| [Page 518 (continued)] This appendix provides an overview of Internet protocols. We begin with a summary of the role of an internet protocol in providing internetworking. Then the two main internet protocols, IPv4 and IPv6, are introduced. The Role of an Internet Protocol An internet protocol (IP) provides the functionality for interconnecting end systems across multiple networks. For this purpose, IP is implemented in each end system and in routers, which are devices that provide connection between networks. Higher-level data at a source end system are encapsulated in an IP protocol data unit (PDU) for transmission. This PDU is then passed through one or more networks and connecting routers to reach the destination end system. The router must be able to cope with a variety of differences among networks, including Addressing schemes: The networks may use different schemes for assigning addresses to devices. For example, an IEEE 802 LAN uses either 16-bit or 48-bit binary addresses for each attached device; an X.25 public packet-switching network uses 12-digit decimal addresses (encoded as 4 bits per digit for a 48-bit address). Some form of global network addressing must be provided, as well as a directory service. Maximum packet sizes: Packets from one network may have to be broken into smaller pieces to be transmitted on another network, a process known as fragmentation. For example, Ethernet imposes a maximum packet size of 1500 bytes; a maximum packet size of 1000 bytes is common on X.25 networks. A packet that is transmitted on an Ethernet system and picked up by a router for retransmission on an X.25 network may have to fragment the incoming packet into two smaller ones. Interfaces: The hardware and software interfaces to various networks differ. The concept of a router must be independent of these differences. Reliability: Various network services may provide anything from a reliable end-to-end virtual circuit to an unreliable service. The operation of the routers should not depend on an assumption of network reliability.
The operation of the router, as Figure 16.13 indicates, depends on an internet protocol. In this example, the Internet Protocol (IP) of the TCP/IP protocol suite performs that function. IP must be implemented in all end systems on all networks as well as on the routers. In addition, each end system must have compatible protocols above IP to communicate successfully. The intermediate routers need only have up through IP.
[Page 519] Figure 16.13. Configuration for TCP/IP Example 
Consider the transfer of a block of data from end system X to end system Y in Figure 16.13. The IP layer at X receives blocks of data to be sent to Y from TCP in X. The IP layer attaches a header that specifies the global internet address of Y. That address is in two parts: network identifier and end system identifier. Let us refer to this block as the IP packet. Next, IP recognizes that the destination (Y) is on another subnetwork. So the first step is to send the packet to a router, in this case router 1. To accomplish this, IP hands its data unit down to LLC with the appropriate addressing information. LLC creates an LLC PDU, which is handed down to the MAC layer. The MAC layer constructs a MAC packet whose header contains the address of router 1. Next, the packet travels through LAN to router 1. The router removes the packet and LLC headers and trailers and analyzes the IP header to determine the ultimate destination of the data, in this case Y. The router must now make a routing decision. There are two possibilities: The destination end system Y is connected directly to one of the subnetworks to which the router is attached. To reach the destination, one or more additional routers must be traversed.
In this example, the packet must be routed through router 2 before reaching the destination. So router 1 passes the IP packet to router 2 via the intermediate network. For this purpose, the protocols of that network are used. For example, if the intermediate network is an X.25 network, the IP data unit is wrapped in an X.25 packet with appropriate addressing information to reach router 2. When this packet arrives at router 2, the packet header is stripped off. The router determines that this IP packet is destined for Y, which is connected directly to a subnetwork to which the router is attached. The router therefore creates a packet with a destination address of Y and sends it out onto the LAN. The data finally arrive at Y, where the packet, LLC, and internet headers and trailers can be stripped off.
[Page 520] This service offered by IP is an unreliable one. That is, IP does not guarantee that all data will be delivered or that the data that are delivered will arrive in the proper order. It is the responsibility of the next higher layer, in this case TCP, to recover from any errors that occur. This approach provides for a great deal of flexibility. Because delivery is not guaranteed, there is no particular reliability requirement on any of the subnetworks. Thus, the protocol will work with any combination of subnetwork types. Because the sequence of delivery is not guaranteed, successive packets can follow different paths through the internet. This allows the protocol to react to congestion and failure in the internet by changing routes. IPv4 For decades, the keystone of the TCP/IP protocol architecture has been the Internet Protocol (IP) version 4. Figure 16.14a shows the IP header format, which is a minimum of 20 octets, or 160 bits. The fields are as follows: Version (4 bits): Indicates version number, to allow evolution of the protocol; the value is 4. Internet Header Length (IHL) (4 bits): Length of header in 32-bit words. The minimum value is five, for a minimum header length of 20 octets. DS/ECN (8 bits): Prior to the introduction of differentiated services, this field was referred to as the Type of Service field and specified reliability, precedence, delay, and throughput parameters. This interpretation has now been superseded. The first 6 bits of the TOS field are now referred to as the DS (Differentiated Services) field. The remaining 2 bits are reserved for an ECN (Explicit Congestion Notification) field. Total Length (16 bits): Total IP packet length, in octets. Identification (16 bits): A sequence number that, together with the source address, destination address, and user protocol, is intended to identify a packet uniquely. Thus, this number should be unique for the packet's source address, destination address, and user protocol for the time during which the packet will remain in the internet. Flags (3 bits): Only two of the bits are currently defined. When a packet is fragmented, the More bit indicates whether this is the last fragment in the original packet. The Don't Fragment bit prohibits fragmentation when set. This bit may be useful if it is known that the destination does not have the capability to reassemble fragments. However, if this bit is set, the packet will be discarded if it exceeds the maximum size of an en route subnetwork. Therefore, if the bit is set, it may be advisable to use source routing to avoid subnetworks with small maximum packet size.
[Page 521] Fragment Offset (13 bits): Indicates where in the original packet this fragment belongs, measured in 64-bit units. This implies that fragments other than the last fragment must contain a data field that is a multiple of 64 bits in length. Time to Live (8 bits): Specifies how long, in seconds, a packet is allowed to remain in the internet. Every router that processes a packet must decrease the TTL by at least one, so the TTL is somewhat similar to a hop count.
[Page 522]Protocol (8 bits): Indicates the next higher level protocol, which is to receive the data field at the destination; thus, this field identifies the type of the next header in the packet after the IP header. Header Checksum (16 bits): An error-detecting code applied to the header only. Because some header fields may change during transit (e.g., time to live, segmentation-related fields), this is reverified and recomputed at each router. The checksum field is the 16-bit one's complement addition of all 16-bit words in the header. For purposes of computation, the checksum field is itself initialized to a value of zero. Source Address (32 bits): Coded to allow a variable allocation of bits to specify the network and the end system attached to the specified network (7 and 24 bits, 14 and 16 bits, or 21 and 8 bits). Destination Address (32 bits): Same characteristics as source address. Options (variable): Encodes the options requested by the sending user; these may include security label, source routing, record routing, and timestamping. Padding (variable): Used to ensure that the packet header is a multiple of 32 bits in length.
Figure 16.14. IP Headers (This item is displayed on page 521 in the print version) 
IPv6 In 1995, the Internet Engineering Task Force (IETF), which develops protocol standards for the Internet, issued a specification for a next-generation IP, known then as IPng. This specification was turned into a standard in 1996 known as IPv6. IPv6 provides a number of functional enhancements over the existing IP (known as IPv4), designed to accommodate the higher speeds of today's networks and the mix of data streams, including graphic and video, that are becoming more prevalent. But the driving force behind the development of the new protocol was the need for more addresses. IPv4 uses a 32-bit address to specify a source or destination. With the explosive growth of the Internet and of private networks attached to the Internet, this address length became insufficient to accommodate all systems needing addresses. As Figure 16.14b shows, IPv6 includes 128-bit source and destination address fields. Ultimately, all installations using TCP/IP are expected to migrate from the current IP to IPv6, but this process will take many years, if not decades. IPv6 Header The IPv6 header has a fixed length of 40 octets, consisting of the following fields (Figure 16.14b): Version (4 bits): Internet Protocol version number; the value is 6. DS/ECN (8 bits): Prior to the introduction of differentiated services, this field was referred to as the Traffic Class field and was reserved for use by originating nodes and/or forwarding routers to identify and distinguish between different classes or priorities of IPv6 packets. The first six bits of the Traffic Class field are now referred to as the DS (Differentiated Services) field. The remaining 2 bits are reserved for an ECN (Explicit Congestion Notification) field. Flow Label (20 bits): May be used by a host to label those packets for which it is requesting special handling by routers within a network. Flow labeling may assist resource reservation and real-time traffic processing.
[Page 523]Payload Length (16 bits): Length of the remainder of the IPv6 packet following the header, in octets. In other words, this is the total length of all of the extension headers plus the transport-level PDU. Next Header (8 bits): Identifies the type of header immediately following the IPv6 header; this will either be an IPv6 extension header or a higher-layer header, such as TCP or UDP. Hop Limit (8 bits): The remaining number of allowable hops for this packet. The hop limit is set to some desired maximum value by the source and decremented by 1 by each node that forwards the packet. The packet is discarded if Hop Limit is decremented to zero. Source Address (128 bits): The address of the originator of the packet. Destination Address (128 bits): The address of the intended recipient of the packet. This may not in fact be the intended ultimate destination if a Routing extension header is present, as explained later.
Although the IPv6 header is longer than the mandatory portion of the IPv4 header (40 octets versus 20 octets), it contains fewer fields (8 versus 12). Thus, routers have less processing to do per header, which should speed up routing. IPv6 Extension Headers An IPv6 packet includes the IPv6 header, just discussed, and zero or more extension headers. Outside of IPSec, the following extension headers have been defined: Hop-by-Hop Options Header: Defines special options that require hop-by-hop processing Routing Header: Provides extended routing, similar to IPv4 source routing Fragment Header: Contains fragmentation and reassembly information Authentication Header: Provides packet integrity and authentication Encapsulating Security Payload Header: Provides privacy Destination Options Header: Contains optional information to be examined by the destination node
The IPv6 standard recommends that, when multiple extension headers are used, the IPv6 headers appear in the following order: IPv6 header: Mandatory, must always appear first Hop-by-Hop Options header Destination Options header: For options to be processed by the first destination that appears in the IPv6 Destination Address field plus subsequent destinations listed in the Routing header Routing header Fragment header Authentication header Encapsulating Security Payload header
[Page 524]Destination Options header: For options to be processed only by the final destination of the packet
Figure 16.15 shows an example of an IPv6 packet that includes an instance of each nonsecurity header. Note that the IPv6 header and each extension header include a Next Header field. This field identifies the type of the immediately following header. If the next header is an extension header, then this field contains the type identifier of that header. Otherwise, this field contains the protocol identifier of the upper-layer protocol using IPv6 (typically a transport-level protocol), using the same values as the IPv4 Protocol field. In the figure, the upper-layer protocol is TCP, so the upper-layer data carried by the IPv6 packet consist of a TCP header followed by a block of application data. Figure 16.15. Ipv6 Packet with Extension Headers (containing a TCP segment) 
The Hop-by-Hop Options header carries optional information that, if present, must be examined by every router along the path. The header consists of the following fields: Next Header (8 bits): Identifies the type of header immediately following this header.
[Page 525]Header Extension Length (8 bits): Length of this header in 64-bit units, not including the first 64 bits. Options: Contains one or more options. Each option consists of three subfields: a tag, indicating the option type; a length, and a value.
Only one option has so far been defined: the Jumbo Payload option, used to send IPv6 packets with payloads longer than 216 - 1 = 65,535 octets. The Option Data field of this option is 32 bits long and gives the length of the packet in octets, excluding the IPv6 header. For such packets, the Payload Length field in the IPv6 header must be set to zero, and there must be no Fragment header. With this option, IPv6 supports packet sizes up to more than 4 billion octets. This facilitates the transmission of large video packets and enables IPv6 to make the best use of available capacity over any transmission medium. The Routing header contains a list of one or more intermediate nodes to be visited on the way to a packet's destination. All routing headers start with a 32-bit block consisting of four 8-bit fields, followed by routing data specific to a given routing type. The four 8-bit fields are Next Header, Header Extension Length, and Routing Type: Identifies a particular Routing header variant. If a router does not recognize the Routing Type value, it must discard the packet. Segments Left: Number of explicitly listed intermediate nodes still to be visited before reaching the final destination.
In addition to this general header definition, the IPv6 specification defines the Type 0 Routing header. When using the Type 0 Routing header, the source node does not place the ultimate destination address in the IPv6 header. Instead, that address is the last address listed in the Routing header, and the IPv6 header contains the destination address of the first desired router on the path. The Routing header will not be examined until the packet reaches the node identified in the IPv6 header. At that point, the IPv6 and Routing header contents are updated and the packet is forwarded. The update consists of placing the next address to be visited in the IPv6 header and decrementing the Segments Left field in the Routing header. IPv6 requires an IPv6 node to reverse routes in a packet it receives containing a Routing header, to return a packet to the sender. The Fragment header is used by a source when fragmentation is required. In IPv6, fragmentation may only be performed by source nodes, not by routers along a packet's delivery path. To take full advantage of the internetworking environment, a node must perform a path discovery algorithm that enables it to learn the smallest maximum transmission unit (MTU) supported by any subnetwork on the path. In other words, the path discovery algorithm enables a node to learn the MTU of the "bottleneck" subnetwork on the path. With this knowledge, the source node will fragment, as required, for each given destination address. Otherwise the source must limit all packets to 1280 octets, which is the minimum MTU that must be supported by each subnetwork. In addition to the Next Header field, the fragment header includes the following fields: Fragment Offset (13 bits): Indicates where in the original packet the payload of this fragment belongs. It is measured in 64-bit units. This implies that fragments (other than the last fragment) must contain a data field that is a multiple of 64 bits long.
[Page 526] Res (2 bits): Reserved for future use. M Flag (1 bit): 1 = more fragments; 0 = last fragment. Identification (32 bits): Intended to identify uniquely the original packet. The identifier must be unique for the packet's source address and destination address for the time during which the packet will remain in the internet. All fragments with the same identifier, source address, and destination address are reassembled to form the original packet.
The Destination Options header carries optional information that, if present, is examined only by the packet's destination node. The format of this header is the same as that of the Hop-by-Hop Options header. |
