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Is the book answer wrong on TCP Header Compression?

Answered Question
Nov 19th, 2006
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Hi, can you help me on this?


Question:


If the command below is issued on the s0/0 interface,

what is the correct behavior.


router(config-if)#ip tcp header-compression passive



a)The router will only compress outgoing TCP packets if incoming

TCP packets on the same interface are compressed.


b) The L2 header will be compressed and therefore cannot be used

for WAN switching networks such as Frame Relay.


c) The L2 header will be left intact and therefore can can be used

for WAN networks such as Frame Relay.



I thought that options a) and c) are correct.

However, the book states that a) and b) are the answer for this exercise.


According to the "BCRAN", Cisco Press book, I see that the L2 header

information is left intact when tcp header-compression is used.


What I am missing?

Correct Answer by pkhatri about 10 years 9 months ago

Hi,


Options (a) and (c) are indeed correct. TCP header compression does not have any impact on the L2 header at all. Obviously, there is an error in the book.


Paresh


PS. Pls do remember to rate posts

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Correct Answer
pkhatri Sun, 11/19/2006 - 13:12
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Hi,


Options (a) and (c) are indeed correct. TCP header compression does not have any impact on the L2 header at all. Obviously, there is an error in the book.


Paresh


PS. Pls do remember to rate posts

smothuku Sun, 11/19/2006 - 13:38
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  • Silver, 250 points or more


Hi ,


TCP/IP header compression, as described by RFC 1144, is designed to improve the efficiency of bandwidth utilization over low-speed serial links. A typical TCP/IP packet includes a 40-byte datagram header. Once a connection is established, the header information is redundant and need not be repeated in every packet that is sent. Reconstructing a smaller header that identifies the connection and indicates the fields that have changed and the amount of change reduces the number of bytes transmitted. The average compressed header is 10 bytes long.


For this algorithm to function, packets must arrive in order. If packets arrive out of order, the reconstruction will appear to create regular TCP/IP packets but the packets will not match the original. Because priority queueing changes the order in which packets are transmitted, enabling priority queueing on the interface is not recommended.


check the below link.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fwan_c/wcffrely.htm#wp1002801


Hope it helps you.


Thanks,

satish




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