The IPv6 Compact Routing
Header (CRH)Juniper Networks2251 Corporate Park DriveHerndon20171VirginiaUSArbonica@juniper.netNTT Communications Corporation3-4-1 Shibaura, Minato-kuTokyo108-8118Japany.kamite@ntt.comLiquid TelecomNairobiKenyaAndrew.Alston@liquidtelecom.comLiquid TelecomJohannesburgSouth Africadaniam.henriques@liquidtelecom.comVerizonRichardsonTexasUSAluay.jalil@one.verizon.com
INT Area
6manIPv6Routing headerThis document defines two new Routing header types. Collectively,
they are called the Compact Routing Headers (CRH). Individually, they
are called CRH-16 and CRH-32.IPv6 source nodes use Routing headers
to specify the path that a packet takes to its destination. The IETF has
defined several Routing header types. This
document defines two new Routing header types. Collectively, they are
called the Compact Routing Headers (CRH). Individually, they are called
CRH-16 and CRH-32.The CRH allows IPv6 source nodes to specify the path that a packet
takes to its destination. The CRH:Can be encoded in relatively few bytes.Is designed to operate within a limited domain. (See ).The following are reasons for encoding the CRH in as few bytes
as possible:Many ASIC-based forwarders copy headers from buffer memory to
on-chip memory. As header sizes increase, so does the cost of this
copy.Because Path MTU Discovery (PMTUD)
is not entirely reliable, many IPv6 hosts refrain from sending
packets larger than the IPv6 minimum link MTU (i.e., 1280 bytes).
When packets are small, the overhead imposed by large Routing
Headers is excessive.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.Both CRH versions (i.e., CRH-16 and CRH-32) contain the following
fields:Next Header - Defined in .Hdr Ext Len - Defined in .Routing Type - Defined in . (CRH-16 value
is 5. CRH-32 value is 6).Segments Left - Defined in .Type-specific Data - Described in .In the CRH, the Type-specific data field contains a list of Segment
Identifiers (SIDs). Each SID represents both of the following:A segment of the path that the packet takes to its
destination.An entry in the CRH Forwarding Information
Base (CRH-FIB) .SIDs are listed in reverse order. So, the first SID in the list
represents the final segment in the path. Because segments are listed in
reverse order, the Segments Left field can be used as an index into the
SID list. In this document, the "current SID" is the SID list entry
referenced by the Segments Left field.The first segment in the path can be omitted from the list. See for examples.In the CRH-16, each SID is encoded in
16-bits. In the CRH-32, each SID is
encoded in 32-bits.In all cases, the CRH MUST end on a 64-bit boundary. So, the Type-
specific data field MUST be padded with zeros if the CRH would otherwise
not end on a 64-bit boundary.Each SID identifies a CRH-FIB entry.Each CRH-FIB entry contains:An IPv6 address.A topological function.Arguments for the topological function. (Optional).The first ten bits of the IPv6 address MUST NOT be fe00. That prefix
is reserved for link-local addresses.The topological function specifies how the processing node forwards
the packet to the next segment endpoint. The following are examples:Forward the packet through the least-cost path to the next
segment endpoint.Forward the packet through a specified interface.Encapsulate the packet in another IPv6 header of any type (e.g.,
MPLS, IPv6) and forward either through the least cost path or a
specified interface.Some topological functions require parameters. For example, a
topological function might require a parameter that identifies the
interface through which the packet should be forwarded.The CRH-FIB can be populated:By an operator, using a Command Line Interface (CLI).By a controller, using the Path
Computation Element (PCE) Communication Protocol (PCEP) or
the Network Configuration Protocol
(NETCONF).By a distributed routing protocol , , .The following rules describe CRH processing:If Segments Left equals 0, skip over the CRH and process the next
header in the packet.If Hdr Ext Len indicates that the CRH is larger than the
implementation can process, discard the packet and send an ICMPv6 Parameter Problem, Code 0, message
to the Source Address, pointing to the Hdr Ext Len field.Compute L, the minimum CRH length ( ).If L is greater than Hdr Ext Len, discard the packet and send an
ICMPv6 Parameter Problem, Code 0, message to the Source Address,
pointing to the Segments Left field.Decrement Segments Left.Search for the current SID in the CRH-FIB. In this document, the
"current SID" is the SID list entry referenced by the Segments Left
field.If the search does not return a CRH-FIB entry, discard the packet
and send an ICMPv6 Parameter Problem, Code 0, message to the Source
Address, pointing to the current SID.If Segments Left is greater than 0 and the CRH-FIB entry contains
a multicast address, discard the packet and send an ICMPv6 Parameter
Problem, Code 0, message to the Source Address, pointing to the
current SID.Copy the IPv6 address from the CRH-FIB entry to the Destination
Address field in the IPv6 header.Decrement the IPv6 Hop Limit.Submit the packet, its topological function and its parameters to
the IPv6 module. See NOTE.NOTE: By default, the IPv6 module determines the next-hop and
forwards the packet. However, the topological function may elicit
another behavior. For example, the IPv6 module may forward the packet
through a specified interface.The algorithm described in this section accepts the following CRH
fields as its input parameters:Routing Type (i.e., CRH-16 or CRH-32).Segments Left.It yields L, the minimum CRH length. The minimum CRH length is
measured in 8-octet units, not including the first 8 octets.In the CRH, the Segments Left field is mutable. All remaining fields
are immutable.A CRH contains one or more SIDs. Each SID is processed by exactly one
node.Therefore, a SID is not required to have domain-wide significance.
Applications can:Allocate SIDs so that they have domain-wide significance.Allocate SIDs so that they have node-local significance.PING and TRACEROUTE both operate
correctly in the presence of the CRH.Networks that process the CRH MUST NOT accept packets containing the
CRH from outside of there limited domain.. Their border routers SHOULD
discard packets that satisfy the following criteria:The packet contains a CRHThe Segments Left field in the CRH has a value greater than 0The Destination Address field in the IPv6 header represents an
interface that resides inside of the network.Many border routers cannot filter packets based upon the
Segments Left value. These border routers MAY discard packets that
satisfy the following criteria:The packet contains a CRHThe Destination Address field in the IPv6 header represents an
interface that resides inside of the network.Juniper Networks has produced experimental implementations of the CRH
on:A LINUX-based software platformThe MX-series (ASIC-based) routerLiquid Telecom has deployed the CRH, on a limited basis, in
their network. Other experimental deployments are in progress.This document makes the following registrations in the "Internet
Protocol Version 6 (IPv6) Parameters" "Routing Types" subregistry
maintained by IANA:Thanks to Dr. Vanessa Ameen, Fernando Gont, Naveen Kottapalli, Joel
Halpern, Tony Li, Gerald Schmidt, Nancy Shaw, Ketan Talaulikar, and
Chandra Venkatraman for their contributions to this document.Gang ChenBaiduNo.10 Xibeiwang East Road Haidian DistrictBeijing 100193 P.R. ChinaEmail: phdgang@gmail.comYifeng ZhouByteDanceBuilding 1, AVIC Plaza, 43 N 3rd Ring W Rd Haidian DistrictBeijing 100000 P.R. ChinaEmail: yifeng.zhou@bytedance.comGyan MishraVerizonSilver Spring, Maryland, USAEmail: hayabusagsm@gmail.comRouting HeadersIANA"Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in conjunction with
the protocol for providing the connectionless-mode Network Service
(ISO 8473)", ISO/IEC 10589:2002, Second Edition,International Organization for
StandardizationThis appendix demonstrates CRH processing in the following
scenarios:The SID list contains one entry for each
segment in the path .The SID list omits the first entry in the path
. provides a reference topology that is used
in all examples.SIDIPv6 AddressForwarding Method22001:db8::2Least-cost path112001:db8::bLeast-cost path describes two entries that appear in each
node's CRH-FIB.In this example, Node S sends a packet to Node D, via I2. In this
example, I2 appears in the CRH segment list.As the packet travels from S to I2:Source Address = 2001:db8::aSegments Left = 1Destination Address = 2001:db8::2SID[0] = 11SID[1] = 2As the packet travels from I2 to D:Source Address = 2001:db8::aSegments Left = 0Destination Address = 2001:db8::bSID[0] = 11SID[1] = 2In this example, Node S sends a packet to Node D, via I2. In this
example, I2 does not appear in the CRH segment list.As the packet travels from S to I2:Source Address = 2001:db8::aSegments Left = 1Destination Address = 2001:db8::2SID[0] = 11As the packet travels from I2 to D:Source Address = 2001:db8::aSegments Left = 0Destination Address = 2001:db8::bSID[0] = 11