Point-to-Point Protocol in Objective-C

Drawing PDF-417 2d barcode in Objective-C Point-to-Point Protocol

Point-to-Point Protocol (PPP) is another data-encapsulation protocol that can be used with a wide variety of links PPP is a highly robust and configurable protocol, and it is currently the major standard for modem-based serial communications PPP is actually a suite of protocols that most closely resembles a datalink-layer protocol Some sub-protocols of PPP are the following:

Link Control Protocol (LCP) Used to negotiate encapsulation options, detect configuration errors, perform peer authentication, and terminate connections

Password Authentication Protocol (PAP) Used to perform user authentication across serial links Cleartext authentication (unencrypted) Challenge Handshake Authentication Protocol (CHA) Also used to perform user authentication across serial links Encrypted authentication Network Control Protocol (NCP) Used to perform functions specific to each type of upper-layer network protocol supported

PPP supports the following features:

IP header compression (RFC 2509) Data compression using a variety of algorithms, including Microsoft Point-to-Point Compression (MPPC), Predictor, and STAC Multiple upper-layer protocols using various NCPs Multiplexing of multiple upper-layer protocols over a single physical connection (in other words, running Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX) and TCP/IP over the same dial-up connection) Dynamic address allocation (using the appropriate NCP) Error detection with a frame check sequence (FCS) Tunneling using Level 2 Tunneling Protocol (L2TP) (RFC 2661) or Point-to-Point Tunneling Protocol (PPTP) (RFC 2637) A wide variety of physical- and datalink-layer technologies including X25, serial, Frame Relay, and Asynchronous Transfer Mode (ATM) using AAL5 Bonding of connections with Multilink PPP (MP) (RFC 1990) Dynamic allocation of bandwidth using Bandwidth Allocation Protocol (BAP) (RFC 2125) Many other less common enhancements

PPP is formally defined by RFCs 1661 and 1662, Standard 51, and is available at ftp://ftpisiedu/in-notes/std/std51txt

Internet Protocol (IP) has a fairly simple job IP defines network-layer addressing in the TCP/IP suite and how, when, and where to route packets Version 4 is the current version of IP (defined in RFC 791, Standard 5), and version 6 is the latest version (defined in RFC 1884) We will concentrate our efforts on version 4 because version 6 is not yet widely used and is not expected to be for quite some time IP is responsible for two major jobs: addressing and fragmentation Fragmentation occurs when the layer below IP (datalink) cannot accept the packet size IP is attempting to send it In this case, IP chops the packets into smaller pieces for the datalink layer It also sets the More Fragments (MF), Fragment Offset (FO), and Fragment Identification fields to aid the endpoint in reassembly IP addresses (in version 4) are 32-bit addresses generally represented with four octets in dotted decimal notation For example, a common IP address is 19216811 Each section, or octet, consists of eight bits, which means the maximum value for any given octet is 255 and the minimum is 0 IP specifies an additional part of the address known as a subnet mask, which masks, or hides, part of the address to split an IP address into its two basic components: Network and Host (We will examine IP addressing in more detail in the next chapter)

Finally, IP specifies four additional fields in the IP header of a packet that require further explanation: Type of Service (TOS), Time to Live (TTL), an Options field for additional customization, and a Header Checksum field Figure 5-1 provides a breakdown of an IP packet, along with a basic description of each of the fields

The TOS field in an IP packet tells network devices along the path what priority, or precedence, the packet has, and how much delay, throughput, and reliability it requires The TOS field is generally used by protocols at other layers (such as ATM) to define quality of service (QOS) parameters and modify queues along the path of a connection The TTL field in an IP packet is used to discard packets that have been en route for too long The TTL is measured in seconds and is an 8-bit field, making the maximum time a packet could be in transit 255 seconds In reality, a packet will never reach this limit because each device (mainly routers) operating at layer 3 or above along the path is required to decrement the TTL by at least one, even if the device has the packet for less than one second When the TTL field reaches zero, the packet is discarded This field exists so that, in case a packet is caught in a routing loop, it will not circle the network endlessly The Options field is of variable length and is rarely used (it would be the last field in an IP header, but is not shown in Figure 5-1) A few standard options have been defined, which consist mainly of fields for source routing (where the source device determines the route for a packet) and security descriptions for the DOD (secret, top secret, and so on) The Header Checksum field is a simple computation that can adequately detect bit errors in the IP header If bit errors are detected in the header, the packet will be discarded

Finally, the IP version 4 specification, as defined in RFC 791, is available at the following address: ftp://ftpisiedu/in-notes/rfc791txt