tcprewrite can fragment packets at the IP level, is there a tool for splitting tcp segments at the TCP level resulting in more smaller sized segments?
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We all know that TCP is a streaming protocol.
Unlike UDP, which guarantees that the whole client message will be delivered to server as a single datagram (we're talking about transport layer, so avoid MTU at the moment), TCP can fragment one "message" (data that was passed to the send function) into several smaller packets so we have to use our own delimiters as a messages' borders.
The question is -- how TCP chooses which data should be fragmented and by which size? Is there any common / standard methods that it uses?
TCP uses a parameter called Maximum Segment Size:
The maximum segment size (MSS) is a parameter of the options field of the TCP header that specifies the largest amount of data, specified in bytes, that a computer or communications device can receive in a single TCP segment. It does not count the TCP header or the IP header. The IP datagram containing a TCP segment may be self-contained within a single packet, or it may be reconstructed from several fragmented pieces; either way, the MSS limit applies to the total amount of data contained in the final, reconstructed TCP segment.
The default TCP Maximum Segment Size is 536. Where a host wishes to set the maximum segment size to a value other than the default, the maximum segment size is specified as a TCP option, initially in the TCP SYN packet during the TCP handshake. The value cannot be changed after the connection is established.
If path MTU discovery is enabled, MSS is set to that minus TCP headers size.
On Linux TCP_MAXSEG socket option controls the parameter:
if this option is set before connection establishment, it also changes the MSS value announced to the other end in the initial packet. Values greater than the (eventual) interface MTU have no effect. TCP will also impose its minimum and maximum bounds over the value provided.
I know that data from the application layer is split into segments by the Transport Layer (like TCP). Also, the Data Link layer might split the datagrams into multiple frames.
What about the Internet layer? Will IP layer simply encapsulate the segment or will it further split it?
Thanks,
Pavan.
The IP layer can't split a single TCP packet into multiple TCP packets, because it doesn't know what TCP is. However, routers along the network path may choose to fragment the IP packet itself into multiple pieces. Each of those pieces contains only a fraction of the TCP packet, so they all need to be received before the TCP layer can go to work. (For that matter, the sending machine can send out the packet pre-fragmented if it likes, though one generally tries to size the TCP packet so it doesn't have to.)
All that's the theory. In practice, IP fragmentation is uncommon, and avoided as much as possible. Additionally, IPv6 doesn't support fragmentation at all.
As far as we know the absolute limitation on TCP packet size is 64K (65535 bytes), and in practicality this is far larger than the size of any packet you will see, because the lower layers (e.g. ethernet) have lower packet sizes. The MTU (Maximum Transmission Unit) for Ethernet, for instance, is 1500 bytes.
I want to know, Is there any any way or any tools, to send packets larger than 64k?
I want to test a device in facing with packet larger than 64k! I mean I want to see, if I send a packet larger than 64K, how it behave? Does it drop some part of it? Or something else.
So :
1- How to send this large packets? What is the proper layer for this?
2- How the receiver behave usually?
The IP packet format has only 16 bit for the size of the packet, so you will not be able to create a packet with a size larger than 64k. See http://en.wikipedia.org/wiki/IPv4#Total_Length. Since TCP uses IP as the lower layer this limit applies here too.
There is no such thing as a TCP packet. TCP data is sent and received in segments, which can be as large as you like up to the limits of the API you're using, as they can be comprised of multiple IP packets. At the receiver TCP is indistinguishable from a byte stream.
NB osi has nothing to do with this, or anything else.
TCP segments are not size-limited. The thing which imposes the limit is that IPv4 and IPv6 packets have 16 bit length fields, so a size larger than this limit is not possible to express.
However, RFC 2675 is a proposed standards for IPv6 which would expand the length field to 32 bits, allowing much larger TCP segments.
See here for a talk about why this change could help improve performance and here for a set of (experimental) patches to Linux to enable this RFC.
I'm trying to reverse engineer an application, and i need help understanding how TCP window size works. My MTU is 1460
My application transfers a file using TCP from point A to B. I know the following:
The file is split into segments of size 8K
Each segment is compressed
Then each segment is sent to point B over TCP. These segment for a text file can be of size 148 Bytes, and for a pdf 6000 Bytes.
For a text file, am i supposed to see the segments of 148 attached to one another to form one large TCP stream? and then it is split according to the Window Size?
Any help is appreciated.
The receiver application should see the data in teh same way, the sender application sent it. TCP uses byte-streaming and so it collects all the bytes in an in-order manner and delivers it to the application. MTU is largely an internal semantics to TCP and does not take into application-layer packet boundaries. If TCP has enough data to send in its send buffer (each TCP socket has its own send buffer, btw), then it will package its next segment worth MTU size and sends it; to be more precise, it deducts TCP and IP header from the MTU size.
What happen when ICMP is disabled in an router and when packet size greater than MTU how the router fragments that packet?Will TCP header be present in IP fragmented packets?
I don't test this scenario but ... I think that if the IP fragmentation is enabled, your IP packet data part will be fragmented and transferred independently because the fragmentation was used at L3 layer. Without ICMP, the sending computer don't know that the packet size is bigger than the allowed network MTU and it cannot send new fragmented data at L4 layer (TCP header will be presented in each packet). But we are fragmenting on L3 layer and we have to use fragment offset field. I think that the TCP header will be presented only in one packet, followed by fragmented TCP data part.
Try to make an experiment to test this behavior.