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
<draft-ietf-pppext-scm-00.txt>
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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.
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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
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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.
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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.
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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.
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peer The other end of the point-to-point link.
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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
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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.
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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
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| 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
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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
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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.
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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.
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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
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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.
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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.
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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
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