Tcp mss Value

From: http://blog.chinaunix.net/u2/72886/showart_1802033.html

Low-end and Middle-end roads
Operator

TCP MSS


Implementation Principle


Why do I need to configure both the Intranet and Internet ports?
What about the tcp mss value?
?



PC1 (192.168.0.1)
―――
Router
――――
Internet
―――-
WWW
Server (238.135.1.1)





Create
TCP
The two ends of the connection negotiate during three handshakes.
TCP MSS
The size is as follows:



PC1
Sending
Output
SYN
Packet, where
Option
Option filled
MSS
The field is generally
1460
, The same
WWW Server
Yes
SYN
The message will be sent
SYN
+
ACK
Message response,
Option
Option filled
MSS
The field is
1460
; Both parties will compare
SYN
And
SYN + ACK
Message
MSS
Field size.
MSS
Send
TCP
The size of the part. By comparison
TCP MSS
All
1460
.





For
MPLS l3vpn
,
Pppoe
+
Nat
,
IPSec
,
L2TP
,
GRE
Such as networking, usually because the packet is too large to need to be split, generally can
Set
TCP MSS
Solution
.

For the above example
TCP MSS
How to Implement



1,



Assume that the network port is configured in the vro.
Tcp mss 1200



A)



Router received
WWW Server
Of
SYN
+
ACK
Message will be modified
Option
Option
MSS
The field is
1200
And then forwarded
PC1
,
PC1
After receiving the packet, it is considered that the peer
TCP MSS
Is
1200

PC1
Send data
WWW Server
The
1200
As part size, but the router modifies
TCP MSS
Is
1200
Operations
WWW Server
Is unknown, so
WWW Server
The
1460
Send messages as part size.





2,



Assume that the Internet port of the router is configured
Tcp mss 1200



A)



Router received
PC1
Of
SYN
Message will be modified
Option
Option
MSS
The field is
1200
And then forwarded
WWW Server
, The same
WWW Server
Send data
PC1
The
1200
As part size; same
PC1
Do not know vro Modification
TCP MSS
Is
1200
Because
PC1
The
1460
Send messages as part size.

3,



Therefore, it must be modified simultaneously on the Intranet and Internet for Bidirectional big packet transmission.
TCP MSS

To sum up:
The



TCP MSS
Command only for the outbound Interface

SYN
Message and

SYN
+

ACK
Valid packets.

SYN
And

SYN
+

ACK
The message is invalid.