[Note that this file is a concatenation of more than one RFC.] Network Working Group F. Audet, Ed. Request for Comments: 4787 Nortel Networks BCP: 127 C. Jennings Category: Best Current Practice Cisco Systems January 2007 Network Address Translation (NAT) Behavioral Requirements for Unicast UDP Status of This Memo This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract This document defines basic terminology for describing different types of Network Address Translation (NAT) behavior when handling Unicast UDP and also defines a set of requirements that would allow many applications, such as multimedia communications or online gaming, to work consistently. Developing NATs that meet this set of requirements will greatly increase the likelihood that these applications will function properly. Audet & Jennings Best Current Practice [Page 1] RFC 4787 NAT UDP Unicast Requirements January 2007 Table of Contents 1. Applicability Statement . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Network Address and Port Translation Behavior . . . . . . . . 5 4.1. Address and Port Mapping . . . . . . . . . . . . . . . . . 5 4.2. Port Assignment . . . . . . . . . . . . . . . . . . . . . 9 4.2.1. Port Assignment Behavior . . . . . . . . . . . . . . . 9 4.2.2. Port Parity . . . . . . . . . . . . . . . . . . . . . 11 4.2.3. Port Contiguity . . . . . . . . . . . . . . . . . . . 11 4.3. Mapping Refresh . . . . . . . . . . . . . . . . . . . . . 12 4.4. Conflicting Internal and External IP Address Spaces . . . 13 5. Filtering Behavior . . . . . . . . . . . . . . . . . . . . . . 15 6. Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . . 16 7. Application Level Gateways . . . . . . . . . . . . . . . . . . 17 8. Deterministic Properties . . . . . . . . . . . . . . . . . . . 18 9. ICMP Destination Unreachable Behavior . . . . . . . . . . . . 19 10. Fragmentation of Outgoing Packets . . . . . . . . . . . . . . 20 11. Receiving Fragmented Packets . . . . . . . . . . . . . . . . . 20 12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 21 13. Security Considerations . . . . . . . . . . . . . . . . . . . 24 14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 25 15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26 16.1. Normative References . . . . . . . . . . . . . . . . . . . 26 16.2. Informative References . . . . . . . . . . . . . . . . . . 26 Audet & Jennings Best Current Practice [Page 2] RFC 4787 NAT UDP Unicast Requirements January 2007 1. Applicability Statement The purpose of this specification is to define a set of requirements for NATs that would allow many applications, such as multimedia communications or online gaming, to work consistently. Developing NATs that meet this set of requirements will greatly increase the likelihood that these applications will function properly. The requirements of this specification apply to Traditional NATs as described in [RFC2663]. This document is meant to cover NATs of any size, from small residential NATs to large Enterprise NATs. However, it should be understood that Enterprise NATs normally provide much more than just NAT capabilities; for example, they typically provide firewall functionalities. A comprehensive description of firewall behaviors and associated requirements is specifically out-of-scope for this specification. However, this specification does cover basic firewall aspects present in NATs (see Section 5). Approaches using directly signaled control of middle boxes are out of scope. UDP Relays (e.g., Traversal Using Relay NAT [TURN]) are out of scope. Application aspects are out of scope, as the focus here is strictly on the NAT itself. This document only covers aspects of NAT traversal related to Unicast UDP [RFC0768] over IP [RFC0791] and their dependencies on other protocols. 2. Introduction Network Address Translators (NATs) are well known to cause very significant problems with applications that carry IP addresses in the payload (see [RFC3027]). Applications that suffer from this problem include Voice Over IP and Multimedia Over IP (e.g., SIP [RFC3261] and H.323 [ITU.H323]), as well as online gaming. Many techniques are used to attempt to make realtime multimedia applications, online games, and other applications work across NATs. Application Level Gateways [RFC2663] are one such mechanism. STUN [RFC3489bis] describes a UNilateral Self-Address Fixing (UNSAF) mechanism [RFC3424]. Teredo [RFC4380] describes an UNSAF mechanism consisting of tunnelling IPv6 [RFC2460] over UDP/IPv4. UDP Relays have also been used to enable applications across NATs, but these are generally seen as a solution of last resort. Interactive Audet & Jennings Best Current Practice [Page 3] RFC 4787 NAT UDP Unicast Requirements January 2007 Connectivity Establishment [ICE] describes a methodology for using many of these techniques and avoiding a UDP relay, unless the type of NAT is such that it forces the use of such a UDP relay. This specification defines requirements for improving NATs. Meeting these requirements ensures that applications will not be forced to use UDP relay. As pointed out in UNSAF [RFC3424], "From observations of deployed networks, it is clear that different NAT box implementations vary widely in terms of how they handle different traffic and addressing cases". This wide degree of variability is one factor in the overall brittleness introduced by NATs and makes it extremely difficult to predict how any given protocol will behave on a network traversing NAT. Discussions with many of the major NAT vendors have made it clear that they would prefer to deploy NATs that were deterministic and caused the least harm to applications while still meeting the requirements that caused their customers to deploy NATs in the first place. The problem NAT vendors face is that they are not sure how best to do that or how to document their NATs' behavior. The goals of this document are to define a set of common terminology for describing the behavior of NATs and to produce a set of requirements on a specific set of behaviors for NATs. This document forms a common set of requirements that are simple and useful for voice, video, and games, which can be implemented by NAT vendors. This document will simplify the analysis of protocols for deciding whether or not they work in this environment and will allow providers of services that have NAT traversal issues to make statements about where their applications will work and where they will not, as well as to specify their own NAT requirements. 3. Terminology 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]. Readers are urged to refer to [RFC2663] for information on NAT taxonomy and terminology. Traditional NAT is the most common type of NAT device deployed. Readers may refer to [RFC3022] for detailed information on traditional NAT. Traditional NAT has two main varieties -- Basic NAT and Network Address/Port Translator (NAPT). NAPT is by far the most commonly deployed NAT device. NAPT allows multiple internal hosts to share a single public IP address simultaneously. When an internal host opens an outgoing TCP or UDP session through a NAPT, the NAPT assigns the session a public IP Audet & Jennings Best Current Practice [Page 4] RFC 4787 NAT UDP Unicast Requirements January 2007 address and port number, so that subsequent response packets from the external endpoint can be received by the NAPT, translated, and forwarded to the internal host. The effect is that the NAPT establishes a NAT session to translate the (private IP address, private port number) tuple to a (public IP address, public port number) tuple, and vice versa, for the duration of the session. An issue of relevance to peer-to-peer applications is how the NAT behaves when an internal host initiates multiple simultaneous sessions from a single (private IP, private port) endpoint to multiple distinct endpoints on the external network. In this specification, the term "NAT" refers to both "Basic NAT" and "Network Address/Port Translator (NAPT)". This document uses the term "session" as defined in RFC 2663: "TCP/ UDP sessions are uniquely identified by the tuple of (source IP address, source TCP/UDP ports, target IP address, target TCP/UDP Port)". This document uses the term "address and port mapping" as the translation between an external address and port and an internal address and port. Note that this is not the same as an "address binding" as defined in RFC 2663. This document uses IANA terminology for port ranges, i.e., "Well Known Ports" is 0-1023, "Registered" is 1024-49151, and "Dynamic and/or Private" is 49152-65535, as defined in http://www.iana.org/assignments/port-numbers. STUN [RFC3489] used the terms "Full Cone", "Restricted Cone", "Port Restricted Cone", and "Symmetric" to refer to different variations of NATs applicable to UDP only. Unfortunately, this terminology has been the source of much confusion, as it has proven inadequate at describing real-life NAT behavior. This specification therefore refers to specific individual NAT behaviors instead of using the Cone/Symmetric terminology. 4. Network Address and Port Translation Behavior This section describes the various NAT behaviors applicable to NATs. 4.1. Address and Port Mapping When an internal endpoint opens an outgoing session through a NAT, the NAT assigns the session an external IP address and port number so that subsequent response packets from the external endpoint can be received by the NAT, translated, and forwarded to the internal endpoint. This is a mapping between an internal IP address and port IP:port and external IP:port tuple. It establishes the translation Audet & Jennings Best Current Practice [Page 5] RFC 4787 NAT UDP Unicast Requirements January 2007 that will be performed by the NAT for the duration of the session. For many applications, it is important to distinguish the behavior of the NAT when there are multiple simultaneous sessions established to different external endpoints. The key behavior to describe is the criteria for reuse of a mapping for new sessions to external endpoints, after establishing a first mapping between an internal X:x address and port and an external Y1:y1 address tuple. Let's assume that the internal IP address and port X:x are mapped to X1':x1' for this first session. The endpoint then sends from X:x to an external address Y2:y2 and gets a mapping of X2':x2' on the NAT. The relationship between X1':x1' and X2':x2' for various combinations of the relationship between Y1:y1 and Y2:y2 is critical for describing the NAT behavior. This arrangement is illustrated in the following diagram: E +------+ +------+ x | Y1 | | Y2 | t +--+---+ +---+--+ e | Y1:y1 Y2:y2 | r +----------+ +----------+ n | | a X1':x1' | | X2':x2' l +--+---+-+ ...........| NAT |............... +--+---+-+ I | | n X:x | | X:x t ++---++ e | X | r +-----+ n a l Address and Port Mapping The following address and port mapping behavior are defined: Endpoint-Independent Mapping: The NAT reuses the port mapping for subsequent packets sent from the same internal IP address and port (X:x) to any external IP address and port. Specifically, X1':x1' equals X2':x2' for all values of Y2:y2. Audet & Jennings Best Current Practice [Page 6] RFC 4787 NAT UDP Unicast Requirements January 2007 Address-Dependent Mapping: The NAT reuses the port mapping for subsequent packets sent from the same internal IP address and port (X:x) to the same external IP address, regardless of the external port. Specifically, X1':x1' equals X2':x2' if and only if, Y2 equals Y1. Address and Port-Dependent Mapping: The NAT reuses the port mapping for subsequent packets sent from the same internal IP address and port (X:x) to the same external IP address and port while the mapping is still active. Specifically, X1':x1' equals X2':x2' if and only if, Y2:y2 equals Y1:y1. It is important to note that these three possible choices make no difference to the security properties of the NAT. The security properties are fully determined by which packets the NAT allows in and which it does not. This is determined by the filtering behavior in the filtering portions of the NAT. REQ-1: A NAT MUST have an "Endpoint-Independent Mapping" behavior. Justification: In order for UNSAF methods to work, REQ-1 needs to be met. Failure to meet REQ-1 will force the use of a UDP relay, which is very often impractical. Some NATs are capable of assigning IP addresses from a pool of IP addresses on the external side of the NAT, as opposed to just a single IP address. This is especially common with larger NATs. Some NATs use the external IP address mapping in an arbitrary fashion (i.e., randomly): one internal IP address could have multiple external IP address mappings active at the same time for different sessions. These NATs have an "IP address pooling" behavior of "Arbitrary". Some large Enterprise NATs use an IP address pooling behavior of "Arbitrary" as a means of hiding the IP address assigned to specific endpoints by making their assignment less predictable. Other NATs use the same external IP address mapping for all sessions associated with the same internal IP address. These NATs have an "IP address pooling" behavior of "Paired". NATs that use an "IP address pooling" behavior of "Arbitrary" can cause issues for applications that use multiple ports from the same endpoint, but that do not negotiate IP addresses individually (e.g., some applications using RTP and RTCP). Audet & Jennings Best Current Practice [Page 7] RFC 4787 NAT UDP Unicast Requirements January 2007 REQ-2: It is RECOMMENDED that a NAT have an "IP address pooling" behavior of "Paired". Note that this requirement is not applicable to NATs that do not support IP address pooling. Justification: This will allow applications that use multiple ports originating from the same internal IP address to also have the same external IP address. This is to avoid breaking peer-to-peer applications that are not capable of negotiating the IP address for RTP and the IP address for RTCP separately. As such it is envisioned that this requirement will become less important as applications become NAT-friendlier with time. The main reason why this requirement is here is that in a peer-to-peer application, you are subject to the other peer's mistake. In particular, in the context of SIP, if my application supports the extensions defined in [RFC3605] for indicating RTP and RTCP addresses and ports separately, but the other peer does not, there may still be breakage in the form of the stream losing RTCP packets. This requirement will avoid the loss of RTP in this context, although the loss of RTCP may be inevitable in this particular example. It is also worth noting that RFC 3605 is unfortunately not a mandatory part of SIP [RFC3261]. Therefore, this requirement will address a particularly nasty problem that will prevail for a significant period of time. Audet & Jennings Best Current Practice [Page 8] RFC 4787 NAT UDP Unicast Requirements January 2007 4.2. Port Assignment 4.2.1. Port Assignment Behavior This section uses the following diagram for reference. E +-------+ +-------+ x | Y1 | | Y2 | t +---+---+ +---+---+ e | Y1:y1 Y2:y2 | r +---------+ +---------+ n | | a X1':x1' | | X2':x2' l +--+---+--+ ...........| NAT |............... +--+---+--+ I | | n +---------+ +---------+ t | X1:x1 X2:x2 | e +---+---+ +---+---+ r | X1 | | X2 | n +-------+ +-------+ a l Port Assignment Some NATs attempt to preserve the port number used internally when assigning a mapping to an external IP address and port (e.g., x1=x1', x2=x2'). This port assignment behavior is referred to as "port preservation". In case of port collision, these NATs attempt a variety of techniques for coping. For example, some NATs will overridden the previous mapping to preserve the same port. Other NATs will assign a different IP address from a pool of external IP addresses; this is only possible as long as the NAT has enough external IP addresses; if the port is already in use on all available external IP addresses, then these NATs will pick a different port (i.e., they don't do port preservation anymore). Some NATs use "Port overloading", i.e., they always use port preservation even in the case of collision (i.e., X1'=X2' and x1=x2=x1'=x2'). Most applications will fail if the NAT uses "Port overloading". A NAT that does not attempt to make the external port numbers match the internal port numbers in any case is referred to as "no port preservation". Audet & Jennings Best Current Practice [Page 9] RFC 4787 NAT UDP Unicast Requirements January 2007 When NATs do allocate a new source port, there is the issue of which IANA-defined range of port to choose. The ranges are "well-known" from 0 to 1023, "registered" from 1024 to 49151, and "dynamic/ private" from 49152 through 65535. For most protocols, these are destination ports and not source ports, so mapping a source port to a source port that is already registered is unlikely to have any bad effects. Some NATs may choose to use only the ports in the dynamic range; the only downside of this practice is that it limits the number of ports available. Other NAT devices may use everything but the well-known range and may prefer to use the dynamic range first, or possibly avoid the actual registered ports in the registered range. Other NATs preserve the port range if it is in the well-known range. [RFC0768] specifies that the source port is set to zero if no reply packets are expected. In this case, it does not matter what the NAT maps it to, as the source port will not be used. However, many common OS APIs do not allow a user to send from port zero, applications do not use port zero, and the behavior of various existing NATs with regards to a packet with a source of port zero is unknown. This document does not specify any normative behavior for a NAT when handling a packet with a source port of zero which means that applications cannot count on any sort of deterministic behavior for these packets. REQ-3: A NAT MUST NOT have a "Port assignment" behavior of "Port overloading". a) If the host's source port was in the range 0-1023, it is RECOMMENDED the NAT's source port be in the same range. If the host's source port was in the range 1024-65535, it is RECOMMENDED that the NAT's source port be in that range. Justification: This requirement must be met in order to enable two applications on the internal side of the NAT both to use the same port to try to communicate with the same destination. NATs that implement port preservation have to deal with conflicts on ports, and the multiple code paths this introduces often result in nondeterministic behavior. However, it should be understood that when a port is randomly assigned, it may just randomly happen to be assigned the same port. Applications must, therefore, be able to deal with both port preservation and no port preservation. a) Certain applications expect the source UDP port to be in the well-known range. See the discussion of Network File System port expectations in [RFC2623] for an example. Audet & Jennings Best Current Practice [Page 10] RFC 4787 NAT UDP Unicast Requirements January 2007 4.2.2. Port Parity Some NATs preserve the parity of the UDP port, i.e., an even port will be mapped to an even port, and an odd port will be mapped to an odd port. This behavior respects the [RFC3550] rule that RTP use even ports, and RTCP use odd ports. RFC 3550 allows any port numbers to be used for RTP and RTCP if the two numbers are specified separately; for example, using [RFC3605]. However, some implementations do not include RFC 3605, and do not recognize when the peer has specified the RTCP port separately using RFC 3605. If such an implementation receives an odd RTP port number from the peer (perhaps after having been translated by a NAT), and then follows the RFC 3550 rule to change the RTP port to the next lower even number, this would obviously result in the loss of RTP. NAT-friendly application aspects are outside the scope of this document. It is expected that this issue will fade away with time, as implementations improve. Preserving the port parity allows for supporting communication with peers that do not support explicit specification of both RTP and RTCP port numbers. REQ-4: It is RECOMMENDED that a NAT have a "Port parity preservation" behavior of "Yes". Justification: This is to avoid breaking peer-to-peer applications that do not explicitly and separately specify RTP and RTCP port numbers and that follow the RFC 3550 rule to decrement an odd RTP port to make it even. The same considerations apply, as per the IP address pooling requirement. 4.2.3. Port Contiguity Some NATs attempt to preserve the port contiguity rule of RTCP=RTP+1. These NATs do things like sequential assignment or port reservation. Sequential port assignment assumes that the application will open a mapping for RTP first and then open a mapping for RTCP. It is not practical to enforce this requirement on all applications. Furthermore, there is a problem with glare if many applications (or endpoints) are trying to open mappings simultaneously. Port preservation is also problematic since it is wasteful, especially considering that a NAT cannot reliably distinguish between RTP over UDP and other UDP packets where there is no contiguity rule. For those reasons, it would be too complex to attempt to preserve the contiguity rule by suggesting specific NAT behavior, and it would certainly break the deterministic behavior rule. In order to support both RTP and RTCP, it will therefore be necessary that applications follow rules to negotiate RTP and RTCP separately, and account for the very real possibility that the RTCP=RTP+1 rule Audet & Jennings Best Current Practice [Page 11] RFC 4787 NAT UDP Unicast Requirements January 2007 will be broken. As this is an application requirement, it is outside the scope of this document. 4.3. Mapping Refresh NAT mapping timeout implementations vary, but include the timer's value and the way the mapping timer is refreshed to keep the mapping alive. The mapping timer is defined as the time a mapping will stay active without packets traversing the NAT. There is great variation in the values used by different NATs. REQ-5: A NAT UDP mapping timer MUST NOT expire in less than two minutes, unless REQ-5a applies. a) For specific destination ports in the well-known port range (ports 0-1023), a NAT MAY have shorter UDP mapping timers that are specific to the IANA-registered application running over that specific destination port. b) The value of the NAT UDP mapping timer MAY be configurable. c) A default value of five minutes or more for the NAT UDP mapping timer is RECOMMENDED. Justification: This requirement is to ensure that the timeout is long enough to avoid too-frequent timer refresh packets. a) Some UDP protocols using UDP use very short-lived connections. There can be very many such connections; keeping them all in a connections table could cause considerable load on the NAT. Having shorter timers for these specific applications is, therefore, an optimization technique. It is important that the shorter timers applied to specific protocols be used sparingly, and only for protocols using well-known destination ports that are known to have a shorter timer, and that are known not to be used by any applications for other purposes. b) Configuration is desirable for adapting to specific networks and troubleshooting. c) This default is to avoid too-frequent timer refresh packets. Some NATs keep the mapping active (i.e., refresh the timer value) when a packet goes from the internal side of the NAT to the external side of the NAT. This is referred to as having a NAT Outbound refresh behavior of "True". Audet & Jennings Best Current Practice [Page 12] RFC 4787 NAT UDP Unicast Requirements January 2007 Some NATs keep the mapping active when a packet goes from the external side of the NAT to the internal side of the NAT. This is referred to as having a NAT Inbound Refresh Behavior of "True". Some NATs keep the mapping active on both, in which case, both properties are "True". REQ-6: The NAT mapping Refresh Direction MUST have a "NAT Outbound refresh behavior" of "True". a) The NAT mapping Refresh Direction MAY have a "NAT Inbound refresh behavior" of "True". Justification: Outbound refresh is necessary for allowing the client to keep the mapping alive. a) Inbound refresh may be useful for applications with no outgoing UDP traffic. However, allowing inbound refresh may allow an external attacker or misbehaving application to keep a mapping alive indefinitely. This may be a security risk. Also, if the process is repeated with different ports, over time, it could use up all the ports on the NAT. 4.4. Conflicting Internal and External IP Address Spaces Many NATs, particularly consumer-level devices designed to be deployed by nontechnical users, routinely obtain their external IP address, default router, and other IP configuration information for their external interface dynamically from an external network, such as an upstream ISP. The NAT, in turn, automatically sets up its own internal subnet in one of the private IP address spaces assigned to this purpose in [RFC1918], typically providing dynamic IP configuration services for hosts on this internal network. Auto-configuration of NATs and private networks can be problematic, however, if the NAT's external network is also in RFC 1918 private address space. In a common scenario, an ISP places its customers behind a NAT and hands out private RFC 1918 addresses to them. Some of these customers, in turn, deploy consumer-level NATs, which, in effect, act as "second-level" NATs, multiplexing their own private RFC 1918 IP subnets onto the single RFC 1918 IP address provided by the ISP. There is no inherent guarantee, in this case, that the ISP's "intermediate" privately-addressed network and the customer's internal privately-addressed network will not use numerically identical or overlapping RFC 1918 IP subnets. Furthermore, customers of consumer-level NATs cannot be expected to have the technical Audet & Jennings Best Current Practice [Page 13] RFC 4787 NAT UDP Unicast Requirements January 2007 knowledge to prevent this scenario from occurring by manually configuring their internal network with non-conflicting RFC 1918 subnets. NAT vendors need to design their NATs to ensure that they function correctly and robustly even in such problematic scenarios. One possible solution is for the NAT to ensure that whenever its external link is configured with an RFC 1918 private IP address, the NAT automatically selects a different, non-conflicting RFC 1918 IP subnet for its internal network. A disadvantage of this solution is that, if the NAT's external interface is dynamically configured or re- configured after its internal network is already in use, then the NAT may have to renumber its entire internal network dynamically if it detects a conflict. An alternative solution is for the NAT to be designed so that it can translate and forward traffic correctly, even when its external and internal interfaces are configured with numerically overlapping IP subnets. In this scenario, for example, if the NAT's external interface has been assigned an IP address P in RFC 1918 space, then there might also be an internal node I having the same RFC 1918 private IP address P. An IP packet with destination address P on the external network is directed at the NAT, whereas an IP packet with the same destination address P on the internal network is directed at node I. The NAT therefore needs to maintain a clear operational distinction between "external IP addresses" and "internal IP addresses" to avoid confusing internal node I with its own external interface. In general, the NAT needs to allow all internal nodes (including I) to communicate with all external nodes having public (non-RFC 1918) IP addresses, or having private IP addresses that do not conflict with the addresses used by its internal network. REQ-7: A NAT device whose external IP interface can be configured dynamically MUST either (1) automatically ensure that its internal network uses IP addresses that do not conflict with its external network, or (2) be able to translate and forward traffic between all internal nodes and all external nodes whose IP addresses numerically conflict with the internal network. Justification: If a NAT's external and internal interfaces are configured with overlapping IP subnets, then there is, of course, no way for an internal host with RFC 1918 IP address Q to initiate a direct communication session to an external node having the same RFC 1918 address Q, or to other external nodes with IP addresses that numerically conflict with the internal subnet. Such nodes can still open communication sessions indirectly via NAT traversal techniques, however, with the help of a third-party server, such as a STUN server having a public, non-RFC 1918 IP address. In Audet & Jennings Best Current Practice [Page 14] RFC 4787 NAT UDP Unicast Requirements January 2007 this case, nodes with conflicting private RFC 1918 addresses on opposite sides of the second-level NAT can communicate with each other via their respective temporary public endpoints on the main Internet, as long as their common, first-level NAT (e.g., the upstream ISP's NAT) supports hairpinning behavior, as described in Section 6. 5. Filtering Behavior This section describes various filtering behaviors observed in NATs. When an internal endpoint opens an outgoing session through a NAT, the NAT assigns a filtering rule for the mapping between an internal IP:port (X:x) and external IP:port (Y:y) tuple. The key behavior to describe is what criteria are used by the NAT to filter packets originating from specific external endpoints. Endpoint-Independent Filtering: The NAT filters out only packets not destined to the internal address and port X:x, regardless of the external IP address and port source (Z:z). The NAT forwards any packets destined to X:x. In other words, sending packets from the internal side of the NAT to any external IP address is sufficient to allow any packets back to the internal endpoint. Address-Dependent Filtering: The NAT filters out packets not destined to the internal address X:x. Additionally, the NAT will filter out packets from Y:y destined for the internal endpoint X:x if X:x has not sent packets to Y:any previously (independently of the port used by Y). In other words, for receiving packets from a specific external endpoint, it is necessary for the internal endpoint to send packets first to that specific external endpoint's IP address. Address and Port-Dependent Filtering: This is similar to the previous behavior, except that the external port is also relevant. The NAT filters out packets not destined for the internal address X:x. Additionally, the NAT will filter out packets from Y:y destined for the internal endpoint X:x if X:x has not sent packets to Y:y previously. In other words, for receiving packets from a specific external endpoint, it is necessary for the internal endpoint to send packets first to that external endpoint's IP address and port. Audet & Jennings Best Current Practice [Page 15] RFC 4787 NAT UDP Unicast Requirements January 2007 REQ-8: If application transparency is most important, it is RECOMMENDED that a NAT have an "Endpoint-Independent Filtering" behavior. If a more stringent filtering behavior is most important, it is RECOMMENDED that a NAT have an "Address-Dependent Filtering" behavior. a) The filtering behavior MAY be an option configurable by the administrator of the NAT. Justification: The recommendation to use Endpoint-Independent Filtering is aimed at maximizing application transparency; in particular, for applications that receive media simultaneously from multiple locations (e.g., gaming), or applications that use rendezvous techniques. However, it is also possible that, in some circumstances, it may be preferable to have a more stringent filtering behavior. Filtering independently of the external endpoint is not as secure: An unauthorized packet could get through a specific port while the port was kept open if it was lucky enough to find the port open. In theory, filtering based on both IP address and port is more secure than filtering based only on the IP address (because the external endpoint could, in reality, be two endpoints behind another NAT, where one of the two endpoints is an attacker). However, such a policy could interfere with applications that expect to receive UDP packets on more than one UDP port. Using Endpoint-Independent Filtering or Address- Dependent Filtering instead of Address and Port-Dependent Filtering on a NAT (say, NAT-A) also has benefits when the other endpoint is behind a non-BEHAVE compliant NAT (say, NAT-B) that does not support REQ-1. When the endpoints use ICE, if NAT-A uses Address and Port-Dependent Filtering, connectivity will require a UDP relay. However, if NAT-A uses Endpoint-Independent Filtering or Address-Dependent Filtering, ICE will ultimately find connectivity without requiring a UDP relay. Having the filtering behavior being an option configurable by the administrator of the NAT ensures that a NAT can be used in the widest variety of deployment scenarios. 6. Hairpinning Behavior If two hosts (called X1 and X2) are behind the same NAT and exchanging traffic, the NAT may allocate an address on the outside of the NAT for X2, called X2':x2'. If X1 sends traffic to X2':x2', it goes to the NAT, which must relay the traffic from X1 to X2. This is referred to as hairpinning and is illustrated below. Audet & Jennings Best Current Practice [Page 16] RFC 4787 NAT UDP Unicast Requirements January 2007 NAT +----+ from X1:x1 to X2':x2' +-----+ X1':x1' | X1 |>>>>>>>>>>>>>>>>>>>>>>>>>>>>>--+--- +----+ | v | | v | | v | | v | +----+ from X1':x1' to X2:x2 | v | X2':x2' | X2 |<<<<<<<<<<<<<<<<<<<<<<<<<<<<<--+--- +----+ +-----+ Hairpinning Behavior Hairpinning allows two endpoints on the internal side of the NAT to communicate even if they only use each other's external IP addresses and ports. More formally, a NAT that supports hairpinning forwards packets originating from an internal address, X1:x1, destined for an external address X2':x2' that has an active mapping to an internal address X2:x2, back to that internal address, X2:x2. Note that typically X1' is the same as X2'. Furthermore, the NAT may present the hairpinned packet with either an internal (X1:x1) or an external (X1':x1') source IP address and port. Therefore, the hairpinning NAT behavior can be either "External source IP address and port" or "Internal source IP address and port". "Internal source IP address and port" may cause problems by confusing implementations that expect an external IP address and port. REQ-9: A NAT MUST support "Hairpinning". a) A NAT Hairpinning behavior MUST be "External source IP address and port". Justification: This requirement is to allow communications between two endpoints behind the same NAT when they are trying each other's external IP addresses. a) Using the external source IP address is necessary for applications with a restrictive policy of not accepting packets from IP addresses that differ from what is expected. 7. Application Level Gateways Certain NATs have implemented Application Level Gateways (ALGs) for various protocols, including protocols for negotiating peer-to-peer sessions, such as SIP. Audet & Jennings Best Current Practice [Page 17] RFC 4787 NAT UDP Unicast Requirements January 2007 Certain NATs have these ALGs turned on permanently, others have them turned on by default but allow them to be turned off, and others have them turned off by default but allow them be turned on. NAT ALGs may interfere with UNSAF methods or protocols that try to be NAT-aware and therefore must be used with extreme caution. REQ-10: To eliminate interference with UNSAF NAT traversal mechanisms and allow integrity protection of UDP communications, NAT ALGs for UDP-based protocols SHOULD be turned off. Future standards track specifications that define ALGs can update this to recommend the defaults for the ALGs that they define. a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow the NAT administrator to enable or disable each ALG separately. Justification: NAT ALGs may interfere with UNSAF methods. a) This requirement allows the user to enable those ALGs that are necessary to aid in the operation of some applications without enabling ALGs, which interfere with the operation of other applications. 8. Deterministic Properties The classification of NATs is further complicated by the fact that, under some conditions, the same NAT will exhibit different behaviors. This has been seen on NATs that preserve ports or have specific algorithms for selecting a port other than a free one. If the external port that the NAT wishes to use is already in use by another session, the NAT must select a different port. This results in different code paths for this conflict case, which results in different behavior. For example, if three hosts X1, X2, and X3 all send from the same port x, through a port preserving NAT with only one external IP address, called X1', the first one to send (i.e., X1) will get an external port of x, but the next two will get x2' and x3' (where these are not equal to x). There are NATs where the External NAT mapping characteristics and the External Filter characteristics change between the X1:x and the X2:x mapping. To make matters worse, there are NATs where the behavior may be the same on the X1:x and X2:x mappings, but different on the third X3:x mapping. Another example is that some NATs have an "Endpoint-Independent Mapping", combined with "Port Overloading", as long as two endpoints are not establishing sessions to the same external direction, but then switch their behavior to "Address and Port-Dependent Mapping" Audet & Jennings Best Current Practice [Page 18] RFC 4787 NAT UDP Unicast Requirements January 2007 without "Port Preservation" upon detection of these conflicting sessions establishments. Any NAT that changes the NAT Mapping or the Filtering behavior without configuration changes, at any point in time, under any particular conditions, is referred to as a "non-deterministic" NAT. NATs that don't are called "deterministic". Non-deterministic NATs generally change behavior when a conflict of some sort happens, i.e., when the port that would normally be used is already in use by another mapping. The NAT mapping and External Filtering in the absence of conflict is referred to as the Primary behavior. The behavior after the first conflict is referred to as Secondary and after the second conflict is referred to as Tertiary. No NATs have been observed that change on further conflicts, but it is certainly possible that they exist. REQ-11: A NAT MUST have deterministic behavior, i.e., it MUST NOT change the NAT translation (Section 4) or the Filtering (Section 5) Behavior at any point in time, or under any particular conditions. Justification: Non-deterministic NATs are very difficult to troubleshoot because they require more intensive testing. This non-deterministic behavior is the root cause of much of the uncertainty that NATs introduce about whether or not applications will work. 9. ICMP Destination Unreachable Behavior When a NAT sends a packet toward a host on the other side of the NAT, an ICMP message may be sent in response to that packet. That ICMP message may be sent by the destination host or by any router along the network path. The NAT's default configuration SHOULD NOT filter ICMP messages based on their source IP address. Such ICMP messages SHOULD be rewritten by the NAT (specifically, the IP headers and the ICMP payload) and forwarded to the appropriate internal or external host. The NAT needs to perform this function for as long as the UDP mapping is active. Receipt of any sort of ICMP message MUST NOT destroy the NAT mapping. A NAT that performs the functions described in the paragraph above is referred to as "support ICMP Processing". There is no significant security advantage to blocking ICMP Destination Unreachable packets. Additionally, blocking ICMP Destination Unreachable packets can interfere with application failover, UDP Path MTU Discovery (see [RFC1191] and [RFC1435]), and traceroute. Blocking any ICMP message is discouraged, and blocking ICMP Destination Unreachable is strongly discouraged. Audet & Jennings Best Current Practice [Page 19] RFC 4787 NAT UDP Unicast Requirements January 2007 REQ-12: Receipt of any sort of ICMP message MUST NOT terminate the NAT mapping. a) The NAT's default configuration SHOULD NOT filter ICMP messages based on their source IP address. b) It is RECOMMENDED that a NAT support ICMP Destination Unreachable messages. Justification: This is easy to do and is used for many things including MTU discovery and rapid detection of error conditions, and has no negative consequences. 10. Fragmentation of Outgoing Packets When the MTU of the adjacent link is too small, fragmentation of packets going from the internal side to the external side of the NAT may occur. This can occur if the NAT is doing Point-to-Point over Ethernet (PPPoE), or if the NAT has been configured with a small MTU to reduce serialization delay when sending large packets and small higher-priority packets, or for other reasons. It is worth noting that many IP stacks do not use Path MTU Discovery with UDP packets. The packet could have its Don't Fragment bit set to 1 (DF=1) or 0 (DF=0). REQ-13: If the packet received on an internal IP address has DF=1, the NAT MUST send back an ICMP message "Fragmentation needed and DF set" to the host, as described in [RFC0792]. a) If the packet has DF=0, the NAT MUST fragment the packet and SHOULD send the fragments in order. Justification: This is as per RFC 792. a) This is the same function a router performs in a similar situation [RFC1812]. 11. Receiving Fragmented Packets For a variety of reasons, a NAT may receive a fragmented packet. The IP packet containing the header could arrive in any fragment, depending on network conditions, packet ordering, and the implementation of the IP stack that generated the fragments. Audet & Jennings Best Current Practice [Page 20] RFC 4787 NAT UDP Unicast Requirements January 2007 A NAT that is capable only of receiving fragments in order (that is, with the header in the first packet) and forwarding each of the fragments to the internal host is described as "Received Fragments Ordered". A NAT that is capable of receiving fragments in or out of order and forwarding the individual fragments (or a reassembled packet) to the internal host is referred to as "Receive Fragments Out of Order". See the Security Considerations section of this document for a discussion of this behavior. A NAT that is neither of these is referred to as "Receive Fragments None". REQ-14: A NAT MUST support receiving in-order and out-of-order fragments, so it MUST have "Received Fragment Out of Order" behavior. a) A NAT's out-of-order fragment processing mechanism MUST be designed so that fragmentation-based DoS attacks do not compromise the NAT's ability to process in-order and unfragmented IP packets. Justification: See Security Considerations. 12. Requirements The requirements in this section are aimed at minimizing the complications caused by NATs to applications, such as realtime communications and online gaming. The requirements listed earlier in the document are consolidated here into a single section. It should be understood, however, that applications normally do not know in advance if the NAT conforms to the recommendations defined in this section. Peer-to-peer media applications still need to use normal procedures, such as ICE [ICE]. A NAT that supports all the mandatory requirements of this specification (i.e., the "MUST"), is "compliant with this specification". A NAT that supports all the requirements of this specification (i.e., including the "RECOMMENDED") is "fully compliant with all the mandatory and recommended requirements of this specification". Audet & Jennings Best Current Practice [Page 21] RFC 4787 NAT UDP Unicast Requirements January 2007 REQ-1: A NAT MUST have an "Endpoint-Independent Mapping" behavior. REQ-2: It is RECOMMENDED that a NAT have an "IP address pooling" behavior of "Paired". Note that this requirement is not applicable to NATs that do not support IP address pooling. REQ-3: A NAT MUST NOT have a "Port assignment" behavior of "Port overloading". a) If the host's source port was in the range 0-1023, it is RECOMMENDED the NAT's source port be in the same range. If the host's source port was in the range 1024-65535, it is RECOMMENDED that the NAT's source port be in that range. REQ-4: It is RECOMMENDED that a NAT have a "Port parity preservation" behavior of "Yes". REQ-5: A NAT UDP mapping timer MUST NOT expire in less than two minutes, unless REQ-5a applies. a) For specific destination ports in the well-known port range (ports 0-1023), a NAT MAY have shorter UDP mapping timers that are specific to the IANA-registered application running over that specific destination port. b) The value of the NAT UDP mapping timer MAY be configurable. c) A default value of five minutes or more for the NAT UDP mapping timer is RECOMMENDED. REQ-6: The NAT mapping Refresh Direction MUST have a "NAT Outbound refresh behavior" of "True". a) The NAT mapping Refresh Direction MAY have a "NAT Inbound refresh behavior" of "True". REQ-7 A NAT device whose external IP interface can be configured dynamically MUST either (1) Automatically ensure that its internal network uses IP addresses that do not conflict with its external network, or (2) Be able to translate and forward traffic between all internal nodes and all external nodes whose IP addresses numerically conflict with the internal network. REQ-8: If application transparency is most important, it is RECOMMENDED that a NAT have "Endpoint-Independent Filtering" behavior. If a more stringent filtering behavior is most important, it is RECOMMENDED that a NAT have "Address-Dependent Filtering" behavior. Audet & Jennings Best Current Practice [Page 22] RFC 4787 NAT UDP Unicast Requirements January 2007 a) The filtering behavior MAY be an option configurable by the administrator of the NAT. REQ-9: A NAT MUST support "Hairpinning". a) A NAT Hairpinning behavior MUST be "External source IP address and port". REQ-10: To eliminate interference with UNSAF NAT traversal mechanisms and allow integrity protection of UDP communications, NAT ALGs for UDP-based protocols SHOULD be turned off. Future standards track specifications that define an ALG can update this to recommend the ALGs on which they define default. a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow the NAT administrator to enable or disable each ALG separately. REQ-11: A NAT MUST have deterministic behavior, i.e., it MUST NOT change the NAT translation (Section 4) or the Filtering (Section 5) Behavior at any point in time, or under any particular conditions. REQ-12: Receipt of any sort of ICMP message MUST NOT terminate the NAT mapping. a) The NAT's default configuration SHOULD NOT filter ICMP messages based on their source IP address. b) It is RECOMMENDED that a NAT support ICMP Destination Unreachable messages. REQ-13 If the packet received on an internal IP address has DF=1, the NAT MUST send back an ICMP message "Fragmentation needed and DF set" to the host, as described in [RFC0792]. a) If the packet has DF=0, the NAT MUST fragment the packet and SHOULD send the fragments in order. REQ-14: A NAT MUST support receiving in-order and out-of-order fragments, so it MUST have "Received Fragment Out of Order" behavior. a) A NAT's out-of-order fragment processing mechanism MUST be designed so that fragmentation-based DoS attacks do not compromise the NAT's ability to process in-order and unfragmented IP packets. Audet & Jennings Best Current Practice [Page 23] RFC 4787 NAT UDP Unicast Requirements January 2007 13. Security Considerations NATs are often deployed to achieve security goals. Most of the recommendations and requirements in this document do not affect the security properties of these devices, but a few of them do have security implications and are discussed in this section. This document recommends that the timers for mapping be refreshed on outgoing packets (see REQ-6) and does not make recommendations about whether or not inbound packets should update the timers. If inbound packets update the timers, an external attacker can keep the mapping alive forever and attack future devices that may end up with the same internal address. A device that was also the DHCP server for the private address space could mitigate this by cleaning any mappings when a DHCP lease expired. For unicast UDP traffic (the scope of this document), it may not seem relevant to support inbound timer refresh; however, for multicast UDP, the question is harder. It is expected that future documents discussing NAT behavior with multicast traffic will refine the requirements around handling of the inbound refresh timer. Some devices today do update the timers on inbound packets. This document recommends that the NAT filters be specific to the external IP address only (see REQ-8) and not to the external IP address and UDP port. It can be argued that this is less secure than using the IP and port. Devices that wish to filter on IP and port do still comply with these requirements. Non-deterministic NATs are risky from a security point of view. They are very difficult to test because they are, well, non-deterministic. Testing by a person configuring one may result in the person thinking it is behaving as desired, yet under different conditions, which an attacker can create, the NAT may behave differently. These requirements recommend that devices be deterministic. This document requires that NATs have an "external NAT mapping is endpoint independent" behavior. This does not reduce the security of devices. Which packets are allowed to flow across the device is determined by the external filtering behavior, which is independent of the mapping behavior. When a fragmented packet is received from the external side, and the packets are out of order so that the initial fragment does not arrive first, many systems simply discard the out-of-order packets. Moreover, since some networks deliver small packets ahead of large ones, there can be many out-of-order fragments. NATs that are capable of delivering these out-of-order packets are possible, but they need to store the out-of-order fragments, which can open up a Audet & Jennings Best Current Practice [Page 24] RFC 4787 NAT UDP Unicast Requirements January 2007 Denial-of-Service (DoS) opportunity, if done incorrectly. Fragmentation has been a tool used in many attacks, some involving passing fragmented packets through NATs, and others involving DoS attacks based on the state needed to reassemble the fragments. NAT implementers should be aware of [RFC3128] and [RFC1858]. 14. IAB Considerations The IAB has studied the problem of "Unilateral Self Address Fixing", which is the general process by which a client attempts to determine its address in another realm on the other side of a NAT through a collaborative protocol reflection mechanism [RFC3424]. This specification does not, in itself, constitute an UNSAF application. It consists of a series of requirements for NATs aimed at minimizing the negative impact that those devices have on peer-to- peer media applications, especially when those applications are using UNSAF methods. Section 3 of UNSAF lists several practical issues with solutions to NAT problems. This document makes recommendations to reduce the uncertainty and problems introduced by these practical issues with NATs. In addition, UNSAF lists five architectural considerations. Although this is not an UNSAF proposal, it is interesting to consider the impact of this work on these architectural considerations. Arch-1: The scope of this is limited to UDP packets in NATs like the ones widely deployed today. The "fix" helps constrain the variability of NATs for true UNSAF solutions such as STUN. Arch-2: This will exit at the same rate that NATs exit. It does not imply any protocol machinery that would continue to live after NATs were gone, or make it more difficult to remove them. Arch-3: This does not reduce the overall brittleness of NATs, but will hopefully reduce some of the more outrageous NAT behaviors and make it easer to discuss and predict NAT behavior in given situations. Arch-4: This work and the results [RESULTS] of various NATs represent the most comprehensive work at IETF on what the real issues are with NATs for applications like VoIP. This work and STUN have pointed out, more than anything else, the brittleness NATs introduce and the difficulty of addressing these issues. Audet & Jennings Best Current Practice [Page 25] RFC 4787 NAT UDP Unicast Requirements January 2007 Arch-5: This work and the test results [RESULTS] provide a reference model for what any UNSAF proposal might encounter in deployed NATs. 15. Acknowledgments The editor would like to acknowledge Bryan Ford, Pyda Srisuresh, and Dan Kegel for their multiple contributions on peer-to-peer communications across a NAT. Dan Wing contributed substantial text on IP fragmentation and ICMP behavior. Thanks to Rohan Mahy, Jonathan Rosenberg, Mary Barnes, Melinda Shore, Lyndsay Campbell, Geoff Huston, Jiri Kuthan, Harald Welte, Steve Casner, Robert Sanders, Spencer Dawkins, Saikat Guha, Christian Huitema, Yutaka Takeda, Paul Hoffman, Lisa Dusseault, Pekka Savola, Peter Koch, Jari Arkko, and Alfred Hoenes for their contributions. 16. References 16.1. Normative References [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 16.2. Informative References [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990. [RFC1435] Knowles, S., "IESG Advice from Experience with Path MTU Discovery", RFC 1435, March 1993. [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995. [RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security Considerations for IP Fragment Filtering", RFC 1858, October 1995. Audet & Jennings Best Current Practice [Page 26] RFC 4787 NAT UDP Unicast Requirements January 2007 [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2623] Eisler, M., "NFS Version 2 and Version 3 Security Issues and the NFS Protocol's Use of RPCSEC_GSS and Kerberos V5", RFC 2623, June 1999. [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [RFC3027] Holdrege, M. and P. Srisuresh, "Protocol Complications with the IP Network Address Translator", RFC 3027, January 2001. [RFC3128] Miller, I., "Protection Against a Variant of the Tiny Fragment Attack (RFC 1858)", RFC 3128, June 2001. [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. [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002. [RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN - Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs)", RFC 3489, March 2003. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, October 2003. Audet & Jennings Best Current Practice [Page 27] RFC 4787 NAT UDP Unicast Requirements January 2007 [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through Network Address Translations (NATs)", RFC 4380, February 2006. [RFC3489bis] Rosenberg, J., "Simple Traversal Underneath Network Address Translators (NAT) (STUN)", Work in Progress, October 2006. [ICE] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", Work in Progress, October 2006. [RESULTS] Jennings, C., "NAT Classification Test Results", Work in Progress, October 2006. [TURN] Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal Underneath NAT (STUN)", Work in Progress, October 2006. [ITU.H323] "Packet-based Multimedia Communications Systems", ITU- T Recommendation H.323, July 2003. Authors' Addresses Francois Audet (editor) Nortel Networks 4655 Great America Parkway Santa Clara, CA 95054 US Phone: +1 408 495 2456 EMail: audet@nortel.com Cullen Jennings Cisco Systems 170 West Tasman Drive MS: SJC-21/2 San Jose, CA 95134 US Phone: +1 408 902 3341 EMail: fluffy@cisco.com Audet & Jennings Best Current Practice [Page 28] RFC 4787 NAT UDP Unicast Requirements January 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Audet & Jennings Best Current Practice [Page 29] ========================================================================= Internet Engineering Task Force (IETF) S. Perreault, Ed. Request for Comments: 6888 Viagenie BCP: 127 I. Yamagata Updates: 4787 S. Miyakawa Category: Best Current Practice NTT Communications ISSN: 2070-1721 A. Nakagawa Japan Internet Exchange (JPIX) H. Ashida Cisco Systems April 2013 Common Requirements for Carrier-Grade NATs (CGNs) Abstract This document defines common requirements for Carrier-Grade NATs (CGNs). It updates RFC 4787. Status of This Memo This memo documents an Internet Best Current Practice. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6888. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (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. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Perreault, et al. Best Current Practice [Page 1] RFC 6888 CGN Requirements April 2013 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Requirements for CGNs . . . . . . . . . . . . . . . . . . . 4 4. Logging . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5. Port Allocation Scheme . . . . . . . . . . . . . . . . . . . 11 6. Deployment Considerations . . . . . . . . . . . . . . . . . 11 7. Security Considerations . . . . . . . . . . . . . . . . . . 12 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 9.1. Normative References . . . . . . . . . . . . . . . . . 12 9.2. Informative Reference . . . . . . . . . . . . . . . . . 13 1. Introduction With the shortage of IPv4 addresses, it is expected that more Internet Service Providers (ISPs) may want to provide a service where a public IPv4 address would be shared by many subscribers. Each subscriber is assigned a private address, and a Network Address Translator (NAT) [RFC2663] situated in the ISP's network translates the traffic between private and public addresses. When a second IPv4 NAT is located at the customer edge, this results in two layers of NAT. This service can conceivably be offered alongside others, such as IPv6 services or regular IPv4 service assigning public addresses to subscribers. Some ISPs started offering such a service long before there was a shortage of IPv4 addresses, showing that there are driving forces other than the shortage of IPv4 addresses. One approach to CGN deployment is described in [RFC6264]. This document describes behavior that is required of those multi- subscriber NATs for interoperability. It is not an IETF endorsement of CGNs or a real specification for CGNs; rather, it is just a minimal set of requirements that will increase the likelihood of applications working across CGNs. Because subscribers do not receive unique IPv4 addresses, Carrier- Grade NATs introduce substantial limitations in communications between subscribers and with the rest of the Internet. In particular, it is considerably more involved to establish proxy functionality at the border between internal and external realms. Some applications may require substantial enhancements, while some others may not function at all in such an environment. Please see "Issues with IP Address Sharing" [RFC6269] for details. Perreault, et al. Best Current Practice [Page 2] RFC 6888 CGN Requirements April 2013 This document builds upon previous works describing requirements for generic NATs [RFC4787][RFC5382][RFC5508]. These documents, and their updates if any, still apply in this context. What follows are additional requirements, to be satisfied on top of previous ones. 2. Terminology 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]. Readers are expected to be familiar with "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP" [RFC4787] and the terms defined there. The following additional term is used in this document: Carrier-Grade NAT (CGN): A NAT-based [RFC2663] logical function used to share the same IPv4 address among several subscribers. A CGN is not managed by the subscribers. Note that the term "carrier-grade" has nothing to do with the quality of the NAT; that is left to discretion of implementers. Rather, it is to be understood as a topological qualifier: the NAT is placed in an ISP's network and translates the traffic of potentially many subscribers. Subscribers have limited or no control over the CGN, whereas they typically have full control over a NAT placed on their premises. Note also that the CGN described in this document is IPv4-only. IPv6 address translation is not considered. However, the scenario in which the IPv4-only CGN logical function is used may include IPv6 elements. For example, Dual- Stack Lite (DS-Lite) [RFC6333] uses an IPv4-only CGN logical function in a scenario making use of IPv6 encapsulation. Therefore, this document would also apply to the CGN part of DS-Lite. Perreault, et al. Best Current Practice [Page 3] RFC 6888 CGN Requirements April 2013 Figure 1 summarizes a common network topology in which a CGN operates. . : | Internet ............... | ................... | ISP network External pool: | 192.0.2.1/26 | ++------++ External realm ........... | CGN |............... ++------++ Internal realm 10.0.0.1 | | | | | | ISP network ............. | .. | ................ | | Customer premises 10.0.0.100 | | 10.0.0.101 ++------++ ++------++ | CPE1 | | CPE2 | etc. ++------++ ++------++ (IP addresses are only for example purposes) Figure 1: CGN Network Topology Another possible topology is one for hotspots, where there is no customer premise or customer premises equipment (CPE), but where a CGN serves a bunch of customers who don't trust each other; hence, fairness is an issue. One important difference with the previous topology is the absence of a second layer of NAT. This, however, has no impact on CGN requirements since they are driven by fairness and robustness in the service provided to customers, which applies in both cases. 3. Requirements for CGNs What follows is a list of requirements for CGNs. They are in addition to those found in other documents such as [RFC4787], [RFC5382], and [RFC5508]. REQ-1: If a CGN forwards packets containing a given transport protocol, then it MUST fulfill that transport protocol's behavioral requirements. Current applicable documents are as follows: a. "NAT Behavioral Requirements for Unicast UDP" [RFC4787] Perreault, et al. Best Current Practice [Page 4] RFC 6888 CGN Requirements April 2013 b. "Network Address Translation (NAT) Behavioral Requirements for TCP" [RFC5382] c. "NAT Behavioral Requirements for ICMP" [RFC5508] d. "Network Address Translation (NAT) Behavioral Requirements for the Datagram Congestion Control Protocol (DCCP)" [RFC5597] Any future NAT behavioral requirements documents for IPv4 transport protocols will impose additional requirements for CGNs on top of those stated here. Justification: It is crucial for CGNs to maximize the set of applications that can function properly across them. The IETF has documented the best current practices for UDP, TCP, ICMP, and DCCP. REQ-2: A CGN MUST have a default "IP address pooling" behavior of "Paired" (as defined in Section 4.1 of [RFC4787]). A CGN MAY provide a mechanism for administrators to change this behavior on an application protocol basis. * When multiple overlapping internal IP address ranges share the same external IP address pool (e.g., DS-Lite [RFC6333]), the "IP address pooling" behavior applies to mappings between external IP addresses and internal subscribers rather than between external and internal IP addresses. Justification: This stronger form of REQ-2 from [RFC4787] is justified by the stronger need for not breaking applications that depend on the external address remaining constant. Note that this requirement applies regardless of the transport protocol. In other words, a CGN must use the same external IP address mapping for all sessions associated with the same internal IP address, be they TCP, UDP, ICMP, something else, or a mix of different protocols. The justification for allowing other behaviors is to allow the administrator to save external addresses and ports for application protocols that are known to work fine with other behaviors in practice. However, the default behavior MUST be "Paired". REQ-3: The CGN function SHOULD NOT have any limitations on the size or the contiguity of the external address pool. In particular, the CGN function MUST be configurable with contiguous or non- contiguous external IPv4 address ranges. Perreault, et al. Best Current Practice [Page 5] RFC 6888 CGN Requirements April 2013 Justification: Given the increasing rarity of IPv4 addresses, it is becoming harder for an operator to provide large contiguous address pools to CGNs. Additionally, operational flexibility may require non-contiguous address pools for reasons such as differentiated services, routing management, etc. The reason for having SHOULD instead of MUST is to account for limitations imposed by available resources as well as constraints imposed for security reasons. REQ-4: A CGN MUST support limiting the number of external ports (or, equivalently, "identifiers" for ICMP) that are assigned per subscriber. a. Per-subscriber limits MUST be configurable by the CGN administrator. b. Per-subscriber limits MAY be configurable independently per transport protocol. c. Additionally, it is RECOMMENDED that the CGN include administrator-adjustable thresholds to prevent a single subscriber from consuming excessive CPU resources from the CGN (e.g., rate-limit the subscriber's creation of new mappings). Justification: A CGN can be considered a network resource that is shared by competing subscribers. Limiting the number of external ports assigned to each subscriber mitigates the denial-of-service (DoS) attack that a subscriber could launch against other subscribers through the CGN in order to get a larger share of the resource. It ensures fairness among subscribers. Limiting the rate of allocation mitigates a similar attack where the CPU is the resource being targeted instead of port numbers. However, this requirement is not a MUST because it is very hard to explicitly call out all CPU-consuming events. REQ-5: A CGN SHOULD support limiting the amount of state memory allocated per mapping and per subscriber. This may include limiting the number of sessions, the number of filters, etc., depending on the NAT implementation. a. Limits SHOULD be configurable by the CGN administrator. b. Additionally, it SHOULD be possible to limit the rate at which memory-consuming state elements are allocated. Perreault, et al. Best Current Practice [Page 6] RFC 6888 CGN Requirements April 2013 Justification: A NAT needs to keep track of TCP sessions associated with each mapping. This state consumes resources for which, in the case of a CGN, subscribers may compete. It is necessary to ensure that each subscriber has access to a fair share of the CGN's resources. Limiting the rate of allocation is intended to prevent CPU resource exhaustion. Item "B" is at the SHOULD level to account for the fact that means other than rate limiting may be used to attain the same goal. REQ-6: It MUST be possible to administratively turn off translation for specific destination addresses and/or ports. Justification: It is common for a CGN administrator to provide access for subscribers to servers installed in the ISP's network in the external realm. When such a server is able to reach the internal realm via normal routing (which is entirely controlled by the ISP), translation is unneeded. In that case, the CGN may forward packets without modification, thus acting like a plain router. This may represent an important efficiency gain. Figure 2 illustrates this use-case. X1:x1 X1':x1' X2:x2 +---+from X1:x1 +---+from X1:x1 +---+ | C | to X2:x2 | | to X2:x2 | S | | l |>>>>>>>>>>>>| C |>>>>>>>>>>>>>>| e | | i | | G | | r | | e |<<<<<<<<<<<<| N |<<<<<<<<<<<<<<| v | | n |from X2:x2 | |from X2:x2 | e | | t | to X1:x1 | | to X1:x1 | r | +---+ +---+ +---+ Figure 2: CGN Pass-Through REQ-7: It is RECOMMENDED that a CGN use an "endpoint-independent filtering" behavior (as defined in Section 5 of [RFC4787]). If it is known that "Address-Dependent Filtering" does not cause the application-layer protocol to break (how to determine this is out of scope for this document), then it MAY be used instead. Justification: This is a stronger form of REQ-8 from [RFC4787]. This is based on the observation that some games and peer-to-peer applications require EIF for the NAT traversal to work. In the context of a CGN, it is important to minimize application breakage. Perreault, et al. Best Current Practice [Page 7] RFC 6888 CGN Requirements April 2013 REQ-8: Once an external port is deallocated, it SHOULD NOT be reallocated to a new mapping until at least 120 seconds have passed, with the exceptions being: a. If the CGN tracks TCP sessions (e.g., with a state machine, as in Section 3.5.2.2 of [RFC6146]), TCP ports MAY be reused immediately. b. If external ports are statically assigned to internal addresses (e.g., address X with port range 1000-1999 is assigned to subscriber A, 2000-2999 to subscriber B, etc.), and the assignment remains constant across state loss, then ports MAY be reused immediately. c. If the allocated external ports used address-dependent or address-and-port-dependent filtering before state loss, they MAY be reused immediately. The length of time and the maximum number of ports in this state MUST be configurable by the CGN administrator. Justification: This is necessary in order to prevent collisions between old and new mappings and sessions. It ensures that all established sessions are broken instead of redirected to a different peer. The exceptions are for cases where reusing a port immediately does not create a possibility that packets would be redirected to the wrong peer. One can imagine other exceptions where mapping collisions are avoided, thus justifying the SHOULD level for this requirement. The 120 seconds value corresponds to the Maximum Segment Lifetime (MSL) from [RFC0793]. Note that this requirement also applies to the case when a CGN loses state (due to a crash, reboot, failover to a cold standby, etc.). In that case, ports that were in use at the time of state loss SHOULD NOT be reallocated until at least 120 seconds have passed. REQ-9: A CGN MUST implement a protocol giving subscribers explicit control over NAT mappings. That protocol SHOULD be the Port Control Protocol [RFC6887]. Justification: Allowing subscribers to manipulate the NAT state table with PCP greatly increases the likelihood that applications will function properly. Perreault, et al. Best Current Practice [Page 8] RFC 6888 CGN Requirements April 2013 A study of PCP-less CGN impacts can be found in [NAT444]. Another study considering the effects of PCP on a peer-to-peer file sharing protocol can be found in [BITTORRENT]. REQ-10: CGN implementers SHOULD make their equipment manageable. Standards-based management using standards such as "Definitions of Managed Objects for NAT" [RFC4008] is RECOMMENDED. Justification: It is anticipated that CGNs will be primarily deployed in ISP networks where the need for management is critical. This requirement is at the SHOULD level to account for the fact that some CGN operators may not need management functionality. Note also that there are efforts within the IETF toward creating a MIB tailored for CGNs (e.g., [NAT-MIB]). REQ-11: When a CGN is unable to create a dynamic mapping due to resource constraints or administrative restrictions (i.e., quotas): a. it MUST drop the original packet; b. it SHOULD send an ICMP Destination Unreachable message with code 1 (Host Unreachable) to the sender; c. it SHOULD send a notification (e.g., SNMP trap) towards a management system (if configured to do so); and d. it MUST NOT delete existing mappings in order to "make room" for the new one. (This only applies to normal CGN behavior, not to manual operator intervention.) Justification: This is a slightly different form of REQ-8 from [RFC5508]. Code 1 is preferred to code 13 because it is listed as a "soft error" in [RFC1122], which is important because we don't want TCP stacks to abort the connection attempt in this case. See [RFC5461] for details on TCP's reaction to soft errors. Sending ICMP errors and SNMP traps may be rate-limited for security reasons, which is why requirements B and C are SHOULDs, not MUSTs. Applications generally handle connection establishment failure better than established connection failure. This is why dropping the packet initiating the new connection is preferred over deleting existing mappings. See also the rationale in Section 6 of [RFC5508]. Perreault, et al. Best Current Practice [Page 9] RFC 6888 CGN Requirements April 2013 4. Logging It may be necessary for CGN administrators to be able to identify a subscriber based on external IPv4 address, port, and timestamp in order to deal with abuse. When multiple subscribers share a single external address, the source address and port that are visible at the destination host have been translated from the ones originated by the subscriber. In order to be able to do this, the CGN would need to log the following information for each mapping created (this list is for informational purposes only and does not constitute a requirement): o transport protocol o subscriber identifier (e.g., internal source address or tunnel endpoint identifier) o external source address o external source port o timestamp By "subscriber identifier" we mean information that uniquely identifies a subscriber. For example, in a traditional NAT scenario, the internal source address would be sufficient. In the case of DS- Lite, many subscribers share the same internal address and the subscriber identifier is the tunnel endpoint identifier (i.e., the B4's IPv6 address). A disadvantage of logging mappings is that CGNs under heavy usage may produce large amounts of logs, which may require large storage volume. REQ-12: A CGN SHOULD NOT log destination addresses or ports unless required to do so for administrative reasons. Justification: Destination logging at the CGN creates privacy issues. Furthermore, readers should be aware of logging recommendations for Internet-facing servers [RFC6302]. With compliant servers, the destination address and port do not need to be logged by the CGN. This can help reduce the amount of logging. This requirement is at the SHOULD level to account for the fact that there may be other reasons for logging destination addresses or ports. One such reason might be that the remote server is not following [RFC6302]. Perreault, et al. Best Current Practice [Page 10] RFC 6888 CGN Requirements April 2013 5. Port Allocation Scheme A CGN's port allocation scheme is subject to three competing requirements: REQ-13: A CGN's port allocation scheme SHOULD maximize port utilization. Justification: External ports are one of the resources being shared by a CGN. Efficient management of that resource directly impacts the quality of a subscriber's Internet connection. Some schemes are very efficient in their port utilization. In that sense, they have good scaling properties (nothing is wasted). Others will systematically waste ports. REQ-14: A CGN's port allocation scheme SHOULD minimize log volume. Justification: Huge log volumes can be problematic to CGN operators. Some schemes create one log entry per mapping. Others allow multiple mappings to generate a single log entry, which sometimes can be expressed very compactly. With some schemes, the logging frequency can approach that of DHCP servers. REQ-15: A CGN's port allocation scheme SHOULD make it hard for attackers to guess port numbers. Justification: Easily guessed port numbers put subscribers at risk of the attacks described in [RFC6056]. Some schemes provide very good security in that ports numbers are not easily guessed. Others provide poor security to subscribers. A CGN implementation's choice of port allocation scheme optimizes to satisfy one requirement at the expense of another. Therefore, these are soft requirements (SHOULD as opposed to MUST). 6. Deployment Considerations Several issues are encountered when CGNs are used [RFC6269]. There is current work in the IETF toward alleviating some of these issues. For example, see [NAT-REVEAL]. Perreault, et al. Best Current Practice [Page 11] RFC 6888 CGN Requirements April 2013 7. Security Considerations If a malicious subscriber can spoof another subscriber's CPE, it may cause a DoS to that subscriber by creating mappings up to the allowed limit. An ISP can prevent this with ingress filtering, as described in [RFC2827]. This document recommends endpoint-independent filtering (EIF) as the default filtering behavior for CGNs. EIF has security considerations that are discussed in [RFC4787]. NATs sometimes perform fragment reassembly. CGNs would do so at presumably high data rates. Therefore, the reader should be familiar with the potential security issues described in [RFC4963]. 8. Acknowledgements Thanks for the input and review by Alexey Melnikov, Arifumi Matsumoto, Barry Leiba, Benson Schliesser, Dai Kuwabara, Dan Wing, Dave Thaler, David Harrington, Francis Dupont, Jean-Francois Tremblay, Joe Touch, Lars Eggert, Kousuke Shishikura, Mohamed Boucadair, Martin Stiemerling, Meng Wei, Nejc Skoberne, Pete Resnick, Reinaldo Penno, Ron Bonica, Sam Hartman, Sean Turner, Senthil Sivakumar, Stephen Farrell, Stewart Bryant, Takanori Mizuguchi, Takeshi Tomochika, Tina Tsou, Tomohiro Fujisaki, Tomohiro Nishitani, Tomoya Yoshida, Wes George, Wesley Eddy, and Yasuhiro Shirasaki. 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. [RFC4008] Rohit, R., Srisuresh, P., Raghunarayan, R., Pai, N., and C. Wang, "Definitions of Managed Objects for Network Address Translators (NAT)", RFC 4008, March 2005. [RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007. [RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, RFC 5382, October 2008. Perreault, et al. Best Current Practice [Page 12] RFC 6888 CGN Requirements April 2013 [RFC5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT Behavioral Requirements for ICMP", BCP 148, RFC 5508, April 2009. [RFC5597] Denis-Courmont, R., "Network Address Translation (NAT) Behavioral Requirements for the Datagram Congestion Control Protocol", BCP 150, RFC 5597, September 2009. [RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, April 2013. 9.2. Informative Reference [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, May 2000. [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, July 2007. [RFC5461] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461, February 2009. [RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport- Protocol Port Randomization", BCP 156, RFC 6056, January 2011. [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers", RFC 6146, April 2011. [RFC6264] Jiang, S., Guo, D., and B. Carpenter, "An Incremental Carrier-Grade NAT (CGN) for IPv6 Transition", RFC 6264, June 2011. Perreault, et al. Best Current Practice [Page 13] RFC 6888 CGN Requirements April 2013 [RFC6269] Ford, M., Boucadair, M., Durand, A., Levis, P., and P. Roberts, "Issues with IP Address Sharing", RFC 6269, June 2011. [RFC6302] Durand, A., Gashinsky, I., Lee, D., and S. Sheppard, "Logging Recommendations for Internet-Facing Servers", BCP 162, RFC 6302, June 2011. [RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual- Stack Lite Broadband Deployments Following IPv4 Exhaustion", RFC 6333, August 2011. [NAT-MIB] Perreault, S., Tsou, T., and S. Sivakumar, "Additional Managed Objects for Network Address Translators (NAT)", Work in Progress, February 2013. [NAT-REVEAL] Boucadair, M., Touch, J., Levis, P., and R. Penno, "Analysis of Solution Candidates to Reveal a Host Identifier (HOST_ID) in Shared Address Deployments", Work in Progress, April 2013. [NAT444] Donley, C., Ed., Howard, L., Kuarsingh, V., Berg, J., and J. Doshi, "Assessing the Impact of Carrier-Grade NAT on Network Applications", Work in Progress, April 2013. [BITTORRENT] Boucadair, M., Zheng, T., Deng, X., and J. Queiroz, "Behavior of BitTorrent service in PCP-enabled networks with Address Sharing", Work in Progress, May 2012. Authors' Addresses Simon Perreault (editor) Viagenie 246 Aberdeen Quebec, QC G1R 2E1 Canada Phone: +1 418 656 9254 EMail: simon.perreault@viagenie.ca URI: http://www.viagenie.ca Perreault, et al. Best Current Practice [Page 14] RFC 6888 CGN Requirements April 2013 Ikuhei Yamagata NTT Communications Corporation Gran Park Tower 17F, 3-4-1 Shibaura, Minato-ku Tokyo 108-8118 Japan Phone: +81 50 3812 4704 EMail: ikuhei@nttv6.jp Shin Miyakawa NTT Communications Corporation Gran Park Tower 17F, 3-4-1 Shibaura, Minato-ku Tokyo 108-8118 Japan Phone: +81 50 3812 4695 EMail: miyakawa@nttv6.jp Akira Nakagawa Japan Internet Exchange Co., Ltd. (JPIX) Otemachi Building 21F, 1-8-1 Otemachi, Chiyoda-ku Tokyo 100-0004 Japan Phone: +81 90 9242 2717 EMail: a-nakagawa@jpix.ad.jp Hiroyuki Ashida Cisco Systems Midtown Tower, 9-7-1, Akasaka Minato-Ku, Tokyo 107-6227 Japan EMail: hiashida@cisco.com Perreault, et al. Best Current Practice [Page 15] ========================================================================= Internet Engineering Task Force (IETF) R. Penno Request for Comments: 7857 Cisco BCP: 127 S. Perreault Updates: 4787, 5382, 5508 Jive Communications Category: Best Current Practice M. Boucadair, Ed. ISSN: 2070-1721 Orange S. Sivakumar Cisco K. Naito NTT April 2016 Updates to Network Address Translation (NAT) Behavioral Requirements Abstract This document clarifies and updates several requirements of RFCs 4787, 5382, and 5508 based on operational and development experience. The focus of this document is Network Address Translation from IPv4 to IPv4 (NAT44). This document updates RFCs 4787, 5382, and 5508. Status of This Memo This memo documents an Internet Best Current Practice. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7857. Penno, et al. Best Current Practice [Page 1] RFC 7857 Updates to NAT Behavioral Requirements April 2016 Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (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. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Penno, et al. Best Current Practice [Page 2] RFC 7857 Updates to NAT Behavioral Requirements April 2016 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. TCP Session Tracking . . . . . . . . . . . . . . . . . . . . 4 2.1. TCP Transitory Connection Idle-Timeout . . . . . . . . . 6 2.2. TCP RST . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Port Overlapping Behavior . . . . . . . . . . . . . . . . . . 6 4. Address Pooling Paired (APP) . . . . . . . . . . . . . . . . 7 5. Endpoint-Independent Mapping (EIM) Protocol Independence . . 8 6. Endpoint-Independent Filtering (EIF) Protocol Independence . 8 7. Endpoint-Independent Filtering (EIF) Mapping Refresh . . . . 8 7.1. Outbound Mapping Refresh and Error Packets . . . . . . . 9 8. Port Parity . . . . . . . . . . . . . . . . . . . . . . . . . 9 9. Port Randomization . . . . . . . . . . . . . . . . . . . . . 9 10. IP Identification (IP ID) . . . . . . . . . . . . . . . . . . 10 11. ICMP Query Mappings Timeout . . . . . . . . . . . . . . . . . 10 12. Hairpinning Support for ICMP Packets . . . . . . . . . . . . 10 13. Security Considerations . . . . . . . . . . . . . . . . . . . 11 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 14.1. Normative References . . . . . . . . . . . . . . . . . . 12 14.2. Informative References . . . . . . . . . . . . . . . . . 12 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 1. Introduction [RFC4787], [RFC5382], and [RFC5508] contributed to enhance Network Address Translation (NAT) interoperability and conformance. Operational experience gained through widespread deployment and evolution of NAT indicates that some areas of the original documents need further clarification or updates. This document provides such clarifications and updates. 1.1. Scope The goal of this document is to clarify and update the set of requirements listed in [RFC4787], [RFC5382], and [RFC5508]. The document focuses exclusively on NAT44. The scope of this document has been set so that it does not create new requirements beyond those specified in the documents cited above. Requirements related to Carrier-Grade NAT (CGN) are defined in [RFC6888]. Penno, et al. Best Current Practice [Page 3] RFC 7857 Updates to NAT Behavioral Requirements April 2016 1.2. Terminology 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]. The reader is assumed to be familiar with the terminology defined in [RFC2663], [RFC4787], [RFC5382], and [RFC5508]. In this document, the term "NAT" refers to both "Basic NAT" and "Network Address/Port Translator (NAPT)" (see Section 3 of [RFC4787]). As a reminder, Basic NAT and NAPT are two variations of traditional NAT in that translation in Basic NAT is limited to IP addresses alone, whereas translation in NAPT is extended to include IP addresses and transport identifiers (such as a TCP/UDP port or ICMP query ID); refer to Section 2 of [RFC3022]. 2. TCP Session Tracking [RFC5382] specifies TCP timers associated with various connection states but does not specify the TCP state machine a NAT44 should follow as a basis to apply such timers. Update: The TCP state machine depicted in Figure 1, adapted from [RFC6146], SHOULD be implemented by a NAT for TCP session tracking purposes. Penno, et al. Best Current Practice [Page 4] RFC 7857 Updates to NAT Behavioral Requirements April 2016 +----------------------------+ | | V | +------+ Client | |CLOSED|-----SYN------+ | +------+ | | ^ | | |TCP_TRANS T.O. | | | V | +-------+ +-------+ | | TRANS | | INIT | | +-------+ +-------+ | | ^ | | data pkt | | | | Server/Client RST | | | TCP_EST T.O. | | V | Server SYN | +--------------+ | | | ESTABLISHED |<---------+ | +--------------+ | | | | Client FIN Server FIN | | | | V V | +---------+ +----------+ | | C FIN | | S FIN | | | RCV | | RCV | | +---------+ +----------+ | | | | Server FIN Client FIN TCP_TRANS | | T.O. V V | +----------------------+ | | C FIN + S FIN RCV |-----------------+ +----------------------+ Legend: * Messages sent or received from the server are prefixed with "Server". * Messages sent or received from the client are prefixed with "Client". * "C" means "Client-side". * "S" means "Server-side". * TCP_EST T.O. refers to the established connection idle-timeout as defined in [RFC5382]. * TCP_TRANS T.O. refers to the transitory connection idle-timeout as defined in [RFC5382]. Figure 1: Simplified Version of the TCP State Machine Penno, et al. Best Current Practice [Page 5] RFC 7857 Updates to NAT Behavioral Requirements April 2016 2.1. TCP Transitory Connection Idle-Timeout The transitory connection idle-timeout is defined as the minimum time a TCP connection in the partially open or closing phases must remain idle before the NAT considers the associated session a candidate for removal (REQ-5 of [RFC5382]). However, [RFC5382] does not clearly state whether these can be configured separately. Clarification: This document clarifies that a NAT SHOULD provide different configurable parameters for configuring the open and closing idle timeouts. To accommodate deployments that consider a partially open timeout of 4 minutes as being excessive from a security standpoint, a NAT MAY allow the configured timeout to be less than 4 minutes. However, a minimum default transitory connection idle-timeout of 4 minutes is RECOMMENDED. 2.2. TCP RST [RFC5382] leaves the handling of TCP RST packets unspecified. Update: This document adopts a similar default behavior as in [RFC6146]. Concretely, when the NAT receives a TCP RST matching an existing mapping, it MUST translate the packet according to the NAT mapping entry. Moreover, the NAT SHOULD wait for 4 minutes before deleting the session and removing any state associated with it if no packets are received during that 4-minute timeout. Notes: * Admittedly, the NAT has to verify whether received TCP RST packets belong to a connection. This verification check is required to avoid off-path attacks. * If the NAT immediately removes the NAT mapping upon receipt of a TCP RST message, stale connections may be maintained by endpoints if the first RST message is lost between the NAT and the recipient. 3. Port Overlapping Behavior REQ-1 from [RFC4787] and REQ-1 from [RFC5382] specify a specific port overlapping behavior; that is, the external IP address and port can be reused for connections originating from the same internal source IP address and port irrespective of the destination. This is known as Endpoint-Independent Mapping (EIM). Penno, et al. Best Current Practice [Page 6] RFC 7857 Updates to NAT Behavioral Requirements April 2016 Update: This document clarifies that this port overlapping behavior may be extended to connections originating from different internal source IP addresses and ports as long as their destinations are different. The following mechanism MAY be implemented by a NAT: If destination addresses and ports are different for outgoing connections started by local clients, a NAT MAY assign the same external port as the source ports for the connections. The port overlapping mechanism manages mappings between external packets and internal packets by looking at and storing their 5-tuple (protocol, source address, source port, destination address, and destination port). This enables concurrent use of a single NAT external port for multiple transport sessions, which allows a NAT to successfully process packets in a network that has a limited number of IP addresses (e.g., deployment with a high address space multiplicative factor (refer to Appendix B of [RFC6269])). 4. Address Pooling Paired (APP) The "IP address pooling" behavior of "Paired" (APP) was recommended in REQ-2 from [RFC4787], but the behavior when an external IPv4 runs out of ports was left undefined. Clarification: This document clarifies that if APP is enabled, new sessions from a host that already has a mapping associated with an external IP that ran out of ports SHOULD be dropped. A configuration parameter MAY be provided to allow a NAT to start using ports from another external IP address when the one that anchored the APP mapping ran out of ports. Tweaking this configuration parameter is a trade-off between service continuity and APP strict enforcement. Note, this behavior is sometimes referred to as "soft-APP". As a reminder, the recommendation for the particular case of a CGN is that an implementation must use the same external IP address mapping for all sessions associated with the same internal IP address, be they TCP, UDP, ICMP, something else, or a mix of different protocols [RFC6888]. Update: This behavior SHOULD apply also for TCP. Penno, et al. Best Current Practice [Page 7] RFC 7857 Updates to NAT Behavioral Requirements April 2016 5. Endpoint-Independent Mapping (EIM) Protocol Independence REQ-1 from [RFC4787] and REQ-1 from [RFC5382] do not specify whether EIM are protocol dependent or protocol independent. For example, if an outbound TCP SYN creates a mapping, it is left undefined whether outbound UDP packets can reuse such mapping. Update: EIM mappings SHOULD be protocol dependent. A configuration parameter MAY be provided to allow protocols that multiplex TCP and UDP over the same source IP address and port number to use a single mapping. The default value of this configuration parameter MUST be protocol-dependent EIM. This update is consistent with the stateful Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers (NAT64) [RFC6146] that clearly specifies three binding information bases (TCP, UDP, and ICMP). 6. Endpoint-Independent Filtering (EIF) Protocol Independence REQ-8 from [RFC4787] and REQ-3 from [RFC5382] do not specify whether mappings with Endpoint-Independent Filtering (EIF) are protocol independent or protocol dependent. For example, if an outbound TCP SYN creates a mapping, it is left undefined whether inbound UDP packets matching that mapping should be accepted or rejected. Update: EIF filtering SHOULD be protocol dependent. A configuration parameter MAY be provided to make it protocol independent. The default value of this configuration parameter MUST be protocol- dependent EIF. This behavior is aligned with the update in Section 5. Applications that can be transported over a variety of transport protocols and/or support transport fallback schemes won't experience connectivity failures if the NAT is configured with protocol-independent EIM and protocol-independent EIF. 7. Endpoint-Independent Filtering (EIF) Mapping Refresh The NAT mapping Refresh direction may have a "NAT Inbound refresh behavior" of "True" according to REQ-6 from [RFC4787], but [RFC4787] does not clarify how this behavior applies to EIF mappings. The issue in question is whether inbound packets that match an EIF mapping but do not create a new session due to a security policy should refresh the mapping timer. Penno, et al. Best Current Practice [Page 8] RFC 7857 Updates to NAT Behavioral Requirements April 2016 Clarification: This document clarifies that even when a NAT has an inbound refresh behavior set to "TRUE", such packets SHOULD NOT refresh the mapping. Otherwise, a simple attack of a packet every two minutes can keep the mapping indefinitely. Update: This behavior SHOULD apply also for TCP. 7.1. Outbound Mapping Refresh and Error Packets Update: In the case of NAT outbound refresh behavior, ICMP Errors or TCP RST outbound packets sent as a response to inbound packets SHOULD NOT refresh the mapping. Other packets that indicate the host is not interested in receiving packets MAY be configurable to also not refresh state, such as a Session Traversal Utilities for NAT (STUN) error response [RFC5389] or IKE INVALID_SYNTAX [RFC7296]. 8. Port Parity Update: A NAT MAY disable port parity preservation for all dynamic mappings. Nevertheless, A NAT SHOULD support means to explicitly request to preserve port parity (e.g., [RFC7753]). Note: According to [RFC6887], dynamic mappings are said to be dynamic in the sense that they are created on demand, either implicitly or explicitly: 1. Implicit dynamic mappings refer to mappings that are created as a side effect of traffic such as an outgoing TCP SYN or outgoing UDP packet. Implicit dynamic mappings usually have a finite lifetime, though this lifetime is generally not known to the client using them. 2. Explicit dynamic mappings refer to mappings that are created as a result, for example, of explicit Port Control Protocol (PCP) MAP and PEER requests. Explicit dynamic mappings have a finite lifetime, and this lifetime is communicated to the client. 9. Port Randomization Update: A NAT SHOULD follow the recommendations specified in Section 4 of [RFC6056], especially: A NAPT that does not implement port preservation [RFC4787] [RFC5382] SHOULD obfuscate selection of the ephemeral port of a packet when it is changed during translation of that packet. Penno, et al. Best Current Practice [Page 9] RFC 7857 Updates to NAT Behavioral Requirements April 2016 A NAPT that does implement port preservation SHOULD obfuscate the ephemeral port of a packet only if the port must be changed as a result of the port being already in use for some other session. A NAPT that performs parity preservation and that must change the ephemeral port during translation of a packet SHOULD obfuscate the ephemeral ports. The algorithms described in this document could be easily adapted such that the parity is preserved (i.e., force the lowest order bit of the resulting port number to 0 or 1 according to whether even or odd parity is desired). 10. IP Identification (IP ID) Update: A NAT SHOULD handle the Identification field of translated IPv4 packets as specified in Section 5.3.1 of [RFC6864]. 11. ICMP Query Mappings Timeout Section 3.1 of [RFC5508] specifies that ICMP Query mappings are to be maintained by a NAT. However, the specification doesn't discuss Query mapping timeout values. Section 3.2 of [RFC5508] only discusses ICMP Query session timeouts. Update: ICMP Query mappings MAY be deleted once the last session using the mapping is deleted. 12. Hairpinning Support for ICMP Packets REQ-7 from [RFC5508] specifies that a NAT enforcing Basic NAT must support traversal of hairpinned ICMP Query sessions. Clarification: This implicitly means that address mappings from external address to internal address (similar to Endpoint- Independent Filters) must be maintained to allow inbound ICMP Query sessions. If an ICMP Query is received on an external address, a NAT can then translate to an internal IP. REQ-7 from [RFC5508] specifies that all NATs must support the traversal of hairpinned ICMP Error messages. Clarification: This behavior requires a NAT to maintain address mappings from external IP address to internal IP address in addition to the ICMP Query mappings described in Section 3.1 of [RFC5508]. Penno, et al. Best Current Practice [Page 10] RFC 7857 Updates to NAT Behavioral Requirements April 2016 13. Security Considerations NAT behavioral considerations are discussed in [RFC4787], [RFC5382], and [RFC5508]. Because some of the clarifications and updates (e.g., Section 2) are inspired from NAT64, the security considerations discussed in Section 5 of [RFC6146] apply also for this specification. The update in Section 3 allows for an optimized NAT resource usage. In order to avoid service disruption, the NAT must not invoke this functionality unless the packets are to be sent to distinct destination addresses. Some of the updates (e.g., Sections 7, 9, and 11) allow for increased security compared to [RFC4787], [RFC5382], and [RFC5508]. Particularly, o the updates in Sections 7 and 11 prevent an illegitimate node to maintain mappings activated in the NAT while these mappings should be cleared, and o port randomization (Section 9) complicates tracking hosts located behind a NAT. Sections 4 and 12 propose updates that increase the serviceability of a host located behind a NAT. These updates do not introduce any additional security concerns to [RFC4787], [RFC5382], and [RFC5508]. The updates in Sections 5 and 6 allow for a better NAT transparency from an application standpoint. Hosts that require a restricted filtering behavior should enable specific policies (e.g., Access Control List (ACL)) either locally or by soliciting a dedicated security device (e.g., firewall). How a host updates its filtering policies is out of scope of this document. The update in Section 8 induces security concerns that are specific to the protocol used to interact with the NAT. For example, if PCP is used to explicitly request parity preservation for a given mapping, the security considerations discussed in [RFC6887] should be taken into account. The update in Section 10 may have undesired effects on the performance of the NAT in environments in which fragmentation is massively experienced. Such an issue may be used as an attack vector against NATs. Penno, et al. Best Current Practice [Page 11] RFC 7857 Updates to NAT Behavioral Requirements April 2016 14. References 14.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4787] Audet, F., Ed. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January 2007, . [RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P. Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, RFC 5382, DOI 10.17487/RFC5382, October 2008, . [RFC5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT Behavioral Requirements for ICMP", BCP 148, RFC 5508, DOI 10.17487/RFC5508, April 2009, . [RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport- Protocol Port Randomization", BCP 156, RFC 6056, DOI 10.17487/RFC6056, January 2011, . [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, April 2011, . [RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field", RFC 6864, DOI 10.17487/RFC6864, February 2013, . 14.2. Informative References [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, DOI 10.17487/RFC2663, August 1999, . Penno, et al. Best Current Practice [Page 12] RFC 7857 Updates to NAT Behavioral Requirements April 2016 [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, DOI 10.17487/RFC3022, January 2001, . [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, DOI 10.17487/RFC5389, October 2008, . [RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and P. Roberts, "Issues with IP Address Sharing", RFC 6269, DOI 10.17487/RFC6269, June 2011, . [RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, DOI 10.17487/RFC6887, April 2013, . [RFC6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa, A., and H. Ashida, "Common Requirements for Carrier-Grade NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888, April 2013, . [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. Kivinen, "Internet Key Exchange Protocol Version 2 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 2014, . [RFC7753] Sun, Q., Boucadair, M., Sivakumar, S., Zhou, C., Tsou, T., and S. Perreault, "Port Control Protocol (PCP) Extension for Port-Set Allocation", RFC 7753, DOI 10.17487/RFC7753, February 2016, . Acknowledgements Thanks to Dan Wing, Suresh Kumar, Mayuresh Bakshi, Rajesh Mohan, Lars Eggert, Gorry Fairhurst, Brandon Williams, and David Black for their review and discussion. Many thanks to Ben Laurie for the SecDir review and Dan Romascanu for the Gen-ART review. Dan Wing proposed some text for the configurable errors in Section 7.1. Penno, et al. Best Current Practice [Page 13] RFC 7857 Updates to NAT Behavioral Requirements April 2016 Contributors The following individual contributed text to the document: Sarat Kamiset Insieme Networks United States Authors' Addresses Reinaldo Penno Cisco Systems, Inc. 170 West Tasman Drive San Jose, California 95134 United States Email: repenno@cisco.com Simon Perreault Jive Communications Canada Email: sperreault@jive.com Mohamed Boucadair (editor) Orange Rennes 35000 France Email: mohamed.boucadair@orange.com Senthil Sivakumar Cisco Systems, Inc. United States Email: ssenthil@cisco.com Kengo Naito NTT Tokyo Japan Email: k.naito@nttv6.jp Penno, et al. Best Current Practice [Page 14]