Post-Stack MPLS Network Action (MNA) Solution

Post-Stack MPLS Network Action (MNA) Solution Cisco Systems, Inc.

Canada jrajaman@cisco.com

Cisco Systems, Inc.

Canada rgandhi@cisco.com

Broadcom

royi.zigler@broadcom.com

Juniper Networks

tony.li@tony.li

Huawei Technologies

Beijing China jie.dong@huawei.com

MPLS Working Group This document defines the Post-Stack MPLS Network Action (MNA) solution for carrying Network Actions and Ancillary Data after the MPLS label stack, based on the In-Stack MNA solution defined in "MPLS Network Action (MNA) Sub-Stack Solution." MPLS Network Actions can be used to influence packet forwarding decisions, carry additional Operations, Administration, and Maintenance (OAM) information in the MPLS packet, or perform user-defined operations. This solution document addresses the Post-Stack network action and Post-Stack data specific requirements found in "Requirements for MPLS Network Actions." This document follows the architectural framework for the MPLS Network Actions (MNA) technologies specified in "MPLS Network Actions (MNA) Framework."

Introduction defines the encoding of the MPLS label stack, the basic structure used to define a forwarding path. Forthcoming applications require MPLS packets to perform special network actions and carry optional Ancillary Data (AD) that can affect the packet forwarding decision or trigger Operations, Administration, and Maintenance (OAM) logging, for example. AD can be used to carry additional information, such as for In Situ OAM (IOAM) as described in . User-defined network actions allow local actions to be defined. In some cases, more AD may be required to be carried in the MPLS header, so these kinds of network actions and their AD are encoded after the Bottom of Stack (BOS). This network action with AD is called Post-Stack Data. This document defines the Post-Stack MPLS Network Action (MNA) solution for carrying Network Actions and Ancillary Data after the MPLS label stack. It is based on the In-Stack MNA solution defined in . This solution document addresses the Post-Stack network action and Post-Stack data specific requirements found in . This document follows the architectural framework for the MPLS Network Actions (MNA) technologies specified in .

Conventions Used in This Document

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 when, and only when, they appear in all capitals, as shown here.

Abbreviations The terminology defined in and are used in this document. Abbreviations

Abbreviation	Meaning	Reference
AD	Ancillary Data
bSPL	Base Special Purpose Label
BOS	Bottom Of Stack
HBH	Hop-By-Hop Scope
I2E	Ingress-To-Egress Scope
IHS	I2E, HBH, or Select Scope
ISD	In-Stack Data
LSE	Label Stack Entry
MNA	MPLS Network Actions
NAI	Network Action Indicator
NAL	Network Action Length
NAS	Network Action Sub-Stack
NASL	Network Action Sub-Stack Length
OAM	Operations, Administration, and Maintenance
P	Post-Stack Header Presence Bit	This document
PSD	Post-Stack Data	and
PSH	Post-Stack Header	This document
TC	Traffic Class
TTL	Time To Live

Overview The Post-Stack MNA solution contains two main parts:

Post-Stack Header Presence in In-Stack MNA Sub-Stack
Post-Stack Header Encoding that includes Post-Stack Header Type and Post-Stack Network Actions

Post-Stack Header Presence in In-Stack MNA Sub-Stack A reserved bit (21st bit from the left in LSE Format B ) in the In-Stack Network Action Sub-Stack (NASS) described in is defined as P bit in this document to indicate the presence of the Post-Stack Header in the packet after the BOS. The Post-Stack Header is encoded after the MPLS Label Stack (BOS), either immediately after the BOS (i.e., start offset of 0) or after any other Post-Stack headers that follow the BOS (i.e., start offset of non-zero).

Post-Stack Header Presence in In-Stack MNA Sub-Stack The following flags as defined in are also applicable to Post-Stack Header:

IHS (2 Bit): Indicates the combined scope of the In-Stack and the Post-Stack Network Actions. NASS for each scope with P bit set will have its corresponding Post-Stack Header.
U (1 Bit): Indicates the combined Unknown Action Handling of the In-Stack and the Post-Stack Network Actions.

Post-Stack Header Encoding The Post-Stack Header is encoded after the Bottom of the MPLS Label Stack (BOS). The Post-Stack Header carry one ore more Post-Stack Network Actions and their Ancillary Data. The PSH consist of two main parts:

Post-Stack Header (PSH) Type
Post-Stack Network Action Encoding

Post-Stack Header Type The Post-Stack Header type is the top-header for all the Post-Stack Network Actions that are encoded in the PSH for each scope.

Post-Stack Header Type

TBA1 (4 bits): This first nibble identifies the start of the PSH. A new value (value TBA1) is to be assigned by IANA for PSH. Editor's note: Generic Associated Channel (0001b) or CW (0000b) can be used.
Version (4 bits): This is PSH version. The initial version is set to 0.
PS-HDR-LEN (8 bits): PSH Total Length in 4-octet units that includes Post-Stack Network Actions. This excludes the PSH type header.
TYPE (16 bits): Type is set to MNA-POST-STACK-HDR as value 1 for MNA, to be allocated by IANA.

Post-Stack Network Action Encoding The format shown in Figure 3, encodes a single Post-Stack Network Action. By repeating this format, multiple Post-Stack Network Action and its corresponding Ancillary data can be encoded.

Post-Stack Network Action Encoding

MNA-PS-OP (7 bits): Post-Stack Network Action Opcode. Opcode "0" is reserved and other opcodes will be assigned by IANA accordingly.
R (2 bits): Reserved bits.
PS-NAL (7 bits): Post-Stack Network Action Length in 4-octet units. This excludes the first 4-octets starting MNA-PS-OP.
POST-STACK DATA (16 bits): Post-Stack Data associated with the Post-Stack Network Action.

In-Stack Network Action Special Opcodes

In-Stack Network Action Opcode for PSH Start Offset Opcode: TBA2 Purpose: This opcode carries the start offset of the PSH from BOS. LSE Format: B (defined in ) Data: The data value of the LSE contains the offset from the MPLS BOS in units of 4 octets. This allows the Generic Control Word (0000b) and G-ACh (0001b) fields to be placed immediately after the BOS. In the absence of this opcode, the PSH is encoded immediately after the MPLS BOS. A data value of 1 indicates that the PSH starts 4 octets after the BOS. Scope: This opcode can be used with any scope.

In-Stack Network Action Opcode for PSH Depth Offset Opcode: TBA3 Purpose: This opcode carries the depth of the Post-Stack Header offset from BOS. LSE Format: B or C (defined in ) Data: The data value of the LSE contains the offset from the MPLS BOS in units of 4 octets. A data value of 5 indicates that the Post-Stack Header ends 20 octets after the BOS. Scope: This opcode can be used with any scope.

Procedure

Processing Rules for P Bit The P bit MUST be set to 1 in In-Stack Network Action Sub-Stack when corresponding Post-Stack Header is added in the packet. By default, the PSH starts immediately after the BOS. The offset for the PSH that does not start immediately after the BOS is indicated using the PSH Start Offset Opcode TBA2. The P bit MUST be set to 1 when the network action with opcode TBA2 is added to the In-Stack Network Action Sub-Stack. The node that recognizes the network action with Opcode TBA2 MUST process the packet according to the U flag if the P bit is not set. The P bit MUST be set to 1 when the network action with opcode TBA3 is added to the In-Stack Network Action Sub-Stack. The node that recognizes the network action with Opcode TBA3 MUST process the packet according to the U flag if the P bit is not set. The node that supports the P bit, processes the Post-Stack Network Actions in the Post-Stack Header as defined in this document. Conversely, the node that does not support the P bit, will skip processing the Post-Stack Header altogether.

Network Action Processing Order The Post-Stack Network Actions are processed in the same order they are encoded after the BOS. By default, they are processed after the In-Stack Network Actions in the Network Action Sub-Stack. However, Post-Stack Network Action Opcodes for applications (such as IOAM as described in ) can be added in the In-Stack Network Action Sub-Stack to process Post-Stack Network Actions in a certain order with respect to the Opcodes in the In-Stack Network Action Sub-Stack.

Node Capability Signaling The ingress node that is adding a Post-Stack Header MUST ensure that the egress node is capable of Post-Stack MNA and can remove the PSH from the packet.

Each participating node MUST signal the network actions that it supports.
Each participating node MUST signal its "Readable Label Depth including Post-Stack Header" that can be encoded.

The above capability signaling will be added in appropriate protocols. Signaling details are outside the scope of this document.

Node Responsibilities This section defines the specific responsibilities for nodes along an MPLS path for processing a Post-Stack Header.

Encapsulating Node Responsibilities The encapsulating node MAY add Post-Stack Header to the packet in accordance with its policies, the placement restrictions, and the limitations. The encapsulating node MUST NOT add a Post-Stack Header to the packet if the decapsulation node does not support Post-Stack Header. If the encapsulating node is also a transit node, then it MUST also respect transit node responsibilities.

Transit Node Responsibilities A transit node MAY modify the Ancillary Data in the Post-Stack Header. A transit node MUST respect the Unknown Action Handling flag encoded in the corresponding NAS when processing the PSH. A node that removes the last copy of a NAS that has the P bit set MUST remove all Post-Stack Headers.

Penultimate Node Responsibilities In addition to the transit node responsibilities above, the penultimate node MUST NOT remove the last copy of an HBH or I2E NAS and associated PSH when it is exposed after removing the forwarding (transport) label. This allows the egress node to process the NAS and associated PSH.

Decapsulating Node Responsibilities The decapsulating node MUST remove any Post-Stack Header it receives.

Security Considerations The security considerations in , , and also apply to this document.

IANA Considerations

First Nibble for Post-Stack Header This document requests that IANA allocate a value (TBA1) for the First Nibble (value TBA1) for Post-Stack Header from the registry "Post-Stack First Nibble" created by to indicate the start of the PSH after BOS. First Nibble for Post-Stack Header

Value	Description	Reference
TBA1	First Nibble for Post-Stack Header	This document

Post-Stack Header Types Registry This document requests that IANA create a new registry with the name "Post-Stack Header Types" as follows. The registration procedure for this registry is "IETF Review", "Experimental Use" and "Private Use". The fields are "Type" (integer), "Description" (string), and "Reference" (string). The assignments for this registry are: Post-Stack Header Types Registry

Type	Description	Reference
0	Reserved, not to be assigned	This document
1-65520	IETF Review	This document
65521-65524	Experimental Use	This document
65525-65535	Private Use	This document

The initial assignment for this registry is: Post-Stack Header Type For MNA

Type	Description	Reference
1	Post-Stack Header Type For MNA	This document

In-Stack Network Action Opcodes The opcodes for In-Stack Network Action to be allocated by IANA from the "In-Stack Network Action Opcode registry" to be created by in [] as defined in this document as follows. In-Stack Network Action Opcodes

Opcode	Description	Reference
TBA2	Offset for Start of Post-Stack Header	This document
TBA3	Offset for Depth of Post-Stack Header	This document

Post-Stack Network Action Opcodes This document requests that IANA create a new registry with the name "Post-Stack Network Action Opcodes". The registration procedure for this registry is "IETF Review", "Experimental Use" and "Private Use". The fields are "Opcode" (integer), "Description" (string), and "Reference" (string). Opcode is an integer 0-127. The assignments for this registry are: Post-Stack Network Action Opcodes Registry

Opcode	Description	Reference
0	Reserved, not to be assigned	This document
1-110	IETF Review	This document
111-114	Experimental Use	This document
115-126	Private Use	This document
127	Reserved, not to be assigned	This document

Appendix A: Examples

Examples of Post-Stack Header Encoding

NAS that only Indicates Post-Stack Header

NAS that only indicates Post-Stack Header In some cases, the NAS may encode only the presence of Post-Stack Header. The Post-Stack Header starts immediately after the BOS.

NAS that Indicates Post-Stack Header Start Offset

NAS Indicates Post-Stack Header Start Offset The NAS may encode the start offset for the Post-Stack Header with a non-zero value, for example, when it is after other header such as GACH or CW header. In this example, the PSH starts at offset of 8 bytes after the BoS.

Post-Stack Network Actions with Two Opcodes

Post-Stack NA with Two Opcodes This is an example, Post-Stack Header encodes two different Post-Stack Network Actions. Details:

PS-HDR-LEN=3: This is the Total Length of Post-Stack Header.
MNA-PS-OP=2: Post-Stack NA Opcode 2.
PS-NAL=0: Post-Stack Network Action does not contain any additional data.
MNA-PS-OP=3: Post-Stack NA Opcode 3.
PS-NAL=1: Post-Stack Network Action contains 1 additional 4-octet Ancillary data.

Post-Stack Network Action with two Different Scopes

Post-Stack NA with two Different Scopes This is an example of Post-Stack Header encoding, that encode two different different scoped Post-Stack Network Actions. The first scope is Hop-By-Hop and the second scope is Ingress-To-Egress scoped Post-Stack Network Action. Details:

The offset of the Hop-By-Hop scoped Post-Stack Network Action 0.
Opcode TBA2 carries the offset of the Ingress-To-Egress scoped Post-Stack Network Action. The data is 2, i.e., the Post-Stack Header starts 8 bytes after the MPLS Bottom of Stack.

In-Stack and Post-Stack Network Actions

NAS with In-Stack and Post-Stack NAs

NAS with In-Stack and Post-Stack NAs In some cases, the NAS may encode In-Stack NAs and indicate the presence of Post-Stack Header. The IHS field indicates the scope of both the In-Stack and Post-Stack NAs.

NASes with Different In-Stack and Post-Stack Scopes

NASes with Different In-Stack and Post-Stack Scopes In some cases the label stack may need to carry In-Stack NAs with Hop-By-Hop scope and Post-Stack NAs with I2E scope. In this case, there will be two NASes in the label stack. In this case, the first NAS will encode the In-Stack NA with the Hop-By-Hop scope and the second NAS will encode the presence of I2E scoped Post-Stack NAs.

References Normative References &I-D.ietf-mpls-mna-fwk; &I-D.ietf-mpls-mna-requirements; &I-D.ietf-mpls-mna-hdr; &I-D.ietf-mpls-1stnibble; &RFC2119; &RFC3032; &RFC4385; &RFC5462; &RFC5586; &RFC6291; &RFC8174; &RFC9017; Informative References &I-D.ietf-mpls-mna-usecases;

Acknowledgments The authors would like to thank the authors and contributors of the draft-ietf-mpls-mna-hdr as this document borrows some text from the earlier version of that document. The authors would like to thank Greg Mirsky, Loa Andersson, Haoyu Song, and Joel Halpern for reviewing this document and providing many useful comments.

Contributors The following people have substantially contributed to this document: