BIER MTU DiscoveryCisco Systems, Inc.Tasman DriveSan JoseCA 95134USAstig@cisco.comCisco Systems, Inc.De kleetlaan 6aDiegem1831Belgiumice@cisco.comCisco Systems, Inc.Tasman DriveSan JoseCA 95134USAginsberg@cisco.comJuniper Networks1133 Innovation WaySunnyvaleCA 94089USAsivakumar.mahesh@gmail.com
Routing
MulticastBIERThis document defines an IGP based mechanism for discovering the
MTU of a BIER sub-domain. This document defines extensions to OSPF
and IS-IS, but other protocols could potentially be extended.
MTU discovery is usually
done for a given path, while this document defines it for a sub-domain.
This allows the computed MTU to be independent of the set of receivers.
Also, the MTU is independent of rerouting events within the sub-domain.
This document defines an IGP based mechanism for discovering the
MTU of a BIER sub-domain. The discovered MTU indicates the largest
possible BIER packet that can be sent across any link in a BIER
sub-domain. This is different from
which performs Path
MTU Discovery (PMTUD) for a set of receivers. PMTUD is based on probing,
and when there are routing changes, e.g., a link going down, the
actual MTU for a path may become less than was previously discovered,
and there will be some delay until the next probe is performed.
Also, the set of receivers for a flow may change at any time, which
may cause the MTU to change. This document instead discovers a BIER
sub-domain MTU, which is independent of paths and receivers within
the sub-domain.
Discovering the sub-domain MTU is much simpler than discovering the
multicast path MTU, and is more robust with regards to path changes as
discussed above. However, the sub-domain MTU may be a lot smaller than
the path MTU would have been for a given flow. The discovery mechanisms
may be combined, allowing the
discovery of the path MTU for certain flows as needed.
The BIER sub-domain MTU defined here provides the maximum size of a
BIER packet that can be forwarded through the sub-domain regardless of
path. A BIER router that performs BIER encapsulation will need to
subtract the encapsulation overhead to find the largest size packet
that can be encapsulated. This would give the IP MTU, and may be used
for IP PMTUD by for instance sending an ICMP Packet too big message
if an IP packet will be too large after BIER encapsulation.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP 14
when,
and only when, they appear in all capitals, as shown here. An interface on a router is said to be a BIER interface if the
router has a BIER neighbor on the interface. That is, there is a
directly connected router on that interface that is announcing a BIER
prefix. Further, the BIER interface is said to belong to a given
sub-domain if the router itself announces a prefix tagged with
the sub-domain, and there is BIER neighbor on the interface also
announcing a prefix tagged with the sub-domain.
The BIER MTU of an interface is the largest BIER packet that can be
sent out of the interface. Further, the local sub-domain MTU
of a router is the minimum of all the BIER MTUs of the BIER
interfaces in the sub-domain. Note that the local sub-domain MTU of a
router is only defined if it has at least one BIER interface in the
sub-domain.
A BIER router announces a BIER prefix in either IS-IS or OSPF as
specified in and .
They both define a BIER Sub-TLV to be included with the prefix. There
is one BIER Sub-TLV included for each sub-domain. This document defines
how a router includes its local sub-domain MTU in each of the BIER
Sub-TLVs it advertizes.
A router can discover the MTU of a BIER sub-domain by identifying
all the prefixes that have a BIER Sub-TLV for the sub-domain. It then
computes the minimum of the advertised MTU values for
that sub-domain. This includes its own local sub-domain MTU.
This allows all the routers in the sub-domain to discover the same
sub-domain wide MTU.Note that a router should announce a new local MTU for a sub-domain
immediately if the value becomes smaller than what it currently
announces. This would happen if the MTU
of an interface is configured to a smaller value, or the first BIER
neighbor for a sub-domain is detected on an interface, and the MTU of
the interface is less than all the other local BIER interfaces in the
sub-domain. However, if BIER neighbors go away, or if an interface goes
down, so that the local MTU becomes larger, a router SHOULD NOT
immediately announce the larger value. A router MAY after some delay
announce the new larger MTU. The intention is that dynamic events such
as a quick link flap should not cause the announced MTU to be increased.
It is a concern that the sub-domain MTU will be based on the link
with the smallest MTU. This means that if for instance a single link is
accidentally configured with an extra small MTU, it will impact the
sub-domain MTU and potentially all the flows through the sub-domain.
As an example, an administrator might decide to use jumbo frames and
has configured that on all the links. But accidentally forget to
configure it on a new link before it is brought up. To provide some
protection against this, an implementation SHOULD provide a configurable
minimum BIER sub-domain MTU. When this is configured, the MTU discovery
is still done according to the above procedure, but if the resulting
MTU value is less than the configured minimum, the configured minimum
MUST be used instead. If the discovery procedure later should provide
an MTU larger than the minimum, then the discovered MTU MUST be used.
An implementation SHOULD provide notification to the administrator
when the discovered MTU is less than the minimum, as this is likely a
configuration mistake that should be corrected.
A router uses the BIER Sub-Domain MTU Sub-sub-TLV to announce the
minimum BIER MTU of all its BIER enabled interfaces in a sub-domain.
The BIER Sub-Domain MTU is the largest BIER packet that can be sent
out of all the interfaces in a sub-domain.
The Sub-sub-TLV MUST be ignored if it is included multiple times.
TBD
2
MTU in octets
A router uses the BIER Sub-Domain MTU Sub-TLV to announce the
minimum BIER MTU of all its BIER enabled interfaces in a sub-domain.
The BIER Sub-Domain MTU is the largest BIER packet that can be sent
out of all the interfaces in a sub-domain.
The Sub-TLV MUST be ignored if it is included multiple times.
TBD2
4
MTU in octets
An allocation from the "sub-sub-TLVs for BIER Info sub-TLV" registry
as defined in is requested for the
IS-IS BIER Sub-Domain MTU Sub-sub-TLV. Please replace the string TBD in
this document with the appropriate value.An allocation from the "OSPF Extended Prefix sub-TLV" registry as
defined in is requested for the
OSPF BIER Sub-Domain MTU Sub-TLV. Please replace the string TBD2 in this
document with the appropriate value.
The authors would like to thank Greg Mirsky in particular for fruitful
discussions and input. Valuable comments were also provided by
Alia Atlas, Eric C Rosen, Toerless Eckert, Tony Przygienda and
Xie Jingrong.