Extensions to Resource Reservation Protocol - Traffic Engineering (RSVP-TE) for Hub and Spoke Multipoint Label Switched Paths (LSPs)

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 . When used in lower case, these words convey their typical use in common language, and are not to be interpreted as described in RFC2119 .

The proposed technique targets one-to-many applications that require reverse one-to-one traffic flow (thus many one-to-one in the reverse direction). There are a few applications that could use such kind of Resource Reservation Protocol - Traffic Engineering (RSVP-TE) based Hub and Spoke Multipoint LSPs.

over MPLS is defined in . It is required that the LSP used to transport PTP event message between a Master Clock and a Slave Clock and the LSP between the same Slave Clock and Master Clock must be co-routed. By using RSVP-TE based Point-to-Multipoint (P2MP) technology to transmit PTP Sync messages will get bandwidth insurance and greatly improve the bandwidth usage. In this case, the root of HSMP LSP could be Master Clock, or connects Master Clock directly or indirectly, as long as the root receives PTP event message, the leaf of HSMP LSP could be Slave Clock, or connects Slave Clock directly or indirectly, as long as the leaf receives PTP event message. Current RSVP-TE based Point-to-Multipoint LSP mechanism only provides unidirectional path from the root to the leaf nodes, and could not provide a co-routed reverse path from leaf to root node. This draft attempts to solve this problem. RSVP-TE based Hub and Spoke P2MP LSP described in this draft provides a co-routed reverse path from the leaf to the root based on current unidirectional Point- to-Multipoint LSP. The bandwidth from leaf to root will be the total bandwidth consumed by all leaves to root.

Point-to-Multipoint (P2MP) Pseudowires (PW) described in requires reverse path from the leaf to the root node. The P2MP PW multiplexed to RSVP-TE based HSMP LSP can provide VPMS with reverse path, without introducing independent reverse paths from each leaf to the root. In that case, the operational cost will be reduced for maintaining only one HSMP LSP, instead of P2MP LSP and n (number of leaf nodes) P2P reverse LSPs.

An HSMP LSP provides a Point-to-Multipoint reachability from the root node to the leaf nodes and a unicast reachability from all the leaf nodes back to the root node. An obvious question that comes up is that how is this better than setting up a P2MP LSP from a root node and Unidirectional reverse LSPs back from the leaves to the root node. This section compares the two mechanisms and demonstrates how establishing one HSMP LSP is better than establishing a P2MP LSP with reverse LSPs from the leaves back to the root. Consider the topology as shown in Figure 1. Router A wants to establish a Point-to-Multipoint connectivity to Routers E, F, G and H and also wants a Unicast path back from these routers to itself. There are two ways to accomplish this. In the first, we set up a HSMP LSP between A, E, F, G and H. In the second, we set up a P2MP LSP between A, E, F, G and H and establish regular LSPs back from these routers to A.

A | B / \ C D / \ / \ E F G H

When an RSVP-capable router receives an initial Path message, it creates a path state block (PSB) for that particular session. Each PSB consists of parameters derived from the received Path message such as SESSION, SENDER_TEMPLATE, SENDER_TSPEC, RSVP_HOP objects, and the outgoing interface provided by the IGP routing. Similarly, as a Resv message travels upstream toward the sender, it creates a reservation state block (RSB) in each RSVP-capable node along the way which stores information derived from the objects in the received Resv message, such as SESSION, RSVP_HOP, FLOWSPEC, FILTERSPEC, STYLE, etc objects. The PSB and the RSB need to be periodically refreshed by the Path and the Resv messages. In case of HSMP LSP, the number of PSBs and the RSBs is the same as that for establishing a single P2MP LSP and is a function of how the P2MP LSP is signaled. It is equal to the number of S2L sub-LSPs of the P2MP LSP if each S2L sub-lsp is signaled independently. It is one, if an aggregated mode is used where multiple sub-lsps of the P2MP LSP are signaled togethar. In the second case routers need to maintain this state for the P2MP LSP and all the Unidirectional LSPs that go via it. Lets look at the state that branch node B needs to maintain. In case of HSMP LSP it is the same as a P2MP LSP. In the other approach it needs to maintain state for the following LSPs: P2MP LSP from A and E, F, G and H Reverse LSP ECBA Reverse LSP FCBA Reverse LSP GDBA Reverse LSP HDBA We can thus clearly see that the amount of state that routers need to maintain in the second approach is much more than the HSMP LSP. It becomes all the more pronounced when the P2MP LSP is signalled using the aggregated approach described in where a single Path and Resv message is used to signal the entire P2MP LSP. In such cases the amount of state that such branch nodes need to maintain increase linearly with the leaf nodes that get added to the P2MP LSP.

In the HSMP LSP the LSR B advertises the same (upstream) label to C and D, thus consumes only one label and needs to only program one entry in the ILM table. In the second approach, LSR B needs to advertise two different labels to LSRs C and D and will thus consume 2 ILM entries in HW. We can clearly see that the number of labels consumed in the second approach will increase linearly with the amount of branching that happens on that LSR. It will further aggravate as the number of P2MP LSPs increase.

In the second approach, LSR B will have RSVP control traffic for the P2MP LSP and all the Unidirectional reverse LSPs that pass through it. In case of HSMP LSR B will only have the RSVP traffic for the P2MP LSP.

The Hub and Spoke Multipoint LSP comprises of one downstream unidirectional P2MP LSP from ingress LSR to each of egress LSR, and a co-routed upstream path from each of egress LSR to ingress LSR. describes a point-to-point bidirectional LSP mechanism for the GMPLS architecture, where a bidirectional LSP setup is indicated by the presence of an Upstream_Label object in the Path message. The Upstream_Label object has the same format as the generalized label, and uses Class-Number 35 (of form 0bbbbbbb) and the C-Type of the label being used. Hub and Spoke Multipoint LSP describe in this draft will use similar mechanism, and reuse the Upstream_Label object defined in . Note: the downstream label assignment is still applied, and upstream direction is based on the h&s topology (hub = upstream, spoke= downstream), rather on forwarding direction.

allows a P2MP LSP to be signaled using one or more Path messages . Each Path message may signal one or more source to leaf (S2L) sub-LSPs. This document assumes that a unique Path message is being used to signal each individual sub-LSP of the HSMP LSP. Later versions of this document can describe mechanisms to use a single Path message to describe each component sub LSP of the HSMP LSP.

The process of establishing a Hub and Spoke Multipoint LSP follows the establishment of a unidirectional P2MP LSP define in with some additions. To support Hub and Spoke Multipoint LSPs an Upstream_Label object is added to the Path message. The Upstream_Label object MUST indicate a label that is valid for forwarding at the time the Path message is sent. When a Path message containing an Upstream_Label object is received, the receiver first verifies that the upstream label is acceptable. If the label is not acceptable, the receiver MUST issue a PathErr message with a "Routing problem/Unacceptable label value" indication. The generated PathErr message MAY include an Acceptable Label Set defined in section 4.1. The transit node must also allocate one label for the co-routed upstream path before propagating the Path message to all downstream nodes. If a transit node is unable to allocate a label or internal resources, then it MUST issue a PathErr message with a "Routing problem/MPLS label allocation failure" indication. With regards to the co-routed return path from the leafs to the root, the forwarding table on transit node will have one incoming labels allocated for all of the outgoing interfaces, and one outgoing label received from Upstream_Label object in Path message sent by upstream node. That means the traffic from different egress LSRs will be merged at each transit node, and will be sent together to upstream node, see section 3.3 for more detail of bandwidth guarantee in this case. The Path messages sending downstream with same [P2MP ID, Tunnel ID, Extended Tunnel ID] tuple as part of the SESSION object and the [Tunnel Sender Address, LSP ID] tuple as part of the SENDER_TEMPLATE object, but may different [Sub-Group Originator ID, Sub-Group ID] MUST use same allocated label value for Upstream_Label object. Leaf nodes process Path messages as usual, with the exception that the upstream label should be used to transport data traffic associated with the Hub and Spoke Multipoint LSP upstream towards the root node. When a Hub and Spoke Multipoint LSP is removed, both upstream and downstream labels are invalidated and it is no longer valid to send data using the associated labels.

The bandwidth allocation for upstream path from leaf to root could be same as the downstream path from root to leaf node , and the bandwidth will be guarantee only when there is no traffic merging happened on transit node. If there are cases where leaf nodes send traffic to root node at the same time which may cause traffic to be merged on one physical link at transit node, then traffic overload may happen on these links.

There are several casse to require asymmetric bandwidth allocation for HSMP LSP. For example, when HSMP LSP is used in VPLS application, each leaf node may send reverse traffic to root node at the same time. The reverse path bandwidth allocation MUST fullfill the traffic requirement, with the assumption that traffic from all leaf nodes will be merged at each link. Some applications may not require bandwidth guarantee for the upstream path from leaf to root (only use reverse path for signaling), then it is not necessary to allocate bandwidth for the upstream path. The mechanism of allocating asymmetric bandwidth for point-to-point link is described in, and this draft will have some extension to support asymmetric bandwidth allocation for HSMP LSP. Each S2L sub-LSP should have its own traffic bandwidth requirement for the reverse path of HSMP LSP, and the S2L sub-LSP should carry the bandwidth information.

The sender descriptor in path message is modified as follows to add bandwidth information to each S2L sub-LSPs.

<sender descriptor> ::= <SENDER_TEMPLATE> <SENDER_TSPEC> [ <ADSPEC> ] [ <RECORD_ROUTE> ] [ <SUGGESTED_LABEL> ] [ <RECOVERY_LABEL> ] <UPSTREAM_LABEL> <UPSTREAM_FLOWSPEC> S2L sub-LSP descriptor list in path message should have the following format.

<S2L sub-LSP descriptor list> ::= <S2L sub-LSP descriptor> [ <S2L sub-LSP descriptor list> ] <S2L sub-LSP descriptor> ::= <S2L_SUB_LSP> [ <P2MP SECONDARY_EXPLICIT_ROUTE> ] <UPSTREAM_FLOWSPEC> FF flow descriptor in resv message is modified as follows to add bandwidth information to each S2L sub-LSPs.

<FF flow descriptor> ::= [ <FLOWSPEC> ] [ <UPSTREAM_TSPEC>] [ <UPSTREAM_ADSPEC> ] <FILTER_SPEC> <LABEL> [ <RECORD_ROUTE> ] [ <S2L sub-LSP flow descriptor list> ] <S2L sub-LSP flow descriptor list> ::= <S2L sub-LSP flow descriptor> [ <S2L sub-LSP flow descriptor list> ] <S2L sub-LSP flow descriptor> ::= <S2L_SUB_LSP> [ <P2MP_SECONDARY_RECORD_ROUTE> ] [ <UPSTREAM_TSPEC>] [ <UPSTREAM_ADSPEC> ]

The Path message of an asymmetric bandwidth HSMP LSP MUST contain an UPSTREAM_FLOWSPEC object defined in. Nodes processing a Path message containing an UPSTREAM_FLOWSPEC object MUST allocate the upstream bandwidth with the sum of bandwidth of all S2L sub-LSP traversing this node. A node that is unable to allocate the internal resources based on the contents of the UPSTREAM_FLOWSPEC object MUST issue a PathErr message with a "Routing problem/MPLS label allocation failure" indication.

The process of UPSTREAM_TSPEC and UPSTREAM_ADSPEC Object is same as defined in. When an UPSTREAM_TSPEC object is received by the root node, the root MAY determine that the original reservation is insufficient to satisfy the traffic flow. In this case, the root MAY require the LSP to be re-routed, and in extreme cases might result in the LSP being torn down when sufficient resources are not available.

The Following is an example of establishing a HSMP LSP using the procedures described in the previous sections.

Receiver | | PE2 PE3 --- Receiver | | P1 -- P3 / Source --- PE1 \ P2 -- PE5 --- Receiver | PE4 --- Receiver Figure 2 The mechanism is explained using Figure 1. PE1 is a root LER (head end) node. PE2, PE3, PE4 and PE5 are the leaf LER nodes. P1 and P2 are branch LSR nodes and P3 is a plain LSR node. PE1 learns that PE2, PE3, PE4 and PE5 are interested in joining a HSMP tree with a P2MP ID of P2MP ID1. We assume that PE1 learns of the egress LERs at different points in time. PE1 computes the P2P path by control plane to reach PE3 and sends a Path message with ERO [PE1, P1, P3, PE3]. It also provides an Upstream Label UL1 in the Upstream_Label object that P1 should use when forwarding packets to PE1. The Path message traverses hop-by-hop and finally reaches PE3. Assume that the Path message from P1 to P3 uses upstream label of UL3, in which case P1 must program the ILM to swap UL3 with UL1. The Path message from P3 to PE3 uses upstream label UL4, and thus P3 programs the ILM to swap UL4 with UL3. PE3 responds with a Resv message that contains label L4, that P3 should use when forwarding packets to PE3. Similarly, the Resv from P3 to P1 contains label L3, that P1 should use when forwarding packets to P3. Similarly when setting up the component sub-LSP from PE1 to PE2, PE1 will use the same Upstream label UL1 as it knows that this sub-LSP belongs to the same HSMP LSP because of the same P2MP session object that both sub-LSPs carry. The Path message, thus for this component sub-LSP goes with ERO [PE1, P1, PE2] along with the Upstream label UL1 that P1 should use when forwarding packets to PE1. P1 forwards the Path message with a new Upstream label UL2. Finally, PE2 sends a Resv message containing label L2, that P1 should use when forwarding packets to PE2. P1 also understands that the Resv messages from PE2 and PE3 refer to the same HSMP LSP, because of the P2MP Session Object carried in each. [ P1 sends a separate Resv message to PE1 corresponding to each of the sub-LSPs, but uses the same label L1 since the two sub LSPs belong to the same HSMP LSP. The other component sub LSPs are set up in a similar way as described above.

The operation of adding leaf LER(s) to an existing HSMP LSP is termed grafting. This operation allows leaf nodes to join a HSMP LSP at different points in time. The leaf LER(s) can be added by signaling only the impacted component sub- LSPs in a new Path message. Hence, the existing component sub-LSPs do not have to be re-signaled.

Receiver | | PE2 PE3 --- Receiver | | P1 -- P3 -- P6 -- PE6 --- Receiver / Root --- PE1 \ P2 -- PE5 --- Receiver | PE4 --- Receiver Figure 3 Assume PE1 needs to set up another sub-LSP from PE1 to PE6. Being a part of the same HSMP LSP, PE1 MUST advertise the same Upstream Label to P1 in its Path message. P1 advertises the same Upstream Label to P3. P3 when sending the Path message to P6 would advertise a fresh Upstream label and similarly P6 would use a new upstream label when forwarding the Path message to PE6. PE6 sends a Resv message with a label back to P6. P6, would send a new label back to P3. P3 because of this new component sub-LSP (PE1-PE6) is now a branch LSR node that performs MPLS multicast replication.

The operation of removing egress LER nodes from an existing HSMP LSP is termed as pruning. This operation allows leaf nodes to be removed from a HSMP LSP at different points in time. This section describes the mechanisms to perform pruning. Assume that the LER PE6 wants to be removed from the HSMP LSP. Since we used a unique Path message for each component sub LSP, the teardown will rely on generating a PathTear message for the corresponding Path message. PE6 will send a Path Tear message with the SESSION and SENDER_TEMPLATE objects corresponding to the HSMP LSP and the [Sub-Group Originator ID, Sub-Group ID] tuple corresponding to the Path message. P3 upon receiving the PathTear message would prune the MPLS multicast replication list and will become a normal RSVP LSR node. In the P2MP and HSMP context the PathTear is used for a specific component sub LSP teardown. This does not necessarily mean the whole path's breakdown from upstream; hence the LSRs MUST retain the Upstream label until all the component sub LSPs of the HSMP LSP are torn down. When a HSMP LSP is removed by the root, a PathTear message MUST be generated for each Path message used to signal the HS Multipoint LSP.

The refresh reduction procedures described in are equally applicable to HS Multipoint LSPs described in this document. Refresh reduction applies to individual messages and the state they install/maintain, and that continues to be the case for HS Multipoint LSPs.

extensions can be used to perform fast reroute for the mechanism described in this document when applied within packet networks. This is still TBD.

We would like to thank Dimitri Papadimitriou, Yuji Kamite, Sebastien Jobert for their comments and feedback on the document.

The same security considerations apply as for the RSVP-TE P2MP LSP specification, as described in .

No requests for IANA at this point of time.

IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems

We had considered another approach where the leaves could set up a unidirectional LSP by reversing the RRO that they recieve in the S2L sub-LSP Path message and use that as the ERO in their Path message. This approach was rejected because this would have entailed setting up another reverse LSP which leads to more state being maintained on all the intermediate routers. The reader is suggested to go through Section 2 which discusses this in detail.