Service Chaining using MPLS Source Routing
draft-xu-mpls-service-chaining-00

Abstract

Source Packet Routing in Networking (SPRING) WG is developing an MPLS source routing mechanism. This MPLS source routing mechanism can be leveraged to realize the service path layer functionality of the service function chaining (i.e., steering the selected traffic through a particular service function path) by encoding the service function path information as an MPLS label stack. This document describes how to use the MPLS source routing mechanism as developed by the SPRING WG to realize the service path layer functionality of service function chaining.

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].

Status of This Memo

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This Internet-Draft will expire on April 16, 2017.

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Solution Description
• 3.1. Encoding SFP 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) [RFC7665] 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. In order to steer the selected traffic through the required ordered list of SFs, the service classifier needs to attach information to the packet specifying which Service Function Forwarders (SFFs) and which SFs are to be visited by the selected traffic. The Source Packet Routing in Networking (SPRING) WG is developing an MPLS source routing mechanism which can be used to steer traffic through an ordered set of routers (i.e., an explicit path) and instruct nodes on that path to execute specific operations on the packet. This MPLS source routing mechanism thus 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 information as a label stack. This document describes how to use the MPLS 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 [RFC7665].

2. Terminology

This memo makes use of the terms defined in [I-D.ietf-spring-segment-routing-mpls] and [RFC7665].

3. Solution Description

     +----------------------------------------------- ----+
     |               MPLS SPRING Networks                 |
     |            +---------+       +---------+           |     
     |            |   SF1   |       |   SF2   |           |      
     |            +----+----+       +----+----+           |  
     |               ^ | |(3)          ^ | |(6)           |
     |       (1)  (2)| | V     (4)  (5)| | V     (7)      |
+----+-----+ ---> +----+----+ ----> +----+----+ --->  +---+---+ 
|Classifier+------+  SFF1   +-------+  SFF2   +-------+   D   | 
+----------+      +---------+       +---------+       +---+---+ 
     |                                                    |                       
     +----------------------------------------------------+
  Figure 1: Service Function Chaining in MPLS-SPRING Networks

As shown in Figure 1, SFF1 and SFF2 are two MPLS-SPRING-capable nodes. They are also SFFs, each with one SF attached. In addition, they have allocated and advertised Segment IDs (SID) for their locally attached SFs. In the MPLS-SPRING context, SIDs are encoded as MPLS labels. For example, SFF1 allocates and advertises a 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 as an MPLS label stack, those SF SIDs as mentioned above would be interpreted as local MPLS labels. More specifically, local MPLS labels are allocated from SFFs' (e.g., SFF1 in Figure 1) label spaces to identify their attached SFs (e.g., SF1 in Figure 1), whilst the SFFs are identified by either nodal SIDs or adjacency SIDs depending on how strictly the network path needs to be specified. In addition, assume node SIDs for SFF1 and SFF2 are SID(SFF1) and SID(SFF2) respectively. 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 describes how to use the MPLS-based source routing mechanism to realize the service path functionality of the service function chaining (i.e., by encoding the SFP information within an MPLS label stack).

3.1. Encoding SFP Information by an MPLS Label Stack

     +----------------------------------------------- ----+
     |               MPLS SPRING Networks                 |
     |            +---------+       +---------+           |     
     |            |   SF1   |       |   SF2   |           |      
     |            +----+----+       +----+----+           |  
     |  +---------+    |                 |    +---------+ |
     |  |SID(SFF2)|    |                 |    |Pkt to D | |
     |  +---------+    |                 |    +---------+ |
     |  |SID(SF2) |    |                 |                |
     |  +---------+    |               ^ | |              |
     |  |Pkt to D | ^  | |             | | |              |
     |  +---------+ |  | |          (5)| | |(6)           |
     |           (2)|  | |(3)          | | V              |
     |       (1)    |  | V     (4)       |      (7)       |
+----+-----+ ---> +----+----+ ----> +----+----+ --->  +---+---+ 
|Classifier+------+  SFF1   +-------+  SFF2   +-------+   D   | 
+----------+      +---------+       +---------+       +---+---+ 
     |    +---------+      +---------+        +---------+ |
     |    |SID(SFF1)|      |SID(SFF2)|        |Pkt to D | |
     |    +---------+      +---------+        +---------+ |
     |    |SID(SF1) |      |SID(SF2) |                    |
     |    +---------+      +---------+                    |
     |    |SID(SFF2)|      |Pkt to D |                    |
     |    +---------+      +---------+                    |
     |    |SID(SF2) |                                     |
     |    +---------+                                     |
     |    |Pkt to D |                                     |
     |    +---------+                                     |                       
     +----------------------------------------------------+
     Figure 2: Packet Walk in MPLS Source Routing based SFC

[RFC7665]. When the encapsulated packet arrives at SFF1, SFF1 would know which SF should be performed according to the top label (i.e., SID (SF1)) of the received MPLS packet. We first consider the case where SF1 is an encapsulation aware SF, i.e., it understands how to process a packet with a pre-pended MPLS label stack. In this case the packet would be sent to SF1 by SFF1 with the label stack SID(SFF2)-> SID(SF2). SF1 would perform the required service function on the received MPLS packet where the payload is constrained to be an IP packet, and the SF needs to process both IPv4 and IPv6 packets (note that the SF would use the first nibble of the MPLS payload to identify the payload type). After the MPLS packet is returned from SF1, SFF1 would send it to SFF2 according to the top label (i.e., SID (SFF2) ).

If SF1 is a legacy SF, i.e. one that is unable to process the MPLS label stack, the remaining MPLS label stack (i.e., SID(SFF2)->SID(SF2)) MUST be saved and stripped from the packet before sending the packet to SF1. When the packet is returned from SF1, SFF1 would re-impose the MPLS label stack which had been previously stripped and then send the packet to SFF2 according to the current top label (i.e., SID (SFF2) ). As for how to associate the corresponding MPLS label stack with the packets returned from legacy SFs, those mechanisms as described in [I-D.song-sfc-legacy-sf-mapping] could be considered.

When the encapsulated packet arrives at SFF2, SFF2 would perform the similar action to that described above.

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-UDP tunnel [RFC7510] or MPLS-in-L2TPv3 tunnel [RFC4817]) would be used instead (for more details about this special usage, please refer to [I-D.xu-mpls-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 [RFC7665].

4. Acknowledgements

The authors would like to thank Loa Andersson, Andrew G. Malis, Adrian Farrel, Alexander Vainshtein and Joel M. Halpern for their valuable comments and suggestions on the document

5. IANA Considerations

This document makes no request of IANA.

6. Security Considerations

It is fundamental to the SFC design that the classifier is a trusted resource which determines the processing that the packet will be subject to, including for example the firewall. It is also fundamental to the SPRING design that packets are routed through the network using the path specified by the node imposing the SIDs. Where an SF is not encapsulation aware the packet may exist as an IP packet, however this is an intrinsic part of the SFC design which needs to define how a packet is protected in that environment. Where a tunnel is used to link two non-MPLS domains, the tunnel design needs to specify how it is secured. Thus the secutity vulnerabilities are addressed in the underlying technologies used by this design, which itself does not introduce any new security vulnerabilities.

7. References

7.1. Normative References

7.2. Informative References

Authors' Addresses