■To DS A 1-bit flag that indicates (when set to 1) that the frame is destined for the distribution system (DS), which is the wired network that connects wireless APs and provides access to wired network nodes. Only wireless nodes that are operating in infrastructure mode set this flag.

■From DS A 1-bit flag that indicates (when set to 1) that the frame is originating from the wired network. This flag is set by the wireless AP only when forwarding a frame to a wireless node operating in infrastructure mode.

■More Fragments A 1-bit flag that indicates (when set to 1) that there are more fragments of the frame for which this frame is also a fragment. If the frame is not fragmented or is the last fragment of a fragmented frame, the More Fragments flag is set to 0.

■Retry A 1-bit flag that indicates (when set to 1) that this frame is a retransmission of a previously transmitted frame.

■Power Management A 1-bit flag that indicates (when set to 1) that the transmitting wireless node is operating in a power-saving mode.

■More Data A 1-bit flag that indicates (when set to 1) that the wireless AP has at least one frame buffered to send to the wireless node.

■WEP A 1-bit flag that indicates (when set to 1) that the payload is encrypted.

■Order A 1-bit flag that indicates (when set to 1) that the frames must be processed in order.

WAN Media

To successfully troubleshoot IPv6 problems in a wide area network, it is important to understand WAN encapsulations. WAN technologies encompass point-to-point technologies—for example, analog phone lines, Integrated Services Digital Network (ISDN), T-Carrier, and Switched-56—and packet-switched technologies—for example, X.25, Frame Relay, and Asynchronous Transfer Mode (ATM). In each of these technologies, the packet needs to be delimited, addressed, and identified as an IPv6 packet.

Current RFCs exist for sending IPv6 packets over the following WAN media:

■Point-to-Point Protocol (PPP) (RFC 5072)

■X.25 (RFC 1356)

■Frame Relay (RFC 2590)

■ATM (RFC 2492)

Note There is no specific RFC for IPv6 packets over an X.25 link; however, RFC 1356 describes how any packet is sent over an X.25 link.

392 Understanding IPv6, Second Edition

Windows Server 2008 and Windows Vista support IPv6 over PPP and Frame Relay links.

PPP

PPP, described in RFC 1661, is a standardized serial line encapsulation method that can be used over asynchronous serial lines—such as analog phone lines—and over synchronous serial lines—such as T-Carrier, ISDN, or Synchronous Optical Network (SONET). PPP is a family of protocols that describe the following:

■A multiprotocol encapsulation method.

■A Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection.

■A family of Network Control Protocols (NCPs) for establishing and configuring different network-layer protocols. RFC 5072 describes the IPv6 Control Protocol (IPV6CP), which is the NCP for configuring the IPv6 protocol over a PPP link.

Only the encapsulation method is described here.

PPP encapsulation uses a variant of the ISO High-Level Data Link Control (HDLC) protocol as described in RFC 1662. IPv6 encapsulation for PPP links is described in RFC 5072. Figure A-8 shows PPP encapsulation of IPv6 packets.

Flag = 0x7E

Address = 0xFF

Control = 0x3

Protocol = 0x57

IPv6 Packet

• • •

Frame Check Sequence

Flag = 0x7E

Figure A-8 PPP with HDLC framing encapsulation of IPv6 packets

The fields in the PPP header and trailer are the following:

■Flag The Flag field indicates the start and end of a PPP frame and is set to 0x7E. In consecutive PPP frames, only a single Flag character is used to mark both the end of a PPP frame and the beginning of the next PPP frame. The size of this field is 8 bits.

■Address The Address field is used in HDLC environments to address the frame to a destination node. On a point-to-point link, the destination node does not need to be addressed. Therefore, for PPP, the Address field is set to 0xFF, which is the broadcast address. The size of this field is 8 bits. Typically, PPP peers suppress the use of this field.

■Control The Control field is used in HDLC environments for data-link layer sequencing and acknowledgments. For PPP, the Control field is set to 0x3 to indicate an unnumbered information (UI) frame. The size of this field is 8 bits. Typically, PPP peers suppress the use of this field.

■Protocol The Protocol field identifies the protocol of the PPP payload. The Protocol field is set to 0x57 to indicate an IPv6 packet. In contrast, the Protocol field is set to 0x21 to indicate an IPv4 packet. The size of this field is 16 bits. Although defined as a 16-bit field, typically the Protocol field size is compressed to 8 bits.

■Frame Check Sequence The Frame Check Sequence field stores a checksum value that is used to check for bit-level errors in the PPP frame. The size of this field is 16 bits.

The MTU for a PPP connection—called the Maximum Receive Unit (MRU)—is negotiated by using LCP. The default MRU is 1500 octets. If negotiated lower, a PPP host must still have the ability to receive 1500-octet frames in the case of link synchronization failure.

X.25

X.25 was developed in the 1970s to provide dumb terminals with WAN connectivity across public data networks (PDNs). However, because of its flexibility and reliability, X.25 has emerged as an international standard for sending data across PDNs.

X.25 is a connection-oriented interface to a packet-switched network (PSN). It provides error checking with guaranteed delivery of packets over the PSN by using either switched virtual circuits (SVCs) or permanent virtual circuits (PVCs). Because of its reliability and guaranteed delivery, X.25 works effectively for applications that require reliable transmission.

A router can connect via a Packet Assembler/Disassembler (PAD) to the X.25 network. The PAD is responsible for breaking down messages into packets and addressing them appropriately. The connection between the router and the PAD, the operations at the PAD, and the connection from the PAD to the carrier office is defined by the X.28, X.3, and X.29 specifications.

The X.25 specification maps to layers 1 through 3 of the Open Systems Interconnect (OSI) model. However, the X.25 specification was developed before the OSI model, so layer names are slightly different. The Physical layer is called X.21 and specifies the electrical and physical interface. The Data Link layer is the Link Access Procedure-Balanced (LAPB) protocol, which takes care of frame composition, flow control, and error checking at the Link Access layer. The Packet layer corresponds to the Network layer and is responsible for the setup and addressing of the virtual circuit.

When IPv6 packets are encapsulated for transmission on X.25 networks, they are wrapped with two sets of headers that correspond to the X.25 Network and Data Link layers. The X.25 Network layer uses the Packet Layer Protocol (PLP). The X.25 Data Link layer uses LAPB, which is the same HDLC format as PPP.