Signaling Maximum Transmission Unit (MTU) using BGP-LSChina Telecom109, West Zhongshan Road, Tianhe District.Guangzhou510000Chinazhuyq8@chinatelecom.cn Huawei TechnologiesHuawei Bld., No.156 Beiqing Rd.Beijing100095Chinahuzhibo@huawei.comHuawei TechnologiesHuawei Bld., No.156 Beiqing Rd.Beijing100095Chinapengshuping@huawei.comMTN Uganda Ltd.UgandaRobbins.Mwehair@mtn.comBGP Link State (BGP-LS) describes a mechanism by which link-state and
TE information can be collected from networks and shared with external
components using the BGP routing protocol. The centralized controller
(PCE/SDN) completes the service path calculation based on the
information transmitted by the BGP-LS and delivers the result to the
Path Computation Client (PCC) through the PCEP or BGP protocol.Segment Routing (SR) leverages the source routing paradigm, which can
be directly applied to the MPLS architecture with no change on the
forwarding plane and applied to the IPv6 architecture, with a new type
of routing header, called SRH. The SR uses the IGP protocol as the
control protocol. Compared to the MPLS tunneling technology, the SR does
not require additional signaling. Therefore, the SR does not support the
negotiation of the Path MTU. Since multiple labels or SRv6 SIDs are
pushed in the packets, it is more likely that the packet size exceeds
the path mtu of SR tunnel.This document specifies the extensions to BGP Link State (BGP-LS) to
carry maximum transmission unit (MTU) messages of link. The PCE/SDN
calculates the Path MTU while completing the service path calculation
based on the information transmitted by the BGP-LS.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.describes the implementation mechanism of
BGP-LS by which link-state and TE information can be collected from
networks and shared with external components using the BGP routing
protocol . BGP-LS allows the necessary
Link-State Database (LSDB) and Traffic Engineering Database (TEDB)
information to be collected from the IGP within the network, filtered
according to configurable policy, and distributed to the PCE as
necessary.The appropriate MTU size guarantees efficient data transmission. If
the MTU size is too small and the packet size is large, fragmentation
may occur too much and packets are discarded by the QoS queue. If the
MTU configuration is too large, packet transmission may be slow. Path MTU
is the maximum length of a packet that can pass through a path without
fragmentation. describes a technique for
dynamically discovering the maximum transmission unit (MTU) of an
arbitrary internet path.The traditional MPLS tunneling technology has signaling for
establishing a path. defines the mechanism for
automatically discovering the Path MTU of LSPs. For a
certain FEC, the LSR compares the MTU advertised by all downstream
devices with the MTU of the FEC output interface in the local device,
and calculates the minimum value for the upstream device. specify the mechanism of MTU signaling in
RSVP-TE. The ingress node of the RSVP-TE tunnel sends a Path message to
the downstream device. The Adspec object in the Path message carries the
MTU. Each node along the tunnel receives a Path message, compares the
MTU value in the Adspec object with the interface MTU value and MPLS MTU
configured on the physical output interface of the local tunnel,
obtains the minimum MTU value, and puts it into the newly constructed
Path message and continues to send it to the downstream equipment. Thus,
the MTU carried in the Path message received by the Egress node is the
minimum value of the path MTU. The Egress node brings the negotiated
Path MTU back to the Ingress node through the Resv message.Segment Routing (SR) described in leverages
the source routing paradigm. Segment Routing can be directly applied to
the MPLS architecture with no change on the forwarding plane and applied to the IPv6 architecture with a new type
of routing header called the SR header (SRH) . defines SR extensions
to BGP-LS and specifies the TLVs and sub-TLVs for advertising SR
information. Based on the SR information reported by the BGP-LS, the SDN
can calculate the end-to-end explicit SR-TE paths or SR Policies.Nevertheless, Segment Routing is a tunneling technology based on the
IGP protocol as the control protocol, and there is no additional
signaling for establishing the path. so the Segment Routing tunnel
cannot currently support the negotiation mechanism of the MTU. Multiple
labels or SRv6 SIDs are pushed in the packets. This causes the length of
the packets encapsulated in the Segment Routing tunnel to increase
during packet forwarding. This is more likely to cause packet size
exceed the traditional MPLS packet size.This document specify the extension to BGP Link State (BGP-LS) to
carry link maximum transmission unit (MTU) messages.This draft refers to the terms defined in , and .This document suggests a solution to extension to BGP Link State
(BGP-LS) to carry maximum transmission unit (MTU) messages. The MTU
information of the link is acquired through the process of collecting
link state and TE information by BGP-LS. Concretely, a router maintains
one or more databases for storing link-state information about nodes and
links in any given area. The router's BGP process can retrieve topology
from these IGP, BGP and other sources, and distribute it to a consumer, either directly or via
a peer BGP speaker (typically a dedicated Route Reflector). specifies a possible way of using the ISIS mechanism and extensions for link
MTU Sub-TLV. In the case of inter-AS scenario (e.g., BGP EPE), the link MTU of the inter-AS link can be collected via BGP-LS directly.As per , the collection of link-state and TE
information and its distribution to consumers is shown in the following
figure.Please note that this signaled MTU may be different from the actual MTU, which is usually from configuration mismatches in a control plane and a data plane component. defines the BGP-LS NLRI that can be a Node
NLRI, a Link NLRI or a Prefix NLRI. The corresponding BGP-LS attribute
is a Node Attribute, a Link Attribute or a Prefix Attribute. defines the TLVs that map link-state information to
BGP-LS NLRI and the BGP-LS attribute. Therefore, according to this
document, a new sub-TLV is added to the Link Attribute TLV. It is an independent attribute TLV that can be used for the link NLRI advertised with all the Protocol IDs.The format of the sub-TLV is as shown below.Whenever there is a change in MTU value represented by Link
Attribute TLV, BGP-LS should re-originate the respective TLV with the
new MTU value.This document requests assigning a new code-point from the BGP-LS
Link Descriptor and Attribute TLVs registry as specified in section
4.This document does not introduce security issues beyond those
discussed in RFC7752.Email:yangang@huawei.comEmail:yaojunda@huawei.com