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NAME
ng_ppp — PPP protocol netgraph node type
SYNOPSIS
#include <sys/types.h>
#include <netgraph/ng_ppp.h>
DESCRIPTION
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.
ENABLING FUNCTIONALITY
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).
LINK HOOKS
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 AND ENCRYPTION
Compression is supported via two hooks, compress and decompress. Compression and
decompression can be enabled by toggling the enableCompression and enableDecompression
fields of the node configuration structure. (See below.) If enableCompression is set to
NG_PPP_COMPRESS_SIMPLE, then all outgoing frames are sent to the compress hook and all
packets received on this hook are expected to be compressed, so the COMPD tag is put on them
unconditionally. If enableCompression is set to NG_PPP_COMPRESS_FULL, then packets received
on the compress hook are resent as is. The compressor node should put the tag, if the
packet was compressed. If enableDecompression is set to NG_PPP_DECOMPRESS_SIMPLE, then the
node will sent to the decompress hook only those frames, that are marked with the COMPD tag.
If enableDecompression is set to NG_PPP_DECOMPRESS_FULL, then the node will sent all
incoming packets to the decompress hook. Compression and decompression can be completely
disabled by setting the enableCompression and enableDecompression fields to the
NG_PPP_COMPRESS_NONE and NG_PPP_DECOMPRESS_NONE, respectively.
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.
VAN JACOBSON COMPRESSION
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.
BYPASS HOOK
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).
MULTILINK OPERATION
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.
HOOKS
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
ipv6 IPv6 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.
CONTROL MESSAGES
This node type supports the generic control messages, plus the following:
NGM_PPP_SET_CONFIG (setconfig)
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 */
uint16_t mru; /* peer MRU */
uint32_t latency; /* link latency (in milliseconds) */
uint32_t bandwidth; /* link bandwidth (in bytes/sec/10) */
};
/* Bundle config structure */
struct ng_ppp_bund_conf {
uint16_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];
};
NGM_PPP_GET_CONFIG (getconfig)
Returns the current configuration as a struct ng_ppp_node_conf.
NGM_PPP_GET_LINK_STATS (getstats)
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.
NGM_PPP_GET_LINK_STATS64 (getstats64)
Same as NGM_PPP_GET_LINK_STATS but returns struct ng_ppp_link_stat64 containing 64bit
counters.
NGM_PPP_CLR_LINK_STATS (clrstats)
This command takes a two byte link number as an argument and clears the statistics for
that link.
NGM_PPP_GETCLR_LINK_STATS (getclrstats)
Same as NGM_PPP_GET_LINK_STATS, but also atomically clears the statistics as well.
NGM_PPP_GETCLR_LINK_STATS64 (getclrstats64)
Same as NGM_PPP_GETCLR_LINK_STATS but returns struct ng_ppp_link_stat64 containing
64bit counters.
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).
SHUTDOWN
This node shuts down upon receipt of a NGM_SHUTDOWN control message, or when all hooks have
been disconnected.
SEE ALSO
netgraph(4), ng_async(4), ng_iface(4), ng_mppc(4), ng_pppoe(4), ng_vjc(4), ngctl(8)
W. Simpson, The Point-to-Point Protocol (PPP), RFC 1661.
K. Sklower, B. Lloyd, G. McGregor, D. Carr, and T. Coradetti, The PPP Multilink Protocol
(MP), RFC 1990.
HISTORY
The ng_ppp node type was implemented in FreeBSD 4.0.
AUTHORS
Archie Cobbs <archie@FreeBSD.org>