Internet DRAFT - draft-qian-6man-ipv6-multipath-mtu-detection
draft-qian-6man-ipv6-multipath-mtu-detection
Network Working Group G. Qian
Internet-Draft T. Zhou
Intended status: Standards Track Huawei
Expires: 2 September 2022 1 March 2022
IPv6 Minimum Multipath MTU Detection
draft-qian-6man-ipv6-multipath-mtu-detection-00
Abstract
I In current multipath load balancing network scenario, all path
detection mechanisms have a defect. A typical load balancing route
selection mechanism cannot cover all forwarding paths, which will
cause missing detection.This document describes how to extend a new
path detection mechanism to instruct intermediate devices to send
probe packets to all downstream paths. This new mechanism is named
load-sharing multipath replication forwarding (LMRF).
Requirements Language
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 2 September 2022.
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Scenario Description . . . . . . . . . . . . . . . . . . . . 3
3.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Solution . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Detail solution . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. IPv4 solution . . . . . . . . . . . . . . . . . . . . . . 5
4.2. IPv6 solution . . . . . . . . . . . . . . . . . . . . . . 5
4.2.1. Detection Solution . . . . . . . . . . . . . . . . . 5
4.2.2. Modifications to existing mechanisms . . . . . . . . 6
5. Supplementary description of the protocol . . . . . . . . . . 9
6. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. Normative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
In the current multipath load balancing scenario, a path detection
mechanism has a defect. A common load balancing route selection
solution cannot cover all forwarding paths, which causes missing
detection.This document describes how to extend a new probe mechanism
to instruct intermediate forwarding devices to send probe packets to
all downstream paths.
Typical problem: During path MTU detection, the path MTU of a path
cannot be used as the path MTU of all load balancing paths. In this
case, the source selects the minimum path MTU of different paths as
the path MTU of the entire path to ensure normal forwarding on the
intermediate network.
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Currently, there are some solutions in the industry, such as the
Paris trace solution.By constructing a large number of packets at the
source and modifying information such as the transport-layer port
number of the packets, the forwarding device on the network can hash
the packets to as many forwarding paths as possible during route
selection. This solution cannot ensure that all paths are covered.
In addition, a large number of packets need to be constructed at the
source, which affects network performance and imposes more workload
and skill requirements on O&M engineers.
2. Terminology
The following terminology is used in this document.
MTU: Maximum Transmission Unit
Path MTU: path maximum transmission unit
TWAPM:Two-Way Active Measurement Protocol
BFD: Bidirectional Forwarding Detection
LMRF: Load-sharing multipath replication forwarding
3. Scenario Description
3.1. Example
+-----------+
| |
| B |
/------->| Router |------\
+-----------+ / | | \ +---------+
| | / +-----------+ \ | |
| A |/ \ | D |
| Router |\ \---->| Router |
| | \ / | |
+-----------+ \ +-----------+ / +---------+
\ | | /
\------>| C |-------/
| Router |
| |
+-----------+
Figure 1:Muiltpah Network Example
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As shown in Figure 1, there are two paths from A to D: A-B-D and
A-C-D. The two paths are ECMP paths from A to D. Data packets from
A to D are transmitted based on the 5-tuple or triplet information in
the packet header. Selects a path based on the hash calculation
result. TCP/UDP/ICMP packets are routed based on quintuple, and raw
IP packets are routed based on triplet. Take ping packets as an
example. The source IP address, destination IP address, protocol
number, ICMP type, and ICMP code are used for hash calculation. The
result is used for ECMP route selection. Therefore, ping packets
from A to D can always cover only one path. Therefore, even if the
ping result is normal, services may be abnormal.Conversely, when a
service fault occurs, the ping detection may be normal.
Similar problems occur in trace route detection, BFD detection, TWAMP
detection, and path MTU detection.
In multi-channel load balancing scenarios, incorrect path MTU
detection may cause service exceptions. To simplify packet
processing and improve processing efficiency, IPv6 packets are
fragmented only on the source node.Therefore, the IPv6 path MTU
discovery protocol must be implemented.The latest document (draft-
ietf-6man-mtu-option-11 - IPv6 Minimum Path MTU Hop-by-Hop Option)
provides the path MTU discovery method for a single path, but does
not solve the path MTU problem in multipath scenarios.
+-----------+
| |
MTU 1600 | B | MTU 1600
/------->| Router |------\
+-----------+ / | | \ +-----------+
| | / +-----------+ \ | |
| A |/ \ | D |
| Router |\ \---->| Router |
| | \ / | |
+-----------+ \ +-----------+ / +-----------+
\ | | /
\------>| C |-------/
MTU 1500 | Router | MTU 1500
| |
+-----------+
Figure 2:MTU in Multipath Network
As shown in Figure 2, if the path MTU probe packet from A to D is
A-B-D, the path MTU of this path is 1600, and the path MTU of the
path A-C-D is 1500, Packet loss occurs when data packets with more
than 1500 bytes are routed to route A-C-D.
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3.2. Solution
A universal replication detection mechanism is required to support
connectivity detection, path MTU detection, and delay detection.
This document discusses enhancements to IP header to support
multipath detection.
Path MTU detection affects service availability. Therefore, this
document focuses on the problem of path MTU detection. Other
problems, such as connectivity monitoring and delay monitoring, will
be discussed in the future.
4. Detail solution
4.1. IPv4 solution
This document focuses on the IPv6 network solution, IPv4 netwrok
solution will be discussed in the future.
4.2. IPv6 solution
4.2.1. Detection Solution
For IPv6, Hop by hop header and Destination header are extended to
carry the multipath replication switch and MTU detection switch. For
details, see section 4.2.2. The source node marks the flag, and the
intermediate device and tail device perform corresponding processing.
After the replication function is enabled on the source node, the
source node and transit node copy probe packets to all downstream
load balancing paths. After the MTU detection function is enabled on
the source node, the source node and intermediate node add the MTU
value of the outbound interface to the packet. You can add the MTU
value to the packet one by one, or you can compare the MTU value and
enter the minimum value. The end node responds to all received
detection packets, carries the MTU added along the path, and sends
the packets to the source node. The end node can also compare the
packets and select the smallest MTU as the final path MTU. To
simplify the packet format, packet size, and data-plane prosection
cessing, it is recommended that only the minimum MTU be reserved in
packets. In addition, the path MTU aging mechanism needs to be
modified. Considering that the network topology may change, the path
MTU may increase.If you always select the minimum value, you can
never increase it. Therefore, if no path MTU smaller than or equal
to the current path MTU is received for a long time, the current path
MTU may be set to an aging state. When the path MTU is in the aging
state, the path MTU may be replaced by a larger path MTU.
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4.2.2. Modifications to existing mechanisms
4.2.2.1. Modification of the packet structure
The hop-by-hop extension header is used in common IP packet. The
TTTTT needs to be allocated by the IANA.
Option Option Option
Type Data Len Data
+--------+--------+--------+--------+---------+
|BBCTTTTT|00000011|RRRRRRMD|-------MTU--------+
+--------+--------+--------+--------+---------+
R:Reserved
M:Path MTU detection flag
D:Load balancing duplicating flag
MTU:Minimum MTU on the path
The reply packet uses the DH extension header, and the TTTTT needs to
be allocated from the IANA.
Option Option Option
Type Data Len Data
+--------+--------+--------+---------+
|BBCTTTTT|00000010|-------MTU--------+
+--------+--------+--------+---------+
MTU:Minimum MTU on the path
4.2.2.2. Source node behavior
1. Enable the load balancing duplicating flag.
2. Enable the MTU detection flag.
3. Set the detection timer: The system periodically sends detection
packets in duplicate mode and carries the MTU information of its own
interface. You are advised to set the timer interval to minutes,
which is configurable using the command line.
4. After receiving the response packet from the tail node, the
ingress node compares the path MTU value with the local path MTU
value and selects the minimum value.
5. Set the path MTU aging timer: The lifetime of the path MTU is
periodically updated. When a smaller path MTU or equivalent path MTU
is received, the timer is cleared. It is recommended that the timer
be set to three times of the detection timer.
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6. When the path MTU aging timer expires, the path MTU is set to the
aging state and the minimum MTU detected in the next detection period
is used to overwrite the path MTU.
4.2.2.3. transit node behavior
1. Duplicating is performed to all load balancing next hops based on
the enabling flag of the load balancing duplicating flag.
2. Compare the MTU in packet with the local output interface MTU,
and replace the MTU in the packet with the smaller one.
4.2.2.4. Destination node behavior
1. Send Reply to source node accouding to all received packets and
fill back MTU value get from the received packets.
4.2.2.5. Process flow
step 5
|---------------------------<<---------------------------|
| |
| +-----------+ |
| | | |
| | B | |
| /------->| Router |------\ |
+-----------+ / | |step3 \ +--------+
| | / +-----------+ \ | |
| A |/ step2 \ | D |
| Router |\ \---->| Router |
| | \ / | |
+-----------+ \ +-----------+ / +--------+
step 1 \ | |step4 /
step 6 \------>| C |-------/
| Router |
| |
+-----------+
step 1. Router A try to dicovery the path mtu to Router D
step 2. Two packets will be send to Router D through Router B and
Router C, A-B-D path MTU set as 1600, A-C-D path MTU set as 1700
step 3. Router B received the packet and transfer to Router D, and
modify the MTU to 1500
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step 4. Router C received the packet and transfer to Router D, and
modify the MTU to 1600
step 5. Router D received two packets and reply to Router A with the
corresponding path MTU
step 6. Router A updates local Path MTU with 1500, which is the
smallest one among all reply packets.
4.2.2.6. Uplayer protocol consideration
This function does not depend on upper-layer protocols and can work
with any upper-layer protocols, such as TCP, UPD, ICMP, Quic, and
TWAMP.
Take TWAMP as an example, TWAMP-test packets carry hop-by-hop
extension headers and enable M and D flags to detect the MTU of
multipath. Sequence numbers are used to identify multiple copies of
a packet.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Estimate | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. .
. Packet Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The receiver replies to the source as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Estimate | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receive Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Error Estimate | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender TTL | |
+-+-+-+-+-+-+-+-+ +
| |
. .
. Packet Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sender Sequence Number is a copy of the Sequence Number of the packet
transmitted by the Session-Sender that caused the Session-Reflector
to generate and send this test packet.
5. Supplementary description of the protocol
1. In SDN scenarios, path MTUs can be sent to the controller by
telemetry, and controller then transfer the packets to source node.
This is not discussed in this document.
2. The detection protocol can be extended by TWAMP, BFD, or other
OAM protocol. This document does not provide any analysis.
3. This solution assumes all devices on the network support this
solution. If intermediate devices do not support, real path MTU will
be not detected, Then, PTB will be used to detect the path MTU.
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4. The detection of connectivity faults and parameters such as
latancy in multipath load balancing scenarios will be discussed in
future.
6. Benefits
This solution provides accurate path MTU detection in load balancing
scenarios to prevent packet loss caused by excessively large packets.
7. Acknowledgements
Thank you to Yang Pingan, Zhao Ranxiao, Xia Yang, Wu Qin, Yudan, and
others for participating in the solution discussion and helping
improve the solution.
8. IANA Considerations
For carrying the Load balancing duplicating flag and Path MTU
detection flag, new option types need to be defined in the existing
RH and Hop by Hop headers.
9. Security Considerations
Considering the impact of packet replication on device and network
performance, packets in replication mode need to be traced,
encrypted, URPF, security filtering, and rate limiting.
10. Normative References
[I-D.ietf-6man-mtu-option]
Hinden, R. M. and G. Fairhurst, "IPv6 Minimum Path MTU
Hop-by-Hop Option", Work in Progress, Internet-Draft,
draft-ietf-6man-mtu-option-12, 27 January 2022,
<https://www.ietf.org/archive/id/draft-ietf-6man-mtu-
option-12.txt>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.
Authors' Addresses
Guofeng Qian
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing
100095
China
Email: Qianguofeng@huawei.com
Tianran Zhou
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing
100095
China
Email: Zhoutianran@huawei.com
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