Friday, June 21, 2013

CCIE DC: Advanced FCIP MTU

Upon further investigation, it appears that there is no point increasing the FCIP MTU beyond 2300 as the FC Traffic will never exceed a certain size as per the IETF RFC:


http://tools.ietf.org/html/draft-ietf-ips-fcovertcpip-00


5.2 FC Frame Mapping to TCP/IP packets

Fig.2 shows the mapping of the FC frame into an TCP/IPv4 Packet. The FC to TCP/IP mapping (and the reverse) mapping is one-to-one since the maximum size of the encapsulated FC Frame along with the header fields does not exceed 2148 bytes. The minimum size FC Frame is 36 bytes resulting in a maximally minimum IP MTU size of 156 bytes. (The maximally minimum MTU size is the IP packet with the minimum size payload and the maximum size IP headers and TCP headers). The maximum size FC frame is 2148 bytes resulting in a nominal IP packet size of 2188 bytes. Fig. 2 shows the format of the IPv4 packet with the standard 20-byte IP fixed header, a 40-byte optional IP header, the standard 20-byte TCP fixed header and TCP options. For the case of the maximum size payload of 2148 bytes, the maximum TCP/IPv4 packet size is 2268 bytes. The maximum size FC frame can cause the IP packet to be fragmented when the data link cannot support this MTU size. Furthermore, TCP may also segment the data if the maximum segment size (MSS) is less than the size of the frame. When an IP packet is fragmented, the required parts of the header must be copied by all fragments and the option field may or may not be copied. When a TCP packet is segmented, normal TCP operations are responsible for receiving and re-sequencing the data prior to passing it on to the FC processing portion of the device.
 

2 comments:

  1. Hi Peter, Thanks for your post, but if we use the MTU 9100/9216 for all the devices in our DC include MDS that configured FCIP, is it OK?

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