Compressed Routing Header (CRH) Helper
Option
CERNET Center/Tsinghua University
Room 225, Main Building, Tsinghua University
Beijing
100084
People's Republic of China
xing@cernet.edu.cn
CERNET Center/Tsinghua University
Room 225, Main Building, Tsinghua University
Beijing
100084
People's Republic of China
congxiao@cernet.edu
Fungible Inc.
3201 Scott Blvd
Santa Clara
California
95054
USA
eddie.ruan@fungible.com
Juniper Networks
2251 Corporate Park Drive
Herndon
20171
Virginia
USA
rbonica@juniper.net
INT Area
6man WG
IPv6
This document defines the IPv6 Compressed Routing Header (CRH) Helper
option. When a source node sends a packet with a CRH, it can use the CRH
Helper option to provide additional information to downstream nodes.
IPv6 source nodes use the Compressed Routing Header
(CRH) to steer packets along a delivery path to their
destination. Two CRH versions have been defined. The CRH-16 encodes
segment endpoints in 16 bits, while CRH-32, encodes segment endpoints in
32 bits.
Both CRH versions contain the following fields:
Next Header - Identifies the header immediately following the
CRH.
Hdr Ext Len - Length of the CRH.
Routing Type - Identifies the Routing header variant (i.e.,
CRH-16 or CRH-32).
Segments Left - The number of segments still to be traversed
before reaching the packet's ultimate destination.
Segment Identifier (SID) List - Represents the delivery path as
an ordered list of Segment Identifiers (SID). SIDs are listed in
reverse order, with SID[0] representing the final segment, SID[1]
representing the penultimate segment, and so forth. SIDs are listed
in reverse order so that Segments Left can be used as an index to
the SID List. The SID indexed by Segments Left is called the current
SID.
As per , when an IPv6 node receives a packet,
it examines the packet's destination address. If the destination address
represents an interface belonging to the node, the node processes the
next header. If the next header is a CRH, it is processed as
follows:
If Segments Left equals 0, skip over the CRH and process the next
header in the packet.
Decrement Segments Left.
Query a local data structure, called the CRH Forwarding
Information Base (CRH-FIB), searching for an entry that is indexed
by the current SID.
If the above-mentioned query returns a CRH-FIB entry, update the
packet's IPv6 Destination Address and forward the packet.
In a typical CRH deployment, every segment ingress node maintains a
complete CRH-FIB and the above-mentioned query returns a CRH-FIB entry.
However, in some CRH deployments, some segment ingress nodes maintain a
complete CRH-FIB while others do not. For example, a node that does not
participate in a control plane or communicate with a controller may not
maintain a CRH-FIB.
This document defines the IPv6 CRH Helper option. When a source node
sends a packet with a CRH, it can use the IPv6 CRH Helper option to
provide CRH-FIB information to downstream nodes that do not maintain a
complete CRH-FIB.
If a segment ingress node queries its CRH-FIB, searching for an entry
that is indexed by the current SID, and that query returns nothing, the
segment ingress node can obtain the required CRH-FIB information from
the IPv6 CRH Helper option. If the segment ingress node cannot obtain
the required CRH-FIB information from either source, it discards the
packet and sends an ICMPv6 Parameter
Problem message to the source node.
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.
The CRH Helper option contains the following fields:
Option Type - (8-bits) CRH Helper option. Value TBD by IANA.
(Suggested value: 0x11). See Note below.
Opt Data Len - (8-bits) Length of the option, in octets,
excluding the Option Type and Option Length fields.
Helper List - (Variable length) A list of Helpers.
Each Helper contains the following fields:
Helper Length - (8-bits) Length of the Helper, in octets,
excluding Helper Length. Value MUST be greater than 2 and less than
19.
Low SID - (8-bits) Index to the CRH SID List. References the
first member of the SID List to which this Helper applies.
High SID - (8-bits) Index to the CRH SID List. References the
last member of the SID List to which this Helper applies.
Prefix field (variable length): An IPv6 Prefix.
NOTE : The highest-order two bits of the Option Type (i.e., the "act"
bits) are 00. These bits specify the action taken by a destination node
that does not recognize the option. The required action is to skip over
this option and continue processing the header.
The third highest-order bit of the Option Type (i.e., the "chg" bit)
is 0. This indicates that Option Data cannot be modified along the path
between the packet's source and its destination.
When a segment endpoint node processes a CRH, it attempts to resolve
the SID using information contained by its CRH-FIB. If it cannot resolve
the SID using CRH-FIB, it attempts to resolve the SID using information
received in an applicable Helper. If no Helper applies to the current
SID, the processing node discards the packet and sends an ICMPv6
Parameter Problem message to the source node.
When the processing node uses a Helper to resolve a SID, it executes
the following procedure:
Set the IPv6 Destination Address to 0::
Overwrite the low order bits of the IPv6 Destination Address with
the current SID found in the CRH
Overwrite the high order bits of the IPv6 Destination Address
with the prefix found in the applicable Helper
If the prefix found in the applicable Helper is 16 bytes long, it
overwrites the entire IPv6 Destination Address.
The CRH Helper option MAY occur in a Destination Options header that
precedes a CRH. It SHOULD NOT occur in a Hop-by-hop options header or in
a Destination Options header that precedes an upper-layer header.
When a segment ingress node resolves a SID using information obtained
from the CRH helper option, it forwards the packet through the
least-cost path to its new destination.
Information obtained from the CRH Helper option is transient. It is
discarded as soon as the packet that carried it has been processed.
When a segment endpoint node processes a CRH, it attempts to resolve
the SID using information contained by its CRH-FIB. If it can resolve
the SID using CRH-FIB, it MUST ignore the CRH Helper option, even if it
contains an applicable Helper.
IANA is requested to allocate a code point from the Destination
Options and Hop-by-hop Options registry
(https://www.iana.org/assignments/ipv6-parameters/ipv6-parameters.xhtml#ipv6-parameters-2).
This option is called "CRH Helper Option". The "act" bits are 00 and the
"chg" bit is 0. (Suggested value: 0x11).
Thanks to TBD for their careful review of this document.