Network Working Group Eric C. Rosen
Internet Draft Yakov Rekhter
Expiration Date: September 1997 Daniel Tappan
Dino Farinacci
Guy Fedorkow
Cisco Systems, Inc.
March 1997
Label Switching: Label Stack Encodings
draft-rosen-tag-stack-01.txt
Status of this Memo
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Abstract
"Label Switching" [1] requires a set of procedures for augmenting
network layer packets with "Label Stacks" (formerly called "Tag
Stacks"), thereby turning them into "Labeled packets". This document
specifies the encoding to be used, on PPP data links and LAN data
links, in order to produce a Labeled packet from a Label Stack and a
network layer packet.
This document also specifies rules and procedures for processing the
various fields of the Label Stack encoding.
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Table of Contents
1 Introduction ........................................... 2
1.1 Specification of Requirements .......................... 3
2 The Label Stack ........................................ 3
2.1 Encoding the Label Stack ............................... 3
2.2 Determining the Network Layer Protocol ................. 6
2.3 Processing the Time to Live Field ...................... 6
2.3.1 Definitions ............................................ 6
2.3.2 Protocol-independent rules ............................. 6
2.3.3 IP-dependent rules ..................................... 7
3 Fragmentation and Path MTU Discovery ................... 7
3.1 Terminology ............................................ 8
3.2 Maximum Initially Labeled IP Datagram Size ............. 9
3.3 When are Labeled IP Datagrams Too Big? ................. 10
3.4 Processing Labeled IP Datagrams which are Too Big ...... 11
3.5 Implications with respect to Path MTU Discovery ........ 12
3.5.1 Tunneling through a Transit Routing Domain ............. 12
3.5.2 Tunneling Private Addresses through a Public Backbone .. 13
4 Transporting Labeled Packets over PPP .................. 13
4.1 Introduction ........................................... 13
4.2 A PPP Network Control Protocol for Label Switching ..... 14
4.3 Sending Labeled Packets ................................ 15
4.4 Label Switching Control Protocol Configuration Options . 15
5 Transporting Labeled Packets over LAN Media ............ 16
6 Security Considerations ................................ 16
7 Authors' Addresses ..................................... 16
8 References ............................................. 17
1. Introduction
[1] describes a set of procedures for augmenting network layer
packets with "Label Stacks" (formerly called "Tag Stacks"), thereby
turning them into "Labeled packets". This document specifies the
encoding to be used, on PPP data links and LAN data links, in order
to produce a Labeled packet from a Label Stack and a network layer
packet.
This document also specifies rules and procedures for processing the
various fields of the Label Stack encoding. Label Switching itself
is independent of any particular network layer protocol; however,
while most of the relevant procedures are independent of the network
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layer protocol, some procedures differ for different protocols. In
this document, we specify the protocol-independent procedures, but we
specify protocol-dependent procedures only for IPv4.
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
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
On both PPP and LAN data links, 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.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Labe=
l
| Label |rsvd |CoS|S| TTL | Stac=
k
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entr=
y
Label: Label Value, 19 bits
rsvd: Reserved, 3 bits
CoS: Class of Service, 2 bits
S: Bottom of Stack, 1 bit
TTL: Time to Live, 7 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.
2. Time to Live (TTL)
This seven-bit field is used to encode a time-to-live value.
The processing of this field is described in section 2.3.
3. Class of Service (CoS)
This two-bit field is used to identify a "Class of Service".
Presumably the setting of this field will 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:
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- 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"
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. Reserved
These three bits are reserved. They MUST be set to zero when
writing, and MUST be ignored when reading.
5. Label Value
This 19-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 this lookup one
learns:
(a) all the information needed to forward the packet
(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 Entries off
the Label Stack, or to push Entries on the Label Stack,
or any combination of these operations.
There are several reserved Label values:
i. A value of 0 represents the "IPv4 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 IP header.
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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, before the
packet is forwarded, the Router Alert Label should be
pushed back onto its Label Stack.
2.2. Determining the Network Layer Protocol
When the last Label is popped from the Label Stack, it is necessary
to determine the particular network layer protocol which is being
carried. Since the Label header carries no explicit field to
identify the network layer header, this must be inferable from the
value of the Label which is popped.
2.3. Processing the Time to Live Field
2.3.1. Definitions
The "incoming TTL" of a Labeled packet is defined to be the value of
the TTL field in the Label Stack Entry which is at the top of the
Label Stack 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.
2.3.2. Protocol-independent rules
If the outgoing TTL of a Labeled packet is 0, then the Labeled packet
MUST NOT be further forwarded; 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 silently discarded, or the packet MAY have its Label Stack
stripped off, and passed as an unlabeled packet to the ordinary
processing for network layer packets which have exceeded their
maximum lifetime in the network. However, even if the Label Stack is
stripped, the packet MUST NOT be further forwarded.
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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.
2.3.3. IP-dependent rules
When an IP packet is first Labeled, the TTL field of the Label Stack
Entry is set to the smaller of 127 and the value of the IP TTL field.
When a Label is popped, and the resulting Label Stack is empty, then:
(a) if the value in the IP TTL field is less than or equal to 127,
it MUST be replaced with the outgoing TTL value, as defined
above;
(b) if the value in the IP TTL field is greater than 127, then the
new value of the IP TTL field MUST be set to:
(Old_IP_TTL_value - 127 + Outgoing_TTL)
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 will be able to
generate IP datagrams that do not need fragmentation, even if they
get Labeled as the traverse the network.
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In general, hosts which do not implement RFC 1191 Path MTU Discovery
send IP datagrams which contain no more than 576 bytes; the
probability that such datagrams will need to get fragmented, even if
they get labeled, is very small, since the MTUs in use on most data
links today are 1500 bytes or greater. 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 significant
risk of fragmentation when such datagrams get labeled.
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.
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 or 802.1p 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 Header and the IP datagram.
- Conventional Maximum Frame Payload Size:
The maximum Frame Payload size allowed by standards. For
example, the Conventional Maximum Frame Payload Size for
ethernet is 1500 bytes.
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- 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 (unless an 802.1q or
802.1p header is present). 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.
- Initially Labeled IP Datagram
Suppose that an unlabeled IP datagram is received at a
particular Label Switching Router (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 Label Switching Router which is capable of
(a) receiving an unlabeled IP datagram,
(b) adding a Label Stack to the datagram, and
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(c) forwarding the resulting Labeled packet,
MUST support a configuration parameter known as the "Maximum IP
Datagram Size for Labeling", which may 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
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
MUST 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 MAY be
considered to be "too big".
A Labeled IP datagram which is not "too big" MUST be transmitted
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without fragmentation.
3.4. Processing Labeled IP Datagrams which are Too Big
If a Labeled IP datagram is "too big", and the DF bit is not set in
its IP header, then the Label Switching Router 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 Label Switching Router in the network which is capable
of adding a Label Stack to an unlabeled IP datagram.
If the Label Switching Router chooses not to discard a Labeled IP
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 header 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.
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",
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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. 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.)
However, 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 route to the packet's source address. If this is
not possible, then the ICMP Destination Unreachable message cannot be
sent to the source.
3.5.1. Tunneling through a Transit Routing Domain
Suppose one is using Label switching to "tunnel" through a transit
routing domain, where the external routes are not leaked into the
domain's interior routers. If a packet needs fragmentation at some
router within the domain, and the packet's source address is an
external address, and the packet's DF bit is set, it is desirable to
be able to originate an ICMP message at that router and have it
routed correctly to the source of the fragmented packet. However,
that source is an external address, which is not known to the
internal routers.
Therefore, in order for Path MTU Discovery to work, in any routing
domain in which external routes are not leaked into the interior
routers, there MUST be a default route which causes all packets
carrying external destination addresses to be sent to a border
router.
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For example, one of the border routers may inject "default" into the
IGP.
3.5.2. Tunneling Private Addresses through a Public Backbone
In other cases where Label switching 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 addresses, and
Label Switching were being used to tunnel those packets through a
public backbone.
In such cases, the Label Switching Router 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.
4. Transporting Labeled Packets over PPP
The Point-to-Point Protocol (PPP) [PPP] 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.
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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
Label Switching Control packets to enable the transmission of Labeled
packets. Once the Label Switching 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 Label Switching 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 Label Switching
The Label Switching Control Protocol (LSCP) 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). LSCP packets may not be exchanged until PPP has reached the
Network-Layer Protocol phase. LSCP packets received before this
phase is reached should be silently discarded.
The Label Switching Control Protocol is exactly the same as the Link
Control Protocol [1] 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 LSCP packet is encapsulated in the PPP Information
field, where the PPP Protocol field indicates type hex 80??
(Label Switching).
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.
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4. Timeouts
LSCP 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 Label Switching 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 00??
(Label Switching -- Unicast) or type hex 00?? (Label Switching --
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.
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 LSCP 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.
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5. Transporting Labeled Packets over LAN Media
A pair of two byte ethertype values will be obtained, one
representing "Label Switching -- Unicast" and one representing "Label
Switching -- Multicast".
These can be used with either the Ethernet encapsulation or the 802.3
SNAP/SAP encapsulation to carry Labeled packets.
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 any VLAN headers
that may exist.
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, 01=
824
E-mail: erosen@cisco.com
Dan Tappan
Cisco Systems, Inc.
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
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Guy Fedorkow
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: fedorkow@cisco.com
8. References
[1] "Tag Switching Architecture - Overview", 1/9/97, draft-rekhter-
tagswitch-arch-00.txt, Rekhter, Davie, Katz, Rosen, Swallow
[2] "Internet Control Message Protocol", RFC 792, 9/81, Postel
[3] "Path MTU Discovery", RFC 1191, 11/90, Mogul & Deering
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