ipv4 到 ipv6 的迁移

Porting applications to IPv6 HowToAuthor: Eva M. Castro

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Some changes are needed to adapt the socket API for IPv6 support: a new socket address structure to carry IPv6 addresses, new address conversion functions and several new socket options that are developed in RFC-2553.

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. Access to more advanced features (raw sockets, header configuration, etc.) is addressed in RFC-2292.

This document includes some examples of code porting, used to illustrate the required changes in the client and server components. Migration guidelines are valid for any programming language, however for simplicity, application porting examples are provided only in C language. All these examples have been tested in a SuSE Linux 7.3 distribution, kernel version 2.4.10.

Download all example source code, .

Functions provided by socket API use socket address structures to determine the communication service access point. Since different protocols can handle socket functions, a generic socket address structure is used as argument of these functions for any of the supported communication protocol families, sockaddr.

struct sockaddr { sa_family_t sa_family; /* Address family */ char sa_data[14]; /* protocol-specific address */ };

Although socket functions handle generic socket address structure, developers must fill the adequate socket address structure according to the communication protocol they are using to establish the socket. Concretely, the IPv4 sockets use the following structure, sockaddr_in:

typedef uint32_t in_addr_t; struct in_addr { in_addr_t s_addr; /* IPv4 address */ }; struct sockaddr_in { sa_family_t sin_family; /* AF_INET */ in_port_t sin_port; /* Port number. */ struct in_addr sin_addr; /* Internet address. */ /* Pad to size of `struct sockaddr'. */ unsigned char sin_zero[sizeof (struct sockaddr) - sizeof (sa_family_t) - sizeof (in_port_t) - sizeof (struct in_addr)]; };

And the IPv6 sockets use the following structure, sockaddr_in6, with a new address family AF_INET6:

struct in6_addr { union { uint8_t u6_addr8[16]; uint16_t u6_addr16[8]; uint32_t u6_addr32[4]; } in6_u; #define s6_addr in6_u.u6_addr8 #define s6_addr16 in6_u.u6_addr16 #define s6_addr32 in6_u.u6_addr32 }; struct sockaddr_in6 { sa_family_t sin6_family; /* AF_INET6 */ in_port_t sin6_port; /* Transport layer port # */ uint32_t sin6_flowinfo; /* IPv6 flow information */ struct in6_addr sin6_addr; /* IPv6 address */ uint32_t sin6_scope_id; /* IPv6 scope-id */ };

The sockaddr_in or sockaddr_in6 structures are utilized when using respectively IPv4 or IPv6. Existing applications are written assuming IPv4, using sockaddr_in structure. They can be easily ported changing this structure by sockaddr_in6. However, when writing portable code, it is preferable to eliminate protocol version dependencies from source code. There is a new data structure, sockaddr_storage large enough to store all supported protocol-specific address structures and adequately aligned to be cast to the a specific address structure.

/* Structure large enough to hold any socket address (with the historical exception of AF_UNIX). 128 bytes reserved. */ #if ULONG_MAX > 0xffffffff # define __ss_aligntype __uint64_t #else # define __ss_aligntype __uint32_t #endif #define _SS_SIZE 128 #define _SS_PADSIZE (_SS_SIZE - (2 * sizeof (__ss_aligntype))) struct sockaddr_storage { sa_family_t ss_family; /* Address family */ __ss_aligntype __ss_align; /* Force desired alignment. */ char __ss_padding[_SS_PADSIZE]; };

Hence, portable applications should use sockaddr_storage structure to store their addresses, IPv4 or IPv6 ones. This new structure hides the specific socket address structure that the application is using. 
The socket API has not been changed since it handles generic address structures, independent from the protocol it is using. However, applications should change the arguments used to call the socket functions. First, applications should allocate enough memory to store the appropriate socket address structure. And, before calling the socket functions, the specific socket address structure should be cast to the generic one, which is accepted by the socket functions as an argument.

int socket (int domain, int type, int protocol); int listen (int s, int backlog); ssize_t write (int fd, const void *buf, size_t count); int send (int s, const void *msg, size_t len, int flags); int sendmsg (int s, const struct msghdr *msg, int flags); ssize_t read (int fd, void *buf, size_t count); int recv (int s, void *buf, size_t len, int flags); int recvmsg (int s, struct msghdr *msg, int flags); int close (int fd); int shutdown(int s, int how);

SOCKET ADDRESS STRUCTURE PASSED FROM APPLICATION TO KERNEL

int bind (int sockfd, struct sockaddr *my_addr, socklen_t addrlen); int connect(int sockfd, const struct sockaddr *serv_addr, socklen_t addrlen); int sendto (int s, const void *msg, size_t len, int flags, const struct sockaddr *to, socklen_t tolen);

SOCKET ADDRESS STRUCTURE PASSED FROM KERNEL TO APPLICATION

int accept (int s, struct sockaddr *addr, socklen_t *addrlen); int recvfrom (int s, void *buf, size_t len, int flags, struct sockaddr *from, socklen_t *fromlen); int getpeername(int s, struct sockaddr *name, socklen_t *namelen); int getsockname(int s, struct sockaddr *name, socklen_t *namelen);

Required modifications when porting to IPv6 When porting to IPv6, some modifications related to the socket API are required in the network applications.Three modification types to be made when porting source code to IPv6 have been identified:

• Creating a socket.
• Socket address structure passed from application to kernel.
• Socket address structure passed from kernel to application.

These are the three kind of operations to be changed in the source code. There are some examples below of these types of operation.

• Creating a socket
  The difference between creating an IPv4 and an IPv6 socket is the value of the family argument in the socket call.
  IPv4 source code:
  

  socket(PF_INET, SOCK_STREAM, 0); /* TCP socket */ socket(PF_INET, SOCK_DGRAM, 0); /* UDP socket */

  IPv6 source code: 
  

  socket(PF_INET6, SOCK_STREAM, 0); /* TCP socket */ socket(PF_INET6, SOCK_DGRAM, 0); /* UDP socket */

• Socket address structure passed from application to kernel
  Socket address structure is filled before calling the socket function.
  IPv4 source code:
  

  struct sockaddr_in addr; socklen_t addrlen = sizeof(addr); /* fill addr structure using an IPv4 address before calling socket function */ bind(sockfd,(struct sockaddr *)&addr, addrlen);

  IPv6 source code:

  struct sockaddr_in6 addr; socklen_t addrlen = sizeof(addr); /* fill addr structure using an IPv6 address before calling socket function */ bind(sockfd,(struct sockaddr *)&addr, addrlen);

  Portable source code:

  struct sockaddr_storage addr; socklen_t addrlen; /* fill addr structure using an IPv4/IPv6 address and fill addrlen before calling socket function */ bind(sockfd,(struct sockaddr *)&addr, addrlen);

• Socket address structure passed from kernel to application
  When calling this kind of socket functions, the socket address structure is filled in with the address of the source entity.
  IPv4 source code:
  

  struct sockaddr_in addr; socklen_t addrlen = sizeof(addr); accept(sockfd,(struct sockaddr *)&addr, &addrlen); /* addr structure contains an IPv4 address */

  IPv6 source code:

  struct sockaddr_in6 addr; socklen_t addrlen = sizeof(addr); accept(sockfd,(struct sockaddr *)&addr, &addrlen); /* addr structure contains an IPv4 address */

  Portable source code:

  struct sockaddr_storage addr; socklen_t addrlen = sizeof(addr); accept(sockfd,(struct sockaddr *)&addr, &addrlen); /* addr structure contains an IPv4/IPv6 address addrlen contains the size of the addr structure returned */

The address conversion functions convert between binary and text address representation. Binary representation is the network byte ordered binary value which is stored in the socket address structure and the text representation, named presentation, is an ASCII string.
The IPv4 address conversion functions are the following ones:

/* From text to IPv4 binary representation */ int inet_aton (const char *cp, struct in_addr *inp); in_addr_t inet_addr( const char *cp); /* From IPv4 binary to text representation */ char *inet_ntoa(struct in_addr in);

The new address conversion functions which work with both IPv4 and IPv6 addresses are the following ones:

/* From presentation to IPv4/IPv6 binary representation */ int inet_pton(int family, const char *src, void *dst); /* From IPv4/IPv6 binary to presentation */ const char *inet_ntop(int family, const void *src, char *dst, size_t cnt);

IPv4 source code example:

struct sockaddr_in addr; char *straddr; memset(&addr, 0, sizeof(addr)); addr.sin_family = AF_INET; // family addr.sin_port = htons(MYPORT); // port, networt byte order /* from text to binary representation */ inet_aton("138.4.2.10", &(addr.sin_addr)); /* from binary to text representation */ straddr = inet_ntoa(addr.sin_addr);

IPv6 source code example:

struct sockaddr_in6 addr; char straddr[INET6_ADDRSTRLEN]; memset(&addr, 0, sizeof(addr)); addr.sin6_family = AF_INET6; // family addr.sin6_port = htons(MYPORT); // port, networt byte order /* from presentation to binary representation */ inet_pton(AF_INET6, "2001:720:1500:1::a100", &(addr.sin6_addr)); /* from binary representation to presentation */ inet_ntop(AF_INET6, &addr.sin6_addr, straddr, sizeof(straddr));

Applications should use names instead of addresses for hosts. Names are easier to remember and remain the same, but numeric addresses could change more frequently.

From applications point of view the name resolution is a system-independent process. Applications call functions in a system library known as the resolver, typically gethostbyname and gethostbyaddr, which is linked into the application when the application is built. The resolver code is the burden of making the resolution dependent of the system configuration.

Figure. Address Conversion Functions.

There are two new functions to make name and address conversions protocol independent, getaddrinfo and getnameinfo. Besides, the use of these new ones instead of gethostbyname and gethostbyaddr is recommended because the latter are not normally reentrant and could provoke problems in threaded applications.

The getaddrinfo function returns a linked list of addrinfo structures which contains information requested for a specific set of hostname, service and additional information stored in an addrinfo structure.

struct addrinfo { int ai_flags; /* AI_PASSIVE, AI_CANONNAME */ int ai_family; /* AF_UNSPEC, AF_INET, AF_INET6 */ int ai_socktype; /* SOCK_STREAM, SOCK_DGRAM ... */ int ai_protocol; /* IPPROTO_IP, IPPROTO_IPV6 */ size_t ai_addrlen; /* length of ai_addr */ struct sockaddr ai_addr; /* socket address structure */ char ai_canonname; /* cannonical name */ struct addrinfo ai_next; /* next addrinfo structure */ }; /* function to get socket address structures */ int getaddrinfo(const char *node, const char *service, const struct addrinfo *hints, struct addrinfo **res);

When writing a typical client application, node and service are normally specified. When writing a server application, they both can be specified too, but in many cases only service is specified allowing clients to connect to any node interfaces.

Applications should examine the linked list returned by getaddrinfo to use the adequate structure. In some cases, not all the addresses returned by this function can be used to create a socket.

The getaddrinfo function allocates a set of resources for the returned linked list. The freeaddrinfo function frees these resources.

/* function to free the resources allocated by getaddrinfo */ void freeaddrinfo(struct addrinfo *res);

Next, an example of getaddrinfo and freeaddrinfo usage.

n = getaddrinfo(hostname, service, &hints, &res); /* Try open socket with each address getaddrinfo returned, until getting a valid socket. */ resave = res; while (res) { sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol); if (!(sockfd < 0)) break; res = res->ai_next; } freeaddrinfo(ressave);

The getnameinfo function provides from a socket address structure the address and service as character strings.

char clienthost [NI_MAXHOST]; char clientservice[NI_MAXSERV]; /* ... */ /* listenfd is a server socket descriptor waiting connections from clients */ connfd = accept(listenfd, (struct sockaddr *)&clientaddr, &addrlen); getnameinfo((struct sockaddr *)&clientaddr, addrlen, clienthost, sizeof(clienthost), clientservice, sizeof(clientservice), NI_NUMERICHOST); printf("Received request from host=[%s] port=[%s]\n", clienthost, clientservice);

Typically applications do not require to know the version of the IP they are using. Hence, applications only should try to establish the communication using each address returned by resolver until it works. However, applications could have a different behavior when using IPv4, IPv6, IPv4-compatible-IPv6 or IPv4-mapped, etc. addresses.

There are defined some macros to help applications to test the type of address they are using, see Table 2.

Table 2. Macros for testing type of addresses

int	IN6_IS_ADDR_UNSPECIFIED	(const struct in6_addr *);
int	IN6_IS_ADDR_LOOPBACK	(const struct in6_addr *);
int	IN6_IS_ADDR_MULTICAST	(const struct in6_addr *);
int	IN6_IS_ADDR_LINKLOCAL	(const struct in6_addr *);
int	IN6_IS_ADDR_SITELOCAL	(const struct in6_addr *);
int	IN6_IS_ADDR_V4MAPPED	(const struct in6_addr *);
int	IN6_IS_ADDR_V4COMPAT	(const struct in6_addr *);
int	IN6_IS_ADDR_MC_NODELOCAL	(const struct in6_addr *);
int	IN6_IS_ADDR_MC_LINKLOCAL	(const struct in6_addr *);
int	IN6_IS_ADDR_MC_SITELOCAL	(const struct in6_addr *);
int	IN6_IS_ADDR_MC_ORGLOCAL	(const struct in6_addr *);
int	IN6_IS_ADDR_MC_GLOBAL	(const struct in6_addr *);

When using UDP multicast facilities some changes must be carried out to support IPv6. First application must change the multicast IPv4 addresses to the IPv6 ones, and second, the socket configuration options.

IPv6 multicast addresses begin with the following two octets: FF0X.

The multicast socket options are used to configure some of parameters for sending multicast packets, see Table 3.

Table 3. Multicast socket options

IPV6 OPTION
IPV6_MULTICAST_IF	Interface to use for outgoing multicast packets.
IPV6_MULTICAST_HOPS	Hop limit for multicast packets.
IPV6_MULTICAST_LOOP	Multicast packets are looped back to the local application.
IPV6_ADD_MEMBERSHIP	Join a multicast group.
IPV6_DROP_MEMBERSHIP	Leave a multicast group.

Applications using multicast communication open a socket and need to configure it to receive muticast packets. In the following section some multicast examples over IPv4 and IPv6 networks will be seen.

Today the Internet is predominantly based on IPv4. As a result, most of end systems and network devices are able to exchange packets all together in a global network. However, there is no a single global IPv6 network on the same scale and it will take some time to get it. Therefore, new applications should be designed to work in all environments: single IPv4, single IPv6 or mixed communication when group collaborative applications are considered.

In the design of applications to use IPv6 some characteristics must be taken into account. First of all it is necessary to separate the transport module from the rest of application functional modules. This separation makes the application independent on the network system used. Then, if the network protocol is changed, only the transport module should be modified. Transport module should provide the communication channel abstraction with basic channel operations and generic data structures to represent the addresses. These abstractions could be instantiated as different implementations depending on the network protocol required at any moment. The application will deal with this generic communication channel interface without knowing the network protocol used. Using this design if a new network protocol is added, application developers only need to implement a new instance of the channel abstraction which manages the features of this new protocol.

Once the transport module has been designed, there are some implementation details related to the type of the nodes which will run the application: IPv4-only nodes, IPv6-only nodes or both, dual stack.

Within the transport module the use of the new API with extensions for IPv6 is mandatory, but it is strongly recommended to make the program protocol independent (for instance, using the BSD socket API consider getaddrinfo and getnameinfo instead of gethostbyname and gethostbyaddr). The new IPv6 functions are only valid if this protocol is supported by all installed systems. IPv6 is now reaching maturity and most popular systems provide it as default in their standard distributions. However, IPv6 support does not force to use it, only after the complete network configuration is defined, applications will use IPv4 or IPv6.

Protocol independent code is feasible if design is based on the principal building block for transitioning, the dual stack. Dual stacks maintain two protocol stacks that operate in parallel and thus it is allowed to operate via either protocol. The operating system running dual stack translates IPv4 addresses to IPv4-mapped IPv6 ones when communicating to IPv4 remote applications. In the following sections the connections between IPv6 server and IPv6 client applications when they are running on a dual stack node will be analyzed.

If a server application is running bound to the IPv6 wildcard address and a known port on a dual stack node, it will be able to accept connections from IPv4 and IPv6 clients, see Figure 2. When an IPv4 client is connecting to this IPv6 server, the dual stack kernel converts the client IPv4 address to the IPv4-mapped IPv6 address since the IPv6 server can only deal with IPv6 connections. The communication between the IPv4 client and the IPv6 server will take place using IPv4 datagrams, but the server will not know that it is communicating with an IPv4 client, unless the server explicitly checks it.

When running an IPv6 client application on a dual stack node, the client can connect to an IPv6 or IPv4 server. Since the client node runs dual stack, client will try to open an IPv6 connection to the server. If server is running over IPv4, the resolved server address returned by the resolver system is the IPv4-mapped IPv6 one. Then, the communication between IPv4 server and IPv6 client, which is running on the dual stack, is carried out using IPv4 but the IPv6 client will not know it, unless the client explicitly checks it.

Figure. IPv6 Server on Dual-Stack node

Figure. IPv6 Client on Dual-Stack node

Although applications are written following program protocol independent rules, other points have to be considered such as the movement of binary code between IPv4-only nodes, dual stacks nodes or IPv6-only nodes.

Compilation options (#ifdefs in C language) can be provided throughout the code to select the proper use environment. If IPv6 is not supported the IPv4-only code will be selected for compilation during installation process. However, if IPv6 is supported (Kernel level support) by installed systems, the code for IPv6 or the IPv4 could be selected for compilation, depending on the requirements of applications. Notice that if IPv6 is supported by the kernel, it only means the IPv6 option could be activated and while this option is disabled the node will be only use IPv4 stack. During the transition period nodes are usually running dual stack, both IPv4 and IPv6 stacks. The problem with the conditional compilation approach is that the code becomes littered with compilation options very quickly and harder to follow and maintain.

If an application is compiled on a system which supports dual stack and move the binary code to an IPv4-only node without IPv6 kernel support, the source code must be recompiled to use the original IPv4 API. The binary code generated on the dual stack uses the new system functions which are not supported in the IPv4-only node.

If the binary code is moved, which has been compiled on a dual stack, to an IPv4-only node with IPv6 kernel support and IPv6 stack not activated, recompilation is not required. Since the IPv6 stack is not activated, the node can not establish IPv6 connections. However, the resolver system could return an IPv6 address to an application query and the application should be prepared to discard this IPv6 address and select the IPv4 one to open connections.
All these alternatives are summarized in Table 4.

Table 4. IPv4 or IPv6 activation

NODE		APPLICATION		CONNECTIONS
IPv6 kernel support	Dual stack activated	Network API used	Application type	IPv4 connection	IPv6 connection
Yes	Yes	IPv6 extensions (portable code)	IPv6-enabled	IPv4 stack	IPv6 stack
Yes	No	IPv6 extensions (portable code)	IPv6-enabled	IPv4 stack	IPv6 address resolution. Connection error.
No	No	Without IPv6 extensions (old API)	IPv4-only (compiled with IPv4 options)	IPv4 stack	Error

In summary, if applications follow the recommendations explained above, using a separated protocol independent transport module, which provides a generic communication API, it is easy to adapt them to new network protocols. Besides, the generic communication API could be implemented as a communication library to be used for many applications. This solution encourages the code reusability and makes communication modules of applications easy to maintain.

Today the Internet is predominantly based on IPv4. However, a big effort has been done to set up more and more IPv6 experiments. At the present moment, there is no a single global IPv6 network on the same scale and it will take some time to get it. Therefore, new applications should be designed to work in all environments: single IPv4, single IPv6 or mixed communication when group collaborative applications are considered.

The principal building block during transitioning is the availability of dual stack. Dual stack mechanisms do not, by themselves, solve the IPv4 and IPv6 interworking problems; other important building block, addresses translation, is required many times. Translation refers to the direct translation of protocols, including headers and sometimes protocol payload. Protocol translation often results in features loss. For instance, translation of IPv6 header into an IPv4 header will lead to the loss of the IPv6 flow label. Translation can be complemented with tunneling; which used to bridge compatible networks across incompatible ones.
However, all previous solutions depend on applications availability. This document provides general recommendations to port applications smoothly. The general objectives to fulfill are the following:

• Communications management is isolated from the rest of the application.
• Network compilation options are not dispersed all over the code.
• Application is independent of the IP addresses management.
• Application use only names, never IP addresses to select remote node.
• Code updating of new protocol advanced features is much simpler.

All these requirements can be implemented as a single library. It is a middleware network interface to isolate communications from the rest of functional application modules. This solution simplifies future maintenance operations and it can be used as a general network interface for similar applications. This is interesting when a complete environment should be ported.
This appendix is devoted to show some simple applications which can be used to illustrate porting procedures. This appendix is used to review characteristics and methods described in previous sections.
Migration guidelines are valid for any programming language, however for simplicity, application porting examples are provided only in C language. All these examples have been tested in a SuSE Linux 7.3 distribution, kernel version 2.4.10. Deliverable D3.1 includes detailed rules and sockets interface description for most popular programming languages.
Daytime is a simple utility, which returns the time and date of a node in a human-readable format. It is defined in RFC 867 and it works in the port number 13.

It is a good example to show porting guidelines with a very simple application. Like most of distributed applications, daytime is composed of a client and a server programs. It can operate over TCP and UDP.

1.1. IPv4 Daytime server (TCP/UDP)

The server is composed of a main program and communication library, which is reduced to a simple function.

The main program is devoted to get program parameters and call listen_server function. Each connection restarts the process and local time and date is returned to the client.

The difference between TCP and UDP versions is the listen_server call: accept (which is replaced by receivefrom) and write (which is replaced by sendto)

const char *DAYTIME_PORT="13"; int main(int argc, char *argv[]) { int listenfd, connfd, port; socklen_t addrlen; char timeStr[256]; char *clienthost; struct sockaddr_in clientaddr; time_t now; /* local server socket listening at daytime port=13 */ listenfd = listen_server(NULL,DAYTIME_PORT,AF_INET,SOCK_STREAM); if (listenfd < 0) { fprintf(stderr, "listen_socket error:: could not create listening " "socket\n"); return -1; } for ( ; ;) { addrlen = sizeof(clientaddr); /* accept daytime client connections */ connfd = accept(listenfd, (struct sockaddr *)&clientaddr, &addrlen); if (connfd < 0) continue; clienthost = inet_ntoa(clientaddr.sin_addr); port = ntohs(clientaddr.sin_port); printf("Received request from host=[%s] port=[%d]\n", clienthost, port); /* process daytime request from a client */ memset(timeStr, 0, sizeof(timeStr)); time(&now); sprintf(timeStr, "%s", ctime(&now)); write(connfd, timeStr, strlen(timeStr)); close(connfd); } return 0; }

First of all, network libraries should be included:

#include #include #include #include #include #include #include #include #include #include "listen_server.h"

The core of communications is the listen_server function. It is devoted to setup the server and wait for clients connections. It initializes the server socket and blocks the process waiting for a client connection. The function returns when a client appears.

If IP number should be used, it is managed inside listen_server, not outside in the main program.

#include #include #include #include #include "listen_server.h" const int LISTEN_QUEUE=128; int listen_server(const char *hostname, const char *service, int family, int socktype) { struct sockaddr_in sin; struct hostent *phe; struct servent *pse; struct protoent *ppe; int sockfd; char *protocol; memset(&sin, 0, sizeof(sin)); sin.sin_family=AF_INET; switch(socktype) { case SOCK_DGRAM: protocol= "udp"; break; case SOCK_STREAM: protocol= "tcp"; break; default: fprintf(stderr, "listen_server:: unknown socket type=[%d]\n", socktype); return -1; } if ( pse = getservbyname(service, protocol) ) { sin.sin_port = pse->s_port; } else if ( (sin.sin_port = htons((u_short)atoi(service))) ==0) { fprintf(stderr, "listen_server:: could not get service=[%s]\n", service); return -1; } if (!hostname) { sin.sin_addr.s_addr= INADDR_ANY; } else { if (phe = gethostbyname(hostname)) { memcpy(&sin.sin_addr, phe->h_addr, phe->h_length); } else if ( (sin.sin_addr.s_addr = inet_addr(hostname)) == INADDR_NONE) { fprintf(stderr, "listen_server:: could not get host=[%s]\n", hostname); return -1; } } if ((ppe = getprotobyname(protocol)) == 0) { fprintf(stderr, "listen_server:: could not get protocol=[%s]\n", protocol); return -1; } if ((sockfd = socket(PF_INET, socktype, ppe->p_proto)) < 0) { fprintf(stderr, "listen_server:: could not open socket\n"); return -1; } if (bind(sockfd, (struct sockaddr *)&sin, sizeof(sin)) != 0) { fprintf(stderr, "listen_server:: could not bind socket\n"); close(sockfd); return -1; } listen(sockfd, LISTEN_QUEUE); return sockfd; }

1.2. IPv4 Daytime client (TCP/UDP)

The daytime service is reduced to a request from the client and an answer from the server. In TCP version, the request is combined with connection a request. Therefore, the protocol answer is also combined with a connection acceptance message.

The daytime client is structured in two parts: connection phase and operation phase. Connection phase is grouped in one function (connect_client) and operation phase is reduced to a single read (or a receivefrom in the UDP version).

#include #include #include #include #include #include #include "connect_client.h" const char *DAYTIME_PORT="13"; int main(int argc, char *argv[]) { int connfd; char *myhost; char timeStr[256]; myhost = "127.0.0.1"; if (argc > 1) myhost = argv[1]; connfd = connect_client(myhost, DAYTIME_PORT, AF_UNSPEC, SOCK_STREAM); if (connfd < 0) { fprintf(stderr, "client error:: could not create connected socket\n"); return -1; } memset(timeStr, 0, sizeof(timeStr)); while (read(connfd, timeStr, sizeof(timeStr)) > 0) printf("%s", timeStr); close(connfd); return 0; }

Like listen_server function, the connect_client function is devoted to solve the connection phase in the client.

#include #include #include #include #include "connect_client.h" int connect_client (const char *hostname, const char *service, int family, int socktype) { struct sockaddr_in sin; struct hostent *phe; struct servent *pse; struct protoent *ppe; int sockfd; char *protocol; memset(&sin, 0, sizeof(sin)); sin.sin_family=AF_INET; switch(socktype) { case SOCK_DGRAM: protocol= "udp"; break; case SOCK_STREAM: protocol= "tcp"; break; default: fprintf(stderr, "listen_server:: unknown socket type=[%d]\n", socktype); return -1; } if ( pse = getservbyname(service, protocol) ) { sin.sin_port = pse->s_port; } else if ((sin.sin_port = htons((u_short)atoi(service)))==0) { fprintf(stderr, "connec_client:: could not get service=[%s]\n", service); return -1; } if (!hostname) { fprintf(stderr, "connect_client:: there should be a hostname!\n"); return -1; } else { if (phe = gethostbyname(hostname)) { memcpy(&sin.sin_addr, phe->h_addr, phe->h_length); } else if ( (sin.sin_addr.s_addr = inet_addr(hostname)) == INADDR_NONE) { fprintf(stderr, "connect_client:: could not get host=[%s]\n", hostname); return -1; } } if ((ppe = getprotobyname(protocol)) == 0) { fprintf(stderr, "connect_client:: could not get protocol=[%s]\n", protocol); return -1; } if ((sockfd = socket(PF_INET, socktype, ppe->p_proto)) < 0) { fprintf(stderr, "connect_client:: could not open socket\n"); return -1; } if (connect(sockfd,(struct sockaddr *)&sin, sizeof(sin)) < 0) { fprintf(stderr, "connect_client:: could not connect to host=[%s]\n", hostname); return -1; } return sockfd; }

2. The unicast daytime ported to IPv6
In the following sections it is shown how daytime service is ported from IPv4 to IPv6. Following previous described porting guidelines and after previous program analysis, the code should be reviewed to support IPv4 and IPv6 without compilation.

IPv6 sockets library is similar to IPv4 version, therefore server or client code does not change for the operation phase. The connection phase maintain the same connection model, however the code to set up connections should be adapted.

2.1. IPv6 Daytime server (TCP/UDP)
The TCP and UDP versions of the daytime server program use a common function to create the server socket, the listen_server function. This function generates a server socket from a hostname, the service, socket family (IPv4 or IPv6) and socket type (TCP or UDP) parameters.

2.1.1. File "listen_server.h"

#ifndef listen__server__h__ #define listen__server__h__ /* listen_server creates a server server socket listening at a hostname:service using the family and socket type specified in the function arguments. */ int listen_server(const char *hostname, const char *service, int family, int socktype); #endif

2.1.2 File "listen_server.cpp"

#include #include #include #include #include "listen_server.h" const int LISTEN_QUEUE=128; int listen_server(const char *hostname, const char *service, int family, int socktype) { struct addrinfo hints, *res, *ressave; int n, sockfd; memset(&hints, 0, sizeof(struct addrinfo)); /* AI_PASSIVE flag: the resulting address is used to bind to a socket for accepting incoming connections. So, when the hostname==NULL, getaddrinfo function will return one entry per allowed protocol family containing the unspecified address for that family. */ hints.ai_flags = AI_PASSIVE; hints.ai_family = family; hints.ai_socktype = socktype; n = getaddrinfo(hostname, service, &hints, &res); if (n <0) { fprintf(stderr, "getaddrinfo error:: [%s]\n", gai_strerror(n)); return -1; } ressave=res; /* Try open socket with each address getaddrinfo returned, until getting a valid listening socket. */ sockfd=-1; while (res) { sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol); if (!(sockfd < 0)) { if (bind(sockfd, res->ai_addr, res->ai_addrlen) == 0) break; close(sockfd); sockfd=-1; } res = res->ai_next; } if (sockfd < 0) { freeaddrinfo(ressave); fprintf(stderr, "socket error:: could not open socket\n"); return -1; } listen(sockfd, LISTEN_QUEUE); freeaddrinfo(ressave); return sockfd; }

The TCP daytime server uses listen_server with SOCK_STREAM and PF_UNSPEC parameters to obtain a server socket, which will accept connections to all of the interfaces. When clients connect to the server, it will answer with the daytime information and close the client connection.

2.1.3. File "tcp_daytime_server.cpp"

#include #include #include #include #include #include #include #include "listen_server.h" const char *DAYTIME_PORT="13"; int main(int argc, char *argv[]) { int listenfd, connfd; socklen_t addrlen; char timeStr[256]; struct sockaddr_storage clientaddr; time_t now; char clienthost[NI_MAXHOST]; char clientservice[NI_MAXSERV]; /* local server socket listening at daytime port=13 */ listenfd = listen_server( NULL, DAYTIME_PORT, AF_UNSPEC, SOCK_STREAM); if (listenfd < 0) { fprintf(stderr, "listen_socket error:: could not create listening " "socket\n"); return -1; } for ( ; ;) { addrlen = sizeof(clientaddr); /* accept daytime client connections */ connfd = accept(listenfd, (struct sockaddr *)&clientaddr, &addrlen); if (connfd < 0) continue; memset(clienthost, 0, sizeof(clienthost)); memset(clientservice, 0, sizeof(clientservice)); getnameinfo((struct sockaddr *)&clientaddr, addrlen, clienthost, sizeof(clienthost), clientservice, sizeof(clientservice), NI_NUMERICHOST); printf("Received request from host=[%s] port=[%s]\n", clienthost, clientservice); /* process daytime request from a client */ memset(timeStr, 0, sizeof(timeStr)); time(&now); sprintf(timeStr, "%s", ctime(&now)); write(connfd, timeStr, strlen(timeStr)); close(connfd); } return 0; }

The UDP daytime server uses listen_server with SOCK_DGRAM and PF_UNSPEC parameters to obtain a server socket, which will receive connections to all of the interfaces. When clients connect to the server, it will answer with the daytime information and close the client connection.

2.1.4. File "udp_daytime_server.cpp"

#include #include #include #include #include #include #include "listen_server.h" const char *DAYTIME_PORT="13"; int main(int argc, char *argv[]) { int listenfd, n; socklen_t addrlen; char *myhost; char timeStr[256]; struct sockaddr_storage clientaddr; time_t now; char b[256]; char clienthost[NI_MAXHOST]; char clientservice[NI_MAXSERV]; myhost=NULL; if (argc > 1) myhost=argv[1]; listenfd= listen_server(myhost, DAYTIME_PORT, AF_UNSPEC, SOCK_DGRAM); if (listenfd < 0) { fprintf(stderr, "listen_server error:: could not create listening " "socket\n"); return -1; } addrlen = sizeof(clientaddr); for ( ; ;) { n = recvfrom(listenfd, b, sizeof(b), 0, (struct sockaddr *)&clientaddr, &addrlen); if (n < 0) continue; memset(clienthost, 0, sizeof(clienthost)); memset(clientservice, 0, sizeof(clientservice)); getnameinfo((struct sockaddr *)&clientaddr, addrlen, clienthost, sizeof(clienthost), clientservice, sizeof(clientservice), NI_NUMERICHOST); printf("Received request from host=[%s] port=[%s]\n", clienthost, clientservice); memset(timeStr, 0, sizeof(timeStr)); time(&now); sprintf(timeStr, "%s", ctime(&now)); n = sendto(listenfd, timeStr, sizeof(timeStr), 0, (struct sockaddr *)&clientaddr, addrlen); } return 0; }

Migration process is simple because all main changes are grouped inside listen_server function. If application is not correctly structured porting effort increases. Sometimes, it is much better to review the program structure that only to change functions calls to make the adaptation.

This is the reason why scripts to change code automatically are not recommended. Automatic scripts look for concrete functions and change them by new function version however, program structure is not analyzed and many times the result is very poor.

2.2. IPv6 Daytime client (TCP/UDP).

The TCP and UDP versions of the daytime client program use a common function to create the client socket, the connect_client function. This function generates a client socket from a hostname, the service, socket family (IPv4 or IPv6) and socket type (TCP or UDP) parameters.

2.2.1. File "connect_client.h"

#ifndef connect__client__h__ #define connect__client__h__ int connect_client (const char *hostname, const char *service, int family, int socktype); #endif

2.2.2. File "connect_client.cpp"

#include #include #include #include #include "connect_client.h" int connect_client (const char *hostname, const char *service, int family, int socktype) { struct addrinfo hints, *res, *ressave; int n, sockfd; memset(&hints, 0, sizeof(struct addrinfo)); hints.ai_family = family; hints.ai_socktype = socktype; n = getaddrinfo(hostname, service, &hints, &res); if (n <0) { fprintf(stderr, "getaddrinfo error:: [%s]\n", gai_strerror(n)); return -1; } ressave = res; sockfd=-1; while (res) { sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol); if (!(sockfd < 0)) { if (connect(sockfd, res->ai_addr, res->ai_addrlen) == 0) break; close(sockfd); sockfd=-1; } res=res->ai_next; } freeaddrinfo(ressave); return sockfd; }

The TCP daytime server uses connect_client with the following input parameter values: SOCK_STREAM, PF_UNSPEC, the hostname and the port where TCP daytime server is listening. The client socket connects to this server using IPv4 or IPv6 depending on if the server hostname is resolved to an IPv4 or an IPv6 address. Clients will wait for the daytime answer from the server.

2.2.3. File "tcp_daytime_client.cpp"

#include #include #include #include #include #include #include "connect_client.h" const char *DAYTIME_PORT="13"; int main(int argc, char *argv[]) { int connfd; char *myhost; char timeStr[256]; myhost = "localhost"; if (argc > 1) myhost = argv[1]; connfd= connect_client(myhost, DAYTIME_PORT, AF_UNSPEC, SOCK_STREAM); if (connfd < 0) { fprintf(stderr, "client error:: could not create connected socket " "socket\n"); return -1; } memset(timeStr, 0, sizeof(timeStr)); while (read(connfd, timeStr, sizeof(timeStr)) > 0) printf("%s", timeStr); close(connfd); return 0; }

The UDP daytime server uses connect_client with the following input parameter values: SOCK_DGRAM, PF_UNSPEC, the hostname and the port where TCP daytime server is listening. And the same as in the TCP case, the client socket connects to this server using IPv4 or IPv6 depending on if the server hostname is resolved to an IPv4 or an IPv6 address. Clients will wait for the daytime answer from the server.

2.2.4. File "udp_daytime_client.cpp"

#include #include #include #include #include #include #include "connect_client.h" const char *DAYTIME_PORT="13"; int main(int argc, char *argv[]) { int connfd, n, m; char *myhost; char timeStr[256]; char letter; myhost = "localhost"; if (argc > 1) myhost=argv[1]; connfd = connect_client(myhost, DAYTIME_PORT, AF_UNSPEC, SOCK_DGRAM); if (connfd < 0) { fprintf(stderr, "client error:: could not create connected socket " "socket\n"); return -1; } letter = '1'; m= write(connfd, &letter, sizeof(letter)); memset(timeStr, 0, sizeof(timeStr)); n = read(connfd, timeStr, sizeof(timeStr)); printf("%s\n", timeStr); close(connfd); return 0; }

3. The multicast daytime
In this section daytime utility is used to analyze porting problems related to multicast applications. Also, daytime service is used to show this porting process. Daytime is a very simple service which is easy to be adapted to multicast.

First some useful functions that will be used in the multicast server/client examples are explained. The get_addr function returns a sockaddr_storage struct filled with a valid socket address struct for the address, service, family and socket type input parameters. The joinGroup function configures the socket with useful multicast options.

The file mcastutil.h defines the basic multicast socket interface. It defines the following functions:

get_addr fills the sockaddress_storage struct, addr, with information related to the hostname, service, family and socktype.

int get_addr (const char *hostname, const char *service, int family, int socktype, struct sockaddr_storage *addr);

joinGroup specifies the address group to be used in the application. It configures the socket with some useful multicast options like loopBack and mcastHop.

int joinGroup(int sockfd, int loopBack, int mcastHop, struct sockaddr_storage *addr);

isMulticast checks if an address is a valid multicast group.

int isMulticast(struct sockaddr_storage *addr);

3.1. File "mcastutil.cpp"

#include #include #include #include #include #include #include #include #include "mcastutil.h" int get_addr (const char *hostname, const char *service, int family, int socktype, struct sockaddr_storage *addr) { struct addrinfo hints, *res, *ressave; int n, sockfd, retval; retval = -1; memset(&hints, 0, sizeof(struct addrinfo)); hints.ai_family = family; hints.ai_socktype = socktype; n = getaddrinfo(hostname, service, &hints, &res); if (n <0) { fprintf(stderr, "getaddrinfo error:: [%s]\n", gai_strerror(n)); return retval; } ressave = res; sockfd=-1; while (res) { sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol); if (!(sockfd < 0)) { if (bind(sockfd, res->ai_addr, res->ai_addrlen) == 0) { close(sockfd); memcpy(addr, res->ai_addr, sizeof(*addr); retval=0; break; } close(sockfd); sockfd=-1; } res=res->ai_next; } freeaddrinfo(ressave); return retval; } int joinGroup(int sockfd, int loopBack, int mcastTTL, struct sockaddr_storage *addr) { int r1, r2, r3, retval; retval=-1; switch (addr->ss_family) { case AF_INET: { struct ip_mreq mreq; mreq.imr_multiaddr.s_addr= ((struct sockaddr_in *)addr)->sin_addr.s_addr; mreq.imr_interface.s_addr= INADDR_ANY; r1= setsockopt(sockfd, IPPROTO_IP, IP_MULTICAST_LOOP, &loopBack, sizeof(loopBack)); if (r1<0) perror("joinGroup:: IP_MULTICAST_LOOP:: "); r2= setsockopt(sockfd, IPPROTO_IP, IP_MULTICAST_TTL, &mcastTTL, sizeof(mcastTTL)); if (r2<0) perror("joinGroup:: IP_MULTICAST_TTL:: "); r3= setsockopt(sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (const void *)&mreq, sizeof(mreq)); if (r3<0) perror("joinGroup:: IP_ADD_MEMBERSHIP:: "); } break; case AF_INET6: { struct ipv6_mreq mreq6; memcpy(&mreq6.ipv6mr_multiaddr, &(((struct sockaddr_in6 *)addr)->sin6_addr), sizeof(struct in6_addr)); mreq6.ipv6mr_interface= 0; // cualquier interfaz r1= setsockopt(sockfd, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, &loopBack, sizeof(loopBack)); if (r1<0) perror("joinGroup:: IPV6_MULTICAST_LOOP:: "); r2= setsockopt(sockfd, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, &mcastTTL, sizeof(mcastTTL)); if (r2<0) perror("joinGroup:: IPV6_MULTICAST_HOPS:: "); r3= setsockopt(sockfd, IPPROTO_IPV6, IPV6_ADD_MEMBERSHIP, &mreq6, sizeof(mreq6)); if (r3<0) perror("joinGroup:: IPV6_ADD_MEMBERSHIP:: "); } break; default: r1=r2=r3=-1; } if ((r1>=0) && (r2>=0) && (r3>=0)) retval=0; return retval; } int isMulticast(struct sockaddr_storage *addr) { int retVal; retVal=-1; switch (addr->ss_family) { case AF_INET: { struct sockaddr_in *addr4=(struct sockaddr_in *)addr; retVal = IN_MULTICAST(ntohl(addr4->sin_addr.s_addr)); } break; case AF_INET6: { struct sockaddr_in6 *addr6=(struct sockaddr_in6 *)addr; retVal = IN6_IS_ADDR_MULTICAST(&addr6->sin6_addr); } break; default: ; } return retVal; }

The multicast server is similar to unicast server. It initializes the service, it stops the process (receivefrom) waiting for client connections and it answers immediately after any request.

3.2. File "mcastserver.cpp"

#include #include #include #include #include #include #include #include #include "mcastutil.h" const char *DAYTIME_PORT="13"; int main(int argc, char *argv[]) { int sockfd, n; char *mcastaddr; char timeStr[256]; char b[256]; struct sockaddr_storage clientaddr, addr; socklen_t addrlen; time_t now; char clienthost[NI_MAXHOST]; char clientservice[NI_MAXSERV]; mcastaddr = "FF01::1111"; if (argc ==2) mcastaddr=argv[1]; memset(&addr, 0, sizeof(addr)); if (get_addr(mcastaddr, DAYTIME_PORT, PF_UNSPEC, SOCK_DGRAM, &addr) <0) { fprintf(stderr, "get_addr error:: could not find multicast " "address=[%s] port=[%s]\n", mcastaddr, DAYTIME_PORT); return -1; } if (isMulticast(&addr)<0) { fprintf(stderr, "This address does not seem a multicast address [%s]\n", mcastaddr); return -1; } sockfd = socket(addr.ss_family, SOCK_DGRAM, 0); if (bind(sockfd, (struct sockaddr *)&addr, sizeof(addr)) < 0) { perror("bind error:: "); close(sockfd); return -1; } if (joinGroup(sockfd, 0 , 8, &addr) <0) { close(sockfd); return -1; } addrlen=sizeof(clientaddr); for ( ; ;) { n = recvfrom(sockfd, b, sizeof(b), 0, (struct sockaddr *)&clientaddr, &addrlen); if (n <0) continue; memset(clienthost, 0, sizeof(clienthost)); memset(clientservice, 0, sizeof(clientservice)); getnameinfo((struct sockaddr *)&clientaddr, addrlen, clienthost, sizeof(clienthost), clientservice, sizeof(clientservice), NI_NUMERICHOST); printf("Received request from host=[%s] port=[%s]\n", clienthost, clientservice); memset(timeStr, 0, sizeof(timeStr)); time(&now); sprintf(timeStr, "%s", ctime(&now)); n = sendto(sockfd, timeStr, sizeof(timeStr), 0, (struct sockaddr *)&addr, sizeof(addr)); if (n<1) perror("sendto error:: \n"); } return 0; }

The multicast client joins the multicast group and waits announcements received by the multicast group.

3.3. File "mcastclient.cpp"

#include #include #include #include #include #include #include #include "mcastutil.h" const char *DAYTIME_PORT="13"; int main(int argc, char *argv[]) { int sockfd, n; char *myhost; char timeStr[256]; char letter; struct sockaddr_storage addr, clientaddr; int addrlen; socklen_t clientaddrlen; myhost = "FF01::1111"; if (argc == 2) myhost=argv[1]; addrlen=sizeof(addr); memset(&addr, 0, addrlen); get_addr(myhost, DAYTIME_PORT, PF_UNSPEC, SOCK_DGRAM, &addr); sockfd = socket(addr.ss_family, SOCK_DGRAM, 0); if (bind(sockfd, (struct sockaddr *)&addr, addrlen) <0) { perror("bind error:: \n"); close(sockfd); return -1; } if (joinGroup(sockfd, 0 , 8, &addr) <0) { close(sockfd); return -1; } letter = '1'; n = sendto(sockfd, &letter, sizeof(letter), 0, (struct sockaddr *)&addr, addrlen); if (n<0) { perror("sendto error:: "); close(sockfd); return -1; } memset(timeStr, 0, sizeof(timeStr)); clientaddrlen=sizeof(clientaddr); n = recvfrom(sockfd, timeStr, sizeof(timeStr), 0, (struct sockaddr *)&clientaddr, &clientaddrlen); if (n<0) { perror("sendto error:: "); close(sockfd); return -1; } printf("%s\n", timeStr); close(sockfd); return 0; }

ADU	Application Data Unit.
API	Application Program Interface.
ASCII	American Standard Code for Information Interchange.
BSD	Berkeley System Distribution.
FQDN	Fully Qualified Domain Name.
IP	Internet Protocol.
IPv4	Internet Protocol version 4.
IPv6	Internet Protocol version 6.
MTU	Maximum Transfer Unit.
PMTU	Path Maximum Transfer Unit.
PMTU-D	Path Maximum Transfer Unit Discovery.
TCP	Transport Control Protocol.
TU	Transfer Unit.
UDP	User Datagram Protocol.

[1]Advanced Sockets API for IPv6. W. Stevens, M. Thomas. February 1998. RFC2292. (Format: TXT=152077 bytes) (Status: INFORMATIONAL)

[2]Basic Socket Interface Extensions for IPv6. R. Gilligan, S., Thomson, J. Bound, W. Stevens. March 1999. RFC2553. (Format: TXT=89215 bytes) (Obsoletes RFC2133) (Status: INFORMATIONAL)

[3]Format for Literal IPv6 Addresses in URL's. R. Hinden, B., Carpenter, L. Masinter. December 1999. RFC2732. (Format: TXT=7984 bytes) (Status: PROPOSED STANDARD)

[4]Router Renumbering for IPv6. M. Crawford. August 2000. RFC2894. (Format: TXT=69135 bytes) (Status: PROPOSED STANDARD)

[5]Daytime Protocol. J. Postel. May-01-1983. RFC867. (Format: TXT=2405 bytes) (Also STD0025) (Status: STANDARD)

[6]Path MTU Discovery for IP version 6. J. McCann, S. Deering, J. Mogul. August 1996. RFC1981. (Format: TXT=34088 bytes) (Status: PROPOSED STANDARD)

[7]IP Version 6 Addressing Architecture. R. Hinden, S. Deering. July 1998. RFC2373. (Format: TXT=52526 bytes) (Obsoletes RFC1884) (Status: PROPOSED STANDARD)

[8]Advanced Sockets API for IPv6. W. Richard Stevens. Expires: October 19, 2002. Obsoletes RFC 2292. INTERNET-DRAFT:

[9]Basic Socket Interface Extensions for IPv6. R.E. Gilligan. INTERNET-DRAFT: draft-ietf-ipngwg-rfc2553bis-05. Obsoletes RFC 2553. February 2002

[10]Default Address Selection for IPv6. Richard Draves. INTERNET-DRAFT: draft-ietf-ipv6-default-addr-select-08.txt. June 17, 2002.

[11]The Unix Network Programming - Volume 1. Richard Stevens.

Author: Eva M. Castro