Network Working Group R. Gilligan Request for Comments: 2553 FreeGate Obsoletes: 2133 S. Thomson Category: Informational Bellcore J. Bound Compaq W. Stevens Consultant March 1999 Basic Socket Interface Extensions for IPv6 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 (1999). All Rights Reserved. Abstract The de facto standard application program interface (API) for TCP/IP applications is the "sockets" interface. Although this API was developed for Unix in the early 1980s it has also been implemented on a wide variety of non-Unix systems. TCP/IP applications written using the sockets API have in the past enjoyed a high degree of portability and we would like the same portability with IPv6 applications. But changes are required to the sockets API to support IPv6 and this memo describes these changes. These include a new socket address structure to carry IPv6 addresses, new address conversion functions, and some new socket options. These extensions are designed to provide access to the basic IPv6 features required by TCP and UDP applications, including multicasting, while introducing a minimum of change into the system and providing complete compatibility for existing IPv4 applications. Additional extensions for advanced IPv6 features (raw sockets and access to the IPv6 extension headers) are defined in another document [4]. Gilligan, et. al. Informational [Page 1] RFC 2553 Basic Socket Interface Extensions for IPv6 March 1999 Table of Contents 1. Introduction.................................................3 2. Design Considerations........................................3 2.1 What Needs to be Changed....................................4 2.2 Data Types..................................................5 2.3 Headers.....................................................5 2.4 Structures..................................................5 3. Socket Interface.............................................6 3.1 IPv6 Address Family and Protocol Family.....................6 3.2 IPv6 Address Structure......................................6 3.3 Socket Address Structure for 4.3BSD-Based Systems...........7 3.4 Socket Address Structure for 4.4BSD-Based Systems...........8 3.5 The Socket Functions........................................9 3.6 Compatibility with IPv4 Applications.......................10 3.7 Compatibility with IPv4 Nodes..............................10 3.8 IPv6 Wildcard Address......................................11 3.9 IPv6 Loopback Address......................................12 3.10 Portability Additions.....................................13 4. Interface Identification....................................16 4.1 Name-to-Index..............................................16 4.2 Index-to-Name..............................................17 4.3 Return All Interface Names and Indexes.....................17 4.4 Free Memory................................................18 5. Socket Options..............................................18 5.1 Unicast Hop Limit..........................................18 5.2 Sending and Receiving Multicast Packets....................19 6. Library Functions...........................................21 6.1 Nodename-to-Address Translation............................21 6.2 Address-To-Nodename Translation............................24 6.3 Freeing memory for getipnodebyname and getipnodebyaddr.....26 6.4 Protocol-Independent Nodename and Service Name Translation.26 6.5 Socket Address Structure to Nodename and Service Name......29 6.6 Address Conversion Functions...............................31 6.7 Address Testing Macros.....................................32 7. Summary of New Definitions..................................33 8. Security Considerations.....................................35 9. Year 2000 Considerations....................................35 Changes From RFC 2133..........................................35 Acknowledgments................................................38 References.....................................................39 Authors' Addresses.............................................40 Full Copyright Statement.......................................41 Gilligan, et. al. Informational [Page 2] RFC 2553 Basic Socket Interface Extensions for IPv6 March 1999 1. Introduction While IPv4 addresses are 32 bits long, IPv6 interfaces are identified by 128-bit addresses. The socket interface makes the size of an IP address quite visible to an application; virtually all TCP/IP applications for BSD-based systems have knowledge of the size of an IP address. Those parts of the API that expose the addresses must be changed to accommodate the larger IPv6 address size. IPv6 also introduces new features (e.g., traffic class and flowlabel), some of which must be made visible to applications via the API. This memo defines a set of extensions to the socket interface to support the larger address size and new features of IPv6. 2. Design Considerations There are a number of important considerations in designing changes to this well-worn API: - The API changes should provide both source and binary compatibility for programs written to the original API. That is, existing program binaries should continue to operate when run on a system supporting the new API. In addition, existing applications that are re-compiled and run on a system supporting the new API should continue to operate. Simply put, the API changes for IPv6 should not break existing programs. An additonal mechanism for implementations to verify this is to verify the new symbols are protected by Feature Test Macros as described in IEEE Std 1003.1. (Such Feature Test Macros are not defined by this RFC.) - The changes to the API should be as small as possible in order to simplify the task of converting existing IPv4 applications to IPv6. - Where possible, applications should be able to use this API to interoperate with both IPv6 and IPv4 hosts. Applications should not need to know which type of host they are communicating with. - IPv6 addresses carried in data structures should be 64-bit aligned. This is necessary in order to obtain optimum performance on 64-bit machine architectures. Because of the importance of providing IPv4 compatibility in the API, these extensions are explicitly designed to operate on machines that provide complete support for both IPv4 and IPv6. A subset of this API could probably be designed for operation on systems that support only IPv6. However, this is not addressed in this memo. Gilligan, et. al. Informational [Page 3] RFC 2553 Basic Socket Interface Extensions for IPv6 March 1999 2.1 What Needs to be Changed The socket interface API consists of a few distinct components: - Core socket functions. - Address data structures. - Name-to-address translation functions. - Address conversion functions. The core socket functions -- those functions that deal with such things as setting up and tearing down TCP connections, and sending and receiving UDP packets -- were designed to be transport independent. Where protocol addresses are passed as function arguments, they are carried via opaque pointers. A protocol-specific address data structure is defined for each protocol that the socket functions support. Applications must cast pointers to these protocol-specific address structures into pointers to the generic "sockaddr" address structure when using the socket functions. These functions need not change for IPv6, but a new IPv6-specific address data structure is needed. The "sockaddr_in" structure is the protocol-specific data structure for IPv4. This data structure actually includes 8-octets of unused space, and it is tempting to try to use this space to adapt the sockaddr_in structure to IPv6. Unfortunately, the sockaddr_in structure is not large enough to hold the 16-octet IPv6 address as well as the other information (address family and port number) that is needed. So a new address data structure must be defined for IPv6. IPv6 addresses are scoped [2] so they could be link-local, site, organization, global, or other scopes at this time undefined. To support applications that want to be able to identify a set of interfaces for a specific scope, the IPv6 sockaddr_in structure must support a field that can be used by an implementation to identify a set of interfaces identifying the scope for an IPv6 address. The name-to-address translation functions in the socket interface are gethostbyname() and gethostbyaddr(). These are left as is and new functions are defined to support IPv4 and IPv6. Additionally, the POSIX 1003.g draft [3] specifies a new nodename-to-address translation function which is protocol independent. This function can also be used with IPv4 and IPv6. Gilligan, et. al. Informational [Page 4] RFC 2553 Basic Socket Interface Extensions for IPv6 March 1999 The address conversion functions -- inet_ntoa() and inet_addr() -- convert IPv4 addresses between binary and printable form. These functions are quite specific to 32-bit IPv4 addresses. We have designed two analogous functions that convert both IPv4 and IPv6 addresses, and carry an address type parameter so that they can be extended to other protocol families as well. Finally, a few miscellaneous features are needed to support IPv6. New interfaces are needed to support the IPv6 traffic class, flow label, and hop limit header fields. New socket options are needed to control the sending and receiving of IPv6 multicast packets. The socket interface will be enhanced in the future to provide access to other IPv6 features. These extensions are described in [4]. 2.2 Data Types The data types of the structure elements given in this memo are intended to be examples, not absolute requirements. Whenever possible, data types from Draft 6.6 (March 1997) of POSIX 1003.1g are used: uintN_t means an unsigned integer of exactly N bits (e.g., uint16_t). We also assume the argument data types from 1003.1g when possible (e.g., the final argument to setsockopt() is a size_t value). Whenever buffer sizes are specified, the POSIX 1003.1 size_t data type is used (e.g., the two length arguments to getnameinfo()). 2.3 Headers When function prototypes and structures are shown we show the headers that must be #included to cause that item to be defined. 2.4 Structures When structures are described the members shown are the ones that must appear in an implementation. Additional, nonstandard members may also be defined by an implementation. As an additional precaution nonstandard members could be verified by Feature Test Macros as described in IEEE Std 1003.1. (Such Feature Test Macros are not defined by this RFC.) The ordering shown for the members of a structure is the recommended ordering, given alignment considerations of multibyte members, but an implementation may order the members differently. Gilligan, et. al. Informational [Page 5] RFC 2553 Basic Socket Interface Extensions for IPv6 March 1999 3. Socket Interface This section specifies the socket interface changes for IPv6. 3.1 IPv6 Address Family and Protocol Family A new address family name, AF_INET6, is defined in . The AF_INET6 definition distinguishes between the original sockaddr_in address data structure, and the new sockaddr_in6 data structure. A new protocol family name, PF_INET6, is defined in . Like most of the other protocol family names, this will usually be defined to have the same value as the corresponding address family name: #define PF_INET6 AF_INET6 The PF_INET6 is used in the first argument to the socket() function to indicate that an IPv6 socket is being created. 3.2 IPv6 Address Structure A new in6_addr structure holds a single IPv6 address and is defined as a result of including : struct in6_addr { uint8_t s6_addr[16]; /* IPv6 address */ }; This data structure contains an array of sixteen 8-bit elements, which make up one 128-bit IPv6 address. The IPv6 address is stored in network byte order. The structure in6_addr above is usually implemented with an embedded union with extra fields that force the desired alignment level in a manner similar to BSD implementations of "struct in_addr". Those additional implementation details are omitted here for simplicity. An example is as follows: Gilligan, et. al. Informational [Page 6] RFC 2553 Basic Socket Interface Extensions for IPv6 March 1999 struct in6_addr { union { uint8_t _S6_u8[16]; uint32_t _S6_u32[4]; uint64_t _S6_u64[2]; } _S6_un; }; #define s6_addr _S6_un._S6_u8 3.3 Socket Address Structure for 4.3BSD-Based Systems In the socket interface, a different protocol-specific data structure is defined to carry the addresses for each protocol suite. Each protocol- specific data structure is designed so it can be cast into a protocol- independent data structure -- the "sockaddr" structure. Each has a "family" field that overlays the "sa_family" of the sockaddr data structure. This field identifies the type of the data structure. The sockaddr_in structure is the protocol-specific address data structure for IPv4. It is used to pass addresses between applications and the system in the socket functions. The following sockaddr_in6 structure holds IPv6 addresses and is defined as a result of including the header: struct sockaddr_in6 { sa_family_t sin6_family; /* AF_INET6 */ in_port_t sin6_port; /* transport layer port # */ uint32_t sin6_flowinfo; /* IPv6 traffic class & flow info */ struct in6_addr sin6_addr; /* IPv6 address */ uint32_t sin6_scope_id; /* set of interfaces for a scope */ }; This structure is designed to be compatible with the sockaddr data structure used in the 4.3BSD release. The sin6_family field identifies this as a sockaddr_in6 structure. This field overlays the sa_family field when the buffer is cast to a sockaddr data structure. The value of this field must be AF_INET6. The sin6_port field contains the 16-bit UDP or TCP port number. This field is used in the same way as the sin_port field of the sockaddr_in structure. The port number is stored in network byte order. Gilligan, et. al. Informational [Page 7] RFC 2553 Basic Socket Interface Extensions for IPv6 March 1999 The sin6_flowinfo field is a 32-bit field that contains two pieces of information: the traffic class and the flow label. The contents and interpretation of this memb