SFC M. Boucadair
Internet-Draft C. Jacquenet
Intended status: Informational France Telecom
Expires: May 08, 2014 R. Parker
Affirmed Networks
L. Dunbar
Huawei Technologies
November 04, 2013

Service Function Chaining: Design Considerations, Analysis & Recommendations
draft-boucadair-sfc-design-analysis-01

Abstract

This document aims at analyzing the various design options and providing a set of recommendations for the design of Service Function Chaining solution(s). Note:

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

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on May 08, 2014.

Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1. Introduction

This document aims at analyzing the various design options and providing a set of recommendations for the design of Service Function Chaining solution(s). The conclusions of this analysis, once stable, will be recorded in the framework document.

The overall problem space is described in [I-D.quinn-sfc-problem-statement]. A list of requirements is available at [I-D.boucadair-sfc-requirements].

2. Terminology

The reader should be familiar with the terms defined in [I-D.boucadair-sfc-framework].

3. Scope

This document identifies potential solutions to fulfill the design requirements documented in [I-D.boucadair-sfc-requirements]. Particularly, it focuses on the following design objectives:

  1. Which information to include in the SFC header? (see Section 4)
  2. How to mark packets to indicate they belong to a given Service Function Chain (SFC) (see Section 5) and in which channel the SFC header is to be conveyed (see Section 6)?
  3. How to select a differentiated set of policies at a given Service Function (SF)? (see Section 5)
  4. How to select the forwarding path of a given flow that needs to be processed according to a set of Service Functions which must be invoked in a given order? (see Section 7)

Other design issues will be documented in future versions if required.

4. Service Function Chaining Header

This section identifies the main design points to be agreed upon so as to guide the forthcoming specification effort of the Service Function Chaining Header.

4.1. Why a Subscriber Identifier Does Not Need to be part of the Header?

Injecting (or leaking a Subscriber Identifier (subscriber-ID)) in the Service Function Chaining Header is not motivated by current deployment practices which consider that per-subscriber policies are enforced by only a few nodes (especially in the access network segment). As such, the service chaining solution does not require per-subscriber policies to be supported by all involved (SF) nodes.

Moreover, the enforcement of some policies can be driven by a subset of the information contained in the packets (e.g., source IP address, IPv6 prefix, etc.). Conveying the SF Map Index is sufficient to guide a Service Function to select the set of policies to be enforced for a given packet that belongs to a flow.

Note, current deployments may require an explicit subscriber-ID only during the authentication and authorization phases. These deployments do not require explicit subscriber-ID information to be conveyed when sending actual data traffic.

The Service Chaining approach does not aim to change how authorization and per-subscriber policies are enforced. As a consequence, explicit Subscriber-ID is not mandatory to be included in the Service Chaining Header.

In some contexts, the subscriber-ID is not available to the node that is responsible for enforcing per-subscriber policies. A typical example where complications may arise is a deployment context characterized as follows:

Enforcing for instance per-subscriber port quota requires an additional information to uniquely disambiguate hosts having the same address (called HOST_ID, [RFC6967]). This problem is not specific to the Service Function Chaining, but it is encountered in many other use cases ([I-D.boucadair-intarea-host-identifier-scenarios]).

Within the context of SFC, two solutions can be adopted:

  1. Implement a solution similar to what is specified in [RFC6674]. This means that the subscriber-ID is passed only to the node that enforces the per-subscriber policies without leaking it to other downstream SFs. In such case, the node that inserts the subscriber-ID is not part of the SFC-enabled domain. This solution does not require the insertion of the subscriber-ID in the SFC header.
  2. Define a subscriber-ID optional field in the SFC header. This optional field can be defined as an optional 64-bit field to accommodate the mobile case (e.g., inject an IMSI (International Mobile Subscriber Identity) identifier as a subscriber-ID).
+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| It is NOT RECOMMENDED to encode a subscriber-ID                   |
| as a mandatory field of the SFC header.                           |
+-------------------------------------------------------------------+

4.2. Fixed vs. Variable Length of the SFC Map Index

The number of Service Chains to be instantiated is deployment-specific. It depends on the business context and engineering practices that are internal to each administrative entity. To ensure a better flexibility as a function of the service chains that are theoretically supported, a first design consideration is to decide whether there is a need for a fixed field or a variable length field.

A field with a variable length is flexible enough to accommodate as many Service Function Chains as required for each deployment context. An administrative entity will need to tweak the length of this field to meet its own deployment requirements (e.g., set the length in all involved nodes to 8 bits, 16 bits, 32 bits or even more).

A field with a fixed length would lead to a better performance (mainly because of a simplified processing).

4.3. Recommended Length

An 8-bit field would be sufficient to accommodate deployment contexts that assume a reasonable set of SF Maps. A 16-bit field would be more flexible and would allow to enable large service chains (e.g., to accommodate the requirement discussed in [I-D.boucadair-sfc-requirements]). A 32-bit field would fulfill the needs for deployments with very large Service Function Chains.

+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| It is RECOMMENDED to use a 32-bit field to encode                 |
| the SF Map Index                                                  |
+-------------------------------------------------------------------+

4.4. Extensibility

The header can be extended in the future, based on the experience that will be gained during operational deployments. As such, the header does not need to include any protocol version field nor any reserved bits to disambiguate between two variants of the header.

Implementations supporting the service chaining solution can be upgraded following current best practices in the field.

+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| It is NOT RECOMMENDED to reserve bits to anticipate future        |
| extension needs. Backward compatibility between two versions of   |
| the header can be ensured by consistent system                    |
| setup & configuration.                                            |
+-------------------------------------------------------------------+

5. Format of the Service Function Chaining Header

This section proposes and discusses some formats to encode the Service Chaining Header. An analysis is also included in this section.

5.1. Format 1: Single Marking Code Point

The RBNF format [RFC5511] of the header is shown in Figure 1:

<SFC Header> ::=  <SF Map Index>

Figure 1: Single Marking Code Point

This format is characterized as follows:

5.2. Format 2: Marking Code Point & Profile Index

The RBNF format of the header is shown in Figure 2:

<SFC Header> ::=  <SF Map Index>
                  <Service Function Map>

<Service Function Map> ::= <Service Function> ...

<Service Function> ::=  <Service Function Identifier>
                        <Profile Identifier>

Figure 2: Marking Code Point & Profile Index

  • The list of SF Locator(s) is provisioned out of band to each SF Node.
  • Classifiers are provisioned with classification rules to decide which code point is to be used for a received packet.
  • Fragmentation risks are not minimized.
  • The classifier needs to be configured with a list of profiles/contexts per Service Function.
  • The classifier behavior is not simplified since it must also encode in each incoming packet the full list of functions to be performed by each Service Function hop.

5.3. Format 3: Explicit Route List

The RBNF format of the header is shown in Figure 3:

<SFC Header> ::=  <Total number of Service Function hops>
                  <Current hop Index>
                  <Service Function Map>

<Service Function Map> ::= <Service Function> ...

<Service Function> ::=  <IP ADDRESS>
                        <Profile Identifier>

Figure 3: Explicit Route List

The procedure at a non-reclassifying node is to validate that the IP address of the SF at the current index matches one of the SF's own IP addresses and then to find the profile identifier by its indicated identifier. Once the local Service Function is invoked, if the packet needs to be forwarded to the next Service Function hop, the local node simply increments the current hop index and rewrites the outer IP header with the next hop's IP address.

This format is characterized as follows:

  • Classifiers are provisioned with classification rules to decide which code point is to be used for a received packet.
  • Fragmentation risks are not minimized.
  • The classifier needs to be configured with a list of profiles/contexts per Service Function.
  • The classifier is also responsible for load balancing. This makes the classifier more complex.
  • The classifier behavior is not simplified since it must also encode in each incoming packet the full list of policies to be performed by each Service Function node.

5.4. Format 4: Compact Explicit Route List

<SFC Header> ::=  <Total number of Service Function hops>
                  <Current hop Index>
                  <Service Function List>

<Service Function List> ::= <Service Function> ...

<Service Function> ::=  <IP Address ID>
                        <Profile Identifier>

Figure 4: Compact Explicit Route List

A variant of the previous format is depicted in the RBNF format of the header shown in Figure 4. Instead of including the explicit route list (Figure 3), IP addresses of SFs are configured out of band but each of these addresses is identified with a unique identifier. These identifiers are indicated in the Service Chaining Header.

This proposal suffers from the same drawbacks as the previous format.

5.5. Analysis

Given the design motto that says:

  • "A protocol design is complete not when you can't think of any more things to add, but when you have removed everything you can and you can’t see how to remove any more",

the proposed format must be as simple as possible while meeting the requirements discussed in [I-D.boucadair-sfc-requirements]. The simplicity argument is further discussed in [RFC3439] and [Robust].

Based on the above analysis, the proposal that is simple, minimizes fragmentation, optimizes the behavior of the classifier and SF Nodes, and that prevents potential DDoS attacks is the one discussed in Section 5.1.

6. Where To Convey the Chaining Marking Information In A Packet?

This section lists a set of candidate solutions to convey the Service Chaining Header.

6.1. Use IPv6 Flow Label

The use of the 20-bit Flow Label field in the IPv6 header [RFC6437] can be considered as a candidate solution to convey the SF Map Index.

The following comments can be made for this candidate solution:

  • This proposal requires all packets are transported over IPv6. This should not be considered as a limitation for some deployments.
  • Intermediate Nodes must not alter the content of the Flow Label field.
  • This proposal can apply to any transport protocol.
  • The use of the IPv6 Flow Label may interfere with other usages of the flow label such as Equal Cost Multipath (ECMP) or Link Aggregation (LAG) [RFC6438]. The Flow Label bits need to be combined at least with bits from other sources within the packet, so as to produce a constant hash value for each flow and a suitable distribution of hash values across flows [RFC6437].
  • A 20-bit field to convey the SF Map Index allows to enable Service Function Chains of a large size range.
  • This proposal does not allow to convey additional information than the SF Map Index (if needed).
  • The Flow Label is present in all fragments, SF Nodes do not need to maintain any state to handle a fragmented packet.
  • Altering the value of the Flow Label field does not interfere with the use of IPsec [RFC6438].
  • Carrying the SF Map Index in the IPv6 Flow Label allows to:
    • De-correlate packet marking from forwarding constraints.
    • Avoid requiring an internal tagging mechanism to each SF Node to preserve the same marking in the outgoing interface as the one received in an incoming interface.
+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| It is tempting to use the Flow Label, but the 20-bit length of    |
| the Flow Label field is conflicting with the recommended 32-bit   |
| length discussed in Section 4.3.                                  |
|                                                                   |
| The use of Flow Label is NOT RECOMMENDED.                         |
+-------------------------------------------------------------------+

6.2. Use the DS Field

Another alternative to convey the SF Map Index is to use the Differentiated Services (DS) field [RFC2474] [RFC2475] (for both IPv4 and IPv6).

The following comments can be made for this proposal:

  • This proposal overloads the semantics of the DS field.
  • Having 64 possible values may not accommodate deployments with a large number of service chains (see Section 4.3).
  • This proposal can apply to any transport protocol.
  • The use of the DS field for service chaining purposes may interfere with other usages such as Traffic Engineering (TE) or Quality of Service (QoS).
    • This issue can be mitigated by fragmenting the DS space into to distinct set of values; each set dedicated for a specific usage. An administrative entity can use the first bits for service chaining and other remaining bits for QoS for instance.
    • Splitting the DS space reduces the number of possible service chains to be configured per administrative domain.
+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| The use of DS field is NOT RECOMMENDED.                           |
+-------------------------------------------------------------------+

6.3. Use IP Identification Field

The IPv4 ID (Identification field of IP header, i.e., IP-ID) can be used to insert the SF Map Index. The classifier rewrites the IP-ID field to insert the SF Map Index (16 bits). The classifier must follow the rules defined in [RFC6864]; in particular, the same SF Map Index is not reassigned during a given time interval. Note:

  • This usage is not consistent with the fragment reassembly use of the Identification field [RFC0791] or the updated handling rules for the Identification field [RFC6864].
  • Complications may arise if the packet is fragmented before reaching the Classifier. To appropriately handle those packet fragments, the classifier will need to maintain a lot of state.
  • Preserving the same value when crossing all intermediate SFs may be difficult (e.g., an invoked SF can be a NAT).
  • This proposal assumes packets are transported over IPv4 (plain or encapsulated mode). This may not be considered as a limitation for some deployments.
+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| Using the IP-ID as a channel to convey the SF Map Index is NOT    |
| RECOMMENDED.                                                      |
+-------------------------------------------------------------------+

6.4. Use IPv4 SSRR/LSRR Option

Another candidate channel to convey the Service Chaining Header is to use the IPv4 SSRR/LSRR options [RFC0791]. These options can be inserted by the classifier following the pre-configured classification rules. Note:

  • Some general recommendations documented in [I-D.ietf-opsec-ip-options-filtering] and [RFC6192] need to be taken into account.
  • This proposal assumes packets are transported over IPv4 (plain or encapsulated mode). This may not be considered as a limitation for some deployments.
  • This proposal can apply to any transport protocol.
  • Encoding the full list of intermediate SF Nodes will exacerbate fragmentation issues.
  • Injecting an additional IP option by the classifier introduces some implementation complexity in the following cases: The packet has the MTU size (or is close to it), and the option space is exhausted.
  • Legacy nodes must be configured to not strip this option.
  • Processing the IP option may degrade the performance of involved SF nodes.
+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| Using the IPv4 SSRR/LSRR options as a channel to convey           |
| the Service Chaining Header is NOT RECOMMENDED.                   |
+-------------------------------------------------------------------+

6.5. Define a new IPv4 Option and IPv6 Extension Header

+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| Define a new IPv4 option and IPv6 extension header                |
| as an Experimental track RFC document. This approach is pragmatic,|
| assuming further experiments can be conducted to:                 |
|                                                                   |
| 1. Assess the impact on performance.                              |
|                                                                   |
| 2. Compare the impact of using the IPv4 option and the IPv6       |
|    extension header vs. an encapsulation mode (i.e., in contexts  |
|    where no encapsulation is required to reach the next SF hop).  |
|                                                                   |
| 3. Assess to what extent the use of an IPv4 option/IPv6 extension |
|    header simplify internal tagging mechanisms specific to each SF|
+-------------------------------------------------------------------+

Another candidate solution to convey the Service Chaining Header is to define a new IPv4 option [RFC0791] and a new IPv6 extension header [RFC6564]. The IPv4 option/IPv6 extension header can be inserted by the classifier following the pre-configured classification rules. Note:

  • This proposal is valid for any transport protocol.
  • This proposal offers the same functionality in both IPv4 and IPv6.
  • Some general recommendations documented at [I-D.ietf-opsec-ip-options-filtering], [RFC6192], and [I-D.ietf-6man-ext-transmit] are to be taken into account. Nevertheless, these security threats do not apply for this usage since the Ingress Node is the entity that is responsible for injecting the new option. Therefore, malicious usage of this option is unlikely.
  • Injecting an additional IP option by the classifier introduces some implementation complexity in the following cases: The packet is at or close to the MTU size, and the option space is exhausted.
  • The option can be designed to be compact and therefore avoid inducing fragmentation.
  • Despite it is widely known that routers and middleboxes filter IP options (e.g., drop IP packets with unknown IP options, strip unknown IP options, etc.), this concern does not apply for the Service Function Chaining case because the support of new IP options can be enabled within a domain operated by the same administrative domain.
  • Intermediary Nodes must not strip this IPv4 option/IPv6 extension header.
  • The use of an IPv4 option or IPv6 Extension Header to drive the processing of an incoming packet may alter the performance of SF Nodes.
    • Some vendors claim the use of Extension Headers (other than Hop-by-Hop) does not impact the overall performance of their IPv6 implementation (e.g., [Report]).
    • Some studies revealed an increase of the single-hop delay when IP options are included (e.g., [Delay]).
    • The severity of the overall performance degradation is to be further assessed ([RFC5180]).
  • Carrying the Service Chaining Header as an IPv4 option/IPv6 extension header allows to:
    • De-correlate packet marking from forwarding constraints.
    • Avoid requiring an internal tagging mechanism to each SF Node to preserve the same marking in the outgoing interface as the one received through the incoming interface.

6.6. Define a New TCP Option

This proposal consists in defining a new TCP option to convey the Service Chaining Header. The drawbacks of this proposal are listed below:

  • Encapsulating every received packet in TCP SYN messages may impact the performance of SF nodes.
  • Injecting a TCP option by intermediate nodes will interfere with end-to-end (E2E) issues. One example of such interference would be terminating and re-originating TCP connections not belonging to the transit device.
  • Injecting this TCP option introduces some implementation complexity if the options space is exhausted. TCP option space is limited and might be consumed by the TCP client.
  • SF Nodes may need to maintain a lot of state entries to handle fragments.
+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| Defining a new TCP option as a channel to convey the Service      |
| Chaining Header is NOT RECOMMENDED.                               |
+-------------------------------------------------------------------+

6.7. Use the GRE Key

[RFC2890] defines key and security extensions to GRE (Generic Routing Encapsulation, [RFC2784]). GRE Key and sequence number fields are optional. This section investigates how a GRE Key optional field can be used to convey a 32-bit SF Map Index.

  • GRE Checksum and Sequence Number fields are not required. These fields must not be included.
  • Relying on GRE optional field to drive the processing of received packets may impact the performance of SF Nodes.
  • This proposal does not allow to convey additional information than the SF Map Index (if needed).
  • In cases where GRE would already have been used, it is preferable to rely on this scheme and avoid yet another encapsulation overhead.
  • An SF Node must rely on an internal tagging procedure to preserve the same header be positioned at the outgoing interface of an SF node.
  • Further experiments may be required to compare the performance that would result in activating this solution vs. the performance observed when an IPv4 option or IPv6 extension header is used jointly with IP-in-IP encapsulation [RFC2003].
+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| To be completed                                                   |
+-------------------------------------------------------------------+

6.8. Define a New IP-in-IP Scheme

This proposal is compliant with [RFC1853]. It consists in adding a fixed header as shown in Figure 5:

                                    +---------------------------+
                                    |      Outer IP Header      |
                                    +---------------------------+
                                    |        SFC Header         |
+---------------------------+       +---------------------------+
|         IP Header         |       |      Inner IP Header      |
+---------------------------+ ====> +---------------------------+
|                           |       |                           |
|         IP Payload        |       |         IP Payload        |
|                           |       |                           |
+---------------------------+       +---------------------------+

Figure 5

  • This proposal covers both IPv4 and IPv6 deployment cases.
  • An SF Node must rely on an internal tagging procedure to preserve the same header be positioned at the outgoing interface of an SF node.
  • This header can be extended easily to accommodate new requirements.
  • Because the SFC Header is part of the mandatory header, the performance are likely to not be severely impacted compared to other tunneling modes such as the joint use of IP-in-IP and an IPv4 option/IPv6 extension header.
+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| To be completed                                                   |
+-------------------------------------------------------------------+

6.9. MAC-based SFC Forwarding

The SFC Classifier capability introduced in [I-D.boucadair-sfc-framework] can be for instance supported by a GGSN node of a mobile network that also embeds the PCEF (Policy Charging Enforcement Function) function. A generic description of the related Gi Interface use case is discussed in [I-D.liu-sfc-use-cases].

This candidate solution assumes the following:

  • The SFC Classifier node is connected to various SF Nodes via a tunnel (e.g., VxLAN or L2VPN tunnel).
  • A large block of MAC addresses are allocated to the SFC Classifier node.
  • The SFC Classifier node can use different MAC addresses in the Source Address field of the data frame to identify different SFCs.
  • Out-of-band message(s) can be exchanged between a SFC Classifier node and SF Nodes to signal the SFC associated to each Source MAC address.

This proposal is a particular case of the "Router-based mechanism" defined in Section 10.2 of [I-D.boucadair-sfc-framework].

The following comments can be made for this candidate solution:

  • It can apply to any transport protocol.
  • A large block of MAC addresses has to be allocated to the SFC Classifier node. The SFC Classifier node must have the extra logic of using different MAC addresses for different SF chains.
  • Each SF node needs to be provisioned with instructions or policies provided to and relayed by the classifier on where to send a packet based on the source MAC address associated to a specific SFC.
  • It is designed for topologies where Classifier and SF nodes can be connected by tunnels to maintain their Layer 2 connections. In particular, these tunnels are used to convey SFC-specific instructions and policies to the SF nodes. From that standpoint, the proposal is only applicable to such topologies.
  • The proposed scheme requires that all traffic traverses the Classifier node first. The return path doesn't have to go back to the SFC Classifier node because all the SF Nodes forward traffic based on the instructions and policies provided to and relayed by the Classifier, instead of making forwarding decisions based upon the destination addresses.
  • When multiple SFs that are part of a given SFC are co-located in the same device, the SFC Classifier may have trouble to decide which SF needs to be invoked in which order. A solution to avoid such complication is to use different source addresses to indicate which SFs and in which order. SF nodes (or Proxy Nodes) may need policies from the PDP, classification nodes, or control plane on how to steer packets based on source address to their designated SFs.
  • The mechanism may be exposed to an overlapping MAC address situation whenever some of these MAC addresses need to be locally administered.
+-------------------------------------------------------------------+
| Proposed Recommendation                                           |
+-------------------------------------------------------------------+
| The MAC-based SFC forwarding is designed for specific topologies  |
| and assumes strong requirements on the SFC Classifier Node.       |
|                                                                   |
| The use of MAC-based SFC forwarding is NOT RECOMMENDED as a       |
| generic SFC mechanism that would remove dependency on the         | 
| underlying physical topology.                                     |
+-------------------------------------------------------------------+

7. Steer Paths To Cross Specific SF Nodes

7.1. Need for a Mandatory Encapsulation Scheme

For interoperability reasons, one encapsulation mode MUST be defined. Refer to [RFC3439] for more discussion on the design principles.

7.2. Candidate Solutions

Given the requirements identified in [I-D.boucadair-sfc-requirements], IP-based encapsulation schemes should be considered. From this standpoint, the following encapsulation candidate solutions are identified so far:

  1. Simple IP-in-IP & a SFC header in the inner packet (e.g., IPv4 option, IPv6 extension header)
  2. IP-in-IP with a fixed SFC header (Section 6.8).
  3. GRE & GRE Key as a channel to convey the SF Map Index (Section 6.7)

7.3. Discussion

The following table summarizes the main characteristics for each mode:

Mode Simple IP-in-IP & a SFC header in the inner packet IP-in-IP with a fixed SFC header GRE & GRE Key
Encapsulation overhead when the next hop SF is in the same subnet No Yes Yes
A proprietary internal tagging mechanism is required No Yes Yes
Natural extensibility Yes Yes No
Risk to strip the header by intermediate nodes Yes No No
Possible Impact on Performance Med to High Low to Med Med

The following comments can be made:

  • Both "IP-in-IP with a fixed SFC header" and "GRE & GRE Key" present almost the same characteristics except "IP-in-IP with a fixed SFC header" can be easily extended. Note, "GRE & GRE Key" can also be extended with new optional fields but this may induce some performance degradation.
  • "Simple IP-in-IP & a SFC header in the inner packet" is more flexible:
    • It allows to convey the SFC header separately from the encapsulation header.
    • It allows to avoid encapsulation overhead when adjacent SFs in a SFC sequence are in the same subnet.
    • No internal tagging is needed within a SF Node.
    • The SFC header can be extended in the future (if needed).
  • Indicated values for "Possible Impact on Performance" are hypothetical. These values are inspired from some experiments such as [Delay]. Ideally, further testing should be conducted to better qualify the impact on performance of these proposals under the same configuration and setup.
Proposed Recommendations
(1) Adopt the IP-in-IP with a fixed SFC header solution (Section 6.8). This mode is to be used as the MANDATORY encapsulation scheme for service chaining purposes. The main selection criteria for this proposed recommendation is to minimize performance impacts on involved nodes.
(2) To accommodate deployment cases where encapsulation is not required, allow to rely exclusively on a dedicated tagging field in the inner packet. This extension is to be defined in the EXPERIMENTAL track (e.g., Section 6.5).
(3) Experimental specifications can be obsoleted or promoted to be in the Standard Tracks based on the conclusions from significant experiments.

8. Summary

As a consequence of the above analysis, the following recommendations are made:

  • **** TO BE COMPLETED ONCE THE ANALYSIS IS STABLE ****

9. IANA Considerations

Authors of this document do not require any action from IANA.

10. Security Considerations

Security considerations related to Service Function Chaining are discussed in [I-D.boucadair-sfc-framework].

11. Acknowledgements

Thanks to J. Halpern for the discussion on the subscriber-ID.

12. References

12.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5511] Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax Used to Form Encoding Rules in Various Routing Protocol Specifications", RFC 5511, April 2009.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.

12.2. Informative References

[I-D.boucadair-sfc-framework] Boucadair, M., Jacquenet, C., Parker, R., Lopez, D., Guichard, J. and C. Pignataro, "Service Function Chaining: Framework & Architecture", Internet-Draft draft-boucadair-sfc-framework-00, October 2013.
[I-D.boucadair-sfc-requirements] Boucadair, M., Jacquenet, C., Jiang, Y., Parker, R. and C. Pignataro, "Requirements for Service Function Chaining", Internet-Draft draft-boucadair-sfc-requirements-00, October 2013.
[I-D.quinn-sfc-problem-statement] Quinn, P., Guichard, J., Surendra, S., Agarwal, P., Manur, R., Chauhan, A., Leymann, N., Boucadair, M., Jacquenet, C., Smith, M., Yadav, N., Elzur, U., Nadeau, T., Gray, K., McConnell, B. and K. Kevin, "Service Function Chaining Problem Statement", Internet-Draft draft-quinn-sfc-problem-statement-01, October 2013.
[I-D.ietf-opsec-ip-options-filtering] Gont, F., Atkinson, R. and C. Pignataro, "Recommendations on filtering of IPv4 packets containing IPv4 options", Internet-Draft draft-ietf-opsec-ip-options-filtering-02, January 2013.
[I-D.ietf-6man-ext-transmit] Carpenter, B. and S. Jiang, "Transmission and Processing of IPv6 Extension Headers", Internet-Draft draft-ietf-6man-ext-transmit-03, August 2013.
[I-D.boucadair-intarea-host-identifier-scenarios] Boucadair, M., Binet, D., Durel, S., Reddy, T. and B. Williams, "Host Identification: Use Cases", Internet-Draft draft-boucadair-intarea-host-identifier-scenarios-02, December 2012.
[I-D.liu-sfc-use-cases] Liu, W., Li, H., Huang, O., Boucadair, M., Leymann, N., Cao, Z. and J. Hu, "Service Function Chaining Use Cases", Internet-Draft draft-liu-sfc-use-cases-00, October 2013.
[RFC3439] Bush, R. and D. Meyer, "Some Internet Architectural Guidelines and Philosophy", RFC 3439, December 2002.
[RFC6967] Boucadair, M., Touch, J., Levis, P. and R. Penno, "Analysis of Potential Solutions for Revealing a Host Identifier (HOST_ID) in Shared Address Deployments", RFC 6967, June 2013.
[RFC6674] Brockners, F., Gundavelli, S., Speicher, S. and D. Ward, "Gateway-Initiated Dual-Stack Lite Deployment", RFC 6674, July 2012.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field", RFC 6864, February 2013.
[RFC6437] Amante, S., Carpenter, B., Jiang, S. and J. Rajahalme, "IPv6 Flow Label Specification", RFC 6437, November 2011.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label for Equal Cost Multipath Routing and Link Aggregation in Tunnels", RFC 6438, November 2011.
[RFC6564] Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J. and M. Bhatia, "A Uniform Format for IPv6 Extension Headers", RFC 6564, April 2012.
[RFC2474] Nichols, K., Blake, S., Baker, F. and D.L. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.
[RFC2475] Blake, S., Black, D.L., Carlson, M.A., Davies, E., Wang, Z. and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998.
[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE", RFC 2890, September 2000.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
[RFC1853] Simpson, W., "IP in IP Tunneling", RFC 1853, October 1995.
[RFC6192] Dugal, D., Pignataro, C. and R. Dunn, "Protecting the Router Control Plane", RFC 6192, March 2011.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996.
[RFC5180] Popoviciu, C., Hamza, A., Van de Velde, G. and D. Dugatkin, "IPv6 Benchmarking Methodology for Network Interconnect Devices", RFC 5180, May 2008.
[Report] , , "IPv6 Extension Headers Review and Considerations", .
[Delay] Papagiannaki, K., Moon, S., Fraleigh, C., Thiran, P. and C. Diot, "Measurement and Analysis of Single-Hop Delay on an IP Backbone Network", August 2003.
[Robust] Walter Willinger, W. and J. Doyle, "Robustness and the Internet: Design and evolution", March 2002.

Authors' Addresses

Mohamed Boucadair France Telecom Rennes, 35000 France EMail: mohamed.boucadair@orange.com
Christian Jacquenet France Telecom Rennes, 35000 France EMail: christian.jacquenet@orange.com
Ron Parker Affirmed Networks Acton,, MA USA EMail: Ron_Parker@affirmednetworks.com
Linda Dunbar Huawei Technologies 5430 Legacy Drive, Suite #175 Plano, TX USA EMail: linda.dunbar@huawei.com