H3C Technologies H3C S5560 Series Switches - Manual (Page 143)

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4. The TCP source device sends subsequent TCP segments that each are smaller than the MSS (MSS
= path MTU – IP header length – TCP header length).
If the TCP source device still receives ICMP error messages when the MSS is smaller than 32 bytes, the
TCP source device will fragment packets.
An ICMP error message received from a router that does not support RFC 1191 has the MTU of the
outgoing interface set to 0. Upon receiving the ICMP message, the TCP source device selects the path
MTU smaller than the current path MTU from the MTU table as described in RFC 1191 to calculate the TCP
MSS. The MTU table contains MTUs of 68, 296, 508, 1006, 1280, 1492, 2002, 4352, 8166, 17914,
32000, and 65535 bytes. Because the minimum TCP MSS specified by the system is 32 bytes, the actual
minimum MTU is 72 bytes.
After you enable TCP path MTU discovery, all new TCP connections will detect the path MTU. The device
uses the path MTU to calculate the MSS to avoid IP fragmentation.
The path MTU uses the following aging mechanism to make sure that the source device can increase the
path MTU when the minimum link MTU on the path increases:
• When the TCP source device receives an ICMP error message, it reduces the path MTU and starts
an aging timer for the path MTU.
• After the aging timer expires, the source device uses a larger MSS in the MTU table, as described
in RFC 1191.
• If no ICMP error message is received within 2 minutes, the source device increases the MSS again
until the MSS is as large as the MSS negotiated during TCP three-way handshake.
To enable TCP path MTU discovery:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enable TCP path MTU
discovery. tcp path-mtu-discovery [ aging age-time |
no-aging ] The default setting is
disabled.
Enabling TCP SYN Cookie
A TCP connection is established through a three-way handshake:
1. The sender sends a SYN packet to the server.
2. The server receives the SYN packet, establishes a TCP semi-connection in SYN_RECEIVED state,
and replies with a SYN ACK packet to the sender.
3. The sender receives the SYN ACK packet and replies with an ACK packet. A TCP connection is
established.
An attacker can exploit this mechanism to mount SYN Flood attacks. The attacker sends a large number
of SYN packets, but does not respond to the SYN ACK packets from the server. As a result, the server
establishes a large number of TCP semi-connections and can no longer handle normal services.
SYN Cookie can protect the server from SYN Flood attacks. When the server receives a SYN packet, it
responds with a SYN ACK packet without establishing a TCP semi-connection. The server establishes a
TCP connection and enters ESTABLISHED state only when it receives an ACK packet from the client.
To enable TCP SYN Cookie: