The MTU parameter overrides the default Maximum Transmission Unit (MTU) for a network interface. The MTU is the maximum packet size in bytes that the transport transmits over the underlying network. The size includes the transport header. An IP datagram can span multiple packets. Values larger than the default value for the underlying network cause the transport to use the network default MTU. Values smaller than 68 cause the transport to use an MTU of 68.
Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\
ID for Adapter Value Type: REG_DWORD Number
Valid Range: 68 to
the MTU of the underlying network Default: 0xFFFFFFFF
Note ID for Adapter is the network adapter to which TCP/IP is bound. To determine the relationship between an adapter ID and a network connection, view HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Network\{4D36E972-E325-11CE-BFC1-08002BE10318}\
ID for Adapter\Connection. The Name value in these keys provides the friendly name for a network connection that is used in the Network Connections folder. Values under these keys are specific to each adapter. Parameters that have a DHCP configured value and a statically configured value may not exist. Their existence depends on whether the computer or the adapter is DHCP configured and whether static override values are specified.
The following network trace illustrates the problem.
001 CLIENT TRMSRV TCP Control Bits: ....S., len: 0, seq:1962957351-1962957352, ack: 0, win:65535, src: 1083 dst: 3389
002 TRMSRV CLIENT TCP Control Bits: .A..S., len: 0, seq:3814299443-3814299444, ack:1962957352, win:17520, src: 3389 dst: 1083
003 TRMSRV CLIENT TCP Control Bits: .A..S., len: 0, seq:3814299443-3814299444, ack:1962957352, win:17520, src: 3389 dst: 1083
004 CLIENT TRMSRV TCP Control Bits: .A...., len: 0, seq:1962957352-1962957352, ack:3814299444, win:65535, src: 1083 dst: 3389
005 CLIENT TRMSRV TCP Control Bits: .AP..., len: 39, seq:1962957352-1962957391, ack:3814299444, win:65535, src: 1083 dst: 3389
006 TRMSRV CLIENT TCP Control Bits: .AP..., len: 11, seq:3814299444-3814299455, ack:1962957391, win:17481, src: 3389 dst: 1083
007 CLIENT TRMSRV TCP Control Bits: .A...., len: 280, seq:1962957391-1962957671, ack:3814299455, win:65524, src: 1083 dst: 3389
008 TRMSRV CLIENT TCP Control Bits: .A...., len: 0, seq:3814299455-3814299455, ack:1962957671, win:17201, src: 3389 dst: 1083
009 CLIENT TRMSRV TCP Control Bits: .AP..., len: 132, seq:1962957671-1962957803, ack:3814299455, win:65524, src: 1083 dst: 3389
010 TRMSRV CLIENT TCP Control Bits: .AP..., len: 333, seq:3814299455-3814299788, ack:1962957803, win:17069, src: 3389 dst: 1083
011 ROUTER TRMSRV ICMP Destination Unreachable: 10.102.45.12 (See frame 009)
Inside 011: Notice the Next Hop MTU being smaller,and router requesting the sender to fragment the packet 10.ICMP: Destination Unreachable: 10.102.45.12 (See frame 009)
ICMP: Packet Type = Destination Unreachable
ICMP: Unreachable Code = Fragmentation Needed, DF Flag Set <<<<
ICMP: Checksum = 0x6FAA
ICMP: Next Hop MTU = 320 (0x140) <<<<
ICMP: Data: Number of data bytes remaining = 28 (0x001C)
ICMP: Description of original IP frame
ICMP: (IP) Version = 4 (0x4)
ICMP: (IP) Header Length = 20 (0x14)
ICMP: (IP) Service Type = 64 (0x40)
ICMP: (IP) Precedence = 0x40
ICMP: (IP) Type of Service = 0x40
ICMP: (IP) Total Length = 373 (0x175)
ICMP: (IP) Identification = 10838 (0x2A56)
ICMP: (IP) Flags Summary = 2 (0x2)
ICMP: .......0 = Last fragment in datagram
ICMP: ......1. = Cannot fragment datagram
ICMP: (IP) Fragment Offset = 0 (0x0) bytes
ICMP: (IP) Time to Live = 127 (0x7F)
ICMP: (IP) Protocol = TCP - Transmission Control
ICMP: (IP) Checksum = 0x8C1D
ICMP: (IP) Source Address = 10.102.1.248
ICMP: (IP) Destination Address = 10.102.45.12
ICMP: (IP) Data: Number of data bytes remaining = 8 (0x0008)
012 CLIENT TRMSRV TCP Control Bits: .AP..., len: 132, seq:1962957671-1962957803, ack:3814299455, win:65524, src: 1083 dst: 3389
013 TRMSRV CLIENT TCP Control Bits: .A...., len: 0, seq:3814299788-3814299788, ack:1962957803, win:17069, src: 3389 dst: 1083
014 TRMSRV CLIENT TCP Control Bits: .AP..., len: 333, seq:3814299455-3814299788, ack:1962957803, win:17069, src: 3389 dst: 1083
TRMSRV ignores the ICMP packet 11, and resends the same packet 10 without fragmentation
015 ROUTER TRMSRV ICMP Destination Unreachable: 10.102.45.12 (See frame 014)
016 TRMSRV CLIENT TCP Control Bits: .AP..., len: 333, seq:3814299455-3814299788, ack:1962957803, win:17069, src: 3389 dst: 1083
017 ROUTER TRMSRV ICMP Destination Unreachable: 10.102.45.12 (See frame 016)
018 TRMSRV CLIENT TCP Control Bits: .AP..., len: 333, seq:3814299455-3814299788, ack:1962957803, win:17069, src: 3389 dst: 1083
019 ROUTER TRMSRV ICMP Destination Unreachable: 10.102.45.12 (See frame 017)
020 CLIENT TRMSRV TCP Control Bits: .AP..., len: 9, seq:1962957803-1962957812, ack:3814299455, win:65524, src: 1083 dst: 3389
021 CLIENT TRMSRV TCP Control Bits: .A...F, len: 0, seq:1962957812-1962957813, ack:3814299455, win:65524, src: 1083 dst: 3389
022 TRMSRV CLIENT TCP Control Bits: .A...., len: 0, seq:3814299788-3814299788, ack:1962957813, win:17060, src: 3389 dst: 1083
023 TRMSRV CLIENT TCP Control Bits: .A.R.., len: 0, seq:3814299788-3814299788, ack:1962957813, win: 0, src: 3389 dst: 1083
024 CLIENT TRMSRV TCP Control Bits: .A...., len: 0, seq:1962957813-1962957813, ack:3814299455, win:65524, src: 1083 dst: 3389
025 TRMSRV CLIENT TCP Control Bits: ...R.., len: 0, seq:3814299455-3814299455, ack:3814299455, win: 0, src: 3389 dst: 1083
Frames 14, 16, 18, are re-sends, and the connection leading to termination in frame 25.