Novell NETWARE 6-DOCUMENTATION Manual page 1429

Path MTU Discovery Process
Figure 5 A sample Path MTU discovery process
Host A,
4150 MTU
Token
Ring
20
NetWare TCP/IP Administration Guide
largest IP packet that can be sent across the path without fragmentation. This
feature conforms to RFC 1191.
This feature is automatically enabled when you enable TCP/IP.
There are two advantages to this feature. The Path MTU avoids fragmentation
anywhere along the path and it reduces the protocol overhead.
The Path MTU discovery process prevents fragmentation between two
routers. Figure 5 illustrates a sample Path MTU discovery process, followed
by an example of the steps involved.
PPP, 1500 MTU
Router 1
The following describes the steps involved in the sample Path MTU discovery
process illustrated above:
1. Host A opens a File Transfer Protocol (FTP) connection to Host B.
2. Host A and Host B negotiate the maximum segment size (MSS) during
their connection. This is the largest TCP segment that a host can send
across a network. The MSS in
bytes minus 40 bytes for the IP and TCP headers.
3. Host A sends a 4,150-byte packet to 4,110 bytes of data and 40 bytes of
header information to Host B. The Don't Fragment (DF) flag in the IP
header is set to yes in Host A.
4. Router 1 receives the packet from Host A. Then Router 1 determines that
the packet is larger than 1,500 bytes, which is the maximum packet size
that can be sent over a PPP network.
5. Router 1 sends Host A an ICMP destination unreachable error message.
This message indicates that Router 1 must fragment packets larger than
1,500 bytes.
Router 2
Token
Ring
Figure 5 is 4,110 bytes, which is 4,150
Host B,
4150 MTU