• TCP mechanisms
• Indirect TCP
• Snooping TCP
• Mobile TCP
• Fast retransmit/recovery
• Transmission freezing
• Selective retransmission
• Transaction oriented TCP
Motivation
• Transport protocols typically designed for
- Fixed end-systems
- Fixed, wired networks
• TCP congestion control
• Packet loss in fixed networks typically due to (temporary) overload situations
• Routers discard packets as soon as the buffers are full
• TCP recognizes congestion only indirectly via missing acknowledgements
• Retransmissions unwise, they would only contribute to the congestion and make it even worse
• Slow-start algorithm as reaction
TCP Slow Start
- Sender calculates a congestion window for a receiver
- Start with a congestion window size equal to one segment
- Exponential increase of the congestion window up to the congestion threshold, then linear increase
- Missing acknowledgement causes the reduction of the congestion threshold to one half of the current congestion window
- Congestion window starts again with one segment
TCP Fast Retransmit/Recovery
- TCP sends an acknowledgement only after receiving a packet
- If a sender receives several acknowledgements for the same packet, this is due to a gap in received packets at the receiver
- However, the receiver got all packets up to the gap and is actually receiving packets
- Therefore, packet loss is not due to congestion, continue with current congestion window (do not use slow-start)
Influences of mobility on TCP
- TCP assumes congestion if packets are dropped
- Typically wrong in wireless networks, here we often have packet loss due to transmission errors
- Furthermore, mobility itself can cause packet loss, if e.g. a mobile node roams from one access point (e.g. foreign agent in Mobile IP) to another while there are still packets in transit to the wrong access point and forwarding is not possible
· The performance of an unchanged TCP degrades severely
- However, TCP cannot be changed fundamentally due to the large base of installation in the fixed network, TCP for mobility has to remain compatible
- The basic TCP mechanisms keep the whole Internet together
Indirect TCP I
- No changes to the TCP protocol for hosts connected to the wired Internet, millions of computers use (variants of) this protocol
- Optimized TCP protocol for mobile hosts
- Splitting of the TCP connection at, e.g., the foreign agent into 2 TCP connections, no real end-to-end connection any longer
- Hosts in the fixed part of the net do not notice the characteristics of the wireless part
I-TCP socket and state migration
- Transparent extension of TCP within the foreign agent
- Buffering of packets sent to the mobile host
- lst packets on the wireless link (both directions!) will be retransmitted immediately by the mobile host or foreign agent, respectively (so called “local” retransmission)
- The foreign agent therefore “snoops” the packet flow and recognizes acknowledgements in both directions, it also filters ACKs
- Changes of TCP only within the foreign agent (+min. MH change)
Advantages
* No changes in the fixed network necessary, no changes for the hosts (TCP protocol) necessary, all current optimizations to TCP still work
* Transmission errors on the wireless link do not propagate into the fixed network
* Simple to control, mobile TCP is used only for one hop between, e.g., a foreign agent and mobile host Therefore, a very fast retransmission of packets is possible, the short delay on the mobile hop is known
Disadvantages
· Loss of end-to-end semantics, an acknowledgement to a sender does not any longer mean that a receiver really got a packet, foreign agents might crash
· Higher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agent
Snooping TCP
- Transparent extension of TCP within the foreign agent
- Buffering of packets sent to the mobile host
- lst packets on the wireless link (both directions!) will be retransmitted immediately by the mobile host or foreign agent, respectively (so called “local” retransmission)
- The foreign agent therefore “snoops” the packet flow and recognizes acknowledgements in both directions, it also filters ACKs
- Changes of TCP only within the foreign agent (+min. MH change)
- Data transfer to the mobile host
* FA buffers data until it receives ACK of the MH, FA detects packet loss via duplicated ACKs or time-out
* Fast retransmission possible, transparent for the fixed network
· Data transfer from the mobile host
* FA detects packet loss on the wireless link via sequence numbers, FA answers directly with a NACK to the MH
* MH can now retransmit data with only a very short delay
Advantages:
· 
Maintain end-to-end semantics
Maintain end-to-end semantics · No change to correspondent node
· No major state transfer during handover
Problems
- Snooping TCP does not isolate the wireless link well
- May need change to MH to handle NACKs
- Snooping might be useless depending on encryption schemes
Mobile TCP
- Special handling of lengthy and/or frequent disconnections
- M-TCP splits as I-TCP does
* Unmodified TCP fixed network to supervisory host (SH)
* Optimized TCP SH to MH
· Supervisory host
* No caching, no retransmission
* Monitors all packets, if disconnection detected
- set sender window size to 0
- sender automatically goes into persistent mode
* Old or new SH reopen the window
Advantages
· Maintains semantics, supports disconnection, no buffer forwarding
Disadvantages
· Loss on wireless link propagated into fixed network
· Adapted TCP on wireless link
Fast retransmit/fast recovery
- Change of foreign agent often results in packet loss
* TCP reacts with slow-start although there is no congestion
· Forced fast retransmit
* As soon as the mobile host has registered with a new foreign agent, the MH sends duplicated acknowledgements on purpose
* This forces the fast retransmit mode at the communication partners
* Additionally, the TCP on the MH is forced to continue sending with the actual window size and not to go into slow-start after registration
Advantage
· Simple changes result in significant higher performance
Disadvantage
· Further mix of IP and TCP (to know when there is a new registration), no transparent approach
Transmission/time-out freezing
· Mobile hosts can be disconnected for a longer time
* No packet exchange possible, e.g., in a tunnel, disconnection due to overloaded cells or mux. with higher priority traffic
* TCP disconnects after time-out completely
· TCP freezing
* MAC layer is often able to detect interruption in advance
* MAC can inform TCP layer of upcoming loss of connection
* TCP stops sending, but does not assume a congested link
* MAC layer signals again if reconnected
Advantage
· Scheme is independent of data and TCP mechanisms (Ack,SN) => works even with IPsec
·
Disadvantage
· TCP on mobile host has to be changed, mechanism depends on MAC layer
Selective retransmission
· TCP acknowledgements are often cumulative
· ACK n acknowledges correct and in-sequence receipt of packets up to n
· If single packets are missing quite often a whole packet sequence beginning at the gap has to be retransmitted (go-back-n), thus wasting bandwidth
· Selective retransmission as one solution
* RFC2018 allows for acknowledgements of single packets, not only acknowledgements of in-sequence packet streams without gaps
* Sender can now retransmit only the missing packets
Advantage:
· Much higher efficiency
Disadvantage
· More complex software in a receiver, more buffer needed at the receiver
Transaction oriented TCP
· TCP phases
§ Connection setup, data transmission, connection release
§ Using 3-way-handshake needs 3 packets for setup and release, respectively
§ Thus, even short messages need a minimum of 7 packets!
· Transaction oriented TCP
§ RFC1644, T-TCP, describes a TCP version to avoid this overhead
§ Connection setup, data transfer and connection release can be combined
§ Thus, only 2 or 3 packets are needed
Advantage
· Efficiency
Disadvantage
· Requires changed TC
· Mobility no longer transparent
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