| Internet-Draft | IPv6 Compressed Routing Header | November 2021 |
| Bonica, et al. | Expires 16 May 2022 | [Page] |
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.¶
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IPv6 [RFC8200] source nodes use Routing headers to specify the path that a packet takes to its destination. The IETF has defined several Routing header types [IANA-RH]. 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:¶
The following are reasons for encoding the CRH in as few bytes as possible:¶
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 [RFC2119] [RFC8174] 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:¶
In the CRH, the Type-specific data field contains a list of Segment Identifiers (SIDs). Each SID represents both of the following:¶
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 Appendix A for examples.¶
In the CRH-16 (Figure 1), each SID is encoded in 16-bits. In the CRH-32 (Figure 2), 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.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len | Routing Type | Segments Left | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID[0] | SID[1] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | ......... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len | Routing Type | Segments Left | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + SID[0] + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + SID[1] + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + SID[n] + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each SID identifies a CRH-FIB entry.¶
Each CRH-FIB entry contains:¶
The IPv6 address can represent either:¶
The first ten bits of the IPv6 address MUST NOT be fe00. That prefix is reserved for link-local [RFC6890] addresses.¶
The topological function specifies how the processing node forwards the packet to the next segment endpoint. The following are examples:¶
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 following flags are defined:¶
The service function is optional. If present, it invokes a node specific procedure. The following are examples of node specific procedures:¶
Node specific procedures are not subject to standardization. A node can support any number of node specific procedures and associate them with any SIDs.¶
Some service functions require parameters. For example, an instruction to emit telemetry might require an IP address to which telemetry should be sent.¶
The CRH-FIB can be populated:¶
The following rules describe CRH processing:¶
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:¶
It yields L, the minimum CRH length. The minimum CRH length is measured in 8-octet units, not including the first 8 octets.¶
<CODE BEGINS>
switch(Routing Type) {
case CRH-16:
if (Segments Left <= 2)
return(0)
sidsBeyondFirstWord = Segments Left - 2;
sidPerWord = 4;
case CRH-32:
if (Segments Left <= 1)
return(0)
sidsBeyondFirstWord = Segments Left - 1;
sidsPerWord = 2;
case default:
return(0xFF);
}
words = sidsBeyondFirstWord div sidsPerWord;
if (sidsBeyondFirstWord mod sidsPerWord)
words++;
return(words)
<CODE ENDS>¶
The processing node SHOULD execute the following procedure, if it is capable of doing so:¶
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:¶
PING and TRACEROUTE [RFC2151] both operate correctly in the presence of the CRH.¶
Networks that process the CRH MUST NOT accept packets containing the CRH from untrusted sources. Their border routers SHOULD discard packets that satisfy the following criteria:¶
Many border routers cannot filter packets based upon the Segments Left value. These border routers MAY discard packets that satisfy the following criteria:¶
Juniper Networks has produced experimental implementations of the CRH on:¶
Liquid 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:¶
+-------+------------------------------+---------------+
| Value | Description | Reference |
+=======+==============================+===============+
| 5 | CRH-16 | This document |
+-------+------------------------------+---------------+
| 6 | CRH-32 | This document |
+-------+------------------------------+---------------+
¶
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.¶
This appendix demonstrates CRH processing in the following scenarios:¶
----------- ----------- -----------
|Node: S | |Node: I1 | |Node: I2 |
|Loopback: |---------------|Loopback: |---------------|Loopback: |
|2001:db8::a| |2001:db8::1| |2001:db8::2|
----------- ----------- -----------
| |
| ----------- |
| |Node: D | |
---------------------|Loopback: |---------------------
|2001:db8::b|
-----------
Figure 3 provides a reference topology that is used in all examples.¶
| SID | IPv6 Address | Forwarding Method |
|---|---|---|
| 2 | 2001:db8::2 | Least-cost path |
| 11 | 2001:db8::b | Least-cost path |
Table 1 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::a | Segments Left = 1 |
| Destination Address = 2001:db8::2 | SID[0] = 11 |
| SID[1] = 2 |
| As the packet travels from I2 to D: | |
|---|---|
| Source Address = 2001:db8::a | Segments Left = 0 |
| Destination Address = 2001:db8::b | SID[0] = 11 |
| SID[1] = 2 |
In 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::a | Segments Left = 1 |
| Destination Address = 2001:db8::2 | SID[0] = 11 |
| As the packet travels from I2 to D: | |
|---|---|
| Source Address = 2001:db8::a | Segments Left = 0 |
| Destination Address = 2001:db8::b | SID[0] = 11 |
Network A--D1
/ \ /
/ \ /
/ \ /
S ---- P +
\ / \
\ / \
\ / \
Network B---DN
S needs to reach each destination node through two SR paths. One SR path traverses Network A while the other traverses Network B.¶
Uncompressed SRv6 can encode this SR Path in two segments,with one segment instantiated on P and the other on the destination. To support this strategy, P instantiates two END.X SIDs (one per network).¶
CRH compressed SRv6 can encode this SR Path in two or three segments. When it encodes the path in two segments, one segment instantiated on P and the other on the destination. To support this strategy, P instantiates 2*N SIDs (one per network per destination). When CRH compressed SRv6 encodes the path in three segments, two segments are instantiated on P and the other on the destination. The first segment on P updates the IPv6 Destination address without forwarding the packet, while the other segment on P forwards the packet without updating the IPv6 destination address. To support this strategy, P instantiates 2+N SIDs (one per network and one per destination).¶