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
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
- 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];
};
NGM_PPP_GET_CONFIG
- Returns the current configuration as a
struct ng_ppp_node_conf
.
NGM_PPP_GET_LINK_STATS
- 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
- Same as NGM_PPP_GET_LINK_STATS but returns
struct ng_ppp_link_stat64
containing 64bit
counters.
NGM_PPP_CLR_LINK_STATS
- This command takes a two byte link number as an argument
and clears the statistics for that link.
NGM_PPP_GETCLR_LINK_STATS
- Same as
NGM_PPP_GET_LINK_STATS
, but
also atomically clears the statistics as well.
NGM_PPP_GETCLR_LINK_STATS64
- 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⟩