Re: MSS vs MTU

> Answer is no. Let me use a real world example. tcp adjust modifies the MSS
> for TCP SYN packets traveling through the interface. The router compares the
> MSS value of incoming or outgoing packets against the adjusted MSS setting
> and replaces smaller values that it detects in any packets with the larger
> setting. If the packet does not contain an MSS value, the router assumes a
> value of 536 for the packet MSS on which to base the comparison. The no
> version removes the MSS modification.When a user requests a web site, a
> client/server negotiation occurs between the PC and the web server that
> hosts the web site. During the negotiation, a maximum MTU size is
> negotiated. Since the PC negotiates and its default MTU size is 1500 bytes,
> the web server negotiates an MTU size of 1500 bytes. Therefore, regardless
> of the MTU size you configure on the router, the web server still sends
> packets up to 1500 bytes in size.The reason why some pages do not fully load
> is that the router fragments IP packets if the PC MTU is misconfigured and a
> packet greater than 1492 bytes is sent to the router. This fragmentation
> does not occur on the return path through the UAC. When the UAC receives a
> packet greater than 1492 bytes, the packet is dropped, and the UAC generates
> and sends an Internet Control Message Protocol (ICMP) message to the web
> server that sent the oversized packet. The ICMP informs the web server that
> it sent an oversized packet and that it needs to resend the packet with a
> smaller MTU.The problem occurs because many web servers block ICMP messages,
> which causes the server to continuously send 1500-byte packets. These
> packets are dropped, and as a result, the requested web site does not load.
> If the web server is properly configured and ICMP messages are not blocked,
> the server adjusts its MTU and retransmits until the page loads completely.A
> partially loaded page occurs when the initial data packets sent from the web
> server are under the 1492 byte maximum. However, a packet is then sent that
> exceeds this maximum. The server continues to retransmit this oversized
> packet that results in a partially loaded page and a "waiting for reply..."
> message in the status bar.

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Received on Thu Oct 29 2009 - 19:15:19 ART