4.9. Link layer commands

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

The 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.

The 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.

The 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 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 down.

If 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 max 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. After 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.

If max-redial is set to -1, then mpd will never redial. This setting should be used with links that are dedicated for dial-in.

If 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.

The enable and disable commands determine whether we want the corresponding option. The accept and 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, when you enable an option you're saying you are going to require a login and password from the peer. When you accept an option you're saying you will allow the peer to require a login and password from us.

The yes command is the same as enable and accept. The no command is the same as disable and deny.

The options available at the link layer are:

pap

PAP style authentication. Note that this style of authentication is insecure, since the password crosses the link in plaintext.

Default disable and accept.

chap

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 md5-chap ms-chapv1 ms-chapv2

chap-md5

Traditional CHAP MD5 style authentication.

Default disable and accept.

chap-msv1

Microsoft CHAP style authentication.

Default disable and deny.

chap-msv2

Microsoft CHAP style authentication Version 2.

Default disable and accept.

eap

Extensible Authentication Protocol.

Default disable and accept.

acfcomp

Address and control field compression. This option only applies to asynchronous link types. It saves two bytes per frame.

Default enable and accept.

protocomp

Protocol field compression. This option saves one byte per frame for most frames.

Default enable and accept.

magicnum

check-magic

The 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 conditions.

The 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 enable.

Note that the two most common reasons for seeing ``loopback condition detected'' on a modem link are:

• The modem is in command mode and is echoing back all of our frames.
• The PPP server is not in PPP mode, but is giving a shell prompt or somesuch and echoing back all of our frames.

passive

Enables passive mode for this link. This is useful on some full time connections. See RFC 1661 for more information about this option.

Default disable.

callback

Enables PPP callback request. If the remote peer can/wants to, it will hangup immediately after connecting and call us back.

Default disable.

no-orig-auth

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.

Default disable.

keep-ms-domain

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.

Default disable.