The PPP Protocol in Java

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The PPP Protocol
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Error detection A PPP receiver must be able to detect bit errors in the received frame Connection liveness PPP must be able to detect a failure at the link level (eg, the inability to transfer data from the sending side of the link to the receiving side of this link) and signal this error condition to the network layer Network Layer Address Negotiation PPP must provide a mechanism for the communicating network layers (eg, IP) to learn or configure each other's network layer address Simplicity PPP was required to meet a number of additional requirements beyond the seven listed above On top of all of these requirements, first and foremost among all of the PPP requirements is that of "simplicity" RFC 1547 states "the watchword for a point-to-point protocol should be simplicity" A tall order indeed given all of the other requirements placed on the design of PPP! More than 50 RFC's now define the various aspects of this "simple" protocol
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While it may appear that many requirements were placed on the design of PPP, the situation could actually have been much more difficult! The design specifications for PPP also explicitly note protocol functionality that was PPP was not required to implement:
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Error correction PPP is required to detect bit errors but is not required to correct them Flow control A PPP receiver is expected to be able to receive frames at the full rate of the underlying physical layer If a higher layer can not receive packets at this full rate, it is then up to the higher layer to drop packets or throttle the sender at the higher layer That is, rather than having the PPP sender throttle its own transmission rate, it is the responsibility of a higher level protocol to throttle the rate at which packets are delivered to PPP for sending Sequencing PPP is not required to deliver frames to the link receiver in the same order in which they were sent by the link sender It is interesting to note that while this flexibility is compatible with the IP service model (which allows IP packets to be delivered end-to-end in any order), other network layer protocols which operate over PPP do require sequenced end-to-end packet delivery Multipoint links PPP need only operate over links that have a single sender and a single receiver Other link layer protocols (eg, HDLC) can accommodate multiple receivers (eg, an Ethernet-like scenario) on a link
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Having now considered the design goals )and non-goals) for PPP, let us see how the design of PPP met these goals
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581 PPP Data Framing
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Figure 58-1 shows a PPP data frame using HDLC-like framing [RFC 1662]
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The PPP Protocol
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Figure 58-1: PPP data frame format The PPP frame contains the following fields:
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Flag field Every PPP frame begins and ends with a 1 byte flag field with a value of 01111110 Address field The only possible value for this field is 11111111 Control Field The only possible value of this field is 00000011 Because both the address and control fields can currently take only a fixed value, one wonders why the fields are even defined in the firstplace The PPP specification [RFC 1622] states that other values "may be defined at a later time," although none have been defined to date Because these fields take fixed values, PPP allows the sender to simply not send the address and control bytes, thus saving two bytes of overhead in the PPP frame Protocol The protocol field tells the PPP receiver the upper layer protocol to which the received encapsulated data (ie, the contents of the PPP frame's info field) belongs On receipt of a PPP frame, the PPP receiver will check the frame for correctness and then pass the encapsulated data on to the appropriate protocol [RFC 1700] defines the 16-bit protocol codes used by PPP Of interest to us are the IP protocol (ie, the data encapsulated in the PPP frame is an IP datagram) which has a value of 21 hexadecimal, other network layer protocols such as Appletalk (29) and DECnet (27), the PPP link control protocol (c021 hexadecimal) that we discuss in detail in the following section, and the IP Control Protocol (8021) which is called by PPP when a link is first activated in order to configure the IP-level connection between the two routers on each end of the link (see below) Information This field contains the encapsulate packet (data) that is being sent by an upper layer protocol (eg, IP) over the PPP link The default maximum length of the information field is 1500 bytes, although this can be changed when the link is first configured, as discussed below Checksum The checksum field is used to detect bit errors in a transmitted frame It uses either a two or four byte HDLC-standard cyclic redundancy code
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