key technologies of ptn - packet forwarding technology v1.0
TRANSCRIPT
Key Technologies of PTN - Packet Forwarding Technology
V1.0
Contents
Evolution History of PTN Technology MPLS Technology MPLS-TP Technology
Evolution History of PTN Technology
Ethernet
SDH like OAM/PS
MPLS
T-MPLS
PBT
PTN
Ethernet
SDH like OAM/PS
MPLS
T-MPLS
PBT
Ethernet
SDH like OAM/PS
MPLS
T-MPLS
PBT
PTN
During the early stage of development, there were two types of PTN technologies: T-MPLS and PBT.
T-MPLS: The T-MPLS technology is evolved from the MPLS technology, and it complies with the features of transmission.
PBT: The PBT technology is evolved from the Ethernet technology. This connection-oriented packet transmission technology adopts the MAC-in-MAC technology, it closes the MAC address self-learning function for the operator and adds the configurations of network administration management and network control. It was introduced by Nortel, and it was supported by very few manufacturers. Nowadays, this technology seems to have no users any more.
Early Stage
MPLS-TP
Present Situation
Along with the technology development, the T-MPLS technology has evolved to be the MPLS-TP technology with the improvement on OAM.
Nowadays, the MPLS-TP technology is widely used as a key technology of PTN.
MPLS/
Enhanced Ethernet
SDH Like OAM/PS
Contents
Evolution History of PTN Technology MPLS Technology MPLS-TP Technology
Background of MPLS Technology Features of Traditional IP Technology:
The IP communication is conducted in the hop-by-hop manner; The rule of "longest matching" is applied in packet forwarding; The network devices need to know the routes of the entire network, otherwise
they cannot forward the packets in the network segment; QoS cannot be guaranteed: Since IP protocol is a non-connection protocol, the
concept of QoS is not applicable to the Internet. The throughput and transmission delay cannot be guaranteed, the network can only take the best effort to satisfy the users' needs.
New services cannot be implemented in large scale unless the network environment is improved.
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Background of MPLS Technology Features of ATM Forwarding:
Hardware switching is simplified by using VPI/VCI It is connection-oriented, QoS can be guaranteed. Provides traffic control measures. Supports multiple types of services, such as real-time service.
ATM was considered to be suitable for all conditions. Some people even raised the idea to create a pure ATM network where the desktop terminal can be reached from the core device.
However, this idea was proved to be incorrect. The reasons are as follows:1. It is too complicated to create the pure ATM network, and the price of the application is too high.
2. The R&D of services fell behind the development of the network.
3. Although ATM switch has been widely used as the core node in the network, the "ATM cell to desktop" service has developed slowly.
虚通路连接 (VCC
)
虚通道连接(VPC
)
VP交换
VC交换
VC交换
NNI
NNI
VPI = 2VCI = 44
VPI = 1VCI = 1
VPI = 26VCI = 44
VPI = 20VCI = 30
UNI UN
I
Virtual circuit connection (VCC)
Virtual path connection (VPC)
VC Switching
VC Switching
VC Switching
Background of MPLS Technology
Due to the above mentioned problems, it was inevitable to combine the IP technology and ATM technology to get further development.
MPLS technology was just created by combining the advantages of the switching technology at the core of the network and the advantages of the IP routing technology at the edge of the network.
Brief Introduction to MPLS Technology
Full name: Multi-Protocol Label Switching
Functions: MPLS combines high-speed switching of IP and ATM
technology. It realizes fast forwarding of IP packets via label switching.
Features: Multi-protocol: MPLS technology can support all
network layer protocols (e.g. IPV6 and IPX) and link layer protocols (e.g. ATM, FR and PPP etc.)
Label Switching: MPLS technology attaches label with fixed length to the packet and replace IP forwarding process with this label.
Advantages of MPLS Technology
MPLS provides connection-oriented services for IP network
MPLS provide high quality Internet service MPLS supports high-bandwidth and high rate IP
forwarding MPLS guarantees QoS and security while
providing IP services MPLS has traffic engineering capability MPLS supports VPN function
Introduction to MPLS Solution MPLS is the short form of Multi Protocol Label Switching.
MPLS is a kind of technology between layer 2 and layer 3, i.e. it is a 2.5 layer technology
It is a standard routing and switching solution that combines the label switching technology and the layer-3 routing technology.
Multi-Protocol means the technology can work together with multiple network protocols.
Layer-3 routing is implemented at the edge of MPLS network, and layer-2 switching is implemented inside the MPLS network.
Key Terms of MPLS Label
The label is an integer identifier with fixed-length. It usually is encapsulated between the layer-2 encapsulated header and layer-3 packet of the data link layer. The label is mapped to the FEC via the binding process.
FEC Forwarding Equivalence Class is a group of packets that are processed in equivalence
manner during the forwarding process. FEC can be identified via the address, tunnel, COS etc. Usually assigned with the same labels on one device.
LSP Label Switching Path: A FEC data stream is attached with specific labels on different nodes
and packets are forwarded according to these labels. The path via which data stream flows is LSP.
LSR Label Switching Router is the core router of the MPLS network. It is responsible for creating
the LSP and initiating the change of next hop. It provides the functions of label switching and label distributing.
LER Label Switching Edge Router is mainly responsible for FEC dividing, traffic engineering, LSP
initiating, IP packet forwarding, Diff-Serv etc. At the edge of the MPLS network, LER divides the traffics into different FECs and request for labels for the FECs. It provides functions of traffic classification, label mapping and removal of label.
LDP Label Distribution Protocol is implemented within the MPLS domain to assign the labels for
the devices.
MPLSDomain
LERa
LSRy
LERb
LERc
LERd
LERe
LERf
LSRx LSRz
LSP
Ingress
EgressLDP Protocol
LDP
LDP
MPLS Network Model
Operational Principles of MPLS
The traditional IP forwarding is adopted for the switching out of the MPLS domain. The switching in the MPLS domain is conducted according to the labels, and there is no need to search for the IP.
(LSR)(LER)
MPLS域
(LER)(LSR)I P
I P标签I P标签
I P标签
I P
标签转发
传统I P转发 传统I P转发Traditional IP forwarding
Traditional IP forwarding
Label
Label
Label
MPLS domain
Label forwarding
Operational Principles of MPLS
The label distribution protocols (e.g. LDP, RSVP etc.) are implemented to assign the corresponding labels for the devices within the MPLS domain.
The propagation process of IP packets through the MPLS domain is as follows:1. The LER at the entrance receives the packets and assigns the
corresponding labels for the packets.
2. The backbone LSR receives the labeled packet, searches the label forwarding table and uses a new outgoing label to replace the label in the incoming packet.
3. The LER at the exit receives the labeled packet, deletes the label and performs the traditional layer-3 search for the IP packet.
MPLS Label
MPLS Label is a 20-bit integer between 0 and 1048575. It is used to identify the FEC.
The label is encapsulated between the layer-2 header and the layer-3 data of the packet, and it only has specific meaning for the local domain.
Label Stack
The label stack consists of two or more MPLS labels. Theoretically, there is no limitation on the label nesting, and it can support all kinds of services.
Network layer header closely follows the label whose bottom of stack is set to 1 bit;
Packet forwarding is based on the top label of stack. When receiving one packet, LSR will check top label to decide the next hop.
Implementation of MPLS
In order to implement the MPLS label switching, we should create the label switching path (LSP) in advance. Actually, the LSP is created by assigning the labels for the nodes on the path.
The next section describes how to create the LSP.
Creation of LSP Method for Creating LSP
The following are the 3 methods commonly used to create the LSP (to assign the labels):
Data stream driver: the creation of LSP is triggered by the received packets.
Topology driver: the creation of LSP is triggered by the topology information (the routing information).
Application driver: the creation of LSP is triggered by the application (e.g. QoS).
Compared with the data stream driver and the application driver, the topology driver has the following advantages in the label value:
The label value setting and label distribution are applied to the control information, it will not cause huge network overhead.
The label value setting and label distribution are implemented before the arrival of the data, it will not cause delay.
So the topology driver method is usually used for assigning the labels.
Creation Process of LSP
There are three steps for establishing LSP in MPLS network: After booting up via network, routing protocols (BGP,
OSPF and IS-IS etc.) are enabled on nodes to set up routing tables.
Under the control of LDP, LIB is set up on each node according to routing table.
Map and link incoming labels and outgoing labels on ingress LSR, medium LSR and egress LSR to form one LSR.
Step 1: Creation of Routing Table
Routing table is set up on each router under the function of dynamic routing protocol.
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Dest Out
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RCRB
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Step 2: Creation of LIB
Intf In
Label In
Dest Intf Out
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Label In
Dest Intf Out
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3Intf In
Dest Intf Out
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Mapping: 40
Mapping: 50
Network: 47.1
Network:47.1
RA
RBRC
Step 3: Creation of LSP
Intf In
Label In
Dest Intf Out
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Intf In
Label In
Dest Intf Out
Label Out
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3Intf In
Dest Intf Out
Label Out
3 47.1 1 50
IP 47.1.1.1
IP 47.1.1.1
IP 47.1.1.1IP 47.1.1.150IP 47.1.1.1IP 47.1.1.140
RA
RB
RC
Label Stack PHP Mechanism
The last hop is assigned with a special label 3.
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IP 47.1.1.1
RCRB
RA
Label Distribution Protocol (LDP)
LDP Overview: The Label Distribution Protocol is a protocol for creating the label
dynamically. It is based on the UDP (User Datagram Protocol ) /TCP ( Transmission Control Protocol). The protocol information is transmitted in the hop-by-hop manner according to the routing table.
LDP can inform the label switching routers of the Forward Error Correction (FEC) and the mapping relationship of the labels, and at last the label switching path is generated. Through LDP, the FEC is associated with the LSP, and the network prefix traffics are mapped to the LSP.
According to the regulation of RFC3036, LDP includes the mechanism of neighbor discovering, label requesting, label deleting, label mapping and error correction.
Label Distribution Protocol (LDP)
Operational Principle of LDP LSR sets up and maintains the LIB according to the
binding information between the label and the FEC. The two LSRs that exchange the FEC/label binding by
using the LDP are called as the "LDP Peer". LDP is used by the LSR to bind the FEC and the
labels and to inform the neighbor LSR of this binding. In this way, the LSRs will get the consensus about the binding relationships of the received labels.
Label Distribution Protocol (LDP)
According to the time sequence, the LDP processing includes the following 4 stages: Discovery stage: Discover LDP peers automatically by
sending Hello messages to neighboring LSRs periodically;
Session Establishment and Maintenance stage: Implement TCP connection and session between LSRs, and conduct initialization (negotiation of parameters);
LSP Establishment and Maintenance state: Assign label for FEC to be transmitted between LSRs and establish LSP;
Cancel of Session: When session hold-time is out, terminate the session.
Neighbor discovery: It is implemented by sending Hello messages mutually (UDP/prot:646/IP:224.0.0.2).
Establish TCP connection: The end with larger address initiates connection actively.(TCP/port:646)
Session initiation: The Master router sends initialization message which carries negotiation parameter.
The Slave router checks if the parameter can be accepted. If it can, slave router will send initialization message and carry negotiation parameter.
The Master router checks if the parameter can be accepted. If yes, the Master router will send keepalive message.
After receiving Keepalive message mutually, session is established.
Once received any error message during this period, session will be closed and TCP connection will be disconnected.
M
M
M
M
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R1 R2
Establishment and Maintenance of LDP Session
Contents
Evolution History of PTN Technology MPLS technology MPLS-TP Technology
Brief Introduction to MPLS-TP Technology MPLS-TP : MPLS Transport Profile is a kind of connection-oriented packet transport technology.
It extends downwards from core network. In April 2008, IETF and ITU-T set up the associated work group named JWT to develop the
MPLS-TP technology. IETF was responsible to develop the T-MPLS/MPLS-TP standards and ITU-T was responsible to raise the requirements for transmission.
MPLS-TP is based on MPLS technology. Its relevant standards provide complete carrier-level scheme for deploying packet switching transport network. MPLS-TP is based on the IP core network. It simplifies the MPLS/PW technology, removes unnecessary IP functions, and meets the requirement of packet transport. To maintain integrity of point-to-point OAM, MPLS-TP adopts the concept of layered network, OAM and linear protection. It is independent from the client signals and the management network signals. It meets the requirements of the transmission network.
MPLS-TP takes full advantage of technical advantages of connection-oriented MPLS technology in QoS, bandwidth sharing, differentiated service and other aspects. Based on the hierarchical network structure of the IP transmission network, MPLS-TP provides the following functions:
Support multiple services Connection oriented Robust scalability Carrier-level QoS, bandwidth multiplexing Efficient bandwidth management and traffic engineering Strong OAM and network management functions Provides 50ms protection switchover and recovery Supports dynamic control plane Low CAPEX+OPEX
Routing and signaling protocollast hop pop-upLabel mergingLayer-3 functions
End-to-end OAMCarrier-level protection switchoverClock synchronization
Frame structureLabel switchingDifferentiated QoS
MPLS-TPIP/MPLS
The following aspects of MPLS are adopted:Frame structureLabel switching principleLabel switching pathDifferentiating service (Diff-Serv)Label space and identifier assignmentTTL processing
Simplified and discarded parts:Simplifies the complicated protocol family of MPLSSimplifies the control planeDoesn't support PHPSimplifies the data forwarding planeDoesn't support label merging
New contents added:End-to-end OAM, which is similar to SDHEnhanced protection switchover, which is similar to SDHLinear subnet protection and ring-network protection, supports the APS protocol, and adopts the concept of layered networkClock synchronizationIncreases the depth of the label stackSupports bi-directional LSP
SDH
TDM kernelGFP encapsulationVirtual cascadeLCAS
MPLS-TP is a subset of MPLS.
MPLS-TP = MPLS/IP + OAM + ProtectionMPLS-TP = MPLS/IP + OAM + Protection
Evolution of MPLS-TP Technology
MPLS-TP Functions V.S. MPLS Functions Function IP/MPLS MPLS-TP
IP routing and control signaling
Supports LDP, RSVP, CR-LDP, RSVPTE etc. Supports simplified control plane GMPLS
PHP function Available Not available
Label merging Available Not available
Frame structure Available Available
Label switchingAvailable, adopts uni-directional LSP, supports LSP convergence
Available, adopts bi-directional LSP, provides bi-directional connection, but it doesn't support the LSP convergence
Qos distinguishing service Available Available
End-to-end OAM
Supports MPLS OAM, but the function is weak. Only supports simple connectivity checking and APS switchover.
Fully integrates the OAM functions of T-MPLS and improves the RT and DT-DPL functions.
1588 time synchronization
The router adopts the NTP protocol, the synchronization can be accurate at the ms level
Supports the G.8261 and 1588v3 time synchronization protocols.
Carrier-level protectionLimited by the MPLS OAM technology, lacks ring-network protection capability
Supports path protection, ring-network protection, LAG protection, FRR protection etc. Provides carrier-level reliability
Interaction with the IP/MPLS core network Available
Based on the advantages of T-MPLS, and the interaction with the IP/MPLS network is carefully considered in the design. Supports the interaction with the IP/MPLS core network of the operator in a better way.
Physical Layer
Data Plane Data Plane
Control Plane
Management Plane Management Plane
Control Plane
Physical Layer
Structure of MPLS-TP System
MPLS-TP system contains 3 planes: the data plane, the management plane and the control plane.
Structure of MPLS-TP System Data Plane
The data plane is responsible to transmit the information from one point to another point in uni-directional or bi-directional manner. It can detect the connection status and provide the result to the control plane, and it also undertakes the transmission of control information and network management information.
The major function of the MPLS-TP data plane is to put the different service signals into the MPLS-TP channel and implement the packets forwarding according to the MPLS-TP labels.
The main method is to conduct indirect mapping via the MPLS-TP channel or encapsulate the signals into the MPLS-TP transmission tunnels and transmit the data in the packet network.
Meanwhile, the data plane also conducts the OAM and protection operations. Management Plane
The management plane is responsible to manage the data plane, the control plane and the whole system. It also coordinates the operations between these planes.
The functions of the management plane include: performance management, fault management, configuration management, charging management and security management. The MPLS-TP management plane provides the functions of end-to-end in-domain or inter-domain fault management, configuration management, performance management, user management and security management.
Control Plane The control plane is made up of a group of control modules that provide specific routing and
signaling functions, and it is supported by a signaling network. The information about the interaction and communication between the control plane modules can be acquired via the interfaces.
The control plane is mainly responsible to create, release and remove the connection. And it manages the monitoring and maintenance entities.
Relationship between Client Layer and Server Layer in MPLS-TP Network
The following figure shows the relationship between the client layer and the server layer in MPLS-TP network:
In MPLS-TP network, client signals and control network are completely independent. Client signals carried by MPLS-TP can be IP/MPLS or Ethernet. That is MPLS-TP
exists as the server-layer network of Eth/IPMPLS.
MPLS-TP Interfaces The MPLS-TP network provides the User Network Interface (UNI)
and Network Node Interface (NNI). NNI can work as both the intra-domain interface of a single
management domain and the inter-domain interface between management domains.
The interfaces between CE and PE are UNI interfaces, where UNI interface of CE device can be expressed as UNI-C and UNI interface of PE device can be expressed as UNI-N. The internal interfaces between PE and PE are NNI interfaces. The definitions of MPLS-TP network interfaces are shown in the following figure:
Hierarchical Structure of MPLS-TP Network MPLS-TP network has the following layers from top to bottom:
MPLS-TP channel (TMC) layer, MPLS-TP path (TMP) layer , MPLS-TP section (TMS) layer and transmission medium layer.
Hierarchical Structure of MPLS-TP Network
TMC layer: It provides point-to-point transport network services, that is to provide client with point-to-point signal transmission. TMC functions as PW layer of PWE3 (or virtual circuit layer).
TMP layer: It shows characteristics of point-to-point logical connection. It provides a transport network tunnel and encapsulates one or more client services into one larger tunnel so that transport network can implement more economic and valid transmission, switching, OAM, protection and restoration. TMP functions as tunnel layer in MPLS.
TMS layer: It is optional. It indicates virtual connection between neighboring modes. It guarantees the integrity of transmitted data between two nodes at TMP layer, such as SDH, Optical Transport Hierarchy (OTH), Ethernet or wavelength channel.
Transmission medium layer: It refers to the transmission media that supports the TMS layer, such as optic fiber, radio and so on.
PE P PE
Section Tunnel ATM PWE3 Ethernet PWE3
E1 PWE3
MPLS-TP Transmission Principle The connection-oriented feature of the MPLS-TP network is realized by the PW
technology. By using the PW, the service provider can transmit the traditional circuit-based network services, as well as the new services, in the converged network based on packet technology.
Transmission process: The customer edge device CE1 is connected to the provider edge device PE1. PE1 encapsulates the original service data and transmits the packet via the Peat the receiving end, PE2 implements frame verification and sequence re-arrangement and restores the received packets to original service data, and then send the data to the customer edge device CE2.The process is shown in the following figure:
Packet Forwarding Technology
MPLS-TP can be considered as a tunnel technology based on MPLS labels. It uses a group of MPLS labels to identify the end-to-end forwarding path (LSP).
MPLS-TP has two layers. The inner layer is the PW layer that identifies the type of the service; The outer layer is the tunnel layer that identifies the forwarding path of the service.
The tunnel is the end-to-end label forwarding tunnel based on MPLS-TP. The local packets are encapsulated as PW PDU via the PW and transmitted via the tunnel. The
service data are encapsulated/de-capsulated by the PE device, then they are restored to the local format and sent to the destination CE.
Emulation service, such as TDM, ATM
Payload encapsulation
PW multiplexing
PSN tunnel
PSN
Physical layer
Emulation service
PW
PSN tunnel
Emulation service, such as TDM, ATM
Payload encapsulation
PW multiplexing
PSN tunnel
PSN
Physical layer
Packet Encapsulation B-DA: the 6-byte destination MAC of the Ethernet
encapsulation ( Tunnel_L determines the forwarding path, B-DA is the MAC address of the next hop node)
B-SA: the 6-byte source MAC of the Ethernet encapsulation
0x8100: the 2-byte identifier of the Ethernet data frame
B-VID: the 2-byte outer VLAN tag 0x8847: the 2-byte "Pw Over MPLS-TP" identifier Tunnel_L: the 4-byte tunnel Label (TMP) PW_L: the 4-byte pseudo wire label (TMC) CW: the 4-byte PW control words Customer Frame: the user data, payload (the user
VLAN may be included) The user packet should be added with 26 bytes in
total.B-DA
B-SA
0x8100
B-VID
0x8847
PW_L
CW
CustomerFrame
tunnel_L
Packet Forwarding Process
B-DA
B-SA
0x8100
B-VID
0x8847
PW_L
CW
CustomerFrame
tunnel_L
B-DA
B-SA
0x8100
B-VID
0x8847
PW_L
CW
CustomerFrame
tunnel_L
B-DA
B-SA
0x8100
B-VID
0x8847
PW_L
CW
CustomerFrame
tunnel_L
CustomerFrame
B-DA
B-SA
0x8100
B-VID
0x8847
PW_L
CW
CustomerFrame
tunnel_L
CustomerFrame
CE node PE node P node P node CE nodePE node
Addlabel
Peel offlabelReserve
PWlabel
ReservePWlabel
ReservePWlabel
Updatenext hop
MAC
Updatenext hop
MAC
Updatenext hop
MAC
Switchingtunnellabel
Switchingtunnellabel
Switchingtunnellabel
Sample of Packet Forwarding Process
In label Out label Out Port
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Sample of Packet Forwarding Process
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Sample of Packet Forwarding Process
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