| Routing Area Working Group | Bin Liu |
| Internet-Draft | ZTE Inc. |
| Intended status: Informational | Yantao Sun |
| Expires: December 23, 2014 | Jing Cheng |
| Yichen Zhang | |
| Beijing Jiaotong University | |
| June 21, 2014 |
Generic Fault-avoidance Routing Protocol for Data Center Networks
draft-sl-rtgwg-far-dcn-01
This draft proposes a generic routing method and protocol for a regular data center network, named as the fault-avoidance routing (FAR) protocol. FAR protocol provides a generic routing method for all types of network architectures that are proposed for large-scale cloud-based data centers over the past few years. FAR protocol is well designed to fully leverage the regularity in the topology and compute its routing table in a simplistic manner. Fat-tree is taken as an example architecture to illustrate how FAR protocol can be applied in real operational scenarios.
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In recent years, with the rapid development of cloud computing technologies, the widely deployed cloud services, such as Amazon EC2 and Google search, bring about huge challenges to data center networking (DCN). Today's cloud-based data centers (DCs) require large-scale networks with larger internal bandwidth and smaller transfer delay, However, conventional networks cannot meet such requirements due to limitations in their network architecture. In order satisfy the requirements of cloud computing services, many new network architectures have been proposed for data centers, such as Fat-tree, MatrixDCN, and BCube. These new architectures can support non-blocking large-scale datacenter networks with more than tens of thousands of physical servers.
Generic routing protocols such as OSPF and IS-IS cannot be scaled to support a very large-scale datacenter network because of the problems of network convergence and PDU overhead. For each type of architecture, researchers designed a routing algorithm according to the features of its topology. Because these routing algorithms are different and lack compatibility with each other, it is very difficult to develop a routing protocol for network routers supporting multiple routing algorithms. Furthermore, the fault tolerances in those routing algorithms are very complicated and have low efficiency.
This draft proposes a generic routing method and protocol, fault-avoidance routing (FAR) protocol, for DCNs. This method leverages the regularity in the topologies of data center networks to simplify routing learning and accelerate the query of routing tables. This routing method has a better fault tolerance property and can be applied to any DCN with a regular topology.
DCN - Data Center Network
FAR - Fault-avoidance Routing
BRT - Basic Routing Table
NRT - Negative Routing Table
NDT - Neighbor Devices Table
ADT - All Devices Table
LFT - Link failure Table
DA - Device Announcement
LFA - Link Failure Announcement
DLR¨C Device and Link Request
IP - Internet Protocol
UDP - User Datagram Protocol
VM – Virtual Machine
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 RFC-2119 [RFC2119]. In this document, these words will appear with that interpretation only when in ALL CAPS. Lower case uses of these words are not to be interpreted as carrying RFC-2119 significance.
The problem to be addressed by FAR as proposed in this draft is described in this section. The expansion of Cloud data center networks has brought significant challenges to the existing routing technologies. FAR mainly solves a series of routing problems faced by large-scale data center networks.
In a large-scale cloud data center network, there may be thousands of routers. Running OSPF and other routing protocols in such network will encounter these two challenges: a) Network convergence time would be too long, which will cause a longer time to elapse for creating and updating the routes. The response to network failures may be excessively high; b) a large number of routing protocol packets need to be sent. The routing information consumes a lot of network bandwidth and CPU resources, which easily leads to packet loss and makes the problem (a) more prominent.
In order to solve these problems, a common practice is partitioning the large network into some small areas, where the route calculation runs independently within different areas. However, nowadays the cloud data centers typically require very large internal bandwidth. To meet this requirement, a large number of parallel equivalent links are deployed in the network, such as the Fat-tree network architecture. Partitioning the network will affect the utilization of routing algorithm on equivalent multi-path and reduce internal network bandwidth requirements.
In the FAR routing calculation process, a Basic Routing Table (BRT) is built on local network topology leveraging the regularity of the network topologies. In addition to BRT, FAR also builds a Negative Routing Table (NRT). FAR gradually builds NRT in the process of learning network link failure information, which does not require learning the complete network fault information. FAR does not need to wait for the completion of the network convergence in the process of building these two tables. Therefor, it avoids the problem of excessive network convergence overheads in the route calculation process. In addition, FAR only needs to exchange a small amount of link change information between routers, and hence consumes less network bandwidth.
There are many real world scenario where tens of thousands of nodes(or much more nodes) need to be deployed in a flat area, such as infiniband routing and switching system, high-performance computer network, and many IDC networks in China. The similar problems have been existed long ago. People have solved the problems through similar solutions, such as the traditional regular topology-based RFC3619 protocol, the routing protocols of infiniband routing and switching system, and high-performance computer network routing protocol.
Why OSPF and other conventional routing methods do not work well in a large-scale network with giant number nodes of routers?
As everyone knows, the OSPF protocol uses multiple databases, more topological exchange information (as seen in the following example) and complicated algorithm. It requires routers to consume more memory and CPU processing capability. But the processing rate of CPU on the protocol message per second is very limited. When the network expands, CPU will quickly approach its processing limits, and at this time OSPF can not continue to expand the scale of the management. The SPF algorithm itself does not thoroughly solve these problems.
On the contrary, FAR does not have the convergence time delay and the additional CPU overheads, which SPF requires. Because in the initial stage, FAR already knows the regular information of the whole network topology and does not need to periodically do SPF operation.
One of the examples of "more topological exchange information":
In the OSPF protocol, LSA floods every 1800 seconds. Especially in the larger network, the occupation of CPU and band bandwidth will soon reach the router’s performance bottleneck. In order to reduce these adverse effects, OSPF introduced the concept of Area, which still has not solved the problem thoroughly). By dividing the OSPF Area into several areas, the routers in the same area do not need to know the topological details outside their area. (In comparison with FAR, after OSPF introducing the concept of Area, the equivalent paths cannot be selected in the whole network scope)
OSPF can achieve the following results by Area : 1) Routers only need to maintain the same link state databases as other routers within the same Area, without the necessity of maintaining the same link state database as all routers in the whole OSPF domain. 2) The reduction of the link state databases means dealing with relatively fewer LSA, which reduces the CPU consumption of routers; 3) The large number of LSAs flood only within the same Area. But, its negative effect is that the smaller number of routers which can be managed in each OSPF area. On the contrary, because FAR does not have the above disadvantages, FAR can also manage large-scale network even without dividing Areas.
The aging time of OSPF is set in order to adapt to routing transformation and protocol message exchange happened frequently in the irregular topology. Its negative effect is: when the network does not change, the LSA needs to be refreshed every 1800 seconds to reset the aging time. In the regular topology, as the routings are fixed, it does not need the complex protocol message exchange and aging rules to reflect the routing changes, as long as LFA mechanism in the FAR is enough.
Therefore, in FAR, we can omit many unnecessary processing and the packet exchange. The benefits are fast convergence speed and much larger network scale than other dynamic routing protocol. Now there are some successful implementations of simplified routings in the regular topology in the HPC environment. Conclusion: As FAR needs few routing entries and the topology is regular, the database does not need to be updated regularly. Without the need for aging, there is no need for CPU and bandwidth overhead brought by LSA flood every 30 minutes, so the expansion of the network has no obvious effect on the performance of FAR, which is contrary to OSPF.
Comparison of convergence time: The settings of OSPF spf_delay and spf_hold_time can affect the change of convergence time. The convergence time of the network with 2480 nodes is about 15-20 seconds(as seen in the following pages); while the FAR does not need to calculate the SFP, so there is no such convergence time.
These issues still exist in rapid convergence technology of OSPF and ISIS (such as I-SPF). The convergence speed and network scale constraint each other. FAR does not have the above problems, and the convergence time is almost negligible.
Can FRR solve these problems? IP FRR has some limitations. The establishment of IP FRR backup scheme will not affect the original topology and traffic forwarding which are established by protocol, however, we can not get the information of whereabouts and status when the traffic is switched to an alternate next hop.
Routers typically configure multiple network interfaces, each connected to a subnet. OSPF and other routing algorithms require that each interface of the router must be configured with an IP address. A large-scale data center network may contain thousands of routers. Tens of thousands of IP addresses may be needed to configure for each router with dozens of network interfaces. It will be a very complex issue to configure and manage a large number of network interfaces. Network maintenance is usually costly and error-prone. It will be difficult to troubleshoot the problems.
In FAR, the device position information is encoded in the IP address of the router. Each router only needs to be assigned a unique IP address according its location, which greatly solves complex network addressing issues in large-scale networks.
There are a large number of subnets in the large-scale data center network. Routers may build a routing entry for each subnet, and therefore the size of the routing tables on each router may be very large. It will increase equipment cost and reduce the querying speed of the routing table.
FAR uses two measures to reduce the size of the routing tables: a) Builds a BRT on the regularity of the network topologies; b) introduces a new routing table, i.e., a NRT. In this way FAR can reduce the size of routing tables to only a few dozen routing entries.
To implement efficient routing in large-scale datacenters, besides FAR, some other routing methods are proposed for some specific network architectures, such as Fat-tree and BCube. These routing methods are different(from both design and implementation viewpoints) and not compatibile with the conventional routing methods, which brings big troubles to network equipment providers to develop new routers supporting various new routing methods.
FAR is a generic routing method. With slight modification, FAR method can be applied to most of regular datacenter networks. Furthermore, the structure of routing tables and querying a routing table in FAR are the same as conventional routing method. If FAR is adopted, the workload of developing a new type of router will be significantly decreased.
Supporting VM migration is very important for cloud-based datacenter networks. However, in order to support layer-3 routing, routing methods including OSPF and FAR require limiting VM migration within a subnet. For this paradox, the mainstream methods still utilize layer-3 routing on routers or switches, transmit packets encapsulated by IPinIP or MACinIP between hosts by tunnels passing through network to the destination access switch, and then extract original packet out and send it to the destination host.
By utilizing the aforementioned methods, FAR can be applied to Fat-tree, MatrixDCN or BCube networks for supporting VM migration in entire network.
Link-state Learning |Routing Table | Routing Table
| Building
| | Querying
+--------+ /---------------\ | +--------+ | Packets
|2 Device|<--| 1 Neighbor & |----->| 5 BRT \ |
|Learning| | Link Detection| | |Building|\ | |
+--------+ \---------------/ | +--------+ \ | \|/
| | | \ |+--------------+
| | | /|| 7 Querying |
\|/ \|/ | / || Routing Table|
+-----------------------+| / |+--------------+
|3 Invisible Neighbor & || / |
|Link Failure Inferring || +--------- |
+-----------------------+|/| 6 NRT | |
| / |Building| |
\|/ /| +--------+ |
+--------------+ / | |
|4 Link Failure| / | |
| Learning | | |
+--------------+ | |
| |
Figure 1: The FAR framework
FAR requires that a DCN has a regular topology, and network devices, including routers, switches, and servers, are assigned IP addresses according to their locations in the network. In other word, we can locate a device in the network according to its IP address.
FAR is a distributed routing method. In order to support FAR, each router needs to have a routing module that implements the FAR algorithm. FAR algorithm is composed of three parts, i.e., link-state learning, routing table building and routing table querying, as shown in Fig. 1.
Link-state learning is responsible for a router to detect the states of its connected links and learn the states of all the other links in the entire network. The second part builds two routing tables, a basic routing table (BRT) and an negative routing table (NRT), according to the learned link states in the first part. The third part queries the BRT and the NRT to decide a next forwarding hop for the received (ingress) packets.
----------------------------------------------------------
Device ID | Device IP | Port ID | Link State | Update Time
----------------------------------------------------------
--------------------------------------------------
Device ID | Device IP | Type | State | Update Time
--------------------------------------------------
--------------------------------------------
No | Router 1 IP | Router 2 IP | Update Time
--------------------------------------------
-------------------------------------------------------
Destination | Mask | Next Hop | Interface | Update Time
-------------------------------------------------------
-------------------------------------------------------------------
Destination| Mask| Next Hop| Interface| Failed Link No| Timestamp
-------------------------------------------------------------------
Some data tables are maintained on each router in FAR. They are:
Neighbor Device Table (NDT): To store neighbor routers and related links.
All Devices Table (ADT): To store all routers in the entire network.
Link Failures Table (LFT): To store all link failures in the entire network.
Basic Routing Table (BRT): To store the candidate routes.
Negative Routing Table(NRT): To store the avoiding routes.
The format of NDT
Device ID: The ID of a neighbor router.
Device IP: The IP address of a neighbor router.
Port ID: The port ID that a neighbor router is attached to.
Link State: The state of the link between a router and its neighbor router. There are two states: Up and Down.
Update Time: The time of updating the entry.
The format of ADT
Device ID: The ID of a neighbor router.
Device IP: The IP address of a neighbor router.
Type: The type of a neighbor router.
State: The state of a neighbor router. There are two states: Up and Down.
Update Time: The time of updating the entry.
The format of LFT
No: The entry number.
Router 1 IP: The IP address of one router that a failed link connects to.
Router 2 IP: The IP address of another router that a failed link connects to.
Update Time: The time of updating the entry.
The format of BRT
Destination: A destination network
Mask: The subnet mask of a destination network.
Next Hop: The IP address of a next hop for a destination.
Interface: The interface related to a next hop.
Update Time: The time of updating the entry.
The format of NRT
Destination: A destination network.
Mask: The subnet mask of a destination network.
Next Hop: The IP address of a next hop that should be avoided for a destination.
Interface: The interface related to a next hop that should be avoided.
Failed Link No: A group of failed link numbers divided by ¡°/¡±, for example 1/2/3.
Timestamp: The time of updating the entry.
|<--- 1 --->| <--- 1 --->|<--------- 2 ---------->|
+-----------+------------+------------------------+
| Version |Message Type| Message Length |
+-----------+------------+------------------------+
| Checksum | AuType |
+------------------------+------------------------+
| Authentication |
+-------------------------------------------------+
| Authentication |
+-------------------------------------------------+
| Timestamp |
+-------------------------------------------------+
Figure 2: The format of FAR header
|<--- 1 --->| <--- 1 --->|<--------- 2 ---------->|
+-----------+------------+------------------------+
| Version |Message Type| Message Length |
+-----------+------------+------------------------+
| Checksum | AuType |
+------------------------+------------------------+
| Authentication |
+-------------------------------------------------+
| Authentication |
+-------------------------------------------------+
| Timestamp |
+-------------------------------------------------+
| Router IP |
+------------------------+------------------------+
| HelloInterval | HelloDeadInterval |
+------------------------+------------------------+
| Neighbor Router IP |
+-------------------------------------------------+
| ... |
+-------------------------------------------------+
Figure 3: The Format of Hello Messages
|<----1---->| <----1---->|<----------2----------->|
+-----------+------------+------------------------+
| Version |Message Type| Message Length |
+-----------+------------+------------------------+
| Checksum | AuType |
+------------------------+------------------------+
| Authentication |
+-------------------------------------------------+
| Authentication |
+-------------------------------------------------+
| Timestamp |
+------------------------+------------------------+
| Router1 IP |
+-------------------------------------------------+
| ... |
+-------------------------------------------------+
Figure 4: The Format of DA Messages
|<----1---->| <----1---->|<----------2----------->|
+-----------+------------+------------------------+
| Version |Message Type| Message Length |
+-----------+------------+------------------------+
| Checksum | AuType |
+------------------------+------------------------+
| Authentication |
+-------------------------------------------------+
| Authentication |
+-------------------------------------------------+
| Timestamp |
+------------------------+------------------------+
| Left IP |
+-------------------------------------------------+
| Right IP |
+------------------------+------------------------+
| State |
+-------------------------------------------------+
| ... |
+-------------------------------------------------+
Figure 5: The Format of LFA Messages
|<----1---->| <----1---->|<----------2----------->|
+-----------+------------+------------------------+
| Version |Message Type| Message Length |
+-----------+------------+------------------------+
| Checksum | AuType |
+------------------------+------------------------+
| Authentication |
+-------------------------------------------------+
| Authentication |
+-------------------------------------------------+
| Timestamp |
+-------------------------------------------------+
Figure 6: The Format of DLR Messages
Some protocol messages are exchanged between routers in FAR.
Hello Message: This message is exchanged between neighbor routers to learn adjacency.
Device Announcement (DA): Synchronize the knowledge of routers between routers.
Link Failure Announcement (LFA): Synchronize link failures between routers.
Device and Link Request (DLR): When a router starts, it requests the knowledge of routers and links from its neighbors by a DLR message.
A FAR Message is directly encapsulated in an IP packet. The protocol field of IP header indicates an IP packet is an FAR message. The protocol of IP for FAR should be assigned by IANA.
The four types of FAR messages have same format of packet header, called FAR header (as shown in Figure 2).
Version: FAR version
Message Type: The type of FAR message.
Packet Length: The packet length of the total FAR message.
Checksum: The checksum of an entire FAR message.
AuType:Authentication type. 0: no authentication, 1: Plaintext Authentication, 2: MD5 Authentication.
Authentication: Authentication information. 0: undefined, 1: Key, 2: key ID, MD5 data length and packet number. MD5 data is appended to the backend of the packet.
AuType and Authentication can refer to the definition of OSPF packet.
For Hello messages, the Message Type in FAR header is set to 1.Besides FAR header, a Hello message(Fig. 3) requires the following fields:
Router IP: The router IP address.
HelloInterval: The interval of sending Hello messages to neighbor routers.
RouterDeadInterval: The interval to set a neighbor router dead(out-of-service). If in the interval time, a router doesn't receive a Hello message from its neighbor router, the neighbor router is treated as dead.
Neighbor Router IP: The IP address of a neighbor router. All the neighbor router's addresses should be included in a Hello message.
For DA messages(Fig. 4), the Message Type in FAR header is set to 2. Besides FAR header, a DA message includes IP addresses of all the announced routers.
For LFA messages(Fig. 5), the Message Type in FAR header is set to 3. Besides FAR header, a LFA message includes all the announced link failures.
Left IP: The IP address of the left endpoint router of a link.
Right IP: The IP address of the right endpoint router of a link.
State: Link state. 0: Up, 1: down
For DLR messages(Fig. 6), the Message Type in FAR header is set to 1.Except for FAR header, DLR has no additional fields.
M1 is responsible for sending and receiving Hello messages, and detecting directly-connected links and neighbor routers. Each Hello message is encapsulated in a UDP packet. M1 sends Hello messages periodically to all the active router ports and receives Hello messages from its neighbor routers. M1 detects neighbor routers and directly-connected links according to received Hello Messages and stores these neighbors and links into a Neighbor Devices Table (NDT). Additionally, M1 also stores the neighbor routers into an All Devices Table (ADT).
M2 is responsible for sending, receiving, and forwarding device announcement (DA) messages, learning all the routers in the whole network, and deducing faulted routers. When a router starts, it sends a DA message announcing itself to its neighbors and a DLR message requesting the knowledge of routers and links from its neighbors. If M2 module of a router receives a DA message, it checks whether the router encapsulated in the message is in an ADT. If the router is not in the ADT, M2 puts this router into the ADT and forwards this DA message to all the active ports except for the incoming one, otherwise, M2 discards this message directly. If M2 module of a router receives a DLR message, it replies a DA message that encapsulates all of the learned routers.
M3 is responsible for inferring invisible neighbors of the current router by means of the ADT. If the link between the router A and its neighbor B breaks, which results in that M1 module of A cannot detect the existence of B, then B is an invisible neighbor of A. Since a device's location is coded into its IP address, it can be judged whether two routers are adjacent, according to their IP addresses. Based on this idea, M3 infers all of the invisible neighbors of the current router and the related link failures. The results are stored into a NDT. Moreover, link failures also are added into a link-failure table (LFT). LFT stores all of the failed links in the entire network.
M4 is responsible for sending, receiving and forwarding link failure announcement (LFA) and learning all the link failures in the whole network. M4 broadcasts each newly inferred link failure to all the routers in the network. Each link failure is encapsulated in a LFA message and one link failure is broadcasted only once. If a router receives a DLR request from its neighbor, it will reply a LFA message that encapsulates all the learned link failures through M4 module. If M4 receives a LFA message, it checks whether the link failure encapsulated in the message is in a LFT. If the link failure is not in the LFT, M4 puts this link failure into the LFT and forwards this LFA message to all the active ports except for the incoming one, otherwise, M4 discards this message directly.
M5 is responsible for building a BRT for the current router. By leveraging the regularity in topology, M5 can calculate the routing paths for any destination without the knowledge of the topology of whole network, and then build the BRT based on a NDT. Since the IP addresses of network devices are continuous, M5 only creates one route entry for a group of destination addresses that have the same network prefix by means of route aggregation technology. Usually, the size of a BRT is very small. The detail of how to build a BRT is described in section 5.
M6 is responsible for building a NRT for the current router. Because M5 builds a BRT without considering link failures in network, the routing paths calculated by the BRT cannot avoid failed links. To solve this problem, a NRT is used to exclude the routing paths that include some failed links from the paths calculated by a BRT. M6 calculate the routing paths that include failed links and stored them into the NRT. The details of how to build a NRT is described in section 5.
M7 is responsible for querying routing tables and selecting the next hop for forwarding the packets. Firstly, M7 takes the destination address of a forwarding packet as a criterion to look up route entries in a BRT based on longest prefix match. All of the matched entries are composed of a candidate hops list. Secondly, M7 look up negative route entries in a NRT taking the destination address of the forwarding packet as criteria. This lookup is not limited to the longest prefix match, any entry that matches the criteria would be selected and composed of an avoiding hops list. Thirdly, the candidate hops minus avoiding hops are composed of an applicable hops list. At last, M7 sends the forwarding packet to any one of the applicable hops. If the applicable list is empty, the forwarding packet will be dropped.
10.0.1.1 10.0.1.2 10.0.2.1 10.0.2.2
+--+ +--+ +--+ +--+
| | | | | | | |
+--+,_ +--+', +--+ ,,+--+
|`,`',`-., / | \ `. .'` - .-'``.` /|
| . `', `'., / | ' ' ,-`,' |`. .` ' |
| \ `', `-., .` / | `, .` ,' |
| `, `'. `'-,_ .'` ,' | ', / |
| . `'. `-.,-` / | \
| \ `'., .` `'., ` | `.
| `, .'`, ,`'., | ',
| . ,-` '., - `'-,_ | `.
| \ .` ,'., `|., .
| .'` / `-, | `'., `.
10.1.0.1 ,-` . .' 10.3.0.1 10.3.0.2`'., ',
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
| | | | | | | | | | | | | | | |
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
| \/ | | \/ | | \/ | | \/ |
+--+/\+--+ +--+/\+--+ +--+/\+--+ +--+/\+--+
| | | | | | | | | | | | | | | |
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
/| 10.1.2.1 /| |\ 10 3.1.3 |\ /| | |
/ | | \ / | | \ / | | \ / | | |
/ | | \ / | | \ / | | \ / | | |
/ | | \ / | | \ / | | \ / | | |
++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++
10.1.2.2 10.3.1.3
Figure 7: Fat-tree Network
In this section, we take a Fat-tree network(Fig. 7) as an example to describe how to apply FAR routing. Since M1 to M4 are very simple, we only introduce how the modules M5, M6, and M7 work in a Fat-tree network.
A Fat-tree network is composed of 4 layers. The top layer is core layer, and the other layers are aggregation layer, edge layer and server layer. There are k pods, each one containing two layers of k/2 switches. Each k-port switch in the edge layer is directly connected to k/2 hosts. The remaining k/2 ports are connected to k/2 of the k-port switches in the aggregation layer. There are (k/2)2 k-port core switches. Each core switch has one port connected to each of the k pods.
Aggregation switches are given addresses of the form 10.pod.0.switch, where pod denotes the pod number, and switch denotes the position of that switch in the upper pod (in [1, k/2]). Edge switches are given addresses of the form 10.pod.switch.1, where pod denotes the pod number, and switch denotes the position of that switch in the lower pod (in [1, k/2]). The core switches are given addresses of the form 10.0.j.i, where j and i denote that switch's coordinates in the (k/2)2 core switch grid (each in[1, (k/2)], starting from top-left). The address of a host follows the pod switch to which it is connected to; hosts have addresses of the form: 10.pod.switch.ID, where ID is the host's position in that subnet (in [2, k/2+1], starting from left to the right).
Destination/Mask Next hop 10.0.0.0/255.0.0.0 10.1.0.1 10.0.0.0/255.0.0.0 10.1.0.2
Destination/Mask Next hop 10.1.1.0/255 255.255.0 10.1.1.1 10.1.2.0/255.255.255.0 10.1.2.1 10.0.0.0/255.0.0.0 10.0.1.1 10.0.0.0/255.0.0.0 10.0.1.2
Destination/Mask Next hop 10.1.0.0/255 255.0.0 10.1.0.1 10.2.0.0/255.255.0.0 10.2.0.1 10.3.0.0/255.255.0.0 10.3.0.1 10.4.0.0/255.255.0.0 10.4.0.1
By leveraging the topology's regularity, every switch clearly knows how it forwards a packet. When a packet arrives at an edge switch, if the destination of the packet lies in the same subnet with the switch, then the switch directly forwards the packet to the destination server through layer-2 switching. Otherwise, the switch forwards the packet to any of aggregation switches in the same pod. When a packet arrives at an aggregation switch, if the destination of the packet lies in the same pod, the switch forwards the packet to the corresponding edge switch. Otherwise, the switch forwards the packet to any of core switches that it is connected to. If a core switch receives a packet, it forwards the packet to the corresponding aggregation switch that lies in the destination pod.
The forwarding policy discussed above is easily expressed through a BRT. The BRT of an edge switch, such as 10.1.1.1, is composed of the following entries:
The BRT of an aggregation switch, such as 10.1.0.1, is composed of the following entries:
The BRT of acore switch, such as 10.0.1.1, is composed of the following entries:
The route entries in an NRT are related with link and node failures. We summarize all types of cases into three (3) catalogs.
Destination/Mask Next hop 10.1.2.0/255.255.255.0 10.1.0.1
Destination/Mask Next hop 10.1.2.0/255.255.255.0 10.3.0.1
Destination/Mask Next hop 10.1.0.0/255.255.0.0 10.0.1.2
In Fat-tree, Links can be classified as 3 types by their locations: 1) servers to edge switches; 2) edge to aggregation switches; 3) aggregation to core switches. Link failures between servers to edge switches only affect the communication of the corresponding servers and don't affect the routing tables of any switch, so we only discuss the second and third type of links failures.
Edge to Aggregation Switches
Suppose that the link between an edge switch, such as 10.1.2.1 (A), and an aggregation switch, such as 10.1.0.1(B),fails. This link failure may affect 3 types of communications.
o Sources lie in the same subnet with A, and destinations do not. In this case, the link failure will only affect the routing tables of A. As this link is attached to A directly, A only needs to delete the route entries whose next hop is B in its BRT and add no entries to its NRT when A's M6 module detect the link failure.
o Destinations lie in the same subnet with A, and sources lie in another subnet of the same pod. In this case, the link failure will affect the routing tables of all the edge switches in the same pod except for A. When an edge switch, such as 10.1.1.1, learns the link failure, it will add a route entry to its NRT:
o Destinations lie in the same subnet with A, sources lie in another pod. In this case, the link failure will affect the routing tables of all the edge switches in the other pods. When an edge switch in one other pod, such as 10.3.1.1, learns the link failure, because all the routings that pass through 10.3.0.1 to A will certainly pass through the link between A and B, 10.3.1.1 need add a route entry to its NRT:
Aggregation to Core Switches
Suppose that the link between an aggregation switch, such as 10.1.0.1 (A), and a core switch, such as 10.0.1.2(B), fails. This link failure may affect 2 types of communications.
o Sources lie in the same pod (pod 1) with A, and destinations lie in the other pods. In this case, the link failure will only affect the routing tables of A. As this link is attached to A directly, A only need to delete the route entries whose next hop is B in its BRT and add no entries to its NRT when A¡¯s M6 module detect the link failure.
o Destinations lie in the same pod (pod 1) with A, and sources lie in another pod. In this case, the link failure will affect the routing tables of all the aggregation switches in other pods except for pod 1. When an aggregation switch in one other pod, such as 10.3.0.1, learns the link failure, because all the routings that pass through 10.0.1.2 to the pod 1 where A lies will certainly pass through the link between A and B, 10.3.0.1 need add a route entry to its NRT:
Destination/Mask Next hop 10.1.0.0/255.255.0.0 10.3.0.1
If all the uplinks of an aggregation switch fail, then this switch cannot forward packets, which will affect the routing of every edge switches. Suppose that all the uplinks of the node A (10.1.0.1) fail, it will affect two types of communications.
o Sources lie in the same pod (pod 1) with A, and destinations lie in the other pods. In this case, the link failures will affect the routing of the edge switches in the Pod of A. To avoid the node A, each edge switch should remove the route entry ¡°10.0.0.0/255.0.0.0£¬10.1.0.1¡± in which the next hop is the node A.
o Destinations lie in the same pod (pod 1) with A, and sources lie in other pods. In this case, the link failures will affect the routing of edge switches in other pods. For example, if the edge switch 10.3.1.1 communicates with some node in the pod of A, it should avoid the node 10.3.0.1, because any communication through 10.3.0.1 to the pod of A will pass through the node A. So a route entry should be added to 10.3.1.1:
At last, we discuss the effect of node failures to a NRT. There are 3 types of node failures: the failure of edge, aggregation and core switches.
o An edge switch fails. The failure doesn't affect the routing table of any switch.
o A core switch fails. Only when all the core switches connected to the same aggregation switch fail, they will affect the routing of other switches. This case is equal to the case that all the uplinks of an aggregation switch fail, so the process of link failures can cover it.
o An aggregation switch fails. This case is similar to the case that all the uplinks of an aggregation switch fail. It affects the routing of edge switches in other pods, but doesn't affect the routing of edge switches in pod of the failed switch. The process of this failure is same to the second case in section 6.2.2.
Destination/Mask Next hop 10.0.0.0/255.0.0.0 10.3.0.1
Destination/Mask Next hop 10.1.0.0/255.255.0.0 10.0.1.1 10.1.0.0/255.255.0.0 10.0.1.2
Destination/Mask Next hop 10.1.0.0/255.255.0.0 10.0.1.2
FAR decides a routing by looking up its BRT and NRT. We illuminate the routing procedure by an example. In this example, we suppose that the link between 10.3.1.1 and 10.3.0.2 and the link between 10.1.2.1 and 10.1.0.2 have failed. Then we look into the routing procedure of a communication from 10.3.1.3 (source) to 10.1.2.2 (destination).
Step 1: The source 10.3.1.3 sends packets to its default router 10.3.1.1
Step 2: The routing of 10.3.1.1.
1) Calculate candidate hops
10.3.1.1 looks up its BRT and gets the following matched entries£º
So the candidate hops = {10.3.0.1}
2) Calculate avoiding hops
Its NRT is empty, so the set of avoiding hop is empty too.
3) Calculate applicable hops
The applicable hops are candidate hops minus avoiding hops, so:
The applicable hops = {10.3.0.1}
4) Forward packets to 10.3.0.1
Step 3: The routing of 10.3.0.1
1) Calculate candidate hops.
10.3. 0.1 looks up its BRT and gets the following matched entries£º
So the candidate hops = {10.0.1.1, 10.0.1.2}
2) Calculate avoiding hops
So the avoiding hops = {10.0.1.2}
3) Calculate applicable hops
The applicable hops are candidate hops minus avoiding hops, so:
The applicable hops = {10.0.1.1}
4) Forward packets to 10.0.1.1
Step 4: 10.0.1.1 forwards packets to 10.1.0.1 by looking up its routing tables.
Step 5: 10.1.0.1 forwards packets to 10.1.2.1 by looking up its routing tables.
Step 6:10.1.2.1 forwards packets to the destination 10.1.2.2 by layer-2 switching.
This draft introduces FAR protocol, a generic routing method and protocol, for data centers that have a regular topology. It uses two routing tables, a BRT and a NRT, to store the normal routing paths and avoiding routing paths, respectively, which makes FAR very simple and efficient. The sizes of two tables are very small. Usually, a BRT has only several tens of entries and a NRT has only several or about a dozen entries.
[FAT-TREE] M. Al-Fares, A. Loukissas, and A. Vahdat."A Scalable,Commodity, Data Center Network Architecture",In ACM SIGCOMM 2008.
This document is supported by ZTE Enterprise-University-Research Joint Project.