Point-to-Point Protocol over Ethernet management

The following two parameters in the PPP policy control the PPPoE session ID allocation method in the discovery phase.

• config>subscr-mgmt>ppp-policy>sid-allocation {sequential | random}

• config>subscr-mgmt>ppp-policy>unique-sid-per-sap [per-msap]

The following list describes the various combinations of these two parameters:

The session ID range is 1 to 8191.

• sid-allocation sequential and no unique-sid-per-sap (the default allocation method)

  Each first PPPoE session with a specific client MAC address and active on a specific SAP or MSAP is configured with a session ID value of 1. The session ID for subsequent PPPoE sessions with the same client MAC address and active on the same SAP or MSAP is allocated in sequentially-increasing order.

  The PPPoE session ID is unique per (client MAC address, SAP) and per (client MAC address, MSAP).

• sid-allocation sequential and unique-sid-per-sap

  Each PPPoE session that is active on a specific SAP is assigned a unique, sequentially-increasing session ID starting with a session ID value of 1.

  A unique, sequentially-increasing session ID starting with a session ID value of 1 is assigned per capture SAP: PPPoE sessions with the same or different client MAC address and that are active on the same or different MSAP have a unique session ID per capture SAP.

  The PPPoE session ID is unique per SAP and per capture SAP.

  For a unique-sid-per-sap, the maximum number of PPPoE sessions per SAP or per capture SAP is 8191. This limit is enforced across all derived MSAPs.

• sid-allocation sequential and unique-sid-per-sap per-msap

  Each PPPoE session that is active on a specific SAP is assigned a unique, sequentially-increasing sessions ID starting with a session ID value of 1.

  A unique, sequentially-increasing session ID starting with a session ID value of 1 is assigned per capture SAP. PPPoE sessions with the same or different client MAC address and that are active on the same MSAP have a unique session ID.

  The PPPoE session ID is unique per SAP and per MSAP.

  For a unique-sid-per-sap per-msap, the maximum number of PPPoE sessions per SAP or per MSAP is 8191.

• sid-allocation random and no unique-sid-per-sap

  Each PPPoE session with the same client MAC address and active on the same SAP or MSAP is assigned a unique, randomly-assigned session ID.

  The PPPoE session ID is unique per client MAC address and SAP and per client MAC address and MSAP.

• sid-allocation random and unique-sid-per-sap

  Each PPPoE session that is active on a specific SAP is assigned a unique, randomly-assigned session ID.

  A unique session ID is randomly-assigned per capture SAP: PPPoE sessions with the same or different client MAC address and that are active on the same or different MSAP have a unique session ID per capture SAP.

  The PPPoE session ID is unique per SAP and per capture SAP.

  For a unique-sid-per-sap configured, the maximum number of PPPoE sessions per SAP or per capture SAP is 8191. This limit is enforced across all derived MSAPs.

• sid-allocation random and unique-sid-per-sap per-msap

  Each PPPoE session that is active on a specific SAP or MSAP is assigned a unique, randomly-assigned session ID.

  The PPPoE session ID is unique per SAP and per MSAP.

  With unique-sid-per-sap per-msap configured, the maximum number of PPPoE sessions per SAP or per MSAP is 8191.

To support MAC-concentrating network equipment, which translates the original MAC address of a number of subscribers to a single MAC address toward the PPPoE service, the SR OS supports up to 8191 PPPoE sessions per MAC address. Each of these sessions are identified by a unique combination of MAC address and PPPoE session ID.

To set up multiple sessions per MAC, the following limits should be set sufficiently high:

• Maximum sessions per MAC in the PPPoE policy

• The PPPoE interface session limit in the PPPoE node of the group interface

• The PPPoE SAP session limit in the PPPoE node of the group interface

• The multiple-subscriber-sap limit in the subscriber management node of the SAP

If host information is retrieved from a local DHCP server, care must be taken that, although a host can be identified by MAC address, circuit ID, remote ID or username, a lease in the DHCP server is, by default, only indexed by MAC address and circuit ID. For example, multiple sessions per MAC address are only supported in this scenario if every host with the same MAC address has a unique Circuit-ID value.

To enable IPv4 address allocation using the internal DCHCPv4 client for multiple PPPoE sessions on a single SAP and having the same MAC address and circuit-ID, the optional CLI parameter allow-same-circuit-id-for-dhcp should be added to the max-sessions-per-mac configuration in the PPP policy. The SR OS local DHCP server detects the additional vendor-specific options inserted by the internal DCHCPv4 client and uses an extended unique key for lease allocation.

The config>subscr-mgmt>ppp-policy max-sessions-per-cid command limits the number of PPPoE sessions with the same Agent Circuit ID that can be active on the same SAP or MSAP.

The limit is enforced in the discovery phase, before PAP or CHAP authentication based on the Agent Circuit ID sub-option that is present in the vendor-specific PPPoE access loop identification tag added in PADI and PADR messages by a PPPoE intermediate agent.

By default, PPPoE PADI messages without the Agent Circuit ID sub-option are dropped when a max-sessions-per-cid limit is configured.

3502 2020/06
16 11:02:57.949 UTC MINOR: SVCMGR #2214 Base Managed SAP  creation failure

"The system could not create Managed SAP:1/1/
4:2111.100, MAC:00:00:64:6b:01:07, Capturing SAP:1/1/
4:*.*, Service:10. Description: Agent Circuit ID sub-
option missing in PADI and per circuit id limit configured"

The default behavior can be overruled with the optional allow-sessions-without-cid keyword. PPPoE sessions without an Agent Circuit ID can be established on a SAP with a max-sessions-per-cid limit configured. The max-sessions-per-cid max-sessions-per-cid limit is not applied to these sessions.

When the max-sessions-per-cid limit is not configured, no limit is placed on the number of PPPoE sessions with the same Agent Circuit ID on the same SAP or MSAP and PPPoE sessions with or without an Agent Circuit ID can be established.

The re-establish-session command allows a host to re-establish a PPP session if the previous PPP session has yet to be terminated. The only way for a host to reconnect is to wait for the health check to fail or a manual termination by the operator. To allow a faster reconnect, the feature allows a PADI request to terminate the host previous session and allow the host to re-establish a new PPP session. As the old PPP session terminates, the accounting record also stops. The new PPP session starts a new accounting record; this is to ensure that the subscriber is not charged for the unused time in the previous PPP session.

IPoE and PPPoE are commonly used in residential networks and have expanded into business applications.

Both IPoE and PPPoE subscriber hosts terminate in a retail VPRN. It is possible for the subscriber to connect to one or more 7750 SRs for dual homing purposes. When the subscriber is dual-homed, routing between the two BNGs is required and can be performed with either a direct spoke SDP between the two nodes or by MP-BGP route learning.

For PPPoE, both PADI and PAP/CHAP authentication are supported. The PPPoE session is negotiated with the parameters defined by the retail VPRN interface. Because the IP address space of the sub-mgmt host may overlap between VPRN services, the node must anti-spoof the packets at access ingress with the session ID.

When the config>service>vprn>sub-if>private-retail-subnets command is enabled on the subscriber interface, the retail IP subnets are not pushed into the wholesale context. This allows IP addresses to overlap between different retail VPRNs (those VPRNs can have IPoE or PPPoE sessions). If an operator requires both residential and business services, two VPRNs connected to the same wholesaler can be created and use the flag in only one of them.

To update a subscriber attribute (such as an SLA-profile or subscriber-profile), either perform a RADIUS CoA or enter the tools>perform>subscriber-mgmt>coa command. To identify a subscriber for a CoA, the subscriber IP address can be used as part of the subscriber's key. Because it is possible for different subscribers to use the same IP address in multiple private retail VPRNs, additional parameters are required in addition to the subscriber IP address. When performing a CoA on a retail subscriber for PPPoE hosts using a private retail subnet, the following conditions apply:

• If NAS-port-ID is used, the VSA Alc-Retail-Serv-Id or Alc-Retail-Serv-Name must be included. The subscriber IP address and prefix must also be included.

• If Alc-serv-ID or Alc-Serv-Name is used, the subscriber retail service ID or name must be referenced. The subscriber IP address and prefix must also be included.

When performing a CoA on a retail subscriber for IPoE hosts using a private retail subnet, the following conditions apply:

• If NAS-port-ID is used, the VSA Alc-Client-Hardware-Addr referencing the subscriber MAC and Alc-Retail-Serv-Name must be included. The VSA Alc-Retail-Serv-Id must not be included. The subscriber IP address and prefix must also be included.

• If Alc-serv-ID or Alc-Serv-Name is used, it must reference the subscriber wholesale service ID or name. The VSA Alc-Client-Hardware-Address and the subscriber IP address and prefix must also be included.

To perform a DHCP force renew on IPoE hosts, enter the tools>perform>subscriber-mgmt>forcerenew command. To identify between subscribers within a private retail VPRN where overlapping IP addresses are possible, the MAC address must be used instead of the IP address in the tools command.

Private retail subnet are supported on both numbered and unnumbered subscriber interfaces. RADIUS host creation is possible for IPoEv4 hosts.

When IPoE and PPPoE session terminates in the retail VPRN, the node must learn the retail VPRN service ID. This can be provided by the LUDB or RADIUS. If the local user database is used, the host configuration provides a reference to the VPRN service ID. If RADIUS is used, RADIUS can return the service ID or name VSA. The retail subscriber interface must also reference the subscriber interface on the wholesale subscriber interfaces.

The following are not supported on private retail subnets:

• ARP hosts

• static hosts

ESM multicast is not supported for IPoE hosts that use overlapping IP address between private retail VPRNs. ESM multicast is only supported for IPoE hosts that have unique IP addresses in the system. It is possible for a IPoE subscriber to contain both type of hosts, only the hosts with overlapping private IP address do not support ESM multicast.

This feature enables negotiation between Broadband Network Gateway (BNG) and customer premises equipment (CPE) so that CPE is allocated both ip-address and associated subnet.

Some CPEs use the network up-link in PPPoE mode and perform dhcp-server function for all ports on the LAN side. Instead of wasting one subnet for P2P uplink, CPEs use allocated subnet for LAN portion as shown in CPEs network up-link mode.

From a BNG perspective, the specified PPPoE host is allocated a subnet (instead of /32) by RADIUS, external dhcp-server, or local-user-db. And locally, the host is associated with managed-route. This managed-route is a subset of the subscriber-interface subnet, and also, subscriber-host ip-address is from managed-route range. The negotiation between BNG and CPE allows CPE to be allocated both ip-address and associated subnet.

In this application, the PPPoE subscriber host terminates in a retail IES service. The IES service ID or name can be obtained by the Alc-Retail-Serv-Id or Alc-Retail-Serv-Name attribute in the RADIUS Access-Accept packet.

If MSAP is used, the SAP is created in the wholesale VPRN.

The PPPoE session is negotiated with the command options defined by the wholesale VPRN group interface. The connectivity to the retailer is performed using the linkage between the two interfaces.

Because of the nature of IES service, there is no IP address overlap between different IES retail services; therefore, the private-retail-subnets flag is not needed in this case.

Unlike regular IP routes which are mainly concerned with next-hop information, subscriber-hosts are associated with an extensive set of parameters related to filtering, qos, stateful state (PPPoE/DHCP), antispoofing, and so on. Forwarding Database (FDB) is not suitable to maintain all this information. Instead, each subscriber host record is maintained in separate set of subscriber-host tables.

By pre-provisioning the IP prefix (IPv4 and IPv6) under the subscriber-interface and sub-if>ipv6 CLI hierarchy, only a single prefix aggregating the subscriber host entries is installed in the FDB. This FDB entry points to the corresponding subscriber-host tables that contain subscriber-host records.

When a IPv4/IPv6 prefix is not pre-provisioned, or the subscriber-hosts falls out of pre-provisioned prefix, each subscriber-host is installed in the FDB. The result of the subscriber-host FDB lookup points to the corresponding subscriber-host record in the subscriber-host table. This scenario is referred to as unnumbered subscriber-interfaces.

Unnumbered does not mean that the subscriber hosts do not have an IP address or prefix assigned. It only means that the IP address range out of which the address or prefix is assigned to the host does not have to be known in advance through configuration under the subscriber-interface or sub-if>ipv6 node.

An IPv6 example would be:

configure
    router/service
        subscriber-interface <name>
            ipv6
                [no] allow-unmatching-prefixes
                delegated-prefix-length <bits>
                subscriber-prefixes 

This CLI indicates the following:

• There is no need for any indication of anticipated IPv6 prefixes in CLI.

• However, the delegated-prefix-length (DPL) command is required. The DPL (or the length of the prefix assigned to each Residential Gateway) must be known in advance. All Residential Gateways (or subscribers) under the same subscriber-interface share this pre-configured DPL.

• The DPL range is 48 to 64.

• If the prefix length in the received PD (through the DHCP server, RADIUS or LUDB) and the DPL do not match, the host creation fails.

• If the assigned IP prefix/address (DHCP server, RADIUS, LUDB) for the host falls outside of the CLI defined prefixes and the allow-unmatching-prefixes command is configured, then the new address and prefix automatically installs in the FDB.

The router supports the redirection of PPPoE packets on ingress to a Layer 3 static subscriber SAP and PW-SAPs (for example, bound to an IES or VPRN service) in the upstream direction toward an Epipe service. The Epipe, which is bound to a specific SAP on a group interface of a Layer 3 service, is used to backhaul the PPPoE traffic toward a remote destination, such as, a wholesale service provider. Other non-PPPoE packets are still forwarded to the group interface on the IES or VPRN.

There is a one-to-one mapping from backhaul Epipe to the subscriber. A single backhaul Epipe service per subscriber SAP is supported. Only static configuration of subscriber SAPs is supported.

In the downstream direction, all traffic arriving on the backhaul Epipe is forwarded to the subscriber SAP and merged with IPoE traffic.

This architecture is shown in Redirection of traffic to Epipe for backhaul.

If the SAP on the Layer 3 service is configured to indicate PPPoE redirection, in addition to ESM anti-spoofing, then the following processing occurs in the upstream direction:

• All PPPoE traffic, such as traffic with Ethertype 0x8863 and 0x8864, skips the anti-spoof lookup and is then forwarded to a configured Epipe service.

• All non-PPPoE is subject to anti-spoofing lookup. If the anti-spoofing fails, then the packet is dropped. Otherwise, processing continues using the host configuration.

The fwd-wholesale context in the Layer 3 SAP is used to configure the forwarding of PPPoE packets toward a specified Epipe service:

config
   service
      ies | vrpn
         subscriber-interface
            group-interface
               sap 1/1/1:10.20
                  anti-spoof ...
                  fwd-wholesale 
                     pppoe 10  
                     exit

The PPPoE option specifies that only packets matching Ethertype 0x8863 and 0x8864 are redirected to the Epipe of service ID '10'. This includes all PPPoE control plane packets. The service IDs specified under fwd-wholesale cannot refer to a vc-switching Epipe service.

When fwd-wholesale is configured to an Epipe with a specified service ID, the system ensures that the Epipe exists and meets all the requirements to participate in the PPPoE redirect. There is no need for additional configuration under the Epipe. For the previous example, only the following configuration is required for the Epipe:

epipe 10 name "10" customer 1 create
   service-mtu 1400
   spoke-sdp 10:9 create
      no shutdown
      exit
   no shutdown
   exit

The system generates a CLI error if a user tries to configure additional SAPs on an Epipe that is already referenced from a fwd-wholesale context.

In the upstream direction, subscriber queues are used for IPoE packets destined for a local host and PPPoE backhaul traffic. However, CPM traffic is not consuming subscriber queue resources; QoS profiles are instantiated while single-sub-parameters profiled-traffic-only is enabled.

In the downstream direction, PPPoE traffic arriving on the Epipe is merged back into the subscriber SAP referencing the Epipe service. ESM traffic from the host uses the subscriber queues that the host is configured to use, and the backhaul traffic uses the SAP queues. If profile-traffic-only is configured, then all traffic uses the SAP queues.

The operational status of the Epipe with a matching service ID can only be up if the corresponding Layer 3 SAP's operational status is up. The operational or administrative status of the Epipe does not affect the status of the Layer 3 service SAP. If the Layer 3 service SAP is up, but the backhaul Epipe is down, then the system continues to redirect packets to the Epipe, but they are dropped by the Epipe service. The status of the Epipe service is indicated in the output of the show>service>service-using command.

Interoperability with non-conforming PPPoE client implementations is supported with the following behavior:

• The PPPoE client continues to send IPCP renegotiation messages (such as ConfReq) after negotiation is complete. BNG terminates the PPP session by default, however, the following command allows BNG to ignore the renegotiation messages:

  Classic CLI:

  config>subscr-mgmt>ppp-policy>ncp-renegotiation ignore

  MD-CLI:

  configure subscriber-mgmt ppp-policy ncp-renegotiation

• The PPPoE client sets incorrect identifier values in the Echo-Reply messages that are sent to BNG. BNG discards such messages and the PPP session is terminated because of the Echo timeout. However, the following command instructs BNG to ignore the Identifier field of the Echo-Reply message to keep the PPP session up:

  Classic CLI:

  config>subscr-mgmt>ppp-policy>lcp-ignore-identifier

  MD-CLI:

  configure subscriber-mgmt ppp-policy lcp-ignore-identifier

The term MLPPPoX is used to reference MLPPP sessions over ATM transport (oA), Ethernet over ATM transport (oEoA) or Ethernet transport (oE). Although MLPPP in subscriber management context is not supported natively over PPP/HDLC links, the terms MLPPP and MLPPPoX terms can be used interchangeably. The reason for this is that link bundling, MLPPP encapsulation, fragmentation and interleaving can be in a broader scope observed independently of the transport in the first mile. However, MLPPPoX terminology prevails in this section in an effort to distinguish MLPPP functionality on an ASAP MDA (outside of ESM) and MLPPPoX in LNS (inside of ESM).

The terms speed and rate are interchangeably used throughout this section. Usually, speed refers to the speed of the link in general context (high or low) while rate quantitatively describes the link speed and associates it with the specific value in b/s.

This functionality is supported through LNS on BB-ISA. LNS MLPPPoX can be used then as a workaround for PTA deployments, whereby LAC and LNS can be run back-to-back in the same system (connected by an external loop or a VSM2 module), and therefore locally terminate PPP sessions.

MLPPPoX can:

• Increase bandwidth in the last mile by bundling multiple links together.

• LFI/reassembly over a single MLPPPoX capable link (plain PPP does not support LFI).

After the MLPPP bundle is created in the 7750 SR, traffic can be transmitted by using MLPPP encapsulation. However, MLPPP encapsulation is not mandatory over an MLPPP bundle.

MLPPP header is primarily required for sequencing the fragments. If a packet is not fragmented, it can be transmitted over the MLPPP bundle using either plain PPP encapsulation or MLPPP encapsulation. MLPPP encapsulation for fragmented traffic is shown in MLPPP encapsulation.

MLPPPoX is negotiated during the LCP session negotiation phase by the presence of the Max-Received-Reconstructed Unit (MRRU) field in the LCP ConfReq. MRRU option is a mandatory field required in MLPPPoX negotiation. It represents the maximum number of octets in the Information field (Data part in MLPPP encapsulation) of a reassembled packet. The MRRU value negotiated in the LCP phase must be the same on all member links and it can be greater or lesser than the PPP negotiated MRU value of each member link. This means that the reassembled payload of the PPP packet can be greater than the transmission size limit imposed by individual member links within the MLPPPoX bundle. Packets are always be fragmented so that the fragments are within the MRU size of each member link.

Another field that could be optionally present in an MLPPPoX LCP Conf Req is an Endpoint Discriminator (ED). Along with the authentication information, this field can be used to associate the link with the bundle.

The last MLPPPoX negotiated option is the Short Sequence Number Header Format Option which allows the sequence numbers in MLPPPoX encapsulated frames/fragments to be 12-bit long (instead 24-bit long, by default).

After the multilink capability is successfully negotiated by LCP, PPP sessions can be bundled together over MLPPPoX capable links.

The basic operational principles are:

• LCP session is negotiated on each physical link with MLPPPoX capabilities between the two nodes.

• Based on the ED and the authentication outcome, a bundle is created. A subsequent IPCP negotiation is conveyed over this bundle. User traffic is sent over the bundle.

• If a new link tries to join the bundle by sending a new MLPPPoX LCP Conf Request, the LCP session is negotiated, authentication performed and the link is placed under the bundle containing the links with the same ED and authentication outcome.

• IPCP and IPv6CP is in the whole process negotiated only once over the bundle. This negotiation occurs at the beginning, when the first link is established and MLPPPoX bundle created. IPCP and IPc6CP messages are transmitted from the 7750 SR LNS without MLPPPoX encapsulation, while they can be received as MLPPPoX encapsulated or non-MLPPPoX encapsulated.

The lowest granularity at which MLPPPoX can be enabled is an L2TP tunnel. An MLPPPoX enabled tunnel is not limited to carrying only MLPPPoX sessions but can carry normal PPP(oE) sessions as well.

In addition to enabling MLPPPoX on the session terminating node LNS, MLPPPoX can also be enabled on the LAC by a PPP policy. The purpose of enabling MLPPPoX on the LAC is to negotiate MLPPPoX LCP parameters with the client. After the LAC receives the MRRU option from the client in the initial LCP ConfReq, it changes its tunnel selection algorithm so that all sessions of an MLPPPoX bundle are mapped into the same tunnel.

The LAC negotiates MLPPPoX LCP parameters regardless of the transport technology connected to it (ATM or Ethernet). LCP negotiated parameters are passed by the LAC to the LNS by Proxy LCP in a ICCN message. This way, the LNS has an option to accept the LCP parameters negotiated by the LAC or to reject them and restart the negotiation directly with the client.

The LAC transparently passes session traffic handed to it by the LNS in the downstream direction and the MLPPPoX client in the upstream direction. The LNS and the MLPPPoX client performs all data processing functions related to MLPPPoX such as fragmentation and interleaving.

After the LCP negotiation is completed and the LCP transition into an open state (configuration ACKs are sent and received), the Authentication phase on the LAC begins. During the Authentication phase the L2TP parameters become known (l2tp group, tunnel, and so on), the session is extended by the LAC to the LNS by L2TP. If the Authentication phase does not return L2TP parameters, the session is terminated because the 7750 SR does not support directly terminated MLPPPoX sessions.

In the case that MLPPPoX is not enabled on the LAC, the LAC negotiates plain PPP session with the client. If the client accepts plain PPP instead of MLPPPoX as offered by the LAC, when the session is extended to the LNS, the LNS re-negotiates MLPPPoX LCP with the client on a MLPPPoX enabled tunnel. The LNS learns about the MLPPPoX capability of the client by a Proxy LCP message in ICCN (first Conf Req received from the client is also send in a Proxy LCP). If the there is no indication of the MLPPPoX capability of the client, the LNS establishes a plain PPP(oE) session with the client.

The purpose of Link Fragmentation and Interleaving (LFI) is to ensure that short high priority packets are not delayed by the transmission delay of large low priority packets on slow links.

For example it takes ~150ms to transmit a 5000B packet over a 256 kb/s link, while the same packet is transmitted in only 40us over a 1G link (~4000 times faster transmission). To avoid the delay of a high priority packet by waiting in the queue while the large packet is being transmitted, the large packet can be segmented into smaller chunks. The high priority packet can be then interleaved with the smaller fragments. This approach can significantly reduce the delay of high priority packets.

The interleaving functionality is only supported on MLPPPoX bundles with a single link. If more than one link is added into an interleaving capable MLPPPoX bundle, then interleaving is internally disabled and the tmnxMlpppBundleIndicatorsChange trap generated.

With interleaving enabled on an MLPPPoX enabled tunnel, the following session types are supported:

• Multiple LCP sessions tied into a single MLPPPoX bundle. This scenario assumes multiple physical links on the client side. Theoretically it would be possible to have multiple sessions running over the same physical link in the last mile. For example, two PPPoE sessions going over the same Ethernet link in the last mile, or two ATM VCs o2q1231qa23n the same last mile link. Whichever the case may be, the LAC/LNS is unaware of the physical topology in the last mile (single or multiple physical links). Interleaving functionality is internally disabled on such MLPPPoX bundle.

• A single LCP session (including dual stack) over the MLPPPoX bundle. This scenario assumes a single physical link on the client side. Interleaving is supported on such single session MLPPPoX bundle as long as the conditions for interleaving are met. Those conditions are governed by max-fragment-delay parameter and calculation of the fragment size as described in subsequent sections.

• An LCP session (including dual stack) over a plain PPP/PPPoE session. This type of session is a regular PPP(oE) session outside of any MLPPPoX bundle and therefore its traffic is not MLPPPoX encapsulated.

Packets on an MLPPPoX bundle are MLPPPoX encapsulated unless they are classified as high priority packets when interleaving is enabled.

As mentioned in the previous section, LFI on LNS is implemented only on MLPPPoX bundles with a single LCP session.

There are two major tasks (Most of this is also applicable to non-lfi case. The only difference between lfi and non-lfi is that there is no artificial delay performed in non-lfi case) associated with LFI on the LNS:

• Executing subscriber QoS in the carrier IOM based on the last mile conditions. The subscriber QoS rates are the last mile on-the-wire rates. After traffic is QoS conditioned, it is sent to the BB-ISA for further processing.

• Fragmentation and artificial delay (queuing) of the fragments so that high priority packets can be injected in-between low priority fragments (interleaved). This operation is performed by the BB-ISA.

Examine an example to further clarify functionality of LFI. The parameters, conditions and requirements that are used in the example to describe the wanted behavior are the following:

• High priority packets must not be delayed for more than 50ms in the last mile because of the transmission delay of the large low priority packets. Considering that tolerated end-to-end VoIP delay must be under 150ms, limiting the transmission delay to 50ms on the last mile link is a reasonable option.

• The link between the LNS and LAC is 1Gb/s Ethernet.

• The last mile link rate is 256 kb/s.

• Three packets arrive back-to-back on the network side of the LNS (in the downstream direction). The large 5000B low priority packet P1 arrives first, followed by two smaller high priority packets P2 and P3, each 100B.

  Note:

  Packets P1, P2 and P3 can be originated by independent sources (PCs, servers, and so on) and therefore can theoretically arrive in the LNS from the network side back-to-back at the full network link rate (10Gb/s or 100Gb/s).

• The transmission time on the internal 10G link between the BB-ISA and the carrier IOM for the large packet (5000B) is 4us while the transmission time for the small packet (100B) is 80ns.

• The transmission time on the 1G link (LNS->LAC) for the large packet (5000B) is 40us while the transmission time for the small packet (100B) is 0.8us.

• The transmission time in the last mile (256 kb/s) for the large packet is ~150ms while the transmission time for the small packet on the same link is ~3ms.

• Last mile transport is ATM.

To satisfy the delay requirement for the high priority packets, the large packets are fragmented into three smaller fragments. The fragments are carefully sized so that their individual transmission time in the last mile does not exceed 50ms. After the first 50ms interval, there is an opportunity to interleave the two smaller high priority packets.

This entire process is further clarified by the five points (1-5) in the packet route from the LNS to the Residential Gateway (RG) as depicted in Packet route from the LNS to the RG.

The five points are:

1. Last mile QoS awareness in the LNS

2. BB-ISA processing

3. LNS-LAC link

4. AN-RG link

5. Home link

   

Fragmentation and interleaving is implemented on the originating end of the traffic. In other words, in the upstream direction the CPE (or RG) is fragmenting and interleaving traffic. There is no interleaving or fragmentation processing in the upstream direction in the 7750 SR. The 7750 SR are on the receiving end and is only concerned with the reassembly of the fragments arriving from the CPE. Fragments are buffered until the packet can be reconstructed. If all fragments of a packet are not received within a preconfigured time frame, the received fragments of the partial packet are discarded (a packet cannot be reconstructed without all of its fragments). This time-out and discard is necessary to prevent buffer starvation in the BB-ISA. Two values for the time-out can be configured: 100ms and 1s.

Interleaving over MLPPPoX bundles with multiple links are not supported. However, fragmentation is supported.

To preserve packet order, all packets on an MLPPPoX bundle with multiple links are MLPPPoX encapsulated (monotonically increased sequence numbers).

Multiclass MLPPP (RFC 2686, The Multi-Class Extension to Multi-Link PPP) is not supported.

The following session types in the last mile are supported:

• MLPPPoE

  Single physical link or multilink. The last mile encapsulation is Ethernet over copper (This could be Ethernet over VDSL or HSDSL). The access rates (especially upstream) are still limited by the xDSL distance limitation and therefore interleaving is required on a slow speed single link in the last mile. It is possible that the last mile encapsulation is Ethernet over fiber (FTTH) but in this case, users would not be concerned with the link speed to the point where interleaving and link aggregation is required.

Finally, this is the slow link of the last mile, the reason why LFI is performed in the first place. Assuming that LFI played its role in the network as designed, by the time the transmission of one fragment on this link is completed, the next fragment arrives just in time for unblocked transmission. In between the two fragments are one or more small high priority packets waiting in the queue for the transmission to complete.

As shown in MLPPPoE — multiple physical links, the AN-RG link in that packets P2 and P3 are ahead of fragments F2 and F3. Therefore the delay incurred on this link by the low priority packets is never greater than the transmission delay of the first fragment (50ms). The remaining two fragments, F2 and F3, can be queued and further delayed by the transmission time of packets P2 and P3 (which is normally small, in the example 3ms for each).

• MLPPP(oEo)A — A single physical link or multilink. The last mile encapsulation is ATM over xDSL.

  

  

Some other combinations are also possible (ATM in the last mile, Ethernet in the aggregation) but they all come down to one of the above models that are characterized by:

• Ethernet or ATM in the last mile.

• Ethernet or ATM access on the LAC.

• MLPPP/PPPoE termination on the LNS.

PPP/PPPoE sessions are by default load balanced across multiple BB-ISAs (max 6) in the same group. The load balancing algorithm considers the number of active session on each BB-ISA in the same group. The load balancing algorithm does not consider the number of queues consumed on the carrier IOM. Therefore, a session can be refused if queues are depleted on the carrier IOM even though the BB-ISA may be lightly loaded in terms of the number of sessions that is hosting.

With MLPPPoX, it is important that multiple sessions per bundle be terminated on the same LNS BB-ISA. This can be achieved by per tunnel load balancing mode where all sessions of a tunnel are terminated in the same BB-ISA. Per tunnel load balancing mode is mandatory on LNS BB-ISAs that are in the group that supports MLPPPoX.

On the LAC side, all sessions in an MLPPPoX bundle are automatically assigned to the same tunnel. In other words an MLPPPoX bundle is assigned to the tunnel. There can be multiple tunnels created between the same pair of LAC/LNS nodes.

All downstream traffic on an MLPPPoX bundle with multiple links is always MLPPPoX encapsulated. Some traffic is fragmented and served in a octet oriented round robin fashion over multiple member links. However, fragments are never delayed when the bundle contains multiple links.

In a per fragment/packet load sharing algorithm, there is always the possibility that there is uneven load utilization between the member links. A single link overload can go unnoticed in the network all the way to the Access Node. The access node is the only node in the network that actually has multiple physical links connected to it. All other session-aware nodes (LAC and LNS) only see MLPPPoX as a bundle with multiple sessions without any mechanism to shape traffic per physical link. Other nodes in this case being 7750 SRs. Other vendors may have the ability to condition (shape) traffic per session.

If one of the member sessions is perpetually overloaded by the LNS, traffic is dropped in the last mile because the corresponding physical link cannot absorb traffic beyond its physical capabilities. This would have detrimental effects on the whole operation of the MLPPPoX bundle. To prevent this perpetual overloading of the member links that can be caused by per packet/fragment load balancing scheme, the load balancing scheme that considers the number of octets transmitted over each member link. The octet counter of a new link is initialized to the lowest value of any existing link counter. Otherwise the load balancing mechanism would show significant bias toward the new link until the byte counter catches up with the rest of the links.

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.

  Note: The rate reported according to RFC 5515 can still be different if the source for both methods is not the same.

• 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.

The link failure in the last mile is detected by the expiration of session keepalives (LCP). The LNS tears down the session over the failed link and notify the LAC by a CDN message.

CoA request for the subscriber aggregate-rate-limit change is honored on the LAC and the LNS.

CoA for the rate change of an individual link within the bundle is supported through the same VSA that can be used to initially assign the rate parameter to each member link. This is supported only on LNS. The rate override with CoA is applied to all active link members within the bundle.

Change of the access link parameters with CoA is supported in the following fashion:

• Change of access loop encap: refused (NAK)

• Change of access loop rate down:

• On L2TP LAC session: refused (NAK). On LAC the access loop rate down is not locally used for any rate limiting function but instead it is just passed to the LNS at the beginning when the session is first established. Mid-session changes on LAC by CoA are not propagated to the LNS.

• On the L2TP LNS session, the plain session is ignored. The rate is stored in the MIB table but no rate limiting action is taken. In other words, this parameter is internally excluded from rate calculations and advertisements. However, it is shown in the output of the relevant show commands.

• Bundle session: applied on all link sessions. The aggregate rate limit of the bundle is set to the minimum of the:

• CoA obtained local loop down rate multiplied by the number of links in the bundle

• The aggregate rate limit configured statically or obtained by CoA.

• Fragment length is affected by this change. In case that interleaving is enabled on a single link bundle, the interleave interval is affected.

• Non-L2TP: ignored. The rate is stored in the MIB table but no rate limiting action is taken. In other words, this parameter is internally excluded from rate calculations and advertisements. However, it is shown in the output of the relevant show commands.

Similar behavior is exhibited if at mid-session, the parameters are changed through LUDB with the exception of the rate-down parameter in LAC. If this parameter is changed on the LAC, all sessions are disconnected.

Accounting counters on the LNS include all packet overhead (wire overhead from the last mile). There is only one accounting session per bundle.

On the LAC, there is one accounting session per pppoe session (link).

In tunnel-accounting mode there is one accounting session per link.

On LNS only the stop-link of the last link of the bundle carries all accounting data for the bundle.

Filters and mirrors (LI) are not supported on an MLPPPoX bundle on LAC. However, filters and ip-only mirror type are supported on the LNS.

Locally terminated MLPPPoX (PTA) solution is offered based on the LAC and the LNS hosted in the same system. An external loop (or VSM2) is used to connect the LAC to the LNS within the same box. The subscribers are terminated on the LNS.

In the MLPPPoX case on LNS, multiple sessions are tied into the same subscriber aggregate-rate-limit using a sub-profile. The consequence is that the aggregate rate of the subscriber can be adjusted dynamically depending on the advertised link speed in the last mile and the number of links in the bundle.

In the LAC, the subscriber aggregate rate cannot be dynamically changed based on the number of links in the bundle and their rate. The LAC has no notion of MLPPPoX bundles. However, multiple sessions that in reality belong to an MLPPPoX bundle under the subscriber are shaped as an aggregate (agg-rate-limit under the sub-profile). This in essence yields the same shaping behavior as on LNS.

Sla-profile

Sessions within the MLPPPoX bundle in LNS share a single sla-profile instances (queues).

In the LAC, as long as the sessions within the subscriber6 are on the same SAP, they can also share the same sla-profile. This is be the case in MLPPPoX.

The manner in which sub/sla-profile are applied to MLPPPoX bundles and the individual sessions within results in aggregate shaping per MLPPPoX bundle as well as allocation of unique set of queues per MLPPPoX bundle. This is valid irrespective of the location where shaping is executed (LAC or LNS). Other vendors may have implemented shaping per session within the bundle and this is something that needs to be taken into consideration during the migration process.

LAC behavior

• A new PPP(oEoA) session request arrives to the LAC (PADI or LCP Conf Req).

• The LAC negotiates PADx session if applicable.

• The LAC may negotiate MLPPPoX LCP phase with its own endpoint discriminator, or it may reject MLPPPoX specific options in LCP if MLPPPoX on the LAC is disabled (such as, no accept-mrru in the LAC’s ppp-policy). If MLPPPoX options (seq num header format, ED, MRRU) are rejected, the assumption is that the client renegotiates plain PPP(oEoA) session with the LAC.

• After LCP (MLPPPoX capable or not) is negotiated, the session is authenticated (PAP/CHAP).

• Upon successful authentication, an L2TP tunnel is identified to which the session belongs.

• If the session is a non-L2TP session (PTA MLPPPoX capable session for which the tunnel cannot be determined), the session is terminated.

• Otherwise, the QoS constructs are created for the subscriber hosts: the session is assigned to a sub/sla-profiles.

• The session LCP parameters are sent to the LNS by call management messages.

The following assumes that MLPPPoX is configured on the LNS under the L2TP group or the tunnel hierarchy.

LNS behavior

• The LNS have the option to accept the LCP parameters or to reject them and start renegotiating LCP parameters directly with the client.

• If the LNS choose to renegotiate LCP parameters with the client directly, this renegotiation is completely transparent to the LAC by the means of a T-bit (control vs. data) in the L2TP header. LCP is renegotiated on the LNS with all the options necessary to support MLPPPoX.

  Note: Endpoint Discriminator is not mandatory in the MLPPPoX negotiation. If the client rejects it, the LNS must still be able to negotiate MLPPPoX capable session (same is valid for the LAC). If the client’s endpoint discriminator is invalid (bad format, invalid class, and so on), the 7750 SR is not negotiated MLPPPoX and instead a plain PPP session is created.

• If the LNS is configured to accept the LCP Proxy parameters, the LNS determines the capability of the client.

If there is no indication of MLPPPoX capability in the Proxy LCP (not even in the original ConfReq), the LNS may accept plain (non MLPPPoX capable) LCP session or renegotiate from scratch the non MLPPPoX capable session.

If there is an indication of MLPPPoX capability in the Proxy LCP (either completely negotiated on the LAC or at least attempted from the client), the LNS tries to accept the MLPPPoX negotiated session by the LAC or renegotiate the MLPPPoX capable session directly with the client.

If the LCP Proxy parameters with MLPPPoX capability are accepted by the LNS then the endpoint as negotiated on the LAC is also accepted.

• After the MLPPPoX capable LCP session is negotiated or accepted, authentication can be performed on the LNS. Authentication on the LNS can be restarted (CHAP challenge/response with the client), or accepted (chap challenge/response accepted and verified by the LNS through RADIUS).

  Note: chap-challenge length is configurable in LNS.

• If the authentication is successful, depending on the evaluation of the parameters negotiated up to this point a new MLPPPoX bundle is created or an existing MLPPPoX bundle is joined. In case that a new bundle is established, the QoS constructs for the subscriber(-host) is created (sub/sla-profile). Session negotiation advances to IPCP phase.

• The decision whether a new session should join an existing MLPPPoX bundle, or trigger creation of a new one is governed by RFC 1990, The PPP Multilink Protocol (MP), section 5.1.3, page 16, cases 1,2,3, and 4.

• IPv6 is supported.

• AA is supported at LNS where full IP packets can be redirected with AA policies.

• Intra-chassis redundancy is supported:

  • CPM stateful failover

  • BB-ISA — non-stateful failover