📄 rfc3022.txt
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Network Working Group P. SrisureshRequest for Comments: 3022 Jasmine NetworksObsoletes: 1631 K. EgevangCategory: Informational Intel Corporation January 2001 Traditional IP Network Address Translator (Traditional NAT)Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved.Preface The NAT operation described in this document extends address translation introduced in RFC 1631 and includes a new type of network address and TCP/UDP port translation. In addition, this document corrects the Checksum adjustment algorithm published in RFC 1631 and attempts to discuss NAT operation and limitations in detail.Abstract Basic Network Address Translation or Basic NAT is a method by which IP addresses are mapped from one group to another, transparent to end users. Network Address Port Translation, or NAPT is a method by which many network addresses and their TCP/UDP (Transmission Control Protocol/User Datagram Protocol) ports are translated into a single network address and its TCP/UDP ports. Together, these two operations, referred to as traditional NAT, provide a mechanism to connect a realm with private addresses to an external realm with globally unique registered addresses.1. Introduction The need for IP Address translation arises when a network's internal IP addresses cannot be used outside the network either for privacy reasons or because they are invalid for use outside the network. Network topology outside a local domain can change in many ways. Customers may change providers, company backbones may be reorganized, or providers may merge or split. Whenever external topology changesSrisuresh & Egevang Informational [Page 1] RFC 3022 Traditional NAT January 2001 with time, address assignment for nodes within the local domain must also change to reflect the external changes. Changes of this type can be hidden from users within the domain by centralizing changes to a single address translation router. Basic Address translation would (in many cases, except as noted in [NAT-TERM] and section 6 of this document) allow hosts in a private network to transparently access the external network and enable access to selective local hosts from the outside. Organizations with a network setup predominantly for internal use, with a need for occasional external access are good candidates for this scheme. Many Small Office, Home Office (SOHO) users and telecommuting employees have multiple Network nodes in their office, running TCP/UDP applications, but have a single IP address assigned to their remote access router by their service provider to access remote networks. This ever increasing community of remote access users would be benefited by NAPT, which would permit multiple nodes in a local network to simultaneously access remote networks using the single IP address assigned to their router. There are limitations to using the translation method. It is mandatory that all requests and responses pertaining to a session be routed via the same NAT router. One way to ascertain this would be to have NAT based on a border router that is unique to a stub domain, where all IP packets are either originated from the domain or destined to the domain. There are other ways to ensure this with multiple NAT devices. For example, a private domain could have two distinct exit points to different providers and the session flow from the hosts in a private network could traverse through whichever NAT device has the best metric for an external host. When one of the NAT routers fail, the other could route traffic for all the connections. There is however a caveat with this approach, in that, rerouted flows could fail at the time of switchover to the new NAT router. A way to overcome this potential problem is that the routers share the same NAT configuration and exchange state information to ensure a fail- safe backup for each other. Address translation is application independent and often accompanied by application specific gateways (ALGs) to perform payload monitoring and alterations. FTP is the most popular ALG resident on NAT devices. Applications requiring ALG intervention must not have their payload encoded, as doing that would effectively disables the ALG, unless the ALG has the key to decrypt the payload. This solution has the disadvantage of taking away the end-to-end significance of an IP address, and making up for it with increased state in the network. As a result, end-to-end IP network levelSrisuresh & Egevang Informational [Page 2] RFC 3022 Traditional NAT January 2001 security assured by IPSec cannot be assumed to end hosts, with a NAT device enroute. The advantage of this approach however is that it can be installed without changes to hosts or routers. Definition of terms such as "Address Realm", "Transparent Routing", "TU Ports", "ALG" and others, used throughout the document, may be found in [NAT-TERM].2. Overview of traditional NAT The Address Translation operation presented in this document is referred to as "Traditional NAT". There are other variations of NAT that will not be explored in this document. Traditional NAT would allow hosts within a private network to transparently access hosts in the external network, in most cases. In a traditional NAT, sessions are uni-directional, outbound from the private network. Sessions in the opposite direction may be allowed on an exceptional basis using static address maps for pre-selected hosts. 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 address and Transport identifier (such as TCP/UDP port or ICMP query ID). Unless mentioned otherwise, Address Translation or NAT throughout this document will pertain to traditional NAT, namely Basic NAT as well as NAPT. Only the stub border routers as described in figure 1 below may be configured to perform address translation. \ | / . / +---------------+ WAN . +-----------------+/ |Regional Router|----------------------|Stub Router w/NAT|--- +---------------+ . +-----------------+\ . | \ . | LAN . --------------- Stub border Figure 1: Traditional NAT Configuration2.1 Overview of Basic NAT Basic NAT operation is as follows. A stub domain with a set of private network addresses could be enabled to communicate with external network by dynamically mapping the set of private addresses to a set of globally valid network addresses. If the number of local nodes are less than or equal to addresses in the global set, each local address is guaranteed a global address to map to. Otherwise, nodes allowed to have simultaneous access to external network areSrisuresh & Egevang Informational [Page 3] RFC 3022 Traditional NAT January 2001 limited by the number of addresses in global set. Individual local addresses may be statically mapped to specific global addresses to ensure guaranteed access to the outside or to allow access to the local host from external hosts via a fixed public address. Multiple simultaneous sessions may be initiated from a local node, using the same address mapping. Addresses inside a stub domain are local to that domain and not valid outside the domain. Thus, addresses inside a stub domain can be reused by any other stub domain. For instance, a single Class A address could be used by many stub domains. At each exit point between a stub domain and backbone, NAT is installed. If there is more than one exit point it is of great importance that each NAT has the same translation table. For instance, in the example of figure 2, both stubs A and B internally use class A private address block 10.0.0.0/8 [RFC 1918]. Stub A's NAT is assigned the class C address block 198.76.29.0/24, and Stub B's NAT is assigned the class C address block 198.76.28.0/24. The class C addresses are globally unique no other NAT boxes can use them. \ | / +---------------+ |Regional Router| +---------------+ WAN | | WAN | | Stub A .............|.... ....|............ Stub B | | {s=198.76.29.7,^ | | v{s=198.76.29.7, d=198.76.28.4}^ | | v d=198.76.28.4} +-----------------+ +-----------------+ |Stub Router w/NAT| |Stub Router w/NAT| +-----------------+ +-----------------+ | | | LAN LAN | ------------- ------------- | | {s=10.33.96.5, ^ | | v{s=198.76.29.7, d=198.76.28.4}^ +--+ +--+ v d=10.81.13.22} |--| |--| /____\ /____\ 10.33.96.5 10.81.13.22 Figure 2: Basic NAT OperationSrisuresh & Egevang Informational [Page 4] RFC 3022 Traditional NAT January 2001 When stub A host 10.33.96.5 wishes to send a packet to stub B host 10.81.13.22, it uses the globally unique address 198.76.28.4 as destination, and sends the packet to its primary router. The stub router has a static route for net 198.76.0.0 so the packet is forwarded to the WAN-link. However, NAT translates the source address 10.33.96.5 of the IP header to the globally unique 198.76.29.7 before the packet is forwarded. Likewise, IP packets on the return path go through similar address translations. Notice that this requires no changes to hosts or routers. For instance, as far as the stub A host is concerned, 198.76.28.4 is the address used by the host in stub B. The address translations are transparent to end hosts in most cases. Of course, this is just a simple example. There are numerous issues to be explored.2.2. Overview of NAPT Say, an organization has a private IP network and a WAN link to a service provider. The private network's stub router is assigned a globally valid address on the WAN link and the remaining nodes in the organization have IP addresses that have only local significance. In such a case, nodes on the private network could be allowed simultaneous access to the external network, using the single registered IP address with the aid of NAPT. NAPT would allow mapping of tuples of the type (local IP addresses, local TU port number) to tuples of the type (registered IP address, assigned TU port number). This model fits the requirements of most Small Office Home Office (SOHO) groups to access external network using a single service provider assigned IP address. This model could be extended to allow inbound access by statically mapping a local node per each service TU port of the registered IP address. In the example of figure 3 below, stub A internally uses class A address block 10.0.0.0/8. The stub router's WAN interface is assigned an IP address 138.76.28.4 by the service provider.Srisuresh & Egevang Informational [Page 5] RFC 3022 Traditional NAT January 2001 \ | / +-----------------------+ |Service Provider Router| +-----------------------+ WAN | | Stub A .............|.... | ^{s=138.76.28.4,sport=1024, | v{s=138.76.29.7, sport = 23, ^ d=138.76.29.7,dport=23} | v d=138.76.28.4, dport = 1024} +------------------+ |Stub Router w/NAPT| +------------------+ |⌨️ 快捷键说明
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