Service Function Chaining Using MPLS-SPRING
draft-xu-sfc-using-mpls-spring-02

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
• 3.3. How to Contain Metadata within an MPLS Packet
• 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 SFP 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.ietf-spring-segment-routing-mpls]. This document describes how to leverage the MPLS-based source routing mechanisms to realize the service path layer functionality of the service function chaining. Note that this approach is aligned with the Transport Derived SFF mode as described in Section 4.3.1 of [I-D.ietf-sfc-architecture].

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.ietf-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 Forwarders (SFF) 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 contrast, to encode the SFC information by an MPLS label stack, those SF SIDs MUST be interpreted as domain-wide unique MPLS labels, which means a big difference from the current usage of the MPLS labels. The reason why local MPLS labels are not applicable is that all of the labels in the stack would have to be swapped by the current SFF before forwarding the packet to the next SFF if local labels are used. 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 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. Since the encoding of the partially specified SFP is just a simple combination of the encoding of the SFP and the encoding of the SFC, this document would not describe how to encode the partially specified SFP anymore.

3.1. Encoding SFP 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 of (i.e., 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)) of the received MPLS packet. Before sending the packet to SF1, the whole MPLS label stack (i.e., 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) ). Note that popping and reimposing a label stack means a difference from the current MPLS forwarding behavior. Details of the above special MPLS forwarding behavior would be discribed in a later version of this document. When the encapsulated packet arrives at SFF2, SFF2 would do the similar action as what has been done by SFF1. To some extent, the MPLS label stack here could be looked as a specific implementation of the SFC encapsulation used for containing the SFP information [I-D.ietf-sfc-architecture].

If there is no MPLS LSP towards the next node segment (i.e., the next SFF 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]) could be used instead (For more details about this special usage, please refer to [I-D.xu-spring-islands-connection-over-ip]). Since the transport (i.e., the underlay) could be IPv4, IPv6 or even MPLS networks, the above approach of encoding the SFP information by an MPLS label stack is fully transport-independent which is one of the major requirements for the SFC encapsulation [I-D.ietf-sfc-architecture].

In addition, the service classifier could further impose metadata on the MPLS packet through the Network Service Header (NSH) [I-D.quinn-sfc-nsh] (As for how to contain the NSH within a MPLS packet, please see Section 3.3). Here the Service Path field wihin the NSH would not be used for the path selection purpose anymore and therefore it MUST be set to a particular value to indicate such particular usage. In addition, the service index value within the NSH is set to a value indicating the total number of SFs within the service function path. The service index SHOULD be decreased by one on each SF or SF-proxy on behalf of the corresponding legacy SF. When the service index become zero, the NSH MUST be removed from the packet by the SF or SF-proxy on behalf of the corresponding legacy SF.

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 (i.e., 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 MPLS encapsulated packet through either an MPLS LSP tunnel or an IP-based tunnel towards SFF1. When the MPLS 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 whole MPLS label stack (i.e., 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. Since the transport (i.e., the underlay) could be IPv4, IPv6 or even MPLS networks, the above approach of encoding the SFC information by an MPLS label stack is fully transport-independent which is one of the major requirements for the SFC encapsulation [I-D.ietf-sfc-architecture].

In addition, the service classifier could further impose metadata on the MPLS packet through the Network Service Header (NSH) [I-D.quinn-sfc-nsh]. The procedures of imposing, consumming and stripping the NSH is the same as that being mentioned in Section 3.1.

3.3. How to Contain Metadata within an MPLS Packet

Since the MPLS encapsulation has no explicit protocol identifier field to indicate the protocol type of the MPLS payload, how to indicate the presence of metadata (i.e., the NSH which is only used as a metadata containner) in MPLS packets is a potential issue. There are two possible ways to address the above issue: 1) SFFs allocate two different labels for a given SF, one indicates the presence of NSH while the other indicates the absence of NSH; 2) Add an protocol identifier field at the bottom of the MPLS label stack to indicate the presence or absence of the NSH (For more details about the protocol identifier field in the MPLS packet, see [I-D.xu-mpls-payload-protocol-identifier] ). The former approach has no change to the current MPLS architecture but it would require more than one label binding for a given SF. The latter approach only requires a single label binding for a given SF but it would require a little change to the current MPLS architecture. Which approach would be chosen depends on the rough consensus of the IETF community.

4. Acknowledgements

The authors would like to thank Loa Andersson and Andrew G. Malis for their valuable comments and suggestions on the draft. The authors would like to thank Adrian Farrel, Stewart Bryant, Carlos Pignataro, Huub van Helvoort, Alexander Vainshtein, Joel M. Halpern, Loa Andersson and Andrew G. Malis for their valuable comments and suggestions on how to indicate the presence of metadata within an MPLS packet.

5. IANA Considerations

TBD.

6. Security Considerations

TBD

7. References

7.1. Normative References

7.2. Informative References

Authors' Addresses