RFC 7926
Problem Statement and Architecture for Information Exchange between Interconnected Traffic-Engineered Networks
Pages: 67
Best Current Practice: 206
Best Current Practice: 206
Part 2 of 3 – Pages 22 to 43
4. Architecture
4.1. TE Reachability
As described in Section 1.1, TE reachability is the ability to reach a specific address along a TE path. The knowledge of TE reachability enables an end-to-end TE path to be computed. In a single network, TE reachability is derived from the Traffic Engineering Database (TED), which is the collection of all TE information about all TE links in the network. The TED is usually built from the data exchanged by the IGP, although it can be supplemented by configuration and inventory details, especially in transport networks. In multi-network scenarios, TE reachability information can be described as "You can get from node X to node Y with the following TE attributes." For transit cases, nodes X and Y will be edge nodes of the transit network, but it is also important to consider the information about the TE connectivity between an edge node and a specific destination node. TE reachability may be qualified by TE attributes such as TE metrics, hop count, available bandwidth, delay, and shared risk. TE reachability information can be exchanged between networks so that nodes in one network can determine whether they can establish TE paths across or into another network. Such exchanges are subject to a range of policies imposed by the advertiser (for security and administrative control) and by the receiver (for scalability and stability).4.2. Abstraction, Not Aggregation
Aggregation is the process of synthesizing from available information. Thus, the virtual node and virtual link models described in Section 3.5 rely on processing the information available within a network to produce the aggregate representations of links and nodes that are presented to the consumer. As described in Section 3, dynamic aggregation is subject to a number of pitfalls. In order to distinguish the architecture described in this document from the previous work on aggregation, we use the term "abstraction" in this document. The process of abstraction is one of applying policy to the available TE information within a domain, to produce selective information that represents the potential ability to connect across the domain.
Abstraction does not offer all possible connectivity options (refer to Section 3.5) but does present a general view of potential connectivity. Abstraction may have a dynamic element but is not intended to keep pace with the changes in TE attribute availability within the network. Thus, when relying on an abstraction to compute an end-to-end path, the process might not deliver a usable path. That is, there is no actual guarantee that the abstractions are current or feasible. Although abstraction uses available TE information, it is subject to policy and management choices. Thus, not all potential connectivity will be advertised to each client network. The filters may depend on commercial relationships, the risk of disclosing confidential information, and concerns about what use is made of the connectivity that is offered.4.2.1. Abstract Links
An abstract link is a measure of the potential to connect a pair of points with certain TE parameters. That is, it is a path and its characteristics in the server network. An abstract link represents the possibility of setting up an LSP, and LSPs may be set up over the abstract link. When looking at a network such as the network shown in Figure 7, the link from CN1 to CN4 may be an abstract link. It is easy to advertise it as a link by abstracting the TE information in the server network, subject to policy. The path (i.e., the abstract link) represents the possibility of establishing an LSP from client network edge to client network edge across the server network. There is not necessarily a one-to-one relationship between the abstract link and the LSP, because more than one LSP could be set up over the path. Since the client network nodes do not have visibility into the server network, they must rely on abstraction information delivered to them by the server network. That is, the server network will report on the potential for connectivity.4.2.2. The Abstraction Layer Network
Figure 7 introduces the abstraction layer network. This construct separates the client network resources (nodes C1, C2, C3, and C4, and the corresponding links) and the server network resources (nodes CN1, CN2, CN3, and CN4, and the corresponding links). Additionally, the architecture introduces an intermediary network layer called the
abstraction layer. The abstraction layer contains the client network edge nodes (C2 and C3), the server network edge nodes (CN1 and CN4), the client-server links (C2-CN1 and CN4-C3), and the abstract link (CN1-CN4). The client network is able to operate as normal. Connectivity across the network can be either found or not found, based on links that appear in the client network TED. If connectivity cannot be found, end-to-end LSPs cannot be set up. This failure may be reported, but no dynamic action is taken by the client network. The server network also operates as normal. LSPs across the server network between client network edges are set up in response to management commands or in response to signaling requests. The abstraction layer consists of the physical links between the two networks, and also the abstract links. The abstract links are created by the server network according to local policy and represent the potential connectivity that could be created across the server network and that the server network is willing to make available for use by the client network. Thus, in this example, the diameter of the abstraction layer network is only three hops, but an instance of an IGP could easily be run so that all nodes participating in the abstraction layer (and, in particular, the client network edge nodes) can see the TE connectivity in the layer. -- -- -- -- |C1|--|C2| |C3|--|C4| Client Network -- | | | | -- | | | | . . . . . . . . . . . | | | | | | | | | | --- --- | | Abstraction | |---|CN1|================|CN4|---| | Layer Network -- | | | | -- | | | | . . . . . . . . . . . . . . | | | | | | | | | | --- --- | | Server Network | |--|CN2|--|CN3|--| | --- --- --- --- Key --- Direct connection between two nodes === Abstract link Figure 7: Architecture for Abstraction Layer Network
When the client network needs additional connectivity, it can make a request to the abstraction layer network. For example, the operator of the client network may want to create a link from C2 to C3. The abstraction layer can see the potential path C2-CN1-CN4-C3 and can set up an LSP C2-CN1-CN4-C3 across the server network and make the LSP available as a link in the client network. Sections 4.2.3 and 4.2.4 show how this model is used to satisfy the requirements for connectivity in client-server networks and in peer networks.4.2.2.1. Nodes in the Abstraction Layer Network
Figure 7 shows a very simplified network diagram, and the reader would be forgiven for thinking that only client network edge nodes and server network edge nodes may appear in the abstraction layer network. But this is not the case: other nodes from the server network may be present. This allows the abstraction layer network to be more complex than a full mesh with access spokes. Thus, as shown in Figure 8, a transit node in the server network (here, the node is CN3) can be exposed as a node in the abstraction layer network with abstract links connecting it to other nodes in the abstraction layer network. Of course, in the network shown in Figure 8, there is little if any value in exposing CN3, but if it had other abstract links to other nodes in the abstraction layer network and/or direct connections to client network nodes, then the resulting network would be richer. -- -- -- -- Client |C1|--|C2| |C3|--|C4| Network -- | | | | -- | | | | . . . . . . . . . | | | | | | | | | | --- --- --- | | Abstraction | |--|CN1|========|CN3|========|CN5|--| | Layer Network -- | | | | | | -- | | | | | | . . . . . . . . . . . . | | | | | | | | | | | | Server | | --- | | --- | | Network | |--|CN2|-| |-|CN4|--| | --- --- --- --- --- Figure 8: Abstraction Layer Network with Additional Node
It should be noted that the nodes included in the abstraction layer network in this way are not "abstract nodes" in the sense of a virtual node described in Section 3.5. Although it is the case that the policy point responsible for advertising server network resources into the abstraction layer network could choose to advertise abstract nodes in place of real physical nodes, it is believed that doing so would introduce significant complexity in terms of: - Coordination between all of the external interfaces of the abstract node. - Management of changes in the server network that lead to limited capabilities to reach (cross-connect) across the abstract node. There has been recent work on control-plane extensions to describe and operate devices (such as asymmetrical switches) that have limited cross-connect capabilities [RFC7579] [RFC7580]. These or similar extensions could be used to represent the same type of limitations, as they also apply in an abstract node.4.2.3. Abstraction in Client-Server Networks
Figure 9 shows the basic architectural concepts for a client-server network. The nodes in the client network are C1, C2, CE1, CE2, C3, and C4, where the client edge (CE) nodes are CE1 and CE2. The core (server) network nodes are CN1, CN2, CN3, and CN4. The interfaces CE1-CN1 and CE2-CN4 are the interfaces between the client and server networks. The technologies (switching capabilities) of the client and server networks may be the same or different. If they are different, the client network traffic must be tunneled over a server network LSP. If they are the same, the client network LSP may be routed over the server network links, tunneled over a server network LSP, or constructed from the concatenation (stitching) of client network and server network LSP segments.
: : Client Network : Server Network : Client Network : : -- -- --- --- -- -- |C1|--|C2|--|CE1|................................|CE2|--|C3|--|C4| -- -- | | --- --- | | -- -- | |===|CN1|================|CN4|===| | | |---| | | |---| | --- | | --- --- | | --- | |--|CN2|--|CN3|--| | --- --- --- --- Key --- Direct connection between two nodes ... CE-to-CE LSP tunnel === Potential path across the server network (abstract link) Figure 9: Architecture for Client-Server Network The objective is to be able to support an end-to-end connection, C1-to-C4, in the client network. This connection may support TE or normal IP forwarding. To achieve this, CE1 is to be connected to CE2 by a link in the client network. This enables the client network to view itself as connected and to select an end-to-end path. As shown in the figure, three abstraction layer links are formed: CE1-CN1, CN1-CN2, and CN4-CE2. A three-hop LSP is then established from CE1 to CE2 that can be presented as a link in the client network. The practicalities of how the CE1-CE2 LSP is carried across the server network LSP may depend on the switching and signaling options available in the server network. The CE1-CE2 LSP may be tunneled down the server network LSP using the mechanisms of a hierarchical LSP [RFC4206], or the LSP segments CE1-CN1 and CN4-CE2 may be stitched to the server network LSP as described in [RFC5150]. Section 4.2.2 has already introduced the concept of the abstraction layer network through an example of a simple layered network. But it may be helpful to expand on the example using a slightly more complex network.
Figure 10 shows a multi-layer network comprising client network nodes (labeled as Cn for n = 0 to 9) and server network nodes (labeled as Sn for n = 1 to 9). -- -- |C3|---|C4| /-- --\ -- -- -- -- --/ \-- |C1|---|C2|---|S1|---|S2|----|S3| |C5| -- /-- --\ --\ --\ /-- / \-- \-- \-- --/ -- / |S4| |S5|----|S6|---|C6|---|C7| / /-- --\ /-- /-- -- --/ -- --/ -- \--/ --/ |C8|---|C9|---|S7|---|S8|----|S9|---|C0| -- -- -- -- -- -- Figure 10: An Example Multi-Layer Network If the network in Figure 10 is operated as separate client and server networks, then the client network topology will appear as shown in Figure 11. As can be clearly seen, the network is partitioned, and there is no way to set up an LSP from a node on the left-hand side (say C1) to a node on the right-hand side (say C7). -- -- |C3|---|C4| -- --\ -- -- \-- |C1|---|C2| |C5| -- /-- /-- / --/ -- / |C6|---|C7| / /-- -- --/ -- --/ |C8|---|C9| |C0| -- -- -- Figure 11: Client Network Topology Showing Partitioned Network
For reference, Figure 12 shows the corresponding server network topology. -- -- -- |S1|---|S2|----|S3| --\ --\ --\ \-- \-- \-- |S4| |S5|----|S6| /-- --\ /-- --/ -- \--/ |S7|---|S8|----|S9| -- -- -- Figure 12: Server Network Topology Operating on the TED for the server network, a management entity or a software component may apply policy and consider what abstract links it might offer for use by the client network. To do this, it obviously needs to be aware of the connections between the layers (there is no point in offering an abstract link S2-S8, since this could not be of any use in this example). In our example, after consideration of which LSPs could be set up in the server network, four abstract links are offered: S1-S3, S3-S6, S1-S9, and S7-S9. These abstract links are shown as double lines on the resulting topology of the abstraction layer network in Figure 13. As can be seen, two of the links must share part of a path (S1-S9 must share with either S1-S3 or S7-S9). This could be achieved using distinct resources (for example, separate lambdas) where the paths are common, but it could also be done using resource sharing. -- |C3| /-- -- -- --/ |C2|---|S1|==========|S3| -- --\\ --\\ \\ \\ \\ \\-- -- \\ |S6|---|C6| \\ -- -- -- -- \\-- -- |C9|---|S7|=====|S9|---|C0| -- -- -- -- Figure 13: Abstraction Layer Network with Abstract Links
That would mean that when both paths S1-S3 and S7-S9 carry
client-edge-to-client-edge LSPs, the resources on path S1-S9 are used
and might be depleted to the point that the path is resource
constrained and cannot be used.
The separate IGP instance running in the abstraction layer network
means that this topology is visible at the edge nodes (C2, C3, C6,
C9, and C0) as well as at a Path Computation Element (PCE) if one is
present.
Now the client network is able to make requests to the abstraction
layer network to provide connectivity. In our example, it requests
that C2 be connected to C3 and that C2 be connected to C0. This
results in several actions:
1. The management component for the abstraction layer network asks
its PCE to compute the paths necessary to make the connections.
This yields C2-S1-S3-C3 and C2-S1-S9-C0.
2. The management component for the abstraction layer network
instructs C2 to start the signaling process for the new LSPs in
the abstraction layer.
3. C2 signals the LSPs for setup using the explicit routes
C2-S1-S3-C3 and C2-S1-S9-C0.
4. When the signaling messages reach S1 (in our example, both LSPs
traverse S1), the server network may support them by a number of
means, including establishing server network LSPs as tunnels,
depending on the mismatch of technologies between the client and
server networks. For example, S1-S2-S3 and S1-S2-S5-S9 might be
traversed via an LSP tunnel, using LSPs stitched together, or
simply by routing the client network LSP through the server
network. If server network LSPs are needed, they can be signaled
at this point.
5. Once any server network LSPs that are needed have been
established, S1 can continue to signal the client-edge-to-client-
edge LSP across the abstraction layer, using the server network
LSPs as either tunnels or stitching segments, or simply routing
through the server network.
6. Finally, once the client-edge-to-client-edge LSPs have been set
up, the client network can be informed and can start to advertise
the new TE links C2-C3 and C2-C0. The resulting client network
topology is shown in Figure 14.
-- -- |C3|-|C4| /-- --\ / \-- -- --/ |C5| |C1|---|C2| /-- -- /--\ --/ -- / \ |C6|---|C7| / \ /-- -- / \--/ --/ -- |C0| |C8|---|C9| -- -- -- Figure 14: Connected Client Network with Additional Links 7. Now the client network can compute an end-to-end path from C1 to C7.4.2.3.1. A Server with Multiple Clients
A single server network may support multiple client networks. This is not an uncommon state of affairs -- for example, when the server network provides connectivity for multiple customers. In this case, the abstraction provided by the server network may vary considerably according to the policies and commercial relationships with each customer. This variance would lead to a separate abstraction layer network maintained to support each client network. On the other hand, it may be that multiple client networks are subject to the same policies and the abstraction can be identical. In this case, a single abstraction layer network can support more than one client. The choices here are made as an operational issue by the server network.4.2.3.2. A Client with Multiple Servers
A single client network may be supported by multiple server networks. The server networks may provide connectivity between different parts of the client network or may provide parallel (redundant) connectivity for the client network. In this case, the abstraction layer network should contain the abstract links from all server networks so that it can make suitable computations and create the correct TE links in the client network.
That is, the relationship between the client network and the abstraction layer network should be one to one.4.2.4. Abstraction in Peer Networks
Figure 15 shows the basic architectural concepts for connecting across peer networks. Nodes from four networks are shown: A1 and A2 come from one network; B1, B2, and B3 from another network; etc. The interfaces between the networks (sometimes known as External Network Network Interfaces - ENNIs) are A2-B1, B3-C1, and C3-D1. The objective is to be able to support an end-to-end connection, A1-to-D2. This connection is for TE connectivity. As shown in the figure, abstract links that span the transit networks are used to achieve the required connectivity. These links form the key building blocks of the end-to-end connectivity. An end-to-end LSP uses these links as part of its path. If the stitching capabilities of the networks are homogeneous, then the end-to-end LSP may simply traverse the path defined by the abstract links across the various peer networks or may utilize stitching of LSP segments that each traverse a network along the path of an abstract link. If the network switching technologies support or necessitate the use of LSP hierarchies, the end-to-end LSP may be tunneled across each network using hierarchical LSPs that each traverse a network along the path of an abstract link. : : : Network A : Network B : Network C : Network D : : : -- -- -- -- -- -- -- -- -- -- |A1|--|A2|---|B1|--|B2|--|B3|---|C1|--|C2|--|C3|---|D1|--|D2| -- -- | | -- | | | | -- | | -- -- | |========| | | |========| | -- -- -- -- Key --- Direct connection between two nodes === Abstract link across transit network Figure 15: Architecture for Peering Peer networks exist in many situations in the Internet. Packet networks may peer as IGP areas (levels) or as ASes. Transport networks (such as optical networks) may peer to provide concatenations of optical paths through single-vendor environments (see Section 6). Figure 16 shows a simple example of three peer networks (A, B, and C) each comprising a few nodes.
Network A : Network B : Network C : : -- -- -- : -- -- -- : -- -- |A1|---|A2|----|A3|---|B1|---|B2|---|B3|---|C1|---|C2| -- --\ /-- : -- /--\ -- : -- -- \--/ : / \ : |A4| : / \ : --\ : / \ : -- \-- : --/ \-- : -- -- |A5|---|A6|---|B4|----------|B6|---|C3|---|C4| -- -- : -- -- : -- -- : : : : Figure 16: A Network Comprising Three Peer Networks As discussed in Section 2, peered networks do not share visibility of their topologies or TE capabilities for scaling and confidentiality reasons. That means, in our example, that computing a path from A1 to C4 can be impossible without the aid of cooperating PCEs or some form of crankback. But it is possible to produce abstract links for reachability across transit peer networks and to create an abstraction layer network. That network can be enhanced with specific reachability information if a destination network is partitioned, as is the case with Network C in Figure 16. Suppose that Network B decides to offer three abstract links B1-B3, B4-B3, and B4-B6. The abstraction layer network could then be constructed to look like the network in Figure 17. -- -- -- -- |A3|---|B1|====|B3|----|C1| -- -- //-- -- // // // -- --// -- -- |A6|---|B4|=====|B6|---|C3| -- -- -- -- Figure 17: Abstraction Layer Network for the Peer Network Example Using a process similar to that described in Section 4.2.3, Network A can request connectivity to Network C, and abstract links can be advertised that connect the edges of the two networks and that can be used to carry LSPs that traverse both networks. Furthermore, if
Network C is partitioned, reachability information can be exchanged to allow Network A to select the correct abstract link, as shown in Figure 18. Network A : Network C : -- -- -- : -- -- |A1|---|A2|----|A3|=========|C1|.....|C2| -- --\ /-- : -- -- \--/ : |A4| : --\ : -- \-- : -- -- |A5|---|A6|=========|C3|.....|C4| -- -- : -- -- Figure 18: Tunnel Connections to Network C with TE Reachability Peer networking cases can be made far more complex by dual-homing between network peering nodes (for example, A3 might connect to B1 and B4 in Figure 17) and by the networks themselves being arranged in a mesh (for example, A6 might connect to B4 and C1 in Figure 17). These additional complexities can be handled gracefully by the abstraction layer network model. Further examples of abstraction in peer networks can be found in Sections 6 and 8.4.3. Considerations for Dynamic Abstraction
It is possible to consider a highly dynamic system where the server network adaptively suggests new abstract links into the abstraction layer, and where the abstraction layer proactively deploys new client-edge-to-client-edge LSPs to provide new links in the client network. Such fluidity is, however, to be treated with caution. In particular, in the case of client-server networks of differing technologies where hierarchical server network LSPs are used, this caution is needed for three reasons: there may be longer turn-up times for connections in some server networks; the server networks are likely to be sparsely connected; and expensive physical resources will only be deployed where there is believed to be a need for them. More significantly, the complex commercial, policy, and administrative relationships that may exist between client and server network operators mean that stability is more likely to be the desired operational practice.
Thus, proposals for fully automated multi-layer networks based on this architecture may be regarded as forward-looking topics for research both in terms of network stability and with regard to economic impact. However, some elements of automation should not be discarded. A server network may automatically apply policy to determine the best set of abstract links to offer and the most suitable way for the server network to support them. And a client network may dynamically observe congestion, lack of connectivity, or predicted changes in traffic demand and may use this information to request additional links from the abstraction layer. And, once policies have been configured, the whole system should be able to operate independently of operator control (which is not to say that the operator will not have the option of exerting control at every step in the process).4.4. Requirements for Advertising Links and Nodes
The abstraction layer network is "just another network layer". The links and nodes in the network need to be advertised along with their associated TE information (metrics, bandwidth, etc.) so that the topology is disseminated and so that routing decisions can be made. This requires a routing protocol running between the nodes in the abstraction layer network. Note that this routing information exchange could be piggybacked on an existing routing protocol instance (subject to different switching capabilities applying to the links in the different networks, or to adequate address space separation) or use a new instance (or even a new protocol). Clearly, the information exchanged is only information that has been created as part of the abstraction function according to policy. It should be noted that in many cases the abstract link represents the potential for connectivity across the server network but that no such connectivity exists. In this case, we may ponder how the routing protocol in the abstraction layer will advertise topology information for, and over, a link that has no underlying connectivity. In other words, there must be a communication channel between the abstraction layer nodes so that the routing protocol messages can flow. The answer is that control-plane connectivity already exists in the server network and on the client-server edge links, and this can be used to carry the routing protocol messages for the abstraction layer network. The same consideration applies to the advertisement, in the client network, of the potential connectivity that the abstraction layer network can provide, although it may be more normal to establish that connectivity before advertising a link in the client network.
4.5. Addressing Considerations
The network layers in this architecture should be able to operate with separate address spaces, and these may overlap without any technical issues. That is, one address may mean one thing in the client network, yet the same address may have a different meaning in the abstraction layer network or the server network. In other words, there is complete address separation between networks. However, this will require some care, both because human operators may well become confused, and because mapping between address spaces is needed at the interfaces between the network layers. That mapping requires configuration so that, for example, when the server network announces an abstract link from A to B, the abstraction layer network must recognize that A and B are server network addresses and must map them to abstraction layer addresses (say P and Q) before including the link in its own topology. And similarly, when the abstraction layer network informs the client network that a new link is available from S to T, it must map those addresses from its own address space to that of the client network. This form of address mapping will become particularly important in cases where one abstraction layer network is constructed from connectivity in multiple server networks, or where one abstraction layer network provides connectivity for multiple client networks.5. Building on Existing Protocols
This section is non-normative and is not intended to prejudge a solutions framework or any applicability work. It does, however, very briefly serve to note the existence of protocols that could be examined for applicability to serve in realizing the model described in this document. The general principle of protocol reuse is preferred over the invention of new protocols or additional protocol extensions, and it would be advantageous to make use of an existing protocol that is commonly implemented on network nodes and is currently deployed, or to use existing computational elements such as PCEs. This has many benefits in network stability, time to deployment, and operator training. It is recognized, however, that existing protocols are unlikely to be immediately suitable to this problem space without some protocol extensions. Extending protocols must be done with care and with consideration for the stability of existing deployments. In extreme cases, a new protocol can be preferable to a messy hack of an existing protocol.
5.1. BGP-LS
BGP - Link State (BGP-LS) is a set of extensions to BGP, as described in [RFC7752]. Its purpose is to announce topology information from one network to a "northbound" consumer. Application of BGP-LS to date has focused on a mechanism to build a TED for a PCE. However, BGP's mechanisms would also serve well to advertise abstract links from a server network into the abstraction layer network or to advertise potential connectivity from the abstraction layer network to the client network.5.2. IGPs
Both OSPF and IS-IS have been extended through a number of RFCs to advertise TE information. Additionally, both protocols are capable of running in a multi-instance mode either as ships that pass in the night (i.e., completely separate instances using different address spaces) or as dual instances on the same address space. This means that either OSPF or IS-IS could probably be used as the routing protocol in the abstraction layer network.5.3. RSVP-TE
RSVP-TE signaling can be used to set up all TE LSPs demanded by this model, without the need for any protocol extensions. If necessary, LSP hierarchy [RFC4206] or LSP stitching [RFC5150] can be used to carry LSPs over the server network, again without needing any protocol extensions. Furthermore, the procedures in [RFC6107] allow the dynamic signaling of the purpose of any LSP that is established. This means that when an LSP tunnel is set up, the two ends can coordinate into which routing protocol instance it should be advertised and can also agree on the addressing to be said to identify the link that will be created.5.4. Notes on a Solution
This section is not intended to be prescriptive or dictate the protocol solutions that may be used to satisfy the architecture described in this document, but it does show how the existing protocols listed in the previous sections can be combined, with only minor modifications, to provide a solution.
A server network can be operated using GMPLS routing and signaling protocols. Using information gathered from the routing protocol, a TED can be constructed containing resource availability information and Shared Risk Link Group (SRLG) details. A policy-based process can then determine which nodes and abstract links it wishes to advertise to form the abstraction layer network. The server network can now use BGP-LS to advertise a topology of links and nodes to form the abstraction layer network. This information would most likely be advertised from a single point of control that made all of the abstraction decisions, but the function could be distributed to multiple server network edge nodes. The information can be advertised by BGP-LS to multiple points within the abstraction layer (such as all client network edge nodes) or to a single controller. Multiple server networks may advertise information that is used to construct an abstraction layer network, and one server network may advertise different information in different instances of BGP-LS to form different abstraction layer networks. Furthermore, in the case of one controller constructing multiple abstraction layer networks, BGP-LS uses the route target mechanism defined in [RFC4364] to distinguish the different applications (effectively abstraction layer network VPNs) of the exported information. Extensions may be made to BGP-LS to allow advertisement of Macro Shared Risk Link Groups (MSRLGs) (Appendix B.1) and the identification of mutually exclusive links (Appendix B.2), and to indicate whether the abstract link has been pre-established or not. Such extensions are valid options but do not form a core component of this architecture. The abstraction layer network may operate under central control or use a distributed control plane. Since the links and nodes may be a mix of physical and abstract links, and since the nodes may have diverse cross-connect capabilities, it is most likely that a GMPLS routing protocol will be beneficial for collecting and correlating the routing information and for distributing updates. No special additional features are needed beyond adding those extra parameters just described for BGP-LS, but it should be noted that the control plane of the abstraction layer network must run in an out-of-band control network because the data-bearing links might not yet have been established via connections in the server network.
The abstraction layer network is also able to determine potential
connectivity from client network edge to client network edge. It
will determine which client network links to create according to
policy and subject to requests from the client network, and will take
four steps:
- First, it will compute a path across the abstraction layer
network.
- Then, if support of the abstract links requires the use of
server network LSPs for tunneling or stitching and if those LSPs
are not already established, it will ask the server layer to set
them up.
- Then, it will signal the client-edge-to-client-edge LSP.
- Finally, the abstraction layer network will inform the client
network of the existence of the new client network link.
This last step can be achieved by either (1) coordination of the
end points of the LSPs that span the abstraction layer (these points
are client network edge nodes) using mechanisms such as those
described in [RFC6107] or (2) using BGP-LS from a central controller.
Once the client network edge nodes are aware of a new link, they will
automatically advertise it using their routing protocol and it will
become available for use by traffic in the client network.
Sections 6, 7, and 8 discuss the applicability of this architecture
to different network types and problem spaces, while Section 9 gives
some advice about scoping future work. Section 10 ("Manageability
Considerations") is particularly relevant in the context of this
section because it contains a discussion of the policies and
mechanisms for indicating connectivity and link availability between
network layers in this architecture.
6. Application of the Architecture to Optical Domains and Networks
Many optical networks are arranged as a set of small domains. Each
domain is a cluster of nodes, usually from the same equipment vendor
and with the same properties. The domain may be constructed as a
mesh or a ring, or maybe as an interconnected set of rings.
The network operator seeks to provide end-to-end connectivity across
a network constructed from multiple domains, and so (of course) the
domains are interconnected. In a network under management control,
such as through an Operations Support System (OSS), each domain is
under the operational control of a Network Management System (NMS).
In this way, an end-to-end path may be commissioned by the OSS instructing each NMS, and the NMSes setting up the path fragments across the domains. However, in a system that uses a control plane, there is a need for integration between the domains. Consider a simple domain, D1, as shown in Figure 19. In this case, nodes A through F are arranged in a topological ring. Suppose that there is a control plane in use in this domain and that OSPF is used as the TE routing protocol. ----------------- | D1 | | B---C | | / \ | | / \ | | A D | | \ / | | \ / | | F---E | | | ----------------- Figure 19: A Simple Optical Domain Now consider that the operator's network is built from a mesh of such domains, D1 through D7, as shown in Figure 20. It is possible that these domains share a single, common instance of OSPF, in which case there is nothing further to say because that OSPF instance will distribute sufficient information to build a single TED spanning the whole network, and an end-to-end path can be computed. A more likely scenario is that each domain is running its own OSPF instance. In this case, each is able to handle the peculiarities (or, rather, advanced functions) of each vendor's equipment capabilities.
------ ------ ------ ------ | | | | | | | | | D1 |---| D2 |---| D3 |---| D4 | | | | | | | | | ------\ ------\ ------\ ------ \ | \ | \ | \------ \------ \------ | | | | | | | D5 |---| D6 |---| D7 | | | | | | | ------ ------ ------ Figure 20: A Mesh of Simple Optical Domains The question now is how to combine the multiple sets of information distributed by the different OSPF instances. Three possible models suggest themselves, based on pre-existing routing practices. o In the first model (the area-based model), each domain is treated as a separate OSPF area. The end-to-end path will be specified to traverse multiple areas, and each area will be left to determine the path across the nodes in the area. The feasibility of an end-to-end path (and, thus, the selection of the sequence of areas and their interconnections) can be derived using hierarchical PCEs. This approach, however, fits poorly with established use of the OSPF area: in this form of optical network, the interconnection points between domains are likely to be links, and the mesh of domains is far more interconnected and unstructured than we are used to seeing in the normal area-based routing paradigm. Furthermore, while hierarchical PCEs may be able to resolve this type of network, the effort involved may be considerable for more than a small collection of domains. o Another approach (the AS-based model) treats each domain as a separate Autonomous System (AS). The end-to-end path will be specified to traverse multiple ASes, and each AS will be left to determine the path across the nodes in that AS. This model sits more comfortably with the established routing paradigm but causes a massive escalation of ASes in the global Internet. It would, in practice, require that the operator use private AS numbers [RFC6996], of which there are plenty.
Then, as suggested in the area-based model, hierarchical PCEs
could be used to determine the feasibility of an end-to-end path
and to derive the sequence of domains and the points of
interconnection to use. But just as in the area-based model, the
scalability of this model using a hierarchical PCE must be
questioned, given the sheer number of ASes and their
interconnectivity.
Furthermore, determining the mesh of domains (i.e., the inter-AS
connections) conventionally requires the use of BGP as an
inter-domain routing protocol. However, not only is BGP not
normally available on optical equipment, but this approach
indicates that the TE properties of the inter-domain links would
need to be distributed and updated using BGP -- something for
which it is not well suited.
o The third approach (the Automatically Switched Optical Network
(ASON) model) follows the architectural model set out by the ITU-T
[G.8080] and uses the routing protocol extensions described in
[RFC6827]. In this model, the concept of "levels" is introduced
to OSPF. Referring back to Figure 20, each OSPF instance running
in a domain would be construed as a "lower-level" OSPF instance
and would leak routes into a "higher-level" instance of the
protocol that runs across the whole network.
This approach handles the awkwardness of representing the domains
as areas or ASes by simply considering them as domains running
distinct instances of OSPF. Routing advertisements flow "upward"
from the domains to the high-level OSPF instance, giving it a full
view of the whole network and allowing end-to-end paths to be
computed. Routing advertisements may also flow "downward" from
the network-wide OSPF instance to any one domain so that it can
see the connectivity of the whole network.
Although architecturally satisfying, this model suffers from
having to handle the different characteristics of different
equipment vendors. The advertisements coming from each low-level
domain would be meaningless when distributed into the other
domains, and the high-level domain would need to be kept
up to date with the semantics of each new release of each vendor's
equipment. Additionally, the scaling issues associated with a
well-meshed network of domains, each with many entry and exit
points and each with network resources that are continually being
updated, reduces to the same problem, as noted in the virtual link
model. Furthermore, in the event that the domains are under the
control of different administrations, the domains would not want
to distribute the details of their topologies and TE resources.
Practically, this third model turns out to be very close to the methodology described in this document. As noted in Section 6.1 of [RFC6827], there are policy rules that can be applied to define exactly what information is exported from or imported to a low-level OSPF instance. [RFC6827] even notes that some forms of aggregation may be appropriate. Thus, we can apply the following simplifications to the mechanisms defined in [RFC6827]: - Zero information is imported to low-level domains. - Low-level domains export only abstracted links as defined in this document and according to local abstraction policy, and with appropriate removal of vendor-specific information. - There is no need to formally define routing levels within OSPF. - Export of abstracted links from the domains to the network-wide routing instance (the abstraction routing layer) can take place through any mechanism, including BGP-LS or direct interaction between OSPF implementations. With these simplifications, it can be seen that the framework defined in this document can be constructed from the architecture discussed in [RFC6827], but without needing any of the protocol extensions defined in that document. Thus, using the terminology and concepts already established, the problem may be solved as shown in Figure 21. The abstraction layer network is constructed from the inter-domain links, the domain border nodes, and the abstracted (cross-domain) links. Abstraction Layer -- -- -- -- -- -- | |===========| |--| |===========| |--| |===========| | | | | | | | | | | | | | ..| |...........| |..| |...........| |..| |...........| |...... | | | | | | | | | | | | | | -- -- | | | | -- -- | | | | -- -- | | | |_| |_| |_| | | |_| |_| |_| | | |_| |_| |_| | | | | | | | | | | | | | | | | | | | | | | | | | -- -- -- -- -- -- -- -- -- -- -- -- Domain 1 Domain 2 Domain 3 Key Optical Layer ... Layer separation --- Physical link === Abstract link Figure 21: The Optical Network Implemented through the Abstraction Layer Network
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