Re: MSS vs MTU

Jumping in on this one.... after reading the docs I have some questions,
hopefully someone can understand my jibberish and shed some light. If tcp
adjust-mss and ip mtu are both configured on an interface? Does the tcp
adjust-mss apply only the tcp sements and ip mtu apply to all other packets,
ie udp, icmp?

I would think that if you configured tcp adjust-mss and then ip mtu sizes
that ip mtu would over right the mss value because ip mtu would encompass
tcp. Or does the tcp adjust-mss value explicity define the max tcp segment
size sans headers?

Also if someone had a configuration of a larger tcp adjust-mss value
compared to the ip mtu value on the same interface, aren't you just
fragmenting all tcp segments larger than the mtu setting that travel out
that interface?

On Thu, Oct 29, 2009 at 9:55 AM, mark jackson <markcciejackson_at_gmail.com>wrote:

> Correct
>
> Mark Jackson, CCIE#4736
>
> Sent from my iPhone
> Please excuse spelling errors
>
> On Oct 29, 2009, at 7:55 AM, "Mark Stephanus Chandra" <
> mark.chandra_at_gmail.com> wrote:
>
> Mark and Alex,
>
>
>
> Thanks a lot for your explanation,
>
>
>
> So, when the 3 way handshake is on progress, the lower MSS is gonna be
> chosen and agreed between two sites, right ?
>
>
>
> Regards
>
>
>
> Mark
>
>
>
> *From:* Mark Jackson [mailto:markcciejackson_at_gmail.com]
> *Sent:* 29 Oktober 2009 20:13
> *To:* Mark Stephanus Chandra
> *Cc:* Cisco certification
> *Subject:* Re: MSS vs MTU
>
>
>
> TCP and IPv4 headers are 20 bytes long each, and IPv6 header is 40 bytes
> long, so the MSS is equal to MTU minus 40 when using IPv4, and MTU minus 60
> when using IPv6 (in most cases, MTU for Ethernet is 1500 bytes).
>
> MSS and MTU are almost the same, yet not so much. MSS, which is the
> largest
> segment (layer 4, yet not including the layer 4 header) that can fit on the
> current physical medium. MTU, which is the largest packet (layer 3,
> including the layer 3 header) that can be transmitted.
> The MSS is used during the 3-way handshake of TCP to let each side know
> that
> maximum segment size they can transmitt in a single frame. It's purpose is
> to minimize IP fragmentation. However, that is only each side. Who only
> knows what is in the middle. That is why each entity in between must know
> what its MTU size is in case fragmentation is required somewhere along the
> path.
>
> Hopefully that helps.
>
> Thanks kindly.
>
>
>
> On Thu, Oct 29, 2009 at 3:13 AM, Mark Stephanus Chandra <
> mark.chandra_at_gmail.com> wrote:
>
> Can Anyone tell me what is the difference between MSS and MTU ?
>
>
>
> I have look in Cisco DOC explanation, it mention about the PPPOE technology
> that need adjusment in MSS, but still the explanation make no sense to me.
>
>
>
>
>
>
>
> Regards
>
> Mark Stephanus Chandra - CCIE#23887
>
>
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> --
> Mark Jackson, CCIE #4736
> Senior Network, Security and Voice Architect
>
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Received on Thu Oct 29 2009 - 20:39:34 ART