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1.3 1.3 Figure 1.3 Communication over connection- oriented sockets. Figure 1.4 Communication over connectionless sockets. u_int16_t sin_port; /* TCP/UDP port number */ struct in_addr sin_addr; /* IPv4 address */ int8_t sin_zero[8]; /* padding */ }; Normally, users will denote the peer s address either as a host name (e.g., www.example.org) or as a numeric string representation (e.g., 10.2.3.4). Mapping between host names and IP addresses is registered in theDNS database, and there are APIs to query the DNS database, such as gethostbyname(3) or gethostbyaddr(3). There are also functions to convert IP address in numeric string representation Chapter
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8 8 int s; /* socket */ /* * AF_INET: protocol family for IPv4 * SOCK_STREAM: connection-oriented socket * IPPROTO_TCP: use TCP on top of IPv4 */ s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); if (s < 0) { perror( socket ); exit(1); /*NOTREACHED*/ } close(s); While read(2) or write(2) is possible for sockets, we normally need to supply more information, such as peer s address, to get the data stream to reach the peer. There are additional system calls specifically provided for sockets, such as sendmsg(2), sendto(3), recvmsg(2), and recvfrom(3). Since we need to identify the peer when accessing the network, we need to denote it either by: Using connect(2) to make the socket a connected socket. The peer s address will be kept in the system, and you can use read(2) or write(2) after connect(2). Using sendto(3) or sendmsg(2) to denote the peer every time you transmit data to the socket. For connection-oriented (TCP) sockets, there are two sides: client side, which makes active connection, and server side, which awaits connection from the client passively. connect(2) is mandatory for the client side. bind(2), listen(2), and accept(2) are mandatory for the server side. (See Figure 1.3.) For connectionless (UDP) sockets, connect(2) is not mandatory. To receive traffic from other peers, bind(2) is mandatory. (See Figure 1.4.) To denote TCP/UDP endpoints, IP address and port number are necessary. To carry the endpoint information, we use a C structure called sockaddr (short for socket addresses ). sockaddr for IPv4 is defined in the following code segment. Fields that appear on wire (sin_port and sin_addr) are in network byte order; other fields are in host byte order. /* * Note: the definition is based on 4.4BSD socket API. * Linux/Solaris has no sin_len field. */ struct sockaddr_in { u_int8_t sin_len; /* length of sockaddr */ u_int8_t sin_family; /* address family */
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1.3 1.3 g 7 section does not try to be complete for the complete description, you may want to check the reading material listed in the References. With only a few exceptions, UNIX operating systems abstract system resources as files. For instance, the hard disk device is abstracted as a file such as /dev/rwd0c. Even physical memory on the machine is abstracted as a file, /dev/mem. You can open(2), read(2), write(2), or close(2) files, and files already opened by a process are identified by an integer file descriptor. int fd; /* file descriptor */ char buf[128]; fd = open( /tmp/foo , O_RDONLY, 0); if (fd < 0) { perror( open ); exit(1); /*NOTREACHED*/ } if (read(fd, buf, sizeof(buf)) < 0) { perror( read ); exit(1); /*NOTREACHED*/ } close(fd); exit(0); Accesses to the network are also abstracted as special kinds of files, called sockets. Sockets are created by a socket(2) system call. Sockets are a special kind of file descriptor, so they are represented as an integer and can be terminated by using close(2). On a socket(2) call, you need to identify the following three parameters: Protocol family AF_INET identifies IPv4. Type of socket SOCK_STREAM means connetion-oriented socket model. SOCK_DGRAM means datagram-oriented socket model. Protocol type such as IPPROTO_TCP or IPPROTO_UDP. For the Internet protocol, there are two kinds of sockets: connection-oriented and connectionless sockets. Connection-oriented sockets abstract TCP connections, and connectionless sockets abstract communication over UDP. Type of socket and protocol type has to be consistent; SOCK_STREAM has to be used with IPPROTO_TCP. Note: There are transport layer protocols other than TCP/UDP proposed in the IETF, such as SCTP or DCCP. They are also abstracted as connection-oriented or connectionless sockets. Chapter
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6 1.3 UNIX Socket Programming 6 1.3 UNIX Socket Programming Figure 1.1 IPv4/v6 dual stack network, separated by IPv4-only Internet. Figure 1.2 IPv6-over-IPv4 tunnel. From a programmer s point of view, tunneling is transparent: It can be viewed as a simple IPv6 point-to-point link. Therefore, when writing IPv6-capable programs, you can ignore tunneling. 1.3 UNIX Socket Programming This section briefly describes how UNIX systems abstract network accesses via socket interface. If you are familiar with UNIX sockets, you can skip this section. Also, the
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1.2 1.2 t 5 www.example.com. IN A 10.1.1.1 If www.example.com resolves to an IPv6 address, connect to the IPv6 address. www.example.com. IN AAAA 3ffe:501:ffff::1234 If www.example.com resolves to multiple IPv4/v6 addresses, IPv6 addresses will be tried first, and then IPv4 addresses will be tried. For example, with the following DNS records, we will try connecting to 3ffe:501:ffff::1234, then 3ffe:501:ffff::5678, and finally 10.1.1.1. www.example.com. IN AAAA 3ffe:501:ffff::1234 www.example.com. IN AAAA 3ffe:501:ffff::5678 www.example.com. IN A 10.1.1.1 Since we assume that IPv6 nodes will be able to use IPv4 as well, the Internet will be filled with IPv4/v6 dual stack nodes in the near future, and the use of IPv6 will become dominant. 1.2.2 Tunneling Even when we have IPv4/v6 dual stack nodes at two locations (e.g., home and office), it may be possible that the intermediate network (ISPs) are not IPv6-ready yet. To circumvent this situation, RFC 2893 defines ways to encapsulate an IPv6 packet into an IPv4 packet. The encapsulated packet will travel IPv4 Internet with no trouble, and then decapsulate at the other end. We call this technology IPv6-over-IPv4 tunneling. For example, imagine the following situation (see Figure 1.1): We have two networks: home and office. We have an IPv4/v6 dual stack host and router at both locations. However, we have IPv4-only connectivity to the upstream ISP. In this case, we can configure an IPv6-over-IPv4 tunnel between X and Y. An IPv6 packet from A to B will be routed as follows (see Figure 1.2): The IPv6 packet will be transmitted from A to X, as is. X will encapsulate the packet into an IPv4 packet. The IPv4 packet will travel the IPv4 Internet, to Y. Y will decapsulate the packet and recover the original IPv6 packet. The packet will reach B. Chapter
Check our reliable web hosting section. Most often, a reliable protocol is also connection-oriented. However, this is not always so. For example, TCP/IP is a connection-oriented protocol, with the virtual circuit ID consisting of source and destination IP addresses and port numbers. However, there are also unreliable protocols that are connection-oriented as well. These include ATM and Frame Relay, on which 90% or more of all Internet traffic is passed.

4 4 1.2 Transition from IPv4-Only Internet to IPv4/v6 Dual Stack Internet Today, most of the nodes on the Internet use IPv4. We will need to gradually introduce IPv6 to the Internet and hopefully make all nodes on the Internet IPv6-capable. To do this, the IETF has carefully designed IPv6 migration to be seamless. This is achieved by the following two key technologies: Dual stack Tunneling With these technologies, we can transition to IPv6 even though IPv4 and IPv6 are not compatible (IPv4-only devices and IPv6-only devices cannot talk with each other directly). We will go into the details soon. It is expected that we will have a long period of IPv4/v6 dual stack Internet, due to the wide deployment of IPv4 devices. For instance, some of the existing devices, such as IPv4-capable game machines, may not be able to be upgraded to IPv6. Therefore, in this book, we would like to focus on the issues regarding the transi tion from IPv4-only Internet to IPv4/v6 dual stack Internet and the changes in socket API programming. 1.2.1 Dual stack At least in the early stage of IPv6 deployment, IPv6-capable nodes are assumed to be IPv4-capable. They are called IPv4/v6 dual stack nodes or dual stack nodes. Dual stack nodes will use IPv4 to communicate with IPv4 nodes, and use IPv6 to communicate with IPv6 nodes. It is just like a bilingual person he or she will use English when talking to people in the States, and will use Japanese when talking to Japanese people. The determination of protocol version is automatic, based on available DNS records. Because this is based on DNS, and normal users would use fully qualified domain name (FQDN) in email addresses and URLs, the transition from IPv4 to IPv6 is invisible to normal users. For instance, assume that we have a dual stack node, and we are to access http://www.example.com/. A dual stack node will behave as follows: If www.example.com resolves to an IPv4 address, connect to the IPv4 address. In such a case, the DNS database record for www.example.com will be as follows:
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1.1 1.1 1.1.4 Simplified Header Structures IPv6 has simpler packet header structures than IPv4. It will allow vendors to implement hardware acceleration for IPv6 routers easier. 1.1.5 Allows Flexible Protocol Extensions IPv6 allows more flexible protocol extensions than IPv4 by introducing a protocol header chain. Even though IPv6 allows flexible protocol extensions, IPv6 does not impose overhead to intermediate routers. It is achieved by splitting headers into two flavors: the headers intermediate routers need to examine and the headers the final destination will examine. This also eases hardware acceleration for IPv6 routers. 1.1.6 Smooth Transition from IPv4 There were a number of transition considerations made during the IPv6 discussions. Also, there is a large number of transition mechanisms available. You can pick the most suitable one for your network during the transition period. 1.1.7 Follows the Key Design Principles of IPv4 IPv4 was a very successful design, as proven by the large-scale global deployment. IPv6 is a new version of IP, and it follows many of the design features that made IPv4 very successful. This will also allow smooth transition from IPv4 to IPv6. 1.1.8 And More There are number of good books available about IPv6. Be sure to check these if you are interested. Protocol Header Chain IPv6 defines a protocol header chain, which is a way to concatenate extension headers repeatedly after the IPv6 base header. With IPv4, the IPv4 header is adjacent to the final header (like TCP). With IPv6, the protocol header chain allows various extension headers to be put between the IPv6 base header and the final header. IPv6 header Routing header Fragment header Fragment of TCP Next Header = Routing Next Header = Fragment Next Header = TCP header + data Chapter
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2 2 1.1.2 Deploy More Recent Technologies After IPv4 was specified 20 years ago, we saw many technical improvements in networking. IPv6 covers a number of those improvements in its base specification, allowing people to assume that these features are available everywhere, anytime. Recent technologies include, but are not limited to, the following: Autoconfiguration With IPv4, DHCP is optional. A novice user can get into trouble if visiting an offsite without a DHCP server. With IPv6, the stateless host autoconfiguration mechanism is mandatory. This is much simpler to use and manage than IPv4 DHCP. RFC 2462 has the specification for it. Security With IPv4, IPsec is optional and you need to ask the peer if it supports IPsec. With IPv6, IPsec support is mandatory. By mandating IPsec, we can assume that you can secure your IP communication whenever you talk to IPv6 peers. Friendly to traffic engineering technologies IPv6 was designed to allow better support for traffic engineering such as diffserv1 or RSVP2. We do not have single standard for traffic engineering yet; so the IPv6 base specification reserves a 24-bit space in the header field for those technologies and is able to adapt to coming standards better than IPv4. Multicast Multicast support is mandatory in IPv6; it was optional in IPv4. The IPv6 base specifications extensively use multicast on the directly connected link. It is still questionable how widely we will be able to deploy multicast (such as nationwide multicast infrastructure), though. Better support for ad hoc networking Scoped addresses allow better support for ad hoc (or zeroconf ) networking. IPv6 supports anycast addresses, which can also contribute to service discoveries. 1.1.3 A Cure to Routing Table Growth The IPv4 backbone routing table size has been a big headache to ISPs and backbone operators. The IPv6 addressing specification restricts the number of backbone routing entries by advocating route aggregation. With the current IPv6 addressing specification, we will see only 8,192 routes in the default-free zone. 1. diffserv: short for differentiated services. It is an IETF standard that classifies packets into a couple of classes and performs rough bandwidth/priority control. 2. RSVP: an IETF standard for bandwidth reservation.
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Introduction Introduction A History of IPv6 and Its Key Features In 1992, the IETF (http://www.ietf.org/) became aware of a global shortage of IPv4 addresses and technical obstacles in deploying new protocols due to limitations imposed by IPv4. An IPng (IP next generation) effort was started to solve these issues. The discussion is outlined in several RFCs, starting with RFC 1550. After a large amount of discussion, in 1995, IPv6 (IP version 6) was picked as the final IPng proposal. The IPv6 base specification is specified in RFC 1883 and revised in RFC 2460. In a single sentence, IPv6 is a reengineering effort against IP technology. Key features are as follows. 1.1.1 Larger IP Address Space IPv4 uses only 2^32 bits for IP address space, which allows only (theoretically) 4 billion nodes to be identified on the Internet. Four billion may look like a large number; however, it is less than the world s population. Moreover, due to the allocation (in)efficiency, it is not possible to use up all 4 billion addresses. IPv6 allows 2^128 bits for IP address space, (theoretically) allowing 340,282,366,920,938,463,463,374,607,431,768,211,456 (340 undecillion) nodes to be uniquely identified on the Internet. Larger address space allows true end-to-end communication, without NAT or other short-term workarounds against IPv4 address shortage. (In these days, NAT has been a headache to new protocol deployment and scalability issues, and we really need to decommission NATs for the Internet to grow further.)
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