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.

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:

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:

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.

For MTU rules over NAT gateways, see Maximum transmission unit (MTU) in the Amazon VPC User Guide. For MTU rules over Transit gateways, see Maximum transmission unit (MTU) in the AWS Transit Gateway User Guide.

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.