--- 1/draft-ietf-behave-nat-udp-01.txt 2006-02-04 22:51:01.000000000 +0100 +++ 2/draft-ietf-behave-nat-udp-02.txt 2006-02-04 22:51:01.000000000 +0100 @@ -1,46 +1,44 @@ BEHAVE F. Audet, Ed. Internet-Draft Nortel Networks -Expires: October 13, 2005 C. Jennings +Expires: December 29, 2005 C. Jennings Cisco Systems - April 11, 2005 + June 27, 2005 NAT Behavioral Requirements for Unicast UDP - draft-ietf-behave-nat-udp-01 + draft-ietf-behave-nat-udp-02 Status of this Memo - This document is an Internet-Draft and is subject to all provisions - of Section 3 of RFC 3667. By submitting this Internet-Draft, each - author represents that any applicable patent or other IPR claims of - which he or she is aware have been or will be disclosed, and any of - which he or she become aware will be disclosed, in accordance with - RFC 3668. + By submitting this Internet-Draft, each author represents that any + applicable patent or other IPR claims of which he or she is aware + have been or will be disclosed, and any of which he or she becomes + aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. - This Internet-Draft will expire on October 13, 2005. + This Internet-Draft will expire on December 29, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document defines basic terminology for describing different types of NAT behavior when handling Unicast UDP, and defines a set of requirements that would allow many applications, such as multimedia @@ -48,48 +46,46 @@ NATs that meet this set of requirements will greatly increase the likelihood that these applications will function properly. 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 . . . . . . . . . . . . . . . . . . . . . 7 - 4.2.1 Port Assignment Behavior . . . . . . . . . . . . . . . 7 - 4.2.2 Port Parity . . . . . . . . . . . . . . . . . . . . . 9 - 4.2.3 Port Contiguity . . . . . . . . . . . . . . . . . . . 9 - 4.3 Mapping Refresh Direction . . . . . . . . . . . . . . . . 10 - 4.4 Mapping Refresh Scope . . . . . . . . . . . . . . . . . . 10 - 5. Filtering Behavior . . . . . . . . . . . . . . . . . . . . . . 11 - 5.1 Filtering of Unsolicited Packets . . . . . . . . . . . . . 11 - 5.2 NAT Filter Refresh . . . . . . . . . . . . . . . . . . . . 11 - 6. Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . . 12 - 7. Application Level Gateways . . . . . . . . . . . . . . . . . . 12 - 8. Deterministic Properties . . . . . . . . . . . . . . . . . . . 13 - 9. ICMP Behavior . . . . . . . . . . . . . . . . . . . . . . . . 14 - 10. Fragmentation of Packets . . . . . . . . . . . . . . . . . . 14 - 10.1 Smaller Adjacent MTU . . . . . . . . . . . . . . . . . . . 14 - 10.2 Smaller Network MTU . . . . . . . . . . . . . . . . . . . 15 - 11. Receiving Fragmented Packets . . . . . . . . . . . . . . . . 15 - 12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 15 - 12.1 Requirement Discussion . . . . . . . . . . . . . . . . . . 17 - 13. Security Considerations . . . . . . . . . . . . . . . . . . 19 - 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . 20 - 15. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 20 - 16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 21 - 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 - 17.1 Normative References . . . . . . . . . . . . . . . . . . . 21 - 17.2 Informational References . . . . . . . . . . . . . . . . . 21 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 23 - Intellectual Property and Copyright Statements . . . . . . . . 24 + 4.2 Port Assignment . . . . . . . . . . . . . . . . . . . . . 8 + 4.2.1 Port Assignment Behavior . . . . . . . . . . . . . . . 8 + 4.2.2 Port Parity . . . . . . . . . . . . . . . . . . . . . 10 + 4.2.3 Port Contiguity . . . . . . . . . . . . . . . . . . . 10 + 4.3 Mapping Refresh . . . . . . . . . . . . . . . . . . . . . 11 + 5. Filtering Behavior . . . . . . . . . . . . . . . . . . . . . . 12 + 5.1 Filtering of Unsolicited Packets . . . . . . . . . . . . . 12 + 5.2 NAT Filter Refresh . . . . . . . . . . . . . . . . . . . . 13 + 6. Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . . 14 + 7. Application Level Gateways . . . . . . . . . . . . . . . . . . 15 + 8. Deterministic Properties . . . . . . . . . . . . . . . . . . . 15 + 9. ICMP Destination Unreachable Behavior . . . . . . . . . . . . 16 + 10. Fragmentation of Outgoing Packets . . . . . . . . . . . . . 17 + 10.1 Smaller Adjacent MTU . . . . . . . . . . . . . . . . . . . 17 + 10.2 Smaller Network MTU . . . . . . . . . . . . . . . . . . . 18 + 11. Receiving Fragmented Packets . . . . . . . . . . . . . . . . 18 + 12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 19 + 13. Security Considerations . . . . . . . . . . . . . . . . . . 20 + 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . 21 + 15. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 21 + 16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 22 + 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 + 17.1 Normative References . . . . . . . . . . . . . . . . . . . 23 + 17.2 Informational References . . . . . . . . . . . . . . . . . 23 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24 + Intellectual Property and Copyright Statements . . . . . . . . 25 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 on-line 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 generally to all NAT @@ -99,31 +95,31 @@ address all issues specific to all possible NAT variations. 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 capabilities. Firewalls is specifically out-of-scope of this specification: however, this specification does cover the inherent filtering aspects of NAT. - Approaches using directly signaled control off the middle boxes such - as Midcom, UPnP, or in-path signaling are out of scope. + Approaches using directly signaled control of middle boxes such as + Midcom, UPnP, or in-path signaling are out of scope. UDP Relays are out of the scope of this document. Application aspects are out of scope as the focus is strictly on the NAT itself. This document only covers the UDP Unicast aspects of NAT traversal and does not cover TCP, IPSEC, or other protocols. Since the - document is for UDP only, packet inspection below the UDP layer + document is for UDP only, packet inspection above the UDP layer (including RTP) is also out-of-scope. 2. Introduction Network Address Translators (NAT) are well known to cause very significant problems with applications that carry IP addresses in the payload RFC 3027 [5]. Applications that suffer from this problem include Voice Over IP and Multimedia Over IP (e.g., SIP [6] and H.323 [20]), as well as online gaming. @@ -213,24 +209,24 @@ 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 NAT. 4.1 Address and Port Mapping - When an internal endpoint opens an outgoing UDP 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 + 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 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 re-use 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 @@ -256,62 +252,94 @@ | | n X:x | | X:x t ++---++ e | X | r +-----+ n a l The following address and port mapping behavior are defined: - External NAT mapping is endpoint independent: - The NAT reuses the port mapping for subsequent sessions - initiated from the same internal IP address and port (X:x) to - any external IP address and port. Specifically, X1':x1' equals + 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. - External NAT mapping is endpoint address dependent: - The NAT reuses the port mapping for subsequent sessions - initiated from the same internal IP address and port (X:x) only - for sessions 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 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. - External NAT mapping is endpoint address and port dependent: - The NAT reuses the port mapping for subsequent sessions - initiated from the same internal IP address and port (X:x) only - for sessions to the same external and port. Specifically, - X1':x1' equals X2':x2' if, and only if, Y2:y2 equals Y1: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 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 "External NAT mapping is endpoint + independent" behavior. + + Justification for REQ-1: In order for UNSAF methods to work, REQ-1 + needs to be met. Failure to meet REQ-1 will force the use of a + Media 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 do not negotiate IP addresses individually (e.g., some applications using RTP and RTCP). + 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 for REQ-2: 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 which 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 because 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 RFC 3605 [9] for indicating RTP and RTCP + addresses and ports separately, but the other peer does not, there + may still be breakage in the form of lost of the RTP stream. This + requirements 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 (i.e., RFC 3261). This requirement will + therefore address a particularly nasty problem that will prevail + for a significant amount of time. + 4.2 Port Assignment 4.2.1 Port Assignment Behavior This section uses the following diagram for reference. E +-------+ +-------+ x | Y1 | | Y2 | t +---+---+ +---+---+ e @@ -341,21 +369,21 @@ the NAT needs to switch from Basic NAT to a Network Address and Port Translator (NAPT) mode (i.e., X'=X1'=X2' and x=x1=x2 but x1'!=x2', or a mapping of X1:x to X':x1' and X2:x to X':x2'). This port assignment behavior is referred to as "port preservation". It does not guarantee that the external port x' will always be the same as the internal port x but only that the NAT will preserve the port if possible. A NAT that does not attempt to make the external port numbers match the internal port numbers in any case (i.e., X1:x to X':x1', X2:x to - X':x2') is referred to as "no port preservation". + X':x2') is referred to as "No port preservation". Some NATs use "Port overloading", i.e. they always use port preservation even in the case of collision (i.e., X'=X1'=X2' and x=x1=x2=x1'=x2', or a mapping of X1:x to X':x, and X2:x to X':x). These NATs rely on the source of the response from the external endpoint (Y1:y1, Y2:y2) to forward a packet to the proper internal endpoint (X1 or X2). Port overloading fails if the two internal endpoints are establishing sessions to the same external destination. Most applications fail in some cases with "Port Overloading". It is @@ -372,148 +400,219 @@ 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 down side 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 dynamics 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. It should be noted that port 0 is reserved and must not be used. + 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 1-1023, it is + RECOMMENDED the NAT's source port also 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 also be in that range. + + Justification for REQ-3: 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 RFC 2623 for an example. + 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 RFC 3550 [8] rule that RTP use - even ports, and RTCP use odd ports. 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 RFC 3550 rule that RTP use even ports and RTCP use odd ports when - the application takes a single port number as a parameter and derives - the RTP and RTCP port pair from that number. RFC 3550 allows any - port numbers to be used for RTP and RTCP if the two numbers are - specified separately, for example using RFC 3605 [9]. However, some + 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 RFC 3605 [9]. 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 for REQ-4: This is to avoid breaking peer-to-peer + applications which do not explicity and separately specify RTP and + RTCP port numbers and which follow the RFC 3550 rule to decrement + an odd RTP port to make it even. The same considerations as per + the IP address pooling requirement apply. + 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. - These NATs do things like sequential assignment, port reservation and - so forth. 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 glare problem if many applications (or + Furthermore, there is a glaring problem if many applications (or endpoints) are trying to open mapping simultaneously. Port reservation is also problematic since it is wasteful, especially considering that a NAT can not 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 follows rules to negotiate both RTP and RTCP separately, and account for the very real possibility that the RTCP=RTP+1 rule will be broken. As this is an application requirement, it is outside of the scope of this document. -4.3 Mapping Refresh Direction +4.3 Mapping Refresh - NAT UDP mapping timeout implementations vary but include the timer's + 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 2 + minutes. + a) The value of the NAT UDP mapping timer MAY be configurable. + b) A default value of 5 minutes for the NAT UDP mapping timer is + RECOMMENDED. + + Justification for REQ-5: This requirement is to ensure that the + timeout is long enough to avoid too frequent timer refresh + packets. + a) Configuration is desirable for adapting to specific networks + and troubleshooting. + b) 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". 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". -4.4 Mapping Refresh Scope + 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". - If the mapping is refreshed for all sessions on that mapping by any - outbound traffic, the NAT is said to have a NAT Mapping Refresh Scope - of "Per mapping". If the mapping is refreshed only on a specific - session on that particular mapping by any outbound traffic, the NAT - is said to have a "Per session" NAT mapping Refresh Scope. + Justification for REQ-6: Outbound refresh is necessary for allowing + the client to keep the mapping alive. + a) Inbound refresh may be useful for applications where there is + no outgoing UDP traffic. 5. Filtering Behavior This section describes various filtering behaviors observed in NATs. 5.1 Filtering of Unsolicited Packets - When an internal endpoint opens an outgoing UDP 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. + 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. - External filtering is endpoint independent: + 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. - External filtering is endpoint address dependent: + 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 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. + 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. - External filtering is endpoint address and port dependent: + 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. + REQ-7: 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. + + OPEN ISSUE: Should REQ-7a be a SHOULD instead of a MAY? + + Justification for REQ-7: 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 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 Filetering instead of Address and Port Dependent + Filtering on a NAT (say NAT-A) also has benefits when the other + enpoint is behind a non-BEHAVE compliant NAT (say NAT-B) which + doesn't support REQ-1. When the endpoints use ICE, if NAT-A uses + Address and Port Dependent Filtering, connectivity will require a + Media Relay. However, if NAT-A uses Endpoint Indepent Filtering + or Address Dependent Filtering, ICE will ultimately find + connectivity without requiring a Media 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. + 5.2 NAT Filter Refresh The time for which a NAT filter is valid can be refreshed based on - packets that are inbound, outbound, or going either direction. In - the case of "External Filtering" of "Address dependent" or "Address - and port dependent" NATs, the scope of the refresh could include the - filters for just the particular port and destination or for all the - ports and destinations sharing the same external address and port on - the NAT. + packets that are inbound, outbound, or going either direction. 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. NAT @@ -537,32 +636,55 @@ 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 or an external source IP address and port. The hairpinning NAT behavior can therefore 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 an implementation that is expecting an external IP address and port. + REQ-8: A NAT MUST support "Hairpinning". + a) A NAT Hairpinning behavior MUST be "External source IP address + and port". + + Justification for REQ-8: 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 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 - UDP sessions such as SIP. + sessions such as SIP. Certain NATs have these ALGs turned on permanently, others have them turned on by default but let them be be turned off, and others have them turned off by default but let them be turned on. - NAT ALGs may interfere with UNSAF methods and must therefore be used - with extreme caution. + NAT ALGs may interfere with UNSAF methods or protocols that try to be + NAT-aware and must therefore be used with extreme caution. + + REQ-9: If a NAT includes ALGs, it is RECOMMENDED that all of those + ALGs (except for DNS [19] and FTP [18]) be disabled by default. + a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow + the NAT administrator to enable or disable each ALG separately. + + Justification for REQ-9: NAT ALGs may interfere with UNSAF methods. + a) This requirement allows the user to enable ALGs which are + necessary to aid operation of some applications without + enabling ALGs which interfere with 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 @@ -582,285 +704,227 @@ addresses to two different external IP addresses. For example, X1:x is going to Y1:y1 and X2:x is going to Y2:y2, where Y1:y1 does not equal Y2:y2. Some NATs will map X1:x to X1':x and will also map X2:x to X1':x. This works in the case where the NAT mapping is address port dependent. However some NATs change their behavior when this type of port reuse is happening. The NAT may look like it has NAT mappings that are independent when this type of reuse is not happening but may change to Address Port Dependent when this reuse happens. - Any NAT that changes the NAT mapping or the External Filtering at any - point in time under any particular conditions is referred to as a - "non-deterministic" NAT. NATs that don't are called "deterministic". + Any NAT that changes the NAT mapping or the External Filtering + 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. -9. ICMP Behavior + REQ-10: A NAT MUST have deterministic behavior, i.e., it MUST NOT + change the NAT mapping or the External External Filtering Behavior + at any point in time or under any particular conditions. - When a NAT sends a UDP packet towards a host on the other side of the + Justification for REQ-10: 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 towards 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 binding. A NAT which performs the functions + NOT destroy the NAT mapping. A NAT which performs the functions described in the paragraph above is referred to as "UDP Support Destination Unreachable". 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 [10] and RFC1435 [15]), and with traceroute. Blocking any ICMP message is discouraged, and blocking ICMP Destination Unreachable is strongly discouraged. -10. Fragmentation of Packets + REQ-11: It is RECOMMENDED that a NAT support ICMP Destination + Unreachable. + a) The ICMP timeout SHOULD be greater than 2 seconds. + + Justification for REQ-11: This is easy to do, is used for many things + including MTU discovery and rapid detection of error conditions, + and has no negative consequences. + a) The ICMP timeout SHOULD be greater than 2 seconds. + +10. Fragmentation of Outgoing Packets When sending a packet, there are two situations that may cause IP fragmentation for packets from the inside to the outside. It is worth noting that many IP stacks do not use Path MTU Discovery with UDP packets. 10.1 Smaller Adjacent MTU The first situation is when the MTU of the adjacent link is too small. This can occur if the NAT is doing 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. + sending large packets and small higher-priority packets, or for other + reasons. The packet could have its Don't Fragment bit set to 1 (DF=1) or 0 (DF=0). - If the packet has DF=1, the NAT should send back an ICMP message + If the packet has DF=1, the NAT SHOULD send back an ICMP message "fragmentation needed and DF set" message to the host as described in RFC 792 [13]. - If the packet has DF=0, the NAT should fragment the packet and send + If the packet has DF=0, the NAT SHOULD fragment the packet and send the fragments, in order. This is the same function a router performs in a similar situation RFC 1812 [14]. NATs that operate as described in this section are described as "Supports Fragmentation" (abbreviated SF). 10.2 Smaller Network MTU The second situation is when the MTU on some link in the middle of the network that is not the adjacent link is too small. If DF=0, the router adjacent to the small-MTU segment will fragment the packet and - forward the fragments RFC 1812. + forward the fragments as specified in RFC 1812 [14]. If DF=1, the router adjacent to the small-MTU segment will send the ICMP message "fragmentation needed and DF set" back towards the NAT. The NAT needs to forward this ICMP message to the inside host. The classification of NATs that perform this behavior is covered in the ICMP section of this document. + REQ-12: A NAT MUST support fragmentation of packets larger than link + MTU. + + Justification for REQ-12: Fragmented packets become more common with + large video packets and should continue to work. Applications can + use MTU discovery to work around this problem. + 11. Receiving Fragmented Packets - For a variety of reasons, a NAT may receive a fragmented UDP packet. - The IP packet containing the UDP header could arrive first or last + 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. - A NAT that is capable only of receiving UDP fragments in order (that - is, with the UDP 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 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 UDP fragments in or out of order - and forwarding the individual packets (or a reassembled packet) to - the internal host is referred to as "Receive Fragments Out of Order". + A NAT that is capable of receiving fragments in or out of order and + forwarding the individual packets (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-13: A NAT MUST support receiving in order fragments, so it MUST + be "Received Fragment Ordered" or "Received Fragment Out of + Order". + + a) A NAT MAY support receiving fragmented packets that are out of + order and be of type "Received Fragment Out of Order". + + Justification for REQ-13: 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. + 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 [16]. A NAT that supports all of the mandatory requirements of this specification (i.e., the "MUST"), is "compliant with this specification." A NAT that supports all of the requirements of this specification (i.e., included the "RECOMMENDED") is "fully compliant with all the mandatory and recommended requirements of this specification." - REQ-1 A NAT MUST have an "External NAT mapping is endpoint + REQ-1: A NAT MUST have an "External NAT mapping is endpoint independent" behavior. - REQ-2 It is RECOMMENDED that a NAT have an "IP address pooling" + 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 IA NAT MUST NOT have a "Port assignment" behavior of "Port + 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 1-1023, it is - RECOMMENDED the NAT's source port also 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 also be - in that range. - REQ-4 It is RECOMMENDED that a NAT have a "Port parity preservation" + RECOMMENDED the NAT's source port also 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 also 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 2 + REQ-5: A NAT UDP mapping timer MUST NOT expire in less than 2 minutes. a) The value of the NAT UDP mapping timer MAY be configurable. - b) A default value of 5 minutes for the NAT UDP mapping timer - is RECOMMENDED. - REQ-6 The NAT mapping Refresh Direction MUST have a "NAT Outbound + b) A default value of 5 minutes 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". - b) The NAT mapping Refresh Direction MUST have a "NAT refresh - method behavior" of "Per mapping" (i.e. refresh all - sessions active on a particular mapping). - REQ-7 It is RECOMMENDED that a NAT have an "External filtering is - endpoint address dependent" behavior. - REQ-8 A NAT MUST support "Hairpinning". - a) A NAT Hairpinning behavior MUST be "External source IP - address and port". - REQ-9 If a NAT includes ALGs, it is RECOMMENDED that all of those - ALGs (except for DNS [19] and FTP [18]) be disabled by - default. - - a) If a NAT includes ALGs, it is RECOMMENDED that the NAT - allow the user to enable or disable each ALG separately. - REQ-10 A NAT MUST have deterministic behavior, i.e., it MUST NOT - change the NAT mapping or the External External Filtering - Behavior at any point in time or under any particular - conditions. - REQ-11 It is RECOMMENDED that a NAT support ICMP Destination + REQ-7: 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. + OPEN ISSUE: Should REQ-7a be a SHOULD instead of a MAY? + REQ-8: A NAT MUST support "Hairpinning". + a) A NAT Hairpinning behavior MUST be "External source IP address + and port". + REQ-9: If a NAT includes ALGs, it is RECOMMENDED that all of those + ALGs (except for DNS [19] and FTP [18]) be disabled by 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-10: A NAT MUST have deterministic behavior, i.e., it MUST NOT + change the NAT mapping or the External External Filtering Behavior + at any point in time or under any particular conditions. + REQ-11: It is RECOMMENDED that a NAT support ICMP Destination Unreachable. a) The ICMP timeout SHOULD be greater than 2 seconds. - REQ-12 A NAT MUST support fragmentation of packets larger than link + REQ-12: A NAT MUST support fragmentation of packets larger than link MTU. - REQ-13 A NAT MUST support receiving in order fragments, so it MUST be - "Received Fragment Ordered" or "Received Fragment Out of + REQ-13: A NAT MUST support receiving in order fragments, so it MUST + be "Received Fragment Ordered" or "Received Fragment Out of Order". - a) A NAT MAY support receiving fragmented packets that are out - of order and be of type "Received Fragment Out of Order". - -12.1 Requirement Discussion - - This section describes why each of these requirements was chosen and - the consequences of violating any of them: - - REQ-1 In order for UNSAF methods to work, REQ-1 needs to be met. - Failure to meet REQ-1 will force the use of a Media Relay - which is very often impractical. - REQ-2 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 which 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 because 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 RFC 3605 [9] - for indicating RTP and RTCP addresses and ports separately, - but the other peer does not, there may still be breakage in - the form of lost of the RTP stream. This requirements 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 (i.e., RFC 3261). This requirement will therefore - address a particularly nasty problem that will prevail for a - significant amount of time. - - REQ-3 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 a NAT 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 RFC 2623 for an example. - REQ-4 This is to avoid breaking peer-to-peer applications which do - not explicity and separately specify RTP and RTCP port numbers - and which follow the RFC 3550 rule to decrement an odd RTP - port to make it even. The same considerations as per the IP - address pooling requirement apply. - REQ-5 This requirement is to ensure that the timeout is long enough - to avoid too frequent timer refresh packets. - a) Configuration is desirable for adapting to specific - networks and troubleshooting. - b) This default is to avoid too frequent timer refresh - packets. - REQ-6 Outbound refresh is necessary for allowing the client to keep - the mapping alive. - a) Inbound refresh may be useful for applications where there - is no outgoing UDP traffic. - b) Using the refresh on a per mapping basis avoids the need - for separate keep alive packets for all the available - sessions. - REQ-7 Filtering based on the IP address is felt to have the maximum - balance between security and usefulness. Filtering - independently of the external IP address and port is not as - secure: an unauthorized packet could get at 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 restrictive policy could - interfere with certain applications that use more than one - port. - REQ-8 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 IP address is necessary for applications - with a restrictive policy of not accepting packets from IP - addresses that differ from what is expected. - REQ-9 NAT ALGs may interfere with UNSAF methods. - a) This requirement allows the user to enable ALGs which are - necessary to aid operation of some applications without - enabling ALGs which interfere with operation of other - applications. - REQ-10 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. - REQ-11 This is easy to do, is used for many things including MTU - discovery and rapid detection of error conditions, and has no - negative consequences. - REQ-12 Fragmented packets become more common with large video packets - and should continue to work. Applications can use MTU - discovery to work around this problem. - REQ-13 See Security Considerations. + a) A NAT MAY support receiving fragmented packets that are out of + order and be of type "Received Fragment Out of Order". 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 work recommends that the timers for mapping be refreshed only on outgoing packets and does not make recommendations about whether or @@ -895,25 +959,25 @@ 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 - DoS opportunity. 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 RFC 3128 [12] and RFC - 1858 [11]. + 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 RFC 3128 [12] and RFC 1858 [11]. 14. IANA Considerations There are no IANA considerations. 15. 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 @@ -948,29 +1012,29 @@ 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. Arch-5: This work and the test results [17] provide a reference model for what any UNSAF proposal might encounter in deployed NATs. 16. Acknowledgments The editor would like to acknowledge Bryan Ford, Pyda Srisuresh and Dan Kegel for the their multiple contributions on peer-to-peer - communications accross a NAT, from which a lot of the material in - this specification is derived. + communications accross 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 and Spencer Dawkins for their important + Casner, Robert Sanders, Spencer Dawkins, Saikat Guha, Christian + Huitema, Yutaka Takeda and Paul Hoffman for their important contributions. 17. References 17.1 Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-