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Network Working Group                                         K. EgevangRequest for Comments: 1631                           Cray CommunicationsCategory: Informational                                       P. Francis                                                                     NTT                                                                May 1994The IP Network Address Translator (NAT)Status of this Memo   This memo provides information for the Internet community.  This memo   does not specify an Internet standard of any kind.  Distribution of   this memo is unlimited.Abstract   The two most compelling problems facing the IP Internet are IP   address depletion and scaling in routing. Long-term and short-term   solutions to these problems are being developed. The short-term   solution is CIDR (Classless InterDomain Routing). The long-term   solutions consist of various proposals for new internet protocols   with larger addresses.   It is possible that CIDR will not be adequate to maintain the IP   Internet until the long-term solutions are in place. This memo   proposes another short-term solution, address reuse, that complements   CIDR or even makes it unnecessary. The address reuse solution is to   place Network Address Translators (NAT) at the borders of stub   domains. Each NAT box has a table consisting of pairs of local IP   addresses and globally unique addresses. The IP addresses inside the   stub domain are not globally unique. They are reused in other   domains, thus solving the address depletion problem. The globally   unique IP addresses are assigned according to current CIDR address   allocation schemes. CIDR solves the scaling problem. The main   advantage of NAT is that it can be installed without changes to   routers or hosts. This memo presents a preliminary design for NAT,   and discusses its pros and cons.Acknowledgments   This memo is based on a paper by Paul Francis (formerly Tsuchiya) and   Tony Eng, published in Computer Communication Review, January 1993.   Paul had the concept of address reuse from Van Jacobson.   Kjeld Borch Egevang edited the paper to produce this memo and   introduced adjustment of sequence-numbers for FTP. Thanks to Jacob   Michael Christensen for his comments on the idea and text (we thoughtEgevang & Francis                                               [Page 1]

RFC 1631               Network Address Translator               May 1994   for a long time, we were the only ones who had had the idea).1. Introduction   The two most compelling problems facing the IP Internet are IP   address depletion and scaling in routing. Long-term and short-term   solutions to these problems are being developed. The short-term   solution is CIDR (Classless InterDomain Routing) [2]. The long-term   solutions consist of various proposals for new internet protocols   with larger addresses.   Until the long-term solutions are ready an easy way to hold down the   demand for IP addresses is through address reuse. This solution takes   advantage of the fact that a very small percentage of hosts in a stub   domain are communicating outside of the domain at any given time. (A   stub domain is a domain, such as a corporate network, that only   handles traffic originated or destined to hosts in the domain).   Indeed, many (if not most) hosts never communicate outside of their   stub domain. Because of this, only a subset of the IP addresses   inside a stub domain, need be translated into IP addresses that are   globally unique when outside communications is required.   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. There are various work-arounds that minimize   the potential pitfalls of this. Indeed, connection-oriented protocols   are essentially doing address reuse at every hop.   The huge advantage of this approach is that it can be installed   incrementally, without changes to either hosts or routers. (A few   unusual applications may require changes). As such, this solution can   be implemented and experimented with quickly. If nothing else, this   solution can serve to provide temporarily relief while other, more   complex and far-reaching solutions are worked out.2. Overview of NAT   The design presented in this memo is called NAT, for Network Address   Translator. NAT is a router function that can be configured as shown   in figure 1. Only the stub border router requires modifications.   NAT's basic operation is as follows. The 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.Egevang & Francis                                               [Page 2]

RFC 1631               Network Address Translator               May 1994        \ | /                 .                                /   +---------------+  WAN     .           +-----------------+/   |Regional Router|----------------------|Stub Router w/NAT|---   +---------------+          .           +-----------------+\                              .                      |         \                              .                      |  LAN                              .               ---------------                        Stub border                      Figure 1: NAT Configuration   For instance, in the example of figure 2, both stubs A and B   internally use class A address 10.0.0.0. Stub A's NAT is assigned the   class C address 198.76.29.0, and Stub B's NAT is assigned the class C   address 198.76.28.0. 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 Operation   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 it's 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 with the globally unique 198.76.29.7Egevang & Francis                                               [Page 3]

RFC 1631               Network Address Translator               May 1994   before the package 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   completely transparent.   Of course, this is just a simple example. There are numerous issues   to be explored. In the next section, we discuss various aspects of   NAT.3. Various Aspects of NAT3.1 Address SpacesPartitioning of Reusable and Non-reusable Addresses   For NAT to operate properly, it is necessary to partition the IP   address space into two parts - the reusable addresses used internal   to stub domains, and the globally unique addresses. We call the   reusable address local addresses, and the globally unique addresses   global addresses. Any given address must either be a local address or   a global address. There is no overlap.   The problem with overlap is the following. Say a host in stub A   wished to send packets to a host in stub B, but the local addresses   of stub B overlapped the local addressees of stub A. In this case,   the routers in stub A would not be able to distinguish the global   address of stub B from its own local addresses.Initial Assignment of Local and Global Addresses   A single class A address should be allocated for local networks. (SeeRFC 1597 [3].)  This address could then be used for internets with no   connection to the Internet. NAT then provides an easy way to change   an experimental network to a "real" network by translating the   experimental addresses to globally unique Internet addresses.   Existing stubs which have unique addresses assigned internally, but   are running out of them, can change addresses subnet by subnet to   local addresses. The freed adresses can then be used by NAT for   external communications.Egevang & Francis                                               [Page 4]

RFC 1631               Network Address Translator               May 19943.2 Routing Across NAT   The router running NAT should never advertise the local networks to   the backbone. Only the networks with global addresses may be known   outside the stub. However, global information that NAT receives from   the stub border router can be advertised in the stub the usual way.Private Networks that Span Backbones   In many cases, a private network (such as a corporate network) will   be spread over different locations and will use a public backbone for   communications between those locations. In this case, it is not   desirable to do address translation, both because large numbers of   hosts may want to communicate across the backbone, thus requiring   large address tables, and because there will be more applications   that depend on configured addresses, as opposed to going to a name   server. We call such a private network a backbone-partitioned stub.   Backbone-partitioned stubs should behave as though they were a non-   partitioned stub. That is, the routers in all partitions should   maintain routes to the local address spaces of all partitions. Of   course, the (public) backbones do not maintain routes to any local   addresses. Therefore, the border routers must tunnel through the   backbones using encapsulation. To do this, each NAT box will set   aside one global address for tunneling. When a NAT box x in stub   partition X wishes to deliver a packet to stub partition Y, it will   encapsulate the packet in an IP header with destination address set   to the global address of NAT box y that has been reserved for   encapsulation. When NAT box y receives a packet with that destination   address, it decapsulates the IP header and routes the packet   internally.3.3 Header Manipulations   In addition to modifying the IP address, NAT must modify the IP   checksum and the TCP checksum. Remember, TCP's checksum also covers a   pseudo header which contains the source and destination address. NAT   must also look out for ICMP and FTP and modify the places where the   IP address appears. There are undoubtedly other places, where   modifications must be done. Hopefully, most such applications will be   discovered during experimentation with NAT.   The checksum modifications to IP and TCP are simple and efficient.   Since both use a one's complement sum, it is sufficient to calculate   the arithmetic difference between the before-translation and after-   translation addresses and add this to the checksum. The only tricky   part is determining whether the addition resulted in a wrap-around   (in either the positive or negative direction) of the checksum. IfEgevang & Francis                                               [Page 5]

RFC 1631               Network Address Translator               May 1994   so, 1 must be added or subtracted to satisfy the one's complement   arithmetic. Sample code (in C) for this is as follows:   void checksumadjust(unsigned char *chksum, unsigned char *optr,   int olen, unsigned char *nptr, int nlen)   /* assuming: unsigned char is 8 bits, long is 32 bits.     - chksum points to the chksum in the packet     - optr points to the old data in the packet     - nptr points to the new data in the packet   */   {     long x, old, new;     x=chksum[0]*256+chksum[1];     x=~x;     while (olen) {       if (olen==1) {         old=optr[0]*256+optr[1];         x-=old & 0xff00;         if (x<=0) { x--; x&=0xffff; }         break;       }       else {         old=optr[0]*256+optr[1]; optr+=2;         x-=old & 0xffff;         if (x<=0) { x--; x&=0xffff; }         olen-=2;       }     }     while (nlen) {       if (nlen==1) {         new=nptr[0]*256+nptr[1];         x+=new & 0xff00;         if (x & 0x10000) { x++; x&=0xffff; }         break;       }       else {         new=nptr[0]*256+nptr[1]; nptr+=2;         x+=new & 0xffff;         if (x & 0x10000) { x++; x&=0xffff; }         nlen-=2;       }     }     x=~x;     chksum[0]=x/256; chksum[1]=x & 0xff;   }Egevang & Francis                                               [Page 6]

RFC 1631               Network Address Translator               May 1994   The arguments to the File Transfer Protocol (FTP) PORT command   include an IP address (in ASCII!). If the IP address in the PORT   command is local to the stub domain, then NAT must substitute this.   Because the address is encoded in ASCII, this may result in a change   in the size of the packet (for instance 10.18.177.42 is 12 ASCII   characters, while 193.45.228.137 is 14 ASCII characters). If the new   size is the same as the previous, only the TCP checksum needs   adjustment (again). If the new size is less than the previous, ASCII   zeroes may be inserted, but this is not guaranteed to work. If the   new size is larger than the previous, TCP sequence numbers must be   changed too.   A special table is used to correct the TCP sequence and acknowledge   numbers with source port FTP or destination port FTP. The table   entries should have source, destination, source port, destination   port, initial sequence number, delta for sequence numbers and a   timestamp. New entries are created only when FTP PORT commands are   seen. The initial sequence numbers are used to find out if the   sequence number of a packet is before or after the last FTP PORT   command (delta may be increased for every FTP PORT command). Sequence   numbers are incremented and acknowledge numbers are decremented. If   the FIN bit is set in one of the packets, the associated entry may be   deleted soon after (1 minute should be safe). Entries that have not   been used for e.g. 24 hours should be safe to delete too.   The sequence number adjustment must be coded carefully, not to harm   performance for TCP in general. Of course, if the FTP session is   encrypted, the PORT command will fail.   If an ICMP message is passed through NAT, it may require two address   modifications and three checksum modifications. This is because most   ICMP messages contain part of the original IP packet in the body.   Therefore, for NAT to be completely transparent to the host, the IP   address of the IP header embedded in the data part of the ICMP packet   must be modified, the checksum field of the same IP header must   correspondingly be modified, and the ICMP header checksum must be   modified to reflect the changes to the IP header and checksum in the   ICMP body. Furthermore, the normal IP header must also be modified as   already described.   It is not entirely clear if the IP header information in the ICMP   part of the body really need to be modified. This depends on whether   or not any host code actually looks at this IP header information.   Indeed, it may be useful to provide the exact header seen by the   router or host that issued the ICMP message to aid in debugging. In   any event, no modifications are needed for the Echo and Timestamp   messages, and NAT should never need to handle a Redirect message.Egevang & Francis                                               [Page 7]

RFC 1631               Network Address Translator               May 1994   SNMP messages could be modified, but it is even more dubious than for   ICMP messages that it will be necessary.Applications with IP-address Content   Any application that carries (and uses) the IP address inside the   application will not work through NAT unless NAT knows of such   instances and does the appropriate translation. It is not possible or   even necessarily desirable for NAT to know of all such applications.   And, if encryption is used then it is impossible for NAT to make the   translation.   It may be possible for such systems to avoid using NAT, if the hosts   in which they run are assigned global addresses. Whether or not this   can work depends on the capability of the intra-domain routing   algorithm and the internal topology. This is because the global   address must be advertised in the intra-domain routing algorithm.   With a low-feature routing algorithm like RIP, the host may require   its own class C address space, that must not only be advertised   internally but externally as well (thus hurting global scaling). With   a high-feature routing algorithm like OSPF, the host address can be   passed around individually, and can come from the NAT table.Privacy, Security, and Debugging Considerations   Unfortunately, NAT reduces the number of options for providing   security. With NAT, nothing that carries an IP address or information   derived from an IP address (such as the TCP-header checksum) can be   encrypted. While most application-level encryption should be ok, this   prevents encryption of the TCP header.   On the other hand, NAT itself can be seen as providing a kind of   privacy mechanism. This comes from the fact that machines on the   backbone cannot monitor which hosts are sending and receiving traffic   (assuming of course that the application data is encrypted).   The same characteristic that enhances privacy potentially makes   debugging problems (including security violations) more difficult. If   a host is abusing the Internet is some way (such as trying to attack   another machine or even sending large amounts of junk mail or   something) it is more difficult to pinpoint the source of the trouble   because the IP address of the host is hidden.Egevang & Francis                                               [Page 8]

RFC 1631               Network Address Translator               May 19944. Conclusions   NAT may be a good short term solution to the address depletion and   scaling problems. This is because it requires very few changes and   can be installed incrementally. NAT has several negative   characteristics that make it inappropriate as a long term solution,   and may make it inappropriate even as a short term solution. Only   implementation and experimentation will determine its   appropriateness.The negative characteristics are:1. It requires a sparse end-to-end traffic matrix. Otherwise, the NAT   tables will be large, thus giving lower performance. While the   expectation is that end-to-end traffic matrices are indeed sparse,   experience with NAT will determine whether or not they are. In any   event, future applications may require a rich traffic matrix (for   instance, distributed resource discovery), thus making long-term use   of NAT unattractive.2. It increases the probability of mis-addressing.3. It breaks certain applications (or at least makes them more difficult   to run).4. It hides the identity of hosts. While this has the benefit of   privacy, it is generally a negative effect.5. Problems with SNMP, DNS, ... you name it.Current Implementations   Paul and Tony implemented an experimental prototype of NAT on public   domain KA9Q TCP/IP software [1]. This implementation manipulates   addresses and IP checksums.   Kjeld implemented NAT in a Cray Communications IP-router. The   implementation was tested with Telnet and FTP. This implementation   manipulates addresses, IP checksums, TCP sequence/acknowledge numbers   and FTP PORT commands.   The prototypes has demonstrated that IP addresses can be translated   transparently to hosts within the limitations described in this   paper.Egevang & Francis                                               [Page 9]

RFC 1631               Network Address Translator               May 1994REFERENCES   [1] Karn, P., "KA9Q", anonymous FTP from ucsd.edu       (hamradio/packet/ka9q/docs).   [2] Fuller, V., Li, T., and J. Yu, "Classless Inter-Domain Routing       (CIDR) an Address Assignment and Aggregation Strategy",RFC 1519,       BARRNet, cisco, Merit, OARnet, September 1993.   [3] Rekhter, Y., Moskowitz, B., Karrenberg, D., and G. de Groot,       "Address Allocation for Private Internets",RFC 1597, T.J. Watson       Research Center, IBM Corp., Chrysler Corp., RIPE NCC, March 1994.Security Considerations   Security issues are not discussed in this memo.Authors' Addresses   Kjeld Borch Egevang   Cray Communications   Smedeholm 12-14   DK-2730 Herlev   Denmark   Phone: +45 44 53 01 00   EMail: kbe@craycom.dk   Paul Francis   NTT Software Lab   3-9-11 Midori-cho Musashino-shi   Tokyo 180 Japan   Phone: +81-422-59-3843   Fax +81-422-59-3765   EMail: francis@cactus.ntt.jpEgevang & Francis                                              [Page 10]