- With the advent of WLANs, a lot of research went into increasing the performance of TCP in wireless and mobile environments, some of its outcome are I-TCP and SNOOP-TCP, Mobile-TCP etc. Let us study I-TCP and Snoop-TCP:
I-TCP
- I-TCP segments a TCP onto fixed part and wireless part.
The example shows a mobile host connected via a wireless link to an access point (AP). Also access node is connected to the internet via the wired Internet.
Standard TCP is used to connect to the AP from fixed computer. No computer over the internet recognises any change to the TCP.
The Access point acts as a proxy of mobile host and terminates the TCP connection.
Therefore, the fixed computer now sees the AP as mobile host; on other hand the mobile host sees AP as the fixed computer.
In between the AP and the mobile host a special TCP adapted to wireless links is used.
A change is TCP is not needed as even as unchanged TCP produces the same round trip time.
Such segmentation methods can be used is connection between mobile node and correspondent host when host is at the FA. So during handover, control transfers from one FA to another FA in the nearby cell.
Acknowlegements:
Here the correspondent node (i.e. Sender) does not notice the wireless link or segmentation of the connection.
The foreign Agent (FA) becomes or acts as a proxy and relays data in both directions.
When the CN sends data, FA sends back a acknowledgement to it.
When the mobile host receives a packet from FA, the mobile host also sends back an acknowledgement
This acknowledgement is a local acknowledgement. It will not be forwarded to the CN.
If a packet is lost in wireless transmission (i.e. no acknowledgement received) then FA will try re-transmitting it again.
Advantages of I-TCP:
I-TCP does not require any changes in TCP protocol as used by the different hosts in network.
Because of a strict partition between the two connections, transmission error on the wireless link will not propagate to the wired link. Therefore, flow will always be in a sequence.
The delay between the FA and Mobile host is small and if optimized properly, precise time-outs can be used to carry out retransmission of lost packets.
Different solutions can be implemented ad tested between the FA and mobile host without jeopardizing the stability of the internet.
With two partitions, we can use a different transport layer protocol in the second half with the FA acting as a translator.
Dis-advantages of I-TCP:
The end-to-end connection for which TCP has been designed will fail if the Foreign Agent (FA) crashes.
The foreign agent (FA) must be a trusted entity as the TCP connections end at this point.
In practical terms increased handover may latency may be much more problematic. (During handover from old FA to new FA, some delay will occur. During this period, some extra data will come at old FA. This data also needs to be send!!)
Snoop-TCP
One of the main feature of I-TCP also goes on to become its major disadvantage i.e. segmentation of TCP.
To overcome it but also to provide enhanced feature a new TCP was designed which worked completely transparent and also left the TCP end-to-end connection intact.
The new idea for making an enhancement is to buffer the data close to the mobile host to perform fast local retransmission in case of packet loss. A good place to carry out this enhancement is at the foreign agent (FA).
Method (How??):
Here, the foreign agent instead of terminating all packet with destination mobile host, it buffers (i.e. temporarily stores all these packets). In addition to this, it also ‘snoops’ each packet flowing in both the directions for reading acknowledgements.
Buffering towards the mobile host is carried out so that a retransmission can be done in case of missing acknowledgements.
The FA buffers every packet until an acknowledgement is received from the mobile host.
If the foreign agent does not receive an acknowledgement within the stipulated time, the packet or the acknowledgement has been lost.
In such a situation, the FA can directly retransmit the packet without waiting for the correspondent host.
Transparency:
To maintain transparency i.e. the communication happens only between the correspondent node (CN) and the mobile host, the FA doesn’t send acknowledgement to the correspondent host as in I-TCP.
The acknowledgement is send by the Mobile host itself. The FA keeps on monitoring it.
When the data flows for mobile host to CN, the FA snoops and checks the sequence of acknowledgement number. If a gap is found, FA sends signal to re-transmit.
Advantages of Snoop-TCP:
The original TCP sematic i.e. end-to-end connection is preserved.
The correspondent node need not be changed as all the new enhancements are made in the FA.
During handover form on cell to another, there is no need to transfer the previous incoming data (as in I-TCP).
In handover, the next foreign Agent (FA) need not use the same enhancements used here i.e. follow Snoop-TCP method.
Dis-Advantages of Snoop-TCP:
If any encryption is applied at both ends, the snooping and buffering process would be a waste of time as no data can be read by FA.
Does not fully isolate wireless link error from the fixed network (e.g. problems like congestion and interference may cause a delay in retransmission).
The Mobile host needs to be modified to handle the NACK signals (No Acknowledgement) for reverse traffic (i.e. from MH to Sender)
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