поля заголовка протокола в OpenFlow и Open vSwitch (protocol header fields in OpenFlow and Open vSwitch)
LAYER 2.5: MPLS FIELDS
Summary:
Name Bytes Mask RW? Prereqs NXM/OXM Support
─────────── ──────────────── ───── ──── ──────── ──────────────────────
mpls_label 4 (low 20 bits) no yes MPLS OF 1.2+ and OVS 1.11+
mpls_tc 1 (low 3 bits) no yes MPLS OF 1.2+ and OVS 1.11+
mpls_bos 1 (low 1 bits) no no MPLS OF 1.3+ and OVS 1.11+
mpls_ttl 1 no yes MPLS OVS 2.6+
One or more MPLS headers (more commonly called MPLS labels)
follow an Ethernet type field that specifies an MPLS Ethernet
type [RFC 3032]. Ethertype 0x8847 is used for all unicast.
Multicast MPLS is divided into two specific classes, one of which
uses Ethertype 0x8847 and the other 0x8848 [RFC 5332].
The most common overall packet format is Ethernet II, shown below
(SNAP encapsulation may be used but is not ordinarily seen in
Ethernet networks):
Ethernet MPLS
<------------> <------------>
48 48 16 20 3 1 8
+---+---+------+ +-----+--+-+---+
|dst|src| type | |label|TC|S|TTL| ...
+---+---+------+ +-----+--+-+---+
0x8847
MPLS can be encapsulated inside an 802.1Q header, in which case
the combination looks like this:
Ethernet 802.1Q Ethertype MPLS
<------> <--------> <-------> <------------>
48 48 16 16 16 20 3 1 8
+----+---+ +------+---+ +---------+ +-----+--+-+---+
|dst |src| | TPID |TCI| | type | |label|TC|S|TTL| ...
+----+---+ +------+---+ +---------+ +-----+--+-+---+
0x8100 0x8847
The fields within an MPLS label are:
Label, 20 bits.
An identifier.
Traffic control (TC), 3 bits.
Used for quality of service.
Bottom of stack (BOS), 1 bit (labeled just ``S'' above).
0 indicates that another MPLS label follows this
one.
1 indicates that this MPLS label is the last one in
the stack, so that some other protocol follows this
one.
Time to live (TTL), 8 bits.
Each hop across an MPLS network decrements the TTL
by 1. If it reaches 0, the packet is discarded.
OpenFlow does not make the MPLS TTL available as a
match field, but actions are available to set and
decrement the TTL. Open vSwitch 2.6 and later makes
the MPLS TTL available as an extension.
MPLS Label Stacks
Unlike the other encapsulations supported by OpenFlow and Open
vSwitch, MPLS labels are routinely used in ``stacks'' two or
three deep and sometimes even deeper. Open vSwitch currently
supports up to three labels.
The OpenFlow specification only supports matching on the
outermost MPLS label at any given time. To match on the second
label, one must first ``pop'' the outer label and advance to
another OpenFlow table, where the inner label may be matched. To
match on the third label, one must pop the two outer labels, and
so on.
MPLS Inner Protocol
Unlike all other forms of encapsulation that Open vSwitch and
OpenFlow support, an MPLS label does not indicate what inner
protocol it encapsulates. Different deployments determine the
inner protocol in different ways [RFC 3032]:
• A few reserved label values do indicate an inner
protocol. Label 0, the ``IPv4 Explicit NULL
Label,'' indicates inner IPv4. Label 2, the ``IPv6
Explicit NULL Label,'' indicates inner IPv6.
• Some deployments use a single inner protocol
consistently.
• In some deployments, the inner protocol must be
inferred from the innermost label.
• In some deployments, the inner protocol must be
inferred from the innermost label and the
encapsulated data, e.g. to distinguish between
inner IPv4 and IPv6 based on whether the first
nibble of the inner protocol data are 4 or 6.
OpenFlow and Open vSwitch do not currently support
these cases.
Open vSwitch and OpenFlow do not infer the inner protocol, even
if reserved label values are in use. Instead, the flow table must
specify the inner protocol at the time it pops the bottommost
MPLS label, using the Ethertype argument to the pop_mpls action.
Field Details
MPLS Label Field
Name: mpls_label
Width: 32 bits (only the least-significant 20 bits may be nonzero)
Format: decimal
Masking: not maskable
Prerequisites: MPLS
Access: read/write
OpenFlow 1.0: not supported
OpenFlow 1.1: yes (exact match only)
OXM: OXM_OF_MPLS_LABEL (34) since OpenFlow 1.2 and Open vSwitch
1.11
NXM: none
The least significant 20 bits hold the ``label'' field from the
MPLS label. Other bits are zero:
OXM_OF_MPLS_LABEL
<--------------->
12 20
+--------+--------+
| zero | label |
+--------+--------+
0
Most label values are available for any use by deployments.
Values under 16 are reserved.
MPLS Traffic Class Field
Name: mpls_tc
Width: 8 bits (only the least-significant 3 bits may be nonzero)
Format: decimal
Masking: not maskable
Prerequisites: MPLS
Access: read/write
OpenFlow 1.0: not supported
OpenFlow 1.1: yes (exact match only)
OXM: OXM_OF_MPLS_TC (35) since OpenFlow 1.2 and Open vSwitch
1.11
NXM: none
The least significant 3 bits hold the TC field from the MPLS
label. Other bits are zero:
OXM_OF_MPLS_TC
<------------>
5 3
+--------+-----+
| zero | TC |
+--------+-----+
0
This field is intended for use for Quality of Service (QoS) and
Explicit Congestion Notification purposes, but its particular
interpretation is deployment specific.
Before 2009, this field was named EXP and reserved for
experimental use [RFC 5462].
MPLS Bottom of Stack Field
Name: mpls_bos
Width: 8 bits (only the least-significant 1 bits may be nonzero)
Format: decimal
Masking: not maskable
Prerequisites: MPLS
Access: read-only
OpenFlow 1.0: not supported
OpenFlow 1.1: not supported
OXM: OXM_OF_MPLS_BOS (36) since OpenFlow 1.3 and Open vSwitch
1.11
NXM: none
The least significant bit holds the BOS field from the MPLS
label. Other bits are zero:
OXM_OF_MPLS_BOS
<------------->
7 1
+--------+------+
| zero | BOS |
+--------+------+
0
This field is useful as part of processing a series of incoming
MPLS labels. A flow that includes a pop_mpls action should
generally match on mpls_bos:
• When mpls_bos is 0, there is another MPLS label
following this one, so the Ethertype passed to
pop_mpls should be an MPLS Ethertype. For example:
table=0, dl_type=0x8847, mpls_bos=0,
actions=pop_mpls:0x8847, goto_table:1
• When mpls_bos is 1, this MPLS label is the last
one, so the Ethertype passed to pop_mpls should be
a non-MPLS Ethertype such as IPv4. For example:
table=1, dl_type=0x8847, mpls_bos=1,
actions=pop_mpls:0x0800, goto_table:2
MPLS Time-to-Live Field
Name: mpls_ttl
Width: 8 bits
Format: decimal
Masking: not maskable
Prerequisites: MPLS
Access: read/write
OpenFlow 1.0: not supported
OpenFlow 1.1: not supported
OXM: none
NXM: NXM_NX_MPLS_TTL (30) since Open vSwitch 2.6
Holds the 8-bit time-to-live field from the MPLS label:
NXM_NX_MPLS_TTL
<------------->
8
+---------------+
| TTL |
+---------------+
