Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

ipsec + gre overhead

Hello,

Can anybody tell how much overhead will the ipsec and gre tunnel add? I need to correctly adjust the mss on a tunnel interface, in order to avoid the fragmentation. The transform set is: ah-sha-hmac esp-aes esp-sha-hmac.

2 REPLIES
ovt Bronze
Bronze

Re: ipsec + gre overhead

Not sure about AH (IMHO using ah-sha-hmac with ESP adds almost nothing to security). If the transform set is esp-aes esp-sha-hmac then the MAXIMUM overhead is calculated as follows:

ESP constant: SPI(4) + SN(4) + PADLength(1) + NextHeader(1) = 10

ESP-AuthData: always truncated to 12 Bytes

AES-CBC (RFC 3602): IV(16) + MaxPadding(15)

Overall MAXIMUM ESP(AES) overhead = 10 + 12 + 31 = 53. Obviously the padding cannot be odd, so use 52 as the MAXIMUM overhead.

(Also, so far as my understanding goes, the Padding for MTU=1500 should never be more than 2, but still use 30 for safety reasons...)

P-to-P GRE overhead is 4, P-to-MP GRE overhead is 8.

ESP in tunnel mode adds IP header which is usually 20 Bytes. Transport mode doesn't add new IP header.

To sum up: "ip mtu" on the point-to-point GRE interface with esp-aes esp-sha-hmac and IPSec tunnel mode should be set to 1500-20(IP-added-by-IPSec)-52(ESP)-4(p-to-p GRE)-20(IP-added-by-GRE)=1500-96=1404.

For transport mode "ip mtu" = 1424.

Using this figures you can calculate the "ip tcp adjust-mss" by yourself ;)

New Member

Re: ipsec + gre overhead

Thank you very much. I shall use 1360B for the mss, in order to be sure that the fragmentation will not occur.

Regards,

Gabriel

1732
Views
0
Helpful
2
Replies
CreatePlease login to create content