Segment Routing for
Enhanced DetNetHuaweigengxuesong@huawei.comHuaweilizhenbin@huawei.comHuaweizhoutianran@huawei.comOne of the goals of DetNet is to provide bounded end-to-end latency
for critical flows. This document defines how to leverage Segment
Routing(SR) and Segment Routing over IPv6 (SRv6) to implement bounded
latency. Specifically, new SRv6 SID function is used to specify bounded
latency information for a packet. When forwarding devices along the path
follow the instructions carried in the packet, the bounded latency is
achieved by different implementations based on bounded latency
information.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 RFC 2119.Deterministic Networking(DetNet) provides a capability to carry
specified data flows with extremely low data loss rates and bounded
latency within a network domain. DetNet is enabled by a group of
technologies, such as resource allocation, service protection and
explicit routes ().Segment Routing(SR) leverages the source routing paradigm. A ingress
node steers a packet through an ordered list of instructions, called
"segments". When SR is used over the MPLS data plane, SIDs are an MPLS
label or an index into an MPLS label space (either SRGB or SRLB).SR can also be applied over IPv6 data plane using Routing Extension
Header(SRH). Besides routing, the segment of SRv6 can indicate functions
which are executed locally in the node where they are defined. SRv6
network programming makes it convenient to add sophisticated operations
in the network. ()DetNet data plane is enhanced to facilitate DetNet transit nodes to
support end-to-end bounded latency transmission. introduces an unified data
plane feild for bounded latency, which is called Bounded Latency
Information(BLI) BLIis designed to cope with a variety of
queuing/scheduling/shaping mechanisms in a uniform format in the data
plane.This document describes how to implement DetNet with SR or SRv6. It
can provide : 1. Source routing, which can steer the DetNet flows go
through the network according to an explicit route with allocated
resource by segment list in SRH; 2. Network programming, which can give
packet instructions in every node along the path to guarantee bounded
latency. DetNet SR MPLS/SRv6 data plane extensions for enhanced DetNet
are defined in this document.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].Terminologies for DetNet go along with the definition in . Other terminologies are defined as follows:NH: The IPv6 next-header field.SID: A Segment Identifier which represents a specific segment
in a segment routing domain().SRH: The Segment Routing Header ().Conventions in the document are defined as follows:NH=SRH means that NH is 43 with routing type 4.A SID list is represented as <S1, S2, S3> where S1 is the
first SID to visit, S2 is the second SID to visit and S3 is the
last SID to visit along the SR path.SRH[SL] represents the SID pointed by the SL field, that is the
SLth SID in the Segment List.(SA,DA) (S3, S2, S1; SL) represents an IPv6 packet with:IPv6 header with source and destination addresses SA and DA
respectively, and next-header SRH, with SID list <S1, S2,
S3> with SegmentsLeft = SLThe payload of the packet is not represented(S3, S2, S1; SL) represents the same SID list as <S1,
S2, S3>, but encoded in the SRH format where the rightmost
SID in the SRH is the first SID and the leftmost SID in the
SRH is the last SIDTo guarantee the end-to-end bounded latency transmission in DetNet
network, bounded latency information is required to be conveyed inband
with the service data to facilitate the queuing algorithm performed on
the DetNet transit nodes. When the bounded latency information is used
in DetNet IP data plane or DetNet MPLS data plane, it is carried in
IP/UDP or MPLS encapsulations. When the bounded latency information is
used in TSN over IP/MPLS data plane, the information used in TSN
networks is transparently transmitted IP/ UDP or MPLS encapsulations.
Note that, which queuing mechanism is used is a local choice determined
by DetNet transit nodes. It is not necessary to be explicitly indicated
in packets.When an SRv6 SID is in the Destination Address field of an IPv6
header of a packet, it is routed through transit nodes in an IPv6
network as an IPv6 address. SRv6 SID consists of LOC:FUNCT:ARG, where a
locator (LOC) is encoded in the L most significant bits of the SID,
followed by F bits of function (FUNCT) and A bits of arguments (ARG),
which is defined in ().Bounded Latency Information (BLI) is defined in to guide forwarding in
network device, which could be initiated in SRv6 data plane. With the
characteristics of Segment Routing, the bounded latency information
could be coupled with explicit path to provide latency guarantee in each
node/ adjacency indicated by the segment list.Bounded Latency Information is indicated by the allocated SID at each
node along the path without maintaining per-flow states at the
intermediate and egress nodes. Hence, it naturally supports flow
aggregation, and that allows DetNet to support large number of DetNet
flows and scale to large networks.As defined in , 8
or more Bounded Latency Information Types (BLI Type) are introduced to
differentiate the types of BLIs, based on the required information of
queuing/scheduling/shaping mechanisms to guarantee bounded latency.
Bounded Latency Information Value (BLI Value) is a specified value of a
specific type of BLI to provide guidance for packet processing with the
meaning of a particular BLI type. The pair <BLI Type, BLI Value>
information should be indicated by SRv6 data plane.The "Endpoint with L3 cross-connect" behavior ("End.X" for short) is
a variant of the End behavior. It is the SRv6 instantiation of an
Adj-SID (), and its main use is for
traffic-engineering policies.Two new variations of End.X SID are defined for DetNet bounded
latency, which are called End.X.BL and End.X.BLI respectively, and
bounded latency information can be defined as functions or arguments in
the new types of SID.Editors Notes: Another option to implement this is to define new
flavors. This method will be considered when not only End.X could be
combined with BLI.This document defines End.X.BL, which is used to identify Bounded
Latency Information for Enhanced DetNet. End.X.BL a variation of
End.X.End.X.BL SID has two meanings: 1) to identify an interface/link,
just like the adjacency SID; 2) to identify the pair <BLI Type and
BLI Value> information on the interface/link to guarantee bounded
latency. So different End.X.BL SIDs could be allocated to the same
interface/link in order to indicated different pairs <BLI Type, BLI
Value>.The SRv6 encapsulation with End.X.BL SIDs is shown as follows:When N receives a packet destined to S and S is a local End.X.BL
SID, N does the following: max_LE) or (Segments Left > Last Entry+1)) {
S10. Send an ICMP Parameter Problem to the Source Address
with Code 0 (Erroneous header field encountered)
and Pointer set to the Segments Left field,
interrupt packet processing, and discard the packet.
S11. }
S12. Decrement IPv6 Hop Limit by 1
S13. Decrement Segments Left by 1
S14. Update IPv6 DA with Segment List[Segments Left]
S15. Submit the packet to the IPv6 module for transmission
to the new destination via a L3 adjacency indicated by the
End.X.BL SID
S16. Send the packet out using indicated by the
End.X.BL SID with the corresponding bounded latency guarantee
mechanism
S17. }
]]>The "Endpoint with forwarding the packet with bounded latency
guarantee by BLI" behavior ("End.X.BLI" for short) is a variant of the
End behavior.End.X.BLI SID has two meanings: 1) to identify an interface/link,
just like the adjacency SID; 2)to identify the BLI Type to guarantee
bounded latency. So different End.X.BLI could be allocated to the same
interface/link in order to indicated different types of BLIs. The BLI
Value corresponding to the End.X.BLI SID is carried explicitly in the
SRv6 packet header.There are 3 possible options for carrying variable BLI Value
associated with the End.X.BLI SID, including: Option1: Arguments in End.X.BLI SIDOption2: SRH TLV for BLI used together with End.X.BLI SIDOption3: New options in DoH before SRH together with End.X.BLI
SIDThe behavior also takes an argument: "Arg.BLI". This argument
provides a local BLI Value information for bounded latency
guarantee. The SRH with End.X.BLI SIDs is showed as follows:whereLocation&Function: the most significant bits that are
used for routing and function indication;Bounded Latency Information : the least significant bits,
which is defined .When N receives a packet destined to S and S is a local End.X.BLI
SID, N does the following: max_LE) or (Segments Left > Last Entry+1)) {
S10. Send an ICMP Parameter Problem to the Source Address
with Code 0 (Erroneous header field encountered)
and Pointer set to the Segments Left field,
interrupt packet processing, and discard the packet.
S11. }
S12. Decrement IPv6 Hop Limit by 1
S13. Decrement Segments Left by 1
S14. Update IPv6 DA with Segment List[Segments Left]
S15. Submit the packet to the IPv6 module for transmission
to the new destination via a L3 adjacency indicated by the
End.X.BLI SID
S16. Send the packet out using BLI Type indicated by the
End.X.BLI SID and BLI Value carried in the argument
with the corresponding bounded latency guarantee mechanism
S17. }
]]>Optional TLV defined in SRH could also be extended for BLI, which
is used together with End.X.BLI.When all or part of the nodes/adjacencies in the explicit path
indicated by the segment list request different BLI values
corresponding to the End.X.BLI SID to guarantee bounded latency, a
BLI List TLV is defined. The SRH with End.X.BLI SIDs is showed as
follows:The Type field is 8 bits in length, and the value is TBD1.The Length field is 8 bits in length and its value is variable,
which depends on the length of BLI list.BLI Left: 8-bit unsigned integer. Number of BLI remaining,
i.e., number of explicitly listed intermediate nodes still to be
visited before reaching the final destination.BLI List[0..m]: 32-bit unsigned integer, representing the nth
BLI in the BLI list.The BLI in the BLI list corresponds to the Segment in the
Segment List one by one. The length of BLI List depends on the
number of End.X.BLI in the segment list.When N receives a packet destined to S and S is a local
End.X.BLI SID, N does the following: max_LE) or (Segments Left > Last Entry+1)) {
S10. Send an ICMP Parameter Problem to the Source Address
with Code 0 (Erroneous header field encountered)
and Pointer set to the Segments Left field,
interrupt packet processing, and discard the packet.
S11. }
S12. Decrement IPv6 Hop Limit by 1
S13. Decrement Segments Left by 1
S14. Update IPv6 DA with Segment List[Segments Left]
S15. Submit the packet to the IPv6 module for transmission
to the new destination via a L3 adjacency
S16. Send the packet out using BLI Type indicated by the
End.X.BLI SID and BLI Value carried by BLI List[BLI Left]
in SRH TLV and BLI Left--
with the corresponding bounded latency guarantee mechanism
S17. }
]]>When all the nodes/adjacencies in the explicit path indicated
by the segment list request the same BLI value to guarantee
bounded latency, the Shared BLI TLV is defined. The SRH with
End.X.BLI SIDs is showed as follows:The Type field is 8 bits in length, and the value is TBD2.The Length field is 8 bits in length and its value is variable,
which depends on the length of BLI list.The Shared BLI field is 32 bits in length and corresponds to
definition of BLI in .When N receives a packet destined to S and S is a local
End.X.BLI SID, N does the following: max_LE) or (Segments Left > Last Entry+1)) {
S10. Send an ICMP Parameter Problem to the Source Address
with Code 0 (Erroneous header field encountered)
and Pointer set to the Segments Left field,
interrupt packet processing, and discard the packet.
S11. }
S12. Decrement IPv6 Hop Limit by 1
S13. Decrement Segments Left by 1
S14. Update IPv6 DA with Segment List[Segments Left]
S15. Submit the packet to the IPv6 module for transmission
to the new destination via a L3 adjacency
S16. Send the packet out using BLI Type indicated by the
End.X.BLI SID and BLI Value indicated by Shared BLI TLV
with the corresponding bounded latency guarantee mechanism
S17. }
]]>According to , BLI could also be
defined through DOH before SRH for the specified segment. For the
case of BLI List, considering that the location of DOH is before
SRH, it is not recommended to be defined in DoH, because it will
affect the processing efficiency of Segment in SRH. For Shared BLI
TLV, it can be carried by the DOH Option. In order for the
consistency, this document recommends to use the SRH TLV to carry
both information.For SR MPLS data plane, this document defines a new segment that is
called a BLI Segment, which is used to identify Bounded Latency
Information for Enhanced DetNet just like End.BL SID. A BLI Segment is
an adjacency segment and allocated from the Segment Routing Local Block
(SRLB). BLI Segment indicateds <BLI Type, BLI
Value> of an interface/link. So different BLI segments could be
allocated to the same interface/link in order to indicated different
pairs <BLI Type, BLI Value>.Editors Notes: SR MPLS extension with meta data which is still under
discussion will be defined based on the progress of MPLS DT. The
possible definition of MPLS segment associated with the variable BLI
values like the SRv6 End.X.BLI will be defined in the future
version.The following codepoints are defined in this document in Segment
Routing Header TLVs registry:TBD