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    MTU

    Last updated: 2024-07-31 14:49:33
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      Maximum Transmission Unit (MTU)

      The MTU determines the maximum size of a data packet that can be transmitted at one time on the network, including the IP packet header and payload, but excluding the Ethernet header. In theory, the larger the MTU, the more data can be transferred in a single packet, resulting in more efficient network communication. This document describes features related to Jumbo Frames.

      Jumbo Frame

      A Jumbo Frame refers to an Ethernet frame with a payload exceeding 1500 bytes. The increased payload percentage helps improve link utilization rate and achieve better network performance. Tencent Cloud supports Jumbo Frames up to 8500 bytes, allowing you to send Ethernet frames with an 8500-byte payload.
      The latest generation instances (SA5/S8/IA5se/ITA5/M8/MA5) have enabled the Jumbo Frame capability by default, setting the MTU to 8500. For more information, refer to Instance Specifications.
      Note:
      Network communication using Jumbo Frames requires considering the network link and peer host's MTU limit. Packets exceeding the MTU limit will be sharded or discarded. It is recommended to test if there are connectivity and performance issues due to MTU mismatches in business scenarios before deploying services within CVM using Jumbo Frames, and manage the packet size in these scenarios to reduce unnecessary performance loss.
      Disabling the Jumbo Frame feature is in grayscale. If you need to disable it by definition, Submit a ticket for support.

      CVM Intercommunication Link MTU Limit

      When CVM instances supporting Jumbo Frames are used as the source host, the link maximum MTU limit for the destination host in different communication scenarios is shown in the table below:
      Destination Host
      Allowed Maximum MTU
      Description
      Within the same VPC, CVM instances with Jumbo Frames enabled
      8500
      No
      Within the VPCs interconnected through CCN, CVM instances with Jumbo Frames enabled
      The Jumbo Frame capability needs to be enabled on the CCN. CCN's support for the Jumbo Frame capability is in beta testing. If needed, you can submit a ticket to apply.
      Hosts within the offline IDC connected through dedicated channels
      The Jumbo Frame capability needs to be enabled on the Direct Connect Gateway. Direct Connect Gateway's support for the Jumbo Frame capability is in beta testing. For details, see Change Channel.
      Within the same VPC, CVM instances without enabling Jumbo Frames
      1500
      No
      Within the VPCs interconnected through CCN, CVM instances without enabling Jumbo Frames
      No
      Hosts within the offline IDC connected through dedicated channels
      No
      Access external hosts through EIP
      No

      Cloud Network Product MTU Limit

      When accessing specific network products using non-TCP protocols (UDP and ICMP), pay attention to the MTU limitations of different network products. Tencent Cloud is currently improving and perfecting the scenarios for supporting Jumbo Frames. The maximum MTU limitations of some network products are as follows:
      Network Product
      Supported Maximum MTU
      Description
      NAT Gateway
      1500
      No
      CLB (including ALB, NLB, and CLB)
      1500
      No
      Direct Connect Gateway
      8500
      The Jumbo Frame capability needs to be enabled on the Direct Connect Gateway. Direct Connect Gateway's support for the Jumbo Frame capability is in beta testing. For details, refer to Change Channel.
      Cloud Connect Network
      8500
      The Jumbo Frame capability needs to be enabled on the CCN. CCN's support for the Jumbo Frame capability is in beta testing. If needed, you can Submit a Ticket to apply.
      Peering connection
      1500
      No
      Note:
      For data packets exceeding the maximum MTU limitations of cloud products, Tencent Cloud's forwarding components will shard or negotiate the message. This ensures connectivity only but not network communication performance. Therefore, in communication scenarios where the maximum MTU of cloud products is 1500, avoid using Jumbo Frames for UDP and ICMP communication.

      FAQs

      Problem Description: When using Jumbo Frames for UDP and ICMP communication, how to resolve the situation if there is a sharp performance decline?
      Solution: Check for data packet sharding and disable Jumbo Frames on CVM instances if necessary.
      If the following scenarios are involved, it is recommended to disable the Jumbo Frame capability on CVM instances:
      Scenario One: If there is a need for interconnection with other public cloud providers through the public network, it is recommended to disable the Jumbo Frame capability on CVM instances.
      Scenario Two: If there is a need for interconnection with other cloud providers through a dedicated line or VPN, it is recommended to disable the Jumbo Frame capability on CVM instances.
      If the following scenarios require enabling Jumbo Frames, ensure that all gateway devices along the link are enabled for Jumbo Frames:
      Scenario One: When multiple gateway devices are passed, all gateway devices along the link need to enable Jumbo Frames, such as in CCN + dedicated line scenarios.
      Scenario Two: When multiple subnets form ECMP routing, all gateway devices need to enable Jumbo Frames, for example, two dedicated line gateways form ECMP.
      How to calculate the appropriate MTU value when data is encapsulated in a tunnel?
      Scenario: How to calculate a reasonable MTU when establishing a tunnel between the customer's IDC and the cloud CVM?
      Calculation method: 1500 - {Tunnel encapsulation overhead} - 36 bytes (GRE encapsulation overhead). The tunnel encapsulation overhead should be calculated based on the actual tunnel type used.
      
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