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 other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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. Carlson expires January 2001 [Page 1]
INTERNET-DRAFT PPP Link Balancing Detection July 2000 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 Carlson expires January 2001 [Page 2]
INTERNET-DRAFT PPP Link Balancing Detection July 2000 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 Carlson expires January 2001 [Page 3]
INTERNET-DRAFT PPP Link Balancing Detection July 2000 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 Carlson expires January 2001 [Page 4]
INTERNET-DRAFT PPP Link Balancing Detection July 2000 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 Carlson expires January 2001 [Page 5]
INTERNET-DRAFT PPP Link Balancing Detection July 2000 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. Carlson expires January 2001 [Page 6]
INTERNET-DRAFT PPP Link Balancing Detection July 2000 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 Carlson expires January 2001 [Page 7]
