PPP Working Group J. Carlson
Internet Draft IronBridge Networks
expires in six months April 1998
PPP Link Balancing Detection (LBD)
<draft-ietf-pppext-lbd-00.txt>
Status of this Memo
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Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links.
PPP also defines an extensible Link Control Protocol, which allows
the detection of optional link handling procedures, as well as a
Multilink procedure (MP) [2] which allows operation over multiple
physical links. This document defines an extension to MP called Link
Balancing Detection (LBD).
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Table of Contents
1. Introduction ........................................... 2
1.1. Conventions ............................................ 3
2. MRRU Configuration Option Modification ................. 3
3. Bundle Establishment ................................... 3
4. Bundle Tear-Down ....................................... 4
5. Message Distribution ................................... 4
6. Security Issues ........................................ 5
7. References ............................................. 5
8. Authors' Addresses ..................................... 5
1. Introduction
Standard PPP negotiation allows for two types of links with regard to
multiple link layer entities. The standard type of PPP link is nego-
tiated without the Maximum-Receive-Reconstructed-Unit (MRRU) option
and appears as a separate network interface to the network layer and
to routing protocols. The Multilink PPP (MP) [2] type of link uses
the MRRU option and allows multiple PPP links to be bundled into one
network interface. An MP link appears as a single network interface
to the network layer and to routing protocols.
There are many advantages having multiple links between two nodes
appear at the network layer to be a single link. While equal-cost
multi-path balancing is certainly possible with modern interior gate-
way protocols, less stress is placed on scarce routing system
resources when link-layer detection is employed, allowing current
routing protocols to scale farther. Also, routing system stability
in the face of link failures is usually higher when individual links
are not visible to the routing protocols.
The main disadvantage to the current MP technique is that it does not
constrain the fragmentation which may be done by the peer. For sys-
tems employing general purpose CPUs in the data path and with
scatter-gather direct memory access (DMA) capability, this is often
not a problem. For systems with very high bandwidth capabilities,
these features are often infeasible, and this problem makes MP unus-
able.
This draft describes a method similar to (and compatible with) MP for
detecting multiple links to the same node, but without the fragmenta-
tion or reordering protection of MP. Instead, datagrams are distri-
buted intact among the links in any convenient manner, including
based on an IP [3] address hash or simple round-robbin service.
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1.1. Conventions
The following language conventions are used in the items of specifi-
cation in this document:
o MUST, SHALL, or MANDATORY -- This item is an absolute require-
ment of the specification.
o SHOULD or RECOMMEND -- This item should generally be followed
for all but exceptional circumstances.
o MAY or OPTIONAL -- This item is truly optional and may be fol-
lowed or ignored according to the needs of the implementor.
2. MRRU Configuration Option Modification
The MRRU option from MP is modified to allow a new distinguished
value. This value is zero, and indicates that the Configure-Request
sender wishes to bundle multiple links but refuses to reconstruct
received fragmented datagrams or to use the header as defined in MP.
The receiver of Configure-Request with MRRU set to zero MAY
Configure-Reject it to disable all MP support, including LBD, or MAY
send Configure-Nak with a hint set non-zero to request standard MP
support, or MAY send Configure-Ack to indicate its willingness to run
LBD.
Both peers MUST agree that MRRU is not in effect, or that it is zero,
or that it has a non-zero value. If MRRU has a non-zero value, how-
ever, it need not be identical in each direction. If LCP goes to
Open state with the MRRUs set to an illegal set of values (id est,
one zero and the other non-zero), then an implementation SHOULD ter-
minate the link or MAY choose to renegotiate LCP.
3. Bundle Establishment
As with MP, bundle establishment is based on a combination of the
peer's supplied endpoint discriminator (ED) and the peer's identity
as determined via link authentication. The algorithm used for LBD is
identical to the MP algorithm, and is documented here only for con-
venience.
When authentication (if any was negotiated via LCP) is complete, a
check is made before attempting to negotiate any Network Control Pro-
tocols (NCPs). If an MRRU (either zero or non-zero) is agreed to by
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both peers and if there is an existing LBD bundle where the ED (or
lack thereof) matches the new link's ED (or lack), and the authenti-
cated peer name (or lack thereof) match the new link's peer name (or
lack), then this new link should be made part of the bundle and no
new NCPs are created. Otherwise, this is a separate link, and NCPs
should be started.
If the local and remote MRRU values do not agree with the bundle,
then the link SHOULD be terminated. An implementation MAY choose
instead to renegotiate LCP to repair the error. If the MRRU values
are zero, then the MRU values MUST also be checked in the same
manner.
When LBD is in effect, in contrast with standard MP, the value adver-
tised to the network layer as the MTU MUST be the peer's requested
MRU (or any smaller value), rather than the negotiated MRRU.
4. Bundle Tear-Down
Tear-down is identical to standard MP and is thus not covered here.
5. Message Distribution
To distribute messages among the links, a few simple rules must be
followed.
First, since PPP negotiation does not withstand reordering, all PPP
negotiation messages MUST be sent over a single link to avoid possi-
ble reordering. The first link in a bundle MUST be used to transmit
PPP messages until this link is terminated. If the first link is
terminated, then one remaining link in the bundle MUST be chosen for
subsequent messages. Once that link is chosen, an implementation
MUST continue sending all PPP negotiation messages over that single
link. Any remaining link in the bundle MAY be chosen, and it is
entirely possible that each peer may choose a different link without
harm to the PPP protocol.
Second, PPP negotiation messages MUST be handled when received on any
link. An implementation MAY choose to terminate the last link over
which negotiation was received if negotiation is received over a dif-
ferent link, since this transition implies that the peer has already
terminated the prior link.
Third, network datagrams SHOULD be distributed over all links as
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evenly as possible. There are no requirements that any particular
distribution algorithm be used. Note, however, that some network
protocols behave poorly when subjected to message reordering, so
techniques which prevent reordering (such as deterministic hashes of
network layer addresses) are encouraged.
Fourth, the common but technically non-standard practice of using LCP
Terminate-Request to gracefully terminate a link without data loss is
encouraged in LBD implementations. To do this, an implementation
leaves Open state on sending LCP Terminate-Request, but, contrary to
RFC 1661 [1], continues processing received datagrams until the peer
replies with LCP Terminate-Ack.
6. Security Issues
The authentication and bundling techniques are identical to standard
MP and the security issues are the same as with RFC 1990.
7. References
[1] W. Simpson, "The Point-to-Point Protocol (PPP)", RFC 1661,
07/21/1994
[2] K. Sklower, et al, "The PPP Multilink Protocol (MP)", RFC 1990,
08/1996
[3] J. Postel, "Internet Protocol", RFC 791, 09/01/1981
8. Author's Address
James Carlson
IronBridge Networks
55 Hayden Avenue
Lexington MA 02173-7999
Phone: +1 781 402 8032
Fax: +1 781 402 8092
Email: carlson@ironbridgenetworks.com
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