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ng_ppp - PPP protocol netgraph node type


Enabling Functionality
Link Hooks
Compression And Encryption
Van Jacobson Compression
Bypass Hook
Multilink Operation
Control Messages
See Also


.In sys/types.h
.In netgraph/ng_ppp.h


The ppp node type performs multiplexing for the PPP protocol. It handles only packets that contain data, and forwards protocol negotiation and control packets to a separate controlling entity (e.g., a user-land daemon). This approach combines the fast dispatch of kernel implementations with the configuration flexibility of a user-land implementations. The PPP node type directly supports multi-link PPP, Van Jacobson compression, PPP compression, PPP encryption, and the IP, IPX, and AppleTalk protocols. A single PPP node corresponds to one PPP multi-link bundle.

There is a separate hook for each PPP link in the bundle, plus several hooks corresponding to the directly supported protocols. For compression and encryption, separate attached nodes are required to do the actual work. The node type used will of course depend on the algorithm negotiated. There is also a bypass hook which is used to handle any protocol not directly supported by the node. This includes all of the control protocols: LCP, IPCP, CCP, etc. Typically this node is connected to a user-land daemon via a ng_socket(4) type node.


In general, the PPP node enables a specific link or functionality when (a) a NGM_PPP_SET_CONFIG message has been received which enables it, and (b) the corresponding hook(s) are connected. This allows the controlling entity to use either method (a) or (b) (or both) to control the node’s behavior. When a link is connected but disabled, traffic can still flow on the link via the bypass hook (see below).


During normal operation, the individual PPP links are connected to hooks link0, link1, etc. Up to NG_PPP_MAX_LINKS links are supported. These device-independent hooks transmit and receive full PPP frames, which include the PPP protocol, address, control, and information fields, but no checksum or other link-specific fields.

On outgoing frames, when protocol compression has been enabled and the protocol number is suitable for compression, the protocol field will be compressed (i.e., sent as one byte instead of two). Either compressed or uncompressed protocol fields are accepted on incoming frames. Similarly, if address and control field compression has been enabled for the link, the address and control fields will be omitted (except for LCP frames as required by the standards). Incoming frames have the address and control fields stripped automatically if present.

Since all negotiation is handled outside the PPP node, the links should not be connected and enabled until the corresponding link has reached the network phase (i.e., LCP negotiation and authentication have completed successfully) and the PPP node has been informed of the link parameters via the NGM_PPP_LINK_CONFIG message.

When a link is connected but disabled, all received frames are forwarded directly out the bypass hook, and conversely, frames may be transmitted via the bypass hook as well. This mode is appropriate for the link authentication phase. As soon as the link is enabled, the PPP node will begin processing frames received on the link.


Compression is supported via two hooks, compress and decompress. When enabled and connected, the PPP node writes outgoing frames on the comp hook and expects to receive back the compressed frame on the same hook. Similarly, the decompress hook is used to uncompress incoming frames when decompression is negotiated (compression and decompression are independently negotiable). The type of node attached to these hooks should correspond to the type of compression negotiated, e.g., Deflate, Predictor-1, etc.

Encryption works exactly analogously via the encrypt and decrypt nodes. Data is always compressed before being encrypted, and decrypted before being decompressed.

Only bundle-level compression and encryption is directly supported; link-level compression and encryption can be handled transparently by downstream nodes.


When all of the vjc_ip, vjc_vjcomp, vjc_vjuncomp, and vjc_vjip hooks are connected, and the corresponding configuration flag is enabled, Van Jacobson compression and/or decompression will become active. Normally these hooks connect to the corresponding hooks of a single ng_vjc(4) node. The PPP node is compatible with the "pass through" modes of the ng_vjc(4) node type.


When a frame is received on a link with an unsupported protocol, or a protocol which is disabled or for which the corresponding hook is unconnected, the PPP node forwards the frame out the bypass hook, prepended with a four byte prefix. This first two bytes of the prefix indicate the link number on which the frame was received (in network order). For such frames received over the bundle (i.e., encapsulated in the multi-link protocol), the special link number NG_PPP_BUNDLE_LINKNUM is used. After the two byte link number is the two byte PPP protocol number (also in network order). The PPP protocol number is two bytes long even if the original frame was protocol compressed.

Conversely, any data written to the bypass hook is assumed to be in this same format. The four byte header is stripped off, the PPP protocol number is prepended (possibly compressed), and the frame is delivered over the desired link. If the link number is NG_PPP_BUNDLE_LINKNUM the frame will be delivered over the multi-link bundle; or, if multi-link is disabled, over the (single) PPP link.

Typically when the controlling entity receives an unexpected packet on the bypass hook it responds either by dropping the frame (if it is not ready for the protocol) or with an LCP protocol reject (if it does not recognize or expect the protocol).


To enable multi-link PPP, the corresponding configuration flag must be set and at least one link connected. The PPP node will not allow more than one link to be connected if multi-link is not enabled, nor will it allow certain multi-link settings to be changed while multi-link operation is active (e.g., short sequence number header format).

Since packets are sent as fragments across multiple individual links, it is important that when a link goes down the PPP node is notified immediately, either by disconnecting the corresponding hook or disabling the link via the NGM_PPP_SET_CONFIG control message.

Each link has configuration parameters for latency (specified in milliseconds) and bandwidth (specified in tens of bytes per second). The PPP node can be configured for round-robin or optimized packet delivery.

When configured for round-robin delivery, the latency and bandwidth values are ignored and the PPP node simply sends each frame as a single fragment, alternating frames across all the links in the bundle. This scheme has the advantage that even if one link fails silently, some packets will still get through. It has the disadvantage of sub-optimal overall bundle latency, which is important for interactive response time, and sub-optimal overall bundle bandwidth when links with different bandwidths exist in the same bundle.

When configured for optimal delivery, the PPP node distributes the packet across the links in a way that minimizes the time it takes for the completed packet to be received by the far end. This involves taking into account each link’s latency, bandwidth, and current queue length. Therefore these numbers should be configured as accurately as possible. The algorithm does require some computation, so may not be appropriate for very slow machines and/or very fast links.

As a special case, if all links have identical latency and bandwidth, then the above algorithm is disabled (because it is unnecessary) and the PPP node simply fragments frames into equal sized portions across all of the links.


This node type supports the following hooks:

link<N> Individual PPP link number <N>
compress Connection to compression engine
decompress Connection to decompression engine
encrypt Connection to encryption engine
decrypt Connection to decryption engine
vjc_ip Connection to ng_vjc(4) ip hook
vjc_vjcomp Connection to ng_vjc(4) vjcomp hook
vjc_vjuncomp Connection to ng_vjc(4) vjuncomp hook
vjc_vjip Connection to ng_vjc(4) vjip hook
inet IP packet data
atalk AppleTalk packet data
ipx IPX packet data
bypass Bypass hook; frames have a four byte header consisting of a link number and a PPP protocol number.


This node type supports the generic control messages, plus the following:
This command configures all aspects of the node. This includes enabling multi-link PPP, encryption, compression, Van Jacobson compression, and IP, IPv6, AppleTalk, and IPX packet delivery. It includes per-link configuration, including enabling the link, setting latency and bandwidth parameters, and enabling protocol field compression. Note that no link or functionality is active until the corresponding hook is also connected. This command takes a "struct ng_ppp_node_conf" as an argument:
/* Per-link config structure */
struct ng_ppp_link_conf {
u_char enableLink;/* enable this link */
u_char enableProtoComp;/* enable protocol field compression */
u_char enableACFComp; /* enable addr/ctrl field compression */
u_int16_t mru; /* peer MRU */
u_int32_t latency; /* link latency (in milliseconds) */
u_int32_t bandwidth; /* link bandwidth (in bytes/second) */

/* Bundle config structure */
struct ng_ppp_bund_conf {
u_int16_t mrru; /* multilink peer MRRU */
u_char enableMultilink; /* enable multilink */
u_char recvShortSeq; /* recv multilink short seq # */
u_char xmitShortSeq; /* xmit multilink short seq # */
u_char enableRoundRobin; /* xmit whole packets */
u_char enableIP;/* enable IP data flow */
u_char enableIPv6; /* enable IPv6 data flow */
u_char enableAtalk; /* enable AppleTalk data flow */
u_char enableIPX; /* enable IPX data flow */
u_char enableCompression; /* enable PPP compression */
u_char enableDecompression; /* enable PPP decompression */
u_char enableEncryption; /* enable PPP encryption */
u_char enableDecryption; /* enable PPP decryption */
u_char enableVJCompression; /* enable VJ compression */
u_char enableVJDecompression; /* enable VJ decompression */

struct ng_ppp_node_conf {
struct ng_ppp_bund_conf bund;
struct ng_ppp_link_conf links[NG_PPP_MAX_LINKS];

Returns the current configuration as a "struct ng_ppp_node_conf".
This command takes a two byte link number as an argument and returns a "struct ng_ppp_link_stat" containing statistics for the corresponding link. Here NG_PPP_BUNDLE_LINKNUM is a valid link number corresponding to the multi-link bundle.
This command takes a two byte link number as an argument and clears the statistics for that link.
Same as NGM_PPP_GET_LINK_STATS, but also atomically clears the statistics as well.

This node type also accepts the control messages accepted by the ng_vjc(4) node type. When received, these messages are simply forwarded to the adjacent ng_vjc(4) node, if any. This is particularly useful when the individual PPP links are able to generate NGM_VJC_RECV_ERROR messages (see ng_vjc(4) for a description).


This node shuts down upon receipt of a NGM_SHUTDOWN control message, or when all hooks have been disconnected.


netgraph(4), ng_async(4), ng_iface(4), ng_mppc(4), ng_pppoe(4), ng_vjc(4), ngctl(8)
.Rs "The Point-to-Point Protocol (PPP)"
.Rs "The PPP Multilink Protocol (MP)"



Created by Blin Media, 2008-2013