Point-to-Point Protocol Extension Group Mikael Latvala INTERNET DRAFT Oy LM Ericsson Ab Expires September 13, 1997 George Liu Ericsson Radio Systems March 13, 1997 Semi Connected Mode for PPP links Status of this Memo This document is an Internet-Draft. 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.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Distribution of this memo is unlimited. Abstract The configuration of a Point-to-Point Protocol (PPP) [1] link requires a considerable amount of time which makes it impractical to establish a new PPP link every time an end-user wants to send or is about to receive data. This document proposes an LCP extension called Semi Connected Mode. When both sides agree to use Semi Connected Mode they can terminate and quickly re-establish the bearer service without having to recon- figure the PPP link. Latvala & Liu expires September 13, 1997 [Page i] Internet Draft Semi Connected Mode for PPP links March 13, 1997 Table of Contents 1. Introduction .......................................... 1 1.1 Motivation ...................................... 1 1.2 Specification of Requirements ................... 3 1.3 Terminology ..................................... 3 2. PPP Link Operation to support Semi Connected Mode ..... 5 2.1 Phase Diagram ................................... 5 2.2 Semi-Connected Phase ............................ 5 3. LCP State Translation Table ........................... 7 3.1 States .......................................... 8 3.2 Events .......................................... 10 3.3 Actions ......................................... 11 4. Semi Connected Mode Option for LCP Configuration ...... 12 5. Security Considerations ............................... 14 REFERENCES ................................................... 14 AUTHORS' ADDRESSES ........................................... 15 Latvala & Liu expires September 13, 1997 [Page ii] Internet Draft Semi Connected Mode for PPP links March 13, 1997 1. Introduction 1.1 Motivation The exponential growth of the Internet within the last three years has placed new requirements on local telephone companies that offer communication services to the public. Originally telephone companies' end offices were designed to transmit only human voice which has very different characteristics than data, e.g. data traffic is bursty and does not require a strict timing relationship between the source and the destination whereas voice traffic is very sensitive to delays. The sheer number of users that access Internet Service Providers' (ISP) services using regular dial-up lines to explore the Internet, and the duration of those connections is in many places testing the limits of end offices. Trunk lines which connect an end office to other switching centers are not intended to support lengthy connections, which are very common when telephone companies do not charge customers based on the duration of their connections. To take some of the burden off the end offices' shoulders telecom manufacturers have started to offer solutions where phone connections are terminated at the premises of a local telephone company. Customers connect to an end office which has a Network Access Server (NAS). Connections to end offices are either analog or digital depending on whether an end-user has an ISDN connection or a POTS connection. NAS is informed of locally terminated connections after which it routes the packets it receives from customers to their proper destinations using transportation techniques which are better suited for data traffic (SMDS, ATM, Frame relay). Local telephone companies which operate NASs can either assume the role of ISP or just offer a fat data pipe to an ISP which is operating independently of a telephone company. Parallel to the Internet's growth, the cellular systems have seen tremendous success. Mobile offices are slowly gaining popularity among people who need to travel with portable computers and still want to maintain connections to their corporate Intranets or to the Internet. To respond to the growing number of mobile office users, telecom manufactures have introduced a new data service in cellular systems (e.g. GSM) which is similar to the solution described above. In this system part of the GSM data call within the Public Land Mobile Network (PLMN) is not routed through a public ISDN or PSTN, but, instead terminated at a NAS for further transportation of data traffic to external data networks, that is, direct access to servers in a LAN or Internet. This service offered by a digital cellular system considerably reduces the connection setup time because data service connections are terminated within the cellular system. Latvala & Liu expires September 13, 1997 [Page 1] Internet Draft Semi Connected Mode for PPP links March 13, 1997 However, as in many other similar cases, these technological improvements do not come without additional expenses which telephone companies are forced to collect from their customers. Most of the end-users who are using either cellular network or ISDN data services are charged based on their connection's duration. From an end-user's point of view this is highly undesirable because for the better part of the connection time end-users do not send or receive data and are thus paying for an idle connection. These tariffs force many end- users to establish a data connection only when needed, e.g. to read or send e-mail, and disconnect immediately after having sent or received data. End offices would not become bottlenecks if end-users connected and disconnected frequently in order to cut down the bearer service charges. Connections in digital public networks can be established in less than a second which would justify an end-user to disconnect from the bearer service after downloading a large web page for example. Unfortunately the most commonly used datalink-layer protocol, PPP, over POTS, ISDN, and cellular network lines is a very time consuming protocol to configure. Point-to-Point Protocol (PPP) is a full featured protocol which allows two hosts to multiplex different network-layer protocols over the same link, to configure the link so that it can be used in a wide variety of environments, to authenticate themselves, and to configure the network-layer protocols they are using. The reason for having such a variety of features is that PPP is intended to provide a common solution for different hosts and physical links [2]. The drawback of PPP is the time it takes to configure a PPP link. Test measurements done at Ericsson Corporation indicate that PPP configuration can take anywhere from 3 to 6 seconds. Although some of the time is attributed to bad PPP implementations where the PPP can not negotiate options in an orderly fashion, most of the time PPP is waiting for an answer from the peer. Semi Connected Mode (SCM) allows PPP to establish a PPP link without having to keep an open physical-layer connection for the duration of a PPP session. PPP can negotiate the timer value which determines the idle time which is tolerated before the physical-layer connection is terminated. It can also negotiate whether a peer can make a transition to a Semi-Connected phase and terminate the physical link. This feature is necessary when a peer belonging to an ISP is not located on a local telephone company's premises, and the local telephone company always charges the site of the originating call. SCM is an attractive option when the end-users subscribe to a digital bearer service (ISDN, GSM) because digital bearer services typically have very short connection establishment times. Latvala & Liu expires September 13, 1997 [Page 2] Internet Draft Semi Connected Mode for PPP links March 13, 1997 SCM can also help PLMN operators in their frequency shortage dilemma by minimizing the duration of data connections. End-users are more likely to terminate and re-establish the physical-layer connection when they do not suffer from the connection delay caused by the PPP configuration. 1.2. 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 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 must 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. 1.3. Terminology datagram The unit of transmission in the network layer (such as IP). A datagram may be encapsulated in one or more packets passed to the data link layer. frame The unit of transmission at the data link layer. A frame may include a header and/or a trailer, along with some number of units of data. packet The basic unit of encapsulation, which is passed across the interface between the network layer and the data link layer. A packet is usually mapped to a frame; the exceptions are when data link layer fragmentation is being performed, or when multiple packets are incorporated into a single frame. Latvala & Liu expires September 13, 1997 [Page 3] Internet Draft Semi Connected Mode for PPP links March 13, 1997 peer The other end of the point-to-point link. Latvala & Liu expires September 13, 1997 [Page 4] Internet Draft Semi Connected Mode for PPP links March 13, 1997 2. PPP Link Operation to support Semi Connected Mode 2.1. Phase Diagram SCM mode introduces a new phase called Semi-Connected to the PPP diagram: +------+ +-----------+ +--------------+ | | UP | | OPENED | | SUCCESS/NONE | Dead |------->| Establish |---------->| Authenticate |--+ | | | | | | | +------+ +-----------+ +--------------+ | ^ | | | | FAIL | FAIL | | +<--------------+ +----------+ | | | | | +-----------+ | +---------+ | | DOWN | | | CLOSING | | | +------------| Terminate |<---+<----------| Network |<-+ | | | | | | +-----------+ +---------+ | TERMINATE | ^ | +-------------+ | | | | | V | | +----------------+ | | DOWN | | | +--------------------| Semi-Connected |-----------+ | | RE-ESTABLISH +----------------+ 2.2. Semi-Connected Phase The Semi-Connected phase is reached only from the Network phase. PPP MUST terminate the physical link and move to the Semi-Connected phase if 1. LCP configuration authorized the peer to make a transition to the Semi-Connected phase, AND 2. the PPP link has been idle longer than the time negotiated in the LCP configuration. In the Semi-Connected phase PPP observes the data traffic. When PPP receives a datagram which is being sent to the peer or must be routed through the peer PPP MUST re-establish new a physical link to the peer. After the physical link has been established PPP moves back to Latvala & Liu expires September 13, 1997 [Page 5] Internet Draft Semi Connected Mode for PPP links March 13, 1997 the Network phase. PPP MUST also go back to the Network phase when the Up event occurs. The Up event indicates that the peer has re-established a physical link and is going to send data. If PPP determines that it cannot establish a link to the peer it MUST move to the Dead phase. Latvala & Liu expires September 13, 1997 [Page 6] Internet Draft Semi Connected Mode for PPP links March 13, 1997 3. LCP State Translation Table The Semi-Connected phase SHOULD be implemented by adding one new state, Semi-Connected, six new events, and three new actions to the LCP's state translation table. The new events can cause a legal transition only in the Request-Sent, Request-Ack, Opened or Semi- Connected state which is the reason why only those four states are shown in the table below. Events Actions IDT = Idle timer expired tel = terminate link SLN = Session timer expired rel = re-establish link DSC = Down event, SCM configured sst = start session timer CSC = Close event, SCM configured, no peer DUP+ = Datagram from the upper layer DUP- = Datagram from the upper layer, no peer Latvala & Liu expires September 13, 1997 [Page 7] Internet Draft Semi Connected Mode for PPP links March 13, 1997 | State | 7 8 9 10 Events| Ack-Rcvd Ack-Sent Opened Semi-Connected ------+--------------------------------------------------------------- Up | - - - 9 Down | 1 1 tld/1 - Open | 7 8 9r - Close | irc,str/4 irc,str/4 tld,irc,str/4 rel,tld,irc,str/4 | TO+ | scr/6 scr/8 - - TO- | tlf/3p tlf/3p - - | RCR+ | sst,sca,tlu/9 sca/8 tld,scr,sca/8 - RCR- | scn/7 scn/6 tld,scr,scn/6 - RCA | scr/6x sst,irc,tlu/9 tld,scr/6x - RCN | scr/6x irc,scr/8 tld,scr/6x - | RTR | sta/6 sta/6 tld,zrc,sta/5 - RTA | 6 8 tld,scr/6 - | RUC | scj/7 scj/8 scj/9 - RXJ+ | 6 8 9 - RXJ- | tlf/3 tlf/3 tld,irc,str/5 - | RXR | 7 8 ser/9 - | CSC | - - - tld/1 DSC | - - 10 - IDT | - - tel/10 - SLN | - - - tld/1 DUP+ | - - - rel/9 DUP- | - - - tld/1 3.1. States Below are more detailed descriptions of the modified states and the new Semi-Connected state. The descriptions of the Ack-Received, Ack- Sent, and Opened state should not conflict with the descriptions in [1]. Rather, the new functionalities can be implemented without sacrificing the integrity of the "traditional" PPP implementation. Ack-Received The Session timer is started when the Receive-Configure-Request (RCR+) event takes place and PPP enters the Opened state. Ack-Sent Latvala & Liu expires September 13, 1997 [Page 8] Internet Draft Semi Connected Mode for PPP links March 13, 1997 The Session timer is started when the Receive-Configure-Ack (RCA) event takes a place and PPP enters the Opened state. Opened If the PPP is configured to use SCM the implementation MUST keep a record which shows how long the PPP link has been idle when it is in the Opened state. If the value of the Idle timer is higher than the value specified by the Idle-Time field in the Semi Connected Mode option the implementation MUST terminate the physical link and enter the Semi-Connected state. If the implementation and its peer have agreed to use SCM and the Down event occurs indicating that the lower layer is not ready to carry packets it must enter the Semi-Connected State. Implementation Note: It is possible that the peer can terminate the physical link after PPP has given a frame to the lower layer but before the lower layer has successfully transferred the frame to the peer. Implementations SHOULD have a functionality which tries to estimate whether a frame can be transferred to the peer before the peer terminates the link. This functionality reduces the number of retransmissions which would take place if ARQ is implemented in the data link or in a higher layer. Semi-Connected state The implementation MUST leave the Semi-Connected state and enter the Opened State if: 1. The implementation receives a datagram from the upper layer and is able to re-establish the physical link, AND 2. The Up event occurred indicating that the peer is going to send data. When the implementation receives a datagram from the upper layer but comes to the conclusion that it cannot re-establish the physical link it MUST indicate to the upper layers that the PPP link is no longer available for network layer traffic and enter the Starting state. See Implementation Note below how the implementation determines when it cannot re-establish the physical link. The implementation MAY use a PPP Session timer which determines Latvala & Liu expires September 13, 1997 [Page 9] Internet Draft Semi Connected Mode for PPP links March 13, 1997 the maximum lifetime of a PPP session. If the Session timer expires when the LCP automaton is in the Semi Connected state PPP MUST indicate to the upper layers that the PPP link is no longer available for network layer traffic and enter the Starting state. When the Close event occurs PPP MUST try to re-establish the link. If it establishes the link successfully the same actions MUST take place as when the Close event occurs in the Opened state. If PPP fails to re-establish the link it MUST indicate to the upper layers that the PPP link is no longer available for its network layer traffic and enter the Starting state. Implementation Note: The Implementation MUST take into consideration the fact that both sides can try to re-establish the physical link at the same time or that the peer's end-user accepted a regular phone call while the implementation was in the Semi-Connected state. If the lower layer cannot establish the link and returns a value indicating that it received a busy signal the implementation SHOULD wait a particular number of seconds before trying to re-establish the link. The implementation MAY use for example n-persistent Carrier Sense Multiple Access (CSMA) protocol to determine how long it waits before trying to setup the link again. The implementation SHOULD not give up immediately when it fails to re-establish the link. The number of times the implementation tries to re-establish the link is up to the implementation. In some implementations PPP could stay in the Semi-Connected state until the Session timer expires. 3.2. Events Close event when SCM configured (CSC) This event occurs when the automaton is in the Semi-Connected state and the network administrator (human or program) indicates that the link is not allowed to be Opened. Down event when SCM configured (DSC) This event occurs when the PPP link is configured to use SCM, the automaton is in the Opened state, and a lower layer indicates that it is no longer ready to carry packets. Latvala & Liu expires September 13, 1997 [Page 10] Internet Draft Semi Connected Mode for PPP links March 13, 1997 Idle timer expired (IDT) This event occurs when the PPP link is configured to use SCM, the automaton is in the Opened state, and the Idle timer expires. Session timer expired (SLN) This event occurs when the PPP link is configured to use SCM, the automaton is in the Semi-Connected state, and the Session timer expires. Datagram from the upper layer (DUP) This event occurs when the PPP link is configured to use SCM, the automaton is in the Semi-Connected state, and a upper layer has given a packet to PPP to transfer to the peer. The DUP+ event indicates that the peer is still available so that the physical link can be re-established and packets can be sent to the peer. The DUP- event indicates that the peer is not available and that the physical link cannot be re-established. 3.3. Actions Re-establish link (rel) The physical link is re-established. Terminate link (tel) The physical link is terminated. Start session timer (sst) This action starts the Session timer. Implementation Note: Each implementation is free to choose a value for the Maximum- Session-Length variable. A session length can vary between PPP links, e.g. a company pays a higher rate for bearer services and in exchange its session length is increased among other things. Latvala & Liu expires September 13, 1997 [Page 11] Internet Draft Semi Connected Mode for PPP links March 13, 1997 4. Semi Connected Mode Option for LCP Configuration Description This configuration option provides a method to negotiate the usage of SCM. By default SCM is not used. The implementation MUST specify the time a PPP link is allowed to be idle before the physical link is terminated and the LCP automaton enters the Semi-Connected state. At the end of the configuration both sides MUST agree on the same idle time value. In order to ensure that both sides will agree on the idle time value the implementation MUST accept the value that the peer sent if it is lower than the value that the implementation sent to the peer. If the implementation is not concerned about the idle time value it SHOULD send the highest possible idle time value to the peer. The implementation MUST also indicate to the peer if it accpets a terminating call. Implementationa which cannot or are not allowed make an originating call MUST send back a Configure-Nak packet. After the PPP link has been configured and both sides have agreed to use SCM the LCP automaton MUST be in the Opened state, one NCP automaton supported by the implementation in the Opened state and the other NCP automatons in the Initial, Starting, or Opened state. This means that LCP automaton cannot enter the Semi- Connected state when LCP or NCP is being configured or terminated. A summary of the Semi Connected Mode 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 | Idle-Time | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Accept-Conn | +-+-+-+-+-+-+-+-+ Type 20 Latvala & Liu expires September 13, 1997 [Page 12] Internet Draft Semi Connected Mode for PPP links March 13, 1997 Length 5 Idle-Time The Idle-Time field specifies the time in seconds during which the PPP link can be idle. Value 0 MUST be used only when PPP accepts the Idle-Time field but does not accept the Accept-Conn field thus returning a Configure- Nak packet where the Idle-Time field is assigned to 0. Accept-Conn The Accept-Conn field is one octet and indicates whether the peer can re-establish the physical link. 0 Value 0 MUST be used only when PPP accepts the Accept- Conn field but does not accept the Idle-Time field thus returning a Configure-Nak packet where the Accept-Conn field is assigned to 0. 1 The peer is NOT allowed to re-establish the physical link. 2 The peer is allowed to re-establish the physical link. Latvala & Liu expires September 13, 1997 [Page 13] Internet Draft Semi Connected Mode for PPP links March 13, 1997 5. Security SCM does not offer a mechanism which allows the implementation to ensure that the peer which re-established the physical link is indeed the same peer with whom the implementation established the PPP link. To prevent an intruder from re-establishing the physical link the implementation can use the caller ID feature which many PSTNs and PLMNs offer. REFERENCES [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP) for the Transmission of Multi-protocol Datagrams over Point-to-Point Links," RFC 1661, July 1994. [2] Perkins, D., "Requirements for an Internet Standard Point-to-Point Protocol", RFC 1547, Carnegie Mellon University, December 1993. Latvala & Liu expires September 13, 1997 [Page 14] Internet Draft Semi Connected Mode for PPP links March 13, 1997 AUTHORS' ADDRESSES Questions about this paper can be directed to: Mikael Latvala Research Department Oy LM Ericsson Ab SF-02420 Jorvas, Finland Voice: +358-9-299-2850 Fax: +358-9-299-3247 E-Mail: Mikael.Latvala@lmf.ericsson.se George Liu System Research Department Core Unit Radio Systems and Technology Ericsson Radio Systems S-164 80 Stockholm, Sweden Voice: +46-8-764 1231 Fax: +46-8-404 7020 E-mail: George.Liu@era-t.ericsson.se Latvala & Liu expires September 13, 1997 [Page 15]