No relevant resource is found in the selected language.

This site uses cookies. By continuing to browse the site you are agreeing to our use of cookies. Read our privacy policy>Search

Reminder

To have a better experience, please upgrade your IE browser.

upgrade

Configuration Guide - IP Service

CloudEngine 8800, 7800, 6800, and 5800 V200R003C00

This document describes the configurations of IP Service, including IP address, ARP, DHCP, DNS, IP performance optimization, IPv6, DHCPv6, and IPv6 DNS.
Rate and give feedback:
Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Path MTU

Path MTU

In IPv4, oversized packets are fragmented. When the transit device receives a packet exceeding the maximum transmission unit (MTU) size of its outbound interface from a source node, the transit device fragments the packet before forwarding it to the destination node. In IPv6, however, the source node fragments the packets to reduce pressure on the transit device. When an interface on the transit device receives a packet whose size exceeds the MTU, the transit device discards the packet and sends an ICMPv6 Packet Too Big message to the source node. The ICMPv6 Packet Too Big message contains the MTU value of the outbound interface. The source node fragments the packet based on the MTU and resends the packet, increasing traffic overhead. The Path MTU Discovery (PMTUD) protocol dynamically discovers the MTU value of each link on the transmission path, reducing excessive traffic overhead.

The PMTU protocol is implemented through ICMPv6 Packet Too Big messages. A source node first uses the MTU of its outbound interface as the PMTU and sends a probe packet. If a smaller PMTU exists on the transmission path, the transit device sends a Packet Too Big message to the source node. The Packet Too Big message contains the MTU value of the outbound interface on the transit device. After receiving this message, the source node changes the PMTU value to the received MTU value and sends packets based on the new MTU. This process repeats until packets are sent to the destination address. The source node obtains the PMTU of the destination address.

Figure 7-15 shows an example of PMTU discovery.

Figure 7-15 PMTU discovery

Packets are transmitted through four links with MTU values of 1500, 1500, 1400, and 1300 bytes. Before sending a packet, the source node fragments the packet based on a PMTU of 1500. When the packet is sent to the outbound interface with MTU 1400, the device returns a Packet Too Big message carrying MTU 1400. The source node then fragments the packet based on MTU 1400 and sends the fragmented packet again. The process repeats when the packet based on MTU 1400 is sent to the outbound interface with MTU 1300, the device returns another Packet Too Big message that carries MTU 1300. The source node receives the message and fragments the packet based on MTU 1300. In this way, the source node sends the packet to the destination address and discovers the PMTU of the transmission path.

NOTE:

IPv6 allows a minimum MTU of 1280 bytes. Therefore, the PMTU must be greater than 1280 bytes. PMTU of 1500 bytes is recommended.

Translation
Download
Updated: 2019-05-08

Document ID: EDOC1100004354

Views: 69392

Downloads: 147

Average rating:
This Document Applies to these Products
Related Documents
Related Version
Share
Previous Next