The last mile rate information along with the encapsulation information is used for fragmentation (to determine the maximum fragment length) and interleaving (delaying fragments in the BB-ISA). In addition, the aggregate subscriber rate (aggregate-rate-limit) on the LNS is automatically adjusted based on the last mile link rate and the number of links in the MLPPPoX bundle.
Downstream Data Rate in the Last Mile
The subscriber aggregate rates (agg-rate-limit) used in (H)QoS on the carrier IOM and in the BB-ISA (for interleaving) must be wire based in the last mile. This rule applies equally to both, the LAC and LNS.
The last mile on-the-wire rates of the subscriber can be submitted to the LAC and the LNS by various means. The following discusses the break down on how the last mile wire rates are passed to each entity:
LAC
The last mile link rate is taken by the following methods in the order of listed priority:
LUDB — rate-down command under the host hierarchy in LUDB.
RADIUS Alc-Access-Loop-Rate-Down VSA. Although this VSA is stored in the state of plain PPP(oE) sessions (MLPPPoX bundled or not), it is applicable only to MLPPPoX bundles.
PPPoE tags — Vendor Specific Tags (RFC 2516, A Method for Transmitting PPP Over Ethernet (PPPoE); tag type 0x0105; tag value is Enterprise Number 3561 followed by the TLV sub-options as specified in TR-101 -> Actual Data Rate Downstream 0x82)
As long as the link rate information is available in the LAC, it is always passed to the LNS in the ICRQ message using the standard L2TP encoding. This cannot be disabled.
In addition, an option is available to control the source of the rate information can be conveyed to the LNS by TX Connect Speed AVP in the ICCN message. This can be used for compatibility reasons with other vendors that can only use TX Connect Speed to pass the link rate information to the LNS. By default, the maximum port speed (or the sum of the maximum speeds of all member ports in the LAG) is reported in TX Connect Speed. Unlike the rate conveyed in ICRQ message, The TX Connect Speed content is configurable with the following command:
config>subscr-mgmt
sla-profile <name>
egress
report-rate agg-rate-limit | scheduler <scheduler-name> | pppoe-actual-
rate | rfc5515-actual-rate
The report-rate configuration option dictates which rate is reported in the TX Connect Speed as follows:
agg-rate-limit
Statically configured agg-rate-limit value or RADIUS QoS override is reported
scheduler scheduler-name
Virtual schedulers are not supported in MLPPPoX
pppoe-actual-rate
The rate taken from PPPoE tags is reported.
rfc5515-actual-speed
The rate is taken from RFC5515.
The RFC 5515 relies on the same encoding as PPPoE tags (vendor ID is ADSL Forum and the type for Actual Data Rate Downstream is 0x82).
The LAC on the 7750 SR is not aware of MLPPPoX bundles. As such, the aggregate subscriber bandwidth on the LAC is configured statically by usual means (sub-profile, scheduler-policy) or dynamically modified through RADIUS. The aggregate subscriber (or MLPPPoX bundle) bandwidth on the LAC is not automatically adjusted according to the rates of the individual links in the bundle and the number of the links in the bundle. As such, an operator must ensure that the statically provided rate value for aggregate-rate-limit is the sum of the bandwidth of each member link in the MLPPPoX bundle. The number of member links and their bandwidth must be therefore known in advance. The alternative is to have the aggregate rate of the MLPPPoX bundle set to a high value and rely on the QoS treatment performed on the LNS.
LNS
The sources of information for the last mile link rate on the LNS is taken in the following order:
LUDB (during user authentication phase, same as in LAC)
RADIUS (same as in LAC)
ICRQ message — Actual Data Downstream Rate (RFC 5515)
ICCN message — TX Connect Speed
There is no configuration option to determine the priority of the source of information for the last mile link rate. TX Connect Speed in ICCN message is only be taken into consideration as a last resort in absence of any other source of last mile rate information.
After the last mile rate information is obtained, the subscriber aggregate rate aggregate-rate-limit is automatically adjusted to the minimum value of:
The smallest link speed in the MLPPPoX bundle multiplied by the number of links in the bundle.
Statically configured aggregate-rate-limit
The link speed of each link in the bundle must be the same, meaning, different link speeds within the bundle are not supported. When different link are received, speed values for last mile links within the bundle, the minimum received speed is adopted and apply it to all links.
When the obtained rate information from the last mile for a session within the MLPPP bundle is out of bounds (1 kb/s to 100 Mb/s), the session within the bundle is terminated.
Encapsulation
Wire-rates are dependent on the encapsulation of the link to which they apply. The last mile encapsulation information can be extracted by various means.
LAC
Static configuration by LUDB.
RADIUS — Alc-Access_Loop-Encap-Offset VSA.
PPPoE tags — Vendor Specific Tags (RFC 2516; tag type 0x0105; tag value is Enterprise Number 3561 followed by the TLV sub-options as specified in TR-101 -> Actual Data Rate Downstream 0x82).
The LAC passes the line encapsulation information to the LNS by an ICRQ message using the encoding defined in the RFC 5515.
LNS
The LNS extracts the encapsulation information in the following order:
Static configuration by LUDB.
RADIUS — Alc-Access-Loop-Encap-Offset VSA.
ICRQ message (RFC 5515)
When the encapsulation information is not provided by any of the existing means (LUDB, RADIUS, AVP signaling, PPPoE Tags), then by default pppoa-null encapsulation is in effect. This applies to LAC and LNS.