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2. Details of Change

The three RFCs 3032, 3270, and 5129 are now updated according to the following.

2.1. RFC 3032​

RFC 3032 states on page 4:

  1. Experimental Use

This three-bit field is reserved for experimental use.

This paragraph is now changed to:

  1. Traffic Class (TC) field

This three-bit field is used to carry traffic class information, and the change of the name is applicable to all places it occurs in IETF RFCs and other IETF documents.

RFC 3270 and RFC 5129 update the definition of the TC field and describe how to use the field.

In Figure 1 on page 3 in RFC 3032, the format of a label stack entry is specified as:

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
 |                Label                  | Exp |S|       TTL     | Stack
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry

                     Label:  Label Value, 20 bits
                     Exp:    Experimental Use, 3 bits
                     S:      Bottom of Stack, 1 bit
                     TTL:    Time to Live, 8 bits

                              Figure 1

Figure 1 in RFC 3032 is now changed to match the change of name to TC field:

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
 |                Label                  | TC  |S|       TTL     | Stack
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry

                     Label:  Label Value, 20 bits
                     TC:     Traffic Class field, 3 bits
                     S:      Bottom of Stack, 1 bit
                     TTL:    Time to Live, 8 bits

                               Figure 1 (new)

Note: The designation of the picture above as "Figure 1 (new)" is introduced as a way to distinguish the figures in this document. It will still be "Figure 1" in RFC 3032.

2.2. RFC 3270​

RFC 3270 says on page 6:

1.2 EXP-Inferred-PSC LSPs (E-LSP)

A single LSP can be used to support one or more OAs. Such LSPs can support up to eight BAs of a given FEC, regardless of how many OAs these BAs span. With such LSPs, the EXP field of the MPLS Shim Header is used by the LSR to determine the PHB to be applied to the packet. This includes both the PSC and the drop preference.

We refer to such LSPs as "EXP-inferred-PSC LSPs" (E-LSP), since the PSC of a packet transported on this LSP depends on the EXP field value for that packet.

The mapping from the EXP field to the PHB (i.e., to PSC and drop precedence) for a given such LSP, is either explicitly signaled at label set-up or relies on a pre-configured mapping.

Detailed operations of E-LSPs are specified in section 3 below.

RFC 3270 is now updated like this:

a. A new paragraph is added at the end of Section 1 "Introduction":

The EXP field has been renamed the TC field, and thus all references in RFC 3270 to the EXP field now refer to the TC field.

b. A new term is added to Section 1.1 "Terminology":

TC Traffic Class (replaces the term EXP)

c. In Section 1.1 "Terminology", the acronym E-LSP is now understood to mean:

E-LSP Explicitly TC-encoded-PSC LSP

Section 1.2 on page 6 in RFC 3270 is now changed to:

1.2 Explicitly TC-encoded-PSC LSPs (E-LSP)

The EXP field has been renamed to the TC field, and thus all references in RFC 3270 to EXP field now refer to the TC field. However, we retain the acronym E-LSP (Explicitly TC-encoded-PSC LSP) as the acronym is in widespread use.

A single LSP can be used to support one or more OAs. Such LSPs can support up to eight BAs of a given FEC, regardless of how many OAs these BAs span. With such LSPs, the TC field of the MPLS Shim Header is used by the LSR to determine the PHB to be applied to the packet. This includes both the PSC and the drop preference.

We refer to such LSPs as "Explicitly TC-encoded-PSC LSPs" (E-LSPs), since the PSC of a packet transported on this LSP depends on the TC field (previously called the EXP field) value for that packet.

The mapping from the TC field to the PHB (i.e., to PSC and drop precedence) for a given such LSP is either explicitly signaled at label set-up or relies on a pre-configured mapping.

This is an update to RFC 3032 [RFC3032], in line with the original intent of how this field in the MPLS Shim Header should be used (as a TC field). RFC 3270 has itself been updated by RFC 5129 [RFC5129].

Detailed operations of E-LSPs are specified in Section 3 of RFC 3270.

2.3. RFC 5129​

RFC 5129 is now updated like this:

A new paragraph is added at the end of Section 1.1 "Background":

The EXP field has been renamed to the TC field, and thus all references in RFC 5129 to the EXP field now refer to the TC field.

Section 2 (bullet 5) on page 7 of RFC 5129 says:

o A third possible approach was suggested by [Shayman]. In this scheme, interior LSRs assume that the endpoints are ECN-capable, but this assumption is checked when the final label is popped. If an interior LSR has marked ECN in the EXP field of the shim header, but the IP header says the endpoints are not ECN-capable, the edge router (or penultimate router, if using penultimate hop popping) drops the packet. We recommend this scheme, which we call `per-domain ECT checking', and define it more precisely in the following section. Its chief drawback is that it can cause packets to be forwarded after encountering congestion only to be dropped at the egress of the MPLS domain. The rationale for this decision is given in Section 8.1.

Section 2 (bullet 5) of RFC 5129 is now updated to:

o A third possible approach was suggested by [Shayman]. In this scheme, interior LSRs assume that the endpoints are ECN-capable, but this assumption is checked when the final label is popped. If an interior LSR has marked ECN in the TC field of the shim header, but the IP header says the endpoints are not ECN-capable, the edge router (or penultimate router, if using penultimate hop popping) drops the packet. We recommend this scheme, which we call `per- domain ECT checking', and define it more precisely in the following section. Its chief drawback is that it can cause packets to be forwarded after encountering congestion only to be dropped at the egress of the MPLS domain. The rationale for this decision is given in Section 8.1. This scheme is an update to RFC 3032 [RFC3032] and RFC 3270 [RFC3270].

2.4. The Scope of This Change​

There are several places in the RFCs that are explicitly updated by this document that reference the "Exp field", sometimes they refer to the field as "Exp bits", "EXP bits", or "EXP". In all those instances, the references now reference the TC field.

There are also other RFCs (e.g., RFC 3272 [RFC3272], RFC 3443 [RFC3443], RFC 3469 [RFC3469], RFC 3564 [RFC3564], RFC 3985 [RFC3985], RFC 4182 [RFC4182], RFC 4364 [RFC4364], RFC 4379 [RFC4379], RFC 4448 [RFC4448], and RFC 4761 [RFC4761]) that reference the "Exp field"; sometimes they refer to the field as "Exp bits", "EXP bits", and "EXP". For all RFCs, including but not limited to those mentioned in this paragraph, such references now reference the TC field.