Colin Walls , in Embedded Software (Second Edition), 2012
8.7.1 Introduction
PPP (Signal-to-Point Protocol) provides a standard method for transporting multiprotocol datagrams over betoken-to-point links. In the context of a network awarding, PPP allows IP datagrams to be exchanged with a node at the other end of a indicate-to-betoken link. Typically, a client will initiate a PPP connection past using a modem to punch into a foreign server through the public telephone system. All the same, PPP is also used in environments where the physical medium is not always point-to-point. One such instance is Ethernet. The PPPoE and L2TP protocols enable back up for transmission of PPP packets over Ethernet.
A PPP implementation may include support for a PPP client and a PPP server, perhaps even existence utilized as both at the same fourth dimension. Applications just have to be aware that PPP is being used as the underlying link-layer driver when establishing and breaking the physical link—that is, during punch-up and hang up. In all other respects, the application is not enlightened that PPP is the low-level driver being used.
Abstracted Link-Layer Interface
Because PPP is now being adapted for utilise over various types of physical mediums, including ATM and broadcast mediums such as Ethernet, it is necessary to recognize PPP as providing for communications over logical point-to-point links as well as physical betoken-to-point links. To provide for flexibility in supporting multiple link layers, the interface to the link layer is commonly abstracted. The interface to each link layer is thus a self-contained module. Series (HDLC) and Ethernet (PPPoE, L2TP) link layers are examples. This modularization of PPP results in greater organisation flexibility, efficient code reuse, and hardware transparency for easier application development. It also makes information technology straightforward for users to plug in support for new link layers; for case, PPPoA (PPP over ATM).
HDLC and Modem Support
PPP originated as a protocol for sending datagrams over serial indicate-to-signal links. These links were usually dial-up links. Today this is still by far the primary employ for PPP. Every bit a result, PPP commonly includes support for HDLC framing, as well equally basic support for driving a Hayes-uniform modem.
Network Communications Protocols and Congenital-in Security
Timothy Stapko , in Practical Embedded Security, 2008
Point to Point Protocol (PPP)
PPP is a relatively erstwhile communications protocol, described in 1994 in RFC 1661,1 and was designed to provide connectivity over serial hardware channels. PPP was originally developed to allow higher-level protocols to utilize these serial channels in a consistent mode. This protocol, though losing out to newer, faster technologies, is even so used widely for embedded systems due to the fact that simple series hardware is much less expensive than the hardware that some of the newer standards require. PPP consists of a few protocols designed to institute the serial link, encapsulate the higher-level information, and to control each of the high level protocols that can be used. Many different high-level protocols are uniform with PPP, but since nosotros are discussing Internet security, it is reasonable to restrict our discussion to cover merely the Internet Protocol, or IP. We will see in a minute how IP and PPP work together, merely first we will look at the link institution and hash out what it means for our embedded applications.
PPP, as with many depression-level protocols, is designed to be the connectedness between the network hardware and the application. Link establishment in PPP is controlled by the Link Control Protocol, or LCP. The LCP divides the link establishment procedure into 4 distinct steps:
1.
Found the serial link using the hardware.
ii.
Optionally, test the quality of the link to decide if the hardware can handle the advice level desired.
3.
Negotiate and configure the higher-level protocol for transmission.
4.
Cease the link and release the hardware.
To illustrate how we tin can adapt the protocol for our needs, let's look at the steps of the LCP. Obviously, we take to have a hardware connection in order to communicate with the remote device. However, the link quality exam is an optional step; leaving information technology out may result in subpar communications, but it volition definitely save on lawmaking size, and the link tin can exist established faster. If we were trying to use PPP on an 8-bit CPU with only 64KB of program and data space (combined), this is a place where we can definitely cut corners to save on precious infinite. The tradeoff here is that while nosotros obviously proceeds more than space for our program and a small performance advantage, we lose some guarantee of robustness, and therefore, security. Imagine that an attacker could interfere with your communications by generating a high amount of electronic noise about the wire used for communications. Without the link quality exam, yous volition non exist able to decide if the link can handle the connection y'all demand. The assailant and so can hinder your awarding without having to cut a wire or hack into the system.
This is the type of tradeoff that we will focus on in developing secure applications that need to fit onto our resources-constrained devices. On one paw, nosotros have the strict size requirement, on the other a potential security take chances. Depending on the requirements of the application, information technology may be more than desirable to remove the optional stride and salvage precious space. Nosotros volition look at some of these requirements and then you lot can empathize how to look for these types of options when dissecting security and communications protocols. For now, let's go on looking at PPP and come across what we can do with the communication protocol itself.
PPP is an inherently configurable protocol, allowing for many implementation options. For each network-layer protocol, at that place is a corresponding Network Control Protocol (NCP) that allows the protocol to utilize PPP equally the link-layer send mechanism. Each NCP is designed to provide the correct functionality to let PPP to send the higher-level packets. This functionality includes any security protocols inherent to the higher level protocols, such as IPSEC (security for IP). The selection of NCP's to support is another pick we accept to conserve code space. If we know that our awarding volition merely need to support ane higher-level protocol (such as IP), then we just have to implement the functionality specific for the NCP for that particular protocol. We can tailor the implementation to fit the protocol, and since nosotros practise non accept to support the other NCP's, we tin can simply reject any protocols that we practice not support. We need to exist sure that the rejection mechanism is robust, however, since if the mechanism is not well behaved, it could prepare a situation where a denial-of-service or buffer overflow might exist possible. For example, if the LCP on the remote terminate (asking for a link to the device) sends a protocol asking mechanism that is very large, it tin can overflow local buffers, allowing a user on the remote cease to crash the device or run arbitrary lawmaking (depending on the local implementation). If the rejection mechanism is dull, or if the implementation allows multiple connections from a single remote device with no limit between retransmissions, a remote device could send a flurry of connection requests, effectively preventing the device from serving legitimate requests. This is an example of where robust design and programming can make the device more secure.
Figure iii. PPP Structure
PPP has its own security mechanisms that we can utilise to authenticate connection requests, allowing the implementation to protect the device from unauthorized utilise. The security mechanisms supported past PPP are password authentication and a challenge-handshake. Again, we can choose to support either of these mechanisms. The password mechanism will have a simpler implementation, since the challenge-handshake will require additional states in the PPP state automobile to handle the additional messages. However, nosotros may besides choose not to support any PPP security, instead relying on the higher-level protocols to provide security for the awarding. Depending on the application, the no-security option may exist more desirable. If the network is not secure, sending a password would let anyone eavesdropping on the network to read the password.
The challenge-handshake protocol, though more complex, is besides more secure than the password protocol. The common challenge-handshake protocol for PPP is defined in RFC 1994 (written in 1996), and is referred to as the Challenge Handshake Authentication Protocol, or CHAP. CHAP provides decent security for devices with a previously defined trusted human relationship, simply since it requires shared secret keys (cryptographic keys stored on each end), it is non practical for full general-purpose security (connecting to arbitrary remote systems). Without an established human relationship, the secret for the challenge machinery must be sent plaintext over the network—which is obviously not secure at all. Whether or not such a relationship can be established should be a factor in deciding whether or non to support the authentication protocol.
Equally was mentioned previously, PPP is an older technology, only is yet pop on smaller embedded devices because of its ability to utilise inexpensive networking hardware. Nonetheless, current technology is moving toward newer, more complex low-level protocols, and these new technologies have more security options, but also more security challenges. Your awarding requirements will determine whether PPP or one of these other technologies should be used. Side by side nosotros volition look at another applicative technology that has gained widespread popularity and is practically the only technology used for Local Area Networks (LANs). This engineering science is practically a household give-and-take—Ethernet.
In Bluetooth Application Developer'due south Guide, 2002
PPP
The Betoken-to-Point Protocol (PPP) is the existing method used when transferring Transmission Control Protocol/Cyberspace Protocol (TCP/IP) data over modem connections. The Bluetooth specification reuses this protocol in the local area network (LAN) Access Profile to route network data over an RFCOMM port. Work is already underway on a TCP/IP layer that volition sit directly above L2CAP, bypassing and removing the overhead of PPP and RFCOMM. This piece of work is hinted at in some areas of the specification, but in v1.1 PPP, is all that's available.
Dale Liu , ... Luigi DiGrande , in Cisco CCNA/CCENT Test 640-802, 640-822, 640-816 Preparation Kit, 2009
Test Objectives Fast Track
Understanding PPP and CHAP
■
PPP is a point-to-point WAN protocol that works at the information link layer of the OSI model. PPP is more than stable than Sideslip and includes error-checking features.
■
PPP can operate on a variety of DTE/DCE concrete interfaces, including asynchronous serial, synchronous series, HSSI, and ISDN.
■
When PPP is used on a link, information technology will negotiate with the other side of the link. PPP negotiation consists of 3 phases: LCP, Hallmark, and NCP.
■
PPP uses LCP to set, configure, and test a data link connection.
■
PPP uses NCP to establish and configure unlike network layer protocols. PPP is designed to allow the simultaneous use of multiple network layer protocols, including IPv4 and v6, IPX, and AppleTalk.
■
PPP operates using dissimilar network layer protocols (e.k., IPX and AppleTalk), whereas Skid uses simply TCP/IP-based IP. PPP and SLIP will encapsulate a datagram and other network layer protocol information over bespeak-to-signal links. These are called NCPs.
■
The phases of PPP are Link Dead, Link Establishment, Authentication, Network Layer Protocol, and Link Termination, at which indicate the Link Dead phase is initiated again.
■
PPP uses HDLC as a basis for encapsulating datagrams over point-to-point links.
■
PAP is the older of the two PPP hallmark protocols. It has major security flaws, including the sending of passwords in articulate text and assuasive a client to choose when it sends a password.
■
When CHAP is used over a WAN connection, the router receiving the connectedness sends a challenge which includes a random number that tin can be input into an MD5 hash algorithm. MD5 hashing and server command is a office of CHAP.
■
CHAP uses a iii-mode handshake comprising the local host requesting authentication, the remote host sending an encrypted response, and the local host comparing the received information and then accepting or rejecting the connection. PAP only uses a two-fashion handshake and is much less secure.
■
MS-CHAP is nearly identical to CHAP in terms of how it operates. The main difference between the 2 is that MS-CHAP is Microsoft'due south proprietary version of CHAP and is non an open standard. You will non be tested on MS-CHAP on the CCNA examination directly, but you should know about its employ and its proprietary nature.
■
CHAP and PAP are open up standards-based protocols.
Configuring and Implementing PPP and CHAP on Cisco Routers
■
Yous utilise the show interface command to verify the current state of PPP LCP negotiations.
■
Yous use the debug ppp negotiations command to troubleshoot and resolve issues with LCP communications betwixt peers. This command volition brandish PPP packets transmitted during PPP startup where PPP options are offset negotiated.
■
You use the debug ppp packet control to display the PPP packets that are being sent and received, and when this occurs. This command also displays depression-level packet dumps.
■
You lot employ the debug ppp errors command to display output relating to protocol errors that occur while in the connection negotiation and performance phases. Protocol errors are shown in detail.
■
Yous use the debug ppp chap command to brandish CHAP and PAP packet exchanges betwixt peers. This is helpful in determining whether your peers have a misconfiguration.
■
You use the debug ppp authentication command to troubleshoot and resolve problems with authentication attempts using protocols such every bit CHAP and PAP.
Walter Goralski , in The Illustrated Network (Second Edition), 2017
PPP and DSL
Why is PPP used with DSL (and SONET)? The cadre of the issue is that ISPs needed some kind of tunneling protocol. Tunneling occurs when the normal message-packet-frame encapsulation sequence of the layers of a networking protocol suite are violated. When a message is placed inside a package, then within a frame, and this frame is placed inside some other blazon of frame (or even another frame-packet-frame sequence), this is a tunneling situation. Although many tunneling methods have been standardized at several different TCP/IP layers, tunneling works as long every bit the tunnel endpoints empathize the correct sequence of headers and content (which can too exist encrypted for secure tunnels).
In DSL, the tunneling protocol had to acquit the signal-to-point "circuits" from the key networking location to the customer's premises and beyond the shared media LAN to the end user device (host). There are many ways to practice this, such as using IP-in-IP tunneling, a virtual individual network (VPN), or lower level tunneling. ISPs chose PPP equally the solution for this role in DSL.
Using PPP made perfect sense. For years, ISPs had used PPP to manage their WAN dial-in users. PPP could hands assign and manage the Internet access provider's IP address infinite, compartmentalize users for billing purposes, and so on. As a LAN technology, Ethernet had none of those features. PPP too allowed user authentication methods such equally RADIUS to exist used, methods completely absent on near LAN technologies (if you're on the LAN, it'south assumed yous belong there).
Of course, keeping PPP meant putting the PPP frame within the Ethernet frame, a scheme called Point-to-Point Protocol over Ethernet (PPPoE), described in RFC 2516. Since tunneling is just some other form of encapsulation, all was well.
PPP is non the just information link layer framing and negotiation procedure (PPP is not a full data link layer specification) from the IETF. Before PPP became popular, the Serial Line Net Protocol (SLIP) and a closely related protocol using compression (CSLIP, or Compressed Slip) were used to link individual PCs and workstations not connected by a LAN, simply still running TCP/IP, to the Internet over a punch-upwardly, asynchronous analog telephone line with modems. Slip/CSLIP was likewise once used to link routers on widely separated TCP/IP networks over asynchronous analog leased telephone lines, again using modems. Sideslip/CSLIP is specified in RFC 1055/STD 47.
MCSA/MCSE 70-291: Configuring the Windows 2003 Routing and Remote Access Service LAN Routing, Punch-up Services, and Routing Protocols
Deborah Littlejohn Shinder , ... Laura Hunter , in MCSA/MCSE (Test 70-291) Written report Guide, 2003
PPP Multilink Protocol
PPP has, past Internet standards, a long history with the Internet Technology Task Forcefulness (IETF). The basic documented history of PPP dates dorsum to 1989 when "A Proposal for Multi-Protocol Transmission of Datagrams Over Betoken-to-Indicate Links" was specified in Request For Comments (RFC) 1134. The official implementation, as used past Microsoft, comes from RFP 1990. Capabilities were added and subsequent modifications to the standard were made leading upwards to PPP equally it exists today. In 1994, a documented standard was proposed for "The PPP Multilink Protocol" in RFC 1717. At the time, other proposals existed to combine streams of data at the flake level (basically a hardware solution). This proposal described a software-based solution for the need to combine multiple streams of data into 1. This solution was well-suited to the twin bearer channels of ISDN (2B+D).
The PPP Multilink Protocol must be enabled on both the remote access customer and the remote admission server. PPP Multilink is enabled on the remote access server via remote access policy, using the Routing and Remote Admission Service management panel or the Internet Authentication Service (IAS). The nature of multilink requires dialing to multiple devices or endpoints. To enable Multilink on a remote admission client, you must enable multiple device dialing on the client system through the Network and Dial-upward Connections binder. Again, if unlimited connectivity is non available, the nature of Multilink presents cost prohibitive problems due to the lack of provisions to link and unlink actress concrete connections on an equally-needed basis.
Note
Exist aware that if you utilize Multilink to dial a server that requires callback, only one of your devices is chosen back. Because you can store but one number in a user business relationship, only 1 device connects and all other devices fail to consummate the connection. Some ISDN service uses a single number for both B channels. If your ISDN uses only a single number for both B channels, and then Multilink callback volition work in this case. This attribute of callback means your connection loses Multilink functionality.
The Fundamentals in Understanding Networking Middleware
Tammy Noergaard , in Demystifying Embedded Systems Middleware, 2010
4.5.2 Point-to-Point Protocol Examplev
PPP (indicate-to-point protocol) is a common OSI data-link (or network access layer under the TCP/IP model) protocol that can encapsulate and transmit data to college layer protocols, such every bit IP, over a concrete serial transmission medium (see Figure four.24). PPP provides support for both asynchronous (irregular interval) and synchronous (regular interval) serial advice.
Figure 4.24. Information-link Middleware
PPP is responsible for processing data passing through it as frames. When receiving data from a lower layer protocol, for example, PPP reads the bit fields of these frames to insure that entire frames are received, that these frames are error gratis, that the frame is meant for this device (using the physical address retrieved from the networking hardware on the device), and to determine where this frame came from. If the data are meant for the device, then PPP strips all data-link layer headers from the frame, and the remaining data field, called a datagram, is passed upwardly to a higher layer. These same header fields are appended to data coming down from upper layers past PPP for transmission outside the device.
In full general, PPP software is defined via a combination of four submechanisms:
•
The PPP encapsulation mechanism (in RFC1661) such equally the high-level data-link control (HDLC) framing in RFC1662 or the link command protocol (LCP) framing defined in RFC1661 to process (i.eastward., demultiplex, create, verify checksum, etc.)
•
Data-link protocol handshaking, such equally the link command protocol (LCP) handshaking defined in RFC1661, responsible for establishing, configuring, and testing the data-link connection
•
Authentication protocols, such every bit PAP (PPP authentication protocol) in RFC1334, used to manage security after the PPP link is established
•
Network control protocols (NCP), such as IPCP (Cyberspace protocol control protocol) in RFC1332, that establish and configure upper-layer protocol (i.east., OP, IPX, etc.) settings.
These submechanisms work together in the following manner: a PPP communication link, connecting both devices, tin be in one of five possible phases at whatsoever given time, as shown in Table 4.i. The current phase of the advice link determines which mechanism – encapsulation, handshaking, authentication, so on – is executed.
Tabular array 4.i. Phase Table8
Stage
Description
Link Dead
The link necessarily begins and ends with this stage. When an external event (such as carrier detection or network ambassador configuration) indicates that the physical layer is ready to exist used. PPP gain to the Link Establishment stage. During this phase, the LCP automaton (described subsequently in this chapter) volition be in the Initial or Starting states. The transition to the Link Institution phase signals an Upward consequence (discussed afterward in this chapter; to the LCP automaton.
Institute Link
The link control protocol (LCP) is used to constitute the connection through an exchange of configuration packets. An institute link stage is entered one time a Confiure-Ack packet (described later in this affiliate) has been both sent and received.
Authentication
Hallmark is an optional PPP mechanism. If it does take place, information technology typically does then soon after the establish link phase.
Phase
Description
Network Layer Protocol
Ono: PPP has completed the establish or authentication phases, each network-layer protocol (such equally IP. IPX. orAppleTalk) MUST he separately configured by the advisable network control protocol <NCP).
Link Termination
PPP can end the link at whatsoever time, afterwards which PPP should continue to the Link Expressionless phase.
How these phases interact to configure, maintain, and terminate a point-to-indicate link is shown in Figure 4.25.
Effigy 4.25. PPP Phaseseight
Equally defined by PPP layer one (i.e., RFC1662), data are encapsulated within the PPP frame, an example of which is shown in Effigy 4.26.
Figure 4.26. PPP HDLC-similar Frame8
The flag bytes marking the beginning and end of a frame, and are each set to 0x7E. The accost byte is a high-level information-link command (HDLC) broadcast address and is always set to 0xFF, since PPP does not assign individual device addresses. The command byte is an HDLC control for UI (unnumbered data) and is set to 0x03. The protocol field defines the protocol of the data within the information field (i.e., 0x0021 means the information field contains IP datagram, 0xC021 means the information field contains link control data, 0x8021 ways the information field contains network command data – see Tabular array iv.2). Finally, the data field contains the data for college-level protocols, and the FCS (frame check sequence) field contains the frame's checksum value.
Tabular array iv.2. Protocol Information8
Value (in hex)
Protocol Name
0001
Padding Protocol
0003 to 001 f
Reserved (transparency inefficient)
007d
Reserved (Control Escape)
00cf
Reserved (PPP NLPID)
00ff
Reserved (compression inefficient)
8001 to 801 f
Unused
807d
Unused
80cf
Unused
80ff
Unused
c021
Link Control Protocol
c023
Password Authentication Protocol
c025
Link Quality Report
c223
Claiming Handshake Authentication Protocol
The data-link protocol may also define a frame format. An LCP frame, for example, is equally shown in Figure four.27.
Figure four.27. LCP Frame8
The data field contains the data intended for higher networking layers, and is made up of data (type, length, and data). The length field specifies the size of the entire LCP frame. The identifier is used to match client and server requests and responses. Finally, the code field specifies the type of LCP packet (indicating the kind of action existence taken); the possible codes are summarized in Table 4.3. Frames with codes 1–iv are called link configuration frames, 5 and half dozen are link termination frames, and the rest are link management packets.
Tabular array 4.3. LCP Codeseight
Lawmaking
Definition
I
Configure-Asking
2
Configure-Ack
3
Configure-Nak
4
Configure-Turn down
5
Terminate-Request
6
Terminate-Ack
seven
Code-Reject
8
Protocol-Decline
9
Repeat-Request
10
Echo-Reply
xi
Discard-Request
12
Link Quality Report
The LCP code of an incoming LCP datagram determines how the datagram is processed, as shown in the pseudocode example beneath.
In order for two devices to be able to plant a PPP link, each must transmit a data-link protocol frame, such as LCP frames, to configure and exam the information-link connection. As mentioned, LCP is 1 possible protocol that can exist implemented for PPP, to handle PPP handshaking. Afterwards the LCP frames take been exchanged (and thereby a PPP link established), hallmark tin can then occur. It is at this betoken where authentication protocols, such as PPP Authentication Protocol or PAP, can be used to manage security, through countersign authentication and so forth. Finally, Network Control Protocols (NCP) such as IPCP (Internet Protocol Control Protocol) establish and configure upper-layer protocols in the network layer protocol settings, such as IP and IPX.
At whatsoever given time, a PPP connection on a device is in a item state, as shown in Figure iv.28; the PPP states are outlined in Table 4.4.
Figure four.28. PPP Connection States and Events8
Table 4.4. PPP Stateseight
States
Definition
Initial
PPP link is in the Initial country, the lower layer is unavailable (Down), and no Open up event has occurred. The Restart timer is non running in the Initial state.
Starting
The Starting country is the Open up counterpart to the Initial state. An authoritative Open has been initiated, just the lower layer is even so unavailable (Down). The Restart timer is not running in the Starting state. When the lower layer becomes available (Upwardly), a Configure-Asking is sent.
Stopped
The Stopped state is the Open up counterpart to the Closed land. Information technology is entered when the automaton is waiting for a Down event after the This-Layer-Finished activeness, or after sending a Finish-Ack. The Restart timer is non running in the Stopped state.
Closed
ln the Closed state, the link is available (Up), just no Open up has occurred. The Restart timer is non running in the Closed state. Upon reception of Configure-Asking packets, a Terminate-Ack is sent. Stop-Acks are silently discarded to avert creating a loop.
Stopping
The Stopping state is the Open counterpart to the Closing state. A Terminate-Request has been sent and the Restart timer is running, but a Terminate-Ack has not yet been received.
Endmost
In the Closing land, an attempt is made to terminate the connexion. A End-Request has been sent and the Restart timer is running, but a End-Ack has non yet been received. Upon reception of a Terminate-Ack, the Airtight state is entered. Upon the expiration of the Restart timer, a new Finish-Request is transmitted, and the Restart timer is restarted. After the Restart timer has expired Max-Stop times, the Closed state is entered.
Request-Sent
In the Request-Sent state an endeavour is made to Configure the connection. A Configure-Request has been sent and the Restart timer is running, just a Configure-Ack has not nevertheless been received nor has one been sent.
Ack-Sent
In the Ack-Received state, a Configure-Request has been sent and a Configure-Ack has been received. The Restart timer is all the same running, since a Configure-Ack has not nonetheless been sent.
Opened
In the Opened land, a Configure-Ack has been both sent and received. The Restart timer is not running. When entering the Opened state, the implementation SHOULD signal the upper layers that it is now Upward. Conversely, when leaving the Opened country, the implementation SHOULD betoken the upper layers that it is at present Downwards.
Events (also shown in Effigy 4.28) are what cause a PPP connection to transition from country to land. The LCP codes (from the RFC1661 spec) in Table 4.five define the types of events that crusade a PPP state transition.
Table 4.v. PPP Events8
Outcome Characterization
Event
Description
Up
lower layer is Up
This event occurs when a lower layer indicates that it is ready to deport packets.
Downwards
lower layer is Downwards
This event occurs when a lower layer indicates that it is no longer set to carry packets.
Open
administrative open
This outcome indicates that the link is administratively bachelor for traffic; that is, the network administrator (human or programme) has indicated that the link is allowed to be Opened. When this event occurs, and the link is not in the Opened state, the automaton attempts to transport configuration packets to the peer.
Close
administrative shut
This outcome indicates that the link is not available for traffic; that is, the network administrator (man or program) has indicated that the link is not allowed to exist Opened. When this event occurs, and the link is non in the Closed land, the automaton attempts to terminate the connexion. Further attempts to re-configure the link are denied until a new Open event occurs.
TO+
timeout with counter > 0
This event indicates the expiration of the Restart timer.The Restart timer is used to time responses to Configure-Request and Termimate-Request packets. The TO+ event indicates that the Restart counter continues to exist greater than zero, which triggers the corresponding Configure-Request or Terminate-Asking parcel to exist retransmitted. The TO− event indicates that the Restart counter is non greater than zero, and no more packets need to exist retransmitted.
TO−
timeout with counter expired
RCR+
receive configure request expert
An implementation wishing to open a connection MUST transmit a Configure-Request. The Options field is filled with any desired changes to the link defaults.Configuration Options SHOULD NOT exist included with default values.
RCR−
receive configure asking bad
RCA
receive configure ack
This event occurs when a valid Configure-Ack packet is received from the peer. The Configure-Ack packet is a positive response to a Configure-Request bundle. An out of sequence or otherwise invalid packet is silently discarded. If every Configuration Option received in a Configure-Request is recognizable and all values are adequate, and so the implementation MUST transmit a Configure-Ack. The acknowledged Configuration Options MUST Non be reordered or modified in any way. On reception of a Configure-Ack, the Identifier field MUST match that of the final transmitted Configure-Request. Additionally, the Configuration Options in a Configure-Ack MUST exactly lucifer those of the final transmitted Configure-Request. Invalid packets are silently discarded.
RCN
receive configure nak/rej
This consequence occurs when a valid Configure-Nak or Configure-Pass up parcel is received from the peer. The Configure-Nak and Configure-Decline packets are negative responses to a Configure-Request packet. An out of sequence or otherwise invalid package is silently discarded.
RTR
receive stop request
This event occurs when a Cease-Request packet is received. The Terminate-Request bundle indicates the desire of the peer to close the connection.
RTA
receive terminate ack
This event occurs when a Finish-Ack parcel is received from the peer. The End-Ack packet is usually a response to a End-Request packet. The Stop-Ack bundle may likewise bespeak that the peer is in Airtight or Stopped states, and serves to re-synchronize the link configuration.
RUC
receive unknown lawmaking
This event occurs when an uninterpretable packet is received from the peer. A Code-Turn down bundle is sent in response.
RXJ+
receive code reject permitted or receive protocol turn down
This upshot occurs when a Code-Reject or a Protocol-Pass up parcel is received from the peer. The RXJ+ event arises when the rejected value is acceptable, such as a Code-Reject of an extended code, or a Protocol-Pass up of an NCR. These are inside the telescopic of normal functioning. The implementation MUST cease sending the offending parcel type. The RXJ− event arises when the rejected value is catastrophic, such as a Code-Reject of Configure-Request, or a Protocol-Reject of LCP! This result communicates an unrecoverable fault that terminates the connection.
RXJ−
receive lawmaking reject catastrophic or receive protocol reject
RXR
receive echo request, receive echo reply, or receive discard request
This outcome occurs when an Echo-Request, Echo-Answer or Discard-Request packet is received from the peer. The Echo-Reply bundle is a response to an Echo-Request parcel. There is no reply to an Echo-Reply or Discard-Request packet.
As PPP connections transition from state to state, sure actions are taken stemming from these events, such as the manual of packets and/or the starting or stopping of the Restart timer, every bit outlined in Table iv.half-dozen.
Tabular array iv.half-dozen. PPP Actionsviii
Action Characterization
Action
Definition
tlu
this layer upwardly
This action indicates to the upper layers that the automaton is entering the Opened state. Typically, this action is used by the LCP to signal the Up upshot to an NCP, Authentication Protocol, or Link Quality Protocol, or MAY exist used past an NCP to indicate that the link is available for its network layer traffic.
tld
this layer down
This action indicates to the upper layers that the automaton is leaving the Opened state. Typically, this action is used by the LCP to signal the Downward event to an NCP, Authentication Protocol, or Link Quality Protocol, or MAY exist used by an NCP to indicate that the link is no longer bachelor for its network layer traflic.
tls
this layer started
This action indicates to the lower layers that the automaton is entering the Starting state, and the lower layer is needed for the link. The lower layer SHOULD respond with an Upwardly event when the lower layer is available. The results of this action are highly implementation dependent.
tlf
this layer finished
This activity indicates to the lower layers that the automaton is inbound the Initial, Closed or Stopped states, and the lower layer is no longer needed for the link. The lower layer SHOULD respond with a Down consequence when the lower layer has terminated. Typically, this action MAY exist used by the LCP to advance to the Link Expressionless phase, or MAY exist used by an NCP to indicate to the LCP that the link may terminate when there are no other NCPs open. This results of this activity are highly implementation dependent.
irc
initialize restart count
This action sets the Restart counter to the appropriate value (Max-Finish or Max-Configure). The counter is decremented for each transmission, including the first.
zrc
zero restart count
This action sets the Restart counter to nada.
scr
send configure request
Configure-Request packet is transmitted. This indicates the desire to open a connexion with a specified ready of Configuration Options. The Restart timer is started when the Configure-Request packet is transmitted, to guard against packet loss. The Restart counter is decremented each fourth dimension a Configure-Asking is sent.
sca
send configure ack
A Configure-Ack package is transmitted. This acknowledges the reception of a Configure-Asking packet with an acceptable set of Configuration Options.
scn
send configure nak/rej
A Configure-Nak or Configure-Turn down bundle is transmitted, equally advisable. This negative response reports the reception of a Configure-Request packet with an unacceptable set of Configuration Options, Configure-Nak packets are used to refuse a Configuration Option value, and to suggest a new, adequate value, Configure-Reject packets are used to refuse all negotiation almost a Configuration Pick, typically considering it is non recognized or implemented. The apply of Configure-Nak versus Configure-Reject is more fully described in the affiliate on LCP Packet Formats.
str
send terminate request
A Terminate-Request packet is transmitted. This indicates the desire to close a connection. The Restart timer is started when the Terminate-Request pocket is transmitted, to guard against package loss. The Restart counter is decremented each time a Terminate-Request is sent.
sta
transport terminate ack
A Terminate-Ack parcel is transmitted. This acknowledges the reception of a Terminate-Request parcel or otherwise serves to synchronize the automatons.
scj
send code reject
A Code-Reject parcel is transmitted. This indicates the reception of an unknown type of packet.
ser
send echo reply
An Repeat-Respond packet is transmitted. This acknowledges the reception of an Echo-Request bundle.
PPP states, actions, and events are usually created and configured by the platform-specific code at kick-time, some of which is shown in pseudocode form on the next several pages. A PPP connection is in an initial state upon creation; thus, amid other things, the 'initial' land routine is executed. This lawmaking tin be called after at runtime to create and configure PPP, as well as respond to PPP runtime events (i.e., as frames are coming in from lower layers for processing). For instance, after PPP software demuxes a PPP frame coming in from a lower layer, and the checksum routine determines the frame is valid, the advisable field of the frame can then be used to decide what state a PPP connection is in and thus what associated software country, issue, and/or action office needs to be executed. If the frame is to be passed to a higher layer protocol, then some mechanism is used to point to the college layer protocol that there are data to receive (IPReceive for IP, for example).
In Side by side Generation SSH2 Implementation, 2009
PPP over SSH
The bespeak-to-point-protocol describes the connection and the post-obit communication steps between two virtual network interfaces. Through a simple configuration, information technology is possible to tunnel communications through ssh.
As you can see, ssh represents a plus, which allows most services to add the security concept in the manual and authentication without turning to compromises other than that of the time required for the initial setup of the environment. We can protect any type of service by differentiating which communications are protected from ssh and which are not. The use of ssh is therefore recommended for all services that do non natively offering the protection and check for the integrity of the media.
Eric Conrad , ... Joshua Feldman , in Eleventh Hour CISSP® (Third Edition), 2017
VPN
Virtual private networks (VPNs secure data sent via insecure networks like the Internet. The goal is to virtually provide the privacy afforded by a circuit, such as a T1. The bones construction of VPNs involves secure hallmark, cryptographic hashes such as SHA-1 to provide integrity, and ciphers such as AES to provide confidentiality.
PPP
PPP (point-to-point protocol) is a layer 2 protocol that provides confidentiality, integrity, and authentication via point-to-signal links. PPP supports synchronous links, such as T1s, in addition to asynchronous links, such equally modems.
IPsec
IPv4 has no built-in confidentiality; higher-layer protocols similar TLS provide security. To accost this lack of security at layer three, IPsec (Cyberspace protocol security) was designed to provide confidentiality, integrity, and authentication via encryption for IPv6. IPsec is ported to IPv4. IPsec is a suite of protocols; the major ii are encapsulating security protocol (ESP) and authentication header (AH). Each has an IP protocol number; ESP is protocol 50 and AH is protocol 51.
SSL and TLS
Secure sockets layer (SSL) protects HTTP information: HTTPS uses TCP port 443. TLS is the latest version of SSL, equivalent to SSL version 3.1. The current version of TLS is 1.2.
Though initially focused on the spider web, SSL or TLS may be used to encrypt many types of data and tin can be used to tunnel other IP protocols to form VPN connections. SSL VPNs tin can be simpler than their IPsec equivalents: IPsec makes central changes to IP networking, so installation of IPsec software changes the operating system, which requires super-user privileges. SSL client software does not crave altering the operating system. Also, IPsec is difficult to firewall, while SSL is much simpler.
MCSA/MCSE 70-291: Configuring the Windows Server 2003 Routing and Remote Access Service VPN Services
Deborah Littlejohn Shinder , ... Laura Hunter , in MCSA/MCSE (Exam seventy-291) Study Guide, 2003
L2TP/IPSec
L2TP, first introduced with Windows 2000, combines the benefits of PPTP with Cisco Organization's Layer Two Forwarding (L2F) protocol. More often than not, a layer-ii connection is used to connect a remote customer with a remote admission server, and consequently the PPP connection also terminates at the aforementioned endpoints as the layer ii connections.
L2TP extends PPP to let the PPP and layer-two endpoints to reside on dissimilar devices. Whereas PPTP connects systems over an IP network only, L2TP allows for connectivity over IP, X.25, Frame Relay, or Asynchronous Transfer Mode (ATM). When IP is used as a send mechanism, L2TP uses UDP packets and special L2TP messages to handle tunnel direction. L2TP also carries the tunneled data in UDP encapsulated PPP flames. Think that Microsoft's PPTP provides encryption via MPPE as well as compression via Microsoft Point-to-Point Compression (MPPC). L2TP has provisions for encrypted and compressed PPP encapsulated payloads; however, the Microsoft implementation of L2TP does not provide for these features directly. To encrypt the encapsulated PPP payload, Microsoft's implementation of L2TP must exist used with IPSec's Encapsulating Security Payload (ESP) protocol.
Notation
L2TP can exist used but if both the VPN server and VPN customer support it. Windows 2000 and Windows XP clients and Windows 2000 and Windows Server 2003 servers include built-in L2TP/IPSec support. Windows 9x and Windows NT clients do not include L2TP support, but yous tin can download an L2TP/IPSec client for Windows 98, Windows ME, and Windows NT Workstation 4.0 from Microsoft's Web site at http://www.microsoft.com/windows2000/server/evaluation/news/bulletins/12tpclient.asp.
L2TP is described in IETF RFC 2661. The combination of L2TP with IPSec is described in IETF RFC 3193.
Head of the Class…
NAT Traversal
Using IPSec encrypts not merely the information payload, but also the UDP header. This presents a trouble if the information needs to be tunneled behind a NAT server or router. The UDP header specifies the UDP port number for packet forwarding to a specific service. Encryption of the UDP header means encryption of the UDP port number information, and consequently no forwarding of L2TP/IPSec traffic.
The solution to this problem is a technology chosen NAT traversal (NAT-T), which was developed by a consortium of technology companies, including Cisco Systems and Microsoft NAT-T uses UDP encapsulation, placing the IPSec bundle inside a UCP/IP header. This mode, NAT devices can change the IP address or port number without changing the IPSec packet. NAT traversal communications are transmitted through UDP port 500 (which is normally open up for IKE when IPSec is used).
If the VPN client and server both back up NAT-T, the client and/or server may be placed behind a NAT server or router. Windows Server 2003, unlike Windows 2000 Server, provides special NAT-T capabilities. Microsoft offers a new VPN customer that supports client-side NAT-T for Windows NT 4.0, Windows 98, and Windows ME clients, to be used when connecting to a Windows Server 2003 server.
Both L2TP/IPSec and PPTP/MPPE exhibit certain advantages and disadvantages. Table 7.2 compares L2TP/IPSec with PPTP/MPPE.
Tabular array 7.2. Comparing of L2TP/IPSec and PPTP/MPPE
Factor
PPTP Advantages and Constraints
L2TP/IPSec Advantages and Constraints
Client operating systems supported
Supported on clients running Windows 2000, Windows XP, Windows Server 2003, Windows NT Workstation 4.0, Windows ME, or Windows 98
Built-in support on clients running Windows 2000, Windows XP, or Windows Server 2003. MIs2tp.exe must exist installed for support on clients running Windows 98, Windows Me, or Windows NT Workstation 4.0.
Certificate back up
PPTP requires a certificate infrastructure for EAP-TLS infrastructure for EAO-TLS authenticating server and user certificates to all VPN clients or to issue smart cards to all user,
L2TP/IPSec requires a certificate infrastructure or a preshared fundamental(PSK) to issue calculator certificates to the VPN server and all VPN clients
Security
Captured packets cannot exist interpreted without the encryption fundamental—confidentiality, Does not provide proof that the information was not modified in transit—data integrity. Does non provide proof that the data was sent by the authorized user—data origin authentication. Utilise MS-CHAP v2 equally the authentication with potent passwords to increase security.
Provides data confidentiality, data integrity, data origin authentication, and replay protection. Offers the highest level of security.
Performance
A VPN server is capable of supporting more PPTP connections than L2TP/IPSec connections.
IPSec encryption is processing-intensive. A VPN server supports fewer L2TP connections than PPTP connections because of additional processing overhead. To support additional L2TP connections, increase CPU processing power or network adapters designed for encrypted traffic.
NAT back up
PPTP-based VPN clients tin exist located backside a NAT if the NAT includes an editor that can translate PPTR
If yous locate L2TP/IPSec-based clients or servers behind a NAT, both customer and server must support IPSec NAT traversal (NAT-T).
Test Warning
L2TP is still the latest and greatest for VPN security. Ensure that you lot empathise the similarities and differences betwixt L2TP and PPTP. Although L2TP/IPSec tends to provide a more than secure VPN solution, PPTP still has its advantages too. Make certain yous empathise the advantages and disadvantages of each.
Now that you lot understand the basic concepts and terminology associated with Windows Server 2003 VPNs, we will move on to practice some easily-on configurations and yous'll learn how to put the concepts to piece of work. The following preconfiguration checklist will simplify the configuration settings outlined in the side by side department:
1.
Review the bones VPN concepts. Determine the type of VPN you wish to configure: router-to-router or client-server.
2.
Ensure hardware is compatible and install necessary hardware.
3.
Install and enable the Routing and Remote Access service as outlined in Practise 7.01.
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