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
SRm6
This document defines the IPv6 CRH Helper option. When an SRm6
ingress node originates a packet, it can use the IPv6 CRH Helper option
to provide SID Forwarding Information Base (SFIB) information to
downstream nodes that do not maintain a complete SFIB.
SRm6 leverages two
new IPv6 Routing types. Generically, they
are called the Compressed
Routing Header (CRH) . More specifically, the 16-bit version of
the CRH is called the CRH-16, while the 32-bit version of the CRH is
called the CRH-32.
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 SRm6 path egress node.
Segment Identifier (SID) List - Represents the SRm6 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 node encounters and recognizes the CRH, it processes
the CRH as follows:
If Segments Left equal 0, skip over the CRH and process the next
header in the packet.
Decrement Segments Left.
Query a local data structure, called the SID Forwarding
Information Base (SFIB), searching for an entry that is indexed by
the current SID.
If the above-mentioned query returns an SFIB entry, update the
packet's IPv6 Destination Address and forward the packet.
In a typical SRm6 deployment, every segment ingress node maintains a
complete SFIB and the above-mentioned query returns an SFIB entry.
However, in some SRm6 deployments, some segment ingress nodes maintain a
complete SFIB while others do not. The following are reasons why a
segment ingress node might not maintain a complete SFIB:
the node is a Network Interface Card (NIC) in a data center.
Although it originates many segments, it does not participate in the
SRm6 control
plane.
the node resides in another autonomous system and does not
participate in the SRm6 control plane.
This document defines the IPv6 CRH Helper option. When an SRm6
path ingress node originates a packet, it can use the IPv6 CRH Helper
option to provide SFIB information to downstream nodes that do not
maintain a complete SFIB.
If a segment ingress node queries its SFIB, searching for an entry
that is indexed by the current SID, and that query returns nothing, the
segment ingress node can obtain the required SFIB information from the
IPv6 CRH Helper option. If the segment ingress node cannot obtain the
required SFIB information from either source, it discards the packet
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.
SFIB Helper (SFIBH) List - (Variable length) A list of
SFIBHs.
Each SFIBH contains the following fields:
SFIBH Length - (8-bits) Length of the SFIBH, in octets, excluding
SFIBH 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 SFIBH applies.
High SID - (8-bits) Index to the CRH SID List. References the
last member of the SID List to which this SFIBH 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 SFIB. If it cannot resolve
the SID using SFIB, it attempts to resolve the SID using information
received in an applicable SFIBH. If no SFIBH 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 an SFIBH 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 SFIBH
If the prefix found in the applicable SFIBH 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, the SID is understood to be a prefix SID.
Therefore, the packet is forwarded to its destination via the least cost
path.
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 SFIB. If it can resolve the
SID using SFIB, it MUST ignore the CRH Helper option, even if it
contains an applicable SFIBH.
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.