Extending the IPv6 Label
Forwarding idea to Edge NetworksTo the parent page (Prefix Label Forwarding (PLF) - Modified Header Forwarding for IPv6)
I intend to write this up as an ID, but for now, this page and its parent is the best account of this work in progress.
Whittle firstname.lastname@example.org 2008-08-06
I contemplate how the proposed 20 bit
Forwarding Label in the IPv6 header (currently the unused Flow Label)
could be used with one half of the range reserved for forwarding
packets in the core (primarily or solely for the Ivip6 scalable routing
solution) with the other half of the range to be used in a similar way,
with greater flexibility, inside any edge network which choses to do
so. Each such edge network would have its own way of using this
half of the range, so it would be purely for forwarding packets within
one edge network (unless several coordinated their routing systems and
their use of this part of the Forwarding Label Range.
obvious use I can think of for this Label
Forwarding in Edge networks is to forward Ivip6 traffic packets
from the border router which receives them from ITRs via the core, to
the correct ETR, wherever that is located inside this recipient
provider network. Being able to use Label Forwarding like this
inside the recipient provider network could be a lot more efficient,
and have greater flexibility, no PMTUD problems etc, compared to
alternative methods of bridging the gap from border router, via the
ETR, to the end-user network.
Note, the bit numbers I
give here are ordinary binary bit numbers,
where 0 is the least significant. I need to update it to also
IETF bit numbers, where 0 is the most significant.
This is not
Forwarding Label could be used in an Edge network>>
A word about namespaces>>
Core routers drop packets with FL
values of 8 0000 and above>>
List of new functions for core routers > FIB functions
> RIB Functions>>
What do we want this to do in Edge Networks? >
Preliminaries > More flexible assignment of prefixes to
the Forwarding Label values > Carrying Ivip6 packets inside the
provider network from the FLPER to the end-user network, via the ETR
> Conclusion for now>>
What about not splitting the 2^20 values
into Core and Edge?
The Ivip6 page discussed how valuable it
would be, for a scalable routing solution, to implement Label Forwarding in the Core
), by adding some new functions
to core routers and by using the current Flow Label for a new purpose -
a 20 bit Forwarding Label.
This page considers in detail how
that could be done, with the bottom half of the range reserved for use
in the core, and the top half for Label
Forwarding in Edge networks
are relatively simple and potentially very useful functions. They
are rather different from any other existing facility, and we need to
think carefully what they might be useful for.
This page is a
speculative discussion based on several assumptions:
as described in the Ivip6 page, it is decided to change the function of
the IPv6 Flow Label into a 20 bit Forwarding Label, for the primary or
sole purpose of using it for an Ivip6-like core-edge separation
scalable routing and addressing solution.
- That it is decided
that all 2^20 combinations will never be needed for this Forwarding
Label in the Core approach, and that it would be acceptable to reserve
half of these for the Core alone. (Maybe it would be decided that
even less would ever be needed, but the following assumes that 2^19 is
a good number for the core.)
- That existing core routers (BGP
border routers and transit routers) are upgraded, where possible to
perform the required new functions - or they are replaced by new models
which perform these functions. (It is tricky having non-upgraded core
routers, but is acceptable as long as no ITRs or ETRs depend on them.)
it is desired to enable each "edge network" to do a similar thing with
its internal routers. An edge network would be defined as a
particular provider network, a particular end-user network with
conventional space or a particular SEN end-user network with the new,
scalable, SPI address space.
- That most of what is needed would
already be implemented in larger routers in general because these
classes of routers could be used in the core.
Therefore little extra functionality (none in the FIB)
beyond that for core routers would need to be added to routers in
general to enable them to do similar work inside each edge network.
it would be best to define an edge network mode of using half the 2^20
In the discussion which follows, I will
assume that the whole 2^20 space of possible values is divided in this
This indicates the Forwarding Label is empty.
packet should be forwarded as per usual: according
0 0000 to 7 FFFF 524287 values
available for the Core. All, or most
of them would correspond to
the same numbered
Core Egress Prefix (CEP). CEP prefixes are all the
same size and
are all aligned in a contiguous block
as exemplified in the Ivip6 page.
there are 2^19 of them, it wouldn't be E00://12 as
that page, but:
E000::/13 = 524288 /32 prefixes
0000 Part of the Edge Network
range, but also to be regarded
as empty: forward the packet as usual.
0001 to F FFFF 524287 values available for each Edge Network.
will also assume that any edge network which decides to use this will
upgrade all its internal routers to the required functionality.
This gets around some problems we have in the core interworking
with non-upgraded routers.
Please let me know your
Note: I have not considered multicast at all yet.
This is not
Please remember that the proposed
conversion of the IPv6 Flow Label to a Forwarding Label is not an
attempt to re-invent MPLS.
said, it would be possible to implement, in private networks, an
MPLS-like usage of some or all of the proposed 2^19 values 8 000
to F FFFF could be rewritten by each router the packet passes
through. I am not suggesting this.)
requires setting up a path. This takes time, ties up resources on
the routers, involves other protocols etc. The proposed IPv6
Label Forwarding approach involves no path or setup. The proposed IPv6 Label Forwarding simply
takes a packet to a router which advertises a prefix, while that
prefix is identified by the value of the Forwarding Label, which can be
a prefix completely unrelated to the destination address of the
MPLS can handle any kind of packet.
This technique is purely applicable to IPv6 - although of course
and IPv6 header could be used to encapsulate any other kind of packet.
typically involves QoS. This proposal has no QoS arrangements.
MPLS involves an additional header. This proposal has no
MPLS involves significant state to be set up
in each router, and involves writing the label to another value before
forwarding the packet. This proposal involves no rewriting of the
packet by the routers it is forwarded by.
MPLS is generally only
applicable within a particular network, and is not used in the core of
the Internet - the interdomain routing system between BGP routers.
The Edge part of this proposal is like MPLS, its scope is a given
edge network which adopts it and gives its own meanings to the 2^19
potential values of the Forwarding Header. However the core part
is a global system for the core of the Net, handling packets sent from
one AS to another where each AS has no business relationship whatsoever
with the other and where the packet is forwarded by core routers of any
number of similarly unrelated ASes. MPLS can't be used like this.
How the Forwarding
Label could be used in an Edge network
This proposal is applicable to the core, as
a particular "zone" in
which all the bottom half (2^19 - 1) valid Forwarding Label values must
have the same meaning and function.
This proposal is also
to any other zone for the top half (2^19 - 1) valid Forwarding Label
values, and each such zone is most likely to be a separate edge network.
principle it would be possible for several edge networks to form a
larger zone, by agreeing that each network only use a separate subset
of the 524,287 available values, and by coordinating their routing
systems so that a given label value would have the same effect
(forwarding the packet according to one prefix) in all the three
networks. However, here I focus on the scenario in which each
edge network uses the full 524,287 values in its own way, without
regard to how the same range of values are used in any other edge
A word about
The 2^20 range of
Forwarding Label values is split into two.
The bottom half is
for use in the core. The free 19 bits in this range constitute a
single namespace 0 to 524,287. While, in principle, any edge
network can do whatever it likes with these values for the Forwarding
Label, as long as this usage never affects packets in the core, the
following discussion assumes that the bottom half is reserved only for
core use, where these values constitute a single global namespace which
is always applied to these 2^19 values.
The top half of the
range is for use in edge networks. Every edge network has its own
namespace for this range of numbers. That is to say that a
particular value such as 9 603B has a particular meaning and function
everywhere in edge network X, and a completely different and
independent function everywhere in edge network Y. Each network
has its own namespace in which these values have their own local
The core is a single namespace. If the
Forwarding Label is set to 1 6703 in packets in core routers operated
by network X and Y it has the same meaning and function in both packets.
edge network has different public and non-public (Unique Local Unicast)
prefixes and will be free to assign particular values in the 8 0001 to
F FFFF range to whatever prefix they like.
All the internal
routers (or the internal facing functionality of border routers) in a
given edge network will be coordinated so that the use of the top half
of the range can be defined according to the wishes of that network's
administrators, without any reference to how it is used by other
Consequently, the 8 0001 to F FFFF range has its own
unique namespace for every edge network which uses this proposed
Naturally, wherever a network connects to another
network, the routers need to be careful not to propagate a value in the
8 0001 to F FFFF range across the boundary to the other edge network.
Core routers drop packets with FL values of 8
0000 and above
One option is for
core routers to drop a packet which has any value of 8 0000 and above.
a core router is expected to drop any packet with a Forwarding Label
value between 0 0001 and 7 FFFF for which the value points to a prefix
for which the router has no best path (no valid FEC value).
what of the idea that packets with values in the range 8 0000 to F FFFF
should be handled by core routers as normal packets? They would
be transported across the core without any changes, to a border router
which advertises the most specific prefix which encloses the
What good would the non-zero Forwarding
Label value have after that journey? It would generally or almost
always be arriving in a network which is different enough from the
source network not to share the same method of assigning meaning to the
values 8 0001 to F FFFF.
For the following discussion, I will
assume that border routers should never send packets to their core BGP
neighbours with a Forwarding Label value of 8 0000 or above, and that
any core router (or core part of a border router) which receives such a
packet should drop it, probably without generating any messages.
can use Traceroute to debug such a situation. To Do: test if
current Traceroute programs report the value of the Flow Label from the
returned packet header.
List of new
functions for core routers
In the Ivip6 page I described the
additional functionality, but in various places. Here is an exact
list - however here I am assuming a 2^19 range for the core, and that
the top half of the full 2^20 range is reserved for edge networks.
Naturally, these new functions would be
configurable in terms of being on and off. They may be made more
flexible than described here - this is the minimum new functionality to
be added. For simplicity I am assuming the terminology and the
values used in the example on the Ivip6 page. Probably, these
things would be adjustable in each router.
So for the core, the 2^19 values correspond
to the prefixes:
Name of Forwarding
0 0000 (E000:0000::/32) Forward
packet as usual:
according to the prefix of
the destination address.
0 0001 E000:0001::/32 }
0 0002 E000:0002::/32 }
Forward as for this prefix.
7 FFFF E007:FFFF::/32 }
Label recognition: dropping the packet
If the value is 0 0000, forward the packet
as usual: according to its destination address.
If the value is
equal to or above 8 0000, or to some lower value which the router is
configured with, for instance according to the limited length of the
FLFEC array (which means the total range of CEP prefixes advertised
is below this limit), then drop the packet.
If the value points
to an FLFEC entry which is zero, invalid etc. drop the packet.
Label recognition: end of path across core
There needs to be a special FEC
value, here called HOME, which indicates that the current
router is the one which advertises the prefix which matches this
Forwarding Label value.
If the Forwarding Label's value points
to an FLFEC entry == HOME, the packet has arrived at the router
(perhaps one of several) which advertised the CEP prefix the Forwarding
Label referred to. So the router should zero the Forwarding
Label, and then handle the packet with its ordinary FIB functions,
according to its destination address.
Filtering out spoofed
point, border routers might be configured to first check the source
address against a list of internal prefixes, to drop the packet if its
source address matches one which is inside the current network.
how the router handles the packet now is not important to the whole
Label Forwarding in the Core system, because the system has achieved
its goal of delivering the packet to a border router which advertised
the specified CEP prefix.
Forwarding according to the
If the packet did
not match the above conditions, then the FEC value read from the
FLFEC array is a valid FEC value, directing the router to forward the
packet out of one of its interfaces, to one of its neighbours.
The router does this.
The RIB has an additional function which
periodically (every second, every 5 seconds or whatever) monitors the
FEC values for the entire currently used range of CEP prefixes.
There may be a configured limit on this, such as there being no
more than 20,000 or some other number of contiguous prefixes assigned
to providers who have end user networks - so the RIB function
should not look to any further than a certain number of CEP prefixes.
Likewise, perhaps there is a defined limit on the size of the
Assuming there are no such limits, the new RIB
function reads these FEC values and copies them to the FLFEC array in
An alternative arrangement is for the RIB code to
monitor all changes to the FEC values, as the BGP implementation
changes those FEC values whenever it decides on a new best path for a
prefix. If the change is for a prefix in the CEP range, then the
new functionality writes the changed value to the appropriate
element of the FLFEC array in the FIB.
What do we want this
to do in Edge Networks?
is a rough discussion of what we might want to do with the Forwarding
Label in edge networks.
There are all sorts of possibilities,
but the focus is on basic usage, which is useful and robust enough to
warrant somewhat extending the functions required for Label Forwarding
in the Core so we can also do Label Forwarding in Edge networks.
would be possible to reserve some values of the 8 0001 to F FFFF range
for some special purposes we haven't thought of yet. That should
probably be done, but below I assume we use the whole range for
forwarding along the lines of the way proposed for the core.
not considering here anything special with CoS or QoS.
is only marginal value in using the Forwarding Label in a way which
directly follows the value of the destination address. Routers
are already capable of forwarding the packet according to FEC of
the prefix its destination address best matches. However,
the Forwarding Label approach will probably be easier for the FIB,
especially if it has to chew through up to 48 bits of destination
address to match a prefix. If FIB speed is an issue, then this
may be a valid use of the Forwarding Label: simply set it according to
the destination address. It will arrive at the same router, but
use less FIB resources of all the routers en-route.
unusual quality of this Label Forwarding approach is:
It can forward a packet
to a router which advertises some
prefix, which is unrelated to
the destination address -
without any extra headers and without
changing the rest
of the packet in any way.
be many uses of this facility - which performs a job normally done by
encapsulation and tunneling.
Broadly speaking, it is a method by
which some ITR-like function sets the Forwarding Header and the routing
system carries the packet to some router inside the network which
advertises some prefix to which this value of the Forwarding Header
assignment of prefixes to the Forwarding Label values
The first change we would want to make to
the LFC (Core only) arrangements is to provide each edge network
with a way of assigning any of its internal prefixes to any of the
values of the Forwarding Label: 8 0001 to F FFFF.
The LFE (for
individual Edge networks) arrangements specify a fixed linkage between
some publicly advertisable prefixes and the values of the Forwarding
This won't do for Edge networks for several reasons:
edge network has different prefixes it would want to use for the
Forwarding Label system.
- Those prefixes surely won't all be
contiguous, public, or the same size.
So we need to provide
some robust system by which all routers in an edge network can be told
which prefix's FEC value they should write into each of the 8 0001 to F
FFFF elements of the FLFEC array.
This will involve the
specification of some protocol and either one or more robust servers to
provide this information or some way of flooding it to all internal
Let's assume we need a bunch of servers and a way of
configuring each internal router to periodically access at least one of
Then we need a standardised protocol for
specifying the mapping
whatever prefixes the administrators like to whatever Flow Label values
These only impact the RIB - route processor - part of
the router. So they are perfectly amenable to firmware updates,
and would not be particularly complex or involved.
"Forwarding Label Value to Prefix
then an elaboration of the extra RIB functionality for core
routers. Instead of copying from a contiguous list of FEC values
for whatever /32 prefixes are advertised in the E000::/13 prefix, the
FLBPM consists of a list of instructions, covering the 8 0001 to F FFFF
entries of the FLFEC array. The new functionality would step
through the FLBPM, finding the FEC of each specified prefix and copying
it into the appropriate FIB FLFEC array.
For instance, the
FLBPM might start by specifying the particular prefixes to use for
values 8 001, 8 002 and 8 003:
8 000 0
The operators of the
network can do whatever they like with this mapping list. If they
want to put global unicast prefixes in it too, that is fine.
Carrying Ivip6 packets inside the provider
network from the FLPER to the end-user network, via the ETR
There could be all sorts of uses, but the
most most obvious to me is as an internal extension to what we do with
the Forwarding Label in the core.
The ITR has a task of getting
a packet with destination address X to some router which connects to an
end-user network, located somewhere in the world (or, if the network is
a single host, to that host).
The address of the destination
host within the destination end-user network is in the packet, but the
destination end-user network is not directly accessible. The
mapping system in Ivip6 tells the ITR the 19 bit identifier for the CEP
prefix which is advertised by the provider network where the ETR is.
full mapping information for the micronet to which the packet is
addressed consists of a 128 bit address - which we call the ETR address.
Ivip6's current design, the ITR does not get all this - just the bits
96 to 114 of it: the 19 bits to write into the lower 19 bits (0 to 18)
of the Forwarding Label, with bit 19 set to 0.
When the packet
arrives at the recipient host network (the border router is the FLPER -
Forwarding Label Path
Exit Router), it needs to be forwarded to the destination network.
If the recipient provider network's internal routing system has a
route for the end-user network, then all that needs to be done is to
zero the Forwarding Label and let the FIB handle the packet normally.
there will be situations where it is not practical to handle every
end-user network's address range in the recipient provider's internal
routing system. In these cases, the FLPER has a more difficult
This is where the Label Forwarding for Edge networks
could be handy.
Let's say the recipient provider's network has
20,000 ETRs. There may be 1,000,000 separate end-user networks
with links to this provider networks. Let's say each such
connections goes through one of these ETRs, and each ETR is able to
recognise from the destination address of the packet which end-user
network it should be sent to, and therefore which link to send it out
on - which tunnel to a DSLAM or whatever to forward it to.
may be multiple border routers, each advertising one (ideally not more)
CEP prefixes. Each such border router could be acting as an
FLPER: zeroing the Forwarding Label and having the task of getting the
packet to the right ETR.
One approach is for the ETRs to all
have unique addresses, and for the FLPER to look up the full 128 bit
mapping for the packets it is handling. Then the FLPER could
encapsulate the packet and tunnel it to the ETR. For this to work, all
those ETRs would need addresses within the CEP prefix, which should be
fine. The CEP prefix is a /48 in the current description of
Ivip6, and so could be split up into longer prefixes, to handle 20,000
ETRs, at up to 20,000 different physical locations in the network.
(These are very loose and probably extreme numbers, for the sake of
discussion.) However, this encapsulation raises some PMTUD
Another approach is for each ETR to advertise its own
prefix within the CEP prefix. For this discussion, let's assume
these are all the same size prefix and are contiguously assigned from
some defined prefix. Then we can play the same game with some or
all of the 2^19 values of the Forwarding Label inside each recipient
provider network as we did in the Core.
The network can
have its own internal mapping lookup system. There would be a
query server, or some distribution of a complete mapping list, so that
each FLPER could request which ETR (in effect) a packet should go to,
and get back a number to identify that ETR, and the range of addresses
(one or more micronets, for instance) which also have the same ETR.
each FLPER router can easily find out, and cache, the Forwarding Label
value in the 8 0000 to F FFFF range which corresponds to the prefix
advertised by the ETR which handles. Then it simply rewrites the
Forwarding Label, and lets its upgraded FIB, and the upgraded FIBs of
other internal routers, forward the packet to the ETR.
packet gets from the sending host to the ETR without any additional
headers, purely use of the Forwarding Header, but with two values:
the ITR to the FLPER with a value 0
0001 to 7 FFFF written by the ITR after looking up the mapping
of the packet's micronet in the global Ivip6 mapping system. (But
nice and fast, since the full database query servers are local.)
the FLPER to the ETR with a value 8
0000 to F FFFF written by the FLPER.
discussed two ways the FLPER could use the destination address to
determine the value to write to the Forwarding Label.
are again in more detail:
A - Use the global mapping system and
use the full ETR address
case, the global mapping system needs to be able to tell the FLPER the
full 128 bit "ETR" address which the end-user network has mapped each
micronet to. This is clearly possible, but since the ITR only
needs 20 bits of this, I was exploring the idea that only those 20 bits
are fast pushed to all the full database Query Servers around the
world, and that some other mechanism would be provided for the much
lower demand for the full 128 bit ETR address, which would arise only
from those FLPERs which needed this.
In this case, the addresses
of the ETRs must be within the CEP prefix. That CEP prefix can be
split up arbitrarily inside this recipient provider network site,
according to wherever the ETRs are.
The recipient provider
network would have its own way of organizing the advertised prefixes on
which these ETRs could be reached. It is conceivable that this would be
standardised across all networks, for instance so that each /32 CEP
prefix was assumed to be split into 2^19 smaller prefixes: /51s.
(There are a variety of other arrangements I won't explore now.)
it could be standard procedure in all provider networks for the FLPER
to do a lookup of the full 128 bit ETR address, and to use a subset of
those bits to set some or all of the 19 bits of the Forwarding Label so
that the packet would be forwarded in the internal routing system
towards - or all the way to - each ETR.
If an ETR goes down, it
would be up to the end-user network's multihoming monitoring system to
change the mapping to another ETR - either to another ETR in this
recipient provider network, or if the end-user network is multihomed,
perhaps to an ETR in another recipient provider network.
B - Maintain a local mapping system for ETR addresses/prefixes
In this case, there is no need to look up
the full 128 bit ETR address to which the packet's destination micronet
is mapped. This is because the destination provider network
maintains its own query servers which can tell each FLPER what value in
the 8 0000 to F FFFF range to set the Forwarding Label to so that
the packet will be Label Forwarded to the correct ETR.
requires the recipient provider network to do a more work, since
maintaining a local query server, keeping track of all these ETRs and
customers etc. is a lot of work.
Here are two different
1 - Ignore the 128 bit ETR address which the
end-user mapped the micronet to (This turns out to be a bad idea.)
The local mapping system could ignore the
128 bit value to which the end-user mapped each micronet. This
way, the local mapping system is free to use any ETR to connect to each
end-user network - which provides some local flexibility. It also
means that the end-user doesn't have to worry about what ETR address
they map the prefix to, as long as bits 96 to 114 of the address they
use (which the ITR gets and copies to the Forwarding Label) causes the
packet to arrive at the correct recipient provider network.
this sounds like a bad idea.
The local mapping system would need some way of tracking the
actual addresses used by all these end-user networks. There's no
particular reason why an end-user network must have a single nicely
defined prefix. If I have some micronet address space I am not
using, and my friend has an end-user network somewhere, I should be
able to define some of my space as a micronet or two and map it to his
ETR, so he can use as he wishes for as long as I like.
all these end-user networks efficiently and securely communicate to
this recipient provider network exactly which of their micronets they
are mapping to this provider's ETRs right now? The great thing
about Ivip's real time mapping system is that each end-user network can
switch their micronet to any ETR address in any ISP they like.
example, a micronet is currently mapped to an ETR address at ISP-A.
ISP-A's local mapping system somehow is told about this,
which means, in part, that any packets generated in ISP-A's network
will be guided by this mapping system to that ETR. But let's say
the end-user network's link to this ETR fails and they change the
mapping of their micronet(s) to another ETR in another ISP? Or
they change the mapping to another ETR at ISP-A or to another ETR in
another ISP, for reasons of TE, or because they simply feel like it?
can the end-user easily tell ISP-A's local mapping system about the
change if its link to ISP-A has just failed and if this mapping system
doesn't monitor the mapping changes concerning the micronets which are
currently mapped to ISP-A's ETRs?
What should happen is that
all packets, including those sent from this provider's networks, should
go to an ITR and be forwarded to the new ETR in the other ISP's
network. That won't happen if the local mapping system can't be
told to do this by the end-user, and if it doesn't constantly monitor
the mapping updates for any which affect its current end-user networks'
I could go on, but all this is a can of worms.
Every packet generated from outside the
destination end-user network must go through an ITR and be tunneled (Ivip4)
or Label Forwarded (Ivip6) to the ETR address to which the micronet
(which matches the destination address) is currently mapped.
is the plan with Ivip4 - but not of the other map-encap systems.
In Ivip6 we are implementing this path
to the ETR in two stages - one from the ITR across the core to the
FLPER, the other from the FLPER to the ETR - it is vital that the FLPER
respond as quickly to mapping changes as the ITR does.
It is not
good enough for the FLPER to look only at the destination address and
think it knows which ETR the packet should be sent to, unless its
notion of this is fully updated by the Ivip6 mapping system.
So for all recipient provider networks with
more than one ETR, it follows that the FLPER must get the full 128 bit
mapping information for the micronets of all the packets it is
receiving from ITRs. (The only exception would be some
arrangement where the FLPER got just enough bits to write these into
the Forwarding Label in order that the LFE system would forward the
packet to the correct ETR, but this is rather inflexible.)
- Rely on the 128 bit ETR address from the Ivip6 mapping system
This is rather like option A above. If the recipient
provider network maintains a mapping system for the use of its FLPERs
to figure out how to get the packet to the correct ETR, and if that
mapping system depends on how each end-user network has mapped each
micronet it uses, then maybe the whole thing should be simplified by
having each FLPER look up the 128 bit ETR address itself.
may be a good reason for doing this.
Suppose the recipient provider network had
other uses in mind for some of the 2^19 values of the Forwarding Header
it can use. It could configure the FLPERs to use a subset of
these values, such as the bottom 2^16 of them, based directly on bits
80 to 95 inclusive of the full 128 bit ETR address. Then there
can be 2^16 contiguous prefixes where ETRs are located, such as a small
subset of the CEP prefix,or perhaps some regular set of Unique Local
Unicast prefixes. This way, the FLPERs could take care of their
incoming packets in a coordinated fashion, without the local mapping
system needing to respond every time a micronet's mapping changed.
way, the rest of the 2^19 available values of the Forwarding Label
could be used for other purposes.
(In Ivip4, for PMTUD reasons,
ITR needs to be able to send packets directly to the ETR and get
responses from that address. With Ivip6, there is no such
for ITR-ETR communication. So exactly how the recipient provider
network gets the packets to the destination network doesn't matter.
might put its ETRs in Unique Local Unicast space and use some system of
its own devising to make sure the traffic packets get to the right ETR.)
Conclusion for now
larger recipient provider networks, if all routers were suitably
upgraded, and if new protocols were used to coordinate how they write
the FEC values for various internal prefixes into the 8 0001 to F FFFF
entries in their FLFEC array, then the system could be used for a
variety of purposes, not least to forward packets from FLPERs to ETRs,
without any tunneling and so without any PMTU problems or additional
packet length overhead.
What about not
splitting the 2^20 values into Core and Edge?
could be argued that core routers know they are core routers and
internal routers inside a given organisation know they are internal
routers - and that routers which do both know which part of the router
the packet is in . . . so why not let all 2^20 values of the Forwarding
Label be used for the Core, and allow each edge network to use the full
2^20 range as it sees fit within the confines of its network?
have several reasons for maintaining that the split into two separate
2^19 spaces is the best approach:
- The idea of not splitting
it, and of having packets with a Forwarding Header set for the internal
system somehow get treated as if it is in the core . . . the idea gives
me a terrible headache.
- I think 2^19 is plenty in the core and
in any private network. (Famous last words . . .)
now, in the early stages of developing this idea, I suggest we keep the
two ranges separate. It is easier to think about it this way. If
we decide it is a good idea to change the function of routers and of
the current Flow Label, then we will know a lot more about this and
will consider carefully whether to split the range, and if so how.
an edge network, people can do what they like - and if they keep it to
themselves, why should anyone care. If an edge network wants to
use all 2^20 values of the Forwarding Label for its own internal
purposes, or share some of that space with another network, that is
The idea is to set some standards which most people
will follow and find not too restrictive, but which will discourage
practices which are likely to cause trouble.