Routing Area Working Group Bin Liu
Internet-Draft ZTE Inc.
Intended status: Informational Yantao Sun
Expires: May 3, 2016 Jing Cheng
Yichen Zhang
Beijing Jiaotong University
Bhumip Khasnabish
ZTE TX Inc.
October 31, 2015
Generic Fault-avoidance Routing Protocol for Data Center Networks
draft-sl-rtgwg-far-dcn-04
Abstract
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.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 3, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Acronyms & Definitions . . . . . . . . . . . . . . . . . 4
2. Conventions used in this document . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
3.1. The Impact of Large-scale Networks on Route Calculation . 5
3.2. Dilemma of conventional routing methods in a large-scale
network with giant number nodes of routers . . . . . . . 6
3.3. Network Addressing Issues . . . . . . . . . . . . . . . . 8
3.4. Big Routing Table Issues . . . . . . . . . . . . . . . . 8
3.5. Adaptivity Issues for Routing Algorithms . . . . . . . . 8
3.6. Virtual Machine Migration Issues . . . . . . . . . . . . 9
4. The FAR Framework . . . . . . . . . . . . . . . . . . . . . . 9
5. Data Format . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Data Tables . . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Messages . . . . . . . . . . . . . . . . . . . . . . . . 13
6. FAR Modules . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Neighbor and Link Detection Module(M1) . . . . . . . . . 17
6.2. Device Learning Module(M2) . . . . . . . . . . . . . . . 17
6.3. Invisible Neighbor and Link Failure Inferring Module(M3) 18
6.4. Link Failure Learning Module(M4) . . . . . . . . . . . . 18
6.5. BRT Building Module(M5) . . . . . . . . . . . . . . . . . 18
6.6. NRT Building Module(M6) . . . . . . . . . . . . . . . . . 19
6.7. Routing Table Lookup(M7) . . . . . . . . . . . . . . . . 19
7. How a FAR Router Works . . . . . . . . . . . . . . . . . . . 19
8. Compatible Architecture . . . . . . . . . . . . . . . . . . . 22
9. Application Example . . . . . . . . . . . . . . . . . . . . . 22
9.1. BRT Building Procedure . . . . . . . . . . . . . . . . . 24
9.2. NRT Building Procedure . . . . . . . . . . . . . . . . . 25
9.2.1. Single Link Failure . . . . . . . . . . . . . . . . . 25
9.2.2. A Group of Link Failures . . . . . . . . . . . . . . 26
9.2.3. Node Failures . . . . . . . . . . . . . . . . . . . . 27
9.3. Routing Procedure . . . . . . . . . . . . . . . . . . . . 27
9.4. FAR's Performance in Large-scale Networks . . . . . . . . 29
9.4.1. The number of control messages required by FAR . . . 29
9.4.2. The Calculating Time of Routing Tables . . . . . . . 29
9.4.3. The Size of Routing Tables . . . . . . . . . . . . . 30
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10. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 30
11. Reference . . . . . . . . . . . . . . . . . . . . . . . . . . 30
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
13. Security Considerations . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction
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 to 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.
All the architectures have a common feature that is with a regular
topology. Here a regular topology is not a mathematicalordefinite
conception, which means a non- arbitrary network fabric, an inerratic
network fabric. In a regular topology, each network node such as a
switch or router can be addressed by its location and through its
address the node's connectionsin a network can be determined, and
furthermore, the route to the node from other nodes in the network
can be determined.
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 and can be applied to any
DCN with a regular topology.
FAR is not a routing protocol to replace gerneric routing protocols
such as OSPF and IS-IS. It cannot e used in general local networks
whose topological structures are arbitrary, and whose scalesare also
notvery large. OSPF works very well in such a network. But in a
large-scale network with regular topology, FAR has a better
performance. Compared with OSPF and IS-IS，FAR has shorter
time of network convergence and lower PDU overhead. Furthermore, FAR
requires less computing and storage resources, which let FAR routers
has lower cost of production than generic routers.
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In addation, 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. FAR has
better adaptability than these specified routing methods.
1.1. Acronyms & Definitions
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
2. Conventions used in this document
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.
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3. Problem Statement
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.
3.1. The Impact of Large-scale Networks on Route Calculation
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 time 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.
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3.2. Dilemma of conventional routing methods in a large-scale network
with giant number nodes of routers
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.
Infiniband defines a switch-based network to interconnect processing
nodes and the I/O nodes. The network is composed of HCAs, Switches
and SMs. SMs are responsible for the discovery, configuration,
activation, and management of the entire subnets. SMs can be on the
nodes of any subnets (such as Switches, Routers or HCAs). SMs
exchange control management packets through subnet management
interfaces SMIs and subnet management agents SMAs. These control
packets are called subnet management packets SMPs. SMPs use
unreliable datagram service to send. SMPs are divided into the lid
routing and directional routing, and SMPs use directional routing for
network topology discovery before the network initialization.
Infiniband can support very large scale networks, use the regularity
in topology to simplify its routing algorithm, which is just the same
to what we do in FAR.
Why conventional routing methods do not work well in a large-scale
network with giant number nodes of routers?
As everyone knows, the conventional routing 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 the conventional routing protocol
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 the conventional routing protocol, LSA floods every 1800
seconds. Especially in the larger network, the occupation of CPU and
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band bandwidth will soon reach the router's performance bottleneck.
In order to reduce these adverse effects, the conventional routing
protocol introduced the concept of Area, which still has not solved
the problem thoroughly). By dividing the 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 the
conventional routing protocol 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
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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.
3.3. Network Addressing Issues
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.
3.4. Big Routing Table Issues
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.
3.5. Adaptivity Issues for Routing Algorithms
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.
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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.
3.6. Virtual Machine Migration Issues
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.
4. 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.
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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
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.
5. Data Format
5.1. Data Tables
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.
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Negative Routing Table(NRT): To store the avoiding routes.
The format of NDT
----------------------------------------------------------
Device ID | Device IP | Port ID | Link State | Update Time
----------------------------------------------------------
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 | Device IP | Type | State | Update Time
--------------------------------------------------
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 | Router 1 IP | Router 2 IP | Timestamp
--------------------------------------------
No: The entry number.
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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.
Timestamp: It identifies when the entry is created.
The format of BRT
-------------------------------------------------------
Destination | Mask | Next Hop | Interface | Update Time
-------------------------------------------------------
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| Mask| Next Hop| Interface| Failed Link No| Timestamp
-------------------------------------------------------------------
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.
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5.2. 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).
|<--- 1 --->| <--- 1 --->|<--------- 2 ---------->|
+-----------+------------+------------------------+
| Version |Message Type| Message Length |
+-----------+------------+------------------------+
| Checksum | AuType |
+------------------------+------------------------+
| Authentication |
+-------------------------------------------------+
| Authentication |
+-------------------------------------------------+
| Timestamp |
+-------------------------------------------------+
Figure 2: The format of FAR header
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.
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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.
|<--- 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
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.
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Neighbor Router IP: The IP address of a neighbor router. All the
neighbor router's addresses should be included in a Hello message.
|<----1---->| <----1---->|<----------2----------->|
+-----------+------------+------------------------+
| Version |Message Type| Message Length |
+-----------+------------+------------------------+
| Checksum | AuType |
+------------------------+------------------------+
| Authentication |
+-------------------------------------------------+
| Authentication |
+-------------------------------------------------+
| Timestamp |
+------------------------+------------------------+
| Router1 IP |
+-------------------------------------------------+
| ... |
+-------------------------------------------------+
Figure 4: The Format of DA Messages
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.
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|<----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
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
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|<----1---->| <----1---->|<----------2----------->|
+-----------+------------+------------------------+
| Version |Message Type| Message Length |
+-----------+------------+------------------------+
| Checksum | AuType |
+------------------------+------------------------+
| Authentication |
+-------------------------------------------------+
| Authentication |
+-------------------------------------------------+
| Timestamp |
+-------------------------------------------------+
Figure 6: The Format of DLR Messages
For DLR messages(Fig. 6), the Message Type in FAR header is set to
1.Except for FAR header, DLR has no additional fields.
6. FAR Modules
6.1. Neighbor and Link Detection Module(M1)
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).
6.2. Device Learning Module(M2)
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.
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6.3. Invisible Neighbor and Link Failure Inferring Module(M3)
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.
6.4. Link Failure Learning Module(M4)
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 by comparing two link ends
and timestamp. If the link failure is not in the LFT or timestamp is
different, M4 puts this link failure into the LFT (or update
timestamp only) and forwards this LFA message to all the active ports
except for the incoming one, otherwise, M4 discards this message
directly.
There is a special case a router will rebroadcast a link failure. If
a router receives a data packet and must forward the packetgoing
ahead to destinationthrough a failed link, it means some previous
router should avoid this failed link according to its NRT but it
doesn't. In this case, maybe the previous router missed the LFA
message of the link failuredue to some uncertain reasons. So the
forwarding routerrebroadcasts the LFA message again.
6.5. BRT Building Module(M5)
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,
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the size of a BRT is very small. The detail of how to build a BRT is
described in section 5.
6.6. NRT Building Module(M6)
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.
6.7. Routing Table Lookup(M7)
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.
7. How a FAR Router Works
Figure 7 shows how a FAR router works by its FSM.
/---------------\
| Start |
| |
\---------------/
|
+--------+ |1
| | |
| \|/ \|/
| +--------------------+
|2 | ND |
| | |
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| +--------------------+
| | |
| | |3
+--------+ |
\|/
+--------------+
| ND-FIN |
| |
+--------------+
|
|4
|
\|/
________10_______\+--------------+/_______11________
| /| |\ |
| | Listen | |
| ____9_______\| |/_______12___ |
| | /| |\ | |
| | +--------------+ | |
| | 5/ | | \8 | |
| | |/_ | | _\| | |
| | +------------+ 6| |7 +------------+ | |
| --| HELLO-RECV | | | | LFA-RECV |-- |
| +------------+ | | +------------+ |
| ______| |______ |
| | | |
| \|/ \|/ |
| +------------+ +------------+ |
|___________| DLR-RECV | | DA-RECV |__________|
+------------+ +------------+
_________
| |
| |
\|/ |
+--------------+ |13
| Hello Thread | |
+--------------+ |
| |
|_________|
_________
| |
| |
\|/ |
+--------------+ |14
| LFD Thread | |
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+--------------+ |
| |
|_________|
_________
| |
| |
\|/ |
+--------------+ |15
| DA Thread | |
+--------------+ |
| |
|_________|
_________
| |
| |
\|/ |
+--------------+ |16
| DFD Thread | |
+--------------+ |
| |
|_________|
Figure 7: The Finite State Machine of FAR Router
1)When a router starts up, it starts a Hello thread and then starts
ND (neighbor detection) timer (3 seconds). Next the router goes into
ND (neighbor detection) state.
2)In the ND state, if a router received a Hello message, then it
performs a Hello-message processing and goes back to the ND state.
3)When the ND timer is over, a router goes into ND-FIN (neighbor
detection finished) state.
4)A router starts the LFD (link failure detection) thread and DFD
(device failure detection) state, and sends DA message and DLR
message to all of its active ports. Then the router goes into Listen
state.
5) If a router receives a Hello message, then goes into HELLO-RECV
state.
6) If a router receives a DLR message, then goes into DLR-RECV state.
7) If a router receives a DA message, then goes into DA-RECV state.
8) If a router receives a LFA message, then goes into LFA-RECV state.
9) A router performs the Hello-message processing. After that, it
goes back to Listen state.
10) A router performs the DLR-message processing. After that, it
goes back to Listen state.
11) A router performs the DA-message processing. After that, it goes
back to Listen state.
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12) A router performs the LFA-message processing. After that, it
goes back to Listen state.
13) Hello thread produces and sends Hello messages to all its ports
periodically.
14) LFD thread calls link-failure-detection processing to check link
failures in all links periodically
15) DA thread produces and sends DA messages periodically (30
minutes).
16) When DFD thread starts up, it sleep a short time (30 seconds) to
wait for a router learning all the active routers in the network.
Then the thread calls the device-failure-detection processing to
check device failures periodically (30 minutes).
8. Compatible Architecture
As a generic routing protocol, FAR can be run in various DCNs with
regular topology. Up to now, we have implemented the FAR protocol
for 4 types of DCN, including Fat-tree, BCube, MatrixDCN and Diamond.
For different network architectures, most processing of FAR is same
besides calculation of routing tables. BRT routing tables are
calculated based on Hello messages and NRT routing tables are
calculated based on LFA messages in FAR. To extend FAR to support a
new network architecture, only processing of Hello and LFA messages
need providing to build BRT and NRT routing tables.
In this protocol, FAR can support maximally 12 network architectures
and at least support 1 built-in network architecture, such as Fat-
tree, BCube and MatrixDCN,etc. Each network architecture is assigned
a unique number from 1 to 12. For example, if the 1 built-in
architectures are assigned 1, and other customized architectures are
assigned 2 to 12.
1: Fat-tree
2: BCube
3: MatrixDCN.
4: xxx.
......
12: xxx.
9. Application Example
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
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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.
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 8: Fat-tree Network
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
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(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).
9.1. BRT Building Procedure
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:
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
The BRT of an aggregation switch, such as 10.1.0.1, is composed of
the following entries:
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
The BRT of acore switch, such as 10.0.1.1, is composed of the
following entries:
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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
9.2. NRT Building Procedure
The route entries in an NRT are related with link and node failures.
We summarize all types of cases into three (3) catalogs.
9.2.1. Single Link Failure
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:
Destination/Mask Next hop
10.1.2.0/255.255.255.0 10.1.0.1
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
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through the link between A and B, 10.3.1.1 need add a route entry to
its NRT:
Destination/Mask Next hop
10.1.2.0/255.255.255.0 10.3.0.1
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 10.0.1.2
9.2.2. A Group of Link Failures
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
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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:
Destination/Mask Next hop
10.1.0.0/255.255.0.0 10.3.0.1
9.2.3. Node Failures
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.
9.3. Routing Procedure
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:
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Destination/Mask Next hop
10.0.0.0/255.0.0.0 10.3.0.1
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:
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
So the candidate hops = {10.0.1.1, 10.0.1.2}
2) Calculate avoiding hops
Destination/Mask Next hop
10.1.0.0/255.255.0.0 10.0.1.2
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
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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.
9.4. FAR's Performance in Large-scale Networks
FAR has good performanceto support large-scale networks. In this
section, we take a Fat-tree networkcomposed of 2,880 48-port switches
and 27,648 servers as an example to show FAR's performance.
9.4.1. The number of control messages required by FAR
FAR exchanges a few messages between routers and only consumes a
little network bandwidth. Tab. 1 shows the required messages in the
example Fat-tree network.
Table 1:Required messages in a Fat-tree network.
_____________________________________________________________________
Message Type| Scope | size(bytes) | Rate | Bandwidth
---------------------------------------------------------------------
Hello |adjacentswitches|less than 48|10 messages/sec|less than 4 kbps
---------------------------------------------------------------------
DLR |adjacentswitches| less than 48 | (1) |48bytes
---------------------------------------------------------------------
DA |entire network| less than 48 | (2) |1.106M
---------------------------------------------------------------------
LFA |entire network| less than 48 | (3) |48 bytes
_____________________________________________________________________
(1)Produce one when arouter starts
(2)The number of switches(2,880) in a period
(3)Produce one when a link fails or recovers
9.4.2. The Calculating Time of Routing Tables
A BRT is calculated according to the states of its neighbor routers
and attached links. An NRT is calculated according to device and
link failures in the entire network. So FAR does not calculate
network topology and has no problem of network convergence, which
greatly reduces the calculating time of routing tables. The
detection and spread time of link failures is very short in FAR.
Detection time is up to the interval of sending Hello message. In
FAR, the interval is set to 100ms, and a link failure will be
detected in 200ms. The spread time between any pair of routers is
less than 200ms.If a link fails in a data center network, FAR can
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detect it, spread it to all the routers, and calculate routing tables
in no more than 500ms.
9.4.3. The Size of Routing Tables
For the test Fat-tree network, the sizes of BRTs and NRTs are shown
in Tab. 2.
Table 2: The size of routing tables in FAR
_____________________________________________________________________
Routing Table| Core Switch | Aggregation Switch | Edge Switch |
---------------------------------------------------------------------
BRT | 48 | 48 | 24
---------------------------------------------------------------------
NRT | 0 | 14 | 333
_____________________________________________________________________
The BRT's size at a switch is determined by the number of its
neighbor switches. In the example network, a core switch has 48
neighbor switches (aggregation switch), so it has 48 entries in its
BRT.Only aggregation and edge switches have NRTs. The NRT size at a
switch is related to the number of link failures in the network.
Suppose that there are 1000 link failures in the example network, the
number of failed links is 1.2% of total links, which is a very high
failure ratio. We suppose that link failures are uniformly
distributed in the entire network. The NRT size at an edge switch is
about 333 and the NRT size of an aggregation switch is about 14in
average.
10. Conclusion
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.
11. Reference
[FAT-TREE] M. Al-Fares, A. Loukissas, and A. Vahdat."A
Scalable,Commodity, Data Center Network Architecture",In ACM SIGCOMM
2008.
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12. Acknowledgments
This document is supported by ZTE Enterprise-University-Research
Joint Project.
13. Security Considerations
Authors' Addresses
Bin Liu
ZTE Inc.
ZTE Plaza, No.19 East Huayuan Road,Haidian District
Beijing 100191
China
Phone: +86 -010-59932039
Email: 13683610386@139.com
Yantao Sun
Beijing Jiaotong University
No.3 Shang Yuan Cun, Hai Dian District
Beijing 100044
China
Email: ytsun@bjtu.edu.cn
Jing Cheng
Beijing Jiaotong University
No.3 Shang Yuan Cun, Hai Dian District
Beijing 100044
China
Email: yourney.j@gmail.com
Yichen Zhang
Beijing Jiaotong University
No.3 Shang Yuan Cun, Hai Dian District
Beijing 100044
China
Email: snowfall_dan@sina.com
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Bhumip Khasnabish
ZTE TX Inc.
55 Madison Avenue, Suite 160
Morristown, New Jersey 07960
USA
Phone: +001-781-752-8003
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