PPP NBFCP: The PPP NetBIOS Frames Control Protocol (NBFCP)
How a PPP link agrees to carry NetBIOS Frames (NBF) traffic, including which NetBIOS names the dial-in side claims.
Pall, G., "The PPP NetBIOS Frames Control Protocol (NBFCP)", RFC 2097, DOI 10.17487/RFC2097, January 1997, <https://www.rfc-editor.org/info/rfc2097>.
@misc{rfc2097,
series = {Request for Comments},
number = 2097,
howpublished = {RFC 2097},
publisher = {RFC Editor},
doi = {10.17487/RFC2097},
url = {https://www.rfc-editor.org/info/rfc2097},
author = {G. Pall},
title = {{The PPP NetBIOS Frames Control Protocol (NBFCP)}},
pagetotal = 13,
year = 1997,
month = jan,
} RFC text
Contents
Network Working Group G. Pall
Request for Comments: 2097 Microsoft Corp.
Category: Standards Track January 1997
The PPP NetBIOS Frames Control Protocol (NBFCP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP
defines an extensible Link Control Protocol, and proposes a family of
Network Control Protocols for establishing and configuring different
network-layer protocols.
The NBF protocol [3] was originally called the NetBEUI protocol. This
document defines the Network Control Protocol for establishing and
configuring the NBF protocol over PPP.
The NBFCP protocol is only applicable for an end system to connect to
a peer system or the LAN that peer system is connected to. It is not
applicable for connecting two LANs together due to NetBIOS name
limitations and NetBIOS name defense mechanisms.
Table of Contents
1. Introduction .......................................... 2
1.1 Specification of Requirements ................... 2
1.2 Terminology ..................................... 3
2. A PPP Network Control Protocol for NBF ................ 3
2.1 Sending NBF Datagrams ........................... 4
2.2 Bridging NBF Datagrams........................... 5
2.3 NetBIOS Name Defense............................. 5
3. NBFCP Configuration Options ........................... 6
3.1 Name-Projection.................................. 6
3.2 Peer-Information................................. 8
3.3 Multicast-Filtering.............................. 10
3.4 IEEE-MAC-Address-Required........................ 11
SECURITY CONSIDERATIONS ...................................... 12
REFERENCES ................................................... 12
ACKNOWLEDGEMENTS ............................................. 13
CHAIR'S ADDRESS .............................................. 13
AUTHOR'S ADDRESS ............................................. 13
1. Introduction
PPP has three main components:
1. A method for encapsulating multi-protocol datagrams.
2. A Link Control Protocol (LCP) for establishing, configuring,
and testing the data-link connection.
3. A family of Network Control Protocols for establishing and
configuring different network-layer protocols.
In order to establish communications over a point-to-point link, each
end of the PPP link must first send LCP packets to configure and test
the data link. After the link has been established and optional
facilities have been negotiated as needed by the LCP, PPP must send
NBFCP packets to choose and configure the NBF network-layer protocol.
Once NBFCP has reached the Opened state, NBF datagrams can be sent
over the link.
The link will remain configured for communications until explicit LCP
or NBFCP packets close the link down, or until some external event
occurs (an inactivity timer expires or network administrator
intervention).
1.1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications should be
understood and carefully weighed before choosing a
different course.
MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST be
prepared to interoperate with another implementation which
does include the option.
1.2. Terminology
This document frequently uses the following terms:
peer The other end of the point-to-point link.
silently discard
This means the implementation discards the packet without
further processing. The implementation SHOULD provide the
capability of logging the error, including the contents of
the silently discarded packet, and SHOULD record the event
in a statistics counter.
end-system
A user's machine. It only sends packets to servers and
other end-systems. It doesn't pass any packets through
itself.
router Allows packets to pass through, usually from one ethernet
segment to another. Sometimes these are called
"intermediate-systems".
bridge Allows packets to pass through with the data field
unmodified. Usually from one ethernet segment to another
or from one ethernet segment to a token-ring segment.
gateway Allows packets to be sent from one network protocol to
the same or different network protocol. For example,
NetBIOS packets from an NBF network to a TCP/IP network
which has implemented RFC 1001 and RFC 1002.
local access only server A server which does not pass any packets
through itself to other servers.
2. A PPP Network Control Protocol for NBF
The NBF Control Protocol (NBFCP) is responsible for configuring,
enabling, and disabling the NBF protocol modules on both ends of the
point-to-point link. NBFCP uses the same packet exchange mechanism
as the Link Control Protocol. NBFCP packets MUST NOT be exchanged
until PPP has reached the Network-Layer Protocol phase. NBFCP
packets received before this phase is reached should be silently
discarded.
The NBF Control Protocol is exactly the same as the Link Control
Protocol [1] with the following exceptions:
Frame Modifications
The packet may utilize any modifications to the basic frame format
which have been negotiated during the Link Establishment phase.
Data Link Layer Protocol Field
Exactly one NBFCP packet is encapsulated in the Information field
of a PPP Data Link Layer frame where the Protocol field indicates
type hex 803f (NBF Control Protocol).
Code field
Only Codes 1 through 7 (Configure-Request, Configure-Ack,
Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack
and Code-Reject) are used. Other Codes should be treated as
unrecognized and should result in Code-Rejects.
Timeouts
NBFCP packets MUST NOT be exchanged until PPP has reached the
Network-Layer Protocol phase. An implementation should be
prepared to wait for Authentication and Link Quality Determination
to finish before timing out waiting for a Configure-Ack or other
response. It is suggested that an implementation give up only
after user intervention or a configurable amount of time. Also,
because NetBIOS name defense takes time (typically a minimum of
3 seconds if names are added in parallel), it is suggested that
if Name-Projection is negotiated, the timeouts are increased to 10
seconds.
Configuration Option Types
NBFCP has a distinct set of Configuration Options.
2.1. Sending NBF Datagrams
Before any NBF packets may be communicated, PPP must reach the
Network-Layer Protocol phase, and the NBF Control Protocol must reach
the Opened state.
Unless otherwise negotiated, exactly one NBF packet is encapsulated
in the Information field of a PPP Data Link Layer frame where the
Protocol field indicates type hex 003f (NBF datagram).
Since NBF datagrams for PPP do not contain a datagram length field,
the encapsulated NBF packet MUST NOT contain any extra octet padding
except when Self-Defining-Padding is negotiated.
The maximum length of an NBF datagram transmitted over a PPP link is
the same as the maximum length of the Information field of a PPP data
link layer frame. Since there is no standard method for fragmenting
and reassembling NBF datagrams, PPP links supporting NBF MUST allow
at least 576 octets in the information field of a data link layer
frame. It is recommended that an implementation allow 1500 octets in
the information field unless the IEEE-MAC-Address-Required boolean
option is negotiated (see below).
2.2 Bridging NBF Datagrams
There exist at least four different MAC header implementations for
NBF packets: 802.3 Ethernet, 802.5 Token-Ring, DIX Ethernet, and
FDDI. Because NBF is not a routable protocol, some PPP
implementations may require IEEE MAC addresses to properly route or
bridge NBF packets. Some PPP implementations may require the entire
MAC media header in order to properly route or bridge NBF packets.
Other smarter implementations may only require the IEEE MAC addreses,
and still other implementations (such as NetBIOS gateways) may not
require any MAC address fields. NBFCP implementations which require
IEEE Addresses should negotiate the NBFCP IEEE-MAC-Address-Required
boolean configuartion option so that the MAC header can be provided
in the NBF packet.
If IEEE-MAC-Address-Required boolean configuration option is
negotiated, all NBF datagrams MUST be sent with the specified 12
octet IEEE MAC address header. Since negotiation of this option
occurs after the LCP phase, NBF packets MAY exceed the negotiated PPP
MRU size. A PPP implementation which negotiates this option MUST
allow reception of PPP NBF packets 12 octets larger than the
negotiated MRU size.
2.3 NetBIOS Name Defense
In order to guarantee uniqueness of NetBIOS Names on the network,
NBFCP requires that end-system implementations MUST negotiate the
Name-Projection configuration option.
3. NBFCP Configuration Options
NBFCP Configuration Options allow modifications to the standard
characteristics of the network-layer protocol to be negotiated. If a
Configuration Option is not included in a Configure-Request packet,
the default value for that Configuration Option is assumed.
NBFCP uses the same Configuration Option format defined for LCP [1],
with a separate set of Options.
Up-to-date values of the NBFCP Option Type field are specified in the
most recent "Assigned Numbers" RFC [2]. Current values are assigned
as follows:
1 Name-Projection
2 Peer-Information
3 Multicast-Filtering
4 IEEE-MAC-Address-Required
3.1. Name-Projection
Description
This Configuration Option provides a method for the peer to
provide the NetBIOS names registered on its network. The sender
of the Configure-Request states which NetBIOS names should be
added by the remote peer. More than one Name-Projection option
MAY appear in a single Configure-Request.
Implementations which do not attempt to add any NetBIOS names MUST
Configure-Reject the Name-Projection Configuration Option.
If the Name-Projection Configuration Option is not offered by the
remote peer, but is required by the local peer, the local peer
should Configure-Nak the request and indicate that it wishes the
remote peer to add zero NetBIOS names because it is the only known
acceptable value. The remote peer may then terminate NBFCP,
attempt to add zero NetBIOS names, or attempt add one or more
NetBIOS names.
When the receiving peer cannot add all the requested names, it
MUST Configure-Nak with the complete list of names requested.
Those names which could be added should have the Added field set
to zero. Those names which could not be added should have the
Added field set to an appropriate non-zero return code. The
sender of this Configuration Option SHOULD then resend the
Configure-Request with the successfully added names.
The implementation may choose to fail configuration if the
complete list of NetBIOS names is not accepted. By failing, the
implementation should terminate NBFCP by sending a Terminate-
Request packet.
Because adding NetBIOS names can take time (usually 3 seconds) and
because PPP may default the restart timer to 3 seconds, the
restart timer SHOULD default to 10 seconds when configuring
NetBIOS names.
A summary of the Name-Projection Configuration Option format is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | 1st NetBIOS-Name
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1st NetBIOS-Name (cont.)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1st NetBIOS-Name (cont.)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1st NetBIOS-Name (cont.)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1st NetBIOS-Name (cont.) | Added |2nd NetBIOS Name...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
1
Length
2 + (Number of NetBIOS names * 17)
NetBIOS-Names
This group of zero or more sixteen octet NetBIOS-Name fields
contains a list of all the NetBIOS names the peer wishes to add to
the remote network if the packet is Configure-Request. If the
packet is Configure-Reject, the peer does not support this
configuration option and it can be assumed that no NetBIOS names
were added.
Because the length field is only one octet, only 14 NetBIOS names
can be added per Name-Projection option. If more than 14 NetBIOS
names should be added, then more than one Name-Projection option
packet will have to be sent in the Configure-Request packet.
Added
This is a one octet field which plays a dual role. The Added
field in the Name-Projection Request packet contains the type of
NetBIOS name added. A summary of name types is listed below.
01 Unique Name.
02 Group Name.
If the packet is a Configure-Reject the Added field should contain
the NetBIOS return code for the NetBIOS Add Name or NetBIOS Add
Group Name command as defined in the NetBIOS 3.0 specification =
[3].
A summary of common result codes is listed below in type hex.
00 Name successfully added.
0D Duplicate name in local name table.
0E Name table full.
15 Name not found or cannot specify "*" or null.
16 Name in use on remote NetBIOS.
19 Name conflict detected.
30 Name defined by another environment.
35 Required system resources exhausted.
3.2. Peer-Information
Description
This Configuration Option provides a way for the peer to
communicate NetBIOS pertinent configuration information. Although
negotiation of this option is not mandatory, it is suggested.
A summary of the Peer-Information Option format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Peer-class |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer-version (major) | Peer-version(minor) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer-name ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2
Length
>=3D8
If the length is 8, there is no Peer-name. If the length is
greater than 8, the Peer-name's length is Length - 8.
Peer-class
The Peer-class field is one octet. It identifies the sender's
implementation type.
Initial values are assigned as follows:
Value Class
1 Reserved for legacy implementations.
2 PPP NetBIOS Gateway Server.
3 Reserved for legacy implementations.
4 PPP Local Access Only Server.
5 Reserved for legacy implementations.
6 PPP NBF Bridge.
7 Reserved for legacy implementations.
8 PPP End-System.
Peer-version
The Peer-version field is four octets and indicates the version of
the communication peer providing one side of the PPP connection.
The first two octets are the major version number and the last two
octets are the minor version number. The major and minor version
represent a 16 bit unsigned number sent with the most significant
octet first.
Peer-name
The name of the peer. A suggested name is the NetBIOS workstation
name of the peer. If the length field is 8, no peer name is
provided. The peer-name may not be greater than 32 octets in
length.
3.3. Multicast-Filtering
Description
This Configuration Option provides a way to negotiate the use of
the Multicast-Forward-Period and the Multicast-Priority. This
Configuration Option provides a way to negotiate how to handle
mulicast packets. It allows the sender of the Configure-Request
to state the current handling of multicast packets. The peer can
request parameters by NAKing the option, and returning valid
Multicast-Filtering parameters.
If negotiation about the remote Multicast-Filtering is required,
and the peer did not provide the option in its Configure-Request,
the option SHOULD be appended to a Configure-Nak.
Controlling the multicast rate is important because some NetBIOS
applications use multicasts to communicate and withholding
multicasts may prevent these applications from working. It is
also true that other NetBIOS applications do not need to receive
any multicast packets and therefore it is best to quench the rate
at which the peer will send multicast packets.
By default, the peer is pre-configured to an administrator
assigned Multicast-Forward-Period and Priority. A Multicast-
Forward-Period specified as hex type FFFF in a Configure-Request
is interpreted as requesting the receiving peer to specify a value
in its Configure-Nak. A Multicast-Forward-Period value specified
as hex type FFFF in a Configure-Nak is interpreted as agreement
that no value exists. A Multicast-Forward-Period of zero indicates
that all multicast packets SHOULD be forwarded.
Peers that rely on all multicast packets being forwarded SHOULD
request a Multicast-Forward-Period of zero and a Multicast-
Priority of one by NAKing the Configure-Request option and
appending the proper parameters to a Configure-Nak.
A summary of the Multicast-Filtering Configuration Option format is
shown below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Multicast-Forward-Period |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority |
+-+-+-+-+-+-+-+-+
Type
3
Length
5
Multicast-Forward-Period
The Multicast-Forward-Period field is two octets and indicates
the maximum period in seconds at which multicast packets can
be sent. The maximum value for this field is 60 (one minute).
A value of zero indicates that there is no maximum period at
which multicast packets can be sent. A value of hex type FFFF
indicates that the Multicast-Forward-Period is unknown. A value
of five indicates that multicast packets will not be sent at a
rate more frequent than once every five seconds. This two
octet value represents a 16 bit unsigned number sent with
the most significant octet first.
Priority
The Priority field is one octet long and indicates if multicast
packets have priority over other packets when being sent. A value
of 0 indicates that directed packets have priority. A value of 1
indicates that multicast packets have priority.
3.4. IEEE-MAC-Address-Required
Description
This boolean Configuration Option provides a method for the peer
to require that all NBF datagrams be sent with a 12 octet IEEE MAC
Address header. By default, it is assumed that no MAC header is
required.
A summary of the IEEE-MAC-Address-Required Boolean Configuration
Option format is shown below. The fields are transmitted from left
to right.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
4
Length
2
Requirements
By default the NBF datagram is sent without any MAC header
information. The NBF datagram information field is equivalent to
the data field in 802.3, 802.5, and FDDI frames.
If this option is negotiated successfully, each NBF datagram is
sent with a 12 octet IEEE MAC Address header prepended to the
information field. A summary of the information field when using
12 octet IEEE MAC Headers is shown below. The fields are
transmitted from left to right. The MAC Address is in non-
canonical form. This means that the first bit to be transmitted in
every byte is the most significant bit.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination MAC Address | Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 802.3/802.5/FDDI data field...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Security Considerations
Security issues are not discussed in this memo.
References
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
RFC 1700, October 1994.
[3] IBM Corp., "IBM Local Area Network Technical Reference",
Third Edition, Document Number SC30-3383-2, November 4, 1988.
[4] Baker, F., and R. Bowen "PPP Bridging Control Protocol (BCP)",
Work in Progress.
Acknowledgments
Some of the text in this document is taken from previous documents
produced by the Point-to-Point Protocol Working Group of the Internet
Engineering Task Force (IETF).
Thomas J. Dimitri (previously at Microsoft Corporation) authored the
original draft.
Special thanks go to coworkers at Microsoft, Bill Simpson
(Daydreamer), Tom Coradetti (DigiBoard), Marty Del Vecchio (Shiva),
Russ Gocht (Shiva) and several members of the IETF PPP Working Group.
Chair's Address
The working group can be contacted via the current chair:
Karl Fox
Ascend Communications
3518 Riverside Drive, Suite 101
Columbus, Ohio 43221
karl@MorningStar.com
karl@Ascend.com
Author's Address
Questions about this memo can also be directed to:
Gurdeep Singh Pall
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052-6399
EMail: gurdeep@microsoft.com
AI ELI5
On this page
§1Introduction and Scope
PPP has three parts: a way to wrap datagrams, a Link Control Protocol (LCP) that sets up and tests the link, and a family of Network Control Protocols that set up each network-layer protocol. RFC 2097 defines the control protocol for NBF, the protocol originally called NetBEUI. It is called the NBF Control Protocol, or NBFCP.
The order is fixed. Each end sends LCP packets first. After the link is established, PPP sends NBFCP packets to choose and configure NBF. Once NBFCP reaches the Opened state, NBF datagrams can be sent over the link. The link stays configured until LCP or NBFCP packets close it, or until an external event such as an inactivity timer or an administrator closes it.
The abstract says NBFCP is only applicable for an end system connecting to a peer system or the LAN that peer is on. It is not applicable for connecting two LANs together, because of NetBIOS name limitations and NetBIOS name defense mechanisms.
Before two computers can talk the old Windows way over a phone line, they use a short getting-ready chat. This RFC is the rulebook for that chat.
§2NBFCP as a Variant of LCP
NBFCP uses the same packet exchange mechanism as LCP and is described as exactly the same as LCP with a few exceptions. NBFCP packets MUST NOT be exchanged until PPP has reached the Network-Layer Protocol phase. Packets that arrive earlier should be silently discarded.
- One NBFCP packet goes in the Information field of a PPP frame whose Protocol field is hex
803f(NBF Control Protocol). - Only Codes 1 through 7 are used: Configure-Request, Configure-Ack, Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack and Code-Reject. Other Codes are treated as unrecognized and answered with a Code-Reject.
- NBFCP has its own set of Configuration Options.
On timeouts, an implementation should be ready to wait for authentication and link quality determination to finish before giving up on a Configure-Ack. Because NetBIOS name defense takes time (typically a minimum of 3 seconds if names are added in parallel), the RFC suggests raising timeouts to 10 seconds when Name-Projection is negotiated.
The getting-ready chat for this protocol follows the same pattern as the one for the phone line itself. Only the list of things you can agree on is different.
§3Sending NBF Datagrams
No NBF packets may be sent until PPP is in the Network-Layer Protocol phase and NBFCP is Opened. Unless otherwise negotiated, exactly one NBF packet goes in the Information field of a PPP frame whose Protocol field is hex 003f (NBF datagram).
NBF datagrams on PPP have no length field, so the packet MUST NOT carry extra padding octets unless Self-Defining-Padding is negotiated. There is no standard way to fragment and reassemble NBF datagrams, so PPP links supporting NBF MUST allow at least 576 octets in the Information field. The RFC recommends 1500 octets unless the IEEE-MAC-Address-Required option is negotiated.
The RFC notes at least four MAC header styles for NBF: 802.3 Ethernet, 802.5 Token-Ring, DIX Ethernet and FDDI. Because NBF is not routable, some implementations need the IEEE MAC addresses, or even the whole MAC header, to route or bridge NBF packets. Those that need addresses should negotiate IEEE-MAC-Address-Required. NetBIOS gateways may need no MAC fields at all.
Each message goes in its own envelope, and the envelope has to be at least a certain size. Some helper machines also want the sender and receiver address labels added, so they can ask for them.
§4Name-Projection and NetBIOS Name Defense
- 1Name-Projection — Asks the peer to add NetBIOS names on its network.
- 2Peer-Information — Shares peer class, version and name.
- 3Multicast-Filtering — Negotiates multicast forward period and priority.
- 4IEEE-MAC-Address-Required — Requires a 12 octet MAC header on each NBF datagram.
NetBIOS names must be unique on a network. To guarantee that, end-system implementations MUST negotiate the Name-Projection option (type 1). The sender of a Configure-Request lists the NetBIOS names it wants the remote peer to add on the remote network. Each name is 16 octets, followed by an Added octet. The option length is 2 plus 17 per name. The length field is one octet, so one option holds at most 14 names; for more, send several Name-Projection options in the same Configure-Request.
If the receiver cannot add all the names, it MUST Configure-Nak with the complete list. Names that were added have Added set to zero; names that failed get a non-zero return code. The sender SHOULD then resend the request with only the names that were added. In a Configure-Request the Added field holds the name type: 01 for a unique name, 02 for a group name. In a Configure-Reject it carries the NetBIOS return code, such as 00 (added), 0D (duplicate name in local name table), 0E (name table full), 16 (name in use on remote NetBIOS) or 19 (name conflict detected).
An implementation that does not attempt to add any names MUST Configure-Reject this option. Because adding names usually takes 3 seconds and PPP may default its restart timer to 3 seconds, the restart timer SHOULD default to 10 seconds when configuring NetBIOS names.
Every computer needs a name nobody else is using. Joining the line means saying the names you want and hearing back which ones were already taken.
§5Peer-Information and Multicast-Filtering
Peer-Information (type 2) lets a peer share NetBIOS-related details. Negotiating it is not mandatory but is suggested. It carries a one-octet Peer-class, a four-octet Peer-version (two octets major, two octets minor) and an optional Peer-name of at most 32 octets, with a suggested value of the NetBIOS workstation name. With no name the length is 8; otherwise the name is Length minus 8 octets. The classes are 2 (PPP NetBIOS Gateway Server), 4 (PPP Local Access Only Server), 6 (PPP NBF Bridge) and 8 (PPP End-System); the odd values 1, 3, 5 and 7 are reserved for legacy implementations.
Multicast-Filtering (type 3, length 5) negotiates how multicast packets are handled. Some NetBIOS applications use multicasts to talk and need them forwarded; others need none, so it is best to slow the rate. The option holds a two-octet Multicast-Forward-Period in seconds (maximum 60; zero means no maximum period, so all multicasts SHOULD be forwarded; hex FFFF means unknown) and a one-octet Priority (0 means directed packets have priority, 1 means multicast packets do). Peers that rely on every multicast being forwarded SHOULD ask for a period of zero and a priority of one by sending a Configure-Nak.
The two sides can also introduce themselves, saying what kind of machine they are. They can agree on how often loud broadcast-style messages get passed along.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | 3 | Option type. |
LENGTH | Length | 1 byte | 5 | Option length. |
PERIOD | Multicast-Forward-Period | 2 bytes | Maximum period in seconds at which multicasts can be sent; maximum 60. | |
PRIORITY | Priority | 1 byte | 0 means directed packets have priority, 1 means multicast packets do. |
§6The IEEE-MAC-Address-Required Option
This boolean option (type 4, length 2) lets a peer require that every NBF datagram carry a 12 octet IEEE MAC address header. By default no MAC header is sent, and the NBF information field is equivalent to the data field of 802.3, 802.5 and FDDI frames.
When negotiated, each datagram has a destination MAC address and a source MAC address prepended, then the 802.3/802.5/FDDI data field. The MAC address is in non-canonical form: the first bit sent in each byte is the most significant bit. Because the option is negotiated after the LCP phase, NBF packets MAY exceed the negotiated PPP MRU. An implementation that negotiates it MUST accept NBF packets 12 octets larger than the negotiated MRU.
Some machines want a proper address label on every message. If both sides agree, each message gets one, and the envelope is allowed to be a little bigger.
§7Security Considerations
The RFC's security section is a single sentence: security issues are not discussed in this memo. Authentication, where used, is handled by PPP before NBFCP begins, since NBFCP packets are only exchanged after the link reaches the Network-Layer Protocol phase.
This RFC does not say anything about keeping the chat secret or checking who is on the line.