Network maximum transmission unit (MTU) for your EC2 instance
The maximum transmission unit (MTU) of a network connection is the size, in bytes, of the largest permissible packet that can be passed over the connection. The larger the MTU of a connection, the more data that can be passed in a single packet. Ethernet frames consist of the packet, or the actual data you are sending, and the network overhead information that surrounds it.
Ethernet frames can come in different formats, and the most common format is the standard Ethernet v2 frame format. It supports 1500 MTU, which is the largest Ethernet packet size supported over most of the internet. The maximum supported MTU for an instance depends on its instance type.
The following rules apply to instances that are in Wavelength Zones:
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Traffic that goes from one instance to another within a VPC in the same Wavelength Zone has an MTU of 1300.
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Traffic that goes from one instance to another that uses the carrier IP within a Wavelength Zone has an MTU of 1500.
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Traffic that goes from one instance to another between a Wavelength Zone and the Region that uses a public IP address has an MTU of 1500.
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Traffic that goes from one instance to another between a Wavelength Zone and the Region that uses a private IP address has an MTU of 1300.
The following rules apply to instances that are in Outposts:
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Traffic that goes from an instance in Outposts to an instance in the Region has an MTU of 1300.
Contents
Jumbo frames (9001 MTU)
Jumbo frames allow more than 1500 bytes of data by increasing the payload size per packet, and thus increasing the percentage of the packet that is not packet overhead. Fewer packets are needed to send the same amount of usable data. However, traffic is limited to a maximum MTU of 1500 in the following cases:
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Traffic over an internet gateway
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Traffic over an inter-region VPC peering connection
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Traffic over VPN connections
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Traffic outside of a given AWS Region
If packets are over 1500 bytes, they are fragmented, or they are dropped if the
Don't Fragment flag is set in the IP header.
Jumbo frames should be used with caution for internet-bound traffic or any traffic that leaves a VPC. Packets are fragmented by intermediate systems, which slows down this traffic. To use jumbo frames inside a VPC and not slow traffic that's bound for outside the VPC, you can configure the MTU size by route, or use multiple elastic network interfaces with different MTU sizes and different routes.
For instances that are collocated inside a cluster placement group, jumbo frames help to achieve the maximum network throughput possible, and they are recommended in this case. For more information, see Placement groups for your Amazon EC2 instances.
You can use jumbo frames for traffic between your VPCs and your on-premises networks over AWS Direct Connect. For more information, and for how to verify Jumbo Frame capability, see Setting Network MTU in the AWS Direct Connect User Guide.
All Amazon EC2 instance types support 1500 MTU and all current generation instance types support jumbo frames. The following previous generation instance types support jumbo frames: A1, C3, I2, M3, and R3.
For more information about supported MTU sizes:
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For NAT gateways, see NAT gateway basics in the Amazon VPC User Guide.
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For transit gateways, see MTU in the Amazon VPC Transit Gateways User Guide.
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For Local Zones, see Considerations in the AWS Local Zones User Guide.
Path MTU Discovery
Path MTU Discovery (PMTUD) is used to determine the path MTU between two devices. The path MTU is the maximum packet size that's supported on the path between the originating host and the receiving host. When there is a difference in the MTU size in the network between two hosts, PMTUD enables the receiving host to respond to the originating host with an ICMP message. This ICMP message instructs the originating host to use the lowest MTU size along the network path and to resend the request. Without this negotiation, packet drop can occur because the request is too large for the receiving host to accept.
For IPv4, when a host sends a packet that's larger than the MTU of the receiving host
or that's larger than the MTU of a device along the path, the receiving host or device
drops the packet, and then returns the following ICMP message: Destination
Unreachable: Fragmentation Needed and Don't Fragment was Set (Type 3, Code
4). This instructs the transmitting host to split the payload into multiple smaller
packets, and then retransmit them.
The IPv6 protocol does not support fragmentation in the network. When a host sends a
packet that's larger than the MTU of the receiving host or that's larger than the MTU of
a device along the path, the receiving host or device drops the packet, and then returns
the following ICMP message: ICMPv6 Packet Too Big (PTB) (Type 2). This
instructs the transmitting host to split the payload into multiple smaller packets, and
then retransmit them.
Connections made through some components, like NAT gateways and load balancers, are automatically tracked. This means that security group tracking is automatically enabled for your outbound connection attempts. If connections are automatically tracked or if your security group rules allow inbound ICMP traffic, you can receive PMTUD responses.
Note that ICMP traffic can be blocked even if the traffic is allowed at the security group level, such as if you have a network access control list entry that denies ICMP traffic to the subnet.
Important
Path MTU Discovery does not guarantee that jumbo frames will not be dropped by some routers. An internet gateway in your VPC will forward packets up to 1500 bytes only. 1500 MTU packets are recommended for internet traffic.
Troubleshoot
If you experience connectivity issues between your EC2 instance and an Amazon Redshift cluster when using jumbo frames, see Queries Appear to Hang in the Amazon Redshift Management Guide.
