ICMP Does Not Provide MTU Hint in ICMP Dest Unreachable, DF Set

CAUSE

Windows NT 3.5 and 3.51 TCP/IP uses PMTU (Path Maximum Transfer Unit) discovery to determine the optimum TCP segment size that can be used for data transfers on a path between two computers. PMTU discovery is described in RFC1191. Following is an excerpt from that RFC:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Type = 3    |   Code = 4    |           Checksum            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           unused = 0          |         Next-Hop MTU          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Internet Header + 64 bits of Original Datagram Data      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
				

When a computer running Windows NT 3.5 or 3.51 is used as a router, it does return the ICMP Destination Unreachable message; however it does not include the MTU of the next-hop network. This forces the sending computer to retransmit the data, guessing at the next-hop MTU. Because it can take several guesses to determine the correct value, a small delay can occur.

The following trace, captured by Microsoft Network Monitor, illustrates this behavior:

# Source Dest Prot Description

1  server client TCP  ....S., len: 4, seq:1286279, ack:0, win: 8192,
2  client server TCP  .A..S., len: 4, seq:659688, ack:1286280, win: 8760
3  server client TCP  .A...., len: 0, seq:1286280, ack:659689, win: 8760
4  client server FTP  Data Transfer To Server, Port = 1028, size 1460
5  router client ICMP Destination Unreachable, Destination: 199.199.40.14
6  client server TCP  .A...., len: 0, seq:643477, ack:1270209, win: 8558
7  client server FTP  Data Transfer To Server, Port = 1028, size 966
8  router client ICMP Destination Unreachable, Destination: 199.199.40.14
9  client server FTP  Data Transfer To Server, Port = 1028, size 468
10 server client TCP  .A...., len: 0, seq:1286280, ack:660157, win: 8292
11 client server FTP  Data Transfer To Server, Port = 1028, size 468
12 client server FTP  Data Transfer To Server, Port = 1028, size 468
13 server client TCP  .A...., len: 0, seq:1286280, ack:661093, win: 8760
14 client server FTP  Data Transfer To Server, Port = 1028, size 468
				

15 client server FTP Data Transfer To Server, Port = 1028, size 468

• The first three frames show the 3-way handshake to set up the connection.
• The fourth frame shows an attempt to send a 1460 byte TCP segment over the path (through a Windows NT 3.51 router).
• The fifth frame shows the Windows NT 3.51 router returning an ICMP error, destination unreachable (fragmentation needed, but DF bit set)
• The seventh frame shows the client trying a 966 byte segment. This is the next "likely" MSS (Maximum Segment Size) down from 1460.
• The eighth frame is another ICMP destination unreachable message.
• The ninth frame shows the client trying a 468 byte segment. This succeeds, and the transfer continues using that value.