One document matched: draft-ietf-pppext-lbd-02.txt
Differences from draft-ietf-pppext-lbd-01.txt
Network Working Group J. Carlson
INTERNET-DRAFT Sun Microsystems
Expires January 2001 July 2000
Updates RFC 1990
PPP Link Balancing Detection (LBD)
<draft-ietf-pppext-lbd-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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This document is the product of the Point-to-Point Protocol
Extensions Working Group of the Internet Engineering Task Force
(IETF). Comments should be submitted to the ietf-ppp@merit.edu
mailing list. 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
also defines an extensible Link Control Protocol (LCP), which allows
the detection of optional link handling procedures, as well as a
Multilink procedure (MP) [2], which allows operation over multiple
parallel links. This document defines an extension to MP called Link
Balancing Detection (LBD) and the LCP options that control this
extension. This extension allows high-speed implementations to use
the single-NCP negotiation model of MP without requiring prohibitive
datagram buffering and reordering costs.
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Table of Contents
1. Introduction ........................................... 2
1.1. Conventions ............................................ 3
2. No-Fragmentation Configuration Option Format ........... 3
3. No-MP-Headers Configuration Option Format .............. 4
4. Interaction With MRRU .................................. 5
5. Interaction With CCP and ECP ........................... 5
6. Bundle Establishment ................................... 5
7. Bundle Tear-Down ....................................... 6
8. Message Distribution ................................... 6
9. Security Issues ........................................ 7
10. References ............................................. 7
11. Author's Address ....................................... 7
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 better. Also, routing system stability is
usually higher in the face of link failures when individual links are
not visible to link-state routing protocols.
The main disadvantage to the current MP technique is that it does not
constrain the fragmentation that 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, the reassembly
process is often not a problem. For systems with very high bandwidth
capabilities, these features are often infeasible, and this problem
makes regular MP unusable.
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 without MP headers among the links in the bundle in any
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convenient manner, including based on a hash or on round-robbin ser-
vice.
This technique is also referred to as "load balancing." The differ-
ence between LBD and traditional load balancing is that MP's single-
NCP (and associated single address negotiation model) is used and
that the configuration is made automatic. This allows peers to dis-
cover during LCP negotiation that, for example, links within a con-
figured bundle violate a hardware design constraint by having dif-
ferent MRU values, or are provisioned to terminate on the wrong net-
work node.
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. No-Fragmentation Configuration Option Format
A summary of the No-Fragmentation Configuration Option format for LCP
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
TBD
Length
2
The sender of this option in an LCP Configure-Request message is
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indicating to its peer that it cannot support MP reassembly, and,
thus the peer must not fragment messages that it sends.
If the peer Configure-Ack's this option, then the peer MUST NOT frag-
ment frames using MP fragmentation. It MAY still use MP headers to
preserve frame sequencing. If the peer Configure-Reject's this
option, then the sender must remove this option from its next
Configure-Request message and MAY decline to run MP by also removing
its MRRU Configuration Option. Implementations MUST NOT Configure-
Nak this option if it appears in the peer's Configure-Request.
3. No-MP-Headers Configuration Option Format
A summary of the No-MP-Headers Configuration Option format for LCP 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
TBD
Length
2
The sender of this option in an LCP Configure-Request message is
indicating to its peer that it cannot support standard MP headers,
and, thus the peer must not use MP headers on the messages that it
sends, and must send network layer messages using their assigned pro-
tocol numbers rather than inside protocol 003D.
If this option is specified, then the No-Fragmentation option is
unnecessary. Fragmentation without MP headers is not supported.
If the peer Configure-Ack's this option, then it MUST NOT add MP
headers or fragment frames using MP. If the peer Configure-Reject's
this option, then the sender must remove this option from its next
Configure-Request message and MAY decline to run MP by also removing
its MRRU Configuration Option. Implementations MUST NOT Configure-
Nak this option if it appears in the peer's Configure-Request.
This option SHOULD not be used on links that are intended to carry
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network protocols that cannot tolerate reordering. See section 8,
"Message Distribution," for details.
4. Interaction With MRRU
The MRRU option from MP is still used to signal the desire to run MP,
regardless of whether or not these options are present. If MRRU is
not negotiated, then these options have no effect. If an MRRU is
negotiated, then, as with RFC 1990 MP, the peer's MRRU is advertised
to the network layer as the MTU for the interface.
When No-Fragmentation is used but No-MP-Headers is not used, MRRU
should be set to the LCP MRU minus 6 (for long sequence numbers) or
minus 4 (for short sequence numbers). When No-MP-Headers is used,
MRRU should be set equal to the LCP MRU.
5. Interaction With CCP and ECP
The No-Fragmentation option has no effect on either CCP or ECP. When
the No-MP-Headers option is negotiated, the peers should restrict
themselves to per-link CCP or ECP, or may use CCP or ECP algorithms
supporting multiple contexts in an implementation-dependent manner by
prior arrangement.
6. 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 is agreed to by 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 authenticated 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. Oth-
erwise, this is a separate link, and NCPs should be started.
If the local and remote MRRU values do not agree with the bundle or
if the presence or absence of the No-Fragmentation or No-MP-Headers
options does not agree with the bundle, then the link SHOULD be ter-
minated. An implementation MAY choose instead to renegotiate LCP to
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repair the error.
7. Bundle Tear-Down
Tear-down is identical to standard MP and is thus not covered here.
8. Message Distribution
To distribute messages among the links when LBD is in effect, 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
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 that prevent reordering (such as deterministic hashes of
network layer addresses) are encouraged. (For TCP, reordering of IP
datagrams usually causes a slow path in the state machine to be
taken, and can trigger side-effects, such as fast retransmit.)
Fourth, network datagrams from protocols that cannot withstand mes-
sage reordering MUST be sent over a single link within the bundle.
The link for each datagram may be chosen in any manner appropriate
for that network layer, and is left to either the network layer
specification or prior arrangement between the peers. Standard MP
may be preferred over LBD in these cases.
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Fifth, 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.
9. Security Issues
The authentication and bundling techniques are identical to standard
MP and the security issues are the same as with RFC 1990.
10. 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
11. Author's Address
James Carlson
Sun Microsystems
1 Network Drive MS UBUR02-212
Burlington MA 01803-2757
Phone: +1 781 442 2084
Fax: +1 781 442 1677
Email: james.d.carlson@sun.com
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