Network Working Group Eric C. Rosen
Internet Draft Yakov Rekhter
Expiration Date: January 1999 Daniel Tappan
Dino Farinacci
Guy Fedorkow
Cisco Systems, Inc.
Tony Li
Juniper Networks, Inc.
Alex Conta
Lucent Technologies
July 1998
MPLS Label Stack Encoding
draft-ietf-mpls-label-encaps-02.txt
Status of this Memo
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Abstract
"Multi-Protocol Label Switching (MPLS)" [1,2,3] requires a set of
procedures for augmenting network layer packets with "label stacks",
thereby turning them into "labeled packets". Routers which support
MPLS are known as "Label Switching Routers", or "LSRs". In order to
transmit a labeled packet on a particular data link, an LSR must
support an encoding technique which, given a label stack and a
network layer packet, produces a labeled packet. This document
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specifies the encoding to be used by an LSR in order to transmit
labeled packets on PPP data links, on LAN data links, and possibly on
other data links as well. On some data links, the label at the top
of the stack may be encoded in a different manner, but the techniques
described here MUST be used to encode the remainder of the label
stack. This document also specifies rules and procedures for
processing the various fields of the label stack encoding.
Table of Contents
1 Introduction ........................................... 3
1.1 Specification of Requirements .......................... 3
2 The Label Stack ........................................ 4
2.1 Encoding the Label Stack ............................... 4
2.2 Determining the Network Layer Protocol ................. 7
2.3 Generating ICMP Messages for Labeled IP Packets ........ 8
2.3.1 Tunneling through a Transit Routing Domain ............. 8
2.3.2 Tunneling Private Addresses through a Public Backbone .. 9
2.4 Processing the Time to Live Field ...................... 9
2.4.1 Definitions ............................................ 9
2.4.2 Protocol-independent rules ............................. 9
2.4.3 IP-dependent rules ..................................... 10
2.4.4 Translating Between Different Encapsulations ........... 10
3 Fragmentation and Path MTU Discovery ................... 11
3.1 Terminology ............................................ 12
3.2 Maximum Initially Labeled IP Datagram Size ............. 13
3.3 When are Labeled IP Datagrams Too Big? ................. 14
3.4 Processing Labeled IPv4 Datagrams which are Too Big .... 14
3.5 Processing Labeled IPv6 Datagrams which are Too Big .... 15
3.6 Implications with respect to Path MTU Discovery ........ 16
4 Transporting Labeled Packets over PPP .................. 17
4.1 Introduction ........................................... 17
4.2 A PPP Network Control Protocol for MPLS ................ 17
4.3 Sending Labeled Packets ................................ 18
4.4 Label Switching Control Protocol Configuration Options . 19
5 Transporting Labeled Packets over LAN Media ............ 19
6 Security Considerations ................................ 19
7 Authors' Addresses ..................................... 19
8 References ............................................. 20
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1. Introduction
"Multi-Protocol Label Switching (MPLS)" [1,2,3] requires a set of
procedures for augmenting network layer packets with "label stacks",
thereby turning them into "labeled packets". Routers which support
MPLS are known as "Label Switching Routers", or "LSRs". In order to
transmit a labeled packet on a particular data link, an LSR must
support an encoding technique which, given a label stack and a
network layer packet, produces a labeled packet.
This document specifies the encoding to be used by an LSR in order to
transmit labeled packets on PPP data links and on LAN data links.
The specified encoding may also be useful for other data links as
well.
This document also specifies rules and procedures for processing the
various fields of the label stack encoding. Since MPLS is
independent of any particular network layer protocol, the majority of
such procedures are also protocol-independent. A few, however, do
differ for different protocols. In this document, we specify the
protocol-independent procedures, and we specify the protocol-
dependent procedures for IPv4 and IPv6.
LSRs that are implemented on certain switching devices (such as ATM
switches) may use different encoding techniques for encoding the top
one or two entries of the label stack. When the label stack has
additional entries, however, the encoding technique described in this
document MUST be used for the additional label stack entries.
1.1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST
This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.
MUST NOT
This phrase means that the definition is an absolute prohibition
of the specification.
SHOULD
This word, or the adjective "recommended", means that there may
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exist valid reasons in particular circumstances to ignore this
item, but the full implications must be understood and carefully
weighed before choosing a different course.
MAY
This word, or the adjective "optional", means that this item is
one of an allowed set of alternatives. An implementation which
does not include this option MUST be prepared to interoperate
with another implementation which does include the option.
2. The Label Stack
2.1. Encoding the Label Stack
The label stack is represented as a sequence of "label stack
entries". Each label stack entry is represented by 4 octets. This
is shown in Figure 1.
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 | CoS |S| TTL | Stack
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry
Label: Label Value, 20 bits
CoS: Class of Service, 3 bits
S: Bottom of Stack, 1 bit
TTL: Time to Live, 8 bits
Figure 1
The label stack entries appear AFTER the data link layer headers, but
BEFORE any network layer headers. The top of the label stack appears
earliest in the packet, and the bottom appears latest. The network
layer packet immediately follows the label stack entry which has the
S bit set.
Each label stack entry is broken down into the following fields:
1. Bottom of Stack (S)
This bit is set to one for the last entry in the label stack
(i.e., for the bottom of the stack), and zero for all other
label stack entries.
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2. Time to Live (TTL)
This eight-bit field is used to encode a time-to-live value.
The processing of this field is described in section 2.4.
3. Class of Service (CoS)
This three-bit field is used to identify a "Class of Service".
The setting of this field is intended to affect the scheduling
and/or discard algorithms which are applied to the packet as it
is transmitted through the network.
When an unlabeled packet is initially labeled, the value
assigned to the CoS field in the label stack entry is
determined by policy. Some possible policies are:
- the CoS value is a function of the IP ToS value
- the CoS value is a function of the packet's input interface
- the CoS value is a function of the "flow type"
Of course, many other policies are also possible.
When an additional label is pushed onto the stack of a packet
that is already labeled:
- in general, the value of the CoS field in the new top stack
entry should be equal to the value of the CoS field of the
old top stack entry;
- however, in some cases, most likely at boundaries between
network service providers, the value of the CoS field in
the new top stack entry may be determined by policy.
4. Label Value
This 20-bit field carries the actual value of the Label.
When a labeled packet is received, the label value at the top
of the stack is looked up. As a result of a successful lookup
one learns:
(a) information needed to forward the packet, such as the
next hop and the outgoing data link encapsulation;
however, the precise queue to put the packet on, or
information as to how to schedule the packet, may be a
function of both the label value AND the CoS field
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value;
(b) the operation to be performed on the label stack before
forwarding; this operation may be to replace the top
label stack entry with another, or to pop an entry off
the label stack, or to replace the top label stack entry
and then to push one or more additional entries on the
label stack.
There are several reserved label values:
i. A value of 0 represents the "IPv4 Explicit NULL Label".
When this label value is the sole label stack entry, it
indicates that the label stack must be popped, and the
forwarding of the packet must then be based on the IPv4
header. When this label value is not the sole label
stack entry, it indicates that the label stack must be
popped, and the forwarding of the packet must then be
based on the label value which then rises to the top of
the stack.
ii. A value of 1 represents the "Router Alert Label". This
label value is legal anywhere in the label stack except
at the bottom. When a received packet contains this
label value at the top of the label stack, it is
delivered to a local software module for processing.
The actual forwarding of the packet is determined by
the label beneath it in the stack. However, if the
packet is forwarded further, the Router Alert Label
should be pushed back onto the label stack before
forwarding. The use of this label is analogous to the
use of the "Router Alert Option" in IP packets [7].
Since this label cannot occur at the bottom of the
stack, it is not associated with a particular network
layer protocol.
iii. A value of 2 represents the "IPv6 Explicit NULL Label".
This label value is only legal when it is the sole
label stack entry. It indicates that the label stack
must be popped, and the forwarding of the packet must
then be based on the IPv6 header.
iv. A value of 3 represents the "Implicit NULL Label".
This is a label that an LSR may assign and distribute,
but which never actually appears in the encapsulation.
When an LSR would otherwise replace the label at the
top of the stack with a new label, but the new label is
"Implicit NULL", the LSR will pop the stack instead of
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doing the replacement. Although this value may never
appear in the encapsulation, it needs to be specified
in the Label Distribution Protocol, so a value is
reserved.
v. Values 4-16 are reserved.
2.2. Determining the Network Layer Protocol
When the last label is popped from a packet's label stack (resulting
in the stack being emptied), further processing of the packet is
based on the packet's network layer header. The LSR which pops the
last label off the stack must therefore be able to identify the
packet's network layer protocol. However, the label stack does not
contain any field which explicitly identifies the network layer
protocol. This means that the identity of the network layer protocol
must be inferable from the value of the label which is popped from
the bottom of the stack, possibly along with the contents of the
network layer header itself.
Therefore, when the first label is pushed onto a network layer
packet, either the label must be one which is used ONLY for packets
of a particular network layer, or the label must be one which is used
ONLY for a specified set of network layer protocols, where packets of
the specified network layers can be distinguished by inspection of
the network layer header. Furthermore, whenever that label is
replaced by another label value during a packet's transit, the new
value must also be one which meets the same criteria. If these
conditions are not met, the LSR which pops the last label off a
packet will not be able to identify the packet's network layer
protocol.
Adherence to these conditions does not necessarily enable
intermediate nodes to identify a packet's network layer protocol.
Under ordinary conditions, this is not necessary, but there are error
conditions under which it is desirable. For instance, if an
intermediate LSR determines that a labeled packet is undeliverable,
it may be desirable for that LSR to generate error messages which are
specific to the packet's network layer. The only means the
intermediate LSR has for identifying the network layer is inspection
of the top label and the network layer header. So if intermediate
nodes are to be able to generate protocol-specific error messages for
labeled packets, all labels in the stack must meet the criteria
specified above for labels which appear at the bottom of the stack.
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2.3. Generating ICMP Messages for Labeled IP Packets
Section 2.4 and section 3 discuss situations in which it is desirable
to generate ICMP messages for labeled IP packets. In order for a
particular LSR to be able to generate an ICMP packet and have that
packet sent to the source of the IP packet, two conditions must hold:
1. it must be possible for that LSR to determine that a particular
labeled packet is an IP packet;
2. it must be possible for that LSR to route to the packet's IP
source address.
Condition 1 is discussed in section 2.2. The following two
subsections discuss condition 2. However, there will be some cases
in which condition 2 does not hold at all, and in these cases it will
not be possible to generate the ICMP message.
2.3.1. Tunneling through a Transit Routing Domain
Suppose one is using MPLS to "tunnel" through a transit routing
domain, where the external routes are not leaked into the domain's
interior routers. For example, the interior routers may be running
OSPF, and may only know how to reach destinations within that OSPF
domain. The domain might contain several Autonomous System Border
Routers (ASBRs), which talk BGP to each other. However, in this
example the routes from BGP are not distributed into OSPF, and the
LSRs which are not ASBRs do not run BGP.
In this example, only an ASBR will know how to route to the source of
some arbitrary packet. If an interior router needs to send an ICMP
message to the source of an IP packet, it will not know how to route
the ICMP message.
One solution is to have one or more of the ASBRs inject "default"
into the IGP. (N.B.: this does NOT require that there be a "default"
carried by BGP.) This would then ensure that any unlabeled packet
which must leave the domain (such as an ICMP packet) gets sent to a
router which has full routing information. The routers with full
routing information will label the packets before sending them back
through the transit domain, so the use of default routing within the
transit domain does not cause any loops.
This solution only works for packets which have globally unique
addresses, and for networks in which all the ASBRs have complete
routing information. The next subsection describes a solution which
works when these conditions do not hold.
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2.3.2. Tunneling Private Addresses through a Public Backbone
In some cases where MPLS is used to tunnel through a routing domain,
it may not be possible to route to the source address of a fragmented
packet at all. This would be the case, for example, if the IP
addresses carried in the packet were private (i.e., not globally
unique) addresses, and MPLS were being used to tunnel those packets
through a public backbone. Default routing to an ASBR will not work
in this environment.
In this environment, in order to send an ICMP message to the source
of a packet, one can copy the label stack from the original packet to
the ICMP message, and then label switch the ICMP message. This will
cause the message to proceed in the direction of the original
packet's destination, rather than its source. Unless the message is
label switched all the way to the destination host, it will end up,
unlabeled, in a router which does know how to route to the source of
original packet, at which point the message will be sent in the
proper direction.
2.4. Processing the Time to Live Field
2.4.1. Definitions
The "incoming TTL" of a labeled packet is defined to be the value of
the TTL field of the top label stack entry when the packet is
received.
The "outgoing TTL" of a labeled packet is defined to be the larger
of:
(a) one less than the incoming TTL,
(b) zero.
2.4.2. Protocol-independent rules
If the outgoing TTL of a labeled packet is 0, then the labeled packet
MUST NOT be further forwarded; nor may the label stack be stripped
off and the packet forwarded as an unlabeled packet. The packet's
lifetime in the network is considered to have expired.
Depending on the label value in the label stack entry, the packet MAY
be simply discarded, or it may be passed to "ordinary" network layer
for error processing (e.g., for the generation of an ICMP error
message, see section 2.3).
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When a labeled packet is forwarded, the TTL field of the label stack
entry at the top of the label stack must be set to the outgoing TTL
value.
Note that the outgoing TTL value is a function solely of the incoming
TTL value, and is independent of whether any labels are pushed or
popped before forwarding. There is no significance to the value of
the TTL field in any label stack entry which is not at the top of the
stack.
2.4.3. IP-dependent rules
We define the "IP TTL" field to be the value of the IPv4 TTL field,
or the value of the IPv6 Hop Limit field, whichever is applicable.
When an IP packet is first labeled, the TTL field of the label stack
entry MUST BE set to the value of the IP TTL field. (If the IP TTL
field needs to be decremented, as part of the IP processing, it is
assumed that this has already been done.)
When a label is popped, and the resulting label stack is empty, then
the value of the IP TTL field MUST BE replaced with the outgoing TTL
value, as defined above. In IPv4 this also requires modification of
the IP header checksum.
2.4.4. Translating Between Different Encapsulations
Sometimes an LSR may receive a labeled packet over, say, a label
switching controlled ATM (LC-ATM) interface [11], and may need to
send it out over a PPP or LAN link. Then the incoming packet will
not be received using the encapsulation specified in this document,
but the outgoing packet will be sent using the encapsulation
specified in this document.
In this case, the value of the "incoming TTL" is determined by the
procedures used for carrying labeled packets on, e.g., LC-ATM
interfaces. TTL processing then proceeds as described above.
Sometimes an LSR may receive a labeled packet over a PPP or a LAN
link, and may need to send it out, say, an LC-ATM interface. Then
the incoming packet will be received using the encapsulation
specified in this document, but the outgoing packet will not be send
using the encapsulation specified in this document. In this case,
the procedure for carrying the value of the "outgoing TTL" is
determined by the procedures used for carrying labeled packets on,
e.g., LC-ATM interfaces.
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3. Fragmentation and Path MTU Discovery
Just as it is possible to receive an unlabeled IP datagram which is
too large to be transmitted on its output link, it is possible to
receive a labeled packet which is too large to be transmitted on its
output link.
It is also possible that a received packet (labeled or unlabeled)
which was originally small enough to be transmitted on that link
becomes too large by virtue of having one or more additional labels
pushed onto its label stack. In label switching, a packet may grow
in size if additional labels get pushed on. Thus if one receives a
labeled packet with a 1500-byte frame payload, and pushes on an
additional label, one needs to forward it as frame with a 1504-byte
payload.
This section specifies the rules for processing labeled packets which
are "too large". In particular, it provides rules which ensure that
hosts implementing RFC 1191 Path MTU Discovery, and hosts using IPv6,
will be able to generate IP datagrams that do not need fragmentation,
even if they get labeled as the traverse the network.
In general, IPv4 hosts which do not implement RFC 1191 Path MTU
Discovery send IP datagrams which contain no more than 576 bytes.
Since the MTUs in use on most data links today are 1500 bytes or
more, the probability that such datagrams will need to get
fragmented, even if they get labeled, is very small.
Some hosts that do not implement RFC 1191 Path MTU Discovery will
generate IP datagrams containing 1500 bytes, as long as the IP Source
and Destination addresses are on the same subnet. These datagrams
will not pass through routers, and hence will not get fragmented.
Unfortunately, some hosts will generate IP datagrams containing 1500
bytes, as long the IP Source and Destination addresses do not have
the same classful network number. This is the one case in which
there is any risk of fragmentation when such datagrams get labeled.
(Even so, fragmentation is not likely unless the packet must traverse
an ethernet of some sort between the time it first gets labeled and
the time it gets unlabeled.)
This document specifies procedures which allow one to configure the
network so that large datagrams from hosts which do not implement
Path MTU Discovery get fragmented just once, when they are first
labeled. These procedures make it possible (assuming suitable
configuration) to avoid any need to fragment packets which have
already been labeled.
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3.1. Terminology
With respect to a particular data link, we can use the following
terms:
- Frame Payload:
The contents of a data link frame, excluding any data link layer
headers or trailers (e.g., MAC headers, LLC headers, 802.1Q
headers, PPP header, frame check sequences, etc.).
When a frame is carrying an an unlabeled IP datagram, the Frame
Payload is just the IP datagram itself. When a frame is carrying
a labeled IP datagram, the Frame Payload consists of the label
stack entries and the IP datagram.
- Conventional Maximum Frame Payload Size:
The maximum Frame Payload size allowed by data link standards.
For example, the Conventional Maximum Frame Payload Size for
ethernet is 1500 bytes.
- True Maximum Frame Payload Size:
The maximum size frame payload which can be sent and received
properly by the interface hardware attached to the data link.
On ethernet and 802.3 networks, it is believed that the True
Maximum Frame Payload Size is 4-8 bytes larger than the
Conventional Maximum Frame Payload Size (as long neither an
802.1Q header nor an 802.1p header is present, and as long as
neither can be added by a switch or bridge while a packet is in
transit to its next hop). For example, it is believed that most
ethernet equipment could correctly send and receive packets
carrying a payload of 1504 or perhaps even 1508 bytes, at least,
as long as the ethernet header does not have an 802.1Q or 802.1p
field.
On PPP links, the True Maximum Frame Payload Size may be
virtually unbounded.
- Effective Maximum Frame Payload Size for Labeled Packets:
This is either be the Conventional Maximum Frame Payload Size or
the True Maximum Frame Payload Size, depending on the
capabilities of the equipment on the data link and the size of
the ethernet header being used.
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- Initially Labeled IP Datagram
Suppose that an unlabeled IP datagram is received at a particular
LSR, and that the the LSR pushes on a label before forwarding the
datagram. Such a datagram will be called an Initially Labeled IP
Datagram at that LSR.
- Previously Labeled IP Datagram
An IP datagram which had already been labeled before it was
received by a particular LSR.
3.2. Maximum Initially Labeled IP Datagram Size
Every LSR which is capable of
(a) receiving an unlabeled IP datagram,
(b) adding a label stack to the datagram, and
(c) forwarding the resulting labeled packet,
MUST support a configuration parameter known as the "Maximum IP
Datagram Size for Labeling", which can be set to a non-negative
value.
If this configuration parameter is set to zero, it has no effect.
If it is set to a positive value, it is used in the following way.
If:
(a) an unlabeled IP datagram is received, and
(b) that datagram does not have the DF bit set in its IP header,
and
(c) that datagram needs to be labeled before being forwarded, and
(d) the size of the datagram (before labeling) exceeds the value
of the parameter,
then
(a) the datagram must be broken into fragments, each of whose size
is no greater than the value of the parameter, and
(b) each fragment must be labeled and then forwarded.
If this configuration parameter is set to a value of 1488, for
example, then any unlabeled IP datagram containing more than 1488
bytes will be fragmented before being labeled. Each fragment will be
capable of being carried on a 1500-byte data link, without further
fragmentation, even if as many as three labels are pushed onto its
label stack.
In other words, setting this parameter to a non-zero value allows one
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to eliminate all fragmentation of Previously Labeled IP Datagrams,
but it may cause some unnecessary fragmentation of Initially Labeled
IP Datagrams.
Note that the parameter has no effect on IP Datagrams that have the
DF bit set, which means that it has no effect on Path MTU Discovery.
3.3. When are Labeled IP Datagrams Too Big?
A labeled IP datagram whose size exceeds the Conventional Maximum
Frame Payload Size of the data link over which it is to be forwarded
MAY be considered to be "too big".
A labeled IP datagram whose size exceeds the True Maximum Frame
Payload Size of the data link over which it is to be forwarded MUST
be considered to be "too big".
A labeled IP datagram which is not "too big" MUST be transmitted
without fragmentation.
3.4. Processing Labeled IPv4 Datagrams which are Too Big
If a labeled IPv4 datagram is "too big", and the DF bit is not set in
its IP header, then the LSR MAY discard the datagram.
Note that discarding such datagrams is a sensible procedure only if
the "Maximum Initially Labeled IP Datagram Size" is set to a non-zero
value in every LSR in the network which is capable of adding a label
stack to an unlabeled IP datagram.
If the LSR chooses not to discard a labeled IPv4 datagram which is
too big, or if the DF bit is set in that datagram, then it MUST
execute the following algorithm:
1. Strip off the label stack entries to obtain the IP datagram.
2. Let N be the number of bytes in the label stack (i.e, 4 times
the number of label stack entries).
3. If the IP datagram does NOT have the "Don't Fragment" bit set
in its IP header:
a. convert it into fragments, each of which MUST be at least
N bytes less than the Effective Maximum Frame Payload
Size.
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b. Prepend each fragment with the same label header that
would have been on the original datagram had
fragmentation not been necessary.
c. Forward the fragments
4. If the IP datagram has the "Don't Fragment" bit set in its IP
header:
a. the datagram MUST NOT be forwarded
b. Create an ICMP Destination Unreachable Message:
i. set its Code field (RFC 792) to "Fragmentation
Required and DF Set",
ii. set its Next-Hop MTU field (RFC 1191) to the
difference between the Effective Maximum Frame
Payload Size and the value of N
c. If possible, transmit the ICMP Destination Unreachable
Message to the source of the of the discarded datagram.
3.5. Processing Labeled IPv6 Datagrams which are Too Big
To process a labeled IPv6 datagram which is too big, an LSR MUST
execute the following algorithm:
1. Strip off the label stack entries to obtain the IP datagram.
2. Let N be the number of bytes in the label stack (i.e, 4 times
the number of label stack entries).
3. If the IP datagram contains more than 1280 bytes (not counting
the label stack entries), then:
a. Create an ICMP Packet Too Big Message, and set its Next-
Hop MTU field to the difference between the Effective
Maximum Frame Payload Size and the value of N
b. If possible, transmit the ICMP Packet Too Big Message to
the source of the datagram.
c. discard the labeled IPv6 datagram.
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4. If the IP datagram is not larger than 1280 octets, then
a. Convert it into fragments, each of which MUST be at least
N bytes less than the Effective Maximum Frame Payload
Size.
b. Prepend each fragment with the same label header that
would have been on the original datagram had
fragmentation not been necessary.
c. Forward the fragments.
Reassembly of the fragments will be done at the destination
host.
3.6. Implications with respect to Path MTU Discovery
The procedures described above for handling datagrams which have the
DF bit set, but which are "too large", have an impact on the Path MTU
Discovery procedures of RFC 1191. Hosts which implement these
procedures will discover an MTU which is small enough to allow n
labels to be pushed on the datagrams, without need for fragmentation,
where n is the number of labels that actually get pushed on along the
path currently in use.
In other words, datagrams from hosts that use Path MTU Discovery will
never need to be fragmented due to the need to put on a label header,
or to add new labels to an existing label header. (Also, datagrams
from hosts that use Path MTU Discovery generally have the DF bit set,
and so will never get fragmented anyway.)
Note that Path MTU Discovery will only work properly if, at the point
where a labeled IP Datagram's fragmentation needs to occur, it is
possible to cause an ICMP Destination Unreachable message to be
routed to the packet's source address. See section 2.3.
If it is not possible to forward an ICMP message from within an MPLS
"tunnel" to a packet's source address, then the LSR at the
transmitting end of the tunnel MUST be able to determine the MTU of
the tunnel as a whole. It SHOULD do this by sending packets through
the tunnel to the tunnel's receiving endpoint, and performing Path
MTU Discovery with those packets. Then any time the transmitting
endpoint of the tunnel needs to send a packet into the tunnel, and
that packet has the DF bit set, and it exceeds the tunnel MTU, the
transmitting endpoint of the tunnel MUST send the ICMP Destination
Unreachable message to the source, with code "Fragmentation Required
and DF Set", and the Next-Hop MTU Field set as described above.
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4. Transporting Labeled Packets over PPP
The Point-to-Point Protocol (PPP) [8] provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP
defines an extensible Link Control Protocol, and proposes a family of
Network Control Protocols for establishing and configuring different
network-layer protocols.
This section defines the Network Control Protocol for establishing
and configuring label Switching over PPP.
4.1. Introduction
PPP has three main components:
1. A method for encapsulating multi-protocol datagrams.
2. A Link Control Protocol (LCP) for establishing, configuring,
and testing the data-link connection.
3. A family of Network Control Protocols for establishing and
configuring different network-layer protocols.
In order to establish communications over a point-to-point link, each
end of the PPP link must first send LCP packets to configure and test
the data link. After the link has been established and optional
facilities have been negotiated as needed by the LCP, PPP must send
"MPLS Control Protocol" packets to enable the transmission of labeled
packets. Once the "MPLS Control Protocol" has reached the Opened
state, labeled packets can be sent over the link.
The link will remain configured for communications until explicit LCP
or MPLS Control Protocol packets close the link down, or until some
external event occurs (an inactivity timer expires or network
administrator intervention).
4.2. A PPP Network Control Protocol for MPLS
The MPLS Control Protocol (MPLSCP) is responsible for enabling and
disabling the use of label switching on a PPP link. It uses the same
packet exchange mechanism as the Link Control Protocol (LCP). MPLSCP
packets may not be exchanged until PPP has reached the Network-Layer
Protocol phase. MPLSCP packets received before this phase is reached
should be silently discarded.
The MPLS Control Protocol is exactly the same as the Link Control
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Protocol [8] with the following exceptions:
1. Frame Modifications
The packet may utilize any modifications to the basic frame
format which have been negotiated during the Link Establishment
phase.
2. Data Link Layer Protocol Field
Exactly one MPLSCP packet is encapsulated in the PPP
Information field, where the PPP Protocol field indicates type
hex 8281 (MPLS).
3. Code field
Only Codes 1 through 7 (Configure-Request, Configure-Ack,
Configure-Nak, Configure-Reject, Terminate-Request, Terminate-
Ack and Code-Reject) are used. Other Codes should be treated
as unrecognized and should result in Code-Rejects.
4. Timeouts
MPLSCP packets may not be exchanged until PPP has reached the
Network-Layer Protocol phase. An implementation should be
prepared to wait for Authentication and Link Quality
Determination to finish before timing out waiting for a
Configure-Ack or other response. It is suggested that an
implementation give up only after user intervention or a
configurable amount of time.
5. Configuration Option Types
None.
4.3. Sending Labeled Packets
Before any labeled packets may be communicated, PPP must reach the
Network-Layer Protocol phase, and the MPLS Control Protocol must
reach the Opened state.
Exactly one labeled packet is encapsulated in the PPP Information
field, where the PPP Protocol field indicates either type hex 0281
(MPLS Unicast) or type hex 0283 (MPLS Multicast). The maximum length
of a labeled packet transmitted over a PPP link is the same as the
maximum length of the Information field of a PPP encapsulated packet.
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The format of the Information field itself is as defined in section
2.
Note that two codepoints are defined for labeled packets; one for
multicast and one for unicast. Once the MPLSCP has reached the
Opened state, both label Switched multicasts and label Switched
unicasts can be sent over the PPP link.
4.4. Label Switching Control Protocol Configuration Options
There are no configuration options.
5. Transporting Labeled Packets over LAN Media
Exactly one labeled packet is carried in each frame.
The label stack entries immediately precede the network layer header,
and follow any data link layer headers, including, e.g., any 802.1Q
headers that may exist.
The ethertype value 8847 hex is used to indicate that a frame is
carrying an MPLS unicast packet.
The ethertype value 8848 hex is used to indicate that a frame is
carrying an MPLS multicast packet.
These ethertype values can be used with either the ethernet
encapsulation or the 802.3 SNAP/SAP encapsulation to carry labeled
packets.
6. Security Considerations
Security considerations are not discussed in this document.
7. Authors' Addresses
Eric C. Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: erosen@cisco.com
Dan Tappan
Cisco Systems, Inc.
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Internet Draft draft-ietf-mpls-label-encaps-02.txt July 1998
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: tappan@cisco.com
Dino Farinacci
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
E-mail: dino@cisco.com
Yakov Rekhter
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
E-mail: yakov@cisco.com
Guy Fedorkow
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: fedorkow@cisco.com
Tony Li
Juniper Networks
385 Ravendale Dr.
Mountain View, CA, 94043
E-mail: tli@juniper.net
Alex Conta
Lucent Technologies
300 Baker Avenue
Concord, MA, 01742
E-mail: aconta@lucent.com
8. References
[1], "Multiprotocol Label Switching Architecture", 7/98, draft-ietf-
mpls-arch-02.txt, Rosen, Viswanathan, Callon
[2] "A Framework for Multiprotocol Label Switching", 11/97, draft-
ietf-mpls-framework-02.txt, Callon, Doolan, Feldman, Fredette,
Swallow, Viswanathan
[3] "Tag Switching Architecture - Overview", 7/97, draft-rekhter-
tagswitch-arch-01.txt, Rekhter, Davie, Katz, Rosen, Swallow
[4] "Internet Protocol", RFC 791, 9/81, Postel
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[5] "Internet Control Message Protocol", RFC 792, 9/81, Postel
[6] "Path MTU Discovery", RFC 1191, 11/90, Mogul & Deering
[7] "IP Router Alert Option", RFC 2113, 2/97, Katz
[8] "The Point-to-Point Protocol (PPP)", RFC 1661, 7/94, Simpson
[9] "Internet Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 1885, 12/95, Conta,
Deering
[10] "Path MTU Discovery for IP version 6", [RFC-1981] McCann, J., S.
Deering, J. Mogul
[11] "Use of Label Switching with ATM", draft-davie-mpls-atm-01.txt,
7/98, Davie, Lawrence, McCloghrie, Rekhter, Rosen, Swallow, Doolan
Rosen, et al. [Page 21]
