Subscriber Access Line Information Handling by the LAC and LNS Overview

 

Starting in Junos OS Release 14.1, L2TP supports a set of AVPs that convey information about subscriber access lines from the LAC to the LNS. The information originates from an ANCP access node (DSLAM) and is distributed to the LAC by means of either DSL Forum VSAs in ANCP messages or PPPoE intermediate agent tags included in the PPPoE PADI and PADR messages.

Access Line Information Forwarding

In the network topology shown in Figure 1, when a subscriber initiates a connection through the CPE, the DSLAM relays the subscriber’s PPPoE session to the router configured as a LAC. When the router has established the PPPoE session, the LAC initiates an L2TP tunnel to forward the subscriber’s encapsulated PPP packets into the provider network.

In parallel to the PPPoE session, an ANCP connection between the DSLAM and the ANCP agent on the router conveys information about the subscriber’s local loop as well as the link speeds of the PPPoE sessions on the local loop. The DSLAM sends the router Agent Circuit Id (ACI) and Agent Remote Id (ARI) strings that uniquely identify the DSLAM’s receiving interface; this information is encoded in the ANCP Port Up and Port Down messages as Access Line Identifying TLVs. The ANCP messages can also include line attributes such as minimum, maximum, and actual net upstream and downstream data rates in the DSL Line Attributes TLV. The DSLAM can also send the access line attributes in vendor-specific tags that it inserts in the PADI and PADR messages.

Figure 1: Sample L2TP Network Topology
Sample L2TP Network
Topology

Access Line Information AVPs

L2TP supports the AVPs listed in Table 1 to carry this information. The access line information is not required for the L2TP session to be initiated, and the establishment of that session is not delayed waiting for the values to be sent from the DSLAM.

The access line information provided by the AVPs in ICRQ messages is passed on to RADIUS in DSL Forum VSAs. It is not used for shaping the traffic rate on the subscriber access lines.

Table 1: L2TP AVPs That Provide Subscriber Access Line Information

Attribute Value Pair

AVP Type

(Corresponding DSL Forum VSA)

Description

L2TP Message Type

Actual Data Rate Downstream

130

(26-130)

64-bit unsigned integer; data rate in bits per sec.

ICRQ

Actual Data Rate Upstream

129

(26-129)

64-bit unsigned integer; data rate in bits per sec.

ICRQ

Actual Interleaving Delay Downstream

142

(26-142)

32-bit unsigned integer; maximum delay in milliseconds.

ICRQ

Actual Interleaving Delay Upstream

140

(26-140)

32-bit unsigned integer; maximum delay in milliseconds.

ICRQ

Access Loop Encapsulation

144

(26-144)

Three one-octet encodings for data link, encapsulation 1, and encapsulation 2.

ICRQ

Agent Circuit ID

1

(26-1)

2-63 octet string; ACI of the logical access loop port on the DSLAM/access node.

ICRQ

Agent Remote ID

2

(26-2)

2-63 octet statically configured string; uniquely identifies subscriber on the DSLAM (access node).

ICRQ

ANCP Access Line Type

145

(26-145)

One octet encoding for transmission system type, followed by three MBZ (must be zero) octets (total 4 bytes). This value is not supplied in the ICRQ when the access line parameters are sourced from PPPoE-IA, because the ANCP-sourced information may not be immediately available.

Starting in Junos OS Release 18.1R1, this AVP is included even when the line type is 0 for OTHER access line types.

ICRQ

Attainable Data Rate Upstream

133

(26-133)

64-bit unsigned integer; data rate in bits per sec.

ICRQ

Attainable Data Rate Downstream

134

(26-134)

64-bit unsigned integer; data rate in bits per sec.

ICRQ

Connect Speed Update Enable

98

(none)

Value does not matter: presence indicates support for CSUN, CSURQ message types for this session.

ICRQ

Connect Speed Update

97

(none)

Data structure listing remote session id and the current transmit and receive connection speeds in bits per second.

CSUN, CSURQ

IWF Session

254

(26-254)

Four-octet field indicating whether or not the internetworking function has been performed.

ICRQ

Maximum Data Rate Downstream

136

(26-136)

64-bit unsigned integer; data rate in bits per sec.

ICRQ

Maximum Data Rate Upstream

135

(26-135)

64-bit unsigned integer; data rate in bits per sec.

ICRQ

Maximum Interleaving Delay Downstream

141

(26-141)

32-bit unsigned integer; maximum delay in milliseconds.

ICRQ

Maximum Interleaving Delay Upstream

139

(26-139)

32-bit unsigned integer; maximum delay in milliseconds.

ICRQ

Minimum Data Rate Downstream

132

(26-132)

64-bit unsigned integer; data rate in bits per sec.

ICRQ

Minimum Data Rate Downstream Low Power

138

(26-138)

64-bit unsigned integer; data rate in bits per sec.

ICRQ

Minimum Data Rate Upstream

131

(26-13)

64-bit unsigned integer; data rate in bits per sec.

ICRQ

Minimum Data Rate Upstream Low Power

137

(26-137)

64-bit unsigned integer; data rate in bits per sec.

ICRQ

Connection Speed Updates on the LAC

You can configure the LAC to notify the LNS when the speed of the subscriber connection changes from the values initially communicated to the LNS by AVP 24 (transmit speed) and AVP 38 (receive speed) in Incoming-Call-Connected (ICCN) messages. When configured to do so, the LAC informs the LNS that it can send these updates by including the Connect Speed Update Enable AVP (98) in the ICRQ message when the L2TP session starts up. The absence of the Connect Speed Update Enable AVP (98) in the ICRQ message indicates that the LAC does not send updates for the life of the session.

When the connection speed changes, the DSLAM notifies the ANCP agent. The ANCP agent then notifies the LAC, and the LAC in turn relays this information to the LNS by sending a Connect-Speed-Update-Notification (CSUN) message that includes the updated speeds in a Connect Speed Update AVP (97) for each session. The LAC collects connection speed updates and sends them in a batch to minimize both the performance overhead on the LAC and the amount of traffic generated as a result of these notifications.

The initial speeds in the ICCN messages and updated speeds in CSUN messages are used by CoS to shape the traffic rate for subscriber access lines.

The presence of the Connect Speed Update Enable AVP (98) in the ICRQ message also informs the LNS that the LAC does respond if it receives a Connect-Speed-Update-Request (CSURQ) message from an LNS.

Note

The Junos OS does not currently support the sending of CSURQ messages by MX Series routers configured as an LNS. All discussion about CSURQ messages is strictly about how an MX Series LAC responds to a CSURQ that it receives from a third-party LNS.

A third-party LNS can send a CSURQ message at any time during the life of a tunnel to request the current transmit and receive connection speed for one or more L2TP sessions. The LNS includes the remote (relative to the LNS) session IDs in the CSURQ message. If the LAC has previously sent the Connect Speed Update Enable AVP (98) for the requested sessions, then it responds to the CSURQ with a CSUN message that includes the Connect Speed Update AVP (97) for each session. If no changes to connection speeds have occurred by this time, the LAC simply includes the initial connection speed values that were reported in AVP 24 and AVP 38.

When you enable connect speed updates either globally or for a specific LNS, the LAC does not send CSUN messages unless you have also configured the tx-connect-speed statement to be either ancp or service-profile.

Connection Speed Updates on the LNS

Starting in Junos OS Release 17.4R1, an MX Series router configured as an LNS can process subscriber access line information and connection speed updates that it receives from the LAC. The MX Series router cannot send CSURQ messages to solicit updates from the LAC.

The initial speeds in the ICCN messages and updated speeds in CSUN messages are used by CoS to shape the traffic rate for subscriber access lines.

Interaction Between Global and Per-Destination Configurations

You can configure the LAC to forward the access line information in the ICRQ message that it sends to the LNS and you can configure the LNS to receive and process that information. You can configure this globally for all destinations (endpoints) or for a specific destination. The per-destination configuration enables you to limit transmission to an individual LNS or to a set of LNSs or reception from an individual LAC or a set of LACs. This is useful when you know that some remote gateways do not support this feature or have an incorrect implementation.

Include the access-line-information statement at one or both of the following hierarchy levels on the LAC or LNS, respectively, to configure the LAC to forward the access line information in the ICRQ message that it sends to the LNS, or to configure the LNS to receive and process that information:

  • [edit services l2tp]—Configures forwarding globally for all destinations.

  • [edit services l2tp destination ip-address]—Configures forwarding for a specific destination.

To configure the LAC to send connection speed updates or the LNS to receive and process the updates, include the connection-speed-update option with the access-line-information statement at the appropriate hierarchy level on the LAC or LNS, respectively.

The global and per-destination settings interact in the following way:

  • Access line information—When forwarding by the LAC or processing by the LNS is enabled globally, you cannot disable the global setting for a specific destination.

  • Connection speed updates—When forwarding by the LAC or processing by the LNS is enabled globally, you can disable the global setting for a specific destination (LNS or LAC) by specifying access-line-information for the destination and omitting connection-speed-update.

Release History Table
Release
Description
Starting in Junos OS Release 17.4R1, an MX Series router configured as an LNS can process subscriber access line information and connection speed updates that it receives from the LAC.
Starting in Junos OS Release 14.1, L2TP supports a set of AVPs that convey information about subscriber access lines from the LAC to the LNS.