MPLS Extension Header
draft-song-mpls-extension-header-00

Abstract

Motivated by the need to support multiple in-network services and functions in an MPLS network, this document describes a generic method to encapsulate extension headers into MPLS packets. The encapsulation method allows stacking multiple extension headers and quickly accessing any of them as well as the original upper layer protocol header and payload. We show how the extension header can be used to support several new network applications.

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.

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 https://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on January 16, 2019.

Copyright Notice

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Table of Contents

• 1. Motivation
• 2. MPLS Extension Header
• 3. Operation on MPLS Extension Headers
• 4. Use Cases
• 5. Summary
• 6. Security Considerations
• 7. IANA Considerations
• 8. Contributors
• 9. Acknowledgments
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Authors' Addresses

1. Motivation

Some applications require adding instructions and/or metadata to user packets within a network. Such examples include In-situ OAM (IOAM) and Service Function Chaining (SFC). New applications are emerging. It is possible that the instructions and/or metadata for multiple applications are stacked together in one packet to support a compound service.

However, the encapsulation of the new header(s) poses some challenges to the ubiquitous MPLS networks. A problem of the MPLS protocol header is that there is no explicit indicator for the upper layer protocols. The succinct MPLS header provide little room to encode any extra information. Moreover, the backward compatibility issue discourages any attempts trying to overload the semantics of the existing MPLS header fields.

The similar "header extension" requirement for MPLS has led to several proposals. A special Generic Associated Channel Label (GAL) is assigned to support the identification of an Associated Channel Header (ACH). Later, it was proposed to use GAL to indicate the presence of a Metadata Channel Header (MCH) as well.

GAL has several limitations:

• It must be located at the bottom of a label stack for its chief use case of MPLS-TP. An LSR needs to scan the entire label stack to be able to identify the presence of a new header. This can impact the performance when the label stack is deep.
• When GAL is present, the first nibble of the word following the GAL needs to be checked to determine the header type. Since the value of the nibble cannot be greater than 3, this approach is neither scalable nor reliable.
• By design, GAL can only indicate the presence of a single header. Therefore, the solution alone is not sufficient to support adding multiple headers at the same time.
• The presence of GAL makes the network load balancing or deep packet inspection based on upper layer protocol headers and payload difficult.

In addition to the above limitations, it is not desirable to keep overloading GAL with new semantics. Instead of trying to patch on existing schemes, we propose a general mechanism to solve the above mentioned issues and create new innovation opportunities. We derive our scheme from the experience of the IPv4 to IPv6 evolution. The adoption of IPv6 is gaining its momentum. Ironically, this is not due much to the extended address space over IPv4. One true power of IPv6 is that it supports extension headers, which offer a huge innovation potential (e.g, network security, SRv6, network programming, SFC, etc.). It is straightforward to introduce new in-network services into IPv6 networks through extension headers. For example, it has been proposed to carry IOAM header and NSH as new extension headers in IPv6 networks.

Nevertheless, IPv6 is not perfect either. For one thing, IPv6's header overhead is large compared to MPLS. We would like to retain the header compactness in MPLS networks. On the other hand, IPv6's extension headers are chained with the original upper layer protocol headers in a flat stack. One has to scan all the extension headers to access the upper layer protocol headers and the payload. This is inconvenient and raises some performance concerns for some applications (e.g., DPI and ECMP). The new scheme for MPLS header extension needs to address these issues too.

2. MPLS Extension Header

From the previous discussion, we have laid out the design requirements to support extension headers in MPLS networks:

Performance:

If possible, unnecessary label stack scanning and extension header scanning should be avoided.

Scalability:

New applications can be easily supported by introducing new extension headers. Multiple extension headers can be easily stacked together to support multiple services simultaneously.

Backward Compatibility:

Legacy devices which do not recognize the extension header option should still be able to forward the packets as usual. If a device recognize some of the extension headers but not the others in an extension header stack, it can process the known headers only while ignoring the others.

To support the extension header in MPLS, we need to assign a new special label, namely the Extension Header Label (EHL). So far 8 special label values are left unsigned by IANA (which are 4 to 6 and 8 to 12). We believe this use case is significant enough to deserve one dedicated special label. Alternatively, a two label scheme with the use of the extension label (XL) plus an EHL is possible, but it does use one more label. It is also possible to use FEC labels to indicate the presence of extension headers. Although this approach avoid the need of a new special label, it introduces a good deal of complexity into the control plane. In the remaining of the document, we assume a special EHL is assigned.

The format of the MPLS packets with extension headers is shown in Figure 1. An EHL can be located in anywhere in an MPLS label stack. However, if there are legacy devices which do not recognize the EHL in the network, then for backward compatibility, the EHL must be located at the bottom of the stack (i.e., only the MPLS tunnel ends and EHL-aware nodes will look up and process it). Otherwise, the EHL can be located close to the top of the stack for better lookup performance.

The format of an EHL is the same as an MPLS label. The first 20-bit label value will be assigned by IANA. The BoS bit is used to indicate the location of the label. The other fields, CoS and TTL, are unused in the context of EHL.

   0                                  31 
   +--------+--------+--------+--------+  
   |                                   |
   ~     MPLS Label Stack              ~
   |                                   |
   +--------+--------+--------+--------+  
   |     EHL                           |
   +--------+--------+--------+--------+  
   |                                   |
   ~     MPLS Label Stack              ~
   |                                   | 
   +--------+--------+--------+--------+
   | Header of Extension Headers (HEH) |
   +--------+--------+--------+--------+
   |                                   |
   ~     Extension Header (EH) 1       ~
   |                                   |
   +--------+--------+--------+--------+
   ~                                   ~
   +--------+--------+--------+--------+
   |                                   |
   ~     Extension Header (EH) N       ~ 
   |                                   |
   +--------+--------+--------+--------+
   |                                   |
   ~    Upper Layer Protocols/Payload   ~
   |                                   |
   +--------+--------+--------+--------+
   

Figure 1: MPLS with Extension Header

Following the MPLS label stack is the 4-octet Header of Extension Headers (HEH), which indicates the total number of extension headers in this packet, the overall length of the extension headers, and the type of the next header. The format of the HEH is shown in Figure 2.

    0          1          2          3		 
    0123 45678901 234567890123 45678901		 
   +----+--------+------------+--------+  
   | R  | EHCNT  |   EHTLEN   |   NH   |
   +----+--------+------------+--------+
   

Figure 2: HEH Format

The meaning of the fields in an HEH is as follows:

R:

4-bit reserved.

EHCNT:

8-bit unsigned integer for the Extension Header Counter. This field keeps the total number of extension headers included in this packet. It does not count the original upper layer protocol headers.

EHTLEN:

12-bit unsigned integer for the Extension Header Total Length in 4-octet units. This field keeps the total length of the extension headers in this packet, not including the HEH itself.

NH:

8-bit selector for the Next Header. This field identifies the type of the header immediately following the HEH.

The EHCNT field can be used to keep track of the number of extension headers when some headers are inserted or removed at some network nodes. The EHLEN field can help to skip all the extension headers in one step if the original upper layer protocol headers or payload need to be accessed.

The format of an Extension Header (EH) is shown in Figure 3.

    0          1          2          3		 
    01234567 89012345 6789012345678901		 
   +--------+--------+----------------+  
   |  NH    |  HLEN  |                |
   +--------+--------+                +  
   |                                  |
   ~        Header Specific Data      ~
   |                                  |
   +--------+--------+----------------+

Figure 3: EH Format

The meaning of the fields in an EH is as follows:

NH:

8-bit selector for the Next Header. This field identifies the type of the EH immediately following this EH.

HLEN:

8-bit unsigned integer for the Extension Header Length in 4-octet units, not including the first 4 octets.

Header Specific Data:

Variable length field for the specification of the EH. This field is 4-octet aligned.

The extension headers as well as the first original upper layer protocol header are chained together through the NH field in HEH and EHs. The encoding of NH uses the same values as the IPv4 protocol field. Values for new EH types shall be assigned by IANA.

Specifically, the NH field of the last EH in a chain can have two special values, which shall be assigned by IANA:

NONE:

Indicates that there is no any other header and payload after this header. This can be used to transport packets with only extension header(s).

UNKNOWN:

Indicates that the type of the header after this header is unknown. This is intended to be compatible with the original MPLS design in which the upper layer protocol type is unknown from the MPLS header alone.

3. Operation on MPLS Extension Headers

When the first EH X needs to be added to an MPLS packet, an EHL is inserted into the proper location in the MPLS label stack. A HEH is then inserted after the MPLS label stack, in which EHCNT is set to 1, EHTLEN is set to the length of X in 4-octet units, and NH is set to the header value of X. At last, X is inserted after the HEH, in which NH and HELN are set accordingly. Note that if this operation happens at a PE device, the upper layer protocol is known before the MPLS encapsulation, so its value can be saved in the NH field if desired. Otherwise, the NH field is filled with the value of "UNKNOWN".

When an EH Y needs to be added to an MPLS packet which already contains extension header(s), the EHCNT and EHTLEN in the HEH are updated accordingly (i.e., EHCNT is incremented by 1 and EHTLEN is incremented by the size of Y in 4-octet units). Then a proper location for Y in the EH chain is located. Y is inserted at this location. The NH field of Y is copied from the previous EH's NH field (or from the HEH's NH field, if Y is the first EH in the chain). The previous EH's NH value, or, if Y is the first EH in the chain, the HEH's NH, is set to the header value of Y.

Deleting an EH simply reverses the above operation. If the deleted EH is the last one, the EHL and HEH can also be deleted.

When processing an MPLS packet with extension headers, the node needs to scan through the entire EH chain and process the EH one by one. The node should ignore any unrecognized EH.

4. Use Cases

In this section, we show how MPLS extension header can be used to support several new network applications.

In-situ OAM:

In-situ OAM (IOAM) records flow OAM information within user packets while the packets traverse a network. The instruction and collected data are kept in an IOAM header [I-D.ietf-ippm-ioam-data]. When applying IOAM in an MPLS network, the IOAM header can be encapsulated as an MPLS extension header.

NSH:

Network Service Header (NSH) provides a service plane for Service Function Chaining (SFC). NSH maintains the SFC context and metadata. If MPLS is used as the transport protocol for NSH, NSH can be encapsulated as an MPLS extension header.

Network Telemetry and Measurement:

A network telemetry and instruction header can be carried as an extension header to instruct a node what type of network measurements should be done. For example, the method described in [RFC8321] can be implemented in MPLS networks since the EH provides a natural way to color MPLS packets.

Network Security:

Security related functions often require user packets to carry some metadata. In a DoS limiting network architecture, a "packet passport" header is used to embed packet authentication information for each node to verify.

Segment Routing:

MPLS extension header can support the implementation of a new flavor of the MPLS-based segment routing, with better performance and richer functionalities. The details will be described in another draft.

With MPLS extension headers, multiple in-network applications can be stacked together. For example, IOAM and SFC can be applied at the same time to support network OAM and service function chaining. A node can stop scanning the extension header stack if all the known headers it can process have been located. For example, if IOAM is the first EH in a stack and a node is configured to process IOAM only, it will stop searching the EH stack when the IOAM EH is found.

5. Summary

Evidenced by the existing and emerging use cases, MPLS networks need a standard way to support extension headers. In Figure 4, we summarize the potential schemes that allow MPLS packets to carry extension headers and list the main issues for each scheme.

   +---+---------------------------+---------------------------------+  
   |No.|    Description            |       Issues                    |
   +---+---------------------------+---------------------------------+  
   | 1 | GAL + MCH with multi-     |- Label location limitation lead | 
   |   | header extension          |  to performance concern         |
   |   |                           |- Interfere with load balancing  |
   |   |                           |  and DPI functions              |
   |   |                           |- Overlaod GAL semantics         | 
   |   |                           |- Need standard extension        |
   +---+---------------------------+---------------------------------+  
   | 2 | GAL + another nibble value|- Same as above                  |
   |   | to encode the EHs (e.g.,  |                                 |
   |   | "0010")                   |                                 |
   +---+---------------------------+---------------------------------+
   | 3 | FEC label to indicate EH  |- Complex control plane          |
   +---+---------------------------+---------------------------------+
   | 4 | XL(15) + EHL              |- One extra label                |
   |   |                           |- Need standard extension        |
   +---+---------------------------+---------------------------------+
   | 5 | Sepcial EHL	           |- Need standard extension        |
   +---+---------------------------+---------------------------------+ 

Figure 4: Potential Schemes for MPLS Extension Headers

Through comprehensive considerations on the pros and cons of each scheme, we currently recommend the scheme No.5. The proposed MPLS extension header scheme provides a generic way to support in-network services and functions in MPLS networks.

6. Security Considerations

TBD

7. IANA Considerations

This document requires IANA to assign a new special MPLS label value ("EHL") which is dedicated to indicate the presence of MPLS extension header(s).

This document also requires IANA to assign two new protocol numbers which are used to indicate no next header ("NONE") or an unknown next header ("UNKNOWN").

The new header type values shall be assigned by IANA on a case-by-case basis.

8. Contributors

The other contributors of this document are listed as follows.

• James Guichard
• Stewart Bryant
• Andrew Malis

9. Acknowledgments

TBD.

10. References

10.1. Normative References

10.2. Informative References

Authors' Addresses