Network Working Group R. Huang
Internet-Draft T. Eckert
Intended status: Experimental N. Wei
Expires: September 6, 2018 Huawei
P. Thubert
Cisco
March 5, 2018
Encapsulation for BIER-TE
draft-huang-bier-te-encapsulation-00
Abstract
This document proposes an enhanced encapsulation for BIER to support
BIER, BIER-TE and a control word.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
2. BIER-TE Encapsulation (normative) . . . . . . . . . . . . . . 3
2.1. BT bit - Simultaneous support for BIER and BIER-TE . . . 3
2.2. BIFT-ID . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3. Control Word and flows . . . . . . . . . . . . . . . . . 3
2.4. Header format & fields . . . . . . . . . . . . . . . . . 4
3. BIER-TE based resilience operations (informational) . . . . . 5
4. BitStringLength (BSL) considerations (informational) . . . . 6
4.1. IPTV . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Multicast in L3VPN . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
[BIER-TE-ARCH] specifies BIER-TE: Traffic Engineering for Bit Index
Explicit Replication (BIER). It builds on the BIER architecture as
described in RFC8279 [RFC8279], but uses every BitPosition of the
BitString of a BIER-TE packet to indicate one or more adjacencies
instead of a BFER as in BIER.
This document proposes an enhanced version of the MPLS and non-MPLS
encapsulation for BIER packets to support both BIER and BIER-TE. It
is based on RFC8296 [RFC8296].
This enhanced encapsulation also adds support for a control word in
the header and discusses it. Finally, the document discusses
BitStringLength (BSL) size requirements in implementations for
informational reasons to help aide implementors to determine an
appropriate BSL.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
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2. BIER-TE Encapsulation (normative)
2.1. BT bit - Simultaneous support for BIER and BIER-TE
This document supports mixed BIER and BIER-TE forwarding in a domain.
Either or both of them may be used in a domain. The overall solution
to support this depends on additional signaling such as existing BIER
ISIS/BGP signaling. Architecturally, every SD SHOULD only use a
single Type of BIER: BIER or BIER-TE. Note that this document will
use the abbreviation BT to refer to the Bier Type.
In the presence of BIER and BIER-TE together in the network, there is
always a risk of receiving a packet which is meant to be of one BT
and processing it through a BIFT of the other BT. This can come from
misconfiguration even in the face of signalling via IGP/BGP. The
risk increases also when packets are generated modular from
applications on PE or other sources and could use both BTs. To
resolve this, the header includes a bit to indicate the BT. If the
BT of a packet is inconsistent with the BT of the BIFT on the BFR,
the BFR MUST NOT forward it. OAM actions MAY be triggered (subject
to future work).
Note that the TTL field of the existing BIER packet header (or of IP
packets) spends 7 bits on loop prevention. One bit for the BT is a
comparably low cost to protect against a similar degree of problems.
Indicating the BT explicitly through a bit in the encapsulation is
called the "explicit" option. Relying solely on the BT of the BIFTs
is called "implicit" option. In this version of the document, we
choose the explicit option for reasons outlined above.
2.2. BIFT-ID
Like in the original BIER header, the semantic of the BIFT-id of the
header is that it is representing a <SI,SD,BSL> on the BFR receiving
the packet. In the case of MPLS forwarding, the expectation is that
the protocols to signal label ranges would be extended to also signal
label ranges for the SD using BIER-TE. This is subject to the work
of other documents. In the case of non-MPLS forwarding, no
additional signaling may be neccessary, and BIER and BIER-TE packets
using the encapsulation of this document would equally use the BIFT-
ID encoding as described in [BIER-non-MPLS].
2.3. Control Word and flows
This document adds a "control word" to the BIER packet header to
allow that BIER or BIER-TE packets with this header could be used as
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a DetNet Data Plane, independent of MPLS encapsulation, see
[I-D.ietf-detnet-dp-sol], section 5.3 (in revision 01).
The control word provides a sequence number, therefore allowing to
correct reordering and discover packet loss. The primary use though
is resilient dual-path transmission of two copies of the same packet
via disjoint paths. This is specifically a desirable use-case with
BIER-TE because it allows the engineering of such disjoint paths.
The flow to which the sequence number in the control word applies is
<BFR-id,entropy>.
Note: The justification to carry a control word in the BIER
encapsulation is similar to carrying the BFIR-ID in it. Initially,
both could be seen as primarily required on BIER domain edge-nodes as
part of the overlay using BIER, but not by BIER/BIER-TE itself
directly. See Section 3 for more explanation how the resilience
mechanism requiring the control word would work. Compared to the
BFIR-ID, there is also the option to leverage it within BIER-TE
itself. The details of that operations is subject to other
specifications.
The authors think that the overhead of the control word is always
acceptable for BIER-TE. For BIER, the use of this extended header
version is optional, therefore BIER packets that need a control word
would use this version of the header, those that do not need it would
use version 1. If this overhead is considered to not be acceptable
for all BIER-TE packets, the encoding could make those 32 bits
optional through the use of one of the reserved bits or version
numbers or by using a bit in the header to indicate whether the
control word is present or not.
2.4. Header format & fields
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BIFT-id | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Nibble | Ver | BSL | Entropy (flow-id) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|OAM|T|R| DSCP | Proto | BFIR-id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Control Word (sequence number) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BitString (first 32 bits) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ BitString (last 32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
All header fields not described below are left unchanged from
[BIER-TE-ARCH]
T: This 1-bit field identifies that the packet is to be forwarded as
a BIER packet (0) or a BIER-TE paket(1).
Ver: The version of this header format is 2.
R: Reserved - unchanged (just reduced by one bit from version). Must
be set to 0.
Entropy: Unchanged, but double-used as part of the flow-identifier
together with the control word
Control Word: The control word in the terminology of MPLS pseudowires
(where it originates from) is the full 32 bits. For detnet, the
current target is 28 bits of sequence number and 4 bits 0 preceeding
it.
3. BIER-TE based resilience operations (informational)
This section discusses how resilience operations with the help of the
sequence number in the control word of the header in this document
can be operated as an overlay (BFIR-BFER) function but also points
out that it could become an integral (optional) part of BIER-TE
itself. This section is solely informational. The planned document
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to describe the BIER-TE forwarding aspects of resilience operations
is [I-D.thubert-bier-replication-elimination].
The BFIR determines - potentially with the help of a BIER-TE
Controller Host (controller) - a bitstring that forms two disjoint
DAGs (Directed Acyclic Graphs) through the BIER-TE domain towards the
same set of BFER. In addition, an entropy value is decided (by BFIR
and/or controller) and signalled to the BFER. The BFER can therefore
set up "duplicate elimination state": The BFIR increments the
sequence number with every packet of the flow it sends. The BFER
assign packets to a flow by <bfir-id,entropy> and perform duplicate
elimination on them.
Note that the bitstring as seen on the receiving BFER can provide
additional diagnostics, for example the bits not reset by forwarding
in BIER-TE give an indication about which path the BIER-TE packet was
forwarded.
Instead of simply considering this protected mode of operations
solely an end-to-end (BFIR/BFER) function, it could also be more
flexibly embedded into BIER-TE itself, allow to provide in-BIER-TE
segmented Packet Replication and (duplicate) Eliminiation Functions
(PREF) definable by the bitstring of a BIER-TE packet. This could be
achieved by adding to BIER-TE forwarding functions new adjacency
types for duplication with sequence-number generation and duplicate-
elimination. The ability to perform such processing as part of BIER-
TE itself is the primary reason to ensure that all the necessary
elements for such operations are part of the BIER-TE header itself.
4. BitStringLength (BSL) considerations (informational)
BIER-TE uses each BitPostion to indicate the adjacencies instead of a
BFER as in BIER, it therefore consumes more BitPostions than BIER.
In BIER-TE, the number of adjacencies passed by one BIER-TE packet
MUST be less than the value of BitString length (BSL). The BIER-TE
architecture discusses a range of options to reduce the number of
bits for intermediate hops through various BIER-TE adjacencies and
how to use them.
The maximum supported BSL has a different impact in BIER-TE than it
has in BIER: A smaller maximum supported BSL in BIER primarily leads
to less replication efficiency: With a BSL of 256, BIER can be up to
256 more efficient than unicast (1 packet for 256 receivers). In
BIER-TE, the BSL also limits the size of the topology towards BFER
and the alternative paths that can explicitly be engineered to reach
the BFER. One simple guess is that 50% of bits in a bitstring may be
required for intermediate hops, therefore requiring about double the
amount of bits as BIER - as the cost of being able to engineer pats.
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So far, there is no comprehensive analysis of the number of required
bits for specific scenarios in BIER-TE. The following subsections
give two examples of such scenarios and how to use and save BIER-TE
bits for intermedia hops.
4.1. IPTV
Multicast is widely used for IPTV services by simultaneously
delivering a single stream of video to thousands of recipients.
Currently, PIM is widely used to provide multicast capability usually
from core router(CR) to provider edges (PEs). And the multicast tree
is usually constructed in the hierarchical way. The end users using
PIM/IGMP to request the multicast data. BIER can be well used in
from CR to those PEs. The number of hops from multicast source (CR)
which could be BFIRs, to the multicast receivers (PE) which can be
regarded as BRERs is usually no more than 10. BIER-TE will be useful
in the cases where different video channels can have different
transport paths to achieve load balancing.
To save the bit consumption, 2 ways could be used:
1. Multiple BFRs and routes are required to receive the same data.
These BFRs or links can share one bit.
2. Different bits can be used for pruning. But these bits can be
reused in similar but different groups.
Considering an example illustrated as following:
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+----+
|BFIR|
+-+--+
|
|
+-------------------------+---|
| |
| |
| |
+---+---+ +----+----+
| BFR1 | .... | BFRn |
+---+---+ +----+----+
| |
| ...
+------|-------+ +------+---------+
| | | | | |
| | | | | |
+--+----------------+ +---------+-----------+
| BFER1 | | BFERn |
+--+----------------+ +---------------------+
BRF1 and BFRn, and other BFRs in between, can share one bit because
they are receiving all the content and don't need pruning. From BFR1
to BFER1, there are 3 ways to reach BFER1. So these 3 ways can be
assigned with different bits. But these 3 bits can be reused in the
group from BFRn to BFERn, and other groups in between, which share
the same topology as the group from BFR1 to BFER1.
BIER-TE can be well implemented using these 2 ways to save the bit
consumption in IPTV networks with the similar topologies like the
above example.
4.2. Multicast in L3VPN
MVPN is a technology to deploy multicast service in an existing VPN
or as part of a transport infrastructure. Multicast data is
transmitted between private networks over a VPN infrastructure by
encapsulating the original multicast packets. PE routers are
connected to these private networks either containing receivers or
senders.
There are several multicast applications widely using the MVPN
deployment. For example, L3VPN multicast service offered by service
providers to enterprise customers, and video transport applications
for separation between different customers: One content provider may
provider video wholesale service to another, or multiple content
provider may share one network to transport video from headend.
Especially the latter case, network SLAs should be guaranteed as the
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original video content is precious. Thus, traffic engineering is
required.
According to the current implementations, the scale of a MVPN network
usually contains less than several hundreds of PEs, and hundreds of
core routers which are connected in full mesh, like following figure
illustrated.
+--+ +----+ +----+ +--+
|PE+----------+ CE +---------+ CE +--------+PE|
++-+ +-+--+ +--+-+ +--+
| | | |
| | | |
++-+ +-+--+ +--+-+ +-++
|PE+----------+ CE +---------+ CE +---------+PE|
+--+ +----+ +----+ +--+
In such a case, the ways in Section 7.5.2 of [BIER-TE-ARCH] can be
used by regarding the CE area as the Core. Based on this, current
BIER design is sufficient to be reused in BIER-TE.
5. Acknowledgements
TBD.
6. Security Considerations
The security considerations are in compliance with BIER-TE
architecture [BIER-TE-ARCH] and BIER encapsulation RFC8296 [RFC8296].
And the content in this document does not create any other attacks or
security concerns.
7. IANA Considerations
TBD.
8. References
8.1. Normative References
[BIER-non-MPLS]
Wijnands, I., Xu, X., and H. Bidgoli, "An Optional
Encoding of the BIFT-id Field in the non-MPLS BIER
Encapsulation", ID draft-wijnandsxu-bier-non-mpls-bift-
encoding-01 (work in progress), August 2017.
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[BIER-TE-ARCH]
Eckert, T., Cauchie, G., Braun, W., and M. Menth, "Traffic
Engineering for Bit Index Explicit Replication BIER-TE",
ID draft-ietf-bier-te-arch-00 (work in progress), January
2018.
[I-D.thubert-bier-replication-elimination]
Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER-
TE extensions for Packet Replication and Elimination
Function (PREF) and OAM", draft-thubert-bier-replication-
elimination-03 (work in progress), March 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.
[RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
for Bit Index Explicit Replication (BIER) in MPLS and Non-
MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
2018, <https://www.rfc-editor.org/info/rfc8296>.
8.2. Informative References
[I-D.ietf-detnet-dp-sol]
Korhonen, J., Andersson, L., Jiang, Y., Finn, N., Varga,
B., Farkas, J., Bernardos, C., Mizrahi, T., and L. Berger,
"DetNet Data Plane Encapsulation", draft-ietf-detnet-dp-
sol-01 (work in progress), January 2018.
Authors' Addresses
Rachel Huang
Huawei
101 Software Avenue, Yuhua District
Nanjing 210012
China
Email: rachel.huang@huawei.com
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Toerless Eckert
Huawei USA - Futurewei Technologies Inc.
2330 Central Expy
Santa Clara 95050
USA
Email: tte+ietf@cs.fau.de
Naiwen Wei
Huawei
Email: weinaiwen@huawei.com
Pascal Thubert
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com
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