2. 变更细节
RFC 3032, 3270 和 5129 现根据下列内容予以更新.
2.1. RFC 3032
RFC 3032 在第 4 页声明:
3. Experimental Use
This three-bit field is reserved for experimental use.
该段现更改为:
3. 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.
在 RFC 3032 第 3 页的图 1 中, 标签栈条目的格式规定为:
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
RFC 3032 中的图 1 现更改为与更名为 TC 字段相一致:
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)
注意: 将上图标为 "Figure 1 (new)" 是为了在本文档中区分各图. 在 RFC 3032 中它仍将是 "Figure 1".
2.2. RFC 3270
RFC 3270 在第 6 页写到:
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 现按如下方式更新:
a. 在第 1 节 "Introduction" 末尾增加一段新段落:
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. 在第 1.1 节 "Terminology" 中增加一个新术语:
TC Traffic Class (replaces the term EXP)
c. 在第 1.1 节 "Terminology" 中, 缩写 E-LSP 现理解为:
E-LSP Explicitly TC-encoded-PSC LSP
RFC 3270 第 6 页的第 1.2 节现更改为:
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 现按如下方式更新:
在第 1.1 节 "Background" 末尾增加一段新段落:
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.
RFC 5129 第 7 页第 2 节 (第 5 点) 写到:
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.
RFC 5129 第 2 节 (第 5 点) 现更新为:
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. 本变更的范围
在被本文档明确更新的 RFC 中, 有多处引用 "Exp field", 有时称为 "Exp bits", "EXP bits" 或 "EXP". 在所有这些情形中, 这些引用现均指向 TC 字段.
还有其他 RFC (例如 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] 和 RFC 4761 [RFC4761]) 引用 "Exp field"; 有时称为 "Exp bits", "EXP bits" 和 "EXP". 对所有 RFC (包括但不限于本段提到的那些), 此类引用现均指向 TC 字段.