TTL - læs her
Each IP packet contains an 8-bit header field that specifies the Time To Live (TTL) for that packet. Implemented as a safeguard against packets looping between routers, it specifies the maximum number of routers that the packet may be forwarded through to it\'s destination. Each time the packet is forwarded the TTL value is decremented by one, and if the value drops to zero, the packet is rejected. If this happens, ideally an ICMP (Internet Control Message Protocol) packet is be sent to the receiver, indicating that the TTL expired and it\'s value would be dynamically adjusted for retransmission. In reality, however, this is rarely implemented and most systems are configured with a fixed value of around 30.
TTL is a field in the IP header which indicates how long a packet should be allowed to survive before it is discarded. This value specifies the maximum number of routers that a packet may be forwarded through on to its destination. TTL essentially determines the maximum number of hops permitted. Windows default is 32, but with the Internet growing larger all the time, it is worth changing it to the larger setting of 64.
Anbefalet værdi=64. Højst mulige værdi er 255.
Maximum Transmission Unit (MTU)
Each of these intermediary hosts are configured to handle what is called a Maximum Transmission Unit (MTU), which is the number of bytes that it will send as one packet. When a host receives a packet that is larger than it\'s MTU, it will break the large packet into multiple smaller ones, each with their own header and data (which is then reassembled by the receiver). Every time a connection is opened between two computers on the Internet, they must agree on an MTU. This is done by comparing both of their MTUs and selecting the smaller of the two. If the MTU is set too large for routers that are between the computers, these routers then fragment this information into a packet size that the router can handle. This fragmentation can double the amount of time it takes to send a single packet. For example, if your computer has an MTU of 1500 and sends a 1500 packet to a router with an MTU of 576, the packet will exceed the router\'s MTU, so it is split into three separate packets and forwarded on. Since 1500 is not evenly divisible by 576, the third packet only contains the last fragment of the original packet. And since each packet must be acknowledged by the receiver, this kind of fragmentation can have a noticeably negative effect on your connection speed.
The Maximum Transmission Unit value is the maximum number of bytes that will be sent in a single packet. Under Windows 95, the default value is 1500, which is not recommended for dial-up connections. Instead, it is recommended that this value be set to 576.
RcvWindow
When a TCP connection is being established, the sender and requester negotiate the maximum data segment size that will be accepted by both hosts. This value, called the Maximum Segment Size (MSS) is different from the MTU because it only addresses the maximum size of the data portion of the packet, and does not include the header. If each data segment, along with the header, does not exceed the smallest MTU as it is being routed to it\'s destination, then it will not be fragmented. Therefore, because the size of a TCP header is 40 bytes, the MSS is calculated as MTU minus 40. In turn, the MSS value is useful in determining the size of the receive window (RWIN), which is the maximum number of bytes of data that can be buffered, pending the receipt of an acknowledgment packet back from the sender. Typically, the RWIN value should be a multiple of 2-10 times the MSS value. For example, a good RWIN value for a system with an MTU of 576 (and an MSS of 536) would be 2144 (536 * 4).
The Receive Window (RWIN) value is the maximum number of bytes of data that can be buffered, pending the receipt of an acknowledgment packet back from the sender. Typically, the RWIN value should be a multiple of 2-10 times the maximum segment size (MTU-40).
Default (8192)
This sets the RWIN value to the Windows default. Under Windows 95, this value is 8192, which should not be used (because it is not evenly divisible by the default maximum segment size, it can force the packet to be resent repeatedly when the buffer is full). You can improve performance by setting this to a lower value such as 4 times your MSS (MSS = MTU - 40).
Black Hole Detection
Black Hole Detection is Disabled by default. Enabling this option forces the system to try to detect black hole routers. Black hole routers are routers that do not send back ICMP (Internet Control Message Protocol) fragmentation-needed messages while automatically determining the optimal MTU value. This increases the maximum number of retransmissions performed for a given segment. Setting this parameter when it is not needed can cause performance degradation. So it is not recommended that you enable this option unless you understand the implications of doing so.
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