Service Function Chaining Using MPLS-SPRING
draft-xu-sfc-using-mpls-spring-00

Abstract

Source Packet Routing in Networking (SPRING) WG specifies a special source routing mechanism. Such source routing mechanism can be leveraged to realize the service path layer functionality of the service function chaining (i.e, steering traffic through a particular service function path) by encoding the service function path or the service function chain information as the explicit path information. This document describes how to leverage the MPLS-based source routing mechanism as developed by the SPRING WG to realize the service path layer functionality of the service function chaining.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Terminology
• 3. Solution Description
• 3.1. Encoding SFP Information by an MPLS Label Stack
• 3.2. Encoding SFC Information by an MPLS Label Stack
• 4. Acknowledgements
• 5. IANA Considerations
• 6. Security Considerations
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Authors' Addresses

1. Introduction

When applying a particular Service Function Chain (SFC) [I-D.ietf-sfc-architecture] to the traffic selected by a service classifier, the traffic need to be steered through an ordered set of Service Functions (SF) in the network. This ordered set of SFs in the network indicates the Service Function Path (SFP) associated with the above SFC. To steer the selected traffic through an ordered list of SFs in the network, the traffic need to be attached by the service classifier with the information about the SFP (i.e., specifying exactly which Service Function Forwarders (SFFs) and which SFs are to be visited by traffic), the SFC, or the partially specified SPF which is in between the former two extremes. Source Packet Routing in Networking (SPRING) WG specifies a special source routing mechanism which can be used to steer traffic through an ordered set of routers (i.e., an explicit path). Such source routing mechanism can be leveraged to realize the service path layer functionality of the SFC (i.e., steering traffic through a particular SFP) by encoding the SFP, the SFC or the partially specified SFP information as the explicit path information contained in packets. The source routing mechanism specified by the SPRING WG can be applied to the MPLS data plane [I-D.gredler-spring-mpls] [I-D.filsfils-spring-segment-routing-mpls] and IPv6 data plane. This document only describes how to leverage the MPLS-based source routing mechanisms to realize the service path layer functionality of the service function chaining. How to leverage the IPv6-based source routing mechanism will be discried in a separate document. Furthermore, how to carry metadata within MPLS packet would be described in a separate document as well.

1.1. Requirements Language

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].

2. Terminology

This memo makes use of the terms defined in [I-D.filsfils-spring-segment-routing] and [I-D.ietf-sfc-architecture].

3. Solution Description

     +----------------------------------------------- ----+
     |                  SPRING Netowrks                   |
     |            +---------+       +---------+           |     
     |            |   SF1   |       |   SF2   |           |      
     |            +----+----+       +----+----+           |  
     |                 |                 |                |
     |       (1)       |      (2)        |      (3)       |
+----+-----+ ---> +----+----+ ----> +----+----+ --->  +---+---+ 
|Classifier+------+  SFF1   +-------+  SFF2   +-------+   D   | 
+----------+      +---------+       +---------+       +---+---+ 
     |                                                    |                       
     +----------------------------------------------------+
      Figure 1: Service Function Chaining in SPRING Networks

As shown in Figure 1, assume SFF1 and SFF2 are two MPLS-SPRING- capable nodes. They are also Service Function Forwarder (SFF) nodes to which two SFs (i.e., SF1 and SF2) are attached respectively. In addition, they have allocated and advertised Segment IDs (SID) for their locally attached SFs. In the MPLS-SPRING context, SIDs are intercepted as MPLS labels. For example, SFF1 allocates and advertises an SID (i.e., SID(SF1)) for SF1 while SFF2 allocates and advertises an SID ( i.e., SID(SF2)) for SF2. These SIDs which are used to indicate SFs are referred to as SF SIDs. To encode the SFP information by an MPLS label stack, those SF SIDs as mentioned above would be interpreted as local MPLS labels. In addition, assume node SIDs for SFF1 and SFF2 are SID(SFF1) and SID(SFF2) respectively. To simplify the illustration in this document, those node SIDs would be interpreted as domain-wide unique MPLS labels as well. Now assume a given traffic flow destined for destination D is selected by the service classifier to go through a particular SFC (i.e., {SF1, SF2}) before reaching its final destination D. Section 3.1 and 3.2 describe two approaches of leveraging the MPLS- based source routing mechanisms to realize the service path functionality of the service function chaining (i.e., by encoding the SFP information within an MPLS label stack or by encoding the SFC information within an MPLS label stack) respectively.

3.1. Encoding SFP Information by an MPLS Label Stack

Since the selected packet needs to travel through an SFC {SF1, SF2}, the service classifier would attach a segment list {SID(SFF1), SID(SF1), SID(SFF2), SID(SF2)} which indicates the corresponding SFP to the packet. This segment list is actually represented by a MPLS label stack. When the encapsulated packet arrives at SFF1, SFF1 would know which SF should be performed according to the current top label (i.e., SID (SF1)). Before sending the packet to SF1, the remaining MPLS label stack (i.e., a segment list {SID(SFF2), SID(SF2)}) MUST be stripped. After receiving the packet returned from SF1, SFF1 would reimpose the MPLS label stack which had been stripped before to the packet and then send it to SFF2 according to the current top label (i.e., SID (SFF2) ). When the encapsulated packet arrives at SFF2, SFF2 would do the similar action as what has been done by SFF1. Provided that there was no MPLS LSP tunnel towards the next node segment (i.e., the next SFF node identified by the current top label), the corresponding IP-based tunnel (e.g., MPLS-in-IP/GRE tunnel [RFC4023], MPLS-in-L2TPv3 tunnel [RFC4817] or MPLS-in-UDP tunnel [I-D.ietf-mpls-in-udp]) towards the next node segment could be used instead. For more details about this special usage, please refer to [I-D.xu-spring-islands-connection-over-ip]. This approach of encoding the SFP information by an MPLS label stack is transport independent since the transport (i.e., the underlay) protocol could be IPv4, IPv6 or even MPLS. In other words, it fully meets the requirement of transport independence for the SFC encapsulation as mentioned in [I-D.ietf-sfc-architecture].

3.2. Encoding SFC Information by an MPLS Label Stack

Since the selected packet needs to travel through an SFC (i.e., {SF1, SF2}), the service classifier would attach a segment list {SID(SF1), SID(SF2)} which indicates that SFC to the packet. This segment list is actually represented by a MPLS label stack. Since it's known to the service classifier that SFF1 is attached with an instance of SF1, the service classifier would therefore send the encapsulated packet through either an MPLS LSP tunnel or an IP-based tunnel towards SFF1. When the encapsulated packet arrives at SFF1, SFF1 would know which SF should be performed according to the current top label (i.e., SID (SF1)). Before sending the packet to SF1, the remaining MPLS label stack (i.e., a segment list {SID(SF2)}) MUST be stripped. Upon receiving the packet returned from SF1, SFF1 would re-impose the MPLS label stack which had been stripped before to the packet, and then send it to SFF2 through either an MPLS LSP tunnel or an IP-based tunnel towards SFF2 since it's known to SFF1 that SFF2 is attached with an instance of SF2. When the encapsulated packet arrives at SFF2, SFF2 would do the similar action as what has been done by SFF1. This approach of encoding the SFC information by an MPLS label stack is transport independent since the transport (i.e., the underlay) protocol could be IPv4, IPv6 or even MPLS. In other words, it fully meets the requirement of transport independence for the SFC encapsulation as mentioned in [I-D.ietf-sfc-architecture].

4. Acknowledgements

The authors would like to thank Loa Andersson and Andrew G. Malis for their valuable comments and suggestions on the draft.

5. IANA Considerations

TBD.

6. Security Considerations

TBD

7. References

7.1. Normative References

7.2. Informative References

Authors' Addresses