Figure 13-1: Six Fields Make Up the PPP Frame
For serial links, such as the connection between Customer routers and the first provider edge router - Cisco routers use HDLC by default, which is a Cisco version of PPP. However, non-Cisco routers use standard PPP.
PPP can run over async (dial-up) or synch (ISDN) lines. It runs using two dala-link layer 2 protocols, LCP (Link Control Protocol) and NCP (Network Control Protocol). There are two authentication methods, PAP (Password Authentication Protocol)and CHAP (Challenge Authentication Protocol).
The only way for Cisco routers to communicate with PPP is to encapsulate it, then at one of the routers enter a connect command by simply stating the other router’s username and password, as follows :
int s0
encap ppp - both ends of a serial link must be encapsulating PPP for this to work !!
username remote_hostname password remote_password - connect to the other router
for example, Router A to Router B where Router B hostname is Todd1 and the password is coffee - at router A type: hostname Todd1 password coffee
NOTE: both routers must have the same password !!!
PPP
*** overhead is 7 to 9 bytes
see http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ppp.htm
PPP runs at the Datalink layer (ISO layer 2), providing symmetric, peer-to-peer connections utilizing encapsulation, transmission and link management services for the upper layer network protocols. Modems, routers and even workstations utilize PPP for various serial connections.
The Point-to-Point Protocol (PPP) originally emerged as an encapsulation protocol for transporting IP traffic over point-to-point links. PPP also established a standard for the assignment and management of IP addresses, asynchronous (start/stop) and bit-oriented synchronous encapsulation, network protocol multiplexing, link configuration, link quality testing, error detection, and option negotiation for such capabilities as network layer address negotiation and data-compression negotiation. PPP supports these functions by providing an extensible Link Control Protocol (LCP) and a family of Network Control Protocols (NCPs) to negotiate optional configuration parameters and facilities. In addition to IP, PPP supports other protocols, including Novell's Internetwork Packet Exchange (IPX) and DECnet.
PPP provides a method for transmitting datagrams over serial point-to-point links. PPP contains three main components:
General Operation
To establish communications over a point-to-point link, the originating PPP first sends LCP frames to configure and (optionally) test the data link. After the link has been established and optional facilities have been negotiated as needed by the LCP, the originating PPP sends NCP frames to choose and configure one or more network layer protocols. When each of the chosen network layer protocols has been configured, packets from each network layer protocol can be sent over the link. The link will remain configured for communications until explicit LCP or NCP frames close the link, or until some external event occurs (for example, an inactivity timer expires or a user intervenes).
Physical Layer Requirements
PPP is capable of operating across any DTE/DCE interface. Examples include EIA/TIA-232-C (formerly RS-232-C), EIA/TIA-422 (formerly RS-422), EIA/TIA-423 (formerly RS-423), and International Telecommunication Union Telecommunication Standardization Sector (ITU-T) (formerly CCITT) V.35. The only absolute requirement imposed by PPP is the provision of a duplex circuit, either dedicated or switched, that can operate in either an asynchronous or synchronous bit-serial mode, transparent to PPP link layer frames. PPP does not impose any restrictions regarding transmission rate other than those imposed by the particular DTE/DCE interface in use.
PPP Link Layer
PPP uses the principles, terminology, and frame structure of the International Organization for Standardization (ISO) HDLC procedures (ISO 3309-1979), as modified by ISO 3309:1984/PDAD1 "Addendum 1: Start/Stop Transmission." ISO 3309-1979 specifies the HDLC frame structure for use in synchronous environments. ISO 3309:1984/PDAD1 specifies proposed modifications to ISO 3309-1979 to allow its use in asynchronous environments. The PPP control procedures use the definitions and control field encodings standardized in ISO 4335-1979 and ISO 4335-1979/Addendum 1-1979. The PPP frame format appears in Figure 13-1.
Figure 13-1: Six Fields Make Up the PPP Frame
The following descriptions summarize the PPP frame fields illustrated in Figure 13-1:
The LCP can negotiate modifications to the standard PPP frame structure. Modified frames, however, always will be clearly distinguishable from standard frames.
The PPP LCP provides a method of establishing, configuring, maintaining, and terminating the point-to-point connection. LCP goes through four distinct phases.
First, link establishment and configuration negotiation occur. Before any network layer datagrams (for example, IP) can be exchanged, LCP first must open the connection and negotiate configuration parameters. This phase is complete when a configuration-acknowledgment frame has been both sent and received.
This is followed by link quality determination. LCP allows an optional link quality determination phase following the link-establishment and configuration-negotiation phase. In this phase, the link is tested to determine whether the link quality is sufficient to bring up network layer protocols. This phase is optional. LCP can delay transmission of network layer protocol information until this phase is complete.At this point, network layer protocol configuration negotiation occurs. After LCP has finished the link quality determination phase, network layer protocols can be configured separately by the appropriate NCP and can be brought up and taken down at any time. If LCP closes the link, it informs the network layer protocols so that they can take appropriate action.Finally, link termination occurs. LCP can terminate the link at any time. This usually is done at the request of a user but can happen because of a physical event, such as the loss of carrier or the expiration of an idle-period timer.Three classes of LCP frames exist. Link-establishment frames are used to establish and configure a link. Link-termination frames are used to terminate a link, and link-maintenance frames are used to manage and debug a link.These frames are used to accomplish the work of each of the LCP phases.
The Point-to-Point Protocol (PPP) originally emerged as an encapsulation protocol for transporting IP traffic over point-to-point links. PPP also established a standard for assigning and managing IP addresses, asynchronous and bit-oriented synchronous encapsulation, network protocol multiplexing, link configuration, link quality testing, error detection, and option negotiation for added networking capabilities.
PPP provides a method for transmitting datagrams over serial point-to-point links, which include the following three components:
PPP is capable of operating across any DTE/DCE interface. PPP does not impose any restriction regarding transmission rate other than those imposed by the particular DTE/DCE interface in use.
Six fields make up the PPP frame. The PPP LCP provides a method of establishing, configuring, maintaining, and terminating the point-to-point connection.
HDLC
*** overhead is 5 to 6 bytes - or if a Cisco router is running PPP, it must encapsulate with HDLC and then the overhead is 12 to 15 bytes
High-level Data Link Control (HDLC) is an enhanced derivative of SDLC from IBM. HDLC is the default serial link protocol for Cisco routers. Nearly all other routers on the market use PPP for link control.HDLC is a Data Link layer protocol. These protocolsallow computers on either end of a physical connection (end stations) to communicate.
STATION TYPES
COMMUNICATIONS MODES
- Primary - Primary stations manage the connection to all secondary sessions by sending commands. Primary stations open and close connections and poll remote stations for data, or availability. Used primarilly in multi-point or mainframe applications.
- Secondary - Secondary machines respond to commands sent from the Primary. Secondary stations are usually Terminals attatched to mainframes.
- Combined - Combined stations can establish and tear down their own connections. A combined station has the capabilities of both primary and secondary stations. These station types are usually used in point-to-point serial links.
Stations may communicate in one of two modes
DATA TRANSFER MODES
- Unbalanced - Communication between a primary and one or more secondary stations.
- Balanced - Communications between two combined stations.
FRAME STRUCTURE
- Normal Response Mode (NRM) - A Secondary station running in an Unbalanced communications environment may may only communicate when signalled to do so by a Primary.
- Asynchronous Balanced Mode (ABM) - When in a Balanced communications environment, either Combined station may initiate communications. (LAPB Mode on combined stations only.)
- Asynchronous Response Mode (ARM) - When in an Unbalanced environment, the Secondary may transmit at will, however the Primary is still responsible for error detection, line and flow control.
An HDLC frame is structured as follows. It has 5 to 6 bytes of overhead:
| FLAG | ADDRESS | CONTROL | INFORMATION | FCS | FLAG |
| 8 bits | 8 bits | 8 / 16 bits | variable | 8 | 8 bits |
FIELDS:
- FLAG
- The 8 bit pattern "01111110" which signals the beginning and end of an HDLC frame. If a piece of data within the frame to be transmitted contains a series of 5 or more 1's, the transmitting station must insert a 0 to distinguish this set of 1's in the data from the flags at the beginning and end of the frame. This technique of inserting bits is called bit-stuffing. These bits are detected and removed upon receipt. If a pattern of five 1's is followed by a 1 and then a 0, it marks the end of the frame. If a pattern of five ones is followed by two more 1's, it is a signal from the transmitting station to abort.
- ADDRESS
- The address field always contains the address of a secondary station. When the primary station transmits, it is the receiving secondary station's address, if it is a secondary responding, it is it's own address. This field is only populated for Unbalanced connections, it is otherwise empty for point-to-point (Balanced) links.
- CONTROL
- The format of the control field varies with the data it contains. There are three categories of HDLC frames:
- Information (I-frame) - Carries data
- Supervisory (S-frame) - Carries commands and responses
- Unnumbered (U-frame) - Carries additional command sequences