huawei ldp over rsvp

Technical White Paper for LDP over RSVP

Huawei Technologies Co., Ltd.


Copyright ©2007 Huawei Technologies Co., Ltd. All Rights Reserved ihttp://datacomm.huawei.com

Table of Contents

1 Preface........................................................................................................................1

2 Technology Introduction ..............................................................................................2

2.1 LDP Remote Session .............................................................................................2

2.2 LDP LSP .................................................................................................................2

2.3 RSVP TE ................................................................................................................2

2.4 IP over TE...............................................................................................................2

2.5 Overview.................................................................................................................3

2.6 Process of Establishing LSP ..................................................................................3

2.7 Data Forwarding Process .......................................................................................4

3 Typical Application ......................................................................................................5

4 Conclusion...................................................................................................................5

Appendix A Reference.......................................................................................................6

Appendix B Acronyms and Abbreviations..........................................................................6


Copyright ©2007 Huawei Technologies Co., Ltd. All Rights Reserved 1http://datacomm.huawei.com


Abstract: Due to the limit on the extension of resource reservation protocol (RSVP), in many

cases, RSVP can only be deployed in partial core network. Thus the LDP-based MPLS

VPN external tunnels cannot traverse the RSVP area of the core network. The

technology of LDP over RSVP well addresses the deployment of the VPN in core

network when the TE is employed.

Keywords: MPLS, LDP, RSVP, TE, Tunnel, LSP, and IP over TE

1 Preface

Traffic engineering (TE) is one important application in MPLS. The TE technology focuses

on optimizing the performance of the entire network and aims at bringing out efficient and

reliable network service, optimizing the utilization of network resource and network traffic.

The TE technology has two focuses: traffic and resource. From the prospect of traffic, it

aims at improving the service quality of the network. From the prospect of resource, it

emphasizes how to make the best of the resource, especially the bandwidth.

The MPLS is in an overlaying mode. It is easy to build a virtual topology over the physical

network topology and then mirror the traffic onto this virtual topology. Therefore, the

MPLS TE, which integrates MPLS and TE technologies, comes into being. The MPLS TE

technology is, building one or multiple, and even full-connected LSPs in the network to

schedule the network traffic thus achieving the balance of traffic in the network. In this

way, the network traffic is balanced by transferring traffic from heavily loaded links to the

links with less traffic. The MPLS TE has a sharp edge in addressing the network

congestion.

Currently, it seems very difficult to deploy the TE throughout the entire carrier network.

Therefore, the carrier can plan a core RSVP area and deploy TE in this area while

enabling LDP out of the area.

LDP over RSVP can be considered that the RSVP Tunnel, as a whole, is one "hop" of the

LDP LSP. At present, LDP is widely used in MPLS VPN. You can configure the attributes

of LDP over RSVP to prevent congestion of VPN traffic on certain nodes. That is to say,

have the LDP LSP traverse the RSVP TE area and then provide the VPN services.



2 Technology Overview

2.1 LDP Remote Session

The LDP session can be divided into Local Session and Remote Session. The Local

Session is used to handle the LSP message through two directly connected routers. The

Remote Session is, detected by sending target hello through LDP, the session that is

established between non-directly connected neighbors. Generally, when LSP traverses

one area, a piece of LSP message will be sent by Remote Session. For example, the

Martini in L2VPN is the VC LSP of L2VPN established by LDP Remote Session.

2.2 LDP LSP

LDP is the protocol based on which the LSP is built. At present, the LDP is widely used for

providing VPN service. The networking and configuration for LDP is easy. LDP can

provide routing topology to establish LSP and is capable of supporting large amount of

LSPs. LDP over RSVP employs LDP LSP which is established by LDP through Remote

Session. Its egress is the egress of tunnel.

2.3 RSVP TE

RSVP is a transmission protocol based on TCP/IP. Through RSVP, the host can apply for

specific QoS from the network, which is to provide assured data flow service to the

specific applications. By RSVP, the resources are reserved on each node where the data

flow passes. The resource remains reserved until it is released by the application.

RSVP-TE is the extension of RSVP. The RSVP-TE packets can carry parameters like

bandwidth, partially explicit route, color, and so on. Establish the LSP that meet these

constraint conditions through the constrained route calculation of TE, complete the link,

back up the nodes and balance the load.

2.4 IP over TE

IP over TE technology is that, after the TE tunnel is established, IGP (OSPF, ISIS, for

example) makes the egress of the route choose the TE tunnel. Then we can deem that,

this router and the destination router of the TE tunnel are connected through the tunnel

interface (logic interface). The packets are actually forwarded by being transparently



transmitted in TE tunnel. IP over TE technology can be realized into methods: one is IGP

Shortcut, realized by local calculation; the other is FA, which is realized by delivering LSA.

The IGP Shortcut is more widely used for its simple configuration and control.

3 Key Technologies

3.1 Overview

Establish the tunnel in RSVP TE area and enable the Remote Session of the LDP and the

IGP Shortcut. Take the tunnel interface of TE as the egress. Then LDP sends Label

Mapping message through Remote Session. The LSP is required and then its egress can

be taken as the tunnel interface. Therefore, the key points of LDP over RSVP technology

are RSVP TE, LDP Remote Session, IGP Shortcut, and the establishment of the LSP

through LDP Remote Session.

3.2 Process of Establishing LSP

As shown in the above figure, P1, P2, P3 are RSVP TE area enabling RSVP TE. PE1,

PE2 are the PE of VPN, enabling MBGP and VPN. LDP is enabled between PE1 and P1

and it is also enabled between P3 and PE2.

The flow is as follows: (take the LSP established from PE1 to PE2 as an example; the

FEC of LSP is FecPE2)

（1）Establish Tunnel0 of RSVP TE from P1 to P3. The label distributed to P2 from P3 is

Lr2, and the label distributed from P2 to P1 is Lr1.



（2）Establish LDP Remote Session between P1 and P3.

（3）Enable IGP Shortcut on P1. Ensure it can be seen that the egress of FecPE2 is

Tunnel0 on P1.

（4）FecPE2 triggers the establishment of LSP on PE2. The Label Mapping message is

sent to P3, and the label is L2.

（5）After P3 receives the Label Mapping message, it forwards that message to P1

through LDP Remote Session. The label is Lx.

（6）P1 receives the Label Mapping message, and finds out that the egress of the route is

Tunnel0. Then the LSP from PE1 to PE2 is transmitted in RSVP TE. The external

label is Lr1.

（7）P1 continues to send Label Mapping message to PE1, the label is L1.

（8）PE1 generates Ingress LSP（FecPE2）.

（9）MBGP sends private network route of CE2 from PE2 to PE1, the label of private

network is Lb.

Then the establishment of LSP between PE1 and PE2 is complete. This LSP traverses

the RSVP TE area（P1～P3）.

3.3 Data Forwarding Process

The forwarding process of packets is as follows:

1） After PE1 receives packets from CE1, it tags BGP label Lb of private network and

then it tags LDP label L1 of public network;

2）（Lb, L1）label of PE1 is received on P1, replace L1 with Lx（the label sent to P1

through LDP Remote session） and then tag tunnel label Lr1 of RSVP TE, the label of

the packet becomes (Lb, Lx, Lr1);

3） From P2 to P3, with the RSVP TE transparently transmitting packets, the Lr1 is

replaced by Lr2, that is, the packets received by P3 are tagged with the following

labels (Lb, Lx, Lr2);

4） Upon arriving P3, the Lr2 is first stripped and then comes out Lx, and the label of LDP

which is replaced by L2. The packet is then sent to PE2 and the label becomes (Lb,

L2);

5） After the packet reaches PE2, L2 is first stripped and then the Lb. After that, the

packet is sent to CE2.



The forwarding process of data packet from CE2 to CE1 is similar.

4 Typical Application

As shown in the above figure, it is MPLS VPN networking, the signaling protocol is LDP. It

provides ordinary VPN service. When the LDP over RSVP technology is applied, the LSP

of LDP can traverse RSVP and provide common VPN service.

5 Conclusion

As in reality the TE cannot be widely deployed in the network, the VPN traffic maybe

congested on certain node. In this case, TE technology can be used to schedule traffic

and then implement the LDP over RSVP technology to address the current problem of

deploying VPN when TE is deployed in the core network.



Appendix A Reference

（1）RFC 3031，Multi - protocol Label Switching Architecture

（2）RFC 3036，LDP Specification

（3）RFC 3209，RSVP-TE: Extensions to RSVP for LSP Tunnels

（4）RFC 3906，Calculating Interior Gateway Protocol (IGP) Routes Over Traffic Engineering

Tunnels

Appendix B Acronyms and Abbreviations

Abbreviation/Acronyms Full Spelling

MPLS Multi – Protocol Label Switching

LDP Label Distribution Protocol

RSVP Resource Reservation Protocol

TE Traffic Engineering

LSP Label Switched Path

FEC Forwarding Equivalence Class

VPN Virtual Private Network

huawei ldp over rsvp

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