This chapter describes commands that configure the link layer. All of these commands apply to the currently active link, i.e., the link, which physical device shown at the command line prompt.
set link latency microseconds
set link bandwidth bits-per-second
These commands are relevant when multi-link PPP is active. They affect the way in which packets are chopped up into fragments before being sent over the various links that make up the bundle.
To motivate the idea, imagine a bundle that had a modem link and a 1.5Mbps T1 link. If mpd sent each packet in two equal sized fragments over these links, then by the time the modem got around to transmitting the first byte of its fragment, the T1 link would have probably already sent the whole other fragment. Clearly this is not very good. By factoring in the latency and bandwidth parameters for each link, mpd can distribute the fragments in a more intelligent way.
Mpd attempts to distribute bytes over the links so that (if the configured parameters are accurate) the last byte of each fragment arrives at the peer at the same time on each link. This minimizes latency. However, if you only care about maximizing throughput, simply set all of the latency values to zero.
If all of your links are of the same type and speed (which is often the case), then they should be configured with the same values (or just not configured at all, since all links default to the same values anyway). Then mpd will distribute packets in equal sized fragments over the links.
set link mtu numbytes
set link mru numbytes
set link mtu command sets the maximum transmit unit
(MTU) value for the link. This is the size of the largest single
PPP frame (minus PPP header) that this link will transmit, unless
the peer requests an even lower value. The default value is 1500 bytes.
set link mru command sets maximum receive unit (MRU)
value for the link, which is the size of the largest single PPP frame
(minus PPP header) that this link is capable of receiving. The default
value is 1500 bytes.
If PPP multilink is negotiated on a link, then these values are less important, because multilink allows PPP frames themselves to be fragmented, so a PPP frame can always pass through no matter how small the MTU is in a particular direction.
Otherwise, mpd is responsible for making sure that the MTU configured on the system networking interface is low enough so that the largest transmitted IP packet does not exceed the peer's negotiated MRU after it becomes a PPP frame. This includes e.g. PPP encryption and/or compression overhead.
However, mpd does not account for overhead that occurs ``outside'' of the PPP frame. For example, when using link types such as PPTP that encapsulate PPP frames within IP packets, a large outgoing ``inner'' IP packet can result in a fragmented ``outer'' IP packet, resulting in suboptimal performance. In this situation it may be useful to set the link MTU to a lower value to avoid fragmentation.
set link accmap value
This sets the desired asynchronous control-character map for the link at the local end. This option is only relevant for the asynchronous link types (i.e., modem and tcp). It determines which control characters need to be escaped.
value is expressed as a 32-bit hex
value; the default is
0x000a0000, which escapes the
Control-S and Control-Q characters.
set link ident string
This enables the sending of an identification string to the peer via the LCP Ident code. The Ident string is sent when the link is brought up. This is useful for debugging, etc. and is meant to be human-readable. However, it confuses some broken PPP implementations.
Setting an empty string disables this feature; this is the default.
set link fsm-timeout seconds
This command is analogous to the same command at the bundle layer, but it applies to link-layer FSM's such as Link Control Protocol (LCP). The default is two seconds; normally this value shouldn't be changed.
set link keep-alive seconds max
This command enables the sending of LCP echo packets on the link.
The first echo packet is sent after
seconds of quiet time (i.e., no frames received from the peer on
that link). After
seconds more seconds, another
echo request is sent. If after
max seconds of
doing this no echo reply has been received yet, the link is brought
seconds is zero, echo packets are disabled.
The default values are five second intervals with a maximum no-reply
time of fourty.
This feature is especially useful with modems when the carrier
detect signal is unreliable. However, in situations where lines are
noisy and modems spend a lot of time retraining, the
value may need to be bumped up to a more generous value.
set link max-redial num
When a link fails to connect, mpd automatically retries the connection
immediately. This command limits the number of consecutive retries.
num attempts, mpd will give up.
When there is another open event, new dial-on-demand traffic, etc. mpd will try again, starting over at zero.
max-redial is set to -1, then mpd will never redial.
This setting should be used with links that are dedicated for dial-in.
max-redial is set to 0, then mpd will redial infinitely.
The default value is -1.
set link accept option ...
set link deny option ...
set link enable option ...
set link disable option ...
set link yes option ...
set link no option ...
These commands configure various link options. Most options are bi-directional in that they can be independently enabled and disabled in each direction.
disable commands determine
whether we want the corresponding option.
deny commands determine
whether we will allow the peer to request the corresponding option.
Note that when talking about the authentication options PAP and CHAP,
enable an option you're saying you are going
to require a login and password from the peer.
accept an option you're saying you will
allow the peer to require a login and password from us.
yes command is the same as
no command is the same as
The options available at the link layer are:
PAP style authentication. Note that this style of authentication is insecure, since the password crosses the link in plaintext.
CHAP style authentication. This style of authentication is safer than PAP, because only a hash of the password is passed over the link. Mpd supports MD5 style CHAP and Microsoft style CHAP versions 1 and 2. Mpd will prefer MD5 CHAP over Microsoft CHAP, unless the link is a pptp link.
This parameter is an alias for
Traditional CHAP MD5 style authentication.
Microsoft CHAP style authentication.
Microsoft CHAP style authentication Version 2.
Extensible Authentication Protocol.
Address and control field compression. This option only applies to asynchronous link types. It saves two bytes per frame.
Protocol field compression. This option saves one byte per frame for most frames.
magicnum option enables using a magic number for the
local end of the PPP link. This causes a unique number to be
included in each LCP packet we send, which helps detect loopback
check-magic option causes mpd to verify that the peer's
magic number is correct in all received LCP frames.
Some old broken PPP implementations don't handle magic numbers correctly, so these options need to be disabled in these cases.
Default for both options is
Note that the two most common reasons for seeing ``loopback condition detected'' on a modem link are:
Enables passive mode for this link. This is useful on some full time connections. See RFC 1661 for more information about this option.
Enables PPP callback request. If the remote peer can/wants to, it will hangup immediately after connecting and call us back.
Normally, if PAP or CHAP is enabled, we require the peer to authenticate to us at the beginning of each connection. This option temporarily disables this requirement if we are the one who originated the connection and the peer rejects our request for a login.
This is useful when the same link is used for both dial-in and dial-out.
Normally, if using MS-CHAP, the MS-Domain is stripped and only the plain username is used. Under certain circumstances the MS-Domain should be kept, for instance if IAS is used as RADIUS server.