the mapping was manually configured by the   administrator, the gateway MUST still delete as many port mappings as   possible, but respond with a non-zero result code.  The exact result   code to return depends on the cause of the failure.  If the gateway   is able to successfully delete all port mappings as requested, it   MUST respond with a result code of zero.3.5.  Result Codes   Currently defined result codes:   0 - Success   1 - Unsupported Version   2 - Not Authorized/Refused       (e.g., box supports mapping, but user has turned feature off)   3 - Network Failure       (e.g., NAT box itself has not obtained a DHCP lease)   4 - Out of resources       (NAT box cannot create any more mappings at this time)   5 - Unsupported opcodeCheshire & Krochmal           Informational                    [Page 14]

RFC 6886                         NAT-PMP                      April 2013   If the version in the request is not zero, then the NAT-PMP server   MUST return the following "Unsupported Version" error response to the   client:    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Vers = 0      | OP = 0        | Result Code = 1               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Seconds Since Start of Epoch (in network byte order)          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   If the opcode in the request is 128 or greater, then this is not a   request; it's a response, and the NAT-PMP server MUST silently ignore   it.  Otherwise, if the opcode in the request is less than 128, but is   not a supported opcode (currently 0, 1, or 2), then the entire   request MUST be returned to the sender, with the top bit of the   opcode set (to indicate that this is a response) and the result code   set to 5 (Unsupported opcode).   For version 0 and a supported opcode (0, 1, or 2), if the operation   fails for some reason (Not Authorized, Network Failure, or Out of   resources), then a valid response MUST be sent to the client, with   the top bit of the opcode set (to indicate that this is a response)   and the result code set appropriately.  Other fields in the response   MUST be set appropriately.  Specifically:   o Seconds Since Start of Epoch MUST be set correctly   o The External IPv4 Address should be set to the correct address, or     to 0.0.0.0, as appropriate.   o The Internal Port MUST be set to the client's requested Internal     Port.  This is particularly important, because the client needs     this information to identify which request suffered the failure.   o The Mapped External Port and Port Mapping Lifetime MUST be set     appropriately -- i.e., zero if no successful port mapping was     created.   Should future NAT-PMP opcodes be defined, their error responses MUST   similarly be specified to include sufficient information to identify   which request suffered the failure.  By design, NAT-PMP messages do   not contain any transaction identifiers.  All NAT-PMP messages are   idempotent and self-describing; therefore, the specifications of   future NAT-PMP messages need to include enough information for their   responses to be self-describing.Cheshire & Krochmal           Informational                    [Page 15]

RFC 6886                         NAT-PMP                      April 2013   Clients MUST be able to properly handle result codes not defined in   this document.  Undefined results codes MUST be treated as fatal   errors of the request.3.6.  Seconds Since Start of Epoch   Every packet sent by the NAT gateway includes a Seconds Since Start   of Epoch (SSSoE) field.  If the NAT gateway resets or loses the state   of its port mapping table, due to reboot, power failure, or any other   reason, it MUST reset its epoch time and begin counting SSSoE from   zero again.  Whenever a client receives any packet from the NAT   gateway, either unsolicited or in response to a client request, the   client computes its own conservative estimate of the expected SSSoE   value by taking the SSSoE value in the last packet it received from   the gateway and adding 7/8 (87.5%) of the time elapsed according to   the client's local clock since that packet was received.  If the   SSSoE in the newly received packet is less than the client's   conservative estimate by more than 2 seconds, then the client   concludes that the NAT gateway has undergone a reboot or other loss   of port mapping state, and the client MUST immediately renew all its   active port mapping leases as described inSection 3.7, "Recreating   Mappings on NAT Gateway Reboot".3.7.  Recreating Mappings on NAT Gateway Reboot   The NAT gateway MAY store mappings in persistent storage so that,   when it is powered off or rebooted, it remembers the port mapping   state of the network.   However, maintaining this state is not essential for correct   operation.  When the NAT gateway powers on or clears its port mapping   state as the result of a configuration change, it MUST reset the   epoch time and re-announce its IPv4 address as described inSection3.2.1, "Announcing Address Changes".  Reception of this packet where   time has apparently gone backwards serves as a hint to clients on the   network that they SHOULD immediately send renewal packets (to   immediately recreate their mappings) instead of waiting until the   originally scheduled time for those renewals.  Clients who miss   receiving those gateway announcement packets for any reason will   still renew their mappings at the originally scheduled time and cause   their mappings to be recreated; it will just take a little longer for   these clients.Cheshire & Krochmal           Informational                    [Page 16]

RFC 6886                         NAT-PMP                      April 2013   A mapping renewal packet is formatted identically to an original   mapping request; from the point of view of the client, it is a   renewal of an existing mapping, but from the point of view of the   freshly rebooted NAT gateway, it appears as a new mapping request.   This self-healing property of the protocol is very important.   The remarkable reliability of the Internet as a whole derives in   large part from the fact that important state is held in the   endpoints, not in the network itself [ETEAISD].  Power-cycling an   Ethernet switch results only in a brief interruption in the flow of   packets; established TCP connections through that switch are not   broken, merely delayed for a few seconds.  Indeed, a failing Ethernet   switch can even be replaced with a new one, and as long as the cables   are transferred over reasonably quickly, after the upgrade all the   TCP connections that were previously going through the old switch   will be unbroken and now going through the new one.  The same is true   of IP routers, wireless base stations, etc.  The one exception is NAT   gateways.  Because the port mapping state is required for the NAT   gateway to know where to forward inbound packets, loss of that state   breaks connectivity through the NAT gateway.  By allowing clients to   detect when this loss of NAT gateway state has occurred, and recreate   it on demand, we turn hard state in the network into soft state, and   allow it to be recovered automatically when needed.   Without this automatic recreation of soft state in the NAT gateway,   reliable long-term networking would not be achieved.  As mentioned   above, the reliability of the Internet does not come from trying to   build a perfect network in which errors never happen, but from   accepting that in any sufficiently large system there will always be   some component somewhere that's failing, and designing mechanisms   that can handle those failures and recover.  To illustrate this point   with an example, consider the following scenario: Imagine a network   security camera that has a web interface and accepts incoming   connections from web browser clients.  Imagine this network security   camera uses NAT-PMP or a similar protocol to set up an inbound port   mapping in the NAT gateway so that it can receive incoming   connections from clients on the other side of the NAT gateway.  Now,   this camera may well operate for weeks, months, or even years.   During that time, it's possible that the NAT gateway could experience   a power failure or be rebooted.  The user could upgrade the NAT   gateway's firmware, or even replace the entire NAT gateway device   with a newer model.  The general point is that if the camera operates   for a long enough period of time, some kind of disruption to the NAT   gateway becomes inevitable.  The question is not whether the NAT   gateway will lose its port mappings, but when, and how often.  If the   network camera and devices like it on the network can detect when the   NAT gateway has lost its port mappings, and recreate themCheshire & Krochmal           Informational                    [Page 17]

RFC 6886                         NAT-PMP                      April 2013   automatically, then these disruptions are self-correcting and largely   invisible to the end user.  If, on the other hand, the disruptions   are not self-correcting, and after a NAT gateway reboot the user has   to manually reset or reboot all the other devices on the network too,   then these disruptions are *very* visible to the end user.  This   aspect of the design is part of what makes the difference between a   protocol that keeps on working indefinitely over a time scale of   months or years, and a protocol that works in brief testing, but in   the real world is continually failing and requiring manual   intervention to get it going again.   When a client renews its port mappings as the result of receiving a   packet where the Seconds Since Start of Epoch (SSSoE) field indicates   that a reboot or similar loss of state has occurred, the client MUST   first delay by a random amount of time selected with uniform random   distribution in the range 0 to 5 seconds, and then send its first   port mapping request.  After that request is acknowledged by the   gateway, the client may then send its second request, and so on, as   rapidly as the gateway allows.  The requests SHOULD be issued   serially, one at a time; the client SHOULD NOT issue multiple   concurrent requests.   The discussion in this section focuses on recreating inbound port   mappings after loss of NAT gateway state, because that is the more   serious problem.  Losing port mappings for outgoing connections   destroys those currently active connections, but does not prevent   clients from establishing new outgoing connections.  In contrast,   losing inbound port mappings not only destroys all existing inbound   connections, but also prevents the reception of any new inbound   connections until the port mapping is recreated.  Accordingly, we   consider recovery of inbound port mappings more important.  However,   clients that want outgoing connections to survive a NAT gateway   reboot can also achieve that using NAT-PMP, in the common case of a   residential NAT gateway with a single, relatively stable, external IP   address.  After initiating an outbound TCP connection (which will   cause the NAT gateway to establish an implicit port mapping), the   client should send the NAT gateway a port mapping request for the   source port of its TCP connection, which will cause the NAT gateway   to send a response giving the external port it allocated for that   mapping.  The client can then store this information, and use it   later to recreate the mapping if it determines that the NAT gateway   has lost its mapping state.3.8.  NAT Gateways with NAT Function Disabled   Note that only devices that are *currently* acting in the role of NAT   gateway should participate in NAT-PMP protocol exchanges with   clients.  A network device that is capable of NAT (and NAT-PMP) butCheshire & Krochmal           Informational                    [Page 18]

RFC 6886                         NAT-PMP                      April 2013   is currently configured not to perform that function (e.g., it is   acting as a traditional IP router, forwarding packets without   modifying them) MUST NOT respond to NAT-PMP requests from clients nor   send spontaneous NAT-PMP address-change announcements.   In particular, a network device not currently acting in the role of   NAT gateway should not even respond to NAT-PMP requests by returning   an error code such as 2, "Not Authorized/Refused", since to do so is   misleading to clients -- it suggests that NAT port mapping is   necessary on this network for the client to successfully receive   inbound connections, but is not available because the administrator   has chosen to disable that functionality.   Clients should also be careful to avoid making unfounded assumptions,   such as the assumption that if the client has an IPv4 address in one   of the private IPv4 address ranges [RFC1918], then that means NAT   necessarily must be in use.  Net 10/8 has enough addresses to build a   private network with millions of hosts and thousands of   interconnected subnets, all without any use of NAT.  Many   organizations have built such private networks that benefit from   using standard TCP/IP technology, but by choice do not connect to the   public Internet.  The purpose of NAT-PMP is to mitigate some of the   damage caused by NAT.  It would be an ironic and unwanted side effect   of this protocol if it were to lead well-meaning but misguided   developers to create products that refuse to work on a private   network *unless* they can find a NAT gateway to talk to.   Consequently, a client finding that NAT-PMP is not available on its   network should not give up, but should proceed on the assumption that   the network may be a traditional routed IP network, with no address   translation being used.  This assumption may not always be true, but   it is better than the alternative of falsely assuming the worst and   not even trying to use normal (non-NAT) IP networking.   If a network device not currently acting in the role of NAT gateway   receives UDP packets addressed to port 5351, it SHOULD respond   immediately with an "ICMP Port Unreachable" message to tell the   client that it needn't continue with timeouts and retransmissions,   and it should assume that NAT-PMP is not available and not needed on   this network.  Typically, this behavior can be achieved merely by not   having an open socket listening on UDP port 5351.3.9.  All Mappings Are Bidirectional   All NAT mappings, whether created implicitly by an outbound packet,   created explicitly using NAT-PMP, or configured statically, are   bidirectional.  This means that when an outbound packet from a   particular internal address and port is translated to an externalCheshire & Krochmal           Informational                    [Page 19]

RFC 6886                         NAT-PMP                      April 2013   address and port, replies addressed to that external address and port   need to be translated back to the corresponding internal address and   port.   The converse is also true.  When an inbound packet is received that   is addressed to an external address and port that matches an existing   mapping (implicit, explicit, or static), it is translated to the   corresponding internal address and port and forwarded.  Outbound   replies from that internal address and port need to be translated to   the correct external address and port so that they are correctly   recognized by the remote peer.   In particular, if an outbound UDP reply that matches an existing   explicit or static mapping is instead treated like a "new" outbound   UDP packet, and a new dynamic mapping is created (with a different   external address and port), then at the time that packet arrives at   the remote peer it will not be recognized as a valid reply.  For TCP   this bug is quickly spotted because all TCP implementations will   ignore replies with the wrong apparent source address and port.  For   UDP this bug can more easily go unnoticed because some UDP clients   neglect to check the source address and port of replies; thus, they   will appear to work some of the time with NAT gateways that put the   wrong source address and port in outbound packets.  All NAT gateways   MUST ensure that mappings, however created, are bidirectional.4.  UNSAF Considerations   The document "IAB Considerations for UNilateral Self-Address Fixing   (UNSAF) Across Network Address Translation (NAT)" [RFC3424] covers a   number of issues when working with NATs.  It outlines some   requirements for any document that attempts to work around problems   associated with NATs.  This section addresses those requirements.4.1.  Scope   This protocol addresses the needs of TCP and UDP transport peers that   are separated from the public Internet by exactly one IPv4 NAT.  Such   peers must have access to some form of directory server for   registering the public IPv4 address and port at which they can be   reached.4.2.  Transition Plan   Any client making use of this protocol SHOULD implement IPv6 support.   If a client supports IPv6 and is running on a device with a global   IPv6 address, that IPv6 address SHOULD be preferred to the IPv4   external address learned via this NAT mapping protocol.  In case   other clients do not have IPv6 connectivity, both the IPv4 and IPv6Cheshire & Krochmal           Informational                    [Page 20]

RFC 6886                         NAT-PMP                      April 2013   addresses SHOULD be registered with whatever form of directory server   is used.  Preference SHOULD be given to IPv6 addresses when   available.  By implementing support for IPv6 and using this protocol   for IPv4, vendors can ship products today that will work under both   scenarios.  As IPv6 becomes more widely deployed, clients of this   protocol following these recommendations will transparently make use   of IPv6.4.3.  Failure Cases   Aside from NATs that do not implement this protocol, there are a   number of situations where this protocol may not work.4.3.1.  NAT behind NAT   Some people's primary IPv4 address, assigned by their ISP, may itself   be a NAT address.  In addition, some people may have an external IPv4   address, but may then double NAT themselves, perhaps by choice or   perhaps by accident.  Although it might be possible in principle for   one NAT gateway to recursively request a mapping from the next one,   this document does not advocate that and does not try to prescribe   how it would be done.   It would be a lot of work to implement nested NAT port mapping   correctly, and there are a number of reasons why the end result might   not be as useful as we might hope.  Consider the case of an ISP that   offers each of its customers only a single NAT address.  This ISP   could instead have chosen to provide each customer with a single   public IPv4 address, or, if the ISP insists on running NAT, it could   have chosen to allow each customer a reasonable number of addresses,   enough for each customer device to have its own NAT address directly   from the ISP.  If, instead, this ISP chooses to allow each customer   just one and only one NAT address, forcing said customer to run   nested NAT in order to use more than one device, it seems unlikely   that such an ISP would be so obliging as to provide a NAT service   that supports NAT-PMP.  Supposing that such an ISP did wish to offer   its customers NAT service with NAT-PMP so as to give them the ability   to receive inbound connections, this ISP could easily choose to allow   each client to request a reasonable number of DHCP addresses from   that gateway.  Remember that Net 10/8 [RFC1918] allows for over 16   million addresses, so NAT addresses are not in any way in short   supply.  A single NAT gateway with 16 million available addresses is   likely to run out of packet forwarding capacity before it runs out of   internal addresses to hand out.  In this way, the ISP could offer   single-level NAT with NAT-PMP, obviating the need to support nested   NAT-PMP.  In addition, an ISP that is motivated to provide their   customers with unhindered access to the Internet by allowing incoming   as well as outgoing connections has better ways to offer thisCheshire & Krochmal           Informational                    [Page 21]

RFC 6886                         NAT-PMP                      April 2013   service.  Such an ISP could offer its customers real public IPv4   addresses instead of NAT addresses, or could choose to offer its   customers full IPv6 connectivity, where no mapping or translation is   required at all.   Note: In the nine years since NAT-PMP was designed, the pool of   available IPv4 addresses has been exhausted, and many ISPs now offer   translated IPv4 addresses out of necessity.  Such ISPs have indicated   a willingness to offer PCP service to their customers.4.3.2.  NATs with Multiple External IPv4 Addresses   If a NAT maps internal addresses to multiple external addresses, then   it SHOULD pick one of those external addresses as the one it will   support for inbound connections, and for the purposes of this   protocol it SHOULD act as if that address were its only address.4.3.3.  NATs and Routed Private Networks   In some cases, a large network may be subnetted.  Some sites may   install a NAT gateway and subnet the private network.  Such   subnetting breaks this protocol because the router address is not   necessarily the address of the device performing NAT.   Addressing this problem is not a high priority.  Any site with the   resources to set up such a configuration should have the resources to   add manual mappings or attain a range of globally unique addresses.   Not all NATs will support this protocol.  In the case where a client   is run behind a NAT that does not support this protocol, the software   relying on the functionality of this protocol may be unusable.4.3.4.  Communication between Hosts behind the Same NAT   NAT gateways supporting NAT-PMP should also implement "hairpin   translation".  Hairpin translation means supporting communication   between two local clients being served by the same NAT gateway.   Suppose device A is listening on internal address and port   10.0.0.2:80 for incoming connections.  Using NAT-PMP, device A has   obtained a mapping to external address and port x.x.x.x:80, and has   recorded this external address and port in a public directory of some   kind.  For example, it could have created a DNS SRV record containing   this information, and recorded it, using DNS Dynamic Update   [RFC3007], in a publicly accessible DNS server.  Suppose then that   device B, behind the same NAT gateway as device A, but unknowing or   uncaring of this fact, retrieves device A's DNS SRV record and   attempts to open a TCP connection to x.x.x.x:80.  The outgoingCheshire & Krochmal           Informational                    [Page 22]

RFC 6886                         NAT-PMP                      April 2013   packets addressed to this public Internet address will be sent to the   NAT gateway for translation and forwarding.  Having translated the   source address and port number on the outgoing packet, the NAT   gateway needs to be smart enough to recognize that the destination   address is in fact itself, and then feed this packet back into its   packet reception engine, to perform the destination port mapping   lookup to translate and forward this packet to device A at address   and port 10.0.0.2:80.4.3.5.  Non-UDP/TCP Transport Traffic   Any communication over transport protocols other than TCP and UDP   will not be served by this protocol.  Examples are Generic Routing   Encapsulation (GRE), Authentication Header (AH), and Encapsulating   Security Payload (ESP).4.4.  Long-Term Solution   As IPv6 is deployed, clients of this protocol supporting IPv6 will be   able to bypass this protocol and the NAT when communicating with   other IPv6 devices.  In order to ensure this transition, any client   implementing this protocol SHOULD also implement IPv6 and use this   solution only when IPv6 is not available to both peers.4.5.  Existing Deployed NATs   Existing deployed NATs will not support this protocol.  This protocol   will only work with NATs that are upgraded to support it.5.  Security Considerations   As discussed inSection 3.2, "Determining the External Address", only   a client on the internal side of the NAT may create port mappings,   and it may do so only on its own behalf.  By using IP address   spoofing, it's possible for one client to delete the port mappings of   another client.  It's also possible for one client to create port   mappings on behalf of another client.  In cases where this is a   concern, it can be dealt with using IPsec [RFC4301].   The multicast announcements described inSection 3.2.1, "Announcing   Address Changes", could be spoofed, facilitating a denial-of-service   attack.  This makes NAT-PMP unsuitable for use on LANs with large   numbers of hosts where one or more of the hosts may be untrustworthy.   Another concern is that rogue software running on a local host could   create port mappings for unsuspecting hosts, thereby rendering them   vulnerable to external attack.  However, it's not clear how realistic   this threat model is, since rogue software on a local host couldCheshire & Krochmal           Informational                    [Page 23]

RFC 6886                         NAT-PMP                      April 2013   attack such unsuspecting hosts directly itself, without resorting to   such a convoluted indirect technique.  This concern is also a little   misguided because it is based on the assumption that a NAT gateway   and a firewall are the same thing, which they are not.   Some people view the property of NATs blocking inbound connections as   a security benefit that is undermined by this protocol.  The authors   of this document have a different point of view.  In the days before   NAT became prevalent, all hosts had unique public IP addresses, and   had unhindered ability to communicate with any other host on the   Internet (a configuration that is still surprisingly common).  Using   NAT breaks this unhindered connectivity, relegating hosts to second-   class status, unable to receive inbound connections.  This protocol   goes some way to partially reverse that damage.  The purpose of a NAT   gateway should be to allow several hosts to share a single address,   not to simultaneously impede those host's ability to communicate   freely.  Security is most properly provided by end-to-end   cryptographic security, and/or by explicit firewall functionality, as   appropriate.  Blocking of certain connections should occur only as a   result of explicit and intentional firewall policy, not as an   accidental side effect of some other technology.   However, since many users do have an expectation that their NAT   gateways can function as a kind of firewall, any NAT gateway   implementing this protocol SHOULD have an administrative mechanism to   disable it, thereby restoring the pre-NAT-PMP behavior.6.  IANA Considerations   UDP ports 5350 and 5351 have been assigned for use by NAT-PMP, and   subsequently by its successor, Port Control Protocol [RFC6887].   No further IANA services are required by this document.7.  Acknowledgments   The concepts described in this document have been explored,   developed, and implemented with help from Mark Baugher, Bob Bradley,   Josh Graessley, Rory McGuire, Rob Newberry, Roger Pantos, John   Saxton, Kiren Sekar, Jessica Vazquez, and James Woodyatt.   Special credit goes to Mike Bell, the Apple Vice President who   recognized the need for a clean, elegant, reliable Port Mapping   Protocol, and made the decision early on that Apple's AirPort base   stations would support NAT-PMP.Cheshire & Krochmal           Informational                    [Page 24]

RFC 6886                         NAT-PMP                      April 20138.  Deployment History   In August 2004, NAT-PMP client software first became available to the   public through Apple's Darwin Open Source code.  In April 2005,   NAT-PMP implementations began shipping to end users with the launch   of Mac OS X 10.4 Tiger and Bonjour for Windows 1.0, and in June 2005   the protocol was first publicly documented in the original draft   version of this document.   The NAT-PMP client in Mac OS X 10.4 Tiger and Bonjour for Windows   exists as part of the mDNSResponder/mdnsd system service.  When a   client advertises a service using Wide Area Bonjour [RFC6763], and   the machine is behind a NAT-PMP-capable NAT gateway, and the machine   is so configured, the mDNSResponder system service automatically uses   NAT-PMP to set up an inbound port mapping, and then records the   external IPv4 address and port in the global DNS.  Existing client   software using the Bonjour programming APIs [Bonjour] got this new   NAT traversal functionality automatically.  The logic behind this   decision was that if client software publishes its information into   the global DNS via Wide Area Bonjour service advertising, then it's   reasonable to infer an expectation that this information should   actually be usable by the peers retrieving it.  Generally speaking,   recording a private IPv4 address like 10.0.0.2 in the public DNS is   likely to be pointless because that address is not reachable from   clients on the other side of the NAT gateway.  In the case of a home   user with a single computer directly connected to their Cable or DSL   modem, with a single global IPv4 address and no NAT gateway (a common   configuration at that time), publishing the machine's global IPv4   address into the global DNS is useful, because that IPv4 address is   globally reachable.  In contrast, a home user using a NAT gateway to   share a single global IPv4 address between several computers loses   this ability to receive inbound connections.  This breaks many peer-   to-peer collaborative applications, like the multi-user text editor   SubEthaEdit [SEE].  For many users, moving from one computer with a   global IPv4 address, to two computers using NAT to share a single   global IPv4 address, loss of inbound reachability was an unwanted   side effect of using NAT for address sharing.  Automatically creating   the necessary inbound port mappings helped remedy this unwanted side   effect of NAT.   The server side of the NAT-PMP protocol is implemented in Apple's   AirPort Extreme, AirPort Express, and Time Capsule wireless base   stations, and in the Internet Sharing feature of Mac OS X 10.4 and   later.  Some third-party NAT vendors, such as Peplink, also offer   NAT-PMP in their products.Cheshire & Krochmal           Informational                    [Page 25]

RFC 6886                         NAT-PMP                      April 2013   In Mac OS X 10.4 Tiger, the NAT-PMP client was invoked automatically   as a side effect of clients requesting Wide Area Bonjour service   registrations.  Using NAT-PMP without an associated Wide Area Bonjour   service registration required use of a third-party client library.   In October 2007, Mac OS X 10.5 Leopard added the "DNSServiceNATPort-   MappingCreate" API, which made NAT-PMP client functionality directly   available, so software could use it with other directory and   rendezvous mechanisms in addition to Wide Area Bonjour DNS Updates.   In 2013, NAT-PMP was superseded by the IETF Standards Track Port   Control Protocol [RFC6887].  PCP builds on NAT-PMP and uses a   compatible packet format, and adds a number of significant   enhancements, including IPv6 support, management of outbound   mappings, management of firewall rules, full compatibility with   large-scale NATs with a pool of external addresses, error lifetimes,   and an extension mechanism to enable future enhancements.9.  Noteworthy Features of NAT Port Mapping Protocol and PCP   Some readers have asked how NAT-PMP and PCP compare to other similar   solutions, particularly the UPnP Forum's Internet Gateway Device   (IGD) Device Control Protocol [IGD].   The answer is that although the Universal Plug and Play (UPnP) IGD   protocol is often used as a way for client devices to create port   mappings programmatically, it's not ideal for that task.  Whereas   NAT-PMP was explicitly designed to be used primarily by software   entities managing their own port mappings, UPnP IGD is more tailored   towards being used by humans configuring all the settings of their   gateway using some GUI tool.  This difference in emphasis leads to   protocol differences.  For example, while it is reasonable and   sensible to require software entities to renew their mappings   periodically to prove that they are still there (like a device   renewing its DHCP address lease), it would be unreasonable to require   the same thing of a human user.  When a human user configures their   gateway, they expect it to stay configured that way until they decide   to change it.  If they configure a port mapping, they expect it to   stay configured until they decide to delete it.   Because of this focus on being a general administration protocol for   all aspects of home gateway configuration, UPnP IGD is a large and   complicated collection of protocols (360 pages of specification   spread over 13 separate documents, not counting supporting protocol   specifications like Simple Service Discovery Protocol (SSDP) and   Extensible Markup Language (XML)).  While it may be a fine way forCheshire & Krochmal           Informational                    [Page 26]

RFC 6886                         NAT-PMP                      April 2013   human users to configure their home gateways, it is not especially   suited to the task of programmatically creating dynamic port   mappings.   The requirements for a good port mapping protocol, requirements that   are met by NAT-PMP, are outlined below.9.1.  Simplicity   Many home gateways, and many of the devices that connect to them, are   small, low-cost devices, with limited RAM, flash memory, and CPU   resources.  Protocols they use should be considerate of this,   supporting a small number of simple operations that can be   implemented easily with a small amount of code.  A quick comparison,   based on page count of the respective documents alone, suggests that   NAT-PMP is at least ten times simpler than UPnP IGD.9.2.  Focused Scope   The more things a protocol can do, the more chance there is that   something it does could be exploited for malicious purposes.  NAT-PMP   is tightly focused on the specific task of creating port mappings.   Were the protocol to be misused in some way, this helps limit the   scope of what mischief could be performed using the protocol.   Because UPnP IGD allows control over all home gateway configuration   settings, the potential for mischief is far greater.  For example, a   UPnP IGD home gateway allows messages that tell it to change the DNS   server addresses that it sends to clients in its DHCP packets.  Using   this mechanism, a single item of malicious web content (e.g., a rogue   Flash banner advert on a web page) can make a persistent change to   the home gateway's configuration without the user's knowledge, such   that all future DNS requests by all local clients will be sent to a   rogue DNS server.  This allows criminals to perform a variety of   mischief, such as hijacking connections to bank web sites and   redirecting them to the criminals' web servers instead [VU347812].9.3.  Efficiency   In addition to low-cost home gateways, many of the clients will also   be similarly constrained low-cost devices with limited RAM resources.   When implementing a NAT-PMP client on a constrained device, it's   beneficial to have well-defined bounds on RAM requirements that are   fixed and known in advance.  For example, when requesting the   gateway's external IPv4 address, a NAT-PMP client on Ethernet knowsCheshire & Krochmal           Informational                    [Page 27]

RFC 6886                         NAT-PMP                      April 2013   that to receive the reply it will require 14 bytes for the Ethernet   header, 20 bytes for the IPv4 header, 8 bytes for the UDP header, and   12 bytes for the NAT-PMP payload, making a total of 54 bytes.   In contrast, UPnP IGD uses an XML reply of unbounded size.  It is not   uncommon for a UPnP IGD device to return an XML document 4000 to 8000   bytes in size to communicate its 4-byte external IPv4 address, and   the protocol specification places no upper bound on how large the XML   response may be, so there's nothing to stop the reply being even   larger.  This means that developers of UPnP client devices can only   guess at how much memory they may need to receive the XML reply.   Operational experience suggests that 10,000 bytes is usually enough   for most UPnP IGD home gateways today, but that's no guarantee that   some future UPnP IGD home gateway might not return a perfectly legal   XML reply much larger than that.   In addition, because the XML reply is too large to fit in a single   UDP packet, UPnP IGD has to use a TCP connection, thereby adding the   overhead of TCP connection setup and teardown.   The process of discovering a UPnP IGD home gateway's external IPv4   address consists of:   o SSDP transaction to discover the TCP port to use, and the "URL" of     the XML document to fetch from the gateway.  Following the SSDP     specification, this is 3 multicast requests, eliciting 9 unicast     responses.   o HTTP "GET" request to get the device description.  Typically, 16     packets: 3 for TCP connection setup, 9 packets of data exchange,     and a 4-packet FIN-ACK-FIN-ACK sequence to close the connection.   o HTTP "POST" to request the external IPv4 address.  Typically, 14     packets: 3 for TCP connection setup, 7 packets of data exchange,     and a 4-packet FIN-ACK-FIN-ACK sequence to close the connection.   To retrieve the external IPv4 address NAT-PMP takes a 2-packet UDP   exchange (44-byte request, 54-byte response); the same thing using   UPnP IGD takes 42 packets and thousands of bytes.   Similarly, UPnP IGD's HTTP "POST" request for a port mapping is   typically a 14-packet exchange, compared with NAT-PMP's 2-packet UDP   exchange.Cheshire & Krochmal           Informational                    [Page 28]

RFC 6886                         NAT-PMP                      April 20139.4.  Atomic Allocation Operations   Some of the useful properties of NAT-PMP were inspired by DHCP, a   reliable and successful protocol.  For example, DHCP allows a client   to request a desired IP address, but if that address is already in   use the DHCP server will instead assign some other available address.   Correspondingly, NAT-PMP allows a client to request a desired   external port, and if that external port is already in use by some   other client, the NAT-PMP server will instead assign some other   available external port.   UPnP IGD does not do this.  If a UPnP IGD client requests an external   port that has already been allocated, then one of two things happens.   Some UPnP IGD home gateways just silently overwrite the old mapping   with the new one, causing the previous client to lose connectivity.   If the previous client renews its port mapping, then it in turn   overwrites the new mapping, and the two clients fight over the same   external port indefinitely, neither achieving reliable connectivity.   Other IGD home gateways return a "Conflict" error if the port is   already in use, which does at least tell the client what happened,   but doesn't tell the client what to do.  Instead of the NAT gateway   (which does know which ports are available) assigning one to the   client, the NAT gateway makes the client (which doesn't know) keep   guessing until it gets lucky.  This problem remains mild as long as   not many clients are using UPnP IGD, but gets progressively worse as   the number of clients on the network requesting port mappings goes   up.  In addition, UPnP IGD works particularly badly in conjunction   with the emerging policy of allocating pre-assigned port ranges to   each client.  If a client is assigned TCP port range 63488-64511, and   the UPnP IGD client requests TCP port 80, trying successive   incrementing ports until it succeeds, then the UPnP IGD client will   have to issue 63,409 requests before it succeeds.9.5.  Garbage Collection   In any system that operates for a long period of time (as a home   gateway should), it is important that garbage data does not   accumulate indefinitely until the system runs out of memory and   fails.   Similar to how DHCP leases an IP address to a client for a finite   length of time, NAT-PMP leases an external port to a client for a   finite length of time.  The NAT-PMP client must renew the port   mapping before it expires, or, like an unrenewed DHCP address, it   will be reclaimed.  If a laptop computer is abruptly disconnectedCheshire & Krochmal           Informational                    [Page 29]

RFC 6886                         NAT-PMP                      April 2013   from the network without the opportunity to delete its port mappings,   the NAT gateway will reclaim those mappings when they are not   renewed.   In principle, UPnP IGD should allow clients to specify a lifetime on   port mappings.  However, a Google search for "UPnP NewLeaseDuration"   shows that in practice pretty much every client uses   "<NewLeaseDuration>0</NewLeaseDuration>" to request an infinite   lease, and the protocol has no way for the NAT gateway to decline   that infinite lease request and require the client to renew it at   reasonable intervals.  Furthermore, anecdotal evidence is that if the   client requests a lease other than zero, there are IGD home gateways   that will ignore the request, fail in other ways, or even crash   completely.  As a client implementer then, you would be well advised   not to attempt to request a lease other than zero, unless you want to   suffer the support costs and bad publicity of lots of people   complaining that your device brought down their entire network.   Because none of the early UPnP IGD clients requested port mapping   leases, many UPnP IGD home gateway vendors never tested that   functionality, and got away with shipping home gateways where that   functionality was buggy or nonexistent.  Because there are so many   buggy UPnP IGD home gateways already deployed, client writers wisely   stick to the well-trodden path of only requesting infinite leases.   Because there are now few (if any) clients attempting to request non-   zero leases, home gateway vendors have little incentive to expend   resources implementing a feature no one uses.   This unfortunate consequence of the way UPnP IGD was developed and   deployed means that in practice it has no usable port mapping lease   facility today, and therefore when run for a long period of time UPnP   IGD home gateways have no good way to avoid accumulating an unbounded   number of stale port mappings.9.6.  State Change Announcements   When using DHCP on the external interface, as is the norm for home   gateways, there is no guarantee that a UPnP IGD home gateway's   external IPv4 address will remain unchanged.  Indeed, some ISPs   change their customer's IPv4 address every 24 hours (possibly in an   effort to make it harder for their customers to "run a server" at   home).  What this means is that if the home gateway's external IPv4   address changes, it needs to inform its clients, so that they can   make any necessary updates to global directory information (e.g.,   performing a Dynamic DNS update to update their address record).   When a NAT-PMP gateway's external IPv4 address changes, it broadcasts   announcement packets to inform clients of this.  UPnP IGD does not.Cheshire & Krochmal           Informational                    [Page 30]

RFC 6886                         NAT-PMP                      April 20139.7.  Soft State Recovery   When run for a long enough period of time, any network will have   devices that fail, get rebooted, suffer power outages, or lose state   for other reasons.  A home gateway that runs for long enough is   likely to suffer some such incident eventually.  After losing state,   it has no record of the port mappings it created, and clients suffer   a consequent loss of connectivity.   To handle this case, NAT-PMP has the "Seconds Since Start of Epoch"   mechanism.  After a reboot or other loss of state, a NAT-PMP gateway   broadcasts announcement packets giving its external IPv4 address,   with the Seconds Since Start of Epoch field reset to begin counting   from zero again.  When a NAT-PMP client observes packets from its   NAT-PMP gateway where the gateway's notion of time has apparently   gone backwards compared to the client's, the client knows the gateway   has probably lost state, and immediately recreates its mappings to   restore connectivity.   UPnP IGD has no equivalent mechanism.9.8.  On-Path NAT Discovery   For any given host, it is only useful to request NAT port mappings in   the NAT gateway through which that host's packets are flowing.  A NAT   port mapping is a request for packets to be translated in a certain   way; the NAT gateway can only perform that translation if it's   actually forwarding inbound and outbound packets for that host.   This is why NAT-PMP sends its requests to the host's default router,   since this is the device that is forwarding (and possibly   translating) inbound and outbound packets for that host.  (In a   larger network with multiple hops between a host and its NAT gateway,   some other mechanism would need to be used to discover the correct   on-path NAT for a host; this is possible, but outside the scope of   this document.)   In contrast, UPnP IGD does not limit itself to using only on-path   NATs.  UPnP IGD uses a multicast SSDP query, and uses any device it   finds on the local network claiming UPnP IGD capability, regardless   of whether any inbound or outbound traffic is actually flowing   through that device.  Over the past few years this led to many bug   reports being sent to Apple with the general form: "Port Mapping   doesn't work on my Mac and that's a bug because everything else on my   network says UPnP IGD is working fine." Upon investigation it always   turned out that: (i) these people had NAT gateways that either didn't   support port mapping requests, or had that capability disabled, and   (ii) for some reason they also had some other old NAT device stillCheshire & Krochmal           Informational                    [Page 31]

RFC 6886                         NAT-PMP                      April 2013   connected to their network, and those other NAT devices were   advertising UPnP IGD capability, even though they were not the active   NAT gateway for the network.  This led to UPnP IGD clients falsely   reporting that they were "working fine", and only the Mac correctly   reporting that it was unable to make any useful port mappings.  In   many cases the people reporting this "bug" had devices like game   consoles on their home network that for many years had been reporting   that UPnP IGD was "working fine", yet during those years they had   never once successfully received any inbound network packet or   connection.  The irony is that, for these people who were reporting   bugs to Apple, UPnP IGD "working fine" had been indistinguishable   from UPnP IGD doing nothing useful at all.  It was only when Back to   My Mac [RFC6281] started reporting that it was unable to make any   functional port mappings that these people discovered they'd never   had any working port mappings on their NAT gateway.10.  References10.1.  Normative References   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,              and E. Lear, "Address Allocation for Private Internets",BCP 5,RFC 1918, February 1996.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.10.2.  Informative References   [Bonjour]  Apple "Bonjour" <http://developer.apple.com/bonjour/>.   [ETEAISD]  J. Saltzer, D. Reed and D. Clark: "End-to-end arguments in              system design", ACM Trans. Comp. Sys., 2(4):277-88,              November 1984.   [IGD]      UPnP Standards "Internet Gateway Device (IGD) Standardized              Device Control Protocol V 1.0", November 2001,              <http://www.upnp.org/standardizeddcps/igd.asp>.   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",RFC2131, March 1997.   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address              Translator (NAT) Terminology and Considerations",RFC2663, August 1999.   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic              Update",RFC 3007, November 2000.Cheshire & Krochmal           Informational                    [Page 32]

RFC 6886                         NAT-PMP                      April 2013   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network              Address Translator (Traditional NAT)",RFC 3022, January              2001.   [RFC3424]  Daigle, L., Ed., and IAB, "IAB Considerations for              UNilateral Self-Address Fixing (UNSAF) Across Network              Address Translation",RFC 3424, November 2002.   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the              Internet Protocol",RFC 4301, December 2005.   [RFC6281]  Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,              "Understanding Apple's Back to My Mac (BTMM) Service",RFC6281, June 2011.   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service              Discovery",RFC 6763, February 2013.   [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and              P. Selkirk, "Port Control Protocol (PCP)",RFC 6887, April              2013.   [SEE]      SubEthaEdit, <http://www.codingmonkeys.de/subethaedit/>.   [VU347812] United States Computer Emergency Readiness Team              Vulnerability Note VU#347812,              <http://www.kb.cert.org/vuls/id/347812>.Authors' Addresses   Stuart Cheshire   Apple Inc.   1 Infinite Loop   Cupertino, CA 95014   USA   EMail: cheshire@apple.com   Marc Krochmal   Apple Inc.   1 Infinite Loop   Cupertino, CA 95014   USA   EMail: marc@apple.comCheshire & Krochmal           Informational                    [Page 33]